MN1237
MILLWRIGHT
MANUAL
Province of British Columbia
Ministry of Labour
Apprenticeship Branch
Second Edition
1996
Canadian Cataloguing in Publication Data
Main entry under title:
Millwright Manual for the Apprenticeship Branch, Ministry
of Labour, Province of British Columbia.
Editor: Jenni Gehlbach, Cf. Credits.
Previous ed. published: Manual of Instruction for the
Millwright Trade I Richard A. Michener. Province of
British Columbia Apprenticeship Training Programs
Branch, 1980.
ISBN 0-7718-9473-2
I. Mills and millwork - Handbooks, manuals, etc.
2. Milling machinery - Maintenance and repair.
L Gehlbach, Jenni. II. Michener, Richard A. Manual of
instruction for the millwright trade. 1I1. British
Columbia. Apprenticeship Branch.
TJl040.M541996
621.8
SAFETY ADVISORY
Be advised that references to the Workers'
Compensation Board of British Columbia
safety regulations contained within Ihese
materials do not/may not rellect the most
recent Occupational Health and Safety
Regulation (the current Standards and
Regulation in Be can be obtained on the
following website:
http://www.worksafebc.com}. prease nole
that it is always the responsibility of any
person using these materials to inform
him/herself about the OclXlpational Health
and Safely Regulation pertaining to hislher
area of work,
Industry Training and Apprenliceship
Commission
August 2001
C96-960180-8
Ordering
Queen's Printer
Government Publication Services
563 Superior Street
PO Box 9452 Sin Prov Govt
Victoria, British Columbia
Canada V8W 9V7
Telephone: 250387-6409 or 1 800 663-6105
Fax: 250387-1120
Email: QPPublications@gems5.gov.bc.ca
web: www.publications.gov.bc.ca
Millwright Manual
Order number: MN 1237
ISBN: 0-7718-9473-2
Payment options are by company cheque or money
order (no personal cheques) made payable to
Minister of Finance; and Visa or Mastercard,
including expiry date.
Copyright 1996, Province of British Columbia
Ministry of Labour
THIS PUBLICATION MAY NOT BE REPRODUCED IN ANY FORM.
Technical experts
Ernie Janzen
Owen Collings
Peter Fill
Colin Haigh
Steve Ramage
AI Shehowsky
Robert Wereley
Review committee
it.~_fill:~!~_ _ _ _ _ _•
Roger Tremblay
John Davies
Norm Fair
Ian Hodgetts
Harold Kirchner
Jim Martin
Doug Wiebe
Project management __'l_ _ _ _ _ _~~.~_ _ _ _ _ _~
British Columbia Institute of Technology
Learning Resources Unit
Brian Thorn
Adrian Waygood
Production
Editor:
Jenni Gehlbach
Graphic Artists:
Su Gillis
Tim Bonham
Margaret Kernaghan
Kathy Rogers
GeorgeTuma
Elena Underhill
Ken Zupan
Production Assistant:
Pat Holting
Contents
Chapter 1
Safety
Chapter 2
Trade Science
Chapter 3
Technical Drawings
Chapter 4
Shop Practices
Chapter 5
Fasteners and Threads
Chapter 6
Lubrication
Chapter 7
Rigging and Lifting
Chapter 8
Shafts and Attachments
Chapter 9
Bearings
Chapter 10
Belt Drives
Chapter 11
Chain Drives
Chapter 12
Gear Drives
Chapter 13
Couplings and Clutches
Chapter 14
Seals
Chapter 15
Pumps
Chapter 16
Hydraulic Systems
Chapter 17
Pneumatic Systems
Chapter 18
Prime Movers
Chapter 19
Material Handling Systems
Chapter 20
Preventive Maintenance
Chapter 21
Ventilation and Pollution Control
Chapter 22
Installation and Levelling
Chapter 23
Alignment
Index
C")
::r
-...
Do)
-=
CD
......
MILLWRIGHT MANUAL: CHAPTER 1
Safety
WCB regulations ............................................................................ 1:1
WCB responsibilities ........................................................................
Employers' responsibilitie ................................................................
Workers' responsibilities ..................................................................
Industrial Health and Safety Regulations .........................................
1: 1
1: 1
1:2
1;2
Job site safety .................................................................................. 1;3
Housekeeping on the job .................................................................. 1:4
Personal safety.... ......................... ................. ....... ...................... ..... 1:4
Personal apparel................................................................................ 1:4
Personal protective equipment .............................................. ........... 1:5
Safe operation ................................................................................. 1:9
Lockout procedures .......................................................................... 1:9
Tool safety ........................................................................................ 1:9
Shop safety equipment ..................................................................... 1 :11
Fire safety.... ................. ..................... ....... ..................... ................. 1: 11
The fire triangle ......................... .......... .......... ......... .............. ............ 1: 11
Principal causes of fire ..................................................................... 1: 12
Classes of fires .................................................................................. 1; 12
First-aid frrefightiug ......................................................................... 1: 14
Confined-space ............................................................................... 1: 16
Safety
Safety in a plant is the concern of government, management, and labour. A
healthy safety attitude toward accidents benefits the employee by helping to
avoid injury, loss of time, and loss of pay.
A millwright is possibly exposed to more hazards than any other worker in a
plant and should be familiar with the Workers' Compensation Board (WCB)
regulations dealing with personal safety and any special safety rules applying
to each job.
weB regulations
The WCB is a provincial body set up to maintain a safe, healthy, working
environment at job sites throughout the Province. It is a powerful legal body
and can order unsafe job sites closed until they are made safe. The WCB
publishes a handbook: Industrial Health and Safety Regulations. It contains
all the rules, regulations, and responsibilities of the WCB, the employer, and
the worker.
weB responsibilities
According to the Workers' Compensation Act, the WCB is responsible for:
• inspecting places of employment
• investigating accidents and the causes of industrial diseases
• assisting and advising employers and workers in developing health and
safety programs
Employers' responsibilities
The WCB dictates that every employer shall keep a copy of the WCB
Industrial Health and Safety Regulations readily available at each place of
employment for reference by all workers. The handbook begins with a
general explanation of terms, the procedure for notification of injury, and
first aid requirements.
Sections 2 through 8 contain the regulations identifying responsibilities that
are common to all places of employment. A few of the employer's
responsibilities are noted below.
io"iMi!'!.2!.F.:4W;:;,",-cm)t.'i;idHiL"ih"50'1S"_2!_~~"tWif)aR"lM~UJ;.Jij;-I'ii'%Bj,"m;"_2,\P:1i'-8,!;):;'8S3"#9WWb'X0jTh."1SSBB6h"m"{f'!E8_,-~qKG~-{'"%%"'%t,t;;mef&:$D$SBfii'f'i_9_,-;;ef.nR"'''':2;~:i;<':;~;",!_C=;'~-U;*Mt~tMMM_8S;;.ng!{!ii.2!.2!;'
MILLWRIGHT-SAFETY
1 -1
The employer shall ensure that:
•
all work shall be carried out without undue risk of injury or industrial
disease
•
machinery and equipment are capable of safely performing the functions
for which they are used
•
all workers are instructed in the safe performance of their duties
•
no person shall enter or remain on the premises of any place of
employment while that person's ability to work is so affected by alcohol,
drugs, or other substances as to endanger his or her health or safety, or
that of any other person.
Industrial Health and Safety Committees
As part of the program, a health and safety committee must be established.
The committee shall
•
assist in creating a safe place to work
•
ensure that an accident prevention program is set up
•
recommend actions that will improve the effectiveness of the health and
safety program
•
promote enforcement of WCB Regulations.
Workers' responsibilities
The worker is responsible fur his or her own safety on the job. You have the
right to refuse to do any act or operate any tool, appliance, or equipment
when you have reasonable cause to believe that to do so would put you in
danger.
Workers' responsibilities include wearing proper clothing for the job site.
This means warm clothing for cold or wet weather, gloves, safety headgear,
safety footwear, and proper eye protection. Also,
•
You must not remove any safety equipment from machines or equipment.
This includes shields from grinders, guards from belts and pulleys, or
guardrails from scaffolds or excavations.
•
You must have had adequate instruction about a piece of maehinery or
equipment before you operate or use it.
•
You must make sure that no machine, equipment, or tool is used in a way
that would cause injury to someone else.
Industrial Health and Safety Regulations
The Workers' Compensation Board assumes responsibility for periodic
inspection of the operation to ensure that regulations for industry are being
correctly observed.
1-
MILLWRIGHT-SAFETY
For their own protection, millwrights should be familiar with the Industrial
Health and Safety Regulations. In particular, note the following sections
relating to the job of the millwright:
8 12 13 14
16 18
30 32
54 -
Places of Employment General Requirements
Harmful Substances
Health Hazards
Personal Protective Equipment
Machinery, Equipment and Industrial Processes, Guards: general
Welding, burning and soldering
Ladders
Scaffolds Swing Stages
Rigging
Appendix D Appeudix J
Standard Hand Signals for Controlling Crane Operations
Correct Spooling of Ropes on Drums
Lockout Procedures (Section 16.102) should be thoroughly understood and
followed. Actual lockout routine will vary from plant to plant, but each
routine must be acceptable to the WCB.
Job site safety
Accidents are caused by carelessness. Always be concerned with your own
safety and with that of others around you. No horseplay! This is the biggest
cause of injuries on the job.
The following is a general list of safety precautions you must take in any
work area.
MILLWRIGHT-SAFETY
•
Make sure your clothing and personal protective equipment are
appropriate, flame-resistant, and functional.
•
Never show up for work while under the influence of drugs, medications,
or alcohol.
•
Always move carefully.
•
Watch for hazards.
•
Walk in the shop areas.
•
Don't work on a cluttered workbench.
•
Never work under a heavy object until proper supports are in place.
•
Read and obey all posted waruing signs.
•
Report defective or unsafe equipment to your supervisor.
•
Develop good housekeeping habits. Clean the floor immediately after
spilling oil or grease, and sweep the floor after fmishing a job. Oily rags
are a fire hazard. Dispose of them in an approved fireproof receptacle.
Do not use compressed air for cleaning unless specially designated
procedures are in place. See WCB regulations 8.56 (1)-(3). High-pressure
compressed air used for cleaning can cause injury and death.
Housekeeping on the job
Proper housekeeping means maintaining a tidy, safe, work area, and a clean
workbench. Clean the surrounding area before starting a job and at various
stages as the job progresses. Tools and equipment should be cleaned and
maintained in good order. Keep cables. wires. and hoses out of traffic areas
while you work. When they are not in use. keep them in their designated
places.
When a job is finished:
•
Clean and return all tools.
•
Remove any nails from boards (or bend them over).
•
Remove all scrap metal and wood to junk: boxes. and return all unused
parts to storage.
•
Wipe up spilled grease. oil. and solvents. and dispose of rags in proper
metal containers.
Personal safety
A number of important safety considerations for clothing. hair. and jewellery
are common to all job sites. You must supply the appropriate safety footwear
and suitable clothing before you can to work in the shop areas. See WCB
regulations. Sections 14.02 through 14.21.
Personal apparel
Clothing
•
Wear close-fitting clothing that is not ragged or frayed if you are working
near moving machinery.
•
Do not wear oily. greasy. and/or synthetic clothes. As well as being fire
hazards, oily or greasy clothes may cause skin irritation or inflammation.
•
Wear clothing that protects your body from as much dirt and as many
chips and sparks as possible.
•
Do not tock pant legs inside your boots if you are working with or near a
torch, grinder, or chipper.
Hair and beards
Hair and beards can get caught in machinery or catch fire from sparks or
open flame. Beards may also help cause asphyxiation (suffocation) by
preventing respirators and gas masks from fitting properly. Wear caps or hair
nets, particularly if you have long hair. Beards should be trimmed.
1-4
MILLWRIGHT-SAFETY
Jewellery or watches
Do not wear rings, metal watchbands, bracelets, neck chains, or necklaces on
a job site. Wearing these items can cause:
•
a shock if you are working on electrical equipment that has electrical
power applied
•
loss of a finger or hand if your ring gets caught in a piece of machinery.
Personal protective equipment
When you are on the job site, you will need to wear special personal
protective equipment. The equipment you wear will depend on the hazards
you expect to encounter.
Protective equipment can be divided into the following categories.
•
head protection
•
lung protection
•
eye protection
•
ear protection
•
hand protection
•
foot protection
Head protection
The WCB and some employers demand that hard hats (Figure \) be worn in
specific areas or when doing specific jobs. All hard hats are adjustable and
several models also have chin straps, liners, or ear muffs. Some hard hats are
metallic and must not be worn when working on electrical equipment.
Electrical workers wear Class B hard hats that are designed to reduce shock
hazard. WCB regulations prohibit you from painting your hard hat and from
drilling holes in it.
Figure 1 Standard hard hats
A
MILLWRIGHT-SAFETY
The rule for hard hats is DON'T JUST CHOOSE IT-USE IT!
Lung protection
You may be exposed to airborne particles, chemicals, toxic gases or fumes.
These would be hannful if you inhaled them, so wear an approved type of
mask or respirator. Be sure to use the correct cartridge in your respirator for
the gas or hazard you are exposed to.
Figure 2 below shows some of the types of masks and respirators available.
Figure 2 Respirators and gas masks
A
Wear a mask or respirator when necessary!
Eye protection
Eye protection is one of the most important safety concerns of people on the
job site. Many types and styles of eye protection are available. Ensure that
the eye protection you choose is CSA approved and adequate for the job.
(Figures 3, 4, and 5).
Figure 3 CSA approved
eye protection
Figure 4 Goggles
Figure 5 Face shield
Never assume that your regular prescription glasses, sunglasses, or contact
lenses will give you adequate eye protection; dust particles, wood chips,
sparks, or "flash" may still cause eye damage.
-6
MILLWRIGHT-SAFETY
Contact lenses may be worn in some job situations. However, some jobs are
too dangerous to allow contact lenses. Sparks or molten metal may strike the
contact lenses, severely damaging the eye ball.
Whatever job you are doing-drilling, grinding, chipping, cutting, or
welding-use adequate eye protection. Note that welding goggles or helmets
are not acceptable for grinding. Remember the following points when
choosing eye protection:
•
Choose a type or style of eye protection that will protect you in the job
that you are doing (e.g., goggles when chipping concrete, a welder's
helmet when welding).
•
Use approved eye protection. Improper or faulty protection devices can
be hazards themselves.
Ear protection
Another form of protection that is required by the WCB is hearing
protection. Noise on the job site may affect you in the following ways:
•
Moderate noise levels over a long period can cause a decrease in your
ability to hear specific types of sounds.
•
High noise levels will impair your hearing. For example, the noise from
saws in a sawmill and mobile equipment such as loaders can do
permanent damage to your hearing.
•
High noise levels can affect your mind, making you irritable and
mentally fatigued, and decrease your ability to concentrate and stay alert.
There are four major points to remember when choosing ear protection.
•
Choose a type or style of protection that will protect you in the job that
you are doing.
•
Ear plugs should be pliable, fit each ear tightly, and be kept clean and
free from damage.
• Ear muffs make it easier to hear certain signals in noisy environments.
•
Headphones designed for music reproduction are not adequate protection.
Figure 6 shows three types of ear protection.
Figure 6 Ear plugs and ear muffs
:*~?t;;(hl"-K('-1"0A&_;:;{;;""";?"'''k&')'ilt"~_)!:;:-g'''~#~-e~'''f.;',%tpHo:;.;'¢:i;;;!<g-jt;-<&~Vr3_{tl)0';';-Y+>i\¢;;;:;;-d4XXA;'~,{M_0>",i\f!:!lM>;kNVS«iS0X:i1{:«",jo/(i:'JIi:0N1Mj,;;;:p;;r;;\iH'fr4,?\ti7iSIT>t::<>1:,W'";W,,;,z,,$040Ji-,,-,,
MILLWRIGHT-SAFETY
1 -7
Hand protection
Accident statistics indicate that over 30% of work injuries happen to fingers,
hands, and anns. Many of these accidents could be avoided by the use of
appropriate hand protection (see Figure 7).
Insulated
Coated, abrasion-resistant
Natural rubber
Leather
Rigger's
Figure 7 Various types of gloves
As a trades person you are ultimately responsible for your own hand
protection. Use the correct gloves:
•
Use thermally insulated gloves when handling hot metal.
•
Use rubber or approved plastic-treated gloves when handling acids and
cleaning solutions.
•
Use gauntlet-type welder's gloves when welding or flame cutting.
•
Use approved rubber gloves when working with electrical apparatus, but
do not use these gloves for any other purpose (you may damage them so
they are useless for electrical work).
•
Use leather or vinyl-coated gloves when
handling lumber or steel.
Foot protection
Due to the danger of sharp or falling
objects, you must wear CSA-approved
safety footwear. For example, Class I
safety boots must be eight inches high
and made of leather or some other
approved material. They must have
steel shanks and steel toes and should
carry a green triangle at the ankle to
indicate they are CSA approved.
r
8"
1
~::::::::~~~teeIShank
Figure 8 Safety boot
1-
MILLWRIGHT-SAFETY
Safe operation
of equipment
Operate and shut down machinery properly:
•
Be sure the equipment or machine is free from obstruction and that all
personnel are well clear before the machinery is activated.
•
Shut off machinery if you are leaving the immediate area.
•
Allow revolving machinery to stop on its own before leaving it. Do not
slow down or stop a machine with your hands.
•
Be sure all machinery is stopped and disconnected before you begin to
adjust or clean it.
Lockout procedures
As a millwright, you may often be in an area where maintenance procedures
are being carried out on powered machinery. At these times, detailed lockout
procedures are essential to prevent anyone from operating a machine that is
being worked on and to prevent the unexpected energizing of a machine.
Lockout must involve more than merely disconnecting the power source.
Workers have been killed by machinery that was dead electrically but whose
hydraulic systems were still pressurized. The machine must be assessed
thoroughly, and all energy sources--electrical, pneumatic, hydraulic or
gravitational-must be made inoperative, a state often called zero
mechanical state.
Each millwright should have his or her own lock and key (combination locks
are not allowed), and only these locks should be used to lock out energy
sources. The machine operator should be infonned of maintenance plans,
and the lock should be tagged to identify the person who has locked out the
machinery.
No one, other than the person who placed the locks and tags, can remove
them. Operators and other workers are strictly forbidden to remove either the
tag or the lock.
Note that these procedures apply not only to stationary industrial equipment
but also to mobile equipment, including passenger cars, truck equipment,
and heavy construction equipment.
Tool safety
It is very important to use tools safely. Even a small accident can become a
major crisis if no one is around to help. Power tool manufacturers usually
build safety features into their equipment. It is a good practice to use all
safety equipment supplied. It is illcgal, as well as an unsafe practice, to
bypass, disconnect, or remove guards, hoods, shields, ete.
!iU"-M2\X¢fe;";;;:;:1€<{1;g2;;tG}!W,L'W:"%'<i~*,J:0'.-"!&'1-S:'1!;~,,'"t':0XgV;;{g":~;Q{f:F.",i"l';;?i~,-j~"':jHRii.-;';';'~'0pK<Z\t/;q:;;¥hd0A;")¥0illS;<,";';{;e;Y?'k?xr'l'iJit'ii1+t¥;;0(;.~JJ;?t!t{pn;jArW$j'0;:;t;tt'PtM+iZ""",0X§1\\i";+~:f~S:-9§-1.'¥
MILLWAIGHT-8AFETY
1- 9
When using hand-held power tools, hold them firmly and with adequate
control. Assume a comfortable, balauced body position.
A
Maire sure that hand and power tools are inspected, serviced, repaired,
sharpened, or dressed as required to make them safe for usc. Before using
any tool, check the condition of all goard" tool retuiuers, power supply
cords, extension cords, and other accessories. Report any damage or defects,
and return the tool.
If you are not familiar with a piece of equipment. leave it alone!
Electrical tools
Electrical tools must meet CSA standards and comply with WCB
Regolations. When using electrically powered tools (sec WCB regolation
22.32), make sure the terminal in the electrical outIet and the ground pin or
terminal on the power cord is in place and in good repair.
Some hand-held electric power tools have an insulated handle or housing
and are referred to as "double insulated." These tools have power cords with
no ground pin in the plug but the plug may be polarized to fit into the socket
only one way. Be sure you correctly identify these tools before using them.
When using electrical tools:
•
Maire sure the insulation on the power or extension cord is not cut or
frayed. Frayed or otherwise damaged cords could result in an electrical
shock. Position the cord so that it is not damaged while it is in use.
•
Do not operate electrical equipment in wet locations.
•
Do not lift or move an electrical tool by its power cord. Power cords are
easily damaged and malfunctions can result from improper handling. The
proper and safe way to lift tools is by their handles.
•
Always remove a plug by grasping the plug and pulling it straight out of
the receptacle. Cords can be damaged if their plugs are jerked out by
yanking on the cord.
•
Always discormect, unplug. or lock out electrical equipment before
changing saw blades or grinding wheels and before making major
adjustments, or performing preventive maintenance.
Pneumatic tools
Portable pneumatic (air-powered) tools present some of the same hazards as
electrically powered tools plus some that are unique to pneumatic tools.
•
Route air hoses overhead or out of the way so they are not a tripping
hazard.
•
Do not allow them to lie where they may be cut or run over by vehicles.
•
Never point an air hose in the direction of another person.
MILLWRIGHT-8AFETY
Shop safety equipment
When you enter a shop or industrial setting for the first time, locate and
learn how to use the emergency shutdown.
Workshop ventilation
Workshops usually have some type of ventilation equipment for exhausting
harmful dust Or fumes. Many types of ventilation equipment may be found in
workplaees. It is important to become familiar with the ventilation
equipment or systems, and to use them.
Fire safety
The fire triangle
Fire, or combustion, is a fonn of oxidation (the union of a substance with
oxygen). During the process of oxidation, energy is released in the fonn of
heat-sometimes accompanied by light.
Oxidation takes place at varying rates. For example:
Very slow
• rusting iron
Slow
• spontaneous heating of materials such as oilsoaked rags
Fast
• burning paper or wood
Extremely fast
• exploding gunpowder
Before a fire can occur, these three components must be present.
1. Fuel (a combustible material such as
wood, gasoline, paper, or cloth).
2. Hent (sufficient to raise the fuel to
its "ignition temperature").
3. Oxygen, usually in the fonn
of air (to sustain combustion).
HEAT
Figure 9 Fire triangle
When they combine, as shown in the fire triangle diagram (Figure 9), the
result is rapid combustion or firel
":;;:01j'ffl;l;"7ASJl);:i;)'0.'<f0f"\AY{:1V:ut"4&~12)~;;;1!~*:0"*;;:Jt:~E&_*Z""'Ji'g;t"'bH'~\s;PYSl1-;;:;~-"'&~"''lt~~'%~Y'F;R::>;~''j,<,"''lil''WMi\~.,'V\W0%)j'!jri:W4'J;;\i1'\,"~~N'!\S!WS%;;k¥l;;;'?lliibt~.CvV*Y!R70FW~_0';;\%'_\i'0<
MILLWRIGHT-sAFETY
1 - 11
Keeping these three components separated will prevent a fire from
occurring. Likewise, an existing fIre can be extinguished by removing any
one of the three components.
1. Remove the fuel (combustible material) from the vicinity of the fIre (e.g.,
shut off valve of gas main). Result: starvation.
2. Remove the heat (e.g., by applying water). Result: cooling.
3. Remove the oxygen (e.g., cover the fire with a lid, a wet blanket, some
sand, or use carbon dioxide, foam, or a dry chemical). Result:
smothering.
Principal causes of fire
Welding and burning:
•
flying sparks or slag, which immediately result in obvious fires
•
welding against a wooden backing, or dust, which may result in a fire that
does not ignite until several hours after the job is completed
•
poor grounding during welding, which sometimes causes electrical motor
fires.
Electrical sources:
•
motors burning
•
broken electrical wiring
•
light bulbs in contact with fine dust or oily surfaces
•
unprotected bulbs and unshielded switches in dust areas.
Friction:
•
fallen material resting on fast-moving equipment, such as a belt
•
a belt running off-centre and rubbing against a fixed surface
•
hot bearings igniting oil or dust
Other sources:
•
workers ignoring "No Smoking" signs
•
workers ignoring gas ventilation and dust-abatement regulations
Classes of fires
Fires have been divided into four main classes: A, B, C, and D. These
important classifications of fire dictate the type of extinguisher required.
The symbols shown opposite may be the only indication you have of the best
use for a fIre extinguisher. Please make note of the class, letter and symbols
for future reference.
1-
2
MILLWRIGHT-SAFETY
green
triangle->
A
L.:.).
Fires involving ordinary combustibles (wood, cloth, paper, rnbber. and
many plastics):
Use the heat-absorbing (cooling) effects of certain chemicals that retard
combustion.
red
square->
blue
circle->
yellow
star->
Fires involving flammable or combustible vapors:
Use extinguishers that prevenllhese vapors from being released or that
interrupt the combustion.
Fires involving energized electrical equipment:
Use noneonductive extinguishing agents to protect the operator (only when
electrical equipment is de-energized may Class A or B cxtinguishers be
used).
Fires involving certain combustible metals, such as magnesium, titanium,
zirconium, sodium. or potassium:
Use a heat-absorbing extinguishing medium uot reactive with the buming
materials. A small Class D fire such as a burning pile of combustible metal
shavings is put out most efficiently by "smothering," using an appropriate
dry chemical (Class D) extinguisher or sand.
Location and operation of extinguishers
It is important that you know the location of and how to operate each
extinguisher in your workplace. Because each manufacturer uses a slightly
different operating procedure, the best thing you can do is to look carefully
at the markings and instructions on the extinguisher. They will tell you
where the extinguisher is most effective and how to use it.
The standard technique is as follows:
•
Stream the extinguisher at the base of the fire, working from edge to
centre. Do not direct the nozzle at the general location of the flames.
•
Position yourself where any breeze or draft is moving away from you
toward the fire, so that the flames are not fanning toward you.
•
Always make sure that you have a means of escape in the event the fire is
not brought under control.
Extinguisher styles, markings, and ratings
The nameplates of fire extinguishers designate, by means of rating code. the
type(s) of fires for which the equipment can be used; e.g., I-A I()"B:C.
See Figures 10 through 13 and Table 1 on the next page.
MILLWRIGHT-SAFETY
Figure 10 Water extinguisher
(Class A)
Figure 11 Halon extinguisher
(Classes A, B, & C)
Figure 12 Dry chemical extinguisher Figure 13 CO. extinguisher
(Classes A (some). S, or C)
(Classes B & C)
First-aid firefighting
First-aid firefighting is extinguishing a fire in its initial stages by using
whatever is readily at hand, before the fire can become 100 large.
Fire extinguishers that are used in homes, offices, etc., are designed to deal
with fIres in their infancy. They are still necessary even though an area may
be protected by sprinkler systems, etc.
During combustion, sufficient heat is generated to raise the temperature of
the fuel. This produces ignitable vapors and the buruing process will
continue as long as there is sufficient fuel, heat, and oxygen to sustain it. The
process must be interrupted as soon as possible by using a fire extinguisher.
MILLWRIGHT-SAFETY
A
Some materials produce toxic gases when burning.
Table 1: Fire extinguisher ratings
Underwriters' Laboratory of Canada (ULC) ratings show the relative hazard, coverage and travel distance specifications of extinguishers:
ULC Rating code
Class-A Fire
CIBSS-B Fire
Clasa~C Fire
1-A 10-B:C
Light hazard -3 000 sq ft
Sufficient for
(27 m 2 ) of Class-A fire; 75 It
Light haz d-10 sq It
(0.93 m2) of Class-B
Class~C
(23 m) travel distance.
fire; 50 It (15 m) travel
conditions.
distance.
Ordinary hazard -
1Osq It (0. 93 m2 ) of
Class-8 fire; 30 ft
(9 m) travel distance.
ULC Rating Code
Class-A Fire
CIBSS-B Fire
Class~C Fire
2-A 10-B:C
Light hazard --6 000 sq It
(55 m2 ) of Class-A fire; 75 It
Light hazard -10 sq It
(0.93 m2 ) of Class-B
Sufficient for
Class~C
(23 m) travel distance.
fire; 50 ft (15 m) travel
conditions.
distance.
Ordinary hazard-
3 000 sq It (27 m2) of
Class-A fire; 75 It (23 m)
travel distance.
Ordinary hazard 10 sq It (0.93 m2 )
of Class-8 fire;
30 It (9 m) travel
distance.
ULC Rating Code
Class-A Fire
Class-B Fire
4-A 40-B:C
Light hazard -11 250 sq It
Exceeds Class-B
Sufficient for
(1045 m2) of Class-A fire;
requirements for light
Class-C
75 ft (23 m) travel distance.
and ordinary hazards.
conditions
Ordinary hazard 6 000 sq It (557 rri!) of
Class-C Fire
Extra hazard -
Class-A fire; 75 It (23 m)
40 sq It (3 m2 ) of
Class~B fire; 50 ft
travel distance.
(15 m) travel distance.
Extra hazard --4 000 sq It
(371 m2) of Class-A fire;
75 ft (23 m) travel distance.
MILLWRIGHT-5AFETY
1 -15
Confined-space
safety
All workers involved with enclosed spaces are encouraged to read the WCB
manual Confined Space Entry. Read also Section 13, Health Hazards and
Work Environment Controls in the WCB Industrial Health and Safety
Regulations.
It is very important for workers 10 be aware of the hazards associated with
confined spaces and enclosed spaces. Strict adherence to testing, entry, and
exit procedures must be observed at all times.
A confined-space safety program must include at least the following
elements.
•
Workers must be trained and upgraded as necessary in confined space
work and in emergency response procedures.
•
Responsibility must be assigned to ensure program coordination and
accountability.
•
Confined spaces must be identified. Workers must be made aware of
their locations and dangers.
•
Written procedures must be available to workers.
•
Personal protective equipment that meets acceptable standards must be
provided and wOrn.
•
The confined space must be isolated from outside sources of danger.
Equipment in a confined space must be locked out when worked on.
•
Entry and exit points must be provided with equipment for safe access
and for rescue.
•
A safe atmosphere must be ensured.
•
Atmospheric testing must be carried out by a qualified person using
properly calibrated test equipment.
•
Cleaning and purging must be carried out, where necessary, to ensure
worker safety.
•
Fires must be prevented by controlling sourees of ignition and flammable
materials.
•
Physical hazards from material collapse, electricity, lighting, noise, or
temperature extremes must be controlled.
MILLWRIGHT-SAFETY
MlLLWRIGHTMANUAL: CHAPTER2
Trade Science
Atoms and molecules.. .............. .............. ............. ........ ...................
Compounds and mixtures .................................................................
Physical states of matter ...................................................................
Molecular attractions in matter .........................................................
Mass, weight, volume, and density..................................................
2: I
2: 1
2:2
2:3
2:3
Properties of solids ..................................... .............. ...................... 2:5
Mechanical properties of solids ........................................................ 2:5
Physical properties of solids ............................................................. 2:8
Chemical properties of solids ........................................................... 2: I 0
Properties of liquids ........................................................................ 2: 11
Cohesive and adhesive forces ........................................................... 2:11
Volatility ........................................................................................... 2: 12
Viscosity ........................................................................................... 2: 12
Properties of gases .......................................................................... 2: 13
Compressibility and elasticity .......................................................... 2: 13
Gas pressure ...................................................................................... 2: 13
Internal (thennal) energy ................................................................ 2:14
Temperature and heat ........................................................................ 2:15
Measuring temperature ..................................................................... 2: 15
Heat units .......................................................................................... 2: 17
Thermal expansion ........................................................................... 2:17
Heat transfer ..................................................................................... 2:19
Force and motion ............................................................................ 2:21
Newton's laws of motion .................................................................. 2:21
Linear motion ................................................................................... 2:22
Rotational motion ............................................................................. 2:23
Energy, work, and power ................................................................ 2:23
Energy ............................................................................................... 2:23
Work ................................................................................................. 2:25
Power ................................................................................................ 2:25
CONTiNUED
Simple machines ............................................................................. 2:26
Levers ............................................................................................... 2:26
Inclined planes .................................................................................. 2:32
Hydraulic presses .............................................................................. 2:34
Compound machines ........................................................................ 2:34
Electricity and electromagnetism ................................................... 2:35
Atomic structure ............................................................................... 2:35
Current. conductors. and insulators .................................................. 2:36
Magnetism ........................................................................................ 2:36
Electrical circnits .............................................................................. 2:37
Electrical principles and laws ........................................................... 2:39
Direct current (DC) and alternating current (AC) ............................ 2:41
Transfonners ..................................................................................... 2:41
Single-phase AC circuits .................................................................. 2:41
Three-phase AC circuits ................................................................... 2:43
Fuses ................................................................................................. 2:44
Circuit breakers ................................................................................ 2:45
Motor controllers .............................................................................. 2:45
CHAPI'ER2
Trade Science
The millwright trade is based on applied scientific principles. These
principles are constantly applied by millwrights, often without realizing it.
Sometimes the millwright needs to think a problem through using these
principles, or to calculate a dimension, a load, or some other quantity using a
formula. This chapter explains some of these important principles.
Atoms and
molecules
All matter is composed of atoms. Atoms are singlc units of special
substances called elements. Elements are materials containing only one type
of atom. The atomic structure of an element determines its chemical
behaviour (that is, the way it interacts with other elements).
A few of the 104 naturally occurring elements on Earth are hydrogen,
oxygen, carbon, copper, lead, silver and gold. Atoms of elements unite to
form more complex structures called molecules. Most matter is composed of
molecules that contain two or more atoms.
The arrangement and structure of the molecules in a substance are unique.
They determine the characteristics of that material. For example, iron is
harder than copper because of differences in the arrangement and structure
of their molecules. Similarly, the colour of gold is determined by its
molecular structure.
Compounds and mixtures
Thousands of substances can be produced by chemically or physically
combining different proportions of the basic elements. Such combinations of
elements form either a compound or a mixture. For example:
•
Atoms of oxygen, nitrogen and carbon dioxide (with a few trace gases)
combine physically to form the mixture we call air.
•
Two atoms of the element hydrogen and a single element of oxygcn
combine chemically to form the compound water.
Compounds
A compound is the product of two or more atoms that unite chemically. This
creates a substance that has properties different from those of the combining
elements.
There are many examples of compounds. Water is a compound of hydrogen,
and oxygen. Sugar and alcohol are both compounds of carbon, hydrogen and
oxygen. The number and arrangement of the atoms in the sugar and alcohol
molecules are different.
Elements in a compound are always in definite proportions. For example,
the ratio of sodium to chlorine in sodium chloride (table salt) is always
0.65:1. If there is extra sodium or chlorine available when table salt is being
created, the extra atoms will not unite. Individual molecules of compounds
have all the properties of the compound. That is, they behave physically and
chemically like the compound. They can be considered as the smallest unit
of the material.
Mixtures
Two or more elements or compounds may combine so that the molecules in
the resulting material retain their original properties. This material is called a
mixture.
Unlike compounds, a mixture can be formed using varying proportions of
each ingredient. For example, concrete is a mixture of cement, water, and
aggregate. The proportion of cement, water and aggregate can vary, but the
mixture is still concrete. However, the physical characteristics (such as
strength) of the concrete are determined by the ratio of the ingredients.
Alloys
Alloys are mixtures of a metal with other metals or non-metals. For example,
brass is an alloy of copper and zinc. Like any other mixture, an alloy's
properties depend on the proportion of its parts. For example, you can vary
the proportions of copper and zinc to produce different effects: a bronze
coloured alloy has 90% copper and 10% zinc; a silvery white alloy has 55%
copper and 45% zinc. Brass is produced in about a dozen formulations each
with its own distinct characteristics.
Many alloys are used in the trades. For example, babbitt is used in bearings,
stainless steel in construction, and aluminum-magnesium alloy in aircraft.
Physical states of matter
Matter exists in one of three physical forms or states: solid, liquid or gas.
Some substances change states without changing their chemical structure. In
appropriate conditions, solids melt into liquids or vaporize, liquids freeze to
solids or vaporize into gases and gases condense into liquids.
When water changes its physical state, the make-up of the molecules remains
the same. Molecules of frozen water (ice) still contain two hydrogen atoms
and one oxygen atom, chemically combined. Steam also contains these types
of molecules. It is mostly the way the molecules move around each other
that changes in the different physical states. Because of this, water can be
made to return to a previous state by changing the surrounding physical
conditions.
2
2
Many substances cannot change from one physical state to another without
becoming permanently unrecognizable. They cannot be changed back to
their previous physical state. For example, a solid oak block cannot be
melted into liquid oak by applying heat. Instead, given enough heat, the oak
becomes carbon ash. Its chemical structure has changed. The carbon ash
cannot be changed back into an oak block.
Molecular attractions in matter
The physical state of a substance is determined by the spacing and motion of
the individual molecules. The spacing among molecules is determined by
several attracting forces. They include cohesive and adhesive forces.
Cohesive forces
Molecules in solids are strongly attracted to one another and this attraction is
called cohesive force. It limits the space around individual molecules,
packing them relatively a closely so that solids hold definite shapes.
Cohesive forces in solids can be extremely strong. For example, a mass of
several thousand kilograms can be hung by means of a vertical steel rod
several millimetres in diameter. This does not cause the molecules in the rod
to separate.
Liquids are also composed of tightly packed molecules but the cohesive
force is not as great as that found in solids. This reduction in intermolecular
force allows some molecules to slip over other molecules. Molecular
slippage allows liquids to flow.
Gas molecules exhibit little cohesion. Molecules are relatively widely
separated. This lack of cohesion allows a gas to diffuse (expand) quickly and
broadly. For example, one gram of water occupies a cubic centimetre of
space. When the single gram of water is vapourized, the resulting gram of
steam can occupy a space of several thousand cubic centimetres.
Adhesive forces
Molecules that are not alike often share a force of attraction similar to
cohesion. This intermolecular force is called the adhesive force. The
adhesive force between unlike molecules allows water to wet concrete, paint
to stick to steel, oil to lubricate bearings and ink to adhere to paper.
Sometimes a substance's adhesive force is greater than its cohesive force.
This is true of many epoxies used in industry.
Mass, weight, volume, and density
Matter has several physical properties, including mass, volume, density.
Physical properties are different from mechanical properties which are to do
with way a material responds to stress. Physical properties also include
electrical and thermal characteristics.
2-
Mass and weight
In every day life you make no distinction between mass and weight.
Scientists and engineers often need to do so. Mass and weight are closely
related because of gravity:
•
Mass is a measurement of the quantity of material in a body.
•
All objects that have mass are attracted to each other. The force of
attraction is called the force of gravity. The Earth has a large mass and
the force of its gravity pulls objects towards its centre.
•
This force of gravity on Earth determines what most of us call weight.
When you measure the weight of an object. you measure the force of
gravity acting on it. The weight is proportional to the mass.
In the metric system the unit of force is called a newton (N). In everyday
life. weights are often expressed (incorrectly) in grams (g) or kilograms (kg)
rather then in newtons.
The imperial system uses the unit pounds-force (lbf) to express force.
including weight. In the US. the unit pounds (lb) is often used to express
weight. although this is inaccurate.
Volume
Volume is a measurement of the amount of space an object occupies. To find
the volume of an object such as a rectangular box. you multiply the length
times the width times the height.
Volume = Length x Width x Height
To find the volume of cylindrical object such as a oil tank. you multiply the
area of the end times the height.
Volume = Area of an end x Height
V= 7rxrxH
Note that 1t (pronounced pi) stands for the number 3.14159 ...• often rounded
off to 3.142. This is the number oftimes the diameter of a circle can be
divided into its circumference.
The volume of an irregularly shaped solid material is often found indirectly.
Refer to math texts for methods of calculating volume for other shapes.
In the metric system. volume is commonly measured in cubic metres (m3)
and litres (L).
The litre is equal to a cubic decimetre: 1 L = 1 dm3.
It is also equal to 1000 cubic centimetres: 1 L = 1000 cm3.
Imperial units include cubic inches (in3). cubic feet (ft3). and imperial and
US gallons.
1 imperial gal = 277 in3
1 US gal = 231 in 3
Density
Density indicates the mass of a body in a given volume. The more molecules
packed into a given volume of substance, the more dense it is. For example,
a lead block has a greater mass (and is heavier) than an aluminum block that
is identical in volume. The lead is said to have greater density than
aluminum. Or, lead is denser than aluminum.
Density is the ratio of a mass to a volume.
Density = mass + volume
Relative density (specific gravity)
Relative density compares the density of materials to another substance.
Water is the reference substance for solids and liquids and air is the
reference for gases, unless otherwise indicated.
Properties of solids
Solids maintain their Own shape without a container. Solids are compared
and selected based on their properties. Hundreds of different properties can
be measured in laboratories. In routine industrial maintenance, only a few
properties need be known to successfully select and work on materials.
These are the mechanical, physical and chemical properties of solids.
Knowledge of these helps you to troubleshoot equipment, analyze problems,
and suggest improvements or repair methods.
Mechanical properties of solids
Mechanical properties are the characteristics of a solid that are displayed
when a force is applied to it. These properties determine whether the
material can perform its intended task. The following are definitions of some
of the main mechanical properties of solids.
Ultimate strength
Strength is the ability to withstand gradually applied forces without
rupturing. These forces may be tensile, compressive, or shear (these terms
are discussed later in this section). The point at which the material ruptures is
known as its ultimate strength. When materials are used close to the limit of
their ultimate strength, they must be reinforced. For example steel rods are
used in concrete to increase its strength.
Stress and strain
When discussing the ultimate strength of material, the tefms stress and strain
are often used:
2
•
Stress is defined as the force per unit area acting on an object to change
its dimensions.
The metric unit used is the pascal (Pa). Note that I Pa = I N/m2 .
The imperial unit is pounds-force per square inch (lbf/in2 ).
Stress = force + area
Note that stress and pressure use the same units but the abbreviation
Ibflin2 is used for stress on solids and psi is used for the units of fluid
pressure.
•
Strain is the ratio of the extension per unit length when a force is
applied. Because it is a ratio, it has no units.
Strain = extension + original length
Tensile strength
Tensile strength is ability of a material to resist being pulled apart by
external forces. For example, wire rope used to lift a load must have
sufficient tensile strength for it not to break under the load.
The ultimate tensile strength (UTS) of material is the maximum amount of
stress the material can withstand before breaking.
Compressive strength
Compressive strength is the ability to resist external forces that push into or
against the material. For example the foundation of a machine base is
subjected to constant compression and requires adequate compressive
strength for dependable service.
The ultimate compressive strength (UeS) is the maximum compressi ve
stress that a material can withstand before its surface area changes.
Shear strength
Shear strength is the ability to resist forces that try to slide part of the object
along itself. For example rivets and bolts holding two pieces of material
together must have adequate shear strength to prevent them from sliding
over one another.
The ultimate shear strength (USS) is the maximum stress the material can
withstand before it is cut apart. USS of a material is about 40% of its UTS.
Fatigue strength
Fatigue strength is the ability to withstand repeatedly alternating stresses.
Elasticity
Elasticity is the ability of a material to return to its original dimensions after
it has been acted on by a force. All solids have some elasticity.
6
However, the term elastic is used to describe a substance that quickly
assumes its original shape when the deforming force is removed. You may
think of rubber as being very elastic, but some special steels used in timing
springs are far more elastic than rubber.
Elastic limit (yield strength)
The maximum force that a solid can withstand without being permanently
deformed is called the material's elastic limit or yield strength. At that point
the material will not return to its original shape when the deforming force is
removed.
Table I shows the metric values of yield strength and ultimate strengths for
several materials. The figures in this table are approximate and should only
be used to compare the differences between the various materials.
To convert into lbf/in2, multiply the quantity in megapascal (MPa) by 145:
1 MPa = 1 000 000 Pa = 145 lbl/in'
Table 1: Elastic limit and ultimate strengths
Material
Steel, mild
Yield strength
MPa
400
Cast iron
Aluminum
Concrete
180
UTS
UCS
USS
MPa
MPa
MPa
650
200
200
650
700
200
140
240
250
110
3
Plasticity
Plasticity is the ability for material to retain a shape permanently after a
deforming force is removed. For example metal is often heated to increase
its plasticity. The heat relieves the internal forces which make it elastic.
Ductility
Ductility is the ability to stretch and maintain the new shape. It allows a
material to be drawn through a die and pulled into a wire or rod. The finer
the wire that can be produced, the more ductile the material. Platinum, gold,
tungsten, silver and copper are all highly ductile.
Malleability
Malleability is the ability of a material to be permanently deformed by
compression forces. Examples of these forces are found in rolling, pressing
and forging processes. Materials having high malleability can be hammered
or bent into numerous shapes.
Malleable steel is used extensively in the automotive, shipbuilding and other
manufacturing industries.
2
7
Brittleness
Brittle material breaks without noticeable plastic deformation. Brittleness is
the absence of malleability. A substance may be quite hard but lack strength
because it is brittle. Glass is an example of a brittle material.
Toughness
Toughness is the ability of a material to withstand shock loads.
Hardness
Hardness is a material's ability to resist a force that is trying to penetrate it.
Penetrating forces attempt to push molecules apart. Hardness is associated
with durability and abrasion resistance. Materials may be given a hardness
rating.
Several methods are used to determine hardness. They are based on how a
controlled force and mass affect the tested object. For example, in the Brinell
test, a hardened steel ball is forced into the surface of the material. The area
of the resulting impression is divided into the load used to make the
impression. This number is known as the Brinell number.
The Rockwell and the Vickers hardness tests follow variations of the Brinell
procedures and have similar scales.
Physical properties of solids
Physical properties of solids are characteristics of their interaction with
various forms of energy. For example, colour is a physical property that
depends on interaction ofthe material with light energy.
These properties are usually measured without destroying or changing the
material. For example colour and weight are physical properties that can be
observed or measured without destroying or changing the material.
The physical properties to be aware of in industry are electrical and thermal
conductivity, and thermal expansion.
Electrical conductivity
Electrical conductivity is the ability of a material to conduct an electric
current. Metals that have very high electrical conductivity include gold,
silver, copper, aluminum, and steel. Copper and aluminum are the materials
most commonly used in electrical wires.
A
-6
Caution!
Be careful when using conductive materials around electrical power
sources. If you accidentally touch a power line with metal (such as
scaffolding), the shock can be fatal.
Substances that have poor electrical conductivity are also important. They
are called insulators or non-conductors. They prevent current-carrying
conductors from contacting other conductors. For example, hard ruhber,
Baltelite, glass and oil are used as insulators.
Thermal conductivity
Thermal condnctivity determines the rate of beat-flow through a material.
The difference in thermal conductivity between iron and copper is
demonstrated in Figure 1. The copper har conducts heat at a much higher
rate than the iron bar and lights the match IITst.
This match
lights first
Iron
Copper
Figure 1 Thermal conductivity
Thermal conductivity has important industrial implications. For example,
when cutting a bearing raee off a shaft, the millwright must be aware of
thermal conductivity. Nearby parts may be overheated, possibly destroying
some of their desirable properties.
Expansion and contraction
Expansion and contraction change the dimensions of a material as a result of
a change in temperature. Most materials expand when heated and contract
when cooled. This expanding and contracting must be talten into account
when parts with different temperatures are assembled. For example, to
assemble a bearing race with an interference fit on the shaft, the shaft must
either be cooled or the hearing heated, or both.
A rule of thumb for the expansion and contraction of steel is as follows:
For every 150°F change in temperature,
each inch of diameter and length changes
by 0.001".
Chemical properties of solids
Chemical properties of solids are characteristics that relate to the interactions
of the particular elements and compounds in the material and the
environment. They are usually measured in a chemical laboratory. It is
usually necessary to change or destroy the material to make these
measurements. The chemical properties that the millwright deals with are
composition, corrosion resistance, and electrochemical properties.
Composition
All materials are composed of elements and compounds in particular
proportions. Changing this composition usually changes the behaviour and
properties of the material. For example, different lubricants have different
chemical compositions. This enables them to behave in a variety of ways for
different conditions.
Metallurgy is the study of the composition and behaviour of metals. If the
metal alloys used in a piece of equipment are changed, the characteristics of
the material also change. The part may then not behave in the desired way in
its environment.
Corrosion resistance
Corrosion is caused by a material's chemical reaction with its environment.
Corrosion resistance is the ability of materials to resist combining with
undesirable elements and chemical compounds. The corrosion resistance of
material is measured in various ways.
Some materials corrode more than others. For example:
•
ferrous metals such as iron oxidize (that is, combine with oxygen) to
form iron oxide-rust.
•
aluminum oxidizes very rapidly causing aluminum oxide. This forms a
film over the material that effectively protects the aluminum from
further corrosion.
Caution!
When selecting and storing materials, take possible corrosion into account.
Electrochemica1 reaction (electrolysis)
When two different metals are in contact with one another and water is
present, an electric current is produced. The electron flow has a chemical
effect, corroding the metals. It may even disintegrate one of them. These
electrochemical reactions (also called electrolysis) are complex. Millwrights
need to be aware of the reactions among metals or metal alloys.
2-
In the following list, each metal is corroded by all those metals listed after it.
This list is sometimes referred to as the galvanic series.
•
•
•
•
•
•
A
aluminum
magnesIUm
chromium
iron or steel
cast iron
cadmium
•
nickel
•
•
•
•
•
•
•
tin
silver
•
platinum
•
gold.
lead
brass
copper
bronze
copper-nickel
Caution!
When working on metallic materials or with metal tools, take care to avoid
corrosion. Store metal tools and materials properly. Be sure that highly
interactive metals are not stored together.
Properties of
liquids
Liquids are nearly incompressible. They have no definite shape, but take the
shape of their container. The different properties of liquids enable some
liquids to perform tasks better then others.
Liquids commonly used in industry include paints, lubricants, coolants,
hydraulic fluids and fuels. Many presses, jacks and other lifting systems
depend upon the special properties of liquids for their operation. For further
explanation of properties and laws pertaining to liquids see Chapter 16:
Hydraulic Systems.
Cohesive and adhesive forces
The abilities of liquids to hold together and to adhere to other materials are
due to cohesive and adhesive forces like those in solids.
2
11
Cohesion
Molecules in a liquid are held together by cohesive forces like those in
solids, but the forces are not as strong. This limited cohesive foree gives
liquids their familiar abilities to pour and to adopt the shape of their storage
containers.
Adhesion
Just as important as their weak cohesive forces are the strong adhesive forces
of liquids. These adhesive forces allow many liquids to attach themselves to
the surfaces of materials. For example certain oils adhere to the surfaces of
some metals bettcr then others. The adhesive quality of oils is an important
consideration in the selection of lubricants.
Adhesive forces also allow liquids to act as coolants. The wet coolant sticks
to the material being worked and removes heat from the materiaL Some
coolants are a mixture of water, oil, and a special soap that allows the oil and
water to mix (emulsify). See Chapter 6: Lubrication.
Volatility
Volatility is a measure of how quickly a liquid vaporizes. A liquid is said to
be highly volatile if it changes quickly from a liquid to a gas. For example,
when heated, gasoline is converted into a vapour which occupies more
space. Gasoline in a sealed container expands and build pressure when
exposed to sunlight.
Highly volatile materials can be very dangerous. Their vapours can be
explosive or extremely flammable when mixed with air and heat. Highly
volatile liquids commonly used in industry are alcohol, turpentine, gasoline
and paint thinners.
A
Caution I
Be especially careful when handling volatile liquids. The vapour produced
can diffuse (expand) very quickly and be ignited by a spark or openjlame
some distance from the source.
Place volatile liquids in well sealed containers and store them away from
heat. sparks or jlames.
Viscosity
Viscosity is a measure ofa liquid's resistance to flow. The higher the
viscosity of a liquid. the greater its resistance to being poured. Although the
term viscosity Can be applied to allUquids, it is most commonly associated
with engine oils, paints. and machiue lubricants. Viscosity is discussed more
in Chapter 6: Lubrication.
Properties of gases
Gas molecules are widely spaced and can act independently, moving freely.
A gas does not assume a shape like a solid, or confine itself to a tight volume
if it is free to expand. Gases can expand or be compressed into any space
regardless of its size. Refer to Chapter 17: Pneumatic Systems for additional
explanation and application of gases.
Gases are used in many industrial situations. For example, compressed air
operates pneumatic tools and actuators and other gases are used as
refrigerants and fuels. A brief list of industrial gases is shown in Table 2.
Table 2: Industrial gases
Gases
Common use8_________~
Acetylene
Welding and cutting
Oxygen
Welding and cutting
Carbon dioxide
Fire extinguishers
Natural gas and propane
Fuels
Water vapour (steam)
Steam turbine engines
Many of the principles of liquids also apply to gases. They are often studied
together under the general heading of fluids. Following are some of the
common terms used to define gases.
Compressibility and elasticity
The main differences between gases and liquids are in compressibility and
elasticity. Gases are highly compressible and very elastic. Gases not only
expand to fill any volume, but they can also be forced to occupy small
spaces. This combination of elasticity and compressibility in confined gases
is called high resilience. This resilience is secn in pneumatic tires, and other
inflated items.
Gas pressure
When a gas is compressed its volume is reduced. It is said to be under
pressure. The gas molecules become more tightly packed. The compression
increases the number of moving molecules that strike the sides of the
confming container. This increase in molecular activity is measurable as a
pressure.
Gases are most often used when they are under pressure. Some common
examples are fire extinguishers, welding fuels, aerosols and pneumatic and
hydraulic equipment.
Caution!
Handle containers of pressurized gases with care. A rupture in the container
will cause the gas to escape suddenly and blow the container apart.
The units for gas pressure are the pascal (Pa), kilopascal (kPa), and poundsforce per square inch (lbf/in2 or psi).
Measurement of gas pressure with gauges
Gas pressure may be measured by means of a gauge. The units of pressure
marked on the scale may be pounds per square inch gauge (psig) or pounds
per square inch absolute (psia). A container of gas at atmospheric pressure
has pressure readings of 0 psig or 14.7 psia. This is explained further in
Chapter 17: Pneumatic Systems.
Atmospheric pressure
It is a common experience that the pressure at the bottom of a container of
liquid depends on the depth of the liquid. The same is ttue of gases. Earth's
atmosphere is made of gases. The atmospheric gases have a pressure that
changes with the height above sea level. The pressure at the bottom (at sea
level) is greater than the pressure higher up (at the top of a mountain). This is
explained in more detail in Chapter 16: Hydraulic Systems.
Common units of atmospheric pressure are the atmosphere (ATM) and the
bar. 1 ATM is the average pressure exerted by the earth's atmosphere at sea
level:
1 ATM = 101.35 kPa
= approx. 1 bar
= 14.7 psi
Internal (thermal)
energy
In most conditions, a material's molecules are in constant, random motion.
This molecular movement is an energy form called thermal energy, or, more
correctly, internal energy.
If a hotter object comes in contact with a cooler object, the more energetic
molecules of the hotter object transfer some of their internal energy to the
cooler object. They do this by colliding with the less energetic molecules.
This causes the molecules in the cool object to vibrate more vigorously. The
hot object eventually loses its advantage. Both objects are soon in a state of
thermal equilibrium. That is, they each have the same amount of internal
energy.
2
14
Temperature and heat
When objects are in a state of thermal equilibrium, the objects both have the
same temperature. Temperature is simply a measure of the relative hotness
or coldness of a body. It is not directly related to internal energy. For
example, the flame supplied by a match has a much higher temperature than
a kettle full of boiling water. However, the small flame has much less
internal energy. It would take lots of matches to supply enough energy to
boil a kettle of water.
The energy that is transferred from a hot object to a cooler one is called heat.
The heat transferred from one object to another always flows from the object
with the higher temperature to the object with the lower temperature.
Heat has a formal definition:
Heat is energy in transit from a body at a
higher temperature to a body at a lower
temperature.
Measuring temperature
Temperature is difficult to observe directly. However, because matter
behaves in predictable ways when its temperature changes, it is fairly simple
to measure temperatures. For example, matter generally expands when its
temperature rises and contracts when its temperature falls. This fact is used
to indirectly determine temperatures.
Thermometer scales
An instrument used to determine temperature is called a thermometer. They
OF
212
32
100
90
80
70
60
50
40
30
20
10
o
have scales that may be divided (calibrated) in various ways. Almost all
thermometers are calibrated in either Celsius (C) or Fahrenheit (F) degrees.
These systems are based on the ice point (DOC and 32°F) and the steam point
(lOO°C and 212°F). Scientists and engineers often use absolute temperature
scales for Celsius and Fahrenheit. They are called the kelvin and Rankine
scales. These are explained in Chapter 17: Pneumatic Systems.
There are several types of thermometers. They use the same scales. The
major difference is in the method used to arrive at the reading.
Liquid expansion thermometers
The most common household and clinical thermometers (see Figure 2)
consist of a small reservoir and a fine tube. The reservoir is filled with a fluid
such as coloured alcohol or mercury. The temperature range they can
measure depends on the fluid used. A change in temperature changes the
volume of the fluid. This change in volume is indicated by the level of the
liquid in the tube. The tube or column is calibrated directly in degrees
Celsius (0C) or Fahrenheit (OF).
Figure 2 Liquid expansion
thermometer
2
15
Bi-metallic strip thermometers
The bi-metallic strip thennometer has a strip of two different metals bonded
together. This strip may be bent into a coil and linked to a pointer. The two
metals expand different amounts for a given temperature change. This causes
the coil to tighten or unwind. moving the pointer across the scale on the dial
face. (Thermal expansion is discussed in more detail later in the chapter.)
Bi-metallic strip thermometers are much more rugged than glass
thennometers but tend to be less accurate. Bi-metallic strips can be made to
respond to a wider range of temperatures.
Pyrometers
A pyrometer is an instrument that measures temperatures beyond the range
of ordinary thermometers. They are used in such things as industrial furnaces
and boilers. There are various types:
•
The optical pyrometer measures the temperature by observation of the
colour produced by the object being heated.
•
Thennoelectric pyrometers are resistance thennometers.
Resistance thermometers
As their temperature changes. electrical conductors change their resistance to
the flow of electrical current. Resistance drops as temperature drops and
rises as temperature rises. The electrical resistance of the metal platinum, for
example, varies directly with temperature (over a specific range of
temperature).
A platinum resistance thermometer consists of a coil of platinum wire sealed
inside a quartz container. The coil of wire is connected to a power souree
and an ohmmeter. An ohmmeter measures the electrical resistance of the
wire. The reading on the ohmmeter can be calibrated to indicate temperature
directly. These thermometers can accurately indicate temperatures up to
about 1350·C (2460·F).
Thermocouples
Thermocouples are also used to measure high temperatures. They consist of
an electrical circuit terminating with the junction of two different metals.
When the temperature of the junction is raised, a voltage is produced across
the junction. This voltage is detected by a sensitive voltmeter placed within
the loop. (Electrical resistance and voltage are discussed in more detail later
in this chapter.)
Thermocouples are widely used in industry. They are quite accurate and,
depending on the two metals chosen, can rcliably indicate temperatures up to
about 1500"C (2700"F).
2
16
Heat units
Heat is energy in transit. The units used in industry to measure heat are
joules (J), kilojoules (kJ), megajoules (MJ), and British thermal units (Btu).
These units are defined as:
•
Ajoule is the work done by 1 newton of force moving through 1 metre.
1 J= 1 N.m
•
A kilojoule is 1000 joules
1kJ=1000J
•
A megajoule is 1000 000 joules
1 MJ = 1000000 J = 1000 kJ
•
A British thermal unit is the amount of heat needed to change the
temperature of 1 Ib of water by 1°F.
I Btu = 1054.8 J
1 J = 0.000948 Btu
1 kJ = 0.948 Btu
Note that this definition involves temperature change. Note also that the
amount of heat absorbed by a substance to raise its temperature 1° is the
same as the amount of heat lost by the substance when its temperature
drops by 1°.
Thermal expansion
If it is free to move, almost all matter expands when it is heated. This is
called thermal expansion. The amount of thermal expansion of a given
material is predictable. Research has shown that the thermal expansion of a
material is proportional to the change in its temperature. Most materials
expand at different rates. Note that when they are cooled, materials contract
in the same manner as they expand.
Linear expansion
Linear expansion refers to a change in the dimension of an object in a
particular direction: length, width, height, or diameter. The different rates of
linear expansion of various materials are expressed as coefficients of linear
expansion. These coefficients are used to predict dimension changes as
temperature changes.
Table 3 on the next page shows the coefficients of linear expansion for
various common metals.
2 -17
Table 3: Coefficients ot linear expansion
Material
Perce
Per of
Aluminum
0.000023 B
0.0000184
0.000 0165
0.0000124
0.00001
0.000 009
0.0000104
0.000012
0.000 026 7
0.000 0655
0.000 0633
0.000 015
Brass
Copper
Cast iron
Steel
Tin
To calculate the change in one dimension of a piece of material caused by a
change in temperature:
Use a coefficient of linear expansion for the
material and multiply it by the original
dimension of the material and the change in
temperature.
L, =L,( 1 + a(Tz-T)J
where
Ll =original length
L, = neW length
T 1 = original temperature
T 2 = neW temperature
C(
= coefficient of linear expansion.
Temperature of metal parts may change due to the temperature of the
surroundings (ambient temperature) or due to such actions as friction.
A
Caution!
Linear expan.sion must be allowed for wherever the temperature of metal
parts may change.
Volume expansion
A heated substance (solid, liquid, or gas) expands in all directions. That is,
the volume of the heated material expands with a change in temperature. The
coefficient of volume expansion for a solid is three times the coefficient of
linear expansion.
Linear expansion has no practical meaning for liquids and gases, but volume
expansion is very important. For example, a tank of gasoline filled in spring
2-16
or winter conditions may overflow in the heat of Bummer if expansion is not
calculated and allowed for. Table 4 shows volume coefficients of expansion
for some liquids within a specific temperature range.
Table 4: Coefficients of volume expansion
PeroC
~i,,!uid
0.0012
0.0008
0.0003
Gasoline
Petroleum (oil)
Water
--------- ...
Per of
---------~
..
~
..
0.0006
0.0004
0.00012
To calculate the change in the volume of a liquid:
Use the coefficient o/volume expansion/or
the liquid and multiply it by the original
volume and the change in temperature.
volume change = <X,x V,x (T,- T)
where
VI = original volume
V 2 = new volume
Tl = original temperature
T2
new temperature
(Xv
coefficient of volume expansion.
When storing liquids, you must take into account the possibility of the liquid
expanding.
A
Caution!
Tightly sealedfull containers can burst if they are stored near a heat source.
Heat transfer
Heat flows from one object to another in three ways: conduction, convection
and radiation.
Conduction
When heat is transferred by conduction, the heat travels directly through a
material. One example of heat transfer by conduction is when a bearing is
heated with a hot plate. The heat of the element is conducted through the
element cover to the bearing.
19
Several factors detennine how fast heat can travel by conduction:
•
For heat to be conducted through an object, the various parts of the
object must be at different temperatures. Heat transfers from areas of
higher temperature to areas of lower temperature.
•
The thicker the material, the slower the heat transfer.
•
The larger the contact surface area, the greater the heat transfer.
•
Some materials conduct much better than others. A material's ability to
conduct heat is called the material's thermal conductivity.
Convection
When a fluid (liquid or gas) is heated, it expands and so becomes less dense.
Fluids can move or flow. Cooler fluid, being more dense, moves downward
and pushes the less dense warmer fluid upward. This upward movement of a
less dense fluid coupled with the downward movement of a denser fluid is
called convection.
For example, convection heating occurs in hot-water tanks. The heating
element is at the bottom of a hot-water tank. The water at the bottom is
warmed and rises to the top. The warmed water displaces cool water at the
top of the tank. The cooler water is dri yen to the bottom of the tank where it,
too, can be warmed by the heating element. The water constantly circulates
in the tank until all the water is at the desired temperature.
Radiation
Heat transfer is similar to light and may occur through space in the same
way. For example, both light and warmth travel from the sun to earth. The
radiation is electromagnetic radiation. Electromagnetic waves include xrays, gamma rays, visible light, radio waves and infra-red rays. The special
waves produced by thermal radiation are emitted by an object because of its
temperature. Heat transfer of this type is called radiant heat.
At low temperatures, objects emit little thermal radiation. But as an object's
temperature increases, so does the level of radiated heat. Physicists have
found that when an object's absolute temperature doubles, its emitted level
of energy increases by a factor of sixteen.
The ability to emit heat is closely related to the ability to absorb heat. The
amount of heat radiated from an object depends on two factors:
•
temperature of the object
•
colour and surface of the object:
A dull, black surface is a good absorber and radiator of heat (but a
poor reflector). For example solar collectors are often painted black so
they will absorb heat most efficiently.
- A highly polished, light coloured surface is a poor absorber and
radiator of heat (but a good reflector). For example steam and hot
water pipes are often painted white to prevent radiated heat losses.
2-
Force and motion
Objects can be at rest or in motion. An object is in motion when its position
continually changes.
Force is defined as an external agent that
causes a change in motion or stress in a body.
(External agent means something outside the body.) Force can be either a
push or a pull on an object.
Stress is caused when the object is not free to move. A change in motion
(acceleration) is caused in objccts that are free 10 move. When force causes
motion, the movement may be linear (in a straight line) or rotational (around
an axis). This is discussed in more detail later in this chapter.
Newton's laws of motion
Isaac Newton was the most famous 17th and 18th century scientist to
investigate the way forces cause motion.
Newton's first law of motion
The mass of an object tends to keep it at rest unless it is acted on by some
external force. This tendency to stay at rest is called inertia. Newton's first
law describes this quality of inertia:
A body at rest will stay at rest and a body in
motion will remain in motion at the same
speed and in the same direction unless
acted on by some unbalanced force.
Newton's second law of motion
A force acts on a mass to cause acceleration. Newton's second law of motion
gives a way to calculate force based on this:
The size of a net force (F) on an object is
equal to the product of the mass (m) and
acceleration (a) of the object.
F=mxa
Also, the direction of the force is in the
direction of the acceleration.
Newton's third law of motion
Forces never occur singularly. For example, pushing an object into position
while standing on a slippery floor may propel you away from the object.
This example supports Newton's third law of motion:
For every action, there is an equal and
opposite reaction.
Linear motion
Linear motion is motion in a straight line. This movement has linear velocity
(speed), acceleration and deceleration. For example when a hydraulic
cylinder operates its rod has linear motion. The rate at which it moves is
called its velocity. When the rod begins its movement it accelerates until it
reaches its set velocity and decelerates as it slows to a stop.
Linear velocity and speed
Velocity is the distance a body moves in one direction per unit time. The
direction of the movement is part of velocity. In ordinary language the word
speed is used, but speed does not take direction into account.
velocity = distance + time
d
t
V= -
The common units for velocity are metric or imperial:
Metric
•
kilometres per hour (km/h)
•
metre per minute (mlmin)
•
metres per second (mls)
Imperial
•
miles per hour (mph)
•
feet per minute (ftImin)
•
feet per second (ft/s)
It is often useful to convert velocities to the units mls or ftIs . The conversion
factors are:
•
1 km/h = 0.28 mls
•
1 mls = 60 mlmin
•
1 mph = 1.47 ftIs
•
1 ftImin = 60 ft/s
Linear acceleration
If velocity or speed changes, we say that the movement is accelerated or
decelerated. In ordinary language this means to speed up or slow down.
acceleration = (final velocity - original
velocity) + time
a=
2
22
When using this formula to calculate acceleration, a negative result means a
deceleration.
The most common units fOf acceleration are:
•
metres per second per second (m/sz)
•
feet per second per second (ft/sZ)
Rotational motion
Rotational motion occurs when a force causes an object to rotate around an
axis. The axis may be internal as when the Eartb spins on its axis, or external
as when the Earth rotates around the sun.
When an object makes one complete rotation around its axis it is called a
revolution. The speed of a rotating object is measured by the number of
revolutions it makes every minute,-that is, revolutions per minute (r/min or,
more conunonly, rpm).
The periphery (outside edge) of a rotating object moves at a particular
velocity.
•
The metric units used for this velocity are for example, metres per
minute (m/min).
•
The imperial system commonly refers to this velocity as surface feet per
minute (sfpm) or simply feet per minute (ft/min).
For example, when the pulley of a belt drive unit rotates, the pulley's
rotation could be expressed in rpm, but it is more useful to express the belt
travel over the pulley in sfpm.
Energy, work, and
power
In order to move an object, a force must be exerted. For example force is
required to pull a wrench in order to loosen a bolt. As the force moves the
object, energy is used and work is done. We express the rate at which work
is done as power.
Energy
Energy is the ability to do work. Energy comes from many sources such as
the sun's radiation, chemical interactions, motion, position, the tide. wind,
and electrical and magnetic fields.
Energy is commonly divided into three major classifications: potential
(static), kinetic (dynamic) and radiant (electromagnetic) energy. The two
classifications discussed in this section are potential energy and kinetic
energy.
Potential energy
Potential energy occurs when an object is in a position to do work but no
work is being done. That is, it has the potential to do work. For example, ball
at the top of a slope has potential energy. It will start to do work as it rolls
down the hill, decreasing its potential energy.
The potential energy of an object is the same as the work required to put it in
that situation. For example, the ball at the top of a slope got its potential
energy from the work done against the force of gravity to put it there.
Other common examples of potential energy are:
•
the energy of an object attached to a tensioned spring. The object keeps
the potential energy as long as it does not mOve.
•
when a wrench is placed onto a seized bolt and force is applied to the
wrench but no movement is made, then the wrench has potential energy.
Kinetic energy
Kinetic energy occurs when an object moves. For example, as the force on
the wrench begins to cause a seized bolt to move, the potential energy is
converted into kinetic energy and work is being done. Also, as a ball rolls
down a slope, work is done as its potential energy is converted to kinetic
energy.
The total kinetic energy of a moving object is equal to the energy used by the
object to reach its velocity from being at rest. It is the same as the work done
by the object.
Conservation of energy
Energy often changes form. The potential chemieal energy in gasoline
becomes kinetic energy after it is ignited to drive a piston in an internal
combustion engine. The potential energy of water stored behind a dam
becomes electrical energy when the water is released through turbines. That
electrical energy can then be converted to intemal (heat), light, or sound
energy.
When energy changes form, the amount of new energy is the same as the
amount of original energy. The law of conservation of energy states that:
Energy cannot be created or destroyed, but
it may be converted from one form to
another.
Often only part of the energy is converted into useful energy. The rest has
not been lost or destroyed, it simply takes another form. For example, some
of the electrical energy in a stove element is converted to light rather than
intemal energy when it glows red. Or mechanical energy may be converted
to intemal energy by friction, causing its temperature to rise.
2-24
Work
One simple definition of work is used when dealing with forces moving
objects:
The work (W) done on an object equals the
force (F) applied to the object times the
distance (d) that the object moves in the
direction of that force.
W =force x distance
=Fxd
By this definition, if a force is applied and nothing moves, then technically
no work has been done.
A more general definition says that:
Work is done whenever one kind of energy
is converted into another type of energy.
For example, work is done when muscular energy is converted to mechanical
energy and used to move a lever. Work is also done when internal energy
stored in steam is used to drive a motor or generator.
Units of work
The units of work are
•
metric: joules (J), kilojoules (kJl, and megajoules (MJ)
1 J is the work done by a force of 1 N acting over a distance of 1 m
1 J= 1 N.m
lkJ=1000J
1 MJ = 1 000 000 J = 1000kJ
•
imperial: foot-pounds-force (ft-Ibf) in the imperial system
1 fUbf is the work done by a force of 1 Ibf acting over a distance of 1 ft
•
US customary: foot-pounds (ft.lb)
Power
Sometimes it is not only important how much work is done, but also the rate
at which it is done. A small motor can mOve a huge mass a significant
distance if there is plenty oftime. But, if the job is to be done effecti vely,
rate of work is important.
Power is the rate at which work is done.
Power = work + time
w
P=-
t
Units of power
Power is expressed in units of work per unit time (Ils, klls and ft.lbfls). The
common units are:
•
Metric: watts (W)
IW
= 1 lIs
1 kW = 1000 lIs
•
Imperial: horsepower (hp)
1 hp
= 550 ft.lbfls
In North America horsepower is often used as a unit for mechanical power.
Watts and kilowatts are used to express electrical power. The conversion
factors are:
=746W=0.746kW
•
1 hp
•
1 kW = 1.34 hp = 737 ft.lbfls
Electrical energy is sold in kilowatt-hours (kW.h). See "Electricity" further
on in the chapter. The conversion factors are:
I kW.h = 1000 lIs x 3600 s
= 3600 kJ
Simple machines
A simple machine magnifies the effects of an applied force. The main
reasons for using machines are to make work easier or more efficient.
•
When the work is made easier, we say that a mechanical advantage
(MA) has been gained. For example when you are able to move a large
load with a small force you have a mechanical advantage.
MA is the ratio of the work done to the
effort required.
•
When the work is done with very little loss of energy to other processes
such as friction, we say that the machine's efficiency is high.
There are three simple machines: the lever, the inclined plane, and the
hydraulic press. They are often combined to form compound machines.
Levers
Levers are the simplest of the basic machines. Figure 5 shows the use of a
lever to move a large object. The point at which the lever (bar or rod) pivots
is called the fulcrum of the lever.
2
26
Effort
force
E
Fulcrum
Resistance
force
R
Fulcrum
Figure 3 Using a lever
The lengths of the effort arm and the resistance arm of the lever are
measured from the fulcrum (pivot point).
•
The effort ann is measured from the point where the effort (applied
foree) is applied to the fulcrum.
•
The resistance ann is measured from the resistance (or load) to the
fulcrum.
The calculations for all levers are derived from mathematical ratios as
follows:
MA=R+E
EA +RA =R+E
ExEA=RxRA
Where:
E A = length of the effort arm
RA = length of the resistance ann
R =resistance force
E =effort force
Other commonly used transpositions of this ratio are:
EA = (RA X R) + E
RA=(EAxE)+R
R=(EAXE)+RA
E = (RA X R) + EA
There are three classes of levers (see Figure 4).
R
I'
Class I lever
E
R
E
Class 2 lever
E
R
~
____
RA _ _
Class 3 lever
Figure 4 Classes of levers
2
-----I.I
Class 1
Class I levers have a fulcrum between the resistant force and the effort. The
lever shown in Figure 3 is a Class 1 lever. Examples of Class I levers are
crowbars, teeter-totters, bolt cutters, scissors, and pliers.
Class 2
In Class 2 levers, the resistance is between the fulcrum and the effort.
Examples of Class 2 levers are nutcrackers and wheelbarrows. Lifting an
objeet by one end is also an example of a Class 2 lever.
Class 3
In Class 3 levers, the effort is applied between the resistance and the
fulcrum. In these levers:
•
The effort arm is always sborter than the resistance arm.
•
The effort is always greater than the resistant force.
Examples of Class 3 levers are tweezers, cr".mcs and back-hoes. When you
use your hand and arm to carry something, you are also using a Class 3
lever.
Variations of levers
One drawback to simple levers is the limited angle at which they can
operate. Various methods are used to overeome this:
•
wheel and axle systems
•
pulley systems
•
drive systems.
Wheel and axle system
A wheel and axle operate as a continuous lever. They can operate in the
same manner as a Class 2 or 3 lever, depending on whether the wheel or the
axle is used to apply the effort.
•
When the wheel is used to apply the effort, it acts as a Class 2 lever. The
axis of the wheel and axle act as the fulcrum, the radius of the wheel is
the effort arm and the radius of the axle is the resistance arm. For
example, hand winches. wrenches, ratchets and drive units are Class 2
levers.
•
When the axle is used to apply the effort. it act~ as a Class 3 lever. The
radius of the axle is now the effort arm and the radius of the wheel is the
resistance arm. For example. the wheel and axle of a vehicle is a Class 3
lever.
The formulas for levers also apply to the wheel and axle.
Single pulleys
A single pulley is considered to be a continuous lever with equal effort and
resistance arms. In pulley systems the term effort arm (EA) is replaced by
effort distallCc <Bol. and resistance arm (RA) by resistallCc distance (RD).
The single. frxed pulley (see Figure 5) does not increase the applied force.
but simply allows the furce to change direction. For example. pulling down
moves the load up.
\
\
~
Figure 5 Single fixed pulley
/
E
Figure 6 Block and tackle
30
Multiple pulleys
In a mUltiple pulley system such as the block and tackle shown in Figure 5,
the mechanical advantage is much greater than 1. Several pulleys are
arranged so that a single rope threads through them.
When the effort end of the rope is pulled, the load end moves a much smaller
distance than the effort end. Ideally:
MA =ED+RD
You can quickly find the MA in a multiple pulley system by counting the
number of rope strands supporting the load. In Figure 6, the number of rope
parts around the lower pulleys supporting the load is four. That is:
MA=4
Simple drive units
Drive units such as belt, chain and gear drives all work in a similar way.
When one wheel transmits power to another, the circumference is used to
calculate the mechanical advantage (MA).
•
In a belt drive. the diameters of the pulleys or sheaves can be used
because their circumference is directly proportionate to their diameter.
To calculate the MA of a belt drive, the diameter (0) of the driven
pulley or sheave is divided by the diameter of the drive pulley or sheave.
MA = driven 0
•
+ drive 0
In chain and gear drives, the number of teeth around the sprockets or
gears are used. To calculate the MA of a chain and gear drive, the
number of teeth on the driven sprocket or gear are divided by the number
of teeth on the drive sprocket or gear.
MA
# teeth an driven + # teeth on drive
The MA is also used as a speed ratio using revolutions per minute (rpm) of
the rotating parts. In the following, the teon element is used to describe
pulleys, sheaves, sprockets and gears. The example applies to all drives. For
sprockets and gears use the number of teeth instead of diameter.
•
If the driven element is larger than the drive element, the rpm of the
driven shaft is less than the rpm of the driver.
•
If the driven element is smaller than the drive element, the rpm of the
driven shaft is greater than the rpm of the driver.
Therefore:
rpm drive X 0 drive = rpm driven X 0 driven
Note that the dircctions of rotation of two shafts connected by a belt or chain
are the same. Two shafts connected by gears rotate in opposite directions.
Wedges
The wedge is a V-shaped device. They are used to split wood, hold doors
open, and level machinery. They are also the basic machine used in cutting
tools such as chisels, axes and knives.
Cams
Carns are rotary wedges used to operate parts such as val ves in an internal
combustion engine. They use a turning motion to produce an oscillating
(linear) force as shown in Figure 8
/
............._ - Valve stem
Closed
Closed
Open
Figure 8 Cams
Screws
A screw is an inclined plane wrapped around a cylinder (see Figure 9).
Bolt
Pitch
Figure 9 A screw
The distance between two adjacent threads is called the pitch of the screw.
When a screw is turned through one complete revolution, the screw travels a
distance equal to the pitch. See Chapter 5: Threads and Fasteners. The finer
the pitch of a thread, the higher the MA. However, as the pitch becomes
finer, the thread depth becomes shallower and easily stripped.
Screws can have huge MAs, but are inefficient machines because of the
friction developed between the external and internal threads. (Even this can
be a practical advantage because it prevents screws from loosening easily.)
Hydraulic presses
The hydraulic press is the third basic machine and it transfers force through a
liquid. It uses Pascal's principle which is described in Chapter 16:
Hydraulic Systems.
Input pressure = output pressure
where pressure = force + area
Figure 10 Hydraulic lift
In Figure 10, note that the smaller piston (A) needs a long chamber to move
through in order to move the large piston (B) a little. In practice this is done
by moving a small piston through a series of short strokes. The chamber of
the small piston is refilled from a reservoir of fluid. Check valves are used in
the lines to enSure the correct fluid direction.
Hydraulic press principles are used in equipment such as hydraulic floor
jacks, braking systems and presses.
Compound machines
Sometimes, two or more simple machines are coupled together. For
example, a hand-operated hydraulic jack sometimes uses a lever to input
force. This compounds (multiplies) mechanical advantage. When this
happens, the output of one machine is often the input of the second machine
and so on. In such cases, the overall mechanical advantage of the compound
machine is the product of the individual mechanical advantages.
2
34
Electricity and
electromagnetism
Many machines and tools in industry are driven by electricity. On occasion,
the millwright installs and aligns electrical machines. A basic understanding
of electricity is necessary to ensure safe maintenance of electrical equipment
and to communicate with other workers.
This section provides a brief overview of electromagnetic, direct-current,
and alternating-current principles along with an explanation of the more
common terminology.
Atomic structure
The theory of electricity begins with the atom. As mentioned earlier, all
matter consists of atoms. All atoms are made up of smaller subatomic
particles:
•
protons
•
neutrons
•
electrons.
At the centre of an atom is the nucleus which consists of protons and
neutrons. The protons carry a positive electrical charge and the neutrons
carry no electrical charge. Since they are electrically neutral, neutrons are
ignored in this section.
Electrons carry a negative charge. Electrons swirl around an atom's nucleus
in various orbits. They are held in those orbits by the force of attraction
between opposite electric charges. Under normal circumstances, atoms
contain equal numbers of protons and electrons which makes them
electrically neutral. Figure 11 shows a simplified picture of the structure of a
copper atom.
Orbit
Nucleus
(protons + neutrons)
Electrons
Figure 11 Structure of a copper atom
2
35
Current, conductors, and insulators
In certain elements, the force holding electrons in the outennost orbit is
weak enough to ailow those electrons to break away from the atom. These
electrons are called free electrons because they are free to move from atom
to atom.
The movement of free-electrons is quite haphazard, however, if they can be
forced to drift in the same general direction, then they constitute what is
called an electric current.
•
Elements that make available a large number of free-electrons ailow
relatively large currents to flow, and are termed conductors.
•
Elements that make available relatively few free-electrons allow only
incredibly small currents to flow and are termed insulators.
Effects of electric current
A flow of electric current produces any combination of the following three
effects:
•
heating effect-When current flows through a conductor, the
temperature of that conductor rises. This effect is made use of in electric
heating and lighting. It also wastes energy by heating conductors when it
is not needed.
•
magnetic effect-When current flows, a magnetic field is set up around
the current path. This made use of in electric machines such as
generators, motors, and transfurrners.
•
chemical effect-When current flows, certain chemical changes may
occur. This is made use of in electroplating. This effect is also the cause
of corrosion.
Anyone of these effects can be used to detoct the presence and magnitude of
an electric current.
Magnetism
Magnetic materials attract or repel other magnetic materials. Magnetism is
caused by the spin of electrons within a molecule. If the molecules in a metal
can be aligned so that their individual magnetic fields all act iu the same
direction, theu the metal become a magnet If the molecules are not aligned,
then the metal does uot act as a magnet
Ferrous metals can be magnetized. Some are magnetized very easily and
others with more difficulty.
Temporary magnets
The molecules in iron are easily aligned, but can just as easily move out of
alignmeut. Therefore iron can be used as a temporary magnet. In practice,
2
S6
silicon steel has similar magnetic characteristics to iron and is used in the
manufacture of temporary magnets. Temporary magnets are used in the
manufacture of larger motors, relays, solenoids, etc.
Permanent magnets
The molecules in steel, are more difficult to align but, once aligned, tend to
stay that way. Steel (or steel alloys incorporating nickel and cobalt),
therefore, can be used as permanent magnets. Permanent magnets are used in
the manufacture of simple electric motors, energy meters, etc.
Magnetic fields and poles
All magnets are surrounded by magnetic fields. In this field, the effects of
the magnetic force can be felt. A magnetic field is strongest at the poles of
the magnet. Each magnet has two opposite poles identified as north and
south. A fundamental law of magnetism states that:
Like poles repel, and unlike poles attract.
Electromagnetism
Whenever an electric current flows through a conductor, a magnetic field is
set up around that conductor. The strength of the field can be increased by
forming the conductor into a coil. The strength can be increased further if an
iron core is inserted into the coil. Such an arrangement can be used to create
an electromagnet.
When current flows, an electromagnet behaves like a very strong bar
magnet, with a north pole and south pole. If the core is made from iron or
silicon steel, the magnetic field disappears when the current is switched off.
Electromagnets are used in various applications, such as scrap metal cranes,
relays, etc. One practical application that a Millwright will encounter is the
magnetic-base drill-press, which can be clamped to a ferrous metal worksurface, when activated by an electric current. Electromagnetism is also the
basis for operation of generators, transformers and large electric motors.
Electrical circuits
The path which an electric current takes is termed a circuit. The sitnplest
form of a circuit consists of a power source, conductors, a circuit element
(load) and a switching device.
Units of current flow
The current flow in the circuit is measured in amperes (A):
1 ampere (A) = 1000 milliamperes (rnA)
1 kiloampere (kA) = 1000 A
2 37
Energy source
The energy source drives electrical current through the circuit, performing
work. The energy source supplies a voltage to the circuit. This has the
potential to move current through the circuit. Its basic unit of measurement
is the volt (V).
The most commonly used units are:
1 volt (V) = 1000 millivolts (mV)
1 kilovolt (kV) = 1000 V
1 megavolt (MV) = 1 000 000 V
Conductors and resistance
Conductors connect the various components in a circuit. This allows the
current to flow throughout the circuit. Good conductors have low resistance
to current flow. The unit used to measure resistance is the ohm (n):
1 ohm (n) = 1000 milliohms (mn)
1 kilohm (ill) = 1000 n
1 megohm (Mn) = 1 000 000 n
Load
The load uses electrical energy to do useful work. A load may be a motor,
lights, warning horns, heaters, etc.
Switches
Switches are used to open or close the circuit. A circuit is:
•
open when the path for the current is broken.
•
closed when the circuit has a continuous path for the current to flow.
Contacts in an electrical relay function as remote-controlled switches within
control circuits.
Types of circuits
Circuits are classified as series, parallel, series-parallel or complex.
Figure 12 shows examples of these circuits.
2
38
Series
Parallel
Series/Parallel
Complex
Figure 12 Types of circuits
•
Series circuits are connected in such a way that the same current flows
through each load.
•
Parallel circuits are connected so that the same potential-difference
(voltage) appears across each load.
•
Series-parallel circuits share the characteristics of both series and
parallel circuits.
•
Complex circuits are any circuits that do not fall into any of the other
categories.
The most common connection is the parallel circuit. For example, electrical
components in a vehicle are connected in parallel. Therefore, each is
subjected to 12 V. Each component is manufaetured to operate correctly at
12V.
Similarly, lighting and power circuits in a shop are connected in parallel.
This ensures that 120 V is applied across any device connected to those
circuits.
Electrical principles and laws
The following are the common principles needed to understand the function
and units used to express electricity.
Ohm's Law and resistance
Ohm's Law is one of the most fundamental laws in electrical engineering.
It states:
At a given temperature, the current flowing
through a conductor is directly proportional to the voltage across that conductor.
2
Increasing the voltage across any conductor increases the current flowing
through it. And vice versa. Expressed as equations, Ohm's Law is written:
1= V ... R
R=V+I
V=lxR
Where R is the resistance of the conductor. I is current, and V is voltage
(potential difference). Sometimes E is used rather than V.
The resistance (R) of a conductor is independent of current <n and voltage
(V). An object's electrical resistance depend~ on the resistivity of the
material it is made from. It also depends upon the its length and erosssectional area.
Resistivity is affected by temperature. In general:
•
As the temperature of most metals (conductors) increase. the value of
resistivity increases, acting to increase resistance in the circuit.
•
As the temperature of most common insulators increase, resistivity
decreases, aeting to reduce resistance. This explains why overheating is
a major cause of insulation failure.
Electrical work and power
1 watl ofpower results when 1 volt
produces a current of I ampere.
P=VxI
This relationship between power (P). voltage (V) and current (I) can be
found by considering the way work and power are related.
•
Work is defined as energy in transit from one form into another. For
example, a motor converts electrical energy into kinetic energy, thus
doing work.
Electrical work is equal to voltage times current times time (I).
W = VxIxt
•
Power is defined as the rate of doing work. Power is expressed as work
divided by time.
P = W+t
Substituting for W, from the equation above:
P = (V x I x t) + t
= VxI
In the SI metric system, both energy and work are measured in joules (J).
Power is measured in watts.
2-40
1 watt CW) = 1 Jls
1 kilowatt (kW) = 1000 W
When power is measured in kW and time in hours (h), work is expressed in
kilowatt-hours (kW.h). This is an older unit but it is still used only when
electrical energy is sold.
Direct current (DC) and alternating current (AC)
Electric current may flow in two ways:
•
in one direction only-this is direct current (DC). A battery is an
example of a DC energy source.
•
back and forth continuously-this is alternating current (AC). All
generators naturally generate alternating voltages. When these voltages
are supplied to external circuits, they cause alternating currents to flow.
AC circuits have a number of advantages when compared with DC. The
main advantage is in the use of transformers. Transformers cannot work with
direct current.
Transformers
Transformers increase and decrease alternating voltages with very little
energy loss. This allows electricity to be distributed on a large scale. A
transformer comprises two coils, or windings, wound around a common
silicon-steel core.
One winding (the primary winding) is supplied from the utility's alternating,
high-voltage system. The resulting magnetic field links with a second
winding (the secondary winding). The changing magnetic field induces an
alternating voltage into the secondary winding.
•
If the secondary winding has fewer turns than the primary winding, then
the voltage induced into it is lower. The transformer is then termed a
step-down transformer.
•
If the secondary winding has more turns than the primary winding, then
the voltage induced into it is higher. The transformer is then termed a
step-up transformer.
Single-phase AC circuits
Homes and small workshops are usually supplied by means of a single-phase
service. This is obtained from a local transformer. The secondary winding
has a voltage of 240 V induced into it. A connection is made to the centre of
the winding (that is, it is centre-tapped). This connection is grounded, and
provides the electrically neutral conductor. This allows the service entering
the building to provide both 240 V and 120 V supplies.
2
41
A
B
c
A
A
B
c
C
N
Figure 13 Wye connection
2
•
Lighter loads (such as lighting circuits) are connected between either
line conductor, and the neutral, and are SUbjected to 120 V.
•
Heavier loads (like clothes dryers) requiring more energy are connected
line to line, and are subjected to 240 V.
Three-phase AC circuits
Larger buildings and factories are supplied with three-phase AC services. In
fact, the entire utility system operates as a three-phase system.
Three-phase generators
Generators work by rotating magnetic fields within conductor coils called
armature windings. This induces voltages in the coils. The generators at
power stations contain three separate armature windings. They are physically
displaced from each other by 120·.
Wye and delta connections
The generator's three windings may be connected in a wye (or star), as
shown in Figure 13. The generator's output can then supply a given load
using four conductors. If three separate single-phase generators (with the
same total capacity) were used, six conductors would be necessary. This
saving in copper is one reason why a three-phase system is more economical
than a single-phase system.
The common connection between the three windings is termed the star point.
The conductor connected to the star point is grounded and termed the neutral
conductor. The other three conductors are termed line conductors. In North
America, the line conductors are identified as either 1, 2, and 3; as A, B, and
C; or as a, b, and c.
Another common way of connecting three-phase circuits is called delta (or
mesh), as shown in Figure 14. The delta connection has the additional
advantage of requiring only three conductors. Because of this, the utility'S
high-voltage transmission and distribution lines are suspended in multiples
of three.
Line
~---------------------A
Line
B
Line
~--------------------------c
Figure 14 Delta connection
2
The utility's three-phase transmission and distribution system makes USe of
both these connections. In general, the output windings of transformers that
supply workshops are wye-conneeted, and the service line has four
conductors.
Three-phase induction mOu>rs
One of the major advantages of a three-phase system is that it allows the
consumer to use three-phase induction motors. Three-phase induction motors
are smaller and more simply constructed than single phase and DC motors.
A three-phase induction motor has:
•
a stator (the stationary part) that supports three field-windings
•
a rotor (the rotating part).
The rotor is a laminated steel drum. This maximizes the strength of the
magnetic field. It has copper bars laid into its outside surface with their ends
short-circuited together, Their arrangement is similar to that of a squirrel
cage or hamster treadmill. For this reason, three-phase induction motors are
often termed squirrel cage motors.
Squirrel cage motors work as follows:
1.
The motor's field windings are connected to a three-phase supply.
2. The resulting current causes a natural, smoothly rotating, magnetic field
to be set up within the stator.
3. This rotating field, passes through the rotor's copper bars, inducing
voltages into them. This causes currents to circulate within them.
4. These current, in tom, set up magnetic fields which react with the main
rotating field. This creates a torque that causes the rotor to be tomed by
the rotating field.
Note that the direction of the rotating field can be reversed by interchanging
any two of the three line conductors that supply the motor.
A
Caution!
After a three-phase induction motor has been disconnected/or service, it is
very important to reconnect the line conductors to the correct terminals.
Failure to ensure this may result in the motor, and the load which it drives,
rotating in the wrong direction. This can cause damage.
Fuses
Fuses are circuit-protection devices. They are connected in series with a
load. In the event of excessive current flow, the fuse melts. This opens the
circuit, and protects the load device and its supply conductors from
overheating.
-44
Fuses have the following characteristics:
•
The fuse element is usually made of an alloy such as silver-tin. This
alloy combines the high conductivity of silver with the low melting point
of tin.
•
A fuse is rated aecording to the value of current that may continually
flow through it without causing the fuse-element to overheat and melt.
•
Fuses have an itlverse time characteristic. This means that the greater the
value of a fault current, the faster the fuse operates.
IDgb-voltage fuses
High-voltage fuses are physically larger and more complex than fuses
designed for low-voltage applications. This is because once a high-voltage
fault current has been interrupted, an electrical arc is formed. This arc can
lead to serious damage, through overheating, unless it is extinguished. Highvoltage fuses incorporate some means of extinguishing arcs. These vary
from sand-filled cylinders, which suffocate the are, to spring-loaded
elements that streteh and snap the arc.
Circuit breakers
Circuit breakers are switehing devices that can interrupt fault currents. Fault
currents may be many times greater than nonnalload currents. In the case of
high-voltage circuit breakers, fault-currents may be many thousands of
amperes. Various special methods are used to extinguish the resulting arcing
between the contacts. High-voltage circuit-breakers are named after the
medium they use to do this. For example:
•
Oil circuit breakers use the motion and cooling effect of oil to
extinguish the arc.
•
Air-blast circuit breakers use a blast of compressed air to extinguish
the arc.
Motor controllers
A motor controller is used to control and protect electric motors. It has a
heavy-duty, electromagnetically controlled switch (called a cotltactor) which
opens and closes the circuit that supplies the motor.
Because the contactor is controlled electromagnetically, the control circuits
don't carry the motor's load current and so they eM use relatively light
conductors. Most importantly, this allows the remote control of the
controller.
Quite sophisticated control systems can be designed with many useful
features in addition to START and STOP. For example, the motor can be made
to reverse direction immediately, or to reverse direction only after stopping
first. It can also be made to move in increments (jog).
Temperature sensors are usually built into the controller. These monitor the
load current. If. for example, the motor stalls, the resulting excessively high
curren! is sensed. The motor controller disconnects the motor from the
supply. These sensors also prevent the motor from being restarted until its
temperature has returned to normal.
2 -46
ii?ijllfl!tl~_~~l4!~~~~ MILLWRIGHT MANUAL: CHAPTER 3
Technical Drawings
Types of technical drawings ..................................... ........... ........... 3: 1
Orthographic ..................................................................................... 3:2
Pictorial............................................................................................. 3:4
Common drawing elements ............................................................ 3:5
Lines ................................................................................................. 3:5
Dimensions ....................................................................................... 3:9
Views ................................................................................................ 3:13
Tolerance .......................................................................................... 3: 19
Information on drawings .................................................................. 3:24
Reading industrial drawings ........................................................... 3:27
Detail drawings ................................................................................. 3:27
Assembly drawings .......................................................................... 3 :28
Schematics, piping drawings, and symbols .................................... 3:30
Schematics ........................................................................................ 3: 30
Piping drawings ................................................................................ 3:34
Symbols and abbreviations ............................................................... 3 :34
Technical Drawings
The technical drawing or blueprint is a valuable tool in any industry. For
example, blueprints, drawings, and symbols are the "maps" needed to
understand how a machine is crafted, assembled, and installed. Blueprints
provide all the necessary information to understand that machine. As the
millwright trade develops and becomes more diverse, the millwright may be
asked to fabricate such things as chutes and conveyors, to interpret electrical
problems, to troubleshoot complex hydraulic problems, and so on.
Interpreting blueprints is an essential tool of the trade, allowing the isolation
of a problem area before a system is dismantled. The millwright should have
a good understanding of how to interpret these important drawings and the
symbols used in them.
Types of technical
drawings
In technical drawings, three-dimensional objects are represented in two
dimensions. This is done so that their shape and size are described as
accurately as possible. Two main types of technical drawings are used in
industry: orthographic and pictorial. Within these types there are several
variations. The following chart shows how they arc related.
Technical
drawings
I
Orthographic
drawings
First-angle
orthographic
Third-angle
orthographic
Schematic
diagrams
Pictorial
drawings
Isometric
Oblique
Orthographic
The term ortho means at right angles (90°). In orthographic projection, the
object being drawn is viewed at right angles from a number of positions.
These views are called elevations:
o
Infront elevation, the object is viewed from the front.
o
In side (or end) elevation, the object is viewed from one side.
o
In plan, the object is viewed from the top or bottom.
Choosing which face of an object is to be the front elevation is arbitrary.
However, as explained later in the chapter, certain common-sense rules are
usually followed.
First- and third-angle projections
There are two types of projection in orthographic drawings-first angle and
third angle. First-angle projection is used in Europe while third-angle
projection is used in North America. Millwrights should be familiar with
both projections because equipment is often supplied by European
manufacturers. Which projection is used is usually indicated on the drawing.
Figure 1 shows the International Standards Organization (ISO) symbols for
these projections. This symbol is shown adjacent to the title block of a
drawing as shown in Figure 2.
E]@
First angle
Third angle
Figure 1 ISO symbols for first- and third-angle projections
I~I
Title block
Figure 2 Placing the ISO symbol for first- or third-angle
projection on a drawing
Figure 3 shows a simple pictorial drawing of an object with a first- and thirdangle projection drawing of the same object.
3
2
Third-angle projection
•
First-angle projection
~
=:?=1.
LI
L.......f--.J
I
Frqnt elevation
m m m •••
••
I
i
End elevation
r~
Front elevation
Plan 109 I
Side elevation
Figure 3 A simple object shown In pictorial form and in first- and third-angle orthographic projection
In fIrst-angle projection the object is viewed from left to right as shown in
Figure 4.
Viewin~
position
Object
Side elevation
Figure 4 Viewing position for first-angle orthographic projection
Conversely, in third-angle projection the view is from right to left as can be
seen in Fignre 5.
~Viewing
position
Object
Side elevation
Figure 5 Viewing position for third-angle orthographic projection
Because third-angle projection is usually used in Canada and the US, this
chapter deals with drawings using a third-angle projection. Usually only
three views are requined: top (plan), front (front elevation), and one side
(side elevation). Sometimes auxiliary or sectional views are needed-these
are discussed later.
Simple rules for sketching orthographic projections
Most objects do not have a natural front or side position. Therefore, it is
necessary to determine which side of the object provides the most
information about its shape. This side is used as the front view. Most front
views are drawn with the object's longer dimension horizontal (see
Figure 3). This is because drawing paper is normally used in landscape
orientation (with its longer side horizontal).
Pictorial
Technical drawings arc usually orthographic projections. To interpret them,
the viewer must understand their conventions. Pictorial drawings are three
dimensional renderings of an object, showing height, length, and width.
These drawings do not des(:ribe the shape of an object either exactly or
completely. But they do help the viewer to visualize the true shape of the
object The most common engineering pictorial drawings are termed
isometric and oblique.
Isometric and oblique projections
Isometric and oblique drawings show the foreshortening of an object as the
viewer looks at it. For example, circles appear as ellipses. The difference
between them is the angle between the sides of the object and the main axis
of the drawing. See Figure 6.
Isometric
Oblique
\
30" \
Figure 6 Isometric and oblique projections
Isometric drawings
In isometric drawings, all lines are drawn either vertically or at 30° to the
horizontal The lengths along the 30° axes are drawn to full scale.
Oblique drawings
In oblique drawings, one face is at right angles to the horizontal and the
others are at an angle (usually 45°) to the horizontal. In most oblique
drawings, the receding dimensions along the 45° axis are drawn to half scale.
Common drawing
elements
Lines
Lines on technical drawings provide exact information concerning the shape
of an object. A variety of lines is used to convey different meanings.
Canadian Standards define line thicknesses as thick or thin. Thick lines are at
least twice as thick as thin lines. Figure 7 on the next page displays various
types of lines used in technical drawings.
3
5
1. Visible outline
<;v(l\.Q
4,,..0
(fA \
J"C\: \:.--v
\".P\ sv\,p +
. .
thick
(:, 'I- <l \:>
2. Hidden line feature
thin
3. Centreline
thin
Also used for path lines, pitch circles,
and axes of symmetry
thin
Alternate centre line
4. Projection line
thin
Also used for outline of revolved sections
5. Dimensioning lines
Extension line
Dimension line
-
~I
---I"~
--------------'
thin
Leader l i n l e e - - - - - - - - - - - - - - - - - - "
///////
6. Section lines (hatching)
thin
7. Cutting plane lines
---------
Simple sections
----,
Offset sections
I
thick
and
thin
I
I
B. Break lines
Long
thin
Short
thick
9. Phantom line
Used for adjacent parts, alternate positions,
developed views and portions to be removed
thin
10. Stitch line
thin
Used to indicate seams in leather, plastics
and textiles (label if possible confusion with #2)
11. Surface zone line
thick
Used to indicate a surface length that requires
special instructions such as finish
Figure 7 Types of lines
~_~<l~%.M?gf&~,s:4"c,:i1t'~Gruxxxs::c{,{,{*t1c~"~~~@fi~llik,_;t*)jj,:'!{~1)j)t,'%A·j~ji;S'*l\f;t)'(1;{H.t_X:MM,_M~h1:teCjg"2t0,'t[z;@iWM.u:X:X:l;":m~'%"i:ih'W1tM.ti."';HXt1mtr8g,'·"''h'h'h''l%'r=''*''''''m.(§)';)'''.\'iMiitit:Mm~w
3- 6
MILLWRIGHT-TECHNICAL DRAWINGS
Construction and projection lines
Construction lines and projection lines are thin lines used to layout the
various views of an object. Construction lines are eventually overlaid by the
object lines.
Object lines
Object lines are thick lines which indicate the visible outline of an object.
Hidden lines
Hidden lines show those surfaces and features of the object that are not seen
in the chosen views. They are thin, equally spaced, broken lines.
Break lines
Break lines are used to shorten the view of long unifonn sections. Figure 8
shows the various types of break lines used on technical drawings.
Thick line
Short break - all shapes
Cylinders - useful when end view is not shown
Long break - all shapes
- - - ---'~
Solid cylinder
- - - --'If-
~-----
Hollow cylinder
Figure 8 Conventional break lines
3
7
Section lines
Section lines are the parallel diagonal lines (hatch marks) that identify a
sectional view of an object in a drawing.
Centrelines
Centrelines are drawn as thin, broken lines, with long and short lines spaced
alternately. Figure 9 shows the centreline of a shaft and of holes in an object.
Centreline should ~
not be broken when
extended beyond
-I- - - - - --1-
--+-
Use two short dashes
Figure 9 Centrelines
Cutting plane lines
Figure 10 shows two types of cutting plane lines commonly found on
Canadian drawings. However, be prepared to interpret American and
European methods as well. Note that:
•
Offset cutting plane lines are usually of the broken type.
•
Letters placed beside arrows key to the corresponding sectional view.
\;:G<,K!"PmtJ&g{0mlT?>O;:0n©;'J@@;Xii~~h;i;W'Yo1m$ftj;©M'iiijiiit;~it;jdi~%Hillk.'~J~jij;X'i1li-~?l~;t-1";8;i->~~~i>'K31,,';)f1~ijK@;tK,>%%~"%?'at3.M~;(G©,%'X;)©,'K"3b";%''%l;'''ill)";a,'''y~:s:v_~_sti<;:;i!R_8lG'FG}¥:_'i_8};§'H[\\i'''t::;"t;W,%~,%H8
3 -8
MILLWRIGHT-TECHNICAL DRAWINGS
For all drawings
Alternative method
A~ _____________ ~A
Alternative method
i---- fA
A
f
-----.J
Offset
Figure 10 Cutting plane lines
Extension and dimension lines
•
Extension lines are thin lines which extend the object lines out to a
convenient space for dimensioning. These lines do not touch the object
lines. If extension lines cross arrowheads or dimension lines, a break in
the extension line is permitted.
•
Dimension lines are thin lines which indicate the distance between the
extension lines. They terrrrinate with arrowheads which touch the
extension lines. These are the lines which give the object's measured
dimensions such as height, width, and length. Where one or more
dimension lines share one extension line, the dimension lines should run
parallel to each other. See Figure lion the next page.
Dimensions
Dimensions indicate the sizes of various elements on the drawing, such as
length, diameter, and angle.
~ Dimension line
~
r
'!O
40
~
V
Extension line
I·
40
.1
o
'"
-L-_...-_ _...1
- - - -r----
Figure 11 The aligned dimensioning method
Methods of marking dimensions
Dimensions are written adjacent to the dimension lines. There are two
methods of dimensioning: the aligned method and the unidirectional method.
•
In the aligned method (see Figure 11), all dimension figures except
angular dimensions are written parallel to the dimension lines.
•
In unidirectional dimensioning, all figures are written parallel to the
bottom of the drafting paper.
Types of dimensions
There are two types of dimensions. Each drawing should use only one type.
•
Overall dimensions indicate overall length, width, or height of an object.
•
Detail dimensions give size and location of any feature or detail which is
not overall length, height or width.
Dimensioning rules
There are various rules of acceptable practice in dimensioning drawings:
•
Only one form of dimension is used on a single drawing; either aligned
or unidirectional.
•
Technical drawings are dimensioned so that the full-size dimensions are
specified on thc drawing regardless of scale.
•
In metric drawings, dimensions are shown in rnillimetres without their
units (for example, just 1100, not 1100 mm)
•
The position and size of angles are shown.
•
When metric values are less than one, a zero is shown before the decimal
point. For example 0.5 or 0.35 (not .5 or .35).
•
Whenever possible, to avoid confusion, dimensions are placed close to
features being shown and outside the object's outline.
•
Dimensions of a feature are shown only once. For example, in Figure 11.
the 100 mm dimension is shown on only one of the views.
•
When space is limited, dimension figures are placed in one of the ways
shown in Figure 12:
- inside the dimension lines with the arrowheads outside the extension
lines
- with the figures and arrowheads outside the extension lines.
3.0--11---
S12
1
Figure 12 Dimensioning in small spaces
Dimensioning cylindrical objects
All bores and radii are dimensioned by using leader lines followed by the
dimension. Also given are any procedures or details needed to complete a
process which is repeated (either machining or fabricating). Figure 13
depicts a counter-bored hole-it gives the diameter of the drill, the diameter
of the counterbore (C' bore), the depth, and. if necessary, the nunther of
holes. When dimensioning holes of the same size the dimensions need only
be shown once.
5/8" drill, 7/8 c: bore
5/16" deep
Figure 13 Dimensioning counter-bored holes
To locate a hole, the centrelines of the hole are used for dimensioning. Thus,
the centreline acts as an extension line as welL Figures 14 and 15 show the
preferred methods of dimensioning a bolt pattern. Notice that the dimension
Jines can be either leader lines or extension lines from the centre.
50--·············30 ~~_.,
l
o
I
0
(')
f
0
II)
.4>
1
I
~.
Figure 14 Dimensioning a square bolt pattem
In a circular pattern, the arc depicting the radius dimension is normally 30"
or 45° off the horizontal plane.
Acceptable
Preferred
Figure 15 Dimensioning a bolt circle
Arcs, rounds, or fillets are shown in the form of a radius measurement.
3
Views
Sectional views
Sectional views (or sections) are used to show an aspect of the object which
is otherwise too complicated to show with the conventional top, front, and
side views. Sectional views may also show differences in materials. A
sectional view cuts an object along an imaginary cutting plane. The drawing
is sectioned off at the cutting plane to reveal an internal view.
Front section removed
+--y?~/ Cutting plane line
Cutti ng plane
-0
Section B - B
SIDE VIEW IN FULL SECTION
Front section removed
+--y;-
Cutting plane line
Cutting plane
Q}
+_..JA
Section A - A
SIDE VIEW IN HALF SECTION
Figure 16 Side views in full and half sections
•
Full-section views use a cutting plane through the whole object giving
the impression that the object has been cut in half.
•
Half-sectionals remove only a certain portion of the drawing. If a halfsection view gives all the information needed to understand the drawing,
then a full section drawing is not given. See Figure 16.
Different materials and solid parts
A sectional view may cut across more than one type of material. Figure 17
shows some line patterns commonly used to indicate types of materials.
These patterns also indicate solid portions of an object.
/"
/"
/"
/"
/"
/"
/"
Iron
Steel
/"
/"
/"
/"
/"
/"
/"
/"
Brass, bronze, copper
Figure 17 Patterns indicating different materials
Offset sectional views
Another type of sectional view is the offset sectional. Offset sectional cutting
lines are always of the broken type. Figure 18 shows the necessity for
offsetting the sectional view. It can be seen that a normal cutting plane line
could not possibly give a proper perspective of the part in question. The lines
must be offset to show the outer bolt holes.
Figure 18 Sectional and offset sectional cutting planes
3
14
Other types of sectional views
Other types of sectionals include aligned sectional (Figure 19), revolved
sectional (Figure 20), removed sectional (Figure 21), and broken-out
sectional (Figure 22).
.. 1/
Cutting plane line
i
/
j . . ._ . .
/
/.
///
Aligned
sectional
view
/
Angled elements must be aligned
Figure 19 Aligned sectional view
~IOE
Figure 20 Revolved sectional view
3
15
~ ~
Section A·A
Section B·B
A
B
A
B
I I
I I
I
" '
I
: I I
Figure 21 Removed sectional view
Figure 22 Broken-out sectional view
Auxiliary views
Auxiliary views are used to detail sloping (or inclining) surfaces which
cannot be depicted in normal orthographic views. Auxiliary drawing clearly
shows the shape of thc object and gives its true dimensions.
3-
Auxiliary views are created by projecting the lines of the object where the
sloping surface appears as an edge. See Figure 23.
Surface A
Surface B
Surface B
a. Regular views do not show true
features of surfaces A and B
Surface B
Partial
top
Surface A
view
Partial auxiliary view
Partial side view
b. Auxiliary view added to show true
features of surfaces A and B
D
Figure 23 The need for auxiliary views
Occasionally an object cannot be completely described in one auxiliary
view, so an additional auxiliary view may be needed.
iG_~·~{gj(©"'«IiXi:vBiiii,:0iMhLo;2i;;j1;"Y.t;'-ill?ii"~;'!ii!ii!m,\'4'!i!iitiA'%J-'"_9'S"!ii!'i'ii'!&'Si-"'-;;_M;.'i'\,!K"'EHXWii'ib'?i*fi+<!iF;e'$.q,'HF!iH8-,!U~8S;_8¥"H~!{!'F;F~g4!_8n8;S'ie;e'ie;;,;;_;;",,\{;'!,mtiitikillo0ioo;~~$B'0;14;H{i8Fi""',Btii.©%!
MILLWRIGHT-TECHNICAL DRAWINGS
3-17
Exploded views
An exploded view takes a very complicated drawing and separates it along a
common axis where possible. In an exploded view, the viewer sees exactly
how a group of parts fit together. Figure 24 shows a very simple pictorial
drawing in exploded view.
Figure 24 Exploded view
~'b'J_;,"!pw.m_j,'nc~n,,'iiN!!i,l1""K8~('-'i;,'I;_WXX?,$!,'i;;8.{_8!!."!r;g_$J",~&",~(M:;i(;r6i;,,,{%'%'i!!{%H_8;G_}1_W;8_}1_;,!;.:;;_t"~<1_:;;;e*:8lP;;8'8_n:G!:~!8;e:2!.~F2;,9!"';;'ih'''@~, ;'Q!E,,"Itii!ii'ii!iiN¥i~'ii!iic1j'Mi"k.jic'~"i%~M$%;?.,rlg~"';rt;-;'@i:G."')!h8:fiji:&gg
3 -18
MILLWRIGHT-TECHNICAL DRAWINGS
Tolerance
Tolerance in dimensions
Tolerance is the total pennissible variance of the basic size of a component.
Tolerance limits are the maximum and minimum sizes that are allowable.
The tolerance is the difference between these upper and lower limits. A
tolerance allowance is the intentional allowable difference in measurement
between correlating parts. Because they are critical to assembly, tolerances
are always shown on the detailed working drawing.
Caution!
Tolerances directly affect how a machine is constructed. These dimensions
are precise and must be strictly adhered to.
A term used widely in connection with tolerance is maximum material
condition (MMC):
•
In external measurements, the MMC measurement is the highest limit.
•
In internal measurements, the MMC measurement is the lowest limit.
Three methods are used to show the tolerance of mating parts: unilateral,
bilateral, or direct. These are described below.
Unilateral tolerance
Unilateral tolerance is tolerance in one direction only. For example:
•
For an external measurement, if the upper limit is 50.00 mm and the
lower limit is 49.95 mm, then the unilateral tolerance is 0.05 mm and is
written as 50.00::2.05ffiffi.
•
For internal measurements, if the upper limit 40.03 mm and lower limit
40.00 mm, the unilateral tolerance 0.03 mm and is written as
40 . OO f{)·03
--{) mm.
Bilateral tolerance
Bilateral tolerances are divided into two parts. The tolerance is read as plus
or rninus a certain number. Each bilateral tolerance alters the basic
dimension size in two directions. For example, If the basic size of an
external contour is 51.00 ±0.05 mm, this means that the final measurement
can range from 50.95 to 51.05 mm, a difference of 0.10 mm.
Direct method
The direct method shows both the upper and lower limit together. This
method eliminates any calculations concerning maximum and minimum
tolerances. Both the upper and lower limits are given, and the MMC is given
the upper position.
3
Figure 25 shows the correct use of MMC numbers in the direct method.
--31.56
•
31.46
I
l
(
•
•
18.40
0 18.39
.......~.....
ff::.
~
Figure 25 Direct method-exlernal contours
- 7~(\), yet\(~
Using a reference point in dimensioning
When severalloleranees are given in a sequence, a datum or reference point
should be given. All dimensions are then referenced to this point. The datum
reduces the probability of compounding error when sizing the work piece.
The datum is identified with an ISO datum-feature symbol as shown
attached to the left-hand extension line in Figure 26.
:3El,l..,.
36.0
i
126.1
126.0
142.1 ... 1
142.0
Figure 26 Comparing sequential and datum-line dimensioning
Clearance and inteiference fits
Tolerances may be either clearance (positive), or interference (negative) fit.
A clearance fit is one in which the machined pieces can be fitted by hand.
An interference fit requires heat, cold, or a combination of both to assemble
the piece.
Characteristic
Symbol
1- 1005 1
R
Straightness
Symbol
-E----
Max
tolerance
1C7 1005 1
Flatness
Explanation
Example
1
Symbol /
i
~~~;
l_~_~-v(
(-'---_
Max
tolerance
~
70tLIAI0.OSI
r
1# I A10.05 1
/
\
Symbol
Max
tolerance
Datum
Perpendicularity
(Squarness)
40°
Max
tolerance
Datum
Parallelism
?
/
\
Max
r tolerance
Datum
I/IAI" 0.051
Runout
syLI/
~x
tolerance
Datum B Datum A
I
)
Basic
c:b
F
~D
1..-1 Alo.05 I
en
YeaSie
_1
~DatumA
gi
r EJ
~D
--
r
I...LI A1°.05 1
Symbol
I
0.05 j
C7 lo.0s l
ILIA I0.0\1
Angularity
-il
-C-
I
/ \
Symbol
y
r
/
r
(
cb
/
-
1.iIAlo.osl
I
Lf 1/IAI·lo.osl ~j
t---r-
f
0.05
I-
~~
__v____
}--ij~I
--
-U-
Figure 27 Geometric characteristics
3
Tolerance in geometric characteristics
Tolerances are also applied to the different geometric characteristics of parts.
These include:
•
straightness
•
flatness
•
angularity
•
parallelism
•
perpendicularity (squareness)
•
runout.
Figure 27 on the previous page shows these geometric characteristics.
Tolerance in surface texture
There are two major reasons for the need of surface texturing: friction
reduction; and to control wear. The surface irregularities must be fine
enough so as not to break the lubricating fluids film which would result in
metal-to-metal contact. Journal bearings. cylinder walls. and piston pins are
but a few examples of surfaced textured pieces.
Two primary measurements are used for surface texture:
•
micrometre (~)-a micrometre is one millionth of a metre
(0.000 001 metres)
•
micro-inch (~)-a micro-inch is one millionth of an inch
(0.000001 inches)
Figure 28 shows the basic surface texture symbol.
Roughness average values
~
Machining
allowance ~
F
r-D-;::~- Roughness
A
D •
sampling length
....- - - - Lay symbol
Figure 28 Basic surface texture symbol
The number at position A appears in tables of acceptable surface roughness
(see Table 1).
n~%'3J:'ro%:,zi~1iij~f",OOoi1'"'B"i"i!Wia0;;'_8;8:'F8";W86S;&~~<ng%'2,.FoJ1'CooOOoo~?o51t"'&'''.Y'~1g<~:8iK%ti;;;-jl1::~~K~~~O;~'\S6"S"0h't;\Jl!,i1;'W-P<!~;;:""'_;HiiR"':8l::8l:'P""}"_GG~!;-'@W_~~Z8,@).,'Q})lJ~DI'8_11~Z8~8_11.11~ZG~;W@{!~"ES~d1f±j,*,,4:
3 - 22
MILLWRIGHT-TECHNICAL DRAWINGS
~
1;!!!.
"C
~
"C
"2-
r
~
"C
"C
~
iii·
Ie;)
0
9:
co =>
co
s· s·
co
::r
::j"
co
:IJ
s· Q.
(ii"
~
=>
~
~
C"
C
:;
:IJ OJ
0
~
s·
co
'3" agJ
_. ::r
=>
_.
=>
co
~
en
~
s·
co
co
"T1
'c"
0
co
s·
co
=>
'"
~
'coiil"
'" '"o z '"
'" iil
~
~
~
Rough surface.
Rarely used.
Coarse finish. Equivalent to
rolled surfaces & forgings.
L-
0
0
0
",z
'" ~
~
~z
0
0
0
(/)
a
ac
",CD
(/)
(/)
c.
0>
'"
Used on shafts & bearings
with light loads & mod. speeds.
,....
Used on highspeed
shafts & bearings.
0>.."
-'3"
5·
!='z
~ 0>0>
L
~
0>
Used on precision gauge
& instrument work. Costly.
f- IL...
0>
i
~
?
~
i
a
s·
0
::r
'"
~
~
~
~
~
-"
~
~
~
w
m
m
..
n
~
~
m
~
~
3
cr
0
~
~
i•
••q
••Ib
•I
•iB
iI
I
I
~
I••
{
I
.. •
:::l
~
0
~
~
~
N
•M
•D
S
b
.... '"
!='Z
c.
3
IA
a
M
~
=>
'" .... 5·
~'"
n
m
~
,
~
8
n
I
(il
f
z ~
~'" 2.
9
:::l
3
!'l.
(/)
I\J
ill
s·
::-
!='Z
.... !='Z
Super-finish. Costly.
Seldom used.
~
;-'"Z
(/)
f-
I
:::!.
,,0>
Good for close fits. Unsuitable
for fast rotating members.
'"
z
'" <no
'" ~ ''""
~
Refined finish.
Costly.
s·
s·
co
!"z co
'"'"
::r
=>
'
"'z "
co
'"'"
iil
Medium finish. Commonly
used. Reasonable appearance.
I•
~
(/)
I-
r
(/)
(/)
0
0
0
Very rough surface.
Eq uivalent to sand casting.
•*•
:IJ
0
c
co
::r
=>
iii"
3
;3=.
=>
iii·
::r
o·
••&
TI
~
j
;
I
~
~
•
h
I
Information on drawings
In addition to the actual technical drawings. vatious other information
appears on the prints. Some of this information appears in special blocks.
Examples are the title block. the revision block. and the materials list block.
Figure 29 shows where these vatious blocks of infonnation might appear.
1
8
0
0
Trimmed size
Borderline
C
~~
~~I
c
I
Space to the right
of this line not to be
used for drawing
I
B
B
Alternative location
of revision list
A
I
Reference
Revision
list
A
THle block
8
1
Figure 29 Location of Information blocks and zone numbers on a drawing
Zone numbers
If zone numbers appear. they refer to certain areas on the print in much the
same fashion as coordinate numbers on a map. Figure 29 shows these zone
identifiers as numbers and letters in the margins of the print. They sturt on
the lower right-hand side of the print under the title block.
Title block
Figure 30 shows a typical title block. The title block should always appear in
the lower right-hand side of the print. A title block usually contains such
information as the drawing number. title or description of the part. the name
of the flIm that prepared the drawing. and the scale. Provisions may also be
made fur the date of issue. signatures. approvals. professional seals. sheet
number. drawing size. job order or contract number. reference numbers for
this or other drawings. and standard notes such as tolerances or finishes.
Original scale
,..
/
/
General specifications and
standard printed notes such
as tolerances, finishes, etc.
Title or name of part
/
/
I
/
/
!
;
/
Firm's name
and address
/
/
DRAWING NO
SHEET
OF
Signature and approvals
Figure 30 Typical title block
Figures 31, 32, and 33 show typical title blocks.
NO.
Figure 31 Another typical title block
UNIT
DR.
. ,.
I
REVISION
I
DATE
I
CHANGE OR ADDITION
NAME OF PIECE
SYMSOL OF MACflINP WORKlitl ON
eUPERCEO&e owo.
srOCK
eAUTING
eM.
Tft.
TR.CH.
DROP FORGING
,aUPiRtlED£tI BY DWO,
MATERIAL
PIECE NO.
THE RIGHT MACHINE TOOL CO.
TRAIL, BRITISH COLUMBIA 1 CANADA
Figure 32 Typical strip title block extending across one side of a drawing
3-25
-
THE ACME COMPANY
-
NORTH VANCOUVER, BRITISH COLUMBIA
-
r-r--
r-r--
SCALE
r--
DATE
DR. BY
~
~w
~
NO.
CHKD.DV
00.
M~
CHANGE
CH.BY
ITR. BY
IAPP. BY
Figure 33 Typical title block with a change-record section
Revision block
This block may be placed either in the lower left- or upper right-hand sides
of the print The purpose of this block is to list any and all revisions made to
the drawing after the initial drafting of the print. In addition to a brief
description of drawing changes, provisions may be made for recording a
revision symbol, zone location, issue number, date, and approval signatures
for the revision. Figure 34 shows examples of completed revision blocks .
.&. Identification of drawing revision
0.05 x 45"
II
-+- f)
Chamfers
<-----------' I
..
.&.
------r
5.50
REVISION
Rev.
Description
Date
Approved
1
Length was 2.60
Jan. 9/96
J.English
2
Chamfer added
Feb. 24/96
G.Burns
Changes made by
Date
Revision number
~I / I
1
Jan. 9/96
/
/
J.English
2
I
Feb. 24/96
REV
Lengthlas 2.60
Added chamfer
/
Description
Vertical revision table
Horizontal revision table
Figure 34 Completed revision blocks
3
26
Materials list
This block is generally located just above the title block. If there is no
revision block in the upper right-hand, then the materials list is placed there.
The materials list may also be called the item list. Figure 29 shows an item
list just above the title block.
All parts in a materials list are identified by their part or stock number. The
materials list also provides for the number and size of each of part including
all fasteners such as bolts, washers, and nut~. This is a complete list of parts
for that print or page.
Scale
The scale is very important, although it does not occupy its own block. The
scale makes it possible to describe details of large and small machines or
components on standard sized paper. The size and complexity of the
machine determine which scale is used. The fIrst fIgure of a scale
designation refers to the dimensions used to draw the object. The second
number refers to the actual size of the object. For example:
o
A scale which reads 1: 1 means that a drawing is actual size.
o
A reduced scale of I: 10 means that I unit on the print represents 10 of
the same measuring units in the drawn equipment. That is, the drawing is
ten times smaller than the drawn object.
•
An eniarged scale of 10: I means that 10 measuring units on the print
equals 1 unit of the object. That is, the drawing is ten times larger than
the actual size.
Reading industrial
drawings
Detail drawings
A detail drawing is a technical representation of one single part of a
machine. The pUlpose is to provide the complete information needed to
make the part or piece. These drawings serve as a guide for the tradesperson
who converts raw materials into fmished products.
A detail drawing sbould contain:
•
a description of the shape
•
all necessary views and lines needed to describe the complete form of
the object
•
all dimensions and numerals including tolerances used to specify the
object
•
general notes, inclnding such things as the materials list, any heat
treating, machining, and surface texture.
---+-,
,
/ .... -1- , "/
/
I
/-1, "- \
'
I
,,-,..,"
II
~'\
\
f-t\ ' ~=-o:-'" 1-\I
\
... /
'-,.......
,~
-T-
/
\ "- "f-/
I
,
/
"- '-1- .... /
" -r
' - ....
--==--=i""'i,F'
I Ii,
I
g
<i
_J
1
1
1
1
1
1
1
1
QTY
Locking pin
Nut-hex slotted
Cotter pin
Bushina
Clevis Din
:suppOrt
t-'ulley
HOOK
ITEM
STL
STL
STL
Bronze
STL
SAE 1020
STL
STL
MATL
DESCRIPTION
CRANE HOOK
Scale
0Figure 35 A typical assembly drawing
3-28
I
I
8
7
6
5
4
3
2
1
PT
NO
Assembly drawings
An assembly drawing shows the various pieces of a machine and the way
they fit together as a complete unit. This drawing is used to show the correct
working relationship of the mating pieces and their functions. It should give
a general idea of how the machine is supposed to operate. Assembly
drawings use sectional drawing more frequently than do working drawings
so as to better display the relation ships between mating parts.
The only dimensions given on this drawing are the overall dimensions and
the centrelines. These drawings need not be detailed because the precise
details are given in the working drawings.
Assembly drawings contain a bill of materials for the complete unit
including such things as nuts and bolts. Figure 35 is an excellent example of
an assembly drawing with the bill of materials.
Schematics, piping
drawings, and symbols
Schematics
The schematic diagram facilitates the tracing of hydraulic, pneumatic, or
electrical lines and the components of each. It shows the relationship
between the various parts of a system. It does not show the actual size,
shape, or location of the components or devices within the system. It shows
connections, functions, and flow.
Figure 36 on the next page shows all the devices and parts that make up one
hydraulic function. Note that no regard is given to spatial considerations.
Only the parts and their functions are important.
Figure 37 shows a pneumatic schematic and Figure 38 shows a typical
electrical schematic. Again notice the lack of information regarding shape,
size, or location of parts.
Bc+;'!_W~:8l:!8::;-~-0,,~wa;''Q1mm'0'l©&,,%?~XY:m1''*RVJ:WX;''S,,'''''''$.;';m,'''';H''Si;r~'q&~"n;K(G.18~W>}@'*:8S;8P':8l:;8-'O:;8';';:!W0_::;W,,';l';'%:Y'iX!i:'iX!iE8idb~S"Wjiji~;Bjhim11;$)f'lo~'%'m%B%IW~;&,P¥W_R:8l:;';-J~'M;hOO4"&"6?$~~:G
MILLWRIGHT-TECHNICAL DRAWINGS
3 - 29
Figure 36 A schematic diagram of a hydraulic system
Figure 37 A schematic diagram of a pneumatic system
Start
Stop
.....L...
M
M
M
OL
Figure 38 A schematic diagram 01 an electric circuit
Piping drawings
Piping drawings are a little different from schematics. The typical example
shown in Figure 39 displays all the important pieces in symbolic form like a
schematic of a hydraulic system. However, this diagram not only shows
functions, connections, and flow, it may also locate the pipe spatially.
Single-line and double-line pipe drawings
The single line pipe drawings shown in Figure 39 display the pipe in
isometric and orthographic projections. Notice how the spacing and location
of pipe gain importance.
Figure 40a shows a double-line drawing. It also shows the difference
between the older single-line drawings (Figure 40c) and the current singleline technique (Figure 40b).
Pipe line
a
-Tee
Elbow
40
--
~
a. Isometric
\
I
,~
_ ' - ' - Adjoining apparatus
(tank)
_Naive
/
A
--.I------l
L __ ~ _1-1_-,
80 _
1___ ___ _ I
c
i
35
if
Tee
A
Pipe line
,
L -_ _ _ _ _ _:- - - - -
-_-+,_--,
-------.,~
b. Orthographic
Adjoining apparatus
(tank)
Valve
f
40
A
'~
I
I
'a
,- - - - - - -
'",
Elbow
A
-,
1_ _ _ _ _ _ -
Figure 39 Piping drawing
3
." ..
,
,~~~
+- - -- --------Cross
~ Plug
a.
~
J_
~ Flanged
Tee
Lateral
Globe
valve
,
Cross
I
Plug
Gate
valve
/ i n lines for fittings
Lateral
Cap
.~ Thick lines for
pipe and flanges
L
Tee
Flanged
joint
Check
valve
45° elbow
Elbow (used only to indicate direction of pipe)
~
- '"1
I
~
Lateral
Cross
Elbow
rc.
"
-,] '-------.!.--1
t
I'
Elbow
L
"
45° elbow
Check
valve
joint
I
I
~
[~======~
I'
b.
-
l
,I
Cap
Globe
valve
Plug
Gate
valve
Tee
Flanged
joint
1-
Check
valve
45° elbow
Elbow
Figure 40 Single-line and double-line piping drawings
3
33
Symbols and abbreviations
Symbols are the shorthand signs used on drawings. These symbols tell the
tradesperson what to do, and where to go or not to go for information. These
symbols are for the most part international but some countries have different
symbols. Accredited organizations like the International Standards
Organization (ISO), Canadian Standards Association (CSA), and American
National Standards Institute (ANSI) publish tables of symbols for welding,
piping, surface texture, and electrical elements. For a listing of symbols,
refer to the appropriate published standards. For a selection of hydraulic and
pneumatic symbols please refer to Chapters 16: Hydraulic Systems and
Chapter 17: Pneumatic Systems.
!t.'Wih','ia.O'f.'!Nh\W/'i!f%.!r!"tr<f.\Wh'ii'i.,'i,K,W'A'i;,F<i;Wi0iWh'Wh1'i:'Kii;,yWii'!*iI!";.'t;\WW,Wio'i~iJn'~iMij"'~,,]ii-.0:~Bbi'j'j'!Fw.ti?ji,,,;-';}nWXilf;;!,');,');,'%",;mX"t,"gESiB~~:W~'8'B;Wi:;q~,,;e0Fii'.Hi_81ia'""%""%tij1iilj1M$BK"2i%,:n!;}~iT'i'.~iJ~..~B.KW
3 - 34
MILLWRIGHT-TECHNICAL DRAWINGS
Shop Practices
Measuring tools ............................... ............. ..................................
Steel rules .........................................................................................
Vernier calipers .................................................................................
Micrometer calipers ..........................................................................
4: 1
4: 1
4:3
4:7
Comparison measuring tools .......................................................... 4: 11
Dial indicators .................................................................................. 4: 11
Feeler gauges .................................................................................... 4:12
Spring calipers .................................................................................. 4: 13
Telescoping gauges ........................................................................... 4: 13
Layout tools .................................................................................... 4:14
Layoat dye ........................................................................................ 4:14
Scribers ............................................................................................. 4: 15
Straightedges .................................................................................... 4: 15
Spring dividers and trammels ........................................................... 4: 15
Hermaphrodite calipers .................................................................... 4: 16
Combination set ................................................................................ 4: 17
BaIl-peen hammers ........................................................................... 4:18
Punches ............................................................................................. 4:19
Hand tools ....................................................................................... 4:21
Honing stones ................................................................................... 4:21
Scrapers ............................................................................................ 4:22
Flies .................................................................................................. 4:24
Chisels .............................................................................................. 4:25
Hand hacksaws ................................................................................. 4:27
Power tools ..................................................................................... 4:30
Cutting tools ..................................................................................... 4:30
Drilling machines ............................................................................. 4: 38
Portable keyseat cutters .................................................................... 4:42
Powder-actuated tuols ....................................................................... 4:44
CHAPfER4
Shop Practices
In order to succe.~sfully maintain and repair equipment within a plant, the
tradesperson must be able to understand the correct application of shop tools,
This chapter describes the proper use and care of some commonly used
measuring, layout. hand, and power tools. Whenever tools are used. pay
attention to safety requirements-refer to Chapter 1.' Safety.
Measuring tools
To maintain control in the manufacture. assembly. and setup of equipment. it
is vitai to use measuring tools correctly and within their accuracy limitations,
A
Caution! Abuse of measuring tools soon makes them useless.
To maintain accuracy and reliability of measuring tools. they must be
handled with care. kept clean, and used only for their specific purposes, It is
recommended to apply a thin coating of light oil on any moving part.
For accurate measurements, the measuring instrument must be exactly in line
with the axis of the measurement. Figure I on the next page shows some
examples of correct and incorrect methods of measuring.
The accuracy limitation of a measuring tool is determined by its smailest
scale graduation. A description of the measuring tools commonly found in a
shop environment follows.
Steel rules
A steel rule is a precision measuring tool. It is made of hardened and
tempered spring steel. Steel rules are made from flexible. semi-flexible or
rigid steel. graduated in metric or iroperial, or both. They are available in a
variety of lengths; 150 mm (6"), 300 mm (12").450 mm (18"), 600 mm
(24"), ete. The 6" rigid steel rule is most commonly used in this industry.
Metric and imperial graduations
The conversion from inch to mm and vice versa is done often. Therefore, it
is useful to know that 1" = 25.4 mm and I mm = 0.03937".
4
1
Incorrect
Correct
Figure 1 Measuring techniques
•
Metric steel rules are graduated in millimetres and half-millimetres.
Figure 2 shows the end of a typical 150 mm steel rule. The accuracy
limitation of this partieular rule is to 0.5 mm.
Figure 2 Metric steel rule
•
Imperial steel rules are graduated in fractions or decimals of an inch.
Fractional graduations are most commonly used in the millwright trade.
111ese steel rules bave a different graduation on eacb of their four edges,
Ya", J{6", Y3z", and Yo. ". Figure 3 is an end view of the Y32" and Yo."
graduations on a 6" steel rule. Notice that every Ya" is numbered for
easy reading. On the Ya" and J{6" side of the scale, only the incb lines
are numbered. The accuracy limitations of this particular rule is %.".
'rrr
T
I 7 r l 'I'I'!'I'I'I'I'I'I'I'I'"
I'a;Oa:9LI1'L9·Im;nOir!U
9
,
841hs
Figure 3 Imperial staal rule
Vernier calipers
A vermer caliper is a precision measuring instrument which can be used to
measure a wide range of sizes rapidly. It consists of a main frame (beam)
and a moveable jaw.
Sizes and scales
Vernier calipers are available in various sizes. The 150 mm (6") or 200 mm
(8") vernier calipers are often supplied by the worker.
A vernier caliper has a main scale on its beam and a vernier scale on the
moveable jaw. The beam may be graduated in the following ways:
•
on both of its sides (reading outside dimensions on the front and inside
dimensions on the baek)
•
on both its edges (metric on one edge and imperial on the other).
The latter of the two is the type most commonly found in this trade. Figure 4
shows an example of a common vernier caliper with both metric and
imperial graduations on either edge of the beam.
Inside measurement
t=F
. . ~~.~~t,.. ~·T~~==~~~I
Depth measurement
Outside measurement
Figure 4 Common vernier caliper
Functions and designs
Vernier calipers are versatile measuring instruments. They may be used to
measure inside and outside dimensions and depths. They may also be used as
a layout instrument. Figure 5 shows a few examples of the functions of
vernier caliper.
Figure 5 Various functions of a vernier caliper
Vernier calipers are also available with dials or digital display for easier
reading. These vernier calipers are more fragile then the standard vernier
calipers.
Metric vernier scale
The beam of a metric vernier caliper is graduated in I mm increments. Every
10th increment (I cm) is numbered for easy reading. The vernier scale is
used to divide a single increment into 50 divisions (Yso mm = 0.02 mm).
This is done by putting 50 increments on the vernier scale so that they
occupy the space of 49 increments on the beam (see Figure 6).
o
o
3
2
5
4
III
II
1
2
3
4
5
6
7
8
9
Movable jaw
Figure 6
4 4
Yso mm vernier scale
10
Beam
In this type, each vernier division equals "%0 mm, which equals 0.98 mm.
The difference between 1 increment on the beam and 1 increment on the
vernier scale is equal to 1 - 0.98 = 0.02 mm. As the moveable jaw slides
along the beam, only on of the graduations on the vernier scale lines up
exactly to a graduation on the beam. This allows the vernier to be read with
an accuracy of 0.02 rom. Some metric vernier calipers have a vernier scale
with 20 increments, allowing them to be read with an accuracy of only Yzo of
a millimetre (0.05 mm).
Figure 7 shows an example of a vernier measurement. It is read as follows:
A. Read the whole number (3 cm = 30 rom).
B. Read the number of graduations the moveable jaw has passed (2 rom).
C. Read along the vernier scale to find the increment which lines up to a
graduation On the beam (0.66 rom).
D. Add these figures together to achieve the total reading:
30.00 + 2.00 + 0.66 = 32.66 rom
4
o
1
5
2
3
7
6
4
5
8
6
Figure 7 Metric vernier reading
Imperial vernier scale
An imperial vernier caliper has its beam graduated in 25 or 50 onethousandths of an inch (0.025" or 0.050") and has every inch and tenth of an
inch numbered for easy reading. Both vernier scales are designed to read to
an accuracy of 0.001".
•
When the beam is graduated by 0.025", the vernier scale on the
moveable jaw has 25 increments. This divides 0.025" into 25 equal parts.
(At first the manufacturers made the 25 increments occupy 24
increments on the beam. These were difficult to read, so they began
making the 25 increments occupy 49 increments on the beam.) Figure 8
shows a 25-division vernier scale. These are commonly used on 6" and
8" vernier ealipers.
-5
•
1
,
When the beam is graduated by 0.050", the vernier scale has 50
increments which occupy the space of 49 increments on the beam, (see
Figure 9). This divides 0.050" into 50 equal parts. These are easier to
read and are often used on the larger vernier calipers due to the
additional available space on the moveable jaw.
Figure 8 and 9 show readings on the two different scales. They are read in
similar ways.
2
1
10
15
20
25
Figure 8 Imperial vernier reading with a 25-division vernier scale
In Figure 8:
I. Note reading at point A (1 ").
2. Note reading at point B (0.3 ").
3. Note reading at point C (0.003")
4. Add them to get accurate total reading = 1.303"
__
--------------------------------------~c~----~
Figure 9 Imperial vernier reading with a 50-division vernier scale
In Figure 9:
1.
Note reading at point A (0").
2. Note reading at point B (0.9").
3. Note reading at point C (0.042")
4. Add them to get accurate total reading = 0.942"
6
Micrometer calipers
A micrometer caliper is a precision measuring instrument. It is commonly
called a micrometer or mic. The same method of calibration is used for a
wide variety of measuring instruments, such as inside, depth, thread and Vanvil micrometers. The outside micrometer caliper is the only type discussed
in this chapter because it is the most commonly used in this industry. They
may be calibrated for metric Or imperial measurements.
They are also available with a digital display for easier reading. The digital
micrometers are used in a controlled environment due to their more fragile
components and cost.
The principle of the micrometer measurement is based on the pitch and lead
of a thread. At a specific pitch of a thread, !be lead is a set distance, (see
Chapter 5: Fasteners and Threads). Figure 10 shows the various parts of an
outside micrometer caliper and following is a description of how they
function together.
Spindle
I
/ ..·--Frame
Figure 10 Parts of an outside micrometer
•
The spindle has a thread ground on its periphery and acts as a bolt.
•
The thimble has graduations around it and is attached to spindle.
•
The sleeve has an internal thread which acts as a nut in which the spindle
and thimble assembly fit.
•
The sleeve also has a line along its axis which acts as a datum.
•
The anvil is connected to the sleeve by means of the frame. It acts as a
fixed position from which the spinrI1e moves. In one rotation of the
thimble, the spindle moves an exact distance to or from the anvil. This
distance is calibrated by the graduations on the thimble to the datum line
on the sleeve.
•
The amount of torque applied to the thimble can be controlled by means
of a ratchet or afriction thimble. These devices allow the user to
malotain consistent readings, regardless of feel.
•
A lock nut around the spindle or lever on the frame allows the spindle to
be locked to prevent movement after a measurement is taken.
4 7
Metric micrometer scale
Outside metric micrometers are available in various 25-mm ranges:
0-25 mm, 25-50 mm, 50-75 mm, and so on.
•
The spindle thread has a pitch of 0.5 mm which equates to a lead of
0.5 mm. Therefore, in one rotation of the spindle, it moves 0.5 rnm.
•
The sleeve of the micrometer is graduated in 0.5 mm increments. For
easy reading, the graduations are divided above and below the datum
line:
- The top graduations start at 0 and mark off every millimetre, with
every fifth millimetre numbered.
The lower graduations begin at 0.5 mm with one-millimetre
increments and none of these numbered, see Figure 10.
•
The thimble of the micrometer is divided into 50 increments around its
periphery. Therefore, every increment represents Koo th of a millimetre
(0.01 mm). Every fifth increment on the thimble is numbered for easy
reading.
On a 0-25 mm micrometer, the 0 position of the datom line on the sleeve
should line up to the 0 mark on the thimble when the spindle face meets the
anvil. As the thimble is rotated once, the lower mark on the sleeve is exposed
when the 0 mark on the thimble is in line with the datom line. One more
revolution exposes the upper line on the sleeve when the 0 mark on the
thimble is in line with the datum line, and so on.
Figure 11 and 12 show some examples of metric readings on a 0-25 mm
micrometer. When using micrometers of a different range, the numbers on
the sleeve reflect the actual dimension. For example a 25-50 mm micrometer
begins with 25.
In Figure 11:
Upper divisions = 12
Lower divisions = not applicable
Thimble reading'" 0.32
Total reading
= 12.32 mm
In Figure 12:
Upper divisions
15
Lower divisions = 0.50
Thimble reading = 0.20
Total reading
= 15.70 mm
35
30
o
5
10
35
Sleeve -J,LMLMWAA-
30
Thimble
25
Reading 12.32 mm
Figure 11 Metric micrometer reading of 12.32
I_
_I
15mm
25
-YmIMYH,w,YI--20
15
o
5
10
15
25
Sleeve -"rWiWiHfMlIHI:-20 Thimble
15
Reading 15.70 mm
Figure 12 Metric micrometer reading of 15.70
When more accurate dimensions are needed, a micrometer is supplied with a
vernier scale around its sleeve. This vernier scale divides each 0.01 mm into
five equal increments which provide a reading accuracy of 0.002 mm.
Imperial micrometer scale
Outside imperial micrometers are available in increments of one inch, 0--1",
1-2",2-3" and so on. On all the various ranges of outside micrometers, the
sleeve scale begins with zero. The minimum size of the micrometer must be
added to the reading to reflect the actual dimension of the part. For example,
when using a 2-3" micrometer, 2" is added to the reading on the micrometer.
•
The spindle thread has 40 threads per inch (tpi) which equates to a lead
of 0.025". In one rotation of the spindle, it moves 0.025".
•
The sleeve of the micrometer is graduated in 0.025" increments, starting
at zero. Below the datum line, the sleeve is marked every 0.025". At
each 0.050" the marks extend above the line, with every 0.100"
increment numbered.
4
9
•
The thimble of the micrometer is divided into 25 increments around its
periphery, Therefore, every increment represent~ 0.001". Every
increment on the thimble is numbered and every fifth one is highlighted
for ellBY reading.
Figure 13 and 14 show some examples of imperial readings on a 0-1 "
micrometer.
Sleeve
.
..
o 1 . , 20
j! "
II'I::! " 15
"
"," 10
,
Thimble
"·11
Reading 0.241"
Figure 13 Imperial micrometer reading of 0.241" using a 0-1" mlc
Minimum mie range
=0"
Divisions shown
= 0.225"
Thimble reading
= 0.016"
= 0.241"
Total reading
15
10
"
20
__ 18-
2 -"
"
Sleeve
i i
.. 15 Thimble
'1'1'1 ""
" 10
Reading 2.288"
Figure 14 Imperial micrometer reading of 2.288" using a 2-3" mle
Minimum mic range
Divisions shown
=0.275"
Thimble reading
=0.013"
Total reading
= 2.288"
When more precise dimensions are needed, a rnicrometer has a vernier seale
around its sleeve. This vernier scale divides each 0.001" into 10 equal
increments which provides reading accuracy of 0.0001".
Comparison
measuring tools
Comparison measaring tools are used to measure or gauge variations or
clearances. Some tools of this type used in a shop are dial indicators and
feeler gauges. Comparison tools have no capacity within themselves to show
a measurement. They are used to compare a size with a known
measurements achieved from measuring tools. A few commonly used
comparison tools are spring calipers and telescoping gauges.
Dial indicators
)
Dial indicators are used to measure movements such as the run out of a shaft
or to compare parts against each another. They use a series of gears to
accurately convert linear movement of the plunger or arm to rotary
movement of the indicator needle. They are available in a variety of different
styles, sizes and calibrations. The styles most often found in the millwright
trade are the long-range, continuous-reading dial indicator and the backplunger dial indicator (see Figure 15).
a. Continuous reading
b. Back plunger
Figure 15 Dial indicators
4
11
•
The continuous-reading dial indicator is often used to measure run-out of
shafts and other rotary parts.
•
The back-plunger dial indicator is used to align and set up parts of
equipment. See also Chapter 23: Alignment.
When using a dial indicator, take care to ensure that the axis of the indicator
spindle is in line with the axis of measurement, see Figure 16.
Correct
\,........----..
Axis of measurement
"
"
Figure 16 Using a dial Indicator
Feeler gauges
A set of feeler gauges are a col1eetion of metal leaves of various thicknesses.
Feeler gauges have each leaf is marked with a size, see Figure 17. However,
if a number of leaves are stacked together, the thickness should be checked
with a micrometer.
o
Figure 17 Set of feeler gauges
Feeler gauges are used to determine the amount of clearance between parts.
For example, in a bearing, feeler gauges are used to check the clearance
between the rolling element and the bearing race.
12
Spring calipers
Spring calipers are used to duplicate the size of shafts and bores. Shaft sizes
are detennined from outside spring caliper and bore sizes are determined
from inside spring calipers (see Figure 18). They require a carefulJeel and
technique in order to reproduce sizes accurately.
•
When using an outside spring caliper:
The smallest size in which the caliper can pass over the shaft without
resistance on its legs represents the size of the shaft.
•
When using an inside spring caliper:
The largest size in which the caliper can rock within the bore without
resistance on its legs represents the size of the bore.
Inside calipers
Outside calipers
Figure 18 Outside and inside spring calipers
Telescoping gauges
Telescoping gauges also duplicate sizes of bores (as do inside spring
calipers). They consist of one or two telescoping plungers, a handle and a
lockiug screw. Available sets can gauge sizes of 8-150 mm (716-6").
The proper method of using a telescoping gauge is shown in Figure 19 on the
next page.
4
1
To use a telescope gauge properly:
1. Insert the gauge into the bore.
2. Move the handle slightly off the axis of the bore.
3. Release the lock nut to allow the plungers to contact the bore.
4. Snug the lock nut.
5. Arc the handle through the axis of the bore (only once).
6. Remove the gauge and measure with a measuring tool.
t
Figure 19 Proper use of telescoping gauge
Layout tools
Layout is the transfer of information and dimensions from a working
drawings to the surface of a workpiece. Layout tools are used to prepare the
surface, and to scribe and mark lines, arcs, circles, angles and points.
Layout tools are precision instruments and need to be handled and
maintained with care. Damaged or abused layout tools result in the
inaccurate transfer of information to the material. This causes the workpiece
to be inaccurately produced or placed.
Layout dye
Layout dye or bluing is a blue dye which can be brushed Or sprayed onto a
surface to enable the layout lines to be seen more easily. Before applying the
layout dye the material must be clean and free from grease or oil. When it is
applied to a clean, dry surface. it will not rub off during the manufacturing
operations. To remove the layout dye, use a rag to rub the surface with
denatured alcohol.
4
When laying out infonnation on porous parts such as cast iron or if layout
dye is not readily available. products such as low-quality. white latex paint.
lime. or chalk are very successful.
Scribers
A scriber is a pencil-like tool used to mark layout lines on the surface of the
workpiece. They are available with a single or double end (see Figure 20).
The double ended scribers have one end bent for easier access to the line to
be scribed.
Their tips or points are made from materials such as hardened steel and
tungsten carbide. It is important to keep the point of a scriber sharp. by
grinding and honing it.
Wben scribing a line, take particular care to ensure the point of the scriber is
as close to the straightedge as possible.
Figure 20 Seribers
Straightedges
A straightedge is used to layout straight lines, and to check surfaces for
flatness and alignment. Its edges are accurately ground straight and have one
edge bevelled and the othec square (see Figure 21). Take great care to avoid
creating any nicks on its edges. Steel rules can also be used as straightedges
as long as they have not been damaged.
Square edge
Bevel edge
Figure 21 Straightedge
Spring dividers and trammels
A spring divider looks much like an inside spring caliper with its legs
sharpened to a point (see Figure 22a). They are used to scribe arcs and
circles and to transfer measurements. They are also useful in dividing lines,
arcs and circles into equal distances. Dividers have a limited range
depending on their size.
4
15
A trammel consists of a beam and two sliding heads or points. Trammels are
used in the same manner as dividers except the beam allows the points to be
spread a larger distance. The beam may be of various lengths to
accommodate the necessary operation. Figure 22b shows the trammel with a
fine adjustment screw.
Sliding heads
Beam
Fine adjustment
a. Spring divider
b. Trammel set
Figure 22 Spring divider and trammel set
For accurate layout ensure that the points are kept sharp and that are
protected when the instruments are stored away.
Hermaphrodite calipers
A hermaphrodite caliper consists of
one leg with a point and the other
bent inwards, see Figure 23.
These calipers are used to layout lines
parallel to an edge and to find the
centre of round or irregular shaped
objects. By passing the bent leg past
the point, they can also be used to
scribe a line parallel to a flange.
Figure 23 Uses of a hermaphrodite caliper
Combination set
The combination set is a useful and versatile layout tool. It consist~ of four
main parts: steel rule, square head, protractor head and centre head (see
Figure 24). The stccl rule is similar to a rigid 300 mm (12") steel rule except
for a shallow groove which lies along the centre of one side. This allows for
the attachment of various heads.
Protractor
head
Square head
Steel rule
Spirillevel -+-r-
Scriber
•
Figure 24 Combination set
Square head
The square head has one surface
which is square (90") to the
steel rule and the another
surface which is precisely at 45°
to the steel rule. These are used
to layout lines or measure parts
to these set angles. The square
head also has a spirit level
parallel to the square edge and a
scriber fitted into it.
........
The square head may be slid
along the stecl rule allowing it
to layout and measure surfaces
parallel to an edge (Figure 25)
or act as a depth gauge.
Figure 25 Square head
Protractor head
The protractor head is llsed to layout and measure angles. The accuracy of
the protrdctor scale is one degree (10). Figure 26 shows it being used to
seribe an angle on a plate.
Figure 26 Protractor head
Centre head
The centre head is used to find the cenlre of round stock such as ends of
shafting. Figure 27 shows the centre head being used 10 mark several lines to
detennine the exact centre of a shaft.
Figure 27 Centre head
Ball-peen hammers
A large number of different types and sizes of harruners are available. The
most common type of hammer llsed for layout and for general fitting is the
ball-peen hammer, see Figure 28. Ball-peen hammers are available in
various sizes dctcnnined hy the weights of their heads. The lighter weights
are used for layout and the heavier weights are used for general shop work.
Peen
Face
Figure 28 Ball-peen hammer
The round, ball-shaped end of the hammer is called the peen. The peen is
nsed to shape metal. The other end is called the face. The face has a slight
dome to it to allow a surface to he struck without leaving a half moon
imprint on the material. It is also used to strike objects such as punches to
drive pins into or out of parts and to centre punch parts which arc laid out.
A
Caution!
Take great care when using a hammer or any other striking tool.
Do the following for safe, successful use:
• Always wear safety glasses.
• Ensure the handle is in good condition (no cracks) and has a good nonslip grip (no grease).
• Ensure the head of the hammer is fastened securely to the handle.
• Do not strike the hammer against hardened suifaces.
Punches
Punches are available in various sizes and for different functions. The
punches used for layout are prick and centre punches.
Prick punch
The prick punch (see Figure 29) is used to accurately mark a line or the
centre of intersecting lines. It is ground with a 30· to 60· poinllo allow for
neat, accurate marking.
Figure 29 Prick punch
Centre punch
The centre punch (see Figure 30) is used to define centre marks for holes to
be drilled. It is ground with a 9° point which makes a wider indentation. This
allows the drill point to locate more easily and accurately.
Head
Figure 30 Centre punch
Sharpening punches
To be able to locate lines and centres
accurately, the points of prick and
centre punches must be kept sharp.
This is done by the use of a grinder.
Grinding
wheel
~: J
J
~ J J J J~~JJ ~8
~~
~
j
j
~ J
j
i
\
~
..
i
When grinding a punch, ensure that
the point does not overheat, (turn
colour) for this will remove the
temper of the steel and cause
premature dulling of the point.
When the points are sharpened, the
cut of the grinding wheel must be
along the axis of the punch.
..
\
I
J
\
i
i
\~ ..
i
Figure 31 shows the correct way to
sharpen the point of a prick and
centre punch. This grind pattern
allows the metal to flow up as it is
being punched.
F
J
\
i
:r
Figure 31 Grinding a punch
Removing mushroomed heads
The heads of all punches must also be maintained to prevent personal injury.
After repeated use, the heads of punches may become mushroomed (flared
outward). This mushroom must be ground off to prevent any particles from
dislodging and causing harm.
4
A
Caution!
Always use protective eye wear when using a grinder. Always grind off the
mushroom from a used punch.
Figure 32 shows a mushroomed head and a repaired head.
a. Mushroomed head
b. Correctly repaired head
Figure 32 RepairIng a mushroomed head
Hand tools
Many hand tools are used in the shop and in the field. The following are a
few of the common hand tools found in most shops.
Honing stones
Honing stones are used in a variety of applications from cylindrical honing
machines used to finish bores of cylinders to hand honing stones. They are
designed to remove small amounts of material from parts or tools. Honing
stones consist of an abrasive and a bond which hold the abrasives together.
The commonly used abrasives (and designation letter) used for hones are
aluminunJ oxide (A), silicon carbide (C) and diamond (D).
Hand honing stones may be disc shaped, cylindrical, square, rectangular,
triangular, etc. These various shapes allow the user to reach any area needed
to be honed.
Hand hones are used to remove burrs from areas such as gasket surfaces.
They are also invaluable for putting an edge on tools such as scrapers,
chisels, drills and any other cutting tools. The hone remOves any rough,
jagged edges left behind from the grinding process. If the jagged edges are
not removed, they will chip off after its first use, and the tool will become
dull prematurely. Figures 33a and 33b show magnified views of the edge of
a scraper after grinding and after honing.
Honing stones require a liquid to help lubricate and clean the honing surface.
They are manufactured to use either oil, solvent or water. Be aware of which
medium is needed for the particular stone.
a. Alter grinding
b, After honing
Figure 33 Edge of a scraper
Scrapers
A scraper is used to remove small amounts of material at each pass-less
than 0.02 mm (0,001 "), Scrapers may be made in a variety of different
shapes to do particular operations, The three most common scrapers are the
flat, triangular and bearing scraper. see Figure 34.
Figure 34 Scrapers
•
The flat scraper resembles a flat (mill) file with its teeth ground off, Its
end is ground square with a slight crown. It is used to remove high spots
on flat surfaces.
•
The triangular scraper has three sharp edges along its blade which taper
to a point. This scraper is used to remove sharp edges of a part. A threesquare (triangular)file may have its teeth ground off to make such a
scraper,
•
The bearing scraper is hollow ground with its edge curved upwards and
tapered to a point. This scraper is used primarily to scrape concave
surfaces such as bronze and babbitt bearings.
When scraping mating surfaces. a reference surface is used to determine
where the high spots are. A surfaee plate is used to help create a flat surface,
A mandrel (shafting the same size as the shaft to be used) is used to create a
bearing surface.
Scraping procedure
The procedure to scrape mating surfaces is as follows:
L
Apply a thin film of colour (such as Prussian bluing) to the reference
surface.
2. Rub the reference surface onto the work. The high spots are marked by
the transfer of bluing from the reference surface onto the work.
3. Scrape these blue marks off.
4. Repeat this procedure.
5. As the two surfaces begin to match the amount of blue spots increases.
When the work shows a uniform blue coverage, the scraping is
complete.
6. To record the quality of the scraping, use a thin sheet of paper, such as
rice paper. After the scraping is complete and before the bluing is wiped
off, place this paper onto the scraped surface and then place the mating
surface onto it This transfers the blue onto the paper to make a
fingerprint of the job. Keep this sheet of paper on file for reference.
Sharpening scrapers
Scrapers must be ground and honed to create a keen, smooth edge. The way
to achieve this is to allow the curvature of the grinding wheel to create a
slight hollow along the edge of the scraper. Then when the edge is honed, the
stone contacts only the cutting edge of the scraper. Figure 35 shows how a
triangular scraper is ground and honed.
# ' + - - Triangular scraper
Honing stone
Figure 35 Sharpening a triangular scraper
A
Caution! Ensure that protective eye wear is worn when grinding. Also, do
not overheat the scraper when sharpening with a grinder. Overheating
reduces the tempering of the tool, softening it.
4
23
Files
Files are used to smooth and shape parts by hand. They are made of a heattreated, high-carbon steel. To ensure an effective cutting surface, files are
made hard and, therefore, brittle.
Caution! Do NOT use a file to pry objects. This bending action wiU break
the file and may cause personal injury.
Files are available in various shapes, sizes and coarseness. Some common
shapes found in a shop are flat (mill), square, triangular (three-square), halfround, and round. The size of a file is determined by the length from the heel
to the point, see Figure 36. The common sires are 6", 8". and 10" files.
Their coarseness is express as single-cut, double-cut, bastard-cut, second-cut,
and smooth-cut.
Tang
Heel/
Edge
Face
Figure 36 Parts of a file
Files should be stored clean and in such a manner that prevents their cutting
surfaces from touching each other or other tools. Cardboard sleeves are a
successful way to achieve this.
Filing procedure
Filing is an important hand operation which can only be mastered with
patience and practice. When filing a part, do the following:
4
24
•
Have a balanced stance.
•
Ensure that the part is held securely.
•
Keep the file clean in order to produce a smooth surface.
•
Apply a light pressure on the forward stroke only. If heavy pressure is
applied, the teeth of the file will become clogged and scratch the surface
of the part. This is called pinning. Pinning can be reduced by applying
chalk to the surface of the file.
•
If pinning begins, remove the pins between the teeth before continuing.
Do this by rubbing a file card or a piece of brass, copper or wood,
through the teeth.
•
Use the full length of the file if possible.
Drawfiling
Use draw filing to produce a flat, smooth surface. To use this method, plaee
the file square to the work and to the direction of movement. Hold the file on
both ends with your index fingers applying pressure to the file directly above
the work. Draw the file toward you. TItis method allows the teeth of the file
to shear material smoothly from the work.
Chisels
Chisels are made of a hardened and tempered, high-carbon steel. There are
four common types; flat cold chisels. cape chisels, round nose chisels and
diamond point chisels. Other types of cold chisels are just a variations of
these. Power tool are replacing much of the need for cold chisels but not all
of the need.
Caution! Proper eye wear must be worn when using chisels. Work mates
near this operation must also wear eye protection or be protected by shields.
Flat cold chisels
The flat cold chisel is the most common of these chisels. It has a flat. wide
cutting face used to chip or cut metal. Some applications consist of cutting
thin plate. shearing off rivet or bolt heads and removing weld spotS. It is
invaluable when dismantling corroded and seized components such as
brackets from equipment.
Its cutting edge is ground in a similar manner to the centre punch. axially
along the chisel. The included angle of the cutting edge is 70°. see Figure 37.
Head
C_____
~
--=-==_~;;;:::::::::---_~~Cuttingedge
Figure 37 Flat cold chisel
Cape chisels
The cape chisel (see Figure 38) has a narrower cutting edge then the flat cold
chisel. It is used to cut narrow, square-bottomed grooves such as keyways. It
is sharpened in the same manner as the flat cold chisel.
(}>----~c
Figure 38 Cape chisel
4
25
Round nose chisels
Round nose chisels (see Figure 39) have a rounded cutting edge. Their
overall shape is similar to that of the cape chisel. It is used to cut round
bottom grooves such as oil grooves into bearings.
Figure 39 Round nose chisel
Diamond chisels
The diamond point chisel (see Figure 40) has a diamond-shaped cutting
edge. It is used to finish off square comers and create square- or diamondshaped grooves.
Figure 40 Diamond point chisel
Maintaining chisels
To ensure the safe and efficient use of chisels, they must be well maintained.
The following are the most important maintenance procedures:
4
26
•
Keep the cutting edge of the chisel sharp. A dull chisel can deflect away
from the workpiece instead of biting in. This can cause the chisel to fly
out of the hand of the worker and injure self andior a fellow worker.
•
Constant hammering on the head of a chisel causes it to flare outwards
(mushroom). Figure 32 in the punch section shows the effects of a
mushroomed head. Grind off this mushroom as soon as it develops.
•
Keep the chisels clean. Grease, oil and other debris can cause the chisel
to slip out of the hand. It can also cause the hammer to deflect off the
head of the chisel. Both sitoations may cause serious injury.
Hand hacksaws
Hand hacksaws are used primarily to cut metal. They consist of a frame. a
handle. a blade. and a blade tensioning device. see Figure 41. The handle of
the hacksaw is meant to protect the hand from injury if the hacksaw slips or
the blade breaks.
!
Frame
Blade
~
•
Figure 41 Hand hacksaw
Their frames are either fixed or adjustable:
•
Fixed frames accommodate one length of blade only. They are generally
more rigid then the adjustable type.
•
Adjustable frames accommodate a variety of blade lengths.
Most frames are designed to have more then one orientation of the blade.
The most common orientations are: in line with the frame. at 45° and at 90°
to the frame. The positions other than in line with the frame are used when
making long. narrow cuts.
Hand hacksaw blades
Hand hacksaw blades are commonly available in lengths of 250 mm (10")
and 300 mm (12") and types such as all-hard (rigid) and flexible.
•
All-hard blades are made of fully hardened. high-speed steel. These
blades are used to cut tough materials such as alloy and tool steels.
These blades are brittle and break easily.
•
Flexible blades are either carbon or bi-metal. The carbon steel blades are
an economy blade used to cut mild steel. copper. brass and aluminum.
The bi-metal blades are used to cut materials ranging from tough to soft.
They consist of a row of high-speed-steel teeth welded to a flexible back
(see Figure 42 on the next page). These blades flex without breaking.
4
27
Flexible steel back
Figure 42 Bi-melal hacksaw blade
Tooth set
The teeth of the hacksaw blades have a set to them to create a keifwider then
the blade (see Figure 43). This prevents the blade from binding in the cut.
-~Kerf
Hacksaw blade
Work piece
Figure 43 Sel of a hacksaw blade
When the teeth become dull, the set wears down causing the kerf to reduce
in width. This may cause the blade 10 bind, heat up and even break midway
through a cut. When this happens the blade must be replaced. The new blade
must not be placed in the original cut because the narrower kerf will cause
the teeth to wear prematurely. Start a new cut in a new location, either on the
other side of the workpiece or in the waste section beside the existing cut.
~rJ;;;;m:®'!lm1%%
4 - 28
;;;;;;;;;s.;~-*s;eBQi?!ii;':;!Nmi,W'\Wi!fi!iii"''ieR,'N%~isi:s!i$~~*tr;j;';@!jj",jtNi;C;.c;.,JW3%'1!3WJR#';a;~!8·"'K-{jG'-:"4;¥'H8_P;,,"!i'-'kg<j1'ii:8"HMj:ijt~G"A%iWi,,+'i&1!tii'i;';~~%%
MILLWRIGHT-8HOP PRACTICES
Coarseness and tooth pitch
The coarseness of a blade is designated by the number of teeth per inch (tpi)
it has. Hacksaw blades may have 14, 18,24, or 32 tpi (see Figure 44).
The puch of the teeth can be determined from the tpi by dividing the (pi into
one (1). For example:
•
14 tpi = 1 + 14 =0.071" pitch
•
32 (pi = 1 + 32 = 0.031" pitch
Pitch~
:.0.071
:
'
I
i
I... -------:-c-1" -;---c:---- ----1
14 teeth per inch
Figure 44 Hacksaw blade coarseness and tooth pitch
Choosing a blade
The pitch of the teeth is used when selecting the correct blade for the job.
When cutting thick materials a cOUrse blade should be used to allow for
plenty of chip clearance. When cutting thin material a fine pitch blade should
be used to prevent the teeth from taking too heavy a chip load, causing tooth
breakage. A rule of thumb for selecting the pitch of a blade is that at least 2
teeth must be in contact with the material at all times (see Figure 45).
Figure 45 Choosing the correct hacksaw blade
4-29
Using a hand hacksaw
To use a hand hacksaw effectively, these steps must be followed:
Prepare the saw and workpiece:
I.
Choose the correct blade for the job.
2. Ensure that the blade is mounted with its teeth pointing away from the
handle.
3. Adjust the tension so that the blade cannot flex or bend.
4. Secure the workpiece and, if necessary, support it. (When cutting
materials with a thin cross-section such as thin wall tubing or thin sheet
steel, the material should be supported to resist tearing. Tubing may be
supported with a piece of dowelling the size of the inside diameter. Sheet
steel can be supported between two pieces of wood.)
Make the cut:
1. Grip the hacksaw handle firmly, and be sure-footed and comfortable.
2.
Position the blade on the workpiece on the preferred side of the layout
mark. (A file may be used to make a V -shaped nick at the mark to guide
the blade.)
3. Begin to cut with sufficient down-pressure, to keep the teeth cutting, on
the forward stroke only (apply no cutting pressure on the return stroke).
4.
Make long, steady strokes using the full length of the blade, to maximize
the life of the blade.
5. Maintain a cutting speed of 40 to 60 strokes per minute.
6. When the cut is completed, store the hacksaw in such a manner so that
the teeth are protected from damage.
Power tools
Power tools are used in every shop environment. They assist in the fitting
and refitting of equipment. This section covers the basic practices and rules
related to the operation of some of the specific cutting and power tools used
by a millwright.
A
Safety is a key issue when using power tools, see CHAPTER I: SAFETY.
Cutting tools
A cutting tool is often used with a power tool to do work. Cutting tools are
made of several different types of materials such as carbon steel, high-speed
steel (HSS), cobalt HSS, carbide, ceramic and diamond. Cobalt HSS is a
higher grade of HSS which is being used more often throughout industry.
4
30
Cutting tools most commonly used in a maintenance shop are twist drills,
reamers and end mills. Twist drills are often sharpened in the shop, but
reamers and end mills are sent out for sharpenlng or replaced when dull.
Twist drills
A twist drill is an end cutting tool designed to produce a hole. Two types of
twist drills found in the shop environment are carbide and HSS:
AILA
C'I ,'ne{t; Ie,
"l~ {l-r."r (o(
l.}t. ( [Cs u w I.;,/,tJ e5
•
Carbide twist drills are made of an alloy steel body with cemented
carbide cutting edges. They are often used for drilling concrete.
•
HSS and cobalt HSS twist drills have their body made of solid HSS and
their shanks made of a softer material. HSS twist drills are used for
general work throughout the shop.
- Twist drills used in drill chucks have a straight shank and are kept in a
case called a drill index.
- Those used in a stationary drill press often have a tapered shank
which fils directly into the spindle. These shanks are called Morse
tapers and their sizes range from a number 0 to 7 (0 is the smallest).
The body of a standard twist drill consists of a point. helical flutes and lands.
The point consists of two cutting edges. heels. margins, lip clearances and a
chisel edge (see Figure 46).
lip clearance angle
Neck
r--
~
\ i..-.
Helix angle
\
'" .l~
/1"••
~i~~:terIt-r--·. . . . .-. . . . . _-.--AxiS-- -hI~:--~~
""""1
I
~
•
c
•_
•
_
-------~
.......-
Flutes
Land
.......--BOdY
Lip clearance
Land
I~~
lip clearance angle
phisal edge
\
'\'6el
."
twist
"c
.,:
Over-all length
Figure 46 Details of a
I
drill
Point angle and lip clearance
The recommended point angle of a drill varies depending on the material to
be drilled (see Figure 47).
•
For general purpose drilling of materials such as mild steel and others of
a similar hardness, a 1180 drill point angle is recommended.
•
For hard materials such as heat-treated alloy steel, use a point angle up
to a 150°.
•
For soft materials such as aluminum or for thin materials, use a 90° drill
point angle.
General
Hard
Soft
Figure 47 Recommended point angles
The lip clearance behind the (.'Utting edge should range from 80 to 12°.
When the correct lip clearance is achieved, the chisel edge angle (the angle
between the chisel edge and the cutting edge) will be about 135°.
Sharpening a twist drill
Sharpening a twist drill by hand is a skill which can only be mastered
through practice. A few guide lines to be aware of when sharpening a twist
drill for general purpose work are as follows:
1, Begin by holding the drill horizontally with the drill's axis slightly above
the axis of the grinder (see Figure 48a).
2. Align the plane of the drill's cutting edge parallel to the axis of the
grinder and at 59° from the face of the grinding wheel (see Figure 48b).
3. Touch the wheel with the drill and sweep it upwards along tlle wheel (do
not rotate the drill).
4.
Repeat a few times then switch to the other cutting edge and repeat
procedure.
5.
When both cutting edges are ground the same amount, check angle and
length of cutting edge with a drill pnint gauge (see Figure 48c).
6. When the length of both cutting edges are the same and the drill point
angle and the lip clearances are correct tile drill should be ready to use.
32
a. Side view
b. Planvl_
c.
Figure 48 Drill sharpening guide lines
Brassing off the drill
Flattened
cutting edge
When using a twist drill to cut brass.
bronze. plastic. and other materials
which are soft and relatively brittle.
the cutting edge must be flattened
(see Figure 49). Flattening the cutting
edge prevents the drill from biting
into the material too rapidly and
overloading the drill. This is often
called brassing off the drill.
Figure 49 Flattening the oulling
edge (brassing off the drill)
Solid parallel reamers
Parallel reamers are designed to create an accurately sized hole. They are
available as machine and hand tools.
•
Machine reamers are designed to be used in equipment such as a drill
press. engine lathe or milling machine. The shank is either straight, for
chucking, or has a Morse taper. The cutting end has a 45° lead chamfer
ground from its actual diameter. This lead chamfer helps it to enter the
hole and does all the cutting.
•
Hand reamers have shanks with a square driving end to be used with the
aid of the tap wrench. Its cutting end has bevel and a starting taper
ground from it actual diameter. The starting taper helps the worker align
the reamer and cut along the axis of the hole.
The helix of a reamer may be straight, right or left handed. Figure 50 shows
a straight helix and Figure 51 shows a left hand helix.
Lead
. - - -.. chamfer
~r'
r-_.-: .M: :o: :rs:.:e:.;t: a~pe:.:r_ _ _r-_______-::;fiJ~;;;;;;;;;;;;;;;;;;;;~~··~i~·
Figure 50 Taper shank. straight flute machine reamer
Square
drive
Figure 51 Helical flute hand reamer
Drilling and reaming
Reamers follow the location of the drilled hole. Ensure that the drilled hole is
located accurately before reaming. Holes to be reamed must be drilled
undersize to allow the reamer to cut it to size. The amount of material left for
reaming depend on the type of reamer to be used. Table I shows the
recommended amount of material to be left for reaming.
A
Caution! 0.005" is the maximum amount of material to be left in a hole for
hand reaming. If any more material is left in, the hole requires too much
force for it to be removed.
Table 1: Recommended amounts of material to be left for reaming
Type of reamer
below 13 mm (0.5")
13 - 25 mm (0.5 - 1 ")
above 25 mm (1 ")
Machine reamer
0.1-0.3mm
0.1 -0.5 mm
0.5-1.0 mm
0.005 - 0.010"
0.005 - 0.015"
0.015 - 0.030"
0.02 - 0.06 mm
0.02-0.1 mm
0.02-0.1 mm
0.001 - 0.003"
0.001 - 0.005"
0.001 - 0.005"
Hand reamer
A
Caution!
Never rotate the reamer backwards because it will dull the reamer.
Adjustable reamers
Adjustable reamers are primarily made as hand reamers. They consist of a
set of blades set into grooves with a tapered base. Nuts on either end of the
blades adjust the blades along the body. This allows the reamer's size to be
changed to the desired amount. Each reamer has a range within which it can
be adjusted.
Adjustable reamers are very useful when holes must be enlarged to exact
dimensions. These reamers are for finishing operations. Therefore, the
amount of material removed should not exceed 0.1 mm (0.005").
Tapered reamers
Tapered reamers are used to create tapered holes
•
Large diameter tapers such as Morse tapers are cut with machine
tapered reamers.
•
Smaller tapers such as for taper pins are cut with hand tapered
reamers.
35
When preparing holes for reaming, step-drill them to remove excess
material. Ensure that enough material is left in the hole, at the bottom of the
steps, for the reamer to clean up.
End mills
End mills are used to cut narrow slots or grooves. They cut on their
periphery and end. They have multiple flutes which can be straight or helical
(left or right). The most common are the two- and four-flute end mills.
Two-Ilute (slot drill)
•
Two-flute end mills are generally made with one end cutting edge
slightly longer then the other, see Figure 52a). This allows it to centrecut and plunge into the workpiece like a drill. These end mills are often
called slot drills.
•
Four-flute end mills normally have a centre hole drilled in the cutting
end of the tool, see Figure 52b). This allows for easier sharpening but
will not allow it to plunger into the workpiece. This type of end mill
must enter the workpiece from the end.
•
Centre cutting four-flute end mills have the strength of a four-flute cutter
with the end cutting capabilities of a slot drill, see Figure 52c).
Regular lour-II ute
Centre cutting lour-II ute end mill
Centre drill hole
a.
b.
c.
Figure 52 End mills
Various methods are used to hold and drive end mills in the spindle of the
machine. The most common type used in equipment such as the portable
keyseat cutter is the Weldon™.1t has a milled flat area on its shank on
which a setscrew is placed to hold and drive the cutter.
4
36
Cutting speeds
In a cutting operation, either the cutter or the cut object may be moving. The
cutting speed (CS) is the speed with which the saw teeth and cut object move
past each other. This is also sometimes called the surface speed of the
operation. The control of the cutting speed is vital to the life of the cutter.
The es is expressed in metres per minute (mlmin) or feet per minute (ftl
min). Each material has a recommended es for each type of cutter.
Table 2 gives the recommended CS for materials cut with HSS cutters. The
range of cutting speeds in the table is meant to be used as guide or reference
point. Note that:
•
The CSfor carbide cutters is approximately 2.5 times faster than for
HSS cutters.
•
The CS for machine reaming is half that of HSS.
Table 2: Cutting speeds for HSS cutters
Material
CSlnmlmln
CS in ftlmin
Plain carbon steel
24-36
15 24
15 -24
18-30
85-110
30-43
80 -120
50-80
50-80
60 -100
280 360
100 -140
Alloy steel
Stainless steel
Cast iron
Aluminum
Brasslbronze
Using correct rpm and coolant
To ensure maximum life of the cutter, use adequate eoolant and the correct
rpm. Failure to supply adequate coolant and the correct speed results in
premature tool wear, breakage andlor metallurgical damage to the
workpiece.
The coolant helps lubricate the tool as it maintains a cool temperature of the
workpiece and the cutting edge. Chapter 6: Lubrication explains the
different types of coolant available for cutting tools.
The rpm at which the power tool must rotate for a particular cutter diameter
is determined from the es. Note that the actual rpms may differ somewhat,
depending on the speed selections on the power tool.
•
The formula to convert the CS in mlmin to rpm is:
rpm =
cs X 1000
1txD
CSx320
D
where D = diameter in mm and "It = 3.142
Example 1:
Find the rpm for a 10 mm drill to cut brass :
1.
From Table 2, CS for brass is 30 mlmin.
2.
Using the formula:
rpm = 30 x 320 = 960
10
The drill should rotate at approximately 960 rpm.
•
The formula to convert the CS in ft/min to rpm is:
rpm =
CSxl2
1txD
CSx4
~
D
where 0 = diameter in inches and 1t = 3.142
Example 2:
Find the rpm for a half-inch end mill to cut alloy
steel:
1. From Table 2, CS for alloy steel is 50 ftJrnin.
2. Using the formula:
SOx4
rpm =--=400
0.5
The drill should rotate at approximately 400 rpm.
Drilling machines
Drilling machines are available in a variety of different styles (stationary or
portable). They all are designed to rotate a bit which is held in a chucking
device. They are powered by electricity or compressed air.
A
Caution!
Use safety eye wear while operating all drilling machines.
Stationary drill presses
Stationary drill presses are usually fixed to a bench or the floor and have a
mechanical device for feeding (pressing) the drill bit into the work. They are
powered by an electric motor mounted at the rear of the drilling head. The
power is transmitted to the drill chuck through beJt(s) and pulleys or by a set
of change gears which also allow for speed variations.
The drill press in Figure 53 shows a step pulley mounted on the motor shaft.
Another step pulley, the other way around, is mounted on the spindle.
4
36
Changing the position of the belt allows the speed of the spindle to change.
The spindle is not shown in Figure 53 because it is mounted inside the quill
and has the drill chuck attached to it.
Bell
Bell guard
On/off
SWiICh~~~~~~~~5~~
Slep pulley
Drilling head
Quill
Drill
Power supply
Column
Table
:::::;'S:;:-~~~~~I-- Table clamp
Figure 53 Parts of a bench drill press
A
•
The hand feed lever moves the quill in and out of the drilling head. A
depth stop is attached to the quill movement and allows the quill to stop
at any desired position.
•
The column is mounted to the base and supports the drilling head and the
table. The column of a floor-mounted drill press is longer to allow the
base to be mounted to the floor.
•
The table is able to slide up and down and around the column for ease of
positioning. The table clamp is used to lock the table in its desired
position. Slots are cut into the table and the base to allow the work to be
held down by the use of clamps and bolts.
Caution!
Ensure that work is securely mounted to the table or base before drilling.
-39
Portable drills
Portable drills are used throughout industry. They consist of a motor, pistol
grip handle and chuck. A set of reduction gears is placed between the motor
and the chuck to reduce the speed and increase the torque of the chuck.
These drills may be electric or pneumatic. Figure 54 shows an electric drill
with its parts labelled.
Motor
/
Chuck
Reduction gear set
Pistol grip handle
/
Power supply
~
Figure 54 Portable electric drill
The power supply for the electric drill is an electrical cord. A pneumatic drill
would have an air hose. The pneumatic drill also has a narrower molar
casing. For information on air preparation for pneumatic tools, see
Chapter 17: Pneumatic Systems.
Portable drills have drill chuck capacities of ~6'" %", y,", y." and Ys". The
most commOn sizes are %" and y,". They also have featnres such as
variable speed control for matching the correct rpm for the size of drill being
used. Many drills also have a switch to reverse their rotation.
Other variations of pnrtable drills are cordless, right-angle and hammer
drills.
•
Cordless drills have a rechargeable battery pack mounted to the base of
their handles. This drill allows the freedom to work in remote areas
without the need of extcnsion cords or air hoses.
•
Right-angle drills have their chuck rotating at right angles to the motor
axis. They are useful in drilling holes in tight places.
•
Hammer drills have the ability to impact the drill bit as it is rotating with
the option to switch the impacting off for regular drilling. This
hammering is valuable when it is necessary to drill into concrete. The
impacts crumble the concrete at the point of the drill making it easier to
drill. Hammer drills can have as many as 48 000 impacts per minute.
Portable magnetic drill presses
A portable magnetic drill press (see Figure 55) has the features of a
stationary drill press combined with the flexibility of a portable drill. It
consist of an electric drill mounted on a column or slide. The column or slide
has provisions to move the drill in and out by means of a hand feed lever. It
is also mounted squarely onto an electromagnetic base which allows it to
drill at right angles to the workpiece.
Note that electromagnets attach themselves only to materials which can be
magnetized, such as iron and steel. They do not attach to such things as
aluminum, many stainless steels, or concrete.
_Column
"
Electromagnetic base
Figure 55 Portable magnetiC drill press
The electromagnetic base has a power source and switch for turning it on or
off. Due to the vnInerability to a loss of power and the limited holding power
of the magnet itself, additional safety concerns must be met (see next page).
•
When the unit is about to drill a hole, the drill normally rotates in a
clockwise direction. This means that the forces on the unit are in a
counterclockwise direction. Stops must be placed against the side of the
base to prevent it from moving if it slips. These rotational stops must be
in plaee each time a hole is to be drilled.
•
When the unit is used in a position where gravity wants to move it out of
position, such as on a slope or overhead, chains must be used. The
ehains secure the unit to the workpiece to prevent it from falling in the
event of a power failure (see Figure 56).
A method of checking that the safety devices are correctly in place is to
imagine the power has been disconnected. Is the situation still safe?
\
Portable
magnetic
drill press
Figure 56 Securing a portable magnetic drill press with safety chain
Portable keyseat cutters
A portable keyseat cutter is a machine tool which cuts keyseats (also called
kcyways), slots and holes. It uses an end mill as the cutting tool. Some
examples of work it can do are cutting keyways into shafts and plate, slots
into tubing, pipe. sITUC.:turaI steels, disks and nul•. This cutter can be taken to
the equipment to do the work. The equipment needs only to be dismantled
enough to allow the unit to be mounted and able to do the appropriate work.
Figure 57 shows the parts of a typical portable keyseat cutter.
4
~A.ongitlldir,al feed lever
. / Depth adjustment
::::::::~=:;:::!!:7J--Tliiple gear reduction
Power head
Cutter speed control
Quill
Vea block
Power supply
Chain clamp
Figure 57 Portable keyseal cutter
•
Its base is a vee-block which can attach to a clamping bar or a clamping
chain. The vee-block also has a dovetail slide on its top surfaee to
accommodate a slide on which the power head, quill and controls are
mounted.
•
The quill has provisions to hold an end mill. It is driven by the power
head through a triple gear reduction set.
•
The longitudinal travel is controlled by a lead screw attached to the
longitudinal feed lever.
•
The depth is controlled by another lead screw with a graduated collar for
accurate depth setting.
•
They have a variable speed control deviee to allow the operator to
achieve the correct cutting speed for the type and size of the cutter.
Refer tu the operator's manual for additional features for each individual
tool.
4
Powder-actuated tools
Powder-actuated tools arc used to drive fastening devices into steel or
concrete. They use a powder cartridge which fires with the force needed to
drive the fastening device into the material.
Caution!
Powder-actuated tools are extremely dangerous if handled carelessly.
They ruust be used in accordance with the Workers' Compensation Act for
your region as well as with the manufacturer's instruction manual. All
workers using these tools must obtain a current Qualified Operator's
Certificate (QOC). A QOC can only be obtained after the successful
completion of a training program for the specific make and model of the tool
to be used, from a qualified instructor from the manufacturer or training
institution.
C")
:::::r'
I»
"CI
CD
-.
en
Fasteners and Threads
Purpose of threads .. ...... ...... ................ ............. .................... ...... ..... 5: 1
Screw thread tenns and systems ....... ....... ..................... ............. .....
Definitions ........................................................................................
Thread designation ...........................................................................
Thread series .....................................................................................
Variations in thread size and pitch ....................................................
5: 1
5: 1
5:5
5:6
5:8
Types of fasteners ........................................................................... 5: 10
Threaded fasteners ............................................................................ 5: 10
Classification of fasteners ................................................................. 5: 10
Washers and locking devices ............................................................ 5:16
Cutting threads ................................................................................ 5:20
Taps for internal threads ................................................................... 5:20
Dies for external threads ................................................................... 5:23
Installation, removal, and repair of threaded fasteners ................... 5:25
Thread repair .................................................................................... 5 :25
Broken stud removal ......................................................................... 5:27
Failures during installation ............................................................... 5:28
Preload in fasteners ........................................................................... 5:29
Torque wrenches ............................................................................... 5: 31
Torque values .................................................................................... 5:33
Fasteners and Threads
A fastener is a device used to fasten or connect two pieces of material to
form one simple unit. There are thousands of different fasteners, from the
familiar button and buttonhole, safety pin, and shoelace to wood screws,
nails, rivets, and highly specialized devices for custom machinery. The
fasteners that are most often found in the mechanical trades are nuts and
bolts, cap screws, and studs. This chapter deals with these types of fasteners
and with screw threads used for other purposes.
Purpose of threads
St:rew threads are made by cutting a single spiral groove around the outside
of a rod or around the inside of a cylindcr. Threads are designed for various
tasks:
•
to transmit power and increase force-as in a lead screw or automobile
jack (acme, square, buttress and worm threads)
•
to control movement--as in a jacking screw
•
to convey material-as in food grinder (cast spiral threads)
•
to hold parts together-with the use of bolts, studs, nuts and screws etc.
•
to form a pressure-tight joint (tapered pipe threads)
• for measuring-as in a micrometer (V-form threads).
Definitions
External threads
threads on the outside of a fastener or part
Internal threads
threads on the inside of a cylinder
Pitch
the distance from a point on a screw thread to a
corresponding point on the next thread, measured
parallel to the axis.
Major diameter
the largest diameter of a screw thread; this term
applies to both internal and external threads
Minor diameter
the smallest diameter of a screw thread; this term
also applies to both internal and external threads
Nominal diameter
this is the diameter by which the fastener is
named; it is not an exact measurement
Pitch diameter
(effective diameter)
the diameter of the surface of an imaginary
coaxial cylinder that passes through the thread
profile at such points as to make the width of the
thread equal to the width of the groove
Lead
the distance a screw thread advances axially in one
tnrn. On a single-start screw thread, the lead is the
same as the pitch. On a two-start screw thread, the
lead is twice the pitch.
Crest
the top surface joining adjacent sides or flanks of
the thread; this may be flat, rounded, or sharp
Root
the bottom surface joining adjacent sides or flanks
of the thread; this may be flat, rounded, or sharp
Side or flank
the surface of a thread connecting the crest to the
root
11_
~
.~~
MajOrdiam
Pitch diam
Minor diam
•
~I I
Flank
Single depth
I 60°
Shank - -..
Figure 1 Screw thread terms
Single depth of thread
the distanee from the crest to the root, measured
perpendicular to the axis
Thickness of thread
the distance between the adjacent sides of the
thread, measured along the pitch line
Fit
the relationship between two mating parts with
respect to the amount of clearance when they are
assembled, or with respect to the amount of
interference preventing assembly
Tolerance
the total permissible variation, represented by the
gi ven maximum and minimum sizes of the parts
Allowance
an internationally agreed difference in the
dimensions of mating parts
Thread angle
the angle that is formed by the sides or flanks of a
screw thread. Most fasteners have a 60° thread
angle.
Helix angle
the angle formed by the helix of slope of the
thread relative to the thread axis
Right-band and left-band tbreads
A screw thread is considered to be a right-hand thread if, when viewed from
either end of the bolt, the screw thread winds in a clockwise direction and
appears to he receding Or going away from you. Threaded fasteners with
right-hand threads are tightened when torned clockwise.
If a screw thread viewed from the end of the bolt winds in a
counterclockwise direction as it reeedes from you, that screw thread is a
left-hand thread. See Figure 2.
leU·hand thread
Right·hand thread
Figure 2 Left-hand and right-hand threads
Most screw threads are of the right-hand variety and are not labelled. Lefthand threads are often labelled LH. Both the threaded shaft and the nut must,
of course, be either left- or right-hand. For certain applications involving
counterclockwise rotation, a left-hand thread is needed for a secure lock.
Multiple threads
Most screw threads are single start. This means that the screw thread
consists of a single ridge and groove. A double start thread has two ridges
and grooves, starting from diametrically opposite points across the eross
section. A triple thread has three grooves, starting at three equally spaced
points around the circumference. The object of using mnltiple threads is to
obtain an increase in lead without weakening the shaft by an increase of
pitch and depth. Sce Figure 3.
Lead
Lead
Single thread
Double thread
Triple thread
cQ)
co
cO
Pitch
Figure 3 Multiple threads
Nominal size
Several measurements can be taken on any fastener, but the nominal size of
any threaded fastener refers to its major diameter and length as shown in
Figure 4. The length of the fastener is the length of the shank plus the length
of the threaded section including the chamfer.
Shank
r - -..'
I
1
Diam alar
J
·1
Nominal
length
Figure 4 Diameter and length
Chamfar
Load distribution per tbread
All threads in an assembly do not take equal amounts of loading. In threaded
fasteners, the first full thread of the nut next to the flat washer takes about
50 percent of the full load; the next thread about 25 percent; and the
remaining threads share the rest of the load. The percentages are
approximate, but generally accepted.
Thread designation
Thread assemblies are designated by a series of numbers and letters. Each set
describes a particular feature of the thread. For example, a fastener might be
It
II
designated as 3/4 - 10 UNe - 2A x 11/2 Ig. This means:
n
3/4
= nominal diameter
10
= number of threads per inch (!pi)
UNC
= thread style (in this case, Unified National Course)
2
= class of fit (I, 2,3,4,5 or 6-see below)
= external thread (B =the internal thread)
A
n
tI/2 Ig = length (dp = depth of full threads in a bole)
Note that:
•
Right-hand thread is always understood, unless left-hand is speeified in
the drawing
•
2A and 2B are understood as standard and may be left off some shop
drawings
Class of fit
Industrial standards indicate the fit between the mating threads. Fit is the
measure of looseness or tightness. Fit~ range from 1 to 6 and are defined as
follows (classes 4, 5, and 6 are rarely used in this trade):
Class 1 fit
maximum looseness in the mating threads;
examples of use are for stove bolts and rail track
bolts
Class 2fit
an average fit with very little play in the mating
threads; may be assembled by hand; examples of
use are for everyday nuts and bolts and cap screws
Class 3fit
no looseness in the mating threads; cannot be
assemhled by hand but can easily be assembled
with the eorreet wrenches; examples of use are for
bearing and shaft ring nut~
Class 4 fit
precision threads usually cut and ground; used for
precision tools and instruments; an example of use
is for a micrometer
Classes 5 & 6 fits
embellishments of class 4, giving the most precise
and accurate threads; used for extremely precise
tools and high-vibration conditions
Thread series
There is a great variety of thread forms worldwide. However, the millwright
uses only a small proportion of them in general maintenance work.
Use a thread gauge to ensure that the correct thread size is used. There is a
proportional relationship between all the dimensions of a thread pitch.
Technical manuals contain detailed calculations for these proportions.
American National Thread
There are four designations for American National Thread:
• National Coarse (NC)
• National Fine (NF)
• National Extra Fine (NEF)
• National Special (NS).
American National Thread has a 60° angle, with flats on the crest and root.
NC and NF threads are the most used.
Unified Screw Thread
There are four designations for Unified Screw Thread:
• Unified National Coarse (UNC)
• Unified National Fine (UNF)
• Unified National Extra Fine (UNEF)
• Unified National Special (UNS).
Unified Screw Thread has a 60° angle, with a rounded root and a rounded or
flat crest. UNC and UNF threads are the most used:
•
American National and the UN series are interchangeable in all sizes
•
NF and UNF have a different pitch in the I-inch diameter:
UNF = 12 !pi and NF = 14 tpi
Constant pitch series
The threads in this series have the same pitch for all diameters:
•
8 !pi sizes, I" to 6" inclusive, are used for high-pressure pipe flanges and
cylinder head studs. This series is a continuation of the course thread.
•
12 tpi sizes, 9/16" to 6" inclusive, are used for thin nuts on shafts and
sleeves and in boiler work. This series is a continuation of the fine thread
series.
•
16 tpi sizes 3/8" to 6" inclusive are used for fine adjusting nuts, bearing
retaining nuts and other applications requiring fine adjustment. This
series is a continuation of the extra fine thread series.
Acme Thread
This thread is not for a fastener. It is used mainly for an adjusting and
positioning screw on a machine such as a lathe. It: P, l> C; ( IZCIv .
This thread has flat crests and roots and a 29° included angle. The crest and
the root have slightly different widths, wI and w2 in Figure 5. Square
threads are difficult to produce
and tend to break off at the
corners. The 29° thread form
prevents binding while
increasing the applied foree.
Figure 5 Acme 29° screw threads
(,\I u.-l.,I:>Cl) ff>Jc, IE "
National Pipe Taper (NPT) Thread
This tapered thread seals the joint, preventing leakage. In standard pipe
threads, the flanks come in contact frrst. There can be spiral clearance around
the threads. In dry-seal pipe threads, the roots and crests engage frrst,
eliminating spiral clearance. These threads are called American dry-seal
threads (see Figure 6).
Spiral clearance
NPT
Dry seat
Figure 6 Pipe threads
Figure 7 shows that if the pipe bottoms out too soon (touches the end of the
internal threads) the threads may still fit loosely without a good seal.
/.kofl'('. ?l::VJf Sc-rrL
314" laper
per foot
Figure 7 Tapered pipe thread
ISO metric series
The International Organization for Standardization (ISO) sets this series as
the standard metric thread shape, pitch and sizes used throughout the world.
The ISO series has thread sizes ranging from 1.6 to 300 nun. It also has a 60°
included angle and flat crests and root~.
The ISO thread has a crest equal to 0.125 times the pitch. The main
differences are:
•
The depth of thread is less.
•
The root is 0.250 )( pitch.
The larger root diameter allows an increase in the tensile strength of the
fastener. Table 1 shows common combinations of pitch and diameter.
Table 1: Some ISO metric pitches and diameters
Nominal diam. Thread pitch Nominal diam. Thread pitch
(mm)
(mm)
(mm)
(mm)
6.0
8.0
10.0
12.0
14.0
16.0
1.00
1.25
1.50
1.75
2.00
2.00
64
72
80
90
100
6.0
6.0
6.0
6.0
6.0
Figure 8 shows ISO metric threads.
Figure 8 ISO metrie threads
In print, a metric fastener is described as in the following example:
M8 )( 1.25 )( 50 nun
where:
M
= the symbol for metric
8
= the nominal (major) diameter in nun
1.25 = the pitch in mm
50 rum = length offastener
Variations in thread size and pitch
As the diameter of the bolt, screw or nut decreases, the size and pitch of the
thread become less. For example, UNC threads on a 1/4" diameter bolt are
smaHer and closer together than UNC threads on a 1/2" bolt.
Table 2 shows standard imperial or inch sizes of bolts and nuts. The chart
lists threads per inch for various diameters of both UNC and UNF fasteners.
Table 2: Sizes for common nuts and bolts
..........-
Major Threads Thread
ciillrneter per inch series
1/4ft
5116"
3/a u
7/16"
1/2"
9h6"
5/a"
3/4 "
7/e"
1"
20
2B
lB
24
15
24
14
20
13
20
12
1B
11
1B
10
16
9
14
8
12
14
Major Threads Thraad
diameter per inch series
UNC
UNF
UNC
UNF
UNC
UNF
UNC
UNF
UNC
UNF
UNC
UNF
UNC
UNF
UNC
UNF
UNC
UNF
UNC
UNF
UNF
11/a"
11/4"
l1f2'
13/4"
2"
2114"
21f2'
23/4 "
3"
31f4"
3'/2"
33/4"
4"
7
12
7
12
6
12
5
12
4 /2
'
12
4 /2
'
4
4
4
4
4
4
4
UNC
UNF
UNC
UNF
UNC
UNF
UNC
UNF
UNC
UNF
UNC
UNC
UNC
UNC
UNC
UNC
UNC
UNC
Not all pipe threads have the same pitch. Larger pipes have threads of a
larger pitch as is shown in Table 3.
Table 3: Sizes for NPT threads
/\iominal Ilille size
tilL
OD to nearest '116"
1/ar!
27
1B
18
14
14
7/1611
114"
3/a
1/2."
l1
3/4 "
1"
1'/4"
11/2.
11
2'
2'/2"
11'/2
11'/2
11'/2
11'/2
B
911611
11116 11
13
11611
1'116"
15116
11
111/16"
11511611
23/8
27/8
11
11
Types of fasteners
Threaded fasteners are classified as either studs, screws or bolts. The
distinction is that a screw is loaded by a head, and a bolt is loaded or
tightened by a nut, but some fasteners can be used either way. Using a cap
screw as both a bolt and a cap screw will reduce the parts inventory.
Threaded fasteners
(r:j} . . . . . . . . . Nut
Bolts and nuts, cap screws and studs are
threaded fasteners with various functions.
•
A bolt is held and tightened with a
threaded nut. Bolts (with nut.) are
used to join two or more
components as shown in Figure 9.
Figure 9 Nut and bolt
• A cap screw is similar to a bolt
except that it is threaded into a
tapped (threaded) hole rather than
into a nul
Cap screw
Threaded ~
hole".
•
Figure 10 Cap screw
• A stud is a rod that is threaded on
both ends. Some have coarse threads
on both ends. Others, for soft metals
such as cast iron, aluminum or
brass, have coarse threads on one
end sO that it can be threaded into
the soft metal without stripping it.
The other end has fine threads to
achieve a good clamping force with
the nut. See Figure 11.
1--="",- Stud
Figure 11 Stud and nut
Classification of fasteners
Bolts, nuts and cap screws are classified in three ways-by their:
• tensile strength
• design (or shape)
• size and thread pitch
Tensile strength
Threaded fasteners can be made from a number of different materials such as
steel, stainless steel, brass, aluminum and even plastic. The most common
material used is steel. Different grades of steel are used for their different
tensile strengths. Tensile strength is the ability of a material to resist forces
that tend to pull it apart.
To identify the tensile strength of a fastener, grade markings are stamped on
their heads during manufacture. Table 4 shows the general specification of
fasteners. The head markings for both imperial and metric fasteners are
shown so you can compare them.
Table 4: ASTM, SAE and ISO specifications for steel bolts and screws
Head marking
Specification
Tensile strength
minMPa
min psi
ASTM-A307
SAE-Grade 2
450
65000
SAE-Grade 3
760
110000
830
120000
1040
150000
L9™
1170
180000
©
Socket head
Cap screws
1240
170000
0
0
ISO R898
Class 8.8
830
120000
ISO R898
Class 10.9
1040
150000
ISO 4762
Class 8.8
830
120000
ISO 4762
Class 12.9
1220
177 000
IMPERIAL
0
0
0
©
®
ASTM-A325
SAE-Grade 5
ASTM-A345
SAE-Grade 8
High strength
METRIC
©
©
• imperial size fasteners often use a number of slash marks on the head to
identify the grade. To determine the grade of a fastener, you can usually
count the number of slash marks and add two. For example, Figure 12
shows a grade 5 bolt.
Note that the identification rule
Is not consistent. For example,
an SAE grade 3 bolt or screw
3 slash marks ... 2 ; grade 5
head has two slashes rather
than one.
Figure 12 Identifying imperial size bolt grade
• Metric size bolts use a numbering system that identifies the class of the
bolt. Tensile strength increases as the number stamped on the bolt head
increases.
Design
Bolt design
A number of different head desigus are used in the manufacture of bolts. The
three main types are shown in Figure 13.
Carriage bolt
Hex-headed boll
Square-headed bolt
Flgure13 Bolt head designs
• Carriage bolts
are used in applications where a smooth, round
head is desired. The square shoulder is designed to
prevent the bolt from turning when the nut is
tightened. Because the head of a carriage bolt
cannot be turned once the bolt is installed, carriage
bolts are often used to provide added security.
• Hexagon-head bolts
are by far the most common. Hexagon heads
provide a new gripping surface for a straight
wrench each time the bolt is turned 60°. They are
much more widely used, because they can be
tightened more easily in cramped quarters.
• Square-headed bolts
are found on some industrial equipment They
must be turned 90° before the wrench can be
relocated on the head.
Cap screw design
The most common head designs for cap screws are hexagonal and socket
head. The nominal sizes of cap screws start at 1/4".
For installations where the cap screw head is to
be set below the surface, socket-head cap screws
are available (Figure 14).
Figure 14 Socket-head cap screw
Nut design
Nuts are available in a wide range of designs, and again, each design is
intended for a particular application. The following are some of the more
common designs in use today.
• Hexagonal (hex) nuts (Figure 15)
are general purpose nuts used in
almost any location where a
strong nut is required and there is
limited access.
Figure 15 Hex nut
• Square nuts (Figure 16) are most
often used in locations where
access is unrestricted. Square nuts
offer a larger flat surface for the
wrench jaws to grip; therefore,
they are able to withstand greater
torque (twisting force) than
hexagon nuts.
Figure 16 Square nut
• Slotted hex nuts (Figure 17) are
similar to a regular hex nut except
that slots are cut into the top to
receive a cotter pin. The cotter pin
is inserted through the slots of the
nut and a hole in the bolt.
Figure 17 Slotted hex nut
• Castellated nuts (Figure 18) are
locking nuts that also use a cotter
pin to prevent the nut from
turning.
Figure 18 Castellated nut
• Stover™ nuts (Figure 19) are selflocking nuts that have their top
inner surfaces bent inward. When
the top part of the nut is tightened
onto a bolt, this oval part is forced
back to round, causing resistance
to turning.
Figure 19 Stover™ nut
• Nylon lock nuts are another type of
self-locking nut as shown in
Figure 20. When the nylon lock
nut is tightened on a bolt, the nylon
is forced to stretch, causing
resistance to turning.
Figure 20 Nylon lock nut
• Jam nuts (Figure 21) are thinner
than regular hex nuts and are used
as a locking device. When used to
lock a regular hex nut, the jam nut
should be between the hex nut and
the joint surface so that the hex
nut takes the full bolt load.
Figure 21 Jam nut
• PallUltsTM, illustrated in Figure 22
are an even thinner version of the
jam nut.
Figure 22 Painut™
Size and thread pitclI
The size of a nut or of any internal thread is identified by the major thread
diameter and thread pitch of the bolt that would fit the nut. The thread pitch
of any bolt or nut can be determined by using a thread-pitch gauge as shown
in Figure 23.
Figure 23 Thread-pitch gauge (imperial)
Machine screws
Machine screws are a special group of screws 3/8" or less in diameter; they
have their own sizing system. The term "screw" is correct as they are
generally used without nuts. Nuts are available, however, for all sizes of
machine screws. Unlike bolts, when nuts are used with a machine screw, the
name "machine screw" still applies.
i
Charts show machine screws designated by numbers from 0 to 12. Above
this, they are designated by fractions. These numbers indicate the diameter
of a machine screw, for example, a #6 has a diameter of 0.138 inch.
'
To find the diameter of a machine screw in inches:
• multiply the screw number by 0.013
• add 0.060
Example 1
Find the major diameter of a #6 machine screw.
6 x 0.013 = 0.D78
0.078 + 0.060 = 0.138
Major diameter = 0.138"
Example 2 shows how the letters aud numbers are used to specify a machine
screw by its threads per inch, length, and head-type:
Example 2
What is a "#8 - 32 x 112 PAN HEAD" screw?
The letters aud numbers indicate a machine screw
with a #8 diameter of 0.164" (calculated). It has a
pauhead, 32 threads per inch, aud a length of 112"
from the underside of the head to the end.
There are several variations of machine screws:
•
The self-threading or selftapping machine screw is
commonly used in industry. The
self-tapping screw is harder thau
a regular machine screw and has
a specially formed tip that
resembles a threading tap. The
thread has the same appearauce
as the thread of a machine screw
See Figure 24.
Figure 24 Self-tapping machine screw tips
The self-tapping machine screw is installed into a drilled hole slightly
smaller thau the major diameter of the screw. When you engage the selftapping machine screw into a drilled hole, the machine screw cuts threads
into the sides of the hole.
•
Sheet metal screws are auother
variation of the self-tapping
screw. These self-drilling, selftapping screws are generally used
on thin material such as sheet
metal (Figure 25). They are also
available in several head styles.
Figure 25 Sheet metal screws
Washers and locking devices
Washers
•
Flat or plain washers are steel
disks with a hole through their
centres (Figure 26). They are used
under bolt heads and under nuts to
increase the bearing su!face of the
fastener. Plain washers also
protect surfaces from being
damaged by bolt heads or nuts.
Figure 26 Flat steel washer
Plain washers are identified by the closest size of bolt that fits the hole in
the washer. The inside diameter of a washer is about 1/32" larger than the
bolt size. For general use, washers are made from mild steel.
•
Split-ring lock washers are used to keep nuts and bolts from becoming
loose as a result of machine vibration. Sizes of lock washers correspond
to the bolt that fits them. See Figure 27.
•
Hi-collar lock washers are like split washers but thicker and stronger.
Split ring
Hi-collar
Loose
Tight
Figure 27 Split-ring lock washer and hi-collar lock washer
•
Tooth-lock washers are used when extra holding power is required. The
teeth are angled to allow the fastener to be tightened. Each tooth grips the
su!faces to prevent the fastener from loosening (Figure 28), They are also
available in a cone shape to fit countersunk heads (Figure 29).
Figure 28 How a tooth·lock washer grips
A
Do not use lockwashers on flat washers
External
Internal
External/internal
Cone washer
Figure 29 Tooth-lock washers
Pins
Pins are used to lock two or more
parts together. Sometimes pins are
used to accurately position one part to
another.
•
Cotter pins (Figure 30) are inserted
through the slots of the nut and a hole
in the bolt. The prongs are then bent
back to keep the pin in place.
j-",.,",
Figure 30 Cotter pin
Figure 31 shows the correct way
to bend the prongs so that the
cotter pin secures the nut without
leaving jagged ends protruding. If
the prongs are too long. cut them
back with side cutters.
Slotted nut
Figure 31 Bending colter pin prongs
•
Headed pins (or clevis pins) are most often used to attach a part to a
U-shaped yoke known as a clevis. Figure 32 shows a clevis pin used to
attach a part to a turnbuckle.
Clevis pin
Figure 32 Use of a clevis pin in a turnbuckle
•
Taper pins are used to fasten machinery
parts that must fit precisely (Figure 33).
The taperratio is 1:48 (1/4" per foot)
The pin is driven into a matching
tapered hole.
Figure 33 Taper pin
•
Dowel pins are used to aid in aligning two machinery parts. They are
straight round stock with a slight chamfer at each end (Figure 34).
The chamfer serves to get the pin
started. Some dowel pins are hardened
and ground for strength and precision.
Some have an exposed end threaded or
tapped for easy removal.
Chamfer
j
(0
Figure 34 Dowel pin
An example of dowel pins
use is shown in Figure 35.
The dowels are fitted
tightly into matching holes
on both machine parts. This
lines the parts up for
accurate assembly.
Figure 35 Using a dowel pin to line up parts
•
Shear pins are used to connect a
gear or other part to a shaft. They
are designed to tolerate oaly the
normal load imposed on
machinery parts. Under greater
than normal loads they shear,
stopping the part being driven.
This prevents more serious
damage to the rest of the machine.
They are made of a softer material
or have a groo ve where they are
designed to shear.
Figure 36 Shear pin
•
Spring pins and roll (coiled) pins are hollow cylinders of spring steel.
The spring pin is split lengthwise (Figure 37a) and bevelled at each end.
The roll pin curl~ around itself as shown in Figure 37b. These pins are
made slightly oversize so that when driven or pressed into place, they
compress. This causes outward pressure which holds the parts in place.
I+--(Cm
(a)
_----+(()
(b)
Figure 37 Spring pin and roll pin
•
Grooved pins are solid pins with a slot along their length. On some pins
the slot or groove runs the full length of the pin, but is only partly
through the pin. On others the slot may be shorter than full length but
may go right through the pin. Grooved pins have tremendous holding
power because they expand when driven into place. Grooved pins are
designated by type, nominal diameter and length. and material. Their
types are identified by letters as shown in Figure 38.
Q
F-~-
D
Type A
TypeC
TypeE
Type D
Type F
_3
3
f
TypeB
Figure 38 Types of grooved pins
•
Convenience pins are quick and easy to install and remove. They are
designed as retaining pins rather than as pins to withstand great loads.
Two common types of convenience pins are spring-locking pins and
quick-lock pins.
-
Spring-locking pins are often
called hairpin cotters (Figure 39).
They are used instead of a cotter
pin to hold other pins in place
when they must be installed and
removed frequently.
Figure 39 Spring locking pin
The straight leg of the pin is
inserted through the hole in the
end of the clevis pin. The bent
leg is forced up and over the
side of the clevis pin.
-
Quick-lock pins are used to
secure removable attachments
to a machine (Figure 40). They
have a split spring-steel ring
mounted at the head end
Figure 40 A quick-lock pin
Once the pin is installed, the ring is rotated down over the end of the
shaft to prevent the pin from coming off the shaft. Sometimes a quicklock pin has a short length of chain to attach the ring to the machine.
This prevents the pin from being lost.
Cutting threads
Taps for internal threads
The tool used to cut internal
threads is a tap. The process is
called tapping threads. The three
types of taps shown in Figure 41
are for different stages of a cut.
Taper
QQ~:~~§§!EEE3~
Plug
QQ~:§§§!~~
Bottoming Qb)~:
§~~~
Figure 41 Tap types
•
Taper taps have a taper, or gradual narrowing of the shaft, that extends
for approximately six thread pitches. The threads on a taper tap are
shallow at the narrow end ofthe tap, and gradually increase in size to full
thread depth at the top. A taper tap is used to start the thread-cutting
process because this long taper permits gradual removal of the metal as
the tap is turned into a pre-drilled hole. If the hole to be threaded is
drilled completely through the material, the entire screw thread may be
cut using a taper tap.
•
Plug taps have a taper only half the length of the taper on a taper tap
(three thread pitches). They are used to cut threads in blind holes (holes
that are not completely through the material). The plug tap is not
designed, however, to cut full threads right to the bottom of a hole. For
this reason, it has to be used in conjunction with a bottoming tap.
•
The bottoming tap has only a very short taper Gust one thread pitch).
When the plug tap has been inserted as far as it can go and then removed,
the bottoming tap is used to cut the threads down to the required depth.
Tap wrenches
A tap wrench is used to hold the tap
securely so that forces are applied
evenly to the tap. It also makes it easier
to keep the tap in line with the hole.
Tilting the tap during a thread-cutting
operation usually results in the tap
breaking. Figure 42 shows a T-type tap
wrench. Once the tap is inserted into
the wrench, the chuck is turned to
tighten its jaws onto the tap.
Larger taps require more force to turn
them during thread-cutting. Therefore,
a longer tap wrench (see Figure 43) is
required. You can adjust the jaws of the
wrench to fit the square end of the tap
by turning one end of the handle.
Figure 42 T-type tap wrench
G
Figure 43 Tap wrench for large taps
Tapping threads
The procedures for tappiug threads are quite straightforward. The main thiug
is to work carefully aud avoid forciug the tap uuevenly. Taps are very brittle
aud cau break.
Determine and drill the size of hole required
1. Detenniue the thread size and pitch.
2a. Detenniue the diameter of the hole to be drilled from a tap drill size
(TDS) chart. The hole diameter may also be listed On the side of the tap.
Making the hole smaller than listed on the chart will cause too much
strain on the tap. If the hole is made larger thau listed, the threads will not
be deep enough aud will tend to strip easily.
2b. If no tap drill chart is available, determine the size of hole by using the
nomiual diameter:
TDS = nomiual diameter - pitch
Example 1:
Select the drill to tap for a 1"- 8 UNC screw.
An 8 UNC screw has 1/8" pitch
IDS = nominal diameter - pitch
= I" - l/S"
= 7/S"
Example 2:
Select the drill to tap for an MIO x 1.5 screw.
An MIO screw has 1.5 mm pitch
IDS = nominal diameter - pitch
=10mm-1.5mm
=8.5mm
A
Make sure the workpiece is clamped securely for all stages of
drilling and tapping.
3. Drill the holes for taps accurately. When you cannot use a drill press, use
a portable drill as accurately as possible.
Check the taps
1. Make sure the tap is the correct size aud the hole is the correct size for
the tap.
2. Always wipe taps cleau before (aud after) usiug them.
3. Check the cuttiug edges of the tap to be sure they are sharp. Dull cutting
edges require more force to turn the tap, which may break it.
Tap the threads
1. Correct alignment is essential when the tap first enters the hole. Once the
tap is started correctly, it will tend to remain aligned. If the tap is out of
alignment, remove it and start over. Apply equal amonnts of pressure on
each end of the tap wrench. Don't try to force the tap into alignment,
or it may break.
2. Carefully and gently start the appropriate tap in the hole. You will find
the tap will soon appear to jam. Turn back slightly more than a quarter
tum, tum forward to where you were and continue forward a half tum.
The tum backwards breaks off the spiral chips formed by the cutting
process. Once the chips have broken free, they can fall away.
3. Lubricate frequently during tapping nnless the material being tapped is
cast iron. The lubrication reduces friction and prevents excess heating of
the tool and material. It also prevents excess tool wear and helps wash
away chips formed by the cutting. Use a lubricant recommended for
thread cutting.
4. When the tapping is complete, remove the tap and clean the hole.
Broken tap removal
Always wear eye protection whenever you strike a broken tap. The tap
material is very hard and brittle and flying chips could cause eye injury.
Tap extractor method
The tap extractor (see Figure 44) is a rather delicate tool, but it can work
well and save a great deal of time if used correctly. To use the extractor, do
the following:
1. Before using the tap extractor, try
to break loose any chips caught
between the tap and the sides of
the hole. Do this by jarring,
probing, or picking the area.
2. Place the "fingers" of the tool in
the flutes of the broken tap as far
down as possible.
3. Pull the collar down to the top of
the broken tap.
4. Use a tap wreneh to tum the tap
extractor. Turn them back and
forth to work the tap out of the
hole.
Figure 44 Tap extractor
Punch and hammer method
A small punch and hammer can
also be used very carefully to
loosen the broken tap as shown in
Figure 45.
Always use plenty of anti-seize
liquid when attempting to remove
a broken tap.
Figure 45 Loosening a broken tap
Heating and cooling method
This method of tap removal must be done carefully to avoid damaging the
workpiece.
I. Heat the broken tap with a torch.
2. Chill it immediately. Methods of chilling the heated tap varies depending
on the situation. Sometimes cold water is used. Other methods include
using CO2 in gas or dry ice form.
3. Immediately after cooling the heated tap, try to turn the tap out of the
hole.
4. It may take two or three heatings and chillings before the tap will move.
Dies for external threads
The tool used to cut external threads is a die. Dies such as those shown in
Figure 46 have four cutting edges that cut the threads as the die is turned.
The two dies shown can be adjusted slightly for accurate piteh diameters.
Figure 46 Adjustable dies
As the din starts to cut threads on a rod. the cut is quite shallow. It is made
deeper as the die is turned until it reaches the full depth of the thread. The
die on the left is made to cut threads shallower by tightening the set screw of
the die stock into the split in the die. The die on the right has a built-in
adjusting screw which forees the split to open, causing it to cut less.
Die stocks
Dies are designed to fit into die stocks such as those illustrated in Figure 47.
Set screw
Set screws
I
/\
Figure 47 Die stocks
The die has a small dimple
drilled into its side which
coincides with a set screw in
the die stock. When the set
screw is tightened into the
dimple. the die is secured
to the stock.
Figure 48 shows a die stock
used while cutting external
threads.
Figure 48 Using a die stock
Cutting external threads
Many of the precautions and procedures used for tapping internal threads
also apply to the cutting of external threads.
Prepare the work
1. If you are cutting external threads to match a threaded part. you must
select a rod of appropriate diameter. A rod too small in diameter will end
up with threads that are too shallow; rods too large in diameter will either
not allow the die to engage or make cutting very difficult.
2. Make sure you select the correct size die. Remember, size includes both
the thread diameter and the thread pitch.
3. Secure the die to the die stock by engaging the set screw(s) in the
dimpJe(s).
4. Secure the rod to be threaded in a vise so that the workpiece will not
move during thread cutting.
5. Start the cut with the die opened as wide as possible. You can al ways cut
the threads deeper if needed, but if the threads are cut too deep at first,
the workpiece is ruined.
6. Apply thread-cutting fluid often and freely to the area where you are
cutting.
7. Make sure all four cutting surfaces of the die are in contact with the rod
end at the start of the cut. Apply even pressure to both handles at all
times to prevent forcing the die out of alignment with the rod. Check
often to make sure the die and the rod are correctly aligned throughout
the cutting process.
Cut the threads
1. Turn the die stock slowly to start the thread-cutting process. Turn the die
backwards just over a quarter turn after every half torn forward. The
backward torning will break and clear the chips from the cutting areas.
2. As soon as enough threads are cut so that you can test their fit, remove
the die and test the threads on a nut or another internal thread.
3. If necessary, adjust the amount of material being removed by adjusting
the split opening ofthe die. When the die is correctly adjusted, continue
cutting threads.
4. Clean the die before storing it.
If a situation arises when you must cut external threads the full length of a
bolt:
)
1. Cut the threads as far down the length as possible in the usual manner.
2. Remove the die from the workpiece.
3. Turn the die over so that the tapered end points away from the bolt, and
thread the die back onto the bolt.
4. When the die reaches the end of the threaded portion, continue cutting
threads until they have been cut to the end of the bolt.
Installation, removal
and repair of
threaded fasteners
Thread repair
Slight damage
There are occasions when screw threads, internal or external, become
damaged and will not mate with other threaded fasteners. In such cases the
threads can be quickly made usable by repairing them with tools such as
taps, dies, thread chasers, thread fIles and die nuts.
Severe damage
When internal threads are worn, stripped, or badly damaged, they are usually
repaired by one of the four methods outlined below.
If the cap screw can be larger:
•
Drill and tap the hole in the machinery part to suit the next size of
suitable fastener. Use a larger diameter cap screw. The part held by the
cap screw will have to be drilled larger to allow the oversized cap screw
to fit.
If the cap screw size must remain the same:
•
If the material is weldable, plug weld the hole, drill and tap.
•
Drill the hole deeper if you can and use a longer fastener.
•
Repair the damaged internal threads by using
a thread-restoring insert.
Thread-restoring inserts (HeliCoils™)
HeliCoils™ may be used to provide
threads that are stronger (and more wearresistant) than the material they are used in.
HeliCoils™ are formed from diamondshaped, stainless-steel or phosphor-bronze
wire. They have a driving tang and a notch
to help in their installation. See Figure 49.
Before installation, HeliCoils™ are slightly
larger in diameter than the threaded hole
into which they are to be inserted (see
Figure 50).
To use a HeliCoil™:
1. Drill out the damaged threads and retap the hole with a special tap.
Figure 49 HeliColITM
2. Insert a HeliCoil™ with the special
tool. Adjust it until its top end is a
quarter- to a half-turn below the top
surface of the tapped hole.
3. Once the HeliCoil™ is correctly
inserted in a threaded hole, it restores
the hole to its original size.
4. Mter insertion, you can easily break the
driving tang off with a punch. If the
insert reaches the bottom of a blind
hole, it may not be necessary to break
the driving tang off.
Figure 50 HeliCoil™ insertion
Broken stud removal
Studs or cap screws often become seized in a threaded hole. Then, when you
try to remove them, they may twist off rather than come out. When a stud is
broken off in a threaded hole, the procedure used to remove it depends on:
o
how tight the threads fit
o
whether the fastener broke above, below, or flush to the surface of the
threaded hole
Tight or rusted studs
o
If rust seems to be the cause of the seized threads, apply lots of
penetrating fluid to the threads. After allowing time for the liquid to
penetrate to all the threads, try to turn the stud out.
o
If rust is not the problem (for example, corroded aluminum), do one of
the following:
Either
1. Heat the surrounding material, let it cool slowly.
2. Then try to turn the stud out.
Or
1. Heat the stud directly and cool it down quickly.
2. Then try to turn the stud out.
Broken studs
Studs that are broken off well above the surface of the threaded hole may be
turned with locking pliers. If the portion above the hole is too short for a
good grip with pliers, do one ofthe following (see Figure 51):
o
slip a nut over the broken stud and plug weld it
o
hacksaw a notch to accept a screwdriver
o
file the sides to accept a wrench.
Then turn the stud out, using a screwdriver or a wrench.
Weld
File two flats
t
t t
Figure 51 Stud removal
Studs that have broken flush or below the hole require that you:
1. Grind or file them flat if possible.
2. Use a centre punch to make a small dimple in the centre of the stud. Drill
a 3 mm (W') diameter pilot hole into the stud.
3. Select a stud extractor that has a diameter approximately half that of the
broken stud. Three types of stud extractors are shown in Figure 52.
Figure 52 Stud extractors
4. Drill a hole into the centre of the stud to accept the selected stud
extractor. Be careful not to drill past the bottom of the stud into the
material.
5. Tap the stud extractor into the drilled stud until it has gained a good grip
on the inside surface of the broken stud.
6. Use a wrench to turn the stud extractor counterclockwise to remove the
stud. You may have to apply penetrating oil. or anti-seize fluid, or you
may have to heat and cool it before the stud breaks its grip.
Failures during installation
Fasteners may fail in three ways:
•
The shank may break.
•
The external thread may strip.
•
The internal thread may strip.
This can be caused by incorrect torque. machine vibration, and many other
factors in the machine's operating environment.
A voiding failure
Shank breakage can be avoided by following manufacturer's torque values
for assembly. Thread stripping can be avoided by:
•
using deeper nuts SO that more threads are engaged and take the load
•
ensuring that the fastener is strong enough
For example, if a bolted assembly is torqued to 136 N.m (100 ft.lbf) to suit
specifications, several factors are involved during tbe tightening procedure:
•
torque or turning force, set by the wrench
•
tension or elongation of tbe bolt
•
compression of the material between the bolt head and nut
•
dilation or the tendency of the wedge shape of the thread to enlarge the
diameter of the nu t
Mter torque force is taken off the assembly, the major force remaining is the
tension set up by the fastener.
Tensile forces
Tensile force on tbe material can be classed as:
•
elastic limit-the amount a fastener can be stretched and still return to its
originallengtb after tensile forces are removed. Proof-load figures for
fasteners are frequently given; they are slightly less than tbe yield load of
tbe fastener, but within the elastic limit.
•
yield point-where the fastener begins to take a permanent set
•
ultimate tensile strength (uts) -tbe failure or breaking point
These forces are expressed in pounds per square inch (psi), tbe units of
stress. They represent tbe forces tbat will break a length of material witb a
cross section of one square inch.
Preload in fasteners
Preloading means tightening a nut and bolt assembly to a predetermined
torque value. This prepares the nut and bolt to accept an opposing load.
Torque wrenches are tbe most frequently used tool to induce this preload
into nut-and-bolt assemblies. For example, torquing the bearings and tbe
bearing end caps, preloads tbe bearing assembly to counteract the tbrust
forces in the loaded shaft.
Uniform preloading
It is often important to have uniform preloading in a system. Preload is
applied by several methods:
•
Preload indicating washers-These washers are designed to crush at
their highest points at a predetermined torque value. For example, in
Figure 53, the washer has protrusions when flattened as the bolt is
tightened.
A feeler gauge is used to measure the remaining gap between the nut and
the assembly surface. With a paired bolt and load-indicator washer, tbe
amount of gap is proportional to bolt preload. A set of manufacturer's
tables is needed for each washer, and you must assemble tbe bolts and
washers carefully.
Type A-325
TypeA-490
(b)
(a)
Washer
Feeler gauge
(el
Figure 53 Load-indicator washers
• Preload indicating bolts-provide visual or physical evidence that the
desired preload value has been reached. They have various designs such
as the wavy flange bolt (see Figure 54) and the spinning cap bolt.
-
The wavy flauge type has a wavy flange under the bolt head which
flattens when desired preload is reached.
The spinning cap type has a spinning cap on the head of the bolt. As
preload is reached, the bolt stretches and the cap locks in place.
Figure 54 Wavy-flange bolt before and after tightening
• Torque nuts and torque boUs-are specialized fastening systems. They
are used in areas where high bolting tension is required, such as in highpressure flanges, turbines, compressors, pumps and anchor bolts. Either
torque nuts or torque bolts are used, not both. A hardened washer must be
used under the torque nut or bolt see Figure 55. This system achieves
very high clamping loads (preload) with low torque.
Example:
A 11/2 - 12 cap screw tightened by a nut using
conventional methods requires 2194 ft.lbf to
achieve 87 750 lbf preload.
A torque nut with 8 jacking screws requires 44 ft.lbf
per jackscrew to achieve the same preload.
When working with these nuts and bolts, refer to the manufacturer's
specifications for torquing procedures and torquing tables.
Torque nul
Jacking screws
Torque bolt
Hardened washers
Figure 55 Using torque nuts and bolts
• Measured elongation offasteners-Uniform preloading gives uniform
elongation of fasteners. A micrometer reading is taken for the length of
each fastener. On assembly, each fastener is stretched to a specified
uniform length.
•
A
Tum of the nut-In this method the fastener assembly is snugged up to a
"tight-by-hand" position. The nut is then marked with chalk or pencil and
turned 1 to 1 112 more turns. This is usually done with an impact wrench
or a slugging wrench (see Figure 56).
Always use safety glasses when using a slugging wrench.
.
C!o/
Striking face box wrenches
45° offset 12 point opening
-------"'~-\
.----~,
~--~
Figure 56 Slugging wrench
Torque wrenches
A torque wrench is used to ensure that the correct amount of preload is put
into each fastener. Manufacturer's specifications must be followed at all
times. Any deviation from these specifications could fracture or deform a
machine component, resulting in premature failure. Use torque wrenches
only for tightening fasteners.
A
Never use a torque wrench to loosen afastener.
Torque-limiting wrench
The torque-limiting wrench automatically releases when the preset torque
limit is reached.
Direction selector
Micrometer barrel
~~!U~~==~~!*~n:""
Ratchet head
Handle of wrench
showing torque
setting
Figure 57 Torque-limiting wrench
Dial-indicating torque wrench
Smooth, even
o
pull
• •
•
o
In the dial-indicating
torque wrench (see
Figure 58), torque is
shown on a dial face on
the handle.
Figure 58 Dial-indicating torque wrench
Deflecting-beam torque wrench
In the deflecting-beam torque wrench, the amount of torque being applied to
the fastener is shown on a scale situated on the wrench main frame. The
scale may be graduated in imperial and/or metric.
Socket drive head
Deflecting beam
---~~..l.~6e.:/'"--- Scale
Figure 59 Deflecting-beam torque wrench
Torque multiplier
A torque multiplier uses the ratio principle. See Figure 60.
1/2" drive
111 drive
_
------+-
Figure 60 Torque multiplier
Hydraulic torque wrenches
Hydraulic torque wrenches are used where high torque is required to tighten
fasteners. Hydraulic pressure applied to a piston forces a lever to rotate the
drive. A reaction arm against a fIxed surface of the machine keeps the tool
from rotating. Pressure is supplied to the wrench from a portable pump and
tank. To get the pressure gauge reading for the required torque, refer to the
manufacturer's conversion tables.
Socket drive
Reaction arm
Figure 61 Hydraulic torque wrench
Using adapters
Sometimes it is not possible to use a socket on the torque wrench. This is
overcome by using a crow's foot adapter as shown in Figure 62. An
adjustment to the scale is needed to account for the extra length of the
wrench.
Figure 62 Crow's foot adapters
Torque values
The torque values shown in service manuals are designed to:
•
give correct preload on fasteners
•
prevent shear across the thread when assembling
•
ensure a uniform loading on all the fasteners when assembling
•
prevent distortion, failure or cracking of metals, e.g., a cast bushing used
with steel cap screws
•
provide standards
Values given in the tables in service manuals usually apply to a new
threaded assembly (Class 2A or 2B) with very light lubrication.
Correct torque
Correct torque values depend on:
•
accuracy of the torque wrench
•
thread finish
•
type of surface finish on the fixed and turning metals
•
class of fit
•
condition of the fasteners
•
positioning of the holes
•
correct amount of thread for the assembly
Effects of lubrication on torque
Lubrication affects the torque value of any fastener assembly. Lubrication is
often a matter of choice or company policy. If the assembly is fastened "for
life," a lubricant is not critical. If the assembly is to be taken apart
frequently, you should use a commercial anti-seize compound. Some
manuals give a definite torque figure for lubrication and specific correction
factors for common lubricants.
Table 5 contains torque values for graded steel bolts up to I" diameter, both
dry and oiled.
Table 5: Suggested torque values for graded steel bolts up to 1" diameter
~
Thread
sizes
Dry
ft.lbf N.m
Oiled
ft.lbf N.m
Dry
ft.lbf N.m
Oiled
N.m
ft.lbf
'/4-20
'/4-28
8
10
9.8
9.8
6
7
7.64
9.8
12
14
19.6
19.6
9
11
9.8
9.8
5/16-18
5/16-24
17
19
19.6
29.4
13
15
19.6
19.6
24
27
29.4
39.2
18
21
19.6
29.4
3/9-16
3/8-24
31
35
39.2
49
24
27
29.4
39.2
44
49
56.8
68.6
34
38
49
7/16-14
7118-20
49
55
66.6
78,4
38
42
49
98
107.8
54
59
70
78
60
69
78.4
'/2-13
'/2-20
75
85
98
117.6
58
85
78,4
88
105
120
137
166.6
82
90
107.8
117.6
·"6-12
9/16-18
110
120
147
166.6
84
93
118
127
155
170
206
225
120
132
166.6
176
5/a-l1
%-18
150
170
206
225
115
130
157
176
210
240
284
323
185
185
225
245
3/4-10
3/4-16
270
295
363
402
205
230
274
314
375
420
510
568
290
320
392
431
1/a-9
1/a-14
395
435
529
588
305
335
412
451
605
670
813
902
455
515
617
696
1-8
1-14
590
660
793
892
455
510
617
686
905
1030
1225
1392
695
785
941
1057
49
Chapter 6
Lubrication
Kinds offriction ..............................................................................
Sliding friction ..................................................................................
Rolling friction .................................................................................
Fluid friction .....................................................................................
6:1
6: 1
6:2
6:2
Properties of oil .......................... ...... ........ ......................................
Theories of adhesion and cohesion ...................................................
Oiliness .............................................................................................
Viscosity ....................................................................................
6:3
6:3
6:4
6:4
Oi11ubrication .................................................................................
Lubrication using an oil wedge ........................................................
Boundary lubrication using an adherent film ...................................
Hydraulic lock ..................................................................................
Additives and inhibitors ...................................................................
Oil lubrication systems .....................................................................
6:6
6:6
6:7
6:7
6:8
6:9
Properties of grease ........................................................................ 6: 13
Grease types ...................................................................................... 6: 14
Grease lubrication ........................................................................... 6:16
Choosing a grease ............................................................................. 6: 16
Grease lubrication systems ............................................................... 6: 16
Special oil and grease lubrication ................................................... 6:18
Automatic lubricators ....................................................................... 6: 18
Open gears ........................................................................................ 6: 19
Enclosed gears .................................................................................. 6: 19
Oil and grease comparison ............................................................... 6:20
Lubrication during cutting .............................................................. 6:21
Cutting oils ....................................................................................... 6:21
Using cutting oil ............................................................................... 6:23
Safe handling of lubricants ............................................................. 6:23
Safety routine .................................................................................... 6:23
Safe storage and disposal .................................................................. 6:24
Lubrication
Correct lubrication reduces friction between components and increases
component life by reducing wear. Lubricants are substances (usually oils)
used to do this.
Kinds of friction
The most carefully finished metal surfaee is not truly flat, but is covered with
microscopic irregularities-projections and depressions as shown in
Figure 1.
These irregularities tend to
interlock and resist sliding
motion. Friction is the tendency to
resist movement when surfaces
are in contact as they move.
Figure 1 Magnified finished surfaces
Under load, friction increases. Friction between moving surfaces is grouped
into three main types: sliding, rolling and fluid friction.
Sliding friction
Sliding friction occurs when two surfaces slido over each other, such as in
journal bearings or pistons sliding in a cylinder (Figure 2). In sliding friction,
the contact pressure is usually spread over a large area. This means that the
pressure per square inch is comparatively light.
Pislon
Flal
surfaces
sliding
Bush
bearing
I
Skid
,
,,
I ,
Figure 2 Sliding friction
MILLWRIGHT-lUBRICATION
6-1
Rolling friction
Rolling friction takes place when a spherical or cylindrical body rolls over a
surface. Common examples are ball and roller bearings. Figures 3 and 4
show various types of anti-friction bearings using balls and rollers.
Figure 3 Types of rolling friction
With ball and roller anti-friction
bearings, the area of contact is quite
small, with the result that the
pressure-loading is high. There is also
a very small amount of sliding
friction between the ball or roller and
the separators.
Load
Figure 4 Ball bearings
The balls and rollers in anti-friction bearings are slightly deformed under
load like a tire under the weight of a car (see Figure 4). This increases the
sliding friction between rolling members and races. The separators used in
anti-friction bearings also contribute a small amount of sliding friction to
their operation.
Fluid friction
Lubrication is a way to reduce some of the effects of friction. When
lubricating oil is applied to two surfaces in contact, a film of oil is formed,
filling up the depressions and covering the projections on both surfaces.
,<----, - - - (
Figure 5 Magnified bearing surfaces with an oil film between them
6-2
MILLWRIGHT-LUBRICATION
Because there is no metal-to-metal contact, sliding occurs between the layers
of oil within the film This is called "fluid friction."
Lubrication l~ the reduction of friction to a minimum by the replacement of
dry friction with fluid friction.
Properties of oil
Lubricatiog oil, like other liquids, consists entirely of extremely tiny
particles called molecules which are in violent motion. These molecules
attract one another so that few drift away. The molecules may also be
attracted by metal and other surfaces, to which they are then held firmly. A
layer of oil several particles thick may build up on the lubricated surface and
follow the movement of that surface.
Theories of adhesion and cohesion
Adhesion
The theory of adhesion states that adhesive forces cause unlike materials to
stick together (polar attraction). Examples are the forces of adhesion
between grease and metal or between oil and metal.
Cohesion
The theory of cohesion states that cohesive forces cause molecules of like
materials to stick together. Examples are the forces of cohesion between
grease molecules (or between copper molecules).
Forces on oil layers
Oil forms io layers of globules and cohesive forces occur between the layers.
If the oil is io contact with metal surfaces, adhesive forces occur between the
oil and metal surfaces.
For example, in Figure 6, as the surfaces
move agaiost each other, layer 1 adheres
to the top metal surface, layer 7 adheres
to the bottom metal surface. The layers
in between roll over each other,
overcomiog the cohesive forces. The
only friction is the fluid friction between
oil globules. This state is maintained as
long as there is a suitable quantity of oil.
Figure 6 Layers of oil globules
between two metal surfaces
MILLWRIGHT-LUBRICATION
6-3
Oiliness
Because of their chemical composition, some molecules are attracted and
held more strongly to metal than others. The adhesive forces are strong. Oils
rich in these molecules are said to be high in oiliness or lubricity.
Viscosity
Viscosity is the resistance to flow. Because of different cohesive forees, not
all lubricating oils flow with equal readiness. The molecules in higher
viscosity oils have more difficulty sliding past one another. Oils with high
viscosity are thicker and pour more slowly than those with low viscosity.
Viscosity is the most important property of lubricating oil. It largely
detemrines the suitability of the oil. It affects the generation of heat in
bearings, the ease with which machines start in the cold, the sealing effects
of the oil, and the rate of consumption or loss of oil.
Effects on lubrication
Oil molecules adhere to the surface of a rotating journal and additional
molecules are carried along by their pull. High-viscosity oils, with their less
active molecules, exert greater pressures and can carry greater load•.
(Greater loads can also be carried if the journal is speeded up.) A layer of oil
also adheres to the surface of the stationary sleeve and pulls against the bulk
of the oil. This pull opposes that of the moving journal. The molecules in
between are dragged along at various speeds, depending upon their nearness
to the surfaces. Faster-moving molecules must be dragged past those moving
more slowly. The friction between molecules with different speeds acts as a
drag on the journal. Power must be expended to overcome it.
Choosing the best viscosity
The highest viscosity oil is not necessarily the best lubricant. The ideal
choice of oil viscosity depends on speed (rpm), load, temperature, pressure,
and environment.
Considering the load and speed:
o
The higher the viscosity of the oil, the greater the load it will carry.
•
The higher the journal speed, the lower the oil viscosity needed to carry a
specific load.
o
To minimize power losses, the oil should have the lowest viscosity able
to carry the load on the bearing.
In practice, the viscosity you must choose is dictated by plant reqUirements
or manufacturer's specifications
6-4
MILLWRIGHT-LUBRICATION
Viscosity-temperature effects
A
When oil is heated, the molecules move with increasing violence. This
causes them to move further apart and the oil expands. Because the
molecules are more active and have more room to flow past one another, the
oil "thins out"-its viscosity drops. This change may be very great.
Choose oil that has the desired viscosity at its operating temperature.
Sometimes the operating temperature fluctuates. The machine may be at a
low temperature, possibly sub-zero, when operation starts. As operation
continues, it may attain fairly high temperatures. In the case of a hydraulic
system, 93°C (200°F) is not unusual. Obviously the oil must be viscous
enough at the highest temperatures to carry the loads imposed.
Viscosity measurement
Viscosity is measured in two ways: dynamic and kinematic.
Dynamic or absolute viscosity
Dynamic or absolute viscosity is determined by measuring the force required
to overcome fluid friction in a film of known dimensions. Because it
depends only on fluid friction, dynamic viscosity is used most frequently in
bearing design and oil-flow calculations.
Kinematic viscosity
Kinematic viscosity is a measure of viscosity that is affected by the density
of the oil. It is used most often to compare lubricants when both are
measured at the same temperature using the same unit system.
The most common units of kinematic viscosity, are metric centistokes,
abbreviated cSt. Most viscosities are determined in centistokes and converted
to other systems using published conversion tables.
1 cSt = 1 mm2/s
Saybolt viscosimeter
Several instruments were developed to measure viscosity, but the one most
often used by oil manufacturers is a Saybolt viscosimeter. The unit of
measurement is based on the Fahrenheit temperature used for this method. It
is the Saybolt universal second abbreviated SSU or SUS.
The viscosimeter is used as follows:
MILLWRIGHT-LUBRICATION
•
Heat a 60 mL sample of oil to exactly 40°C (lOO°F). (Several other
specific temperatures may be used for various service conditions.)
•
Allow the oil to flow through a calibrated orifice 1.765 mm in diameter
and 12.25 mm long.
•
Measure the time it takes in seconds for 60 mL to drain off.
6- 5
Viscosity index
When subjected to the same change in temperature, all oils do not change
viscosity at the same rate. The specific viscosity reaction of an oil whose
temperature changes is indicated by the viscosity index (VI).
The higher the VI number, the smaller the change in viscosity due to
temperature. For example, a premium-grade oil for turbine lubrication has a
VI of 92 to 96. Some all-weather hydraulic oils have a VI of 200.
Oil lubrication
Lubrication using an oil wedge
Oil molecules are free to move in any direction as long as they keep close
together. Therefore, they respond collectively to any application of force.
They readily occupy any clearance between a moving component and its
surroundings, such as ajoumal and sleeve, fonning an oil wedge (see
Figure 7).
Little oil separates journal from
sleeve in starting position, (a).
As journal begins to rotate, (b),
it rolls up sleeve to Y where oil
wedge raises journal from
sleeve. In full operation, (c),
point of wedge shifts to Z.
a
b
c
Figure 7 Oil wedge in a simple bearing
When a shaft is at rest, most of the oil is squeezed out of the contact area.
During rotation the following happens:
1. As the shaft begins to rotate slowly, oil climbs up the side of the bearing.
It moves in the opposite direction to the rotation.
2. Eventually, oil surrounds the shaft, lifting it hydraulically.
3. At full speed, oil enters at the area of lowest pressure.
4. The shaft now carries the oil in the direction of rotation toward the area
of maximum pressure.
5. The oil wedge fonns and forces the shaft up and toward the centre of the
bearing. It does this until pressure is equalized and shaft position is
maintained.
These actions depend on constant oil feed and drain.
6- 6
MILLWRIGHT-LUBRICATION
Efficiency of the oil wedge depends on:
•
•
load
rubbing speed or rpm
•
operating temperature
•
clearance between surfaces
•
oil viscosity or grease grade
•
volume of lubricant supplied
•
where the lubricant is introduced
•
shaft surface finish
Boundary lubrication using an adherent film
Under many circumstances it is not possible to prevent metal-to-metal
contact entirely. When the fIlm of oil fails for a short time, this is called
boundary or marginal lubrication.
Boundary lubrication occurs because:
•
Most bearings at rest are still under some pressure. Since there is no
motion to maintain the oil wedge, the static load squeezes out all
lubricant except that attracted to the metal surfaces. When the machine is
started, this residual film is the only source of lubrication until the oil
wedge is re-established.
•
Some bearings are subject to frequent overloads, often as the result of
shock loading. This causes higher fluid friction. Rather than USe an oil of
high enough viscosity to carry load peaks, it may be necessary or
preferable to carry momentary or brief overloads on residual oil films.
•
Newly machined surfaces may be so rough that an oil wedge sufficient to
prevent all metal-to-metal contact is difficult to maintain. In this case, the
oils may moderate the wear process so that the new surfaces are worked
gently and uniformly to a smooth condition during the running-in period.
Under all these conditions, friction may be high, causing great wear, unless a
residual oil film can be maintained to minimize contact of metal with metal.
Oils with a good ability to cling to bearing surfaces give superior
performance.
Hydraulic lock
)
A
MILLWRIGHT-LUBRICATION
If you over-lubricate a threaded hole or the screw that threads into it, excess
oil drains to the bottom of the threaded hole. As the bolt is tightened, it
presses the liquid causing a hydraulic lock that may crack the casing.
Do not to aver-lubricate threaded holes ar the screws thot thread into them.
6-7
Additives and inhibitors
Additives and inhibitors are divided into three main groups: air control,
water contro/' and load capacity.
Air control
Petroleum oils contain a small quantity of air. For general lubricating
purposes, air content is usually not considered. However, all petroleum and
vegetable oils react chemically with oxygen. This reaction is called
oxidation. It is increased by high temperatures, air, contaminants, or water.
Oxidation produces two general classes of waste products:
•
•
oiHnsoluble materials such as varnish or sludge
oil-soluble acids.
Basic oxidation stability is established by the refining process. It is increased
by the addition of oxidation inhibitors or anti-corrosion additives that reduce
the formation of sludge or acids. Foam depressants or foam inhibitors are
added to reduce the foam content in an oiL
Water control
Water usually gets into a lubricating system due to faulty seals and
condensation inside the metal housing or tank. This happens when the
machine cools to ambient temperature from an operating temperature of
40°C to 60°C (120°F to 140"F).
In most oil lubrication systems, you wish to remove the water. In some
systems, the water can be absorbed, but metal must be protected from rust
when water is present in the system.
•
Demulsibility is the ability of a petroleum oil to separate from water upon
standing. It can be increased by additives.
•
Emulsibility is the ability of a petroleum oil to mix with water using a
special emulsifying agent. A common example is the cutting fluid used in
a machine shop.
•
Rust inhibitors are added to the oil to reduce the rusting of ferrous parts,
which will occur if there is water in the system.
Load capacity control
Chemicals are added to the oil to increase its load capacity. Load capacity
additives can be separated into two general groups:
• Anti-wear additives minimize surface wear under normal circumstances.
They are phosphorous, active sulphur, or zinc compounds that polish the
contacting faces.
•
6-8
Extreme pressure (EP) additives are used for heavy load or shock load
conditions.
MILLWRIGHT-LUBRICATION
The EP additives can be
- compounds of sulphur andlor phosphorous
- lead soaps
- loug-chain elastomers
A
Do not mix EP oils with other EP oils or standard oils. There may be chemical reactions, Also, do not mix different brands of oils,
Other additives
Detergents and dispersants are additives that are usua!ly found in gasoline
and diesel engine oils, but not in industrial oils. A detergent cleans the
surfaces of components. A dispersant keeps the contaminants in suspension.
A few premium industrial oils used in a full circulating system have a mild
detergent action. But, in general, detergent motor oils are not added to
industrial oils because of the following possible complications:
•
contamination of the oil, causing excessive foaming
•
deposits in critical clearance spaces
•
filter problems.
Oil lubrication systems
Oil lubrication is applied in two main types of systems: the once-through or
wasted oil system; and the enclosed system in which the oil is used over and
Over again. Some methods may be either type depending on the application.
Once-through systems
Oil used to lubricate II once-through system can be a relatively low-grade
(low-cost) oil. This oil must meet viscosity and film-strength requirements.
Usually, additives are not used in a once-through system. To suit plant policy
and prevent lubricating errors, the once-through oil used may be the same
grade of oil used in enclosed systems,
Various applicators are used to oil once-through systems. Common bottle
oilers, such as wick-feed or drip-feed oilers, add oil gradually to suit
operating conditions. Bottle oilers can only be used above the bearing, as the
oil's flow from them is by gravity.
MILLWRIGHT-lUBRICATION
•
The hand oiler Or squirt can is the oldest method of applying oil and is
still in use. However, this method leads to extremes of over- or underlubrication.
•
The wickjeed oiler shown in Figure 8 uses the capillary action of a
strand Or strands of wool to transfer the oil from the reservoir to the shaft.
The flow of oil depends on the number and length of wool strands and on
the height of the oil in the reservoir. The capillary action of the wick
filters the oil, so after a time the wick gets dirty and must be replaced.
6-9
Figure 8 Wick-feed oiler
•
Figure 9 Drip-feed oiler
The drip-feed oiler shown in Figure 9 provides a visual check and a
means of controlling the flow of oil by adjusting a needle valve. The
needle valve is easily fouled by a small piece of dirt. The oiler is filled
through a small hole in the top. This requires some care and the use of a
strainer to keep foreign material from entering. To minimize oil loss, shut
the valve off when the machine is not used.
Enclosed system
An enclosed system is one in
which the oil is circulated and
used over and over again. If
the oil is used over a period, a
highly stable oil with
additives is required. This
reduces oxidation, corrosion,
foaming, and the formation of
emulsions.
Ring-oiled sleeve bearing
Ring oiler lubrication
A ring oiler is a mechanical
means of oiling a shaft. The
ring has an internal diameter
(ID) larger than the outer
diameter (00) of the shaft. It
rests on top of the shaft with
the bottom of the ring in the
oil at the bottom of the
housing. As the shaft turns,
friction pulls the ring around
with oil clinging to its surface
(Figure 10).
L-_ _ Oil level
I ring
Figure 10 Oil-ring lubrication of a
sleeve bearing in an electric motor
6-10
MILLWRIGHT-LUBRICATION
The rings are made in one piece or in two hinged pieces (see Figure 11).
They are usually metal, out can also be made of flexible, light, ladder chain.
Two or more rings may be used on multiple bearing systems.
o
Figure 11 Oiler rings
Splash lubrication
Splash lubrication is the most
common method of lubrication in
enclosed gear systems. In most
units, the larger gear picks up the
oil and carries it to the mesh point
(see Figure 12). It also splashes oil
to a trough which drains to the
bearings. The oil level must be kept
high enough to ensure that the gear
will pick up sufficient oil.
Figure 12 Splash lubrication
Too much oil in the housing results in excessive foaming and a marked
temperature rise of the unit. This decreases the viscosity of the oil, resulting
in a less effeetive oil wedge and a marked rise in the operating temperature.
This reduces the service life of the oil and causes early lip-seal failure.
Oil bath lubrication
Worm-wheel gears and moderate-speed chains are lubricated by paqsing
them through an oil bath. Worm-wheel units with the worm on the bottom
are lubricated by the worm passing oil to the wheel from an oil bath. An
oiler ring is attached to the shaft, and is called a flinger. It works as follows:
•
The flinger picks up oil from the bath and transfers it to the worm
bearings and to the gear.
•
As the gear rotates, it contacts two scrapers with a clearance of 0.0 I 0" to
0.015".
•
The scrapers remove the oil from the gear and direct it to troughs which
are cast in the housing and lead to the gear's bearings.
Re-circulatmg lubrication
Enclosed circulating lubrication is used mainly when a large number of
bearings all use the same grade of oil. Another general application is running
bearings at a high temperature and pumping cool oil from the reservoir over
the shaft and bearings to control the heat rise. This system can be used for
either friction or anti-friction bearings, but not for both together.
MILLWRIGHT-LUBRICATION
6 -11
The basic enclosed circulating system consists of a reservoir, a pump, a pipe
with a flow-control valve to each bearing and a drain from each bearing back
to the tank. In addition, a filter and a heat exchanger to cool the oil may be
required. This type of pressure system allows for higher speeds and heavier
loads. Usually, a premium grade oil is used to obtain maximum hours of
service life between oil changes.
Also, when a shaft is vertical, a positive displacement pump carries
lubrication from the reservoir to the upper bearing against gravity.
Oil mist lubrication
Air and oil mist lubrication is frequently recommended for high-speed
bearings and high-velocity roller chain. The volume of oil supplied is
sufficient to provide an oil wedge, but not enough to create foaming or oil
chuming thut will lead to a heat rise. Oil mist lubrication may be enclosed or
once-through.
A
Make sure that the oil and air supplies are clean. Small solid particles of
material can plug the jets or cause wear in the components.
Oil tanks
Several types of lubrication reservoirs can be made in the plant. The basic
style is a tank with a removable or hinged metal lid covering a smaller
opening for adding oil (Figures 13 and 14). Rate of flow is controlled by a
valve and sight glass on the drain line. The drain line can be either pipe or
tubing, but tubing is better as it can be led around obstructions readily and it
can withstand more vibration. The tubes in a multiple-valve tank are
connected by a pipe called the header (Figure 14).
Tank
Tank
P
' - -_ _.....I
Header
b
~
~
~
iI
L .......•_ Lines 10 bearing _ _,
Figure 13 Single-valve oil tank
6-12
Figure 14 Oil tank with multiple valves
MILLWRIGHT-LUBRICATION
Oil quality checks
Oil in continuous use (such as in a splash system of a fuel-circulating system
with a pump) gradually deteriorates through oxidation and exposure to other
contaminants. Checks on oil quality may be either accurate or approximate.
To make an accurate oil quality check, do ONE' of the following:
•
Set up an in-plant laboratory test routine; or
•
Send samples to an approved testing laboratory for a detailed analysis.
To make an approximate oil quality check:
1. Fill one clean, cleat glass container with new oil.
2. Fill a similat glass container with used oil that has been taken from a
machine that is either running or has just stopped running.
3. Compare the two oils using the following four tests:
• By smell-Many things can cause a change in odour. For example, as
an oil oxidizes and picks up impurities its odour changes; also, if an
oil has been overheated, it smells burnt.
•
By touch-Rub a sample of used oil between thumb and fingers and
feel for impurities. Breakdown in bearings and geat weat can cause a
gritty feel.
• By sight-As an oil oxidizes and picks up impurities, it tums a datker
colour. The datker the oil, the more weat or the greater the amount of
impurities.
If the used oil is the same colour as the new oil but is cloudy, there
may be dissolved air or water in the oil. If the oil does not clear after
allowing time for it to settle, the usual cause is water.
If a little used oil poured onto a sloped sheet of paper shows bands of
colour, metallic particles are in suspension. Steel particles give silver
bands and brass or bronze particles gi ve gold bands.
• By using a magnet-Drop a clean small magnet into the used oil
sample to check for ferrous particles in suspension. Some equipment
has magnetic plugs at the lowest point of the casing to trap ferrous
particles.
Greases are usually made by thickening lubricating oils witb a soap. The
thickener controls water resistance. resistance to machine breakdown from
constant use, temperature range. and the ability of the grease to stay in place.
Soap content for gcneral use greases is usually from 7% to 18%, but can be
as low as 3% and as high as 50% for special greases.
MILLWRIGHT-LUBRICATION
6
13
Grease types
Simple soap greases
Simple soap greases are made by combining a fatty acid with one of the
following base metals: calcium, sodium, aluminum, litbium or barium.
•
Calcium soap (lime base)-requires a small amount of water to stabilize
the oil/soap structure. At about 175°F (soaC), the internal water starts to
work out, so that the soap and oil separate. Calcium soap greases are
recommended for damp conditions but not for bigh temperatures.
•
Sodium soap (soda base)-is soluble in water but has a good, bigh
temperature range. Sodium soap greases are only recommended for dry
conditions and high operating temperatures
•
Aluminum soap-gives stringiness to a grease and is used where
adhesiveness is important. Aluminum soaps are water-resistant but not
recommended for bigh temperatures
•
Lithium and barium soaps-are recommended for water-resistance and
bigh temperatures, and are the most commonly used soap greases
Mixed soap greases
Mixed soap greases are made by combining various types of soaps to extend
the service life of a grease. A grease using a mixture of calcium and sodium
soaps combines some of the water resistance of a calcium base with sOme of
the high temperature resistance of a sodium base.
A
Do not try mixing a small amount of sodium-base grease with a small
amount of calcium-base grease and expect to get a mixed-soap-base grease.
Precise ratios are required.
Complex soap greases
Complex soap greases are made from special soaps and are mUlti-purpose.
Non-soap greases
Non-soap greases are used to suit special conditions of temperature,
environment or service life. Some common thickeners are carbon black,
silica gel and special clays.
Multi-purpose greases
Multi-purpose greases are designed to allow one grease to be used instead of
three or four separate grades. It cuts down on inventory and time of
application, and prevents lubrication errors. A mUlti-purpose grease
corresponds to a mUlti-purpose oil.
6-14
MILLWRIGHT-LUBRICATION
Extreme-pressure (EP) greases
Extreme-pressure (EP) greases are designed for shock loading or high-localpressure areas. They usually contain compounds of chlorine, phosphorous or
sulphur as additives. Molybdenum disulphide (moly) is a common additive
for improving the anti-wear capacity of a grease.
Penetration numbers and NLGI grades
Grease con~istency is measured by penetration numbers. A metal cone of a
definite weight and surface area is allowed to sink for 5 seconds into the
surface of the grease at 25°e (77°F). The amount of penetration measured in
tenths of a millimetre is the penetration number of the grease.
The National Lubricating Grease Institute (NLGI ) has a system to classify
grease consistency. Soft greases have high penetration numbers and low
NLGI grades. The NLGI grades range from 000 for the softest grease
through 00, 0, 1, 2, 3, 4, and 5, to 6 for the hardest.
The temperature and speed of operation determine the required NLGI grade
or penetration number of the grease. Temperature and environment
determine the choice of soap base and the frequency of lubrication required.
Dropping point
The dropping point is the temperature at which a grease becomes fluid
enough to drip. Common greases in general industrial use have dropping
points that range from 135°e (275°F) to !82°e (36Q°F), but greases with
lower or higher dropping points are available.
Directional fluidity
Grease has a peculiar characteristic called directional fluidity. When moving
in a bearing, the grease tends to "shear" into thin layers that move in the
direction of rotation (see Figure 15 and Figure 7). Under shearing stress, the
apparent viscosity of the grease falls rapidly until it approaches the viscosity
of the oil used in its manufacture.
Figure 15 Layers of grease shearing in a bearing
MILLWRIGHT-LUBRICATION
6-15
Grease lubrication
Choosing a grease
Choice of grease is detennined by:
•
the company requirements for choice of lubricants, means of application,
and frequency of application
•
the type, speed, temperature, pressure, and environment of the bearing or
machine unit.
•
manufacturer's recommendations
Select grease for its consistency at the operating temperatures of the system.
It should be fluid enough to flow gradually into the bearing or onto a gear.
The temperature is affected by friction, by churning in soft greases, and by
the ambient temperature.
A
When using different grease types, take the following precautions:
• Do not mix greases from different companies in the same bearing.
• Do not mix various grease grades of the same company in the same
bearing.
• Do not use extra additives.
Grease lubrication systems
~rt(M ~vf\
CIA v1
The initial packing of a bearing is usually done by hand. Grease guns and
cups are usually not recommended for the first filling of a bearing as there is
no reliable check on the position or amount of grease. Overfilling until the
grease leaks past the seals can damage them. Use a grease gun or cup to add
grease to a working bearing only.
pVvvl P Dre c. K
@ 2-0 ~ '1,0 00:.) ?S I Hand-packing bearings
To correctly lubricate a bearing assembly, hand-pack the bearing with the
correet grease:
1. Fill the spaces between the rings with grease after assemhly.
2. Pack the housing one-third full for high speeds and one-half full for slow
speeds.
3. Do not over pack-this causes the grease to churn, giving higher
temperatures. Over packing reduces the lubricating value of the grease,
and the life span of the seals.
After a few minutes of operation, the excess grease carried by the separator
and rolling elements is forced out into the housing void, leaving the bearing
to run on the correct amount of lubricant.
6-16
MILLWRIGHT-LUBRICATION
On low-speed applications with extreme
conditions of moisture, dirt, or poor
sealing, you may fill the housing with
grease. Excess grease works out past the
seal to form a secondary seal to keep out
contaminants.
Figures 16 and 17 show how grease is
packed in anti-friction bearings.
Pack well with
grease in between
and around the
balls and rollers so
that both sides of
the bearing are
completely covered.
Figure 16 A packed deep-groove
ball bearing
Misalign the outer race and pack well
with grease in between and around
the balls or rollers. Align the outer
race and pack grease around
retainers and in all available space on
both sides of the bearing.
Figure 17 Packing a double-row, self-aligning, roller bearing
Greasing with a gnn
Greasing with a gun has the advantage of not
depending on gravity for flow conditions (see
Figure 18).
The lubrication person (oiler) can walk at floor
level while greasing bearings at any level using
piping to allow for greasing at a distance. Fixed
bearings use 1/8" black or galvanized piping.
Movable bearings use a loop of oil-resistant
pressure hose between the bearing and the fixed
pipe. The pipe or gun connects to the bearing by
various types of nipples that attach directly to the
bearing cap.
Figure 18 A grease gun
MILLWRIGHT-LUBRICATION
6-17
When using a grease gun;
1. Check bearings at close range frequently in case the grease line breaks or
works out of the bearing.
2. When using two different greases for different jobs, use two guns.
A
3. Make sure that the correct nipple connection is used on the bearing.
Be careful when using a grease gun. It can generate high pressures.
Greasing with a spring-compression cop
Compression grease cups attach directly to the bearing and give a steady
metered supply of grease for up to four hours. Pressure is applied to the
grease by screwing the cap down by hand, or by an automatic spring
compressor (see Figure 19). There is a screw ncar the attachment point to
allow for flow adjustment.
When using a spring-compression
grease cup, do the following:
1. Screw the grease cup into the
bearing or into the short pipe
conneetion to the bearing,
2. Do not connect or disconnect
parts where you must reach across
moving machinery, or where they
may fall into the machinery.
3. When the caps are off, keep
contaminants out of the grease,
4. Protect the housing from
contaminants.
Figure 19 A spring-compression
grease cup
Special oil and
grease lubrication
Automatic lubricators
In some plants, a mechanism similar to the spring-compression cup
automatically and regularly lubricates components. These mechanisms are
triggered electronically or mechanically.
6-18
MILLWRIGHT-LUBRICATION
Open gears
Open gears are lubricated with a
grea.~e or very heavy oil. You must
consider the following conditions:
•
temperature
•
method of application
•
environment
•
gear materIals
•
choice of lubricant
'\
Figure 20 Oil-drip application by cup
The lubriCllllt may be applied by using brush or paddle, drip cup, oil can,
bottom pan, or spray:
•
When applied by a brush or paddle, the lubricant must be fluid enough to
flow easily. But, during operation, the lubriCllllt should be viscous. You
can thin some oils and greases enough for application by heating them
and applying them hot. When heating is not practical, you can dilute
viscous oils with a non-flammable solvent. The solvent evaporates after
exposure to air, leaving the heavy oil covering the surface.
•
Apply oil to gears using a drip cup or oil can as shown in Figure 20.
•
Lubricate very slow-moving gears from a bottom pan; the lubricant is
picked up by the teeth of the larger gear and brought around to the
smaller gear or gears.
Enclosed gears
Lubrication of enclosed gears depends on several important factors:
•
load
•
operating speed
•
operating temperature
•
lubrication methods
•
type of gear
.,
environment
Pressure effects
Worm-wheel gears and hypoid gears generate high pressure and
considerable friction on the contact line. You must use an oil with special
additives.
MILLWRIGHT-LUBRICATION
6-19
Regardless of gear fonn, the higher the unit load on any gear tooth, the
greater the pressure. When the pressure is too high, the oil film can fail,
allowing metal-to-metal contact. To reduce gear wear, if loads are extremely
heavy, you may require a heavier-bodied oil than is usually recommended,
or one with EP additives.
Effects of type of gear
As a general rule, for parallel-shaft reduction units or bevel-gear units:
•
a single reduction requires a light oil to suit the input gears
•
a compound reduction needs a heavier oil to suit the output gear mesh
•
the use of two oils can be avoided by using a multi-purpose industrial oil.
Manufacturer's recommendations
Any new industrial equipment, including gears, has the manufacturer's
lu brication recommendations for brand names and weight of three or four
different lubricants.
A
Watch for extreme temperature rise or other early indications of trouble
when doing a trial-and-error lubrication.
Oil lubrication
Oil is used for high-speed operation, or for temperatures below zero and
above 93°C (200°F). Select the oil to meet the temperature extremes.
Ensure that it contains additives to
prevent corrosion, foaming, and
rusting.
The oil in the bath nonnally covers
the bottom of the outer race, but does
not cover the centre point of the
lowest ball or roller (see Figure 21).
Over-oiling produces churning and a
temperature rise. In high-speed
conditions that generate heat, oil can
be circulated and cooled, drawing
heat away from the bearing and shaft.
Figure 21 An oil-lubricated bearing
showing the oil level
Oil and grease comparison
Choosing whether to use oil or grease to lubricate bearings depends on
several factors. The types of bearing, load, seals, machine speed,
temperature, and environment all affect the choice.
6-20
MILLWRIGHT-LUBRICATION
Grease advantages
The advantages of grease as a lubricant are that it:
•
is a good lubricant for heavy loads at low or medium speeds
•
requires relatively simple seals
•
provides better start-up protection after short periods of downtime
•
gives better protection against rust during periods of downtime
•
tends to stay in place
Oil advantages
The advantages of oil as a lubricant are that it:
•
is a good lubricant for high to very high speeds
•
develops less fl uid friction than grease
•
has a flushing action that washes dirt and solid contaminants to thc
bottom of the housing
•
can be used as a heat-exchange medium
•
can be removed easily from the housing
Lubrication during
cutting
Lubrication during cutting has several purposes, but its main purpose is to
control the temperatures in the machining process by reducing friction.
The functions of lubrication during cutting are to:
•
cool the workpiece
•
cool the cutting tool
•
protect against rust
•
prevent welding of metal chips with the tool
•
wash away metal chips
•
allow high-speed cutting for better productivity and surface finish
Cutting oils
It is important to use the correct oil for high-speed cutting. This increases
tool life, produces a better finish, and reduces power needs. The type and
severity of the machining process determines which cutting oil to use. There
are three types of cutting oils: straight cutting oils, emulsifiable oils, and
chemical oils. They are also called cuttingfluids.
MILLWRIGHT-LUBRICATION
6-21
Straight cutting oils
Straight cutting oils may be active or inactive. Active oils react with nonferrous metals. To test whether an oil is active or inactive, immerse a copper
strip in the oil for three hours at lOonc. Active oils darken the strip.
Active oils
Active oils may only be used on ferrous metals such as iron and steel.
•
sulphurized mineral oil-has good cooling, lubrication and anti-weld
properties; used for light machining operations on ferrous metals
•
sulpho-chlorinated mineral oil-used for machining or threading soft
grades of steel
•
sulphurized or sulpho-chlorinated fatty oil blends-tolerate extreme
pressures for heavy-duty machining of ferrous metals
Inactive oils
Inactivc oils may be used on both ferrous and non-ferrous metals.
•
mineral oil-used for light-duty machining of non-ferrous metals
• fatty oils-used for ferrous and non-ferrous
•
sulphurized, fatty, mineral oil blends-have good lubrication properties;
used for ferrous and non-ferrous metals
• fatty and mineral oil blends-give excellent machine finishes; used for
ferrous and non-ferrous metals
Emulsifiable oils
Emulsifiable oil~ act as though they are soluble under the right conditions.
•
emulsifiable mineral oil-low cosl; used for general machining
•
super-fatted emulsifiable oils-used for heavier-duty machining
operations
•
extreme-pressure emulsifiable oils-used for heavier-duty machining
operations, broaching gears, and heavy turning
Chemical oils
Chemical oils are truly soluble. They contain rust inhibitors and are used for
grinding.
6-22
•
wetting agent types-have good lubrication properties and good heat
dissipation; used for general machining
•
wetting agent types with EP lubricants-used for tougher machining
operations
MILLWRIGHT-LUBRICATION
Using cutting oil
When using cutting oils, do the following:
•
Use protective clothing and splash guards, and avoid prolonged contact
with the fluids.
•
Use the oils at room temperature for best performance.
•
Apply the oil in a continuous stream, covering all the work.
•
Use soft water-hard water leaves a residue of minerals or salts.
•
Always add oil to water; never add water to oil.
•
After cutting, empty and clean the sump if it is to be left for some time.
To avoid odours, flush with a germicidal solution to kill bacteria, fungi,
and algae.
Safe handling of
lubricants
Take care to avoid extended contact of lubricants with the skin. Occupational
dennatitis is a common problem for millwrights. Also, avoid inhaling the
fumes which can be toxic after prolonged exposure. If you use synthetic oils,
check with suppliers for any possible additional health hazard.
Safety routine
Always use the following safety routine to reduce health hazards:
MILLWRIGHT-LUBRICATION
•
Get first aid for any cut or scratch; this is important.
•
Avoid skin contact by wearing protective gloves, aprons, etc.
•
If a protective hand cream is available, apply it before starting work and
after each time the hands are washed.
•
As soon as possible, remove any petroleum product that gets onto the
skin.
•
Wash your hands and arms carefully at the end of the shift or before
eating.
•
Do not use gasoline, turpentine or other solvents to remove oil or grease.
Use a waterless hand cleaner or a mild soap with warm water and a
brush. Clean the brush after use.
•
Use clean paper towels and discard them in approved safety containers.
Do not use dirty cloth towels or wiping rags.
•
Do not wear oil-soaked work clothes; check at the wrists and cuffs for oil
buildup.
•
Avoid breathing oil mist or solvent fumes.
•
Keep the work area clean.
6-23
Safe storage and disposal
Storage and handling of lubricants and disposal methnds are strictly
controlled by local, provincial, and federal legislation. They are also
industry-specific. Refer to Chapter 1: Safety and check all such legislation in
the area where you work.
6-24
MILLWRIGHT-LUBRICATION
C"')
:::r
Q)
.......
"S
CD
MILLWRIGHT MANUAL: CHAPTER 7
Rigging and Lifting
Rope and chain ....... ..................... ....... ..... ....... ...... ...... ........ ............ 7: 1
Wire rope .......................................................................................... 7: 1
Fibre rope ....... ..... ..... ............... .......... ...... .......... .......... ...................... 7: 8
Chains ............................................................................................... 7:11
Lifting aecessories .......................................................................... 7:12
End fittings and conneetions for wire rope ................................... 7: 12
Hoisting attachments ........................................................................ 7:14
Slings .............................................................................................. 7:17
'IYpes of slings .................................................................................. 7: 18
Estimating safe working load (SWL) ............................................... 7:20
Hoists .............................................................................................. 7:23
Blocks ............................................................................................... 7:23
Chain hoists ...................................................................................... 7:25
Moving a load ................................................................................. 7:27
Safuty ................................................................................................ 7:27
Workers' Compensation Board Regnlations .................................... 7:27
CHAPTER?
Rigging and Lifting
Millwrights frequently lift, move and access equipment in hard-to-reach
places. They must understand the required apparatus and be aware of how to
use it safely and maintain it properly.
Rope and chain
Many methods are used to lift and move equipment and machinery. They
range from a simple fibre rope with a hook on the end to wire rope on large
cranes. This section examines the use of various ropes and chains.
Wire rope
The lifting means most commonly used is wire rope. II is better adapted for
haulage and transmission than are natural and synthetic ropes. It is strong
MWM #1
and (unlike chains) gives good warning of failure when it is defective.
Inspect a wire rope before use to cheek that it is safe.
Construction of wire rope
Wire rope consists of small diameter wires wound around a small core to
create a strand. Strands (normally six) are wound around a steel or fibre core
to create a rope.
The criteria for the use of wire rope in a plant are:
•
MWM #2a
Crush resistance
- Steel-core rope resists crushing better than fibre-core rope.
•
Abrasion resistance
- Large diameter wires on the outside or contact layer have better
abrasion resistance than small wires.
MWM #2b
•
Flexibility
The more wires in the strand, the more flexible the rope will be.
MWM #2c
The more strands in the rope, the more flexible the rope will be.
•
Strength
- Steel core rope is stronger than fibre core.
MWM #2d
MILLWRIGHT-RIGGING AND LIFTING
For a given diameter rope, the fewer wires in a rope, the greater the
strength.
7-1
•
Lay
- Right-lay ropes are the most common style used.
- Left-lay ropes are used for special applications.
•
Environment
MWM #2e Galvanized rope must be used in salt water applications.
- Stainless steel must be used in a corrosive, acidic environment.
Grades
MWM #3
The breaking strength or breaking load of rope is the force needed to break
the rope. Grades of wire rope used in hoisting run from traction grade, with a
breaking load of 80 to 90 tons per square inch of wire material, to special
improved plow grade, with a breaking load of 120 to 130 tons per square
inch. Improved plow quality grade, with a breaking load of 110 to 120 tons
per square ineh, is the most commonly used grade in a mill.
Cores
A core is built into a wire rope to suppert the strands of the rope and
maintain the basic rope shape.
MWM #4a
•
Fibre cores (FC) are usually made from sisal, but can be made from
polypropylene Or other synthetic rope material if the wire rope will be
subject to weathering. On request, manufacturers can supply wire rope
with an oil-impregnated fibre core to self-lubricate the inner wires.
MWM #4b
•
Independent wire rope cores (lWRC) or wire rope cores (WRC) are used
when wire rope is subje~'1: to sudden heavy loads, crushing, or heat. In
smaIl-diameter wire ropes, the wire rope core is replaced by a strand core.
MWM #4c
•
Strand cores use another strand of the rope as core.
Lay
MWM #5
Lay refers to the direction in which the strands of a rope are twisted together.
The tenn lay length refers to the distance along the rope in which a strand
makes one revolution. Various rope lays are shown in Figure 1.
left lay lang lay
Right lay lang lay
left lay regular lay
Right lay regular lay
Figure 1 Different rope lays
7-2
MILLWRIGHT-RIGGING AND LIFTING
MWM #6
•
Right regular lay is the most common wire rope. This consists of a
number of wires twisted to the left around a small core to form each of
six strands. These strands are then twisted to the right around the main
core to form the final rope. Wire ropes of six strands are the most
Common
•
Left regular lay consists of wires twisted to the right in the strands, and
the strands then twisted to the left to produce the rope.
MWM #7 •
Lang lay (or Lang's lay) has the wires and strands twisted in the same
direction. The core design is the same as that of the regnlar lay.
Advantages of Lang lay are that it:
MWM #7a •
•
is more flexible than regular lay
has more area in contact with the drum spools or sheaves, thus
wearing longer
Disadvantages of Lang lay are that:
MWM #7b
•
both ends must be permanently fastened to prevent unwinding
•
it must not be used with a single-part lift
•
it must not be used with swivels
•
it cannot be used for slings
•
it does not resist crushing
Preformed wire ropes
MWM #8
Prefonned wire ropes have the twist or helix set in each wire and strand to
eliminate interuaI stress. Preformed ropes do not fly apart when cut. They
can be spliced without seizing (binding); when wires break, they lie flat in
position on the rope. Figures 2 and 10 show seizing.
Not preformed
Preformed
Figure 2 Wire ropes
MILLWRIGHT-RIGGING AND LIFTING
7-3
Wire rope classification
Wire ropes are classified by four factors (see Figure 3):
MWM #9
•
The number of wires per strand
•
The number of strands
•
The type of core
•
The lay
•
Figure 3 Common wire rope designs
For example, in the classification:
6 x 31 Warrington Seale, 1WRC Right, regular lay
•
6 is the number of strands
•
31 is the number of wires per strand
•
Warrington Seale indicates the configuration of wires in each strand
•
lWRC indicates the type of core used in the rope: Independent Wire
Rope Core
•
Right, regular lay specifies the way wires and strands are wound
Wire rope size
Wire rope is measured across its greatest diameter as shown in Figure 4a.
MWM #10
(a) Correct
(b) Incorrect
Figure 4 Measuring the diameter of wire rope
7-4
MILLWRIGHT-RIGGING AND LIFTING
Using wire ropes
Wire ropes, like the machines and hoists on which they are used, require
careful use, handling and maintenance for satisfactory performance, long life
and safety. When using wire ropes, you must choose the right rope, maintain
the rope properly, and use the rope and related equipment correctly. Observe
the following precautions:
MWM #11a
MWM #11b
A
Choose the right rope
•
Ensure that the correct size and type of rope is used.
•
Ensure tbat tbe rope is in good condition.
Maintain the rope properly
•
Inspect tbe rope regularly according to tbe manufacturer's guidelines and
WCB Regulations.
•
Discard damaged or kinked sections by cutting them out.
Once a kink has been made in a wire rope, the damage is permanent. A weak
spot remains no matter how well the rope is straightened.
•
To remove rope tbat is worn due to vibration, cut off a section next to
the anchorage and refasten the rope.
•
Lubricate the rope regularly according to manufacturer's
recommendations.
•
Store all unused rope in a clean, dry place, wbere it will be protected
from tbe elements.
•
Minimize uneven wear by moving the rope at regular intervals so that
different sections of it are at tbe critical wear points.
•
Cbange layer and crossover points by cutting a few feet of rope from the
drum end and refastening it. Make the cut long enough to:
cbange the layer at least one full coil from its former position
move tbe crossover points one-quarter turn around the drum
Use the rope correctly
MWM #11c
MILLWRIGHT-RIGGING AND LIFTING
•
Never overload tbe rope.
•
Avoid shock loading.
•
Apply tbe power smoothly and steadily.
•
Ensure that the rope winds properly on the drum.
•
Ensure that rope ends are properly seized.
•
Ensure wire rope has a run-in period before operating at full load and
full speed.
7-5
•
On equipment having multiple falls of other than non-rotating ropes, a
new rope will stretch and unlay slightly, causing turns to appear in the
load block. If the anchorage is not fitted to a suitable swivel, disconnect
it, remove the turns, and reconnect the anchorage.
Use related equipment correctly
MWM #11d
Proper maintenance of the equipment over which the ropes operate is an
important factor in rope life. Worn grooves, poor alignment of sheaves and
worn parts resulting in shock loads and excessive vibration will have a
deteriorating effect. See Figure 5.
Check flanges for wear
chips and cracks
proper fitting sheave groove
should support the rope over
135 to 150 degrees of rope
circumference
Incorrect
Correct
Check grooves
for proper size
Check sheave
grooves for wear
Figure 5 Checking sheaves for wire ropes
7-6
MILLWRIGHT-RIGGING AND LIFTING
•
Repair or replace faulty guides and rollers.
•
Inspect sheaves and replace those that have deeply worn or scored
grooves, or have cracked or broken rims,.
•
Use thimbles in eye fittings at all times.
•
If the sheaves or drums bear the imprint of the rope, they should be
machined clean or replaced with harder material.
•
Check sheave and roller bearings for free operation. Sticking will cause
unnecessary wear.
•
Check the fleet angle. See Figure 6. An excessive fleet angle will cause
severe abrasion on the rope as it winds onto the drum. This condition can
severely shorten rope life.
Head
sheave
\
\
II \
\
J
I
.
I I' \
Left
fleet
angle
I
Right
fleet
angle
Centre line
/ofdrum
Figure 6 Fleet angle
Lubrication
Lubricating wire rope is as important as lubricating any other piece of
machinery. Consult your rope manufacturer for lu bricants designed
especially for an operating rope or a standing rope.
MILLWRIGHT-RIGGING AND LIFTING
7-7
MWM #12
A
The lubrication ropes get during manufacturing is adequate for initial storage
and for the early stages of the rope's working life. However, it must be
supplemented regularly according to the rope manufacturer's instructions. If
it is not regularly lubricated, the rope will deteriorate rapidly in the following
ways:
•
The wires become embrittled from excessive corrosion and break easily.
(Embrittled means made more brittle by molecular change,
•
The wires in a rope are subject to frictional wear as they move together
during operation. Lack oflubrication increases wear and reduces
strength.
•
Ropes not in regular service or those not considered as operating ropes
are vulnerable to weathering. If the weathering wears through the
extemallubrication, excessive moisture can gradually leach out the
internal lubricant. This causes the core and wires to rust and deteriorate.
Always use the correct lubricant for wire rope.
Refer to manufacturers' specifications.
To lubricate a rope properly, do the following:
1. Ensure that the rope is clean and dry before the lubricant is applied.
2.
Compressed air
Use a light, penetrating cleaner to soften the built up grime before
removing it. Contact the lubricant supplier regarding the proper cleaning
oil to use. Do not use gasoline or kerosene, since too much of it will
remove the intemallubricant.
-e:=:::;:;:::::;;=:::;;=::J
Air blast drying
@
-- -- --
Bath of penetrating cieaner
Figure 7 Methods of cleaning wire ropes
3. The main objective in external lubrication is to fill any gaps in and
between the strands so that a complete seal is provided. The frequency
required depends on the particular installation.
4. Apply the lubricant using one of the following methods depending on
the viscosity of the compound applied, the length of rope involved and
the limitation of facilities.
7-6
MILLWRIGHT-RIGGING AND LIFTING
Preferred method
I since rope opens up
t oversheave
allowing oil to
penetrate
bath
ts~~~~ Oil
application
Brush or spray
application
~
Oil bath· application
Using splirwooden or metal boxes
Rope passes through burlap or similar
wiper at outlet end 01 box
Figure 8 Methods of lubricating a wire rope
4a. Light oils
MWM #13
• Apply light oils by brushing, running the rope through an oil bath,
spraying, drip method or mechanical force feed. See Figure 8.
o
For maximum penetration, apply the lubricant to the rope where it
"opens up" as it travels around a sheave or winds on a drum.
4b. Medium and heavy oils
Apply medium- or heavy-weight lubricants warm. Apply them by brush
or by running the rope through a fulmel containing the lubricant. You
may also use an air blast provided that only dry air is used and all
proper safety precautions are taken.
5. Even though brushing and dripping methods of lubricating long ropes
are tedious, they give you time to inspect the rope.
6. The rope should be properly lubricated at all times. Inspect the rope
often to see when it must be done.
Storing and handling wire rope
Store all wire rope rolled in a coil or on a spool, in a dry place away from
extreme heat and moisture.
MILLWRIGHT-RIGGING AND LIFTING
7-9
Unwinding from a spool
1. A quick way of mounting a spool is to use a length of pipe and some
blocking.
2. Do not take wire rope off the side of the spool a wrap at a time, as this
often results in kinks.
MWM #14 3.
When taking a length of wire rope from a spool, rotate the spool on a
spindle or turntable, or roll the spool along the floor.
Figure 9 Proper methods of removing rope from spools and coils
4. Before cutting a length of wire rope from a reel, "seize" or wrap each
side of the proposed cut as shown in Figure 10. Use friction tape, or, if
the end is to be welded, light wire. If one end of the wire is to go through
a hole in a drum, a good practice is to "braze" or weld the end and then
remove the seizing.
Seizing
I
Seizing
Cut
Figure 10 Seizing wire before cutting
5. If using an impact cutter to cut the wire rope, keep the cutter in the same
position throughout the cut.
6. After removing a length of rope from a spool, wind the slack back onto
the spool and tie down the free end.
Determining rope anchorage on a drum
Improper attachment on a drum will cause the loaded rope to climb over the
next wrap and pile up on itself. This results in excessive rope wear, usually
MILLWRIGHT-RIGGING AND LIFTING
concentrated in one or more places. To obtain good drum winding, the rope
must be started from the proper flange. Deciding which flange to use
depends on whether the rope is left-lay or right-lay, aod whether the rope
leads from the top or from the bottom of the drum. See Figure 11.
Standing behind the drum, observe the following:
Picture #1 •
Wind left-lay rope leading
from the top side of the drum,
starting at the right flange.
Picture #3 •
Wind left-lay rope leading
from the underside of the
drum, starting at the left
flange.
MWM #15 Picture #4 •
Wind right-lay rope
leading from the top side
of the drum, starting at the
left flange.
Picture #2 •
Wind right-lay rope
leading from the underside
of the drum, starting at the
right flaoge.
One way to remember how to
do this is to use the rule of hand
as shown in Figure 11. The
human haod represents the lay
of the rope either overwound or
underwound on the drum.
)
•
The right hand represents
right-lay rope.
•
The left hand represents
left-lay rope.
•
The index finger shows the
approach of the ropes.
•
The position of the back of
the hand indicates whether
the rope is overwound or
underwound.
•
The position of the
thumb shows where
the rope is attached
to the drum.
L_ _ R
Overwind
right to left
use left-lay rope
L
_R
Right
hand
I
I
~.{,~\~,
Underwind
right to left
Use right-lay rope
Overwind
left to right
Use right-lay rope
Start rope at
left flange
I
I
, L __ _
~
Overwind
left to right
Use right-lay rope
Right-lay-overwound
Figure 11 Rule of hand for the correct
attachment of rope to drum
MILLWRIGHT-RIGGING AND LIFTING
7 -11
Fibre rope
MWM #16
Hemp has been
replaced by Manila.
Hemp is stronger
though
MWM #17
Fibre ropes are divided into two broad groups: natural and synthetic fibres.
Natural fibre ropes
These are made from several different fibres. In decreasing order of strength.
they are hemp, sisal. jute, and cotton. Hemp is the natural fibre rope used
most commonly in industrial rigging and will be used in the following
descriptions.
The usual method of construction for fibre rope is reverse twisting. This
gives rope stability and keeps it from twisting under a strain. The method of
construction is as follows:
•
Fibres from 6 to 20 feet long are combed to bring them parallel to each
other.
•
A definite number of fibres are right-hand twisted to form a yarn.
•
A number of yarns are then left-hand twisted into a strand.
•
Finally. three or four of these strands are right-hand twisted to form the
rope.
This twisting pattern produces a right-lay rope. Reverse order of twisting
produces a left-lay rope (see Figure 12). Various other types of lay were also
discussed earlier in the section on wire ropes.
•
** Important
Drawing **
Fibres
'\ Yarn
'\
_
(made of fibres twisted to the right)
Strand
(made of yarns twisted to the left)
Right·lay rope
(made 01 strands twisted to the right)
Cross section of
a 318" fibre rope
Figure 12 Construetion of a reverse-twisted fibre rope
7-12
MILLWRIGHT-RIGGING AND LIFTING
The varying degrees of tightness of the twisted yarns and strands determine
whether it is a hard-laid, medium-laid (also called common or standard) or
soft-laid rope. Soft-laid rope has the greatest tensile strength but gives poor
service if run over sheaves, and it does not withstand abrasion well.
Synthetic fibre ropes
Synthetic fibre ropes are used extensively in industry, due to their strength,
shock-loading capacity, and resistance to natural weathering. They are made
of nylon, polyethylene or polypropylene. Most industrial synthetic fibre
ropes are the standard three-strand, right-hand-lay rope, but braided and
other special construction styles are also available.
Diameter natural fiber
must be larger than
synthetic to handle the
same load
On synthetic fibre ropes, finish all knots with an additional safety knot to
prevent slippage.
Nylon rope
Nylon rope is very strong and elastic. It is used where shock loading is
common or when a rope needs to be smaller than a hemp one but of equal
strength.
Nylon resists mineral oils and greases, but is affected by paint, linseed oil
and acids. Nylon rope becomes slippery when wet and loses a small part of
its strength, but it does not rot or mildew. Nylon is also the most expensive
of the common industrial synthetic ropes.
Poly ropes
Polyethylene, polypropylene and other materials of this group are used as an
inexpensive substitute for nylon rope. Poly ropes have the advantage of
buoyancy and are therefore used a great deal around water. The tensile
strength varies slightly among the poly ropes.
Braided synthetic rope
In recent years, the use of braided nylon ropes has been increasing steadily.
Braided nylon rope does not stretch as much as other types, and certain types
of braided rope are stronger than many laid ropes of the same fibre.
A synthetic rope that has a braided sheath over a core is soft, strong, and
flexible and does not twist or kink. The load is divided equally between the
sheath and the core so that, even if the outer sheath is damaged, 50% of the
rope's strength remains in the core.
Fibre rope size
Rope is measured by diameter or by circumference. This can be confusing
because sometimes ropes up to one inch are measured by the diameter and
ropes over one inch by the circumference. Note that a rope 1 inch in
diameter has a circumference of about 3 inches. Select your rope carefully.
AND LIFTING
7
13
Choosing and using fibre ropes
When seleeting rope for a lifting job, use charts to decide what you have.
Refer to manufacturer's specifications for the breaking strength of a rope.
A
•
Select the right type and size of rope for the job.
•
Apply whipping (seizing) before eutting the rope to the required length.
•
Apply loads with a steady strain. A sharp heavy jerk will break a rope
more readily than a steady pull.
Do not overload a rope. For new rope, the working strength is ONE FIFTH of
its breaking strength. For a used rope, increase this safety factor.
•
Store rope in a dry room away from moisture and any extreme heat. If
possible, hang a rope on a large wooden peg to ensure air circulation.
Dry rope thoroughly before storing it.
Knots for fibre rope
Many knots are used in lifting and hauling and to attach tag lines. You must
understand how to fonn the knots properly and which of them are suitable
and safe for partieuJar jobs. The most useful are described below.
Overhand knot
In addition to being the starting point
of many knots, an overhand knot is
frequently used as a stopper at the
end of a rope. See Figure 13.
Figure 13 Overhand knot
Figure-oJ-eight knot
A figure-of-eight knot is used at the
end of a line to prevent the end from
slipping through a fastening or loop
in another line. This should be used
to finish off other knots, especially
when using synthetic ropes. See
Figure 14.
~17mmwam
Figure 14 Figure-of-eight knot
Square or reeJ knot
A
7
14
A square or reef knot is used for tying together two lines of the same size so
that they will not slip. See Figure 15.
Do NOT use the granny knot or the thief knot. Both of them can slip.
MILLWRIGHT-RIGGING AND LIFTING
Knot in a rope will
cut the rope strength
by 50%
Bowline used for rescue
Figure 15 Square or reef knot
Bowline
A bowline is the knot most
commonly used for forming a loop in
the end of a line. It is easy to tie and
untie, will not slip. and can be used to
secure loads without crushing them.
See Figure 16, at left.
Bowline-on-the-bight
A bowline-on-the-bight is used for
putting a non-slip loop anywhere
between the ends of a rope. See
Figure 17. at right.
Figure 16 Bowline
Figure 17 Bowline-on-the-bight
MILLWRIGHT-RIGGING AND LIFTING
7-15
Round turn and two half-hitches
A round turn and two half-hitches
is one of the most efficient and
most used knots. It can be used
wherever a line must be made
fast. See Figure 18.
Figure 18 Round turn and two half-hitches
Becket hitch
A becket hitch is used for making a line
fast to the becket of a block or to a ring.
See Figure 19.
Figure 19 Becket hitch
Chains
Chains are made of a series of interconnected links. Each link is made of
wire or rod bent in an oval shape and welded together at one side. The
diameter of the wire or rod determines the chain size.
Use chain for hoisting ONLY when no other method of slinging or rigging is
available. Use chains in hoisting operations ONLY when their ability to
withstand high temperatures and abrasion is require.
A
"A" or "T"
for over head
grade
Chains can break without warning. Only one link in a chain needs to break
for the load to drop.
Several different grades of chain are
available, but the only grade
acceptable for overhead hoisting is
grade A. Each link must bear an "An
stamped into its surface as illustrated
in Figure 20.
Figure 20 Grade A link
Chains used as slings
should be supplied with
a master ring at one end
and a hook at the other
as shown in Figure 21.
Figure 21 Chain with master ring and hook
7 -16
MILLWRIGHT-RIGGING AND LIFTING
The large master ring is designed to
fit over a crane's main hook. Then
either the chain's own hook is hooked
directly to the load, or the chain is
wrapped ar01llld the load and its hook
secured to the master link as shown in
Figure 22.
Chains show signs of wear at the
bearing surfaces of each link. When
the amount of wear equals 5% of the
link diameter for chains less than 1",
or 10% for chains over 1", stop using
the chain for hoisting. Wear will
show up as illustrated in Figure 23.
Figure 22 Using a chain to
hang a load from a hook
Figure 23 Worn link
Lifting accessories
End fittings and connections for wire rope
Safety requires knowing how to:
•
select the correct fittings and connections
•
properly install them
•
evaluate their safe load capacity
It is extremely important that all fittings be of adequate strength for the
application. Whenever possible, use load-rated fittings. This means that safe
working load is stamped on the fitting. For overhead lifting, use only weldfree forged fittings.
MILLWRIGHT-RIGGING AND LIFTING
7-17
Poured sockets
Zinc (or spelter) sockets are standard drop-forged sockets. They are
pennanent terminal attachments for wire rope. They are most commonly
used to secure cables onto passenger or freight elevators. When properly
attached, they are 100% efficient. Epoxy resins are also used for poured
sockets. Figure 24 shows thc formation of a zinc socket.
5
3
Figure 24 Formation of a drop-forged zinc socket
Only trained and properly qualified personnel should make these
connections since it is a skill that requires good facilities and a thorough
understanding of the manufacturer's instructions.
Also called
Wedge Capping
Cappel sockets
A cappel socket is shown in Figure 25. When properly installed and
frequently inspected, cappel sockets also give 100% efficiency. Their
efficiency depends entirely upon the wedges being kept tight.
Figure 25 Cappel socket
Cappel sockets also allow easy, frequent inspection of the whole section of
rope where it is gripped.
Wedge sockets
Wedge sockets (Figure 26) are intended for on-the-job attachment and for
quick rope replacement. Their principal advantages are ease and speed of
applying and detaching. They are used to secure a ball or hook on a mobile
crane. The efficiency of a wedge socket is low-only 70% of the strength of
the rope.
When installing a rope in a wedge socket, it is important to secure the dead
end properly as shown in Figure 27.
7 -18
MILLWRIGHT-RIGGING AND LIFTING
rn.c
COl
**Important**
on IP exam
Whole page
.2"
-0
Oc
Z"
Figure 26 Installing a wedge socket on a rope
The clip will
transfer the
load to the
dead end
of the rope
Figure 27 Proper method of securing dead end of rope at a wedge socket
MILLWRIGHT-RIGGING AND LIFTING
7-19
Swaged sockets
Swaged sockets are also pennanent terminal attachments for wire rope. See
Figure 28. They are made by compressing a steel sleeve over the rope with a
hydraulic press. Properly made, they provide 100% efficiency.
If you see one broken wire, that is enough to condemn the rope section.
U-bolt
Saddle
Figure 28 Swaged sockets
Eyes and thimbles
Eyes in various forms are frequently used as wire-rope end attachments.
With the exception of some slings, all eyes must include rope thimbles to
maintain rope strength and to reduce wear. If a thimble is not used on a
spliced eye, the efficiency of the connection can be reduced by as much as
10% because the rope flattens under load. See Figure 29.
When there is no thimble
in the eye the rope will
flatten, crush and wear
With a thimble in the eye
the rope is completely
protected and retains
Its shape
Figure 29 The use of thimbles in eyes
There are great differences in the efficiencies of eye fonnation but little
difference in appearance.
The weB requires the identification of each eye formation with tags.
7-20
MILLWRIGHT-RIGGING AND LIFTING
Each rope manufacturer attaches a different name to their particular type of
eye. They are usually variations of the types shown in Figure 30.
Swage
sleeve
Figure 30 Common eyes
Cable clips
The most common method used to make an eye or attach a wire rope to a
piece of equipment is with the use of clips (Crosby clips) or clamps.
Various types include (see Figure 31):
•
U-bolt (saddle) clips
•
double-saddle (safety or fist-grip) clips
•
double-base clamps
Figure 31 Cable clips
They are easy to examine and, when installed according to manufacturers'
specifications and current WCB regulations, they have 80% of the rope
strength. All clips must be of drop-forged steel. Double-saddle clips arc
preferable to U-bolt clips because they cause less damage to the rope. The
number of clips required is determined by the rope diameter. Refer to
manufacturers' specifications aod WCB Regulations.
Still greater efficiency can be obtained by the use of long, double-base
clamps. These give a greater clamping force on the rope without damaging
it. This increases rope life and safety.
MILLWRIGHT-RIGGING AND LIFTING
7-21
There are right- and lefthand clips. Take care to fit
right hand clips to right-lay
rope and left-hand clips to
left-lay rope.
Never saddle the
dead end, use U-clamp
on dead end
Improper application of
even one clip can reduce the
efficiency of the connection
to 40%.
Apply the second clip
Figure 32 shows the correct
method of installing cable
clips.
Diameter of the wire rope x 3 +1= # of clips
Always round up.
Apply tension and tighten clips
6 x the wire rope diameter = clip spacing
tfIfI/!:.~
Apply
tension
Figure 32 Proper method of installing cable clips
1. Always put the U-bolt section of the clip on the dead or
short end of the rope.
2. Never use any kind of clip to directly connect two straight
lengths of rope.
If you need to connect two ropes end-to-end. use the clips to form an eye
(with thimble) in each length and connect the eyes with a shackle as shown
in Figure 33.
Incorrect
Correct
~
Figure 33 Connecting two ropes end-to·end with cable clips
7-22
MILLWRIGHT-RIGGING AND LIFTING
Hoisting attachments
Hooks
Various hooks are available for hoisting
and rigging operations. Several safety
procedures apply to all hooks:
l+.c-Check for wear
and deformation
1. They are forged alloy
_Check for signs
~ of "opening up"
steel and generally are
stamped with their rated
safe working loads.
~
¥-cl--Safety catch
Mousing the hook
is wrapping wire around 2. Make sure that all hoisting
hooks (except grab and
the hook to act as a
sorting hooks) are equipped
safety clip
with safety catches as shown
......._...-:7 ~ CheCk for wear
and cracks
in Figure 34.
Figure 34 Hook inspection areas
3. Inspect all hooks
Standard
frequently (see
Figure 34).
• Look for wear in the
saddle of the hook.
• Look for cracks,
severe corrosion and
twisting of the hook
body.
Adjustable sliding
• Measure the tlrroat
opening. If there is
any evidence of
throat opening or
distortion, destroy
the hook.
4. Commonly used types
of choker hooks are
standard, adjustable
sliding, and dual sliding
as shown in Figure 35.
Dual sliding
5. Make sure the loads are
balanced on the hook
(see Figure 36 on the
next page).
Figure 35 Commonly used choker hooks
MILLWRIGHT-RIGGING AND LIFTING
7-23
BaJanced load
t/2 off centre
Point loading
Load
Load
Load
Can carry
Can carry
Can carry
approx. 40%
tOO%.
of rated load
apprO)(. 80%
of rated load
of rated load
Figure 36 Balanced and unbalanced loads on a hook
It is recommended that all hoisting hooks be equipped with swivels and
headache balls. Ensure that the headache ball is securely attached to either
the hook or the rope (see Figure 37).
Q
Figure 37 Hoisting hook hardware
Shackles
Two types of shackles are commonly used in rigging. They are the anchor
(bow-type) shackle and chain (D-type) shackle both of which are available
with screw pins or round pins (see Figure 38). Shackles are sized by the
diameter of the steel in the bow section.
7-24
MILLWRIGHT-flIGGING AND liFTING
Screw pin anchor shackle
Round pin anchor shackle
Safety type anchor shackle
Screw pin chain shackle
Round pin chain shackle
Safety type chain shackle
Figure 38 Typical shackles
When using shackles. take the following precautions:
•
All pins must be straight and all screw pins must be completely seated.
Cotter pins must be used with all round pin shackles. See Figure 38.
•
Never replace the shackle pin with a bolt; use only the proper fitted pin.
Bolts are not intended to take the bending that is normally applied to the
pin.
•
Destroy worn shackles. See Figure 39.
•
When using a screw pin shackle. secure the pin with wire through the
hole to the shackle body to prevent the pin from unthreading (turning
out). See Figure 39.
..{:.~~~~-- Check for wear
Check for wear and straightness
10% of wear shown,
shackle must be
destroyed
Check that pin is always seated
Check that shackle is not"opening up"
Figure 39 Screw-pin shackle inspection areas
•
MILLWRIGHT-RIGGING AND LIFTING
Never allow a shackle to be pulled at an angle Centralize whatever is
being hoisted on the pin by using suitable spacers. See Figure 40.
7-25
•
Do not use screw pin shackles if the pin can roll under load and unscrew.
See Figure 41.
Hook
Figure 40
Figure 41
Eccentric shackle loads
Do not use screw pin shackles If the pin
can roll under the load and unscrew
Eye bolts
SWL=
Safe Working Load
All eye bolts used for hoisting should be of forged alloy steel and equipped
with shoulders or collars (shoulder-type eye bolts). The plain (shoulderless)
eye bolt is fine for vertical loading but as soon as it is loaded at an angle, the
safe working laad (SWL) is severely reduced. See Figure 42.
Note: SWL for plain eye bolts are the same as for shoulder-type eye bolts
under vertical load. Angular loading is NOT reconunended.
Incorrect
Correct
Results in
Figure 42 Use of plain eye bolts
Even with shoulders, the safe working loads (SWLs) of eye bolts are reduced
by angular loading. When installing a shoulder-type eye baIt, make sure that:
•
the shoulder fmnly contacts the working surface (you may need to use
washers)
•
the nuts are properly torqued
**Important** •
•
7-26
the tapped hole for the bolt has a minimum depth of one and one-half
(1.5) times the bolt diameter
the thread in the tapped hole and on the bolt are in good condition
MILLWRIGHT-RIGGING AND LIFTING
Loading
Incorrect
y"
/Pack with
washers
Shoulder must
be in full contact
with surface
Ensure that
lapped hole is
deep enough
Nut must be
properly torqued
Figure 43 Use of shoulder-type eye bolts
When using an eye bolt to lift, keep bending to a minimum. Always apply
the loads to the plane of the eye, especially when bridle slings are used.
See Figure 44.
i····
Correct orientation
Incorrect orientation
M
•••• '"
•
Figure 44 Orientation of eye bolts
When lifting with
eyebolts, always use a
shackle and ensure that
the lifting angle is above
45°. Also, make sure
that the working loads
are safe.
Above 45·
\
90 degrees is perfect
............
_ - --'--'--
Figure 45 Lifting with eye bolts
MILLWRIGHT-RIGGING AND LIFTING
7-27
Where eye bolts cannot be kept in line with each other when tightened, insert
thin washers or shims under the collars. This allows you to align the eye
bolts while maintaining proper tightness. See Figure 46.
Eye correctly
aligned
Tightened eye 90·
out of alignment
~~
Figure 46 Alignment of eye bolts
Turnbuckles
Turnbuckles may have end fittings that are eyes, jaws, stnbs, hooks, or any
combination of these. (See Figure 47.) Their rated loads depend upon the
outside diameter of the threaded portion of the end fitting and on the type of
end fitting. The jaw, eye and stub types are rated equally and the hook types
have reduced ratings. All turnbuckles used in hoisting or rigging operations
should be of weldless construction and fabricated from alloy steel.
Eye
Jaw
Stub
I
Hook
(Has reduced
capacity)
Figure 47 Turnbuckle end fillings
When using a turnbuckle with hook end fittings, apply mousing to any hook
without a safety catch. To mouse a hook means to wire the throat of the hook
closed to prevent rigging from slipping off.
If the turnbuckle is to be used where there is vibration. I'I'IS
EXTRFMELY IMPORTANT TO WCK THE FRAME 117 THE END Fl1TINGS
to prevent it from turning and loosening.
7-28
M1LLWRIGHT-RIGGING AND LIFTING
Use a lock-wire to ensure that the turnbuckle will not loosen. Lock nuts or
jam nuts are not effective and add greatly to the load in the screw thread.
See Figure 48.
Incorrect
Do not use jam nuts
Correct
Use lock wire
\
~I_"
True turnbuckle
has on left hand and
one right hand thread
Figure 48 Securing of tumbuokle end fittings
When tightening up a turnbucklc. apply the same torque to it as you would to
a bolt of equal size. Inspect turnbuckles frequently for cracks in the end
fittings, especially at the neck of the shank. Also check fur deformed end
fittings, deformed and bent rods and bodies, cracks and bends around the
internally threaded portion. and signs of thread damage. See Figure 49.
r::T··················
-
Check for cracks
and bends
Check for thread damagE
and bent rods
~-----r-~ ChecK for cracks
and deformations
d
d
Q!;
19
Figure 49 Tumbuckle inspeotion areas
Refer to manufacturer's specifications for the correct SWL of turnbuckles.
Spreader and equalizer beams
Spreader beams are usually used to support long loads during lifts. See
Figure 50 on the next page. They remove the risk of the load tipping. sliding
or bending. They also remove the possibility of low sling angles and the
tendency of the slings to crush the load.
Equalizer beams are used to equalize the load on sling legs and to keep equal
loads On dual hoist lines when making tandem lifts. They are fabricated to
suit a specific application and must meet WeB specifications.
The load capacity of beams with multiple attachment points depends upon
the distance between the points. For example, if the distance between the
attachment points is doubled. the capacity of the beam is halved.
MILLWRIGHT-RIGGING AND LIFTING
7-29
Spreader beam
Equalizer beams
Figure 50 Spreader and equalizer beams
Slings
Most items that require hoisting have no provisions for attaching the load to
the lifting device. Slings serve to connect the load to the lifting device.
To protect both the load and the sling, place padding or soft comers
(prefonned metal brackets) between the sling and any sharp comers on the
load. See Figure 51.
Load
Wood
Metal bracket
.
(soft corner)
'': /
/
Sling
Figure 51 Padding sharp-edged loads
~Wr)B¥%~WlX'W!'I'!!!,"Wf!')!ff!!'7~f'kx§70:iiit!~~Bwi~:0:iJ:jtk:j;tiijj;jj!j;(itj"fJb\~~WiUi"i!;"W#RifJi?X0it'&PHrrnJ%ffifil1HIf0110A;0Sfo%'$'?1ffZfit:?!J&,u";i/fI#D1!'P)JN1510ffi4W;!;§i)j,:W;::&413";U;>;;J;W:;/,}f,!WCX;);:/¥t;':Ibi!'!Vfi'M0;!{@f
7 - 30
MILLWRIGHT-RIGGING AND LIFTING
Types of slings
o
Slings may be made of fibre rope, wire
rope, chain or webbing. Slings may be
attached to loads in a variety of
configurations, each with its OWn name.
A
The following simplified diagrams
do not show padding, but you must
remember to use it in practice.
Single vertical bitch
The single vertical hitch consists of a
single leg of sling matcrial with a hook
and an eye at each end,
Figure 52 Single vertical hitch
Bridle bitch
Two, three, or more legs may be used together to form a bridle hitch Bridle
hitches are generally used on loads which provide suitable attachment points.
The load will be stable jfthe attachment points are above the load's centre of
gravity.
When a bridle hitch has more than two legs you cannot assume that all legs
are sharing the load equally. Regardless of the total number oflegs, the full
weight of the load might be shared by only two legs. The other legs may
simply be balancing the load as illustrated in Figure 53.
Carrying fuUload
\
FlgUN 53 Bridle hitches
Single basket hitch
Single basket hitches are made from a single length of sling material passed
through a load. Both ends of the sling attach to the main hook. Do not use
single basket hitches on loads which could tilt and slide out of the hitch.
MILLWRIGHT-RIGGING AND LIFTING
7-31
Uses independent wire core
for wire rope being used
for single basket
Figure 54 Single basket hitches
Double basket hitch
Loads which require support from underneath can be lifted with a double
basket hitch (see Figure 55). Locate the double basket hitch so that the load
is balanced between the two points of support. Keep the two support points
far enough apart so that the load cannot tip and slide out.
60° or more
Figure 55 Double basket hitch showing angle of sling legs
NEVER incline the legs of a double basket hitch less than 60° to the
horizontal. This will prevent the legs from sliding towards each other.
Use longer slings to spread the legs apart without having the legs at an
excessively low slope.
Theoretical Best
option for loose loads
7-32
Double-wrap basket hitch
Even loose loads can be securely rigged for hoisting with double basket
hitches. To do this, wrap the sling completely around the load. This double
wrapping presses all the components together, preventing even the top pieces
from sliding out of the rigging. See Figure 56.
MILLWRIGHT-RIGGING AND LIFTING
Figure 56 Double wrap basket hitch securely holding a load
Single choker hitch
The single choker hitches illustrated in Figure 57 are made with a single
length of sling material hooked back to itself just above the load.
'\
Figure 57 Single choker hitches
. There are several methods of securing one end of the sling back into itself to
form the choker, but nonc will totally secure the top ofloose loads.
Double choker hitch
Long loads or loose loads which need
to be lifted level may be hoisted with
double choker hitches as illustrated in
Figure 58.
Figure 58 Double choker hitches
MILLWRIGHT-RIGGING AND LIFTING
7-33
Double-wrap cboker hitch
The double-wrap choker hitch is similar to the double-wrap basket hitch in
that both squeeze the load from all sides. Double-wrap choker hitches may
be used singly or in pairs, as illustrated in Figure 59.
Figure 59 Double-wrap choker hitches
Endless slings
Endless slings (also known as
grommet slings) can be used in
a variety of configurations as
illustrated in Figure 60.
Endless slings are usually made
of fibre rope or synthetic
webbing. They are light to
handle and do not damage the
loads, but because they are
subjected to sharp bends, they
tend to deteriorate more rapidly
than most other types of slings.
Figure 60 Endless slings
Synthetic web slings
Synthetic web slings are available in a variety of shapes and widths. Their
relative softness and width protect the loads they are lifting from being
marred or scratched by the sling. Heat and friction damage synthetic slings.
The shapes most commonly found are shown in Figure 61.
7-34
MILLWRIGHT-RIGGING AND LIFTING
E--=~==J
Endless
Standard eye
Twisted eye
Figure 61 Synthetic web slings
Some web slings have metal end fittings instead of sewn eyes. Two types are
available:
•
A basket web sling has metal triangles of equal size at each end of the
webbing (see Figure 62).
•
A choker web sling has a larger triangle containing a slot at one end and
a smaller triangle at the other end. The smaller triangle can be passed
through the slot of the larger triangle to form a choker hitch (see
Figure 63).
Figure 62 Basket web sling
Figure 63 Choker web sling
Metal mesh slings
Loads which are too abrasive or too hot for synthetic webbing, yet require
the wide bearing surface of a web belt, are rigged with slings made of metal
mesh. These metal mesh slings are usually equipped with triangle ends
which permit the use of either a basket or choker hitch as shown in
Figures 62 and 63.
MILLWRIGHT-RIGGING AND LIFTING
7-35
Estimating safe working load (SWL)
You must consider the safe working load (SWL) of a sling before attempting
any lift. Manufactured slings have their SWLs listed on tags. You must
calculate SWL for slings built on the job, and never exceed them when
lifting.
Safe working loads of rope
Manufacturers rate their ropes by their breaking strength. When using the
rope to hoist, you must not approach this limit. If you are lifting materials
and equipment, they must be no heavier than one-fifth of this breaking
strength. If you are lifting people, they must not weigh more than one-tenth
of the breaking strength.
This SWL is for rope used as a single vertical hitch to lift MATERlALS,
not people.
If you do not know the breaking strength of a rope, use one of the following
simple rules to calculate its SWL.
Wire rope
A rule of thumb for calculating the SWL of wire rope is that 3/s-inch
diameter rope will support I ton (2000 lbf). See note* below. Each 'Is -inch
increase in diameter is equal to a I ton increase in SWL.
Example 1
Rope diameter rope = '/2" = 3/S" + '/s"
SWL = I ton + I ton
SWL = 2 ton = 4000 lbf
If you use either fibre rope or wire rope to make slings you must remember
the following:
This manual uses the
correct abbreviation lbf
for the imperial unit of
poundsforce. This
imperial unit is for aU
forces including weight,
which is a force due to
gravity's effects.
•
A knot can reduce the breaking strength of fibre rope by up to 50%.
•
Cable clips correctly attached to wire rope reduce its breaking strength
by 20%.
•
Sharp corners of a load should have padding or soft corners in place to
reduce abrasion on the rope.
Safe working loads of slings
The SWL of any sling material (rope, webbing, etc.) refers to the load which
that sling can safely lift while the sling is used as a single vertical hitch (with
only one leg). We will refer to this as SWL,.
(The abbreviation lb refers
to mass, rather than
weight. See Chapter 2.)
7-36
MILLWRIGHT-RIGGING AND LIFTING
Basket hitch
Willie Nelson
loves hookers
(W/N) x (L/H)=Tension
W=Weight
N=# of Legs
L=Length
H=Height
If this sling is used in a basket hitch
configuration with the legs of the
basket hitch inclined, as shown in
Figure 64, the SWL is as follows:
SWL= SWL, x (H+L)x 2
In the formula. H refers to the vertical
distance between the hook and the
load (Figure 64). L refers to the
length of the sling legs
Figure 64 Basket hitch with Inclined legs
If the sling shown in Figure 64 has an SWL rating
Example 2
of 2000 lbf when used as a single vertical hitch,
the distance H is 3 feet, and the length of L is
5 feet, calculate the true SWL of the sling.
SWL= SWL, X 3/5 X 2
SWL = 2000 lbf X 3/5 X 2
SWL = 2400 lbf
Two-leg bridle hitch
Two-leg bridle hitches use the same formula as above, providing that both
legs of the bridle hitch are equal in length.
You may need to calculate the SWL of bridle hitches with legs of unequal
length or with load attachments of unequal height, as shown in Figure 65. In
this case, use the SMALLEST height or length as H or L in the same formula.
Do not use these
measurements ~
Use this height as H
Use this length as L
Figure 65 Bridle hitch with lags 01 unequallanglh
MILLWRIGHT-RIGGING AND LIFTING
7-37
Choker hitches
Q
Single choker hitches use a similar
formula except that the ratio is never
less than 3/4.
•
If the choker angle is over 45'
(as in Figure 66), use the formula:
Choker angle
SWL = SWL1 X 3/4
•
If the choker angle is 45° or less,
(as in Figure 67) use the formula:
SWL =SWL I X AlB
Figure 66 Choker angle greater than 45°
Choker angie
Figure 67 Choker angle less than 45°
Chokers are often used in pairs to lift loads in a horizontal position as shown
in Figure 68. To calculate the SWL of such sling configurations use the
following formula:
Figure 68 Pair of choker hitches
7-38
MILLWRIGHT-RIGGING AND LIFTING
Double basket hitches
Double basket hitches are self adjusting in the sense that each leg carries its
share of the load. Therefore. to calculate the SWL of the double basket hitch
illustrated in Figure 69, use the formula:
SWL = SWL1 X Il/L X 4
Figure 69 Double basket hitch
Estimating the weight of lifted objects
You can use simple rules of thumb to calculate the approximate weight of
most objects that must be lifted. The rules are based On the approximate
densities of the objects:
For steel use 490lbf •
•
The approximate weight of a cubic foot of steel is 500 Ibf (the precise
weight is 489.6 Ibf).
The approximate weight of a cubic foot of wood is 35 lbf.
Exnmple 3
Estimate the weight of a stcel block 6 inches high,
2 feet wide and 3 feet long.
Volume of the block is OS X 2' X 3' = 3 ft'
Approximate weight of the block
= 3 X 500 Ibf'" 1500 Ibf
Hoists
Blocks
Blocks are used to lift heavy loads using a small amount of effort. The ratio
of load to effort is called the mechanical advantage of the block.
The blocks used in construction rigging range from the custom-<iesigned,
400-ton capacity units through all types of crane and hook blocks, to wire
rope blocks and snatch blocks, down to the simplest of tackle blocks.
MILLWRIGHT-RIGGING AND LIFTING
7
39
The essential parts of any block are a shell, the sheaves, a centre pin, the
straps, and a becket. See Figure 70.
*Important to
know the parts
on this drawing*
Figure 70 Typical wire rope block
The shell provides protection for the sheave or sheaves and acts as a guide to
keep the rope in the sheave groove. Steel shells are used on all blocks with
wire rope. They add strength, rigidity and protection for the block.
The sheaves transmit the load iruposed by the rope to the centre pin, straps
and connections. On fibre-rope blocks the sheaves are normally cast iron. On
wire-rope blocks they should be cast steel because of its greater strength,
hardness and abrasion resistance. Fibre-rope sheaves are NOT used with wire
rope because their diameters are too small.
Never run fibre ropes over sheaves that have been used with wire ropesrapid rope damage will result.
The centre pin, sometimes called the sheave pin, transmits the sheave load to
the strap. It is prevented from turning by a keeper.
The straps and/or cheek (side) weights transmit the sheave load to the
connections and add rigidity to the block.
They can be equipped with various combinations of end fittings including
hooks, wedge sockets, clevises, shackles, and swivels of all types. The
blocks may also be equipped with a becket which is the anchorage point for
a rope-end.
7-40
MILLWRIGHT-RIGGING AND LIFTING
Block types
Blocks usually take their names from the purpose for which they are used,
the position they occupy, or from a particular shape or type of construction.
They can be designated according to the number of sheaves they have
(single, double. triple, etc.) Or in accordance with the shell shape (diamond
pattern, oval pattern, etc.).
For the millwright. the most commonly used types of blocks are:
•
crane and hook blocks
•
wire-rope blocks
•
tackle blocks
•
snatch blocks
Standing block
All these blocks except the snatch
block can also be classified as (see
Figure 71):
•
A travelling or fall block is a
block that is attached to the
load being lifted or moved. It
moves with the load.
•
A standing block is a block
that is fixed to a stationary
object. This object takes the
force of the loading.
Travelling or fall block
Figure 71 Standing and travelling blocks
Crane-and-hook blocks and wire-rope blocks
Crane-and-hook blocks are equipped
with hea\ly iron cheek weights (see
Figure 72). Wire-rope blocks are
normally much lighter (see
Figure 73).
Cheek
weights
Both types are well suited to high
speed applications and heavy loads.
However, wire-rope blocks are not
intended to withstand the heavy
service and abuse expected of the
crane and hook blocks.
Figure 72 Crane and hook block
MILLWRIGHT-FUGGING AND LIFTING
7-41
Figure 73 Wire rope block
Figure 74 Tackle block
Tackle blocks
Tackle blocks are used in conjunction with fibre ropes, both natural and
synthetic fibre. They are similar to wire-rope blocks but are lighter weight
and have less eapacity. See Figure 74. The sheaves in the blocks should be of
the proper size for the rope being used and must be free and well lubricated.
Snatch blocks
A snatch block can be a single- or multiple-sheave block. It opens on one
side to permit the rope to be slipped over the sheave so that it does not need
to be threaded through the block. See Figure 75. They are normally used
when it is neeessary to change the direction of the pull on a line.
When closed
Double sheave
When open
Figure 75 Snatch blocks
7-42
MILLWRIGHT-RIGGING AND LIFTING
As the direction ehanges, the angle between the lines inereases, the stress on
the block and hook is reduced as illustrated in Figure 76.
2000 pounds
1000 pounds
1840 pounds
1000 pounds
1410 pounds
760 pounds
)
1000 pounds
~900
Figure 76 Variation of snatch block loads with rope angle
Chain hoists
There are four types of ehain hoists: the spur-geared, the serew-geared, the
differential, and the pull-lift types. See Figure 77. The first three types are
used for hoisting, while the pull-lift type is used primarily for pulling in a
horizontal direction.
•
For frequent use and minimum effort, the spur-geared hoist most often
used because it is the most efficient.
•
Where the hoist is not used very often, the screw-geared hoist is
commonly used.
MILLWRIGHT-RIGGING AND LIFTING
7
43
Spur-geared
chain hoist
•
For very infrequent use and where light weight and low cost are
important, the differential hoist is used.
•
For pulling horizontally, either a screw-geared chain hoist Or a special
lever-operated pull-lift hoist is used. Only one person should pull on this
type of hoist because it is easy to overload it.
Screw-geared
Differential-type
chain hoist
chain hoist
Lever-operated.
pull-lift chain hoist
Figure 77 Typical chain hoists
Only forged steel should be used for hoist parts that are subject to stress,
such as the hooks, swivels, chain. sprocket, gears, and similar parts.
All chain hoists are designed with their lower hooks as the weakest parts. If
the hoist is overloaded it is first indicated by the spreading or opeuing up of
the lower hook.
A
7-44
Do not interchange the two hooks on a chain hoist.
MILLWRIGHT-RIGGING AND LIFTING
Inspection and overhauls
You may use a set of test gauges as shown in Figure 78 to inspect the load
chains and hooks of chain hoists.
Figure 78 Test gauges for chain holst
If there is evidence of severe overloading, you should thoroughly inspect all
parts of the hoist.
A
To inspect a chain hoist, examine the hooks and chain very thoroughly.
To do overhauls:
•
either use the parts specified by the manufucturer
•
or send it to a qualified repair shop.
Load chains
Use the opeaing at the wide end of the gauge to check that a chain link has
the proper gauge. Then, with the ehain hanging freely, attempt to insert the
small end of the gauge between the links as illustrated. If the gauge is too
wide, it indicates that the links have stretched and narrowed hecause of
overloading.
MILLWRIGHT-RIGGING ANO LIFTING
7-45
•
If the gauge enters the link up to the first step stamped "OK", it indicates
little or no wear on the links.
•
If it enters to the second step stamped "R", there is evidence of
considerable wear and/or stretch. The chain hoist should be thoroughly
inspected and overhauled.
•
If the gauge enters the link to the third step stamped "C", it shows
excessive wear and/or stretch. Replace the chain and overhaul the hoist.
Hooks
The wide end of the gauge is used to caliper the opening of the hoist hook. If
the opening is the gauge, destroy and replace the hook. The size stamped on
the gauge must agree with the capacity of the hoist.
Excess oil
Occasionally in a manufacturing plant a chain hoist is used for lowering
material into oil baths or for holding material while it is sprayed with oil.
This may cause the load chain to be coated with too much oil. When the
hoist is operated, the oil is transferred to the sprocket and may eventually
find its way into the load brake, thus reducing its holding power. For service
such as this, use the screw-geared or differential hoist.
Moving a load
Safety
Routine maintenance often involves lifting or moving heavy parts and
equipment. Before doing this, always consider the following factors:
•
the characteristics (weight, shape, centre of gravity, etc.) of the object to
be moved
•
its present location
•
the location it will be moved to
•
the equipment available to help you move the object
•
the safety precautions which may have to be in place for the move
•
the path of the move
The weight of the object to be moved determines the type and size of lifting
tackle used. Pay careful attention to the overall dimensions and to the
dimensions of the mating parts. The pathway and the final resting place must
accommodate these dimensions.
Hand signals
Ensure that only one signaller is used when making a lift or move. If the
signaller and hoisting-equipment operator cannot see each other, a relay
person or a two-way radio may be used.
7-46
MILLWRIGHT-RIGGING AND LIFTING
Figure 79 shows a few of the most commonly used hand signals. A complete
list of hand signals is given in Appendix D of the WeB' s Industrial Health
and Safety Regulations.
Stop
Hoist load slowly
1
t
Hoist load
Travel
Lower load
Dog everything
Figure 79 Commonly used standard hand signals
MILLWRIGHT-RIGGING AND LIFTING
7-47
Workers' Compensation Board Regulations
Read Industrial Health and Safety Regulations, Workers' Compensation
Board (WCB), Province of British Columbia.
7-48
•
Section 30, articles 30.02 through 30.16: Ladders
•
Section 32, articles 32.02 through 32.82: Scaffolding
•
Section 54, articles 54.02 through 54.22: Rigging
•
Section 56, articles 56.02 through 56.100: Cranes and hoisting
•
Appendix D: Hand signals
MILLWRIGHT-RIGGING AND LIFTING
Shafts and Attachments
Keys and keyseats ........ ............................ ....... ...... .......................... 8: 1
TYpes of keys.. ......................................................................... 8:2
Keyseats in shafts .................................................................... 8:7
Keyseats in attachments..... ....... ........ ............ ........ ....... ............ 8:9
Standard sizes of keys and keyseats ........................................ 8: 10
Cutting keyseats ....................................................................... 8:11
Installing keys .......................................................................... 8: 14
Fitting keys .............................................................................. 8: 15
Securing components and keys ................................................ 8:16
Removing keys ........................................................................ 8:20
Shafts .............................................................................................. 8:22
TYpes of shafting ..................................................................... 8:23
Uses of shafts ........................................................................... 8:25
Identifying shafting .................................................................. 8:25
Shaft stresses ............................................................................ 8:26
Shaft maintenance .................................................................... 8:30
Shaft repair methods ................................................................ 8:33
Shaft attachments ............................................................................ 8:33
Bearings ................................................................................... 8:33
Hubs ......................................................................................... 8:35
Couplings ................................................................................. 8:35
Gears ........................................................................................ 8:36
Installing attachments .............................................................. 8:37
Tolerances and fits .......................................................................... 8:39
TYpes of tolerances .................................................................. 8:39
TYpes of fits ............................................................................. 8:40
Methods of assembly ............................................................... 8:42
Assembly and disassembly equipment .................................... 8:45
Shafts and Attachments
MWM #1
A shaft transmits motion and power from one machine to another. It usually
has attachments such as gears, belt pulleys and sheaves, or chain sprockets.
The fit with which parts are joined is crucial to smooth running and long life
of the machinery.
•
Keys and keyseats (keyways) are used to locate parts precisely.
•
Splines are axial grooves manufactured into attachments and shafts.
They are used to join attachments and shafts, but the millwright does not
normally modify a spline.
The installation, alignment, and maintenance of shafts and their attachments
are central to the millwright's work.
Keys and keyseats
MWM #2
A key is a removable machine part that provides a positive means of
transmitting torque between two mating components. Shafts transmitting
large torques are often fitted with two or more keys. The key is removable to
facilitate assembling and disassembling the shaft and components.
Keys may also be designed to precisely align components with mating
components. The key lines up the parts for easy assemble much like a dowel
pin. In other applications the key may be utilized as a safety device. If
excessive torque is applied, the key shears.
A keyseat is an axial (longitudinal) groove accurately cut (machined) into the
shaft or hub to retain a key. The key is usually assembled into the keyseat.
A
MWM #3
Caution!
Loose-fitting keys are a hazard and can result in system failure. Always
ensure the key is positively located in its keyseat.
The choice of key is dictated by specific circumstances. The most important
concerns are:
•
shaft size
• material type
•
hub diameter
• permanent or semi-permanent installation
•
style of mounting
• intended use
•
torque load exerted
• engineering specifications.
Refer to technical handbooks such as Machinery's Handbook.
MILLWRIGHT-SHAFTS AND ATTACHMENTS
8-1
Types of keys
Parallel keys
MWM #4a
In parallel keys, the top and bottom are parallel and the sides are parallel.
Key stock is readily available in a variety of sizes that is manufactured to a
plus tolerance (slightly oversize). It usually requires hand-fitting to the
keyseat. Typical key-stock material is a low- to medium-carbon steel, such
as AISI 1020 in the cold-drawn condition. (AISI is the American Iron and
Steel Institute, that sets standards for steel manufacture.)
Cold-finished, square bar stock such as AISI 1018 is also often used for
making parallel keys. However, bar stock is made to a minus tolerance:
•
For sizes up to 0.75", the tolerance is usually -0.002".
•
For sizes 0.75" to 1.5", a toleranceof-0.003" applies.
Any substitute key material must meet the requirements of strength and other
physical properties necessary for the application.
Square and rectangular (flat) keys
MWM #4b The most common forms of parallel keys have square or rectangular cross
sections. Rectangular keys are sometimes referred to as flat keys. Parallel
keys are retained in keyseats. In some cases set screws are used to secure
both the hub and the key axially to the shaft. See Figure 1.
_ _ L. _ _ _ L _
Square
Flat
Figure 1 Square and rectangular keys
8-2
MILLWRIGHT-SHAFTS AND ATTACHMENTS
Square and rectangular keys can be used with all shaft sizes. However,
square keys are the keys most often recommended for shaft diameters up to
165 mm (6.Y:1. "). Rectangular keys are used in shafts of larger diameters.
Offset keys
~- Stepped key
Shaft
Figure 2 Offset (stepped) key
Figure 2 depicts the offset key, also called a stepped key. This adjusts for a
MWM #4c shaft
keyseat of a different width than that of the hub key.eat. This key may
also be used to align machine parts. They facilitate rapid disassembly and
assembly of the machine.
Saddle keys
Saddle key
Figure 3 Saddle key
MWM #4d
A saddle key is shaped to fit the curved surface of the shaft (Figure 3). It is
fitted into the keyseat cut in the hub. No keyseat is required in the shaft.
Several set screws are normally plaeed on top of the key to force it upon the
shaft. This assembly depends upon the friction between shaft and key to
provide the drive when the shaft rotates. Saddle keys are suitable only for
light drive fits.
MILLWAIGHT-SHAFTS AND ATTACHMENTS
8-3
Boxed (blind) keys
MWM #4e
A boxed key is a square or rectangular key that has both its ends rounded.
The buried key and the Pratt and Whitney key are both types of boxed keys.
The key fits into a keyseat with closed-profile ends. The key length is equal
to the length of the keyseal. and less than the width of the hub. See Figure 4.
Figure 4 Boxed key
Sunk keys
MWM #4f Sunk keys (Figure 5) are similar in shape to square boxed keys. They are
adapted for applications where the mating machine components reslrict
accessibility at both ends of the key. Sunk keys are set deeper into the shaft
than plain keys are.
(C==I)~+W
f
Math here is
Important
--I~L~Figure 5 Sunk key
Sunk keys come in sizes from a No.1 that is ~"long by J{6" wide to a
No. 34 that is 3" long by %" wide.
Feather keys
MWM #4g Feather keys permit end movement of the two parts either in operation or
during assembly. They come in two types: fixed. and sliding (Figure 6).
8-4
MILLWRIGHT~AFTS AND ATTACHMENTS
Fixed fealther key
Sliclina feather keys
Figure 6 Feather keys
•
Fixedfeather-The fixed feather lrey has a tight fit in the lreyseatin the
shaft. It is sometimes seeured by screws. It allows the hub a certain
amount of axial sliding motion on the shaft, but prevents the rotation of
one part without the other.
Sunk keys can also serve as fixed feather keys. The extra deep keyseat in
the shaft provides support 10 secure the key.
•
Sliding feather--Sliding feather keys also prevent the rotation of one
part on the other. They permit greater axial (end) movement between the
two parts. The key slides with the part along the shaft in a keyway cut
the length of the shaft.
This key is secured in the outer hub or part by means of a keyseat and
shoulders at eaeh end of the key. Another method of securing it is by a
pin on the feather key. The pin fits into a hole drilled 90° 10, and in the
centre of, the keyseat.
MWM #4h
Gib-headed keys
A square or rectangular key with an enlarged head on one end is called a gibheaded lrey. The gib head allows for quick extraction and is used where parts
are removed at frequent intervals.
Caution!
For safety, restrict the use of gib-headed keys protruding from shafts. This
avoids the problem of material tangling during rotation.
Tapered keys
Taper
Taper
/'
J
Plain tapered
C-----~/---.J
<lIb-headed tapered
Figure 7 Tapered keys
MILLWRIGHT-SHAFTS AND ATTACHMENTS
8-5
Plain tapered
A plain tapered key is given a slight taper on its top surface. It can locate a
component on a shaft to prevent axial movement. When driven tightly into
place, the taper enables the key to secure the component finnly to the shaft.
No set screw is required. The taper also allows quick installation and
disassembly of the key and the component.
MWM #4i
A
Caution!
Take care not to split cast-iron parts by driving the key in too tight.
standard taper for tapered keys is 1 in 96 or Ys" in 12". A matching
**Know ratios** The
taper is applied to the keyseat in the hub. For metric sizes, the tapered key
has a ratio of 1 in 100. The length of the key should be the same as the hub
in which it fits.
Tapered gib-headed keys
To further aid installation and removal, tapered keys may be made with a gib
head. Tapered, gib-headed keys have the same body dimensions as standard
tapered keys.
Woodruff keys
MWM #4j
Woodruff keys are used when keying shafts to mating parts. The key is semicircular as shown in Figure 8 but it is also available with a flattened bottom.
may have their ends
Woodruff keys with a diameter larger than 1
flattened as shown in Figure 8. It fits into a standard keyseat in the hub and
into a semi-circular keyseat in the shaft.
Yz"
Woodruff key
;'
I
/
~i- " '
;'
\
Full radius type
I
/
~i-" ,
\
Flat bottom type
Full radius type
with flattened ends
Flat bottom type
with flattened ends
Figure 8 Woodruff keys
The circular shape pennits the key to align itself to suit either a parallel or a
tapered shaft. The design minimizes any tendency of the key to tip when
load is applied. It permits the two parts to be mounted and dismounted
8-6
MILLWRIGHT-8HAFTS AND ATTACHMENTS
freely. Because of these features, Woodruff keys are extensively used for
light-duty applications. Examples ofthe..e are small electric motors, hand
wheels, and small gears to shafts with tapered ends.
The Woodruff keys can be purchased in standard sizes wbich are designated
by an ANSI code numbering system;
•
**Important**
The last two digits give the nominal diameter (in eighths of an inch).
Example
•
a #406 key has nominal diameter =06 x
Ys = %"
The digit(s) before the last two give the width of the key (in tbirtysecondths of an inch).
Example
a #!lO6 key has width
=4 x ]12 = Ys"
As a rule of thumb:
•
The width of the key is one quarter the nominal diameter of the shaft.
For example, if shaft diameter = I", key width =
•
The key diameter size is the same as that of the shaft.
For example, if shaft diameter = 1", key diameter = I".
X".
When a Woodruff key is not available it can be made from round bar stock
of the required diameter. For dimensions of keys, consult a technical
handbook such as Machinery's Handbook.
Keyseats in shafts
In the past, the groove cut into the bore was referred to as the keyway and the
groove cut in the shaft, the keyseat. However, the tenns were
interchangeable and depended on where the key was actually seated. ANSI
has revised its definition and now refers to both grooves as keyseats.
MWM #5
All keyseats cut into the outside of shafts or cylinders have their sides in line
with the axis of the shaft. Parallel keyseats have their bases flat and parallel
to the shaft's axis. Woodruffkeyseats have a radius at the base.
Parallel keyseats
Parallel keyseats may be cut:
•
at the end of the shaft-this makes them an open keyseat
•
anywhere along the shaft-this makes them a boxed keysea!.
Open keyseats
The shape of the ends of the keyseat is determined by the type of milling
cutter used to cut the groove. End mills produce a profile keyseat end.
Horizontal milling cutters produce a sled runner or runout key. eat. See
Figure 9 (next page).
MILLWRIGHT-8HAFTS AND ATTACHMENTS
8-7
Profile keyseal
Sled-runner keyseal
Figure 9 Shapes of keyseat ends
The open keyseat is used where the exact position of the part is not known or
where the position may vary slightly. It is also used where a key is to be
installed after the hub has been located or where the key is to be removed
before disassembling the part from the shaft. An example of this is a
sprocket fitted with a gib-headed, tapered key.
50% in the Shaft
50% in the Hub
The keyseat in the shaft and hub is designed so that exactly one half of the
height of the key is bearing on the side of the shaft keyseat and the other half
on the side of the hub keyseat. Consult a technical handbook such as
Machinery's Handbook for the correct dimension for the various sizes of
standard shafting.
Boxed (blind or closed) keyseat
The boxed or closed keyseat may be located anywhere on a shaft. It is used
when the exact location of the part is known. The length of the keyseat is
generally shorter than the hub width. This type of keyseat is used in
conjunction with the square, rectangular, or fixed feather key, and with the
Pratt and Whitney. The sunk key uses a similar keyseat but it is cut deeper
into the shaft.
Measurement of parallel keyseats
The location of the keyseat in the shaft is usually determined by the position
of the attachment to be mated to the shaft. When cutting a keyseat, alignment
within the shaft should be maintained within prescribed tolerances. These
maximum tolerances are:
•
0.25 mm (0.010") for offset alignment
•
0.10 mm (0.002") in a distance of 100 mm (4") for angular
misalignment.
The proper depth of the keyseat in a shaft is measured diametrically from the
bottom of the keyseat to the opposite side of the shaft. Figure 10 shows this
dimension. Refer to Machinery's Handbook for more information on these
measurements.
8-8
MILLWRIGHT-5HAFTS AND ATTACHMENTS
Amount removed
to acquire flat
Required
keyseat
depth
Diametrical
measurement
for accurate
keyseat depth
-+---~+
Nominal shaft
diameter
Figure 10 Measuring keyseat depth
Woodruff keyseats
A Woodruff keyseat is a cireular recess cut into a shaft. There is no
requirement for a set screw to secure the key. This key is generally used for
light applications or tapered shaft ends.
For keys up to 175," in diameter, shank-type Woodruff keyseat cutters are
available. Larger cutters are mounted on arbors. The cutters are designated
by numbers that match the key numbering system. Woodruffkeyseats (see
Figure 8) are faster to cut than square and rectangular keyseats, The key
should not require any fitting after the keyseat are cut.
Keyseats in attachments
Keyseats eut into bores of attachments may be either parallel or tapered. The
location of the keyseat in an attachment is normally not critical. However,
where the strength or timing of the attachment is important, keyseat location
is also important.
Parallel keyseat
Parallel keyseats in bores are cut in line and parallel to the axis of the bore.
They are cut deep enough to accommodate half the height of the key plus
clearance. Refer to technical handbooks such as Machinery' $ Handbook for
the correct dimensions. Figure 11 (next page) shows the configuration of the
key to thc keyseat.
Qj_s,_¥iN-'_:",,*,~,0;ft1"t'0..1M~_'f0:(~f~%~"0;%4i'?FHJi1ji;;Mfi;~~'0\.~"''t'lii.'1lli.ic~~;b'0'\S!i'34%if!f7;>$N?fi}"-'th~~ry;1:fJfu4'~4Vl~I::;$Am;;~'t':f0;;il,Ot)0%0l0f4jt&~~"0(&7
MILLWRIGHT-SHAFTS AND AlTACHMENTS
8
9
Vertical Clearance= 0.005"
Horizontal Clearance= 0.002"
Figure 11 Configuration of parallel key to keyseat
Tapered keyseat
The tapered keyseat is used with a tapered key. The shaft has a groove cut
axially and parallel to the centreline of the shaft, as has a standard keyseat.
The mating part has a key seat groove cut with parallel sides and with the top
of the groove matching the key's taper.
Parts not located on the end of a shaft must have longer than normal keyseats
cut into the shaft to facilitate the installation and removal of the key.
Standard sizes of keys and keys eats
ANSI sets standards for keys and their keyseats in relation to nominal shaft
diameter.
Table I lists some of the more common standard sizes and keyseat
dimensions needed for design and maintenance. As a rule of thumb the key
width is nominally one quarter of the diameter of the shaft. The standards in
this table apply to square and flat keys. Woodruff keys have other standards
(refer to Machinery's Handbook).
8-10
MILLWRIGHT-8HAFTS AND ATTACHMENTS
Table 1: Shaft size, key diameter and keyseat depth
Nominal shaft diameter
OVER
;{
Nominal key size
UPTO&
SQUARE'.
INCLUDING
WIDTH & HEIGHT
RECTANGULAR
Nominal keyaeat depth
HEIGHT +2
WIDTH
HEIGHT
SQUARE
RECTANGULAR
Ys
If
%
If
%
1
;{
1
1;{
Yt6
Yt6
;{
7h
1;{
1lf
Ys
Ys
;{
7\6
1lf
1%
"YI6
"YI6
Yt6
~2
Ys
Ys
1:b
1%
2
If
If
Yt6
;{
%2
2
2lf
%
"YI6
Yt6
~2
2lf
3
%
%
%
If
Ys
%
3
3lf
Ys
Ys
"YI6
Yt6
3lf
4
1
1
%
%
If
Ys
4
5
1%
1;{
Ys
%
"YI6
5
6
llf
1lf
1
%
If
7\6
Width= 1/4 of the shaft diameter
Cutting keyseats
A portable keyseat cutter (illustrated in Chapter 4: Shop Practices) can be
used for a variety of purposes. It is used to cut keyseats in straight or tapered
shafts without dismantling the machine components.
These mills are either 115 V or 230 V, single-phase. The voltage is listed on
the mill motor name plate. Most have llf horsepower motors that draw
10 A at 115 Vor 5 A at 230 V. These machines must be correctly grounded.
A
Caution
Before using a key mill, check that the power source is correct and that the
mill is properly grounded. Follow the manufacturer's specifications.
MILLWRIGHT-8HAFTS AND ATTACHMENTS
8-11
End mills
End mills are available in various sizes with standard Weldon-type shanks.
A split collar is required to accept shanks with a diameter smaller than
X".
End mills used to cut keyseats are called slot cutters. They have a +0.0000
and a -0.0015 tolerance on the diameter. They can he plunge-aIt into the
shaft 10 produee a boxed keyseat. To produce a keyseat to correct
dimen.~ions, it is recommended that a keyseat be cut first with the nexl
smallest-size end mill.
Figure 12 shows a typical layout of a keyseat in a shaft. A centreline is
extended to the end of the shaft as a reference line for key orientation.
Figure 12 Laying out a keyseat on a shaft
A
Caution!
When using a portable keyseat cutter on a shaft, take the following
precautions:
1. Secure and align the mill so the cutter will traverse longitudinally,
parallel to the centreline of the shaft.
2. Do not overtighten the clamp. Doing so may distort the milling machine
base causing misalignment. Distortion will cause the keyseat bottom not
to run parallel with the shaft. Refer to the manufacturer's specifications.
3
Rough-out the depth of the keyseat to allow 0.25 rnm (0.010") for
finishing.
Broach and arbor press
Hand-broaching is the method of cutting keyseats in attachments. It is done
with a special multi-toothed cutter called a broach. An arbor press is used to
force the broach thmugh the bore of the attachment to cut the desired keyseat
shape.
8-12
MILLWRIGHT-8HAFTS AND AITACHMENTS
The cutting action of a broach is perfonned by a series of teeth. each
protruding about 0.07 nun (0.003") further than the preceding tooth. The last
three teeth are of the same depth and provide the fini~h cut. Bushings are
used to support and guide the broach. Assorted sizes of bushings and shims
are available in broaching kits (see Figure 13). The sizes are marked on the
sides of the bushings.
Each pass of most broaches cuts to a depth of ){6". Note that:
•
To adjust the depth of cut. enough shims in the standard thicknesses are
supplied with each individual broach.
• J1l" broaches do not require shims as the standard depth of keyway is
accomplished in one pass. This is also the case with one-pass keyway
broaches.
Figure 13 Keyway broaches
Caution!
When uving a portable keyseat cutter on a shaft. take the following
precautions.
L Ensure that the arbor and broach are aligued to the work.
2. Always lubricate the back of the broach and use cutting fluid on the
teeth, particularly when cutting steel.
3. Push the broach through the hub with a finn steady pressure.
4. Stop and check for alignment periedically during each pass.
5. Make sure that at least two broach teeth are engaged at all times. Where
the length of cut does not engage the proper number of teeth. stack (nest)
two or more workpieces to increase the thickness. The maximum length
of cut should not exceed the standard length of the indi vidual bushing as
supplied with each broach set.
!~~'>;%\U#iiil1i/%1".;;;;t,;sY40Sit:t4(fA1Ki\'hf&';{iff~lq;.{qp«:",**";-~;jffi.jo"':@~Fj*,"~M"~'\~~;tI'0'1"i1')ful'@.':::O*tlAW_';;(%>-'~}:(",",V:%"1,j-;;;:'':jG,m,"-~W':'u,ni\1\i~''5'S"'!i\NWV!:'i:G~tl+\10D%
MILLWRIGHT-8HAFTSANDATTACHMENTS
8
13
Arbor press
""
".'-",
Arbor
~,
Bushing
Figure 14 Broaching a keyseat in an arbor press
Eliminating hngging-in
Keyway broaches are furnished with an 8' to 10" rake for use on iron or
steel. This can cause biting Or hogging-in of the teeth when broaching softer
metals. Hogging in can be prevented or reduced by doing one of the
following:
•
Reduce back clearance of the broach teeth.
•
Reduce the pressure or spring on the broach just before the finishing
teeth pass through the work.
•
In some cases, it may be necessary to use a collared bushing above the
work. This gives morc support to the back of the broach.
Another reason the broach may hog in is that if the faces of the work are not
square to the bore. Shim the face of the work to ensure squareness.
Installing keys
Most parallel keys are fitted and assembled into the shaft's keyseat before
the hub is assembled. A light coating with an anti-seize product or oil allows
easier assenthly of the parts. A small chamfer on the front end of the key
allows easy entry.
Tapered keys are fit into to their respective components after assembly.
Where several components are to be assembled on the same shaft, matchmark every key, hub, and keyseat position in order to keep the fitted parts in
sequence.
8
14
MILLWRIGHT-8HAFTS AND ATTACHMENTS
Fitting keys
The degree of interference or clearance that a key has in relation to the shaft
and hub is recognized as the class offit:
o
Class 1 is a clearance fit, where there is a relatively free fit for the key to
slide in the keyseat. Standard square steel bar-stock and keyseat
tolerances produce this fit for parallel keys.
o
Class 2 is a relatively tight fit, with a pcssible slight interference with
the keyseat. The hub should have a tight slide fit on the key. Key stock
and standard keyseat tolerances produce this fit.
o
Class 3 is an interference fit for permanent assemblies., Thc degree of
interference is not standardized, but there should be no relative
movement between the shaft and hub. Key stock and precision-cut
keyseats produce this fit.
Most situations require either a Class 1 or a Class 2 fit. Class 3 fits are
specified by the designer only for special situations. They require careful
attention to tolerances and assemhly methods. Consult technical manuals or
blueprints for tables of tolerances on the key and keyscats in standard sizes
Fitting procedures
Fitting a key requires care and patience. Regardless of the type of key, fitting
procedures follow roughly the same sequence.
Square or rectangular key fitting
A parallel key is fitted as follows:
1. Accurately measure the keyseat width and depth in the shaft and hub to
establish the basic standard key size required.
2. Measure the hub length to determine the key length.
3. Cut the key from key stock that is slightly longer than necessary to allow
for finishing to length.
4. Assess the class of fit required.
5.
Ifneeessary, file the key, measuring the width occasionally to control
parallelism. (It is better to file along the length of the key rather than
across the face. You have more file control and produce flat faces. )
6. Draw file the key to fIuished size.
7.
0.002" on side
0.005 on top
for single direction
low torque
Application
After completing each filing operation, slightly chamfer the edges of the
key (that is, break the comers slightly). Heavy chamfering reduces the
area of the side, with consequent heavier loading per unit of area.
Figure 11 shows the clearance between the key and the keyseat. A clearance
of 0.005" applies above the key. Note that these fits differ with each situation
and size of key. Refer to engineering specifications or to technical
handbooks such as Machinery's Handbook.
MILLWRIGHT-8HAFTS AND ATTACHMENTS
8-15
Tapered key fitting
A tapered key is fitted as follows:
1. Assemble the hub and check the taper by measuring at each end of the
keyway.
2. Cut a piece of stock generously longer than required.
3. Fit the key to the bottom and sides of the keyseat.
4. File the key to match the taper of the hub. Do this as follows:
a. Mark the taper of the hub on the side of the key.
b. File a step on each end of the key blank to a little over the marks of
the taper.
c. File the excess metal on the top of the key blank until a flat surface is
formed between the two points.
5. Remove sharp corners with a file so that the key will not bind during
fitting.
6. Clean the key, then smear the tapered surface lightly with Prussian blue,
and drive the key fmnly into the keyseat.
7. Remove the key, and file the high spots which will be indicated by the
bright spots or lines on the key.
8. Repeat steps 6 and 7 until the key bears evenly on top and bottom.
9. After the taper of the key is fit, trim the key to the required length.
10.
1/4" from end of hub
to accommodate fox wedges
Smear the key with light oil and drive it tightly into position. If it is a gib
head key, the point should be level with the end of the hub and the gib
head at least 6 mrn (X") from the front of the hub.
,,, CI(UYV'tuel",t fo)C v.,eJ)c s .
A
Caution!
Always lightly coat a key with oil or an anti-seize agent before driving it into
the keyseat, otherwise it is liable to seize and damage both key and keyseat.
Securing components and keys
Some keys are secured by the keyseat (as are the Woodruff, fixed feather,
sunk, and boxed keys). Sliding feather keys may be pinned to the hub or
secured with fasteners. Some types have shoulders on each end of the key to
act as a keeper. Keys may also be secured in their keyseats with adhesives or
set screws.
Using adhesives
Adhesive bonding of assemblies offers several advantages:
•
6 -16
Adhesive bonding does not require costly and time-consuming
operations such as drilling and tapping for set screws.
MILLWRIGHT-SHAFTS AND ATTACHMENTS
o
The bond is distributed evenly over the entire joining area. There is
continuous contact between the mating surfaces.
o
Holes for set screws are eliminated and the full strength of the mating
part is maintained.
Disadvantages of using adhesives include:
o
Surfaces must be prepared correctly to produce a proper bond.
o
Heat can drastically reduce the strength of the bond (most should not be
used for temperatures above 93°C).
Using set screws
Set Screws insure
positive contact
Set screws are a common method of securing keys and locking mating
machine parts to shafts.
Collar
Figure 15 Headless, cup-point set screw fastening an
attachment to a shaft
Sometimes the set screw is used alone to transmit torque by friction between
the point and the shaft (for example, with collars). In situations where
reliable axial location of the assembly is required, other more positive means
should be used (such as shoulders, pins, or spacers).
Selecting set screws by heads and points
Set screws are categorized by the style of their heads (forms) and their
points. Selection of a specific form or point is influenced by function, safety
and other considerations. The type of driver to turn the set screw determines
the form.
A
Caution!
Take extra care to guard moving parts that are fitted with protruding parts
such as square-head set screws.
MILLWRIGHT--5HAFTS AND ATTACHMENTS
8 -17
Head styles include hexagon socket, slotted socket, flat socket, and square
head. Note that:
Cup
o
Square-head set screws protrude from the component to be secured. This
is a major disadvantage because the projection can catch loose materials
such as clothing.
o
Flush-seating, hexagon-socket-head set screws are threaded their entire
length. They have an internal driving socket at one end, making them
compact and safe. They are the most common form used in industry.
Flat
Cone
Oval
Half-dog
Full-dog
Figure 16 Standard points of set screws
There are various styles of set-screw points. Use the correct style of point for
the specific application. Figure 16 shows some of the most used points.
o
Cup-point set screws are used to lock pulleys, sheaves, collars, gears,
and other parts directly onto soft shafts and sometimes onto keyed
shafts. The sharp edges on the set screw cut into the metal of the key or
shaft. This gives axial and torsional holding power without increasing
the installation torque values on the set screw. Cup-point set screws
applied directly to a soft shaft produce a burr around the edge of the
indentation that tends to "bind up" when parts are to be dissembled. This
can score the bore and require extra fitting precautions on reassembly.
Where vibration is a concern or permanent positioning of the component
is required, the knurled cup point is selected. This configuration
produces a much more effective locking action.
o
Flat-point set screws are used to secure components such as stops,
screws, collars, cams, and gears to hardened shafts and keyed
assemblies. These assemblies can be disassembled without damage to
the shaft by the set screw point.
o
Cone-point set screws are used for the same types of applications as cuppoint set screws. They are applied directly to the shaft. The location of
the set screw is scribed and the shaft is then spot-drilled to engage the
conical point of the screw.
o
8 -18
Oval-point set screws are used to lock parts that are adjusted frequently
relative to each other. A groove of the same contour as the set screw
point which bears directly against the shaft is generally provided.
MILLWRIGHT-SHAFTS AND ATTACHMENTS
•
Half-dog-point set screws are frequently used to engage directly in slots
milled longitudinally in shafts. The point also acts as a stop to limit
travel. They allow lengthwise movement, but prevent rotation.
•
Full-dog-point set screws are used for exactly the same purpose as half
dog-point set screws. They are not appreciably more efficient and
therefore are now seldom used and must be specially ordered.
Holding power of set screws
Set screws, unlike most other screws, are compression fasteners. They are
not as well understood as standard tension-loaded fasteners. Many variables
such as point type, thread finish, thread lubrication, and selection can
seriously affect set-screw holding power without the user being aware of
them. The diameter, size, location, and number of set screws playa large
role in the holding power of set screws. Selection should depend on the use.
When a set screw is tightened in a typical application, a thrust is developed
against the shaft. The magnitude of the thrust determines how well and how
permanently the set screw will hold.
Lubrication of the threads helps to ensure that maximum thrust is exerted on
the shaft for maximum holding power. This can be done with a thread
lubricant or by selecting a pre-lubricated set screw.
Set screw location
Square and rectangular keys are often held in position with a sets screw in
the part over the key. When a part is to be keyed to a shaft, the set screw is
placed at the hub's longitudinal centre and located over the keyseat.
Consideration should be given to the effect on the hub's strength, ease of
access to the set screw, interference with the operation of the part, and the
number of set screws to be fitted.
Some applications require more than one set screw to secure a keyed hub to
a shaft. In this instance, place the set screws at 90° to each other, as shown in
Figure 17.
Figure 17 Setting set screws at 90°
MILLWRIGHT-5HAFTS AND ATTACHMENTS
8 -19
Socket locking screws
Socket locking screws have the same diametral dimensions as set screws,
except that they are much shorter. Also, the hexagonal driving socket is
broached clear tmough the screw. For most sizes, their length is one-half
their diameter. They are used to lock set screws in high-vibration areas or
where a loose set screw could cause serious damage.
A big advantage of some applications is that the locking screw need not be
completely removed for resetting the sel screw to a new position. It is simply
backed up until the set screw socket can be reached through the locking
screw bole.
Set screw replacement
Always replace set screws when equipment is given a major overhaul, or
when there is any sign of wear to the head or tbreads. Hardened set screws
are difficult to remove if the drive socket becomes damaged.
Stainless steel set screws should be replaced every time that they are
serviced. Work-hardening can cause the head to fail and reduce the thrust
exerted. This, in turn, reduces holding power. Removai is also impaired and
the set screw may have to be drilled out.
Removing keys
Gib-headed key removal
Where possible, a gib-headed key is removed from the assembled mating
part before the part is removed from the shaft. Remove the key by driving a
Fox Wedge fax wedge between the gib and the face of the hub. See Figure 18.
Fox wedges
I
Gibhead key
Figure 18 Glb-headed key removal
8-20
MILLWRIGHT-SHAFTS AND ATTACHMENTS
You may need to lubricate the face of the hub and use more than one wedge
to facilitate a straight pull on the key gib.
A
Caution!
Take care to support the/ox wedges so that they don'tfly loose and injure
someon£ or damage other equipment.
If the tapered key cannot be driven out, the hub can be pushed out along the
shaft to release the grip of the key.
Parallel key removal
Several methods are used to remove parallel keys and each situation can
present challenging problems. Always plan the task, taking into account all
the problems that may be encountered. For example, remove any device that
is used to secure the key. As a guide, a few methods of key removal are
suggested here. Figure 19 shows the use of an extracting screw.
Extracting screw
. Plate
Spacer blocks
Figure 19 Parallel key removal
o
Where the key is located at the end of a shaft, a hole may be drilled and
tapped into the end of the key. This allows the attachment of a slide
hammer to draw the key out.
o
Other methods may be employed to grip the key, such as welding a rod
to the key, Or gripping it with a pair oflocking pliers.
o
Dutchman
Keyseats that are longer than the hub allow using a key drift to drive the
key out. Depending on the length of the keyway and the available travel
of the key drift, a spacer may be needed. The slang name for this spacer
is a dutchman. It is inserted into the keyseat behiod the key, allowing it
to be driven all the way out.
MILLWRIGHT-SHAFTS AND ATTACHMENTS
8-21
Hub
/
Key chaser
~
Dutchman
Figure 20 Removing a key by using a spacer (dutchman)
When a key cannot be removed by using these methods, it may be necessary
to remove the part with pullers. If this is done, take care not to damage
adjoiniug parts or the removal equipment.
Shafts
In diseussing shafts, the terms axle, spindle, and journal are aLqo used. These
terms are defined as follows:
8-22
Shaft
A shaft is the component of all mechanical devices that
transmits motion and power. A shaft usually carries
power-transmitting attachments such as gears, belt
sheaves, or chain sprockets.
Axle
An axle is a rotating device on which a wheel is
mounted. Axles are loaded transversely and are subject to
bending.
Spindle
A spindle is a slender pin or rod which turns, or on which
something else turns. Spindles are usually used to
directly carry a tool for doing work. It must be very
accurately installed.
Journal
A journal is the part of the spindle, shaft, or axle that
rotates in or on a bearing. Plain friction bearings are often
referred to as journal bearings, because they corne in
direct contact with the journaL
MILLWRIGHT-SHAFTS AND ATTACHMENTS
Types of shafting
Normally, shaft type and size are selected when the equipment is designed. If
modifications are necessary, the millwright can obtain adequate information
for selecting the proper size and materials from manufacturers' catalogues.
Shafting is available in various materials and finishes. The materials include
low- to high-carbon alloy steels and various stainless steels. Their finishes
include hot- and cold-rolled, ground, and plated.
The correct steel is usually selected by an engineer, but the millwright must
know the properties of the material and its characteristics. Selection is
influenced by:
•
torque (twisting action)
•
speed requirements
•
power-transmitting components and their mounting methods
•
compression and tensile limits
•
contraction, bowing, or expansion limitations (distance between shaft
supports)
•
cost.
Hot-rolled shafting
The term hot rolled refers to the finishing process in the manufacture of the
steel. The surface has a dark, rough, oxidized finish resulting from rolling
the metal while it is hot. One of the processes called pickling and bright
dipping may be used to remove the surface scale but the rough surface
remains.
Bar stock that has been hot rolled is not intended for direct incorporation into
finished products. The size tolerance, concentricity, and straightness have
not been strictly controlled at the mill. Hot-rolled stock used as shafting
generally requires finishing by some machining process.
Finished shafting
Finished shafting has a smooth surface finish, and is manufactured to close
tolerances. This allows it to be incorporated directly into finished products.
However, the cost can be greatly increased, depending on the degree of
fInishing applied.
Finishing methods include cold rolling (COld finished), machining, centreless
grinding, grinding and polishing, and chrome plating.
Mild Steel cold rolled
can be finished shafting
The two most common types of carbon-steel, finished shafting are described
on the following page.
MILLWRIGHT-5HAFTS AND ATTACHMENTS
8-23
AlSI ClO18
AISI C1018
has .18% Carbon This is a basic cold-finished steel in the low-carbon range that welds readily.
It is used for general shafting purposes, spindles, pins, etc. It is easily
content
machined and has the lowest cost. Commercially produced bars have a
bright, smooth surface finish. This type of shafting is not recommended for
applications involving high speed or high stress.
Diameters are maintained to a minus tolerance (undersize). This allows
imperial-size mounted bearings to be installed directly to the shaft. To fit
metric-size mounted and unmounted bearings, a shaft diameter larger than
the bearing bore is selected. The section to receive the bearing is then
machined to the appropriate size.
AISI CI045 / C1050
AISI CI04SI CI050 shafting is known as precision shafting because its
surface finish is precision-ground and polished. It is made from mediumcarbon steels that have high strength and are distortion-free. It can be used as
shafting for high-speed applications. This shafting is also available with a
chrome-plated finish, to be used as hydraulic piston rods and shafts.
The diameters of this shafting is held to close tolerances as in Table 2.
Table 2: Size tolerances for AISI C1045/C1050
Diameter
Plus
Minus
Y2" to 1 Y2"
over 1 Y2 "to under 2 Y2"
over 2 Y2" to under 21~6"
0.000"
0.0010"
0.000"
0.0015"
0.000"
0.0020"
over 21~6" to under 4"
0.000"
0.0030"
over 4" to under 51~6 "
0.000"
0.0050'
All sizes of chromed shafting and precision shafting over 1 7:\'''' are supplied
in fibre tubes. Do not slide pieces of sbafting out of the tube. Suppliers
recommend that the shafting be left in the tube for cutting and unwrapped
after cutting.
Alloy shafting
Where wear and corrosion are great (such as in centrifugal pumps used to
pump slurries of corrosive fluids), alloy shafts are used. They are
manufactured from a variety of alloys including:
8-24
•
chrome alloys (stainless steel) which are hard, wear-resistant, and
corrosion-resistant
•
high-manganese alloys which give long durability (for crankshafts, etc.)
MILLWRIGHT-SHAFTS AND ATTACHMENTS
•
nickel-chrome-steel alloys with high inherent strength (also used for
crankshafts)
•
brass and bronze which are tough, corrosion-resistant, and wearresistant.
An example of an available alloy shafting material is AISI 4140. This is a
general purpose, medium-carbon, chrome-molybdenum-steel alloy. It has
high strength and is quite ductile, making it suitable for use as gears, axles
and shafts. This shafting is available in an annealed condition, either ground
and polished or hot-rolled. Refer to a metallurgy textbook for more
information on these materials.
Hollow shafting
**Need to know
this Example**
Hollow shafting is used for power transmissions, mostly on shaft-mounted
reducers. The hollow shaft makes handling easier. Compared to a solid shaft,
the weight loss of a hollow shaft is considerably more than its strength loss.
(For example, a 4" diameter shaft with a 2" diameter bore in it loses 25% of
its weight, but its strength is reduced by only 6.25%.)
Uses of shafts
Shafts are used to:
•
transfer torque from a driver to a driven sheave, gear, pulley, or sprocket
•
support pieces of equipment
•
permit equipment to pivot on a point to transfer motion
•
permit a driven component to slide along the shaft while transferring
power.
•
extend the length of a drive, (as in a piston rod and a piston)
•
change rotary motion to reciprocating motion (as in a crankshaft)
•
provide a support for loads applied axially
•
act as guides for slides.
Identifying shafting
Proper labelling is the best way to identify shafting materials. This is aided
by proper storage and good housekeeping practices. Other methods are the
observation of :
Non destructive •
Destructive •
surface finish, colour, weight, and magnetic properties of the material
results from spark tests, hammer-and-chisel tests, and file tests.
MILLWRIGHT-SHAFTS AND ATTACHMENTS
8-25
Designations
The most widely used systems for designating carbon and alloy steels are
those of the American Iron and Steel Institute (AISI) and the Society of
Automotive Engineers (SAE). Technically, they are two separate systems,
but they are nearly identical and closely coordinated. Both systems use a
series of four or five numbers to designate the type of steel.
Colour code
Colour coding placed on the ends of shafting by the steel mill is an
unreliable method of identifying types of shafting. There is no recognized
standard---each mill has its own system. Since suppliers obtain the product
world-wide, trying to rely on this method invites problems.
Shaft stresses
In a given situation, a shaft can experience several stress conditions at the
same time. Torsional shear stress and bending stress occur. Also, at times,
vertical shear stresses and direct normal stresses due to axial loads occur.
These stresses are not evenly distributed-some sections experience no
stress at all, while in other areas, stress can be so concentrated that the
integrity of the shaft is affected.
There are several types of stress, but all may be defined as follows:
Stress can be defined as the internal
resistance offered by a unit area of a
material to an externally applied load.
Normal stresses are either tensile or compressive:
•
If the stress (load) tends to stretch or lengthen the shaft, it is called
tensile stress.
•
If the stress tends to compress or shorten the shaft, it is called a
compressive stress.
Bending stress
Bending stresses are a combination of tensile and compressive stresses.
Figure 21 shows how a load can bend a shaft.
Shear stress
Shear stress occurs when the applied force tends to cut through the shaft (see
Figure 22). An example of shear in machine design is the tendency of a key
to be sheared off at the section between the shaft and the hub.
8-26
MILLWRIGHT-SHAFTS AND ATIACHMENTS
Load
~ Bearings /
Figure 21 Bending stress
Shear plane
Reaction of
hub on key
Force of
shalt
key
Force distributed
over bearing area
is driving hub
Side view
Pictorial view
Figure 22 Shear stress
Torsional shear stress
There is a direct relationship between the power, rotational speed, and torque
in a shaft transmitting power. When torque. or twisting moment. is applied
to a shaft. it tends to deform by twisting. causing rotation of one part of the
shaft relative to another. Such twisting causes a shear stress in the shaft
(torsional deformation). See Figure 23 (next page).
For small sections of the shaft. the nature of the stress is the same as that
experienced under shear stress. However. in torsional shear, the distribution
of stress is not uniform.
H;-_~"';""E.u",i<'t~~~d2:gK~::Q:'*;:;!;1~)ji)}j:;J£t::k~4"'P);YY"4:t\t"'~t1';;0:{¥fti"L%'0i:;0A0:sn;p;;;tfdRi~'M:n00'V1gp2G'ii/i0;his4:>Cr;(0Vli{0t'40m"'0,cf\1KV:,,'G!{';;l$inz$"vi\0v#0#R\m2Lqj:S,%~
MILLWRIGHT-SHAFTS AND ATTACHMENTS
8 - 27
Shaft at rest (no stress)
Same shaft under torsional stress
Figure 23 Torsional shear stress
Sources of stress
The weight of the shaft, components fitted to the shafts, loads applied to the
shaft, and location of supporting bearings all contribute to stress load.
Fatigue
When the shaft is subject to many cycles of loading, stresses encountered are
called fatigue loading. There are many types of fatigue loading-the most
fundamental is reverse bending. The ability of a shaft to resist fatigue is
called its fatigue strength.
Reverse bending is produced when a load applied to a shaft bends it. The
shaft is then rotated and the bending continues, causing cyclic loading of the
shaft. Another common type of fatigue loading is the repeated, one-direction
loading such as pulling or tensile loading.
Shoulders and other shaft modifications
When a change in diameter occurs in a shaft (to create a shoulder against
which to locate a machine component) stress concentrations develop. The
amount of stress concentration depends on two factors:
•
the ratio between the two diameters
•
the size of the fillet in the comer where the diameter changes.
Figure 24 shows recommended fillet radius to diameter ratio and the major
to minor diameter ratio. Note the small recommended radius for anti-friction
bearings and the large radius for hubs and other mating elements. Other
modifications to the shaft such as retaining ring grooves, holes, and notches
can also produce high stress concentrations.
Surface finish
Any deviation from a polished surface reduces the strength of the shaft. It is
critical that parts suhject to fatigue loading be protected from nicks,
scratches, and corrosion hecause they drastically reduce fatigue strength.
8-28
MILLWRIGHT-8HAFTS AND ATTACHMENTS
r = radius = .03
r = radius = .17
~ ,...----,----,.
r--r-----i
1.00 dia.
--+--
D
~ ,...------,----..
1.50 dla.
1.00 dia.
---+---r-
Shaft fillet shown
4 times size With
typical bearing.
Bore radius added.
Note clearance.
D
1.50dia.
Shaft fillet shown
4 times size with
large chamfer on
bore of mating element
Figure 24 Fillets on shafts concentrating stress
Keyseats
The design of the keyseat can reduce stress concentrations. The two types of
parallcl keyseat most frequently used are the profile and rnnout. The stress
concentration factor is less for the ronout keyseat than for a profile keyseat
because of its smooth longitudinal radius at the ends.
Stress reduction
A number of solutions can he used to reduce stress concentrations. Although
they do not eliminate the stress, they facilitate a better distribution of the stress
throughout the shaft. This results in a longer service life.
•
Start by maintaining the surface finish of the shaft. Protect it from nicks,
scratches, corrosion, and excess bending during service, repair, or
installation.
•
Keys are usually made with the edges broken (sharp comers removed). To
further reduce stress concentrations, the edges may be chamferedmatching fillets must be provided in the keyseats. Do not modify ronout
keyseats; provide fillets whenever possible.
•
When sections of shafting must be turned down for a bearing or other
machine component, the reduction in diameter should be no more than
1.5:1. The fillet radius should be as large as possible to reduce stress
concentrations. See Figure 24.
MILLWRIGHT-SHAFTS AND ATTACHMENTS
8-29
Bearing placement
In situations where the millwright has the choice of where to place the
bearing consideration should be given to placing the support as closc to thc
loaded components as possible. There should be sufficient support to prcvent
shaft deflection from causing fatigue stress.
Failure to locate bearings correctly on a shaft can result in changes in the
clearance of mating parts. The effect on a machine can be catastrophicseized components, damaged seals, broken fittings and parts,
When anti-friction bearings are to be fitted up to a shoulder on a shaft, the
bore of the inner ring is made with a radius, but it is a small radius. The fillet
radius on the shaft must be smaller yet in order for the bearing to be seated
properly against the shoulder. To locate the bearing correctly, a small fillet
should be maintained and the bearing inner ring should bear firmly On the
shoulder. See Figure 24.
Shaft maintenance
To maintain shafts in good working order, the millwright must attend to
several factors:
•
alignment
•
shaft centres
•
critical speed
•
runout.
Alignment
Two or more shafts transmitting power from One to the other must be
properly aligned to each other. The axis of the shafts must be parallel and in
line, not offset. Shafts often require realignment because of settling
foundations, the effects of heat, vibration, bearing wear, etc. Although some
bearings and couplings will handle limited misalignment. precise alignment
reduces wear, vibration, and fatigue loading. Refer to Chapter 23:
Alignment.
Shaft centres
Many shafts have their ends centre-drilled during the machining process (see
Figure 25).
Figure 25 Centre drill in end of shaft
8-30
MILLWRIGHT-8HAFTS AND ATTACHMENTS
These centres facilitate checking and repairing the shaft and should be
maintained in good order. Pullers applied to the shaft should utilize a shaft
centre protector. This is a metal button that fits On the end of the shaft to take
any damage caused by the rotating adjusting screw point.
Critical speed
As a shaft rotates, small imbalances can cause it to vibrate. For perfect
running balance, the centre of gravity of the shaft must be at the shaft centre.
In most cases this is not so, and the addition of such things as couplings and
pulleys further unbalances the assembly. As the shaft and its accessories
rotate, the centrifugal force generated by the heavy side is greater than that
generated by the lighter side. The shaft deflects toward the heavy side and
this is the souree of vibration as it rotates.
This small imbalance is usually tolerable even at high speeds, hut as speed
increases a point is eventoally reached where there is excessive vibration.
The speed at which this vibration occurs is called the critical speed. As the
speed is further increased, the unit runs quietly again.
Critical speeds depend upon:
•
size of the load or loads carried by the shaft
•
length of the shaft
•
diameter of the shaft
•
the kind of support bearings.
The normal operating speed of a machine mayor may not be higher than the
critical speed. For example, some steam turbines exceed their critical speed.
Usually, most machines are not set within 20% of their critical speed.
Machines that must run close to their critical speed must be in precise
alignment and balanced, and have very little play in the bearings.
Types of runout
A shaft and its attached components should maintain their relative position
to the shaft's centre as they rotate. Any deviation (wobble) from true is
called runout. There are three kinds of runout:
•
Radial runout oeeurs when the shaft and the attachments are not
concentric in their rotation. Some radial ronout may be tolerated, but the
acceptable limit is indicated on the drawings or in the machine
speeifications.
•
Circular run out occurs because of imperfections (ovality, bumps, etc.) in
the cross section of the part.
•
Axial runout occurs because attachments do not rotate perpendicularly to
the shaft axis.
MILLWRIGHT-SHAFTS AND ATTACHMENTS
8-31
Runout can result from any of the following:
•
bent shaft
•
worn bearings
•
poor machining
•
poor assembly of components.
Excessive runout can cause vibration, premature wear, and possible seizing
of components such as wear rings in a centrifugal pump.
Checking for runout
Runout is usually checked by removing the shaft and attachments from the
machine and rotating them between fixed centres (such as a centring
apparatus or a lathe). However, many components can be checked for runout
without removing them from the machine. A dial indicator is used to read the
amount of deviation in a surface. The dial indicator is fixed to a magnetic
base or clamp which is attached to a fixed surface. Figures 26 and 27 show
examples of checking various surfaces.
- - - - - - - - - - - - - --- - - - - - - -- - - - - - - - - - - - - - - -- - - - - - - - -- Figure 26 Checking a shaft for runout
Figure 27 Checking the face of an attachment for runout
In Figure 26, the shaft is rotated and the dial indicator shows the amount of
runout. Each surface must then be checked by measuring with a micrometer
about the surface to ensure that it is circular. The circular runout can then be
compared to the radial runout to confirm bent shaft, worn bearings,
incorrectly machined or poorly fitted components. Surfaces that are not
within prescribed tolerances must be replaced.
8-32
MILLWRIGHT-5HAFTS AND ATTACHMENTS
Shaft repair methods
Irregular shafts
Shafts often develop irregular areas (damage) at the points at which they
contact seals, bearings, and other components. These irregularities are
caused by wear, misalignment with bearings, exposure to chemicals and
other causes. They can be removed if the worn area is metalized and
machined or ground in a lathe.
Metalizing
The process of metalizing is the technique of spraying a metal coating onto a
metal object. Special metal powders or wire are fed into a spray gun, where
they are melted by a flame. They are then sprayed and deposited on the
surface. The shaft is prepared for this coating by machining and cleaning.
Several types of coatings may be employed such as base coats and corrosionresistant and wear-resistant top coats. The process varies according to the
type of repair and the manufacturer of the metalizing product.
Using a sleeve
Another method of repairing damaged sections of a shaft is to machine the
section of the shaft that is damaged and fit a sleeve over that section. The
sleeve has an interference fit onto the shaft. The sleeve is re-machined after
it is fitted to maintain concentricity with the rest of the shaft.
Bent shafts
Bent shafts wear out bearings and seals, contribute to metal fatigue, and
cause vibration. They should be straightened (where the situation warrants it)
or replaced. When straightening a shaft, use the following precautions:
1. Remove the bearings and other attachments from the shaft to protect
them from damage and to facilitate straightening, if necessary.
2. Rotate the shaft between centres and use a dial indicator to pinpoint the
maximum radial mnout and its location.
3. Straighten the shaft in a hydraulic or arbor press, peening with a hammer
or applying small amounts of heat in appropriate locations.
Shaft attachments
Bearings
In any mechanical arrangement, the static and dynamic positions of the
shafts are established and maintained by the bearings. Any geometric or
dimensional inaccuracy in the shaft may directly affect the performance of
bearings and the machine.
MILLWRIGHT-5HAFTS AND ATTACHMENTS
8-33
Take the simple case of a shaft rotating under a radial load and supported by
two bearings. In this situation, many specific factors related to shaft, bearing
components, and housing can affect the runout of the shaft. These factors
and their influence become more complex with different types of loading
conditions. Some of the conditions that influence shaft position are:
•
straightuess
•
roundness
•
size or fit with the bearing
•
bearing seat concentricity
•
bearing seat squareness
•
the radius fillets at the shoulders on the shaft
•
deflection of the shaft
•
the means of retaining the bearing in position
•
balancing.
Other conditions relating to the bearings and the bearing housings are
discussed in Chapter 9: Bearings. Almost all the conditions relate directly
or indirectly to bearing installation and should be addressed at that time.
Caution!
Take great care when handling and installing bearings.
Size of bearing shafting
The primary requirement for bearing shafting is that it be the proper size.
Dimensions must be accurate for both shaft diameters and axial locations
along the shaft. Oversize diameters invite overheating or preloading.
Undersize shafting may contribute to fretting of the shaft, loosening of the
bearing, or excessive internal clearance.
Shaft surface fmish
Another basic requirement is that it has the correct surface finish. Surface
finish is given in micrometres (1JIll) AA or in microinches (1Jin). AA refers to
the arithmetical average height of surface roughness. See Chapter 3:
Technical Drawings. Other readings may be given as the root mean square
(rms), which is considered the better method of determining surface
roughness since it emphasizes extreme surface deviations.
Shaft surface finish requirements depend on hearing materials and types:
8-34
•
Most anti-friction bearings require a shaft surface finish of up to
0.30 IJIll AA (12IJin). Too rough a surface finish may result in the loss
of interference fit, excessive shaft wear. and fretting of the bearing seat.
•
Babbitt and bronze sleeve bearings require a finish of up to 0.811JIll AA
(32IJin).
MILLWRIGHT-SHAFTS AND ATTACHMENTS
The shafting should be straight and free of nicks, gouges, scratches, or burrs.
Imperfections in shaft surfaces can interfere with bearing lubrication and
cause localized scoring.
Retention
Retention of bearings on a shaft is affected by the fit, by the hardness and
finish of the material, and by the deflection of the shaft. Most anti-friction
bearings are mounted on a shaft with a slight interference fit, the degree of
interference varying with the type of bearing and the application. Too loose a
fit may cause the inner ring of the bearing to creep on the shaft. Bearing
manufacturers' tables indicate maximum shaft diameters (fit) for specific
bearings.
Non-ferrous shaft materials require special attention. The difference in the
thermal coefficient of expansion of ferrous and non-ferrous metals means
that the fit of a bearing on the shaft varies with the temperature. The specific
effects of varying the fit to suit the temperature should be determined, and
then the correct shaft tolerances can be established.
Hubs
Hubs
A - B - C - D
A hub is a machine element fitted to components such as gears, sheaves or
sprockets for the purpose of enabling the component to be attached to a
shaft. Typically hubs are assembled to shafts prepared with a keyseat. To
install the hub:
1. Install the key into the shaft keyseat first.
2. Lubricate the shaft with a non-seize or oil product.
3. Align the hub keyseat with the key.
4. Slide the hub into position.
The method used to position the hub is dictated by fit, type of equipment,
and style of hub.
Couplings
Couplings are used to connect two shafts at their ends for the purpose of
transmitting power. Couplings are discussed in more detail in Chapter J 3:
Couplings and Clutches. There are two general types of coupling:, rigid and
flexible.
Rigid coupling
Rigid couplings are designed to connect shaft~ together rigidly, making a
single unit. As with bearings, the coupling must be correctly mounted tu
maintain the concentricity of the shafts. The coupling is fitted directly to the
shaft with a key in the connection. The shaft should be straight, frec of rust,
paint, dirt, burrs or imperfections that could alter the coupling position.
MILLWRIGHT-SHAFTS AND ATTACHMENTS
8-
For general purposes, the coupling bore should have a locational interference
fit to the shaft. The class of fit is dictated by the type of rigid coupling, the
diameter of shaft, and the torque. This is discussed in more detail later in this
chapter.
Flexible coupling
Flexible and rigid couplings have similar shaft requirements. However,
flexible couplings may use other mounting methods: plain taper bushings,
flanged taper bushings (trade name QD bushing), or tapered shaft and locknut. In most cases a key is included in the connection. See Chapter 10: Belt
Drives.
Gears
Many machine components are attached to shafts for the purpose of
transmitting torque. The means used for axial location depends heavily on
whether or not axial thrust is transmitted. V-belt sheaves, chain sprockets,
and spur gears produce small thrust loads. However, many types of gears can
produce varying amounts of thrust loads. Each application has its own
circumstances that the designer and maintenance person must consider. See
also Chapter 12: Gear Drives.
Figure 28 shows some of the methods used to attach gears to shafts. In most
cases, no one standardized method is recommended. Gears can be keyed to
the shaft, then locked in place by any of the following;
•
interference fit
•
set screws
•
plain tapered bushing
•
flanged tapered bushing
•
locking collar and set screw.
•
pinning
•
retaining rings
•
tapered shaft with key and lock nut or screw (see Figure 29)
•
molding polymer-a cast material attaching directly to a knurled shaft.
In other applications the gear is required to slide on the shaft as in a gear
box. This is accomplished by meanS of a fixed feather key, a sliding feather
key, or a spline. Before dismantling a part, examine it to determine what
method of retention is used.
8-36
MILLWRIGHT-SHAFTS AND ATTACHMENTS
a. Straight pin
c. Spring pin
b. Tapared pin
Figure 28 Three methods of pinning to secure gears to shafts
Shaft
~
Taper
Figure 29 Two methods using tapered shafts for fastening machine elements
Installing attachments
Compression fittings
Compression fittings are a method of attaching machine components to standard
size shafting. They are quick to install and dismantle even in situations where
the shaft is slightly undersized.
In the simplest form the hub on a sprocket can be slotted axially, and a clamp
(bolt) drilled through the hub. When the clamp is drawn down tight, it forces the
split hub into contact with the shaft. The pressure of the hub on the surface of the
shaft permits transmission of torque. To maintain the sprocket's relative position
to the shaft, a key is inserted in the connection. Maintaining concentricity of the
component can be difficult with this type of connection.
MILLWRIGHT-SHAFTS AND ATIACHMENTS
8-37
Plain tapered bushing
The difficulties encountered with simple clamp compression fittings can be
overcome by using a plain tapered bushing. The bushing has a small taper on
its outer surface, and the hub has a matching taper in its bore (see Figure 30).
Shaft
I
----t--
/
Attachment
a-
+----::J---t-
\
Tapered bushing
Assembly bolts
Figure 30 Plain tapered bushing
When the bushing is pulled into a mating hub with a prescribed number of
set screws, it compresses tightly to the shaft to hold the attachment true and
in its proper axial position. A key is used as a positive means of torque
transmission-no slipping can occur between the hub and the shaft.
For the correct installation and removal techniques for a particular bushing,
refer to the manufacturer's specifications. Every unit should have a
procedure sheet for that particular type of bushing.
Flanged, tapered bushings
The flanged, tapered bushing (Figure 31) is a quick means of installing
sprockets and sheaves to shafts. Concentricity of the sheave is maintained.
Figure 31 Standard, flanged, tapered bushing
8-38
MILLWRIGHT-5HAFTS AND ATTACHMENTS
The hub of the sprocket is bored with a small taper that matches the taper of
the bushing. The lightly assembled bushing and sprocket are slid over the
shaft and secured to the shaft by tightening cap screws that compress the
bushing tight to the shaft.
The sprocket is easily removed from the hub by using the jackscrews in the
holes tapped in the face of the sprocket. Most hubs are drilled to allow
reverse mounting.
A
Caution!
Do not overtighten the cap screws.
Tolerances and fits
The millwright is often called upon to make decisions about the fit of
machine components. Sketches of replacement components such as hubs,
couplings, and sprockets must be fully dimensioned with the appropriate
tolerances. This information must be correctly interpreted and understood in
order to successfully assemble these components.
Types of tolerances
A tolerance is the total pennissible deviation of a dimension on a part. It is
the difference between the maximum and minimum limits of size. Before
tolerance is applied, the basic size of the mating parts must be determined.
Basic size
When applying tolerances to the dimensions of mating parts, two systems
may be used: the basic hole system or the basic shaft system.
•
In the basic hole system, the design size of the hole is the basic size. The
allowance (excess material on a part) is on the shaft. For example, when
a replacement shaft is to be fitted with a standard anti-friction bearing,
the bearing bore is the basic hole size and allowance is on the shaft.
•
In the basic shaft system, the design size of the shaft is the basic size and
allowance is left in the bore of the mating part. For example, when a
sprocket is replaced, the shaft becomes the basic shaft size and the
allowance is in the bore of the sprocket.
Unilateral and bilateral tolerances
Tolerance may be expressed as unilateral or bilateral:
•
Unilateral tolerance means that all the tolerance for the drawing
dimension is applied in one direction only. For example, 4. 500"j:gg~.
•
Bilateral tolerance means that the tolerance for the drawing dimension is
applied in both directions. For example, 4.500"j:GGi.
For more explanation of this, refer to Chapter 3: Technical Drawings.
MILLWRIGHT-SHAFTS AND ATTACHMENTS
8-39
Types of fits
The tcnns allowance and fit are used interchangeably. Allowance is the
prescribed difference between maximum material limits of mating parts.
Allowance determines the fit (that is, the tightness or looseness) between
mating parts.
•
Positive allowance refers to minimum clearance between mating parts.
•
Negative allowance refers to maximum interference.
The degree of fit is a result of specifying a tolerance to each mating part.
Two basic types of fit are clearance fit and inteiference fit:
•
Clearance fit allows the mating parts to maintain some clearance
between them at all times. Parts with clearance fit are relatively easy to
assemble and disassemble. They may require only hand-fitting
techniques.
•
Inteiference fit maintains some resistance between the mating parts at all
times. Additional equipment may be required to assemble or
disassemble these parts.
However. to allow specific desined clearance or interference, ANSI has
established terminology to further separate and refine the classes of fil The
main classes are:
•
running and sliding clearance fits
•
locational clearance fit
•
transition fit
•
locational interference fit
•
force or shrink fit.
The degree of clearance or interference of mating parts and specific
tolerances for particular sizes are listed in technical references such as
Machinery's Handbook. Figure 32 shows the relationship between the
various fits.
Running and sliding clearance fits
Running and sliding clearance fits provide clearance between mating parts
for lubrication.
8-40
•
a running fit applies mainly to rotating parts such as a shaft in a friction
bearing. The fit must not be so close that the shaft cannot turn, or so
loose that the shaft floats about.
•
a sliding fit applies to parts which slide on each other, such as a dovetail
slide. In this case, the weight of one part brings it in contact with the
surface beneath. The designated clearance relates to the surface above.
The clearance in the top surface and slight imperfections in the contact
(base) surfaces allow for lubrication.
MILLWRIGHT-SHAFTS AND ATTACHMENTS
Size on size
between hub and shaft
Note: In this diagram, the
length of the black bars
represents the range of
allowance between mating
parts for the type offit.
Figure 32 Clearance and interference fits
Dovetail--slide
Base
Figure 33 Dovetail slide
Figure 33 shows a dovetail slide with a gib. A gib is nonnally used to adjust
the clearance between the mating parts. This class of fit can be further
di vided into the following types of fit:
•
•
Note: These classes offit
are described in technical
manuals such as Machinery's
Handbook.
•
•
•
•
•
close sliding
sliding
precision
close running
medinm running
free running
loose running.
0t:Wt~":'\'Iil:«"1!Ai0t1BMt¥Jl""0;;bl~_t%it%1Y""'2'd:""d:(il0,,;)3&>'Am~e'Ii''±K-;l\X¢:Ftl,,it,!B~$;W,Jij£;'1$!~q1y(i!'S'6\t;:;Z",,,,,;;z;rA2:4:4'h"0i;~:U-::~~-0"'!40'Yd%\S:JG=f:;;;:J>*''RJfS8S~~"0~:£¢;z,"G~>wmig0/
MILLWRIGHT_HAFTS AND AlTACHMENTS
8 - 41
Locationa) clearance fits
These fits are intended for nonnally stationary parts which can be freely
assembled or disassembled. Various classes from snug to medium clearance
fits satisfy the needs of various applications. An example of this fit is a
machine screw fit into a bolting hole.
Transition fits
When the tolerance on the mating parts partially overlaps, so that either a
clearance or an interference may result, the fit is called a transition fit.
Transition fits are used to locate mating parts. An example of this fit is the
dowel pin and mating hole used to align machine parts.
Locationa) interference fits
Locational interference fits are used in situations where accurate location and
rigid alignment is of utmost importance. Because of their tightness, these fits
are not intended to be used to transmit frictional loads from one part to
another. An anti-friction (rolling element) bearing pressed into a housing is
an example of this type of fit.
Force or shrink fits
When the tolerance of mating parts fully overlaps, causing an interference
fit, the fit is classed as aforce or shrink fit. This fit requires the mating parts
to be forced or shrunk together so that they act as one unit. Pressure is put on
the mating parts, which defonns them when they are assembled.
If interference is excessive, the elastic limit of the material will be exceeded
and the assembled parts distorted. In extreme cases, the ultimate strength of
the metal will be exceeded and the outer part will burst.
A
Caution!
Ensure that mating parts are within the prescribed tolerance for the
designated fit.
Methods of assembly
The method of assembling parts is determined by the amount of interference,
the availability of equipment, and the size of the parts to be assembled.
Three common methods of assembling mating parts having an interference
fit are:
8-42
•
forced fitting-One part is pressed onto another.
•
shrink fitting-The hub is heated to expand it sufficiently to allow it to
be easily assembled on the shaft. After assembly the hub cools and
shrinks onto the shaft.
•
expansion fitting-The shaft is cooled sufficiently, the parts assembled,
and allowed to expand together.
MILLWRIGHT--SHAFTS AND ATTACHMENTS
Forced fitting
The most accurate method of assembly is by means of a hydraulic press
where the operator has control of the operation. Where light work is
concerned. a small lever or screw press gives the operator a sense of feel
which enables the parts to be assembled without damage.
Several factors help in achieving a forced fit:
•
As with all fits. the contacting surfaces of both members must be free
from grit.
•
It is most important that the axes of the external and internal parts stay in
line with each another. If either part is canted, one or both pieces may be
damaged. An assembly arbor is often used to maintain the correct
alignment
•
Tests have demonstrated that smooth mating surfaces perfectly free from
surface lubricant give the hest grip. However. an anti-seize lubricant
applied before assembly helps to reduce the possibility of seizure and
surface abrasion.
Shrink fitting
To shrink fit parts, the outer part is heated, causing it te expand sufficiently
to enable the two parts to fit together with little Or no effort.
Heat sources include:
•
immersion in hot water, oil baths
•
steam
•
oxygen and gas hand torches (open flame)
•
oil, gas, or electric furnaces
•
electric resistance heating
•
electric induction heating.
Table 3 on the next page shows the advantages and disadvantages of some of
the heating methods used for shrink fitting.
Allowances for a shrink fit (interference fit) are usually determined by
consulting the working drawing or suitable tables. The desired amount of
expansion (or contraction) enables the parts to he assembled freely without
the aid offorce.
Calculating required temperature
The required temperature change depends upon the total expansion required
and on the coefficient of linear expansion of the metal. This is explained in
Chapter 2: Trade Science.
MILLWRIGHT-8HAFTS AND ATTACHMENTS
8-43
Table 3: Advantages and disadvantages of some heating methods
Heating method
Advantages
Disadvantages
GAS
• immediate heat
• oxidation causing structural
(gas ring, gas and air torch,
• easily portable
oxyacetylene torch, gas torch.
• high temperatures obtainable
• possible uneven heating when
gas furnace etc.)
• furnaces can obtain even heat
hand heating appliances used
change or scaling of the parts
• relatively cheap to use
• possibility of fire if handled
carelessly
INDUCTION HEATING
• cost for large parts
• speed
• cleanliness
• difficulty of heating large or
heavy sections
• even heating
• ideal for small parts
OIL AND ELECTRIC FURNACES
• temperature control
• difficulty of getting large objects
• good variety of furnaces is available
in and out of the furnace
• oxidation of machined surfaces
Expansion fitting (freeze fitting)
Instead of heating the hub, the shaft may be contracted by cooling. Then,
after assembly, it is allowed to expand into the hub. Various methods are
used to cool the shaft:
8-44
•
industrial refrigerator-On suitable parts this method is very
convenient. Control of temperature is simple and tbere is little likelihood
of damaging tbe structure of the material. This metbod achieves
temperatures to about -50°C (_58°P).
•
liquid air-This method can achieve temperatures as low as -120°C
(-184°P).
•
liquid nitrogen-This process is similar to the liquid air process but a lot
safer due to the absence of oxygen.
•
alcohol and dry ice (C02)-The part is placed in a container partly filled
with alcohol to which frozen carbon dioxide (CO~ is added. This
method produces temperatures of -50°C to -60°C (_58°P to -76°F)
without expensive equipment. However, frost formation on the parts
during assembly could be a problem.
MILLWRIGHT-5HAFTS AND ATIACHMENTS
Assembly and disassembly equipment
Presses
Several types of presses are used for assembling and disassembling
eomponents on shafting. In addition to fixed-in-place hydraulic presses, they
are the:
•
mechanical arbor press
•
free-standing hydraulic press
•
portable hydraulic press.
Hydraulic presses are classified by type, force output, and function. They are
rated according to their pushing capacity in tons. They range in size from
small 9 tonnes (10 tons) to large 135 tonnes (150 tons) Availability, size of
the work, and pressures required determine the type of press used.
Mechanical arbor press
Mechanical arbor presses (see Figure 34) are used for light job, such as
assembling small bearings, broaching keyseats, and dismantling small
components from shafts. They are operated by a hand lever. The lever is
attached to a pinion gear driving a rack at the back of the ram. Working the
hand lever of an arbor pre.~s gives a sense of the amount of pressure being
applied. This sense of pressure is the chief advantage of using an arbor press.
Pinion gear
_Frame
Base
,
I
Figure 34 Mechanical arbor press
MILLWRIGHT-SHAFTS AND ATTACHMENTS
8-45
Free-standing hydraulic press
This type of press consists of a frame of substantial construction with
devices for holding the work piece, and a ram for pushing, that is actuated by
a pump. See Figure 35. These presses may use either a power-operated or
manual pump to generate the hydraulic pressure in the cylinder.
A
Caution!
A safety cage should be attached to all powered presses. The cage is
designed to protect the operator from injury if a workpiece that is misaligned
or unstable shatters.
I
Safety cage
Figure 35 Free-standing hydraulic press with safety cage
By controlling the hydraulic fluid flow with a directional control valve, the
cylinder may be extended, retracted, or stopped. A press fitted with a ram
instead of a cylinder has springs that retract the ram when the control valve
is set to release. Power-operated units are fitted with an adjustable pressurecontrol valve to set the maximum working pressure. This valve also operates
as a safety release when pressure becomes too high.
The workpiece and work-holding devices are supported by a slide rack
called a platen. The platen in turn is supported by safety pins that must
withstand the maximum pressure from the hydraulic ram or cylinder.
To adjust the position of the platen, some presses are fitted with lift cables
attached to a small hand winch. The lifting cables are not designed to support
a hydraulic load. They must be slack and the platen resting on the safety pins
before a load is applied.
6 - 46
MILLWRIGHT-5HAFTS AND ATIACHMENTS
Portable hydraulic press
This type of press consists of a hand pump and reservoir connected by a
high-pressure hydraulic hose to a detachable hydraulic cylinder. It is
available in a variety of capacities and cylinder sizes. Accessories such as
pullers are mounted on the cylinder to facilitate the removal of hubs.
bearings, etc. This tool is often called a portable hydraulic hand pump (trade
name Porta-power).
Safety around presses
Presses can generate enormous pressures. Safety must be practised to avoid
serious injury or equipment damage.
Caution!
Be careful when using a press. Ensure guards are in place.
Take the following precautions:
•
Plan the task.
•
Understand how pieces come apart or fit together before pressing.
•
Stay alert and consider any person working in the vicinity.
•
Stand to the side as you work, or use a cage or other device such as a
restraining blanket.
•
Use a face shield.
•
Observe the force (pressure gauge) being applied. Appreciate the
capacity of that force.
•
Maintain alignment.
•
Fully support the part being pressed.
•
Keep platen-lifting cables slack and safety pius fully engaged.
Pullers
Pullers are available in various styles. shape. and size. Manual pullers are
used for light pulling jobs that don't require large forces to dismantle
components. For the more difficult jobs. and where the parts to be
disassembled cannot be fitted in a hydraulic press, pullers are used in
conjunction with a portable hydraulic press.
There are tbree basic situations where a particular style of puller is used:
•
pulling a component off a shaft
•
pulling a component out of a hole
•
pulling a shaft out of a component.
MILLWRIGHT-SHAFTS AND ATTACHMENTS
8-47
The most basic styles are the two-leg and three-leg jaw pullers and the rod
puller (see Figure 36). Any style can be used manually or with a ponable
hydraulic press.
Rod pullers
Two-jaw puller
Three-jaw puller
Figure 36 Styles of pullers
Jaw pullers
A typical two- or three-leg puller may consist of loose legs (also called grip
arms) that are attached to a cross-head by straps. The legs have jaws to grip
the part being pulled. In the centre of the cross-head is fitted an adjustable
screw to provide the pushing or pulling power to remove a machine part. A
hydraulic ram or double-acting cylinder may be substituted for the adjustable
screw to provide greater pushing or pulling power.
A slide hammer (see Figure 37) attached to the cross-head may replace the
centre adjustable screw to provide the pushing or pulling power.
Figure 37 A slide hammer
6-46
MILLWRIGHT-5HAFTS AND ATTACHMENTS
Rod pullers
Rod pullers usually consist of a strong back or cross-head fitted with rods
instead of grip arms. A bearing splitter attachment is often used to grip or
support the work being pulled (see Figure 38). Adapters are used to lengthen
the rods.
Figure 38 Bearing splitter removal attachment
Other accessories
Other accessories include the following:
•
Bearing splitters for removing anti-friction bearings and other
components.
•
Shaft protectors-Inserted between the end of the puller screw and the
shaft, they protect the puller screw tip and the shaft centre hole from
distortion (see Figure 39).
•
Step plate adapters for bridging a hole when removing bearings, gears,
or other hollow parts on a hollow shaft or housing.
protector
Figure 39 Shaft protector button
MILLWRIGHT-5HAFTS AND ATIACHMENTS
8-49
Puller selection
Selection of a puller depends on the type of component to be removed from a
shaft. Plan the task and identify how the part should be pulled. Determine the
puller type by identifying how the part can be gripped. Then the reach,
spread, and the force required together determine the puller size (see
Figure 40).
~Reach~~~
Figure 40 Selecting the right puller
Reach is based on the length of the part or the distance the part may need to
be pulled to remove it from the shaft. Spread refers to the opening needed to
grip the part (usually the width of the part). You may use rules of thumb to
help select the force and strength of a puller:
o
o
8 50
For a manual puller, the diameter of the adjusting screw should be at
least half as large as the shaft.
For hydraulic applications, select a puller that will withstand a force
exerted in tons- 7 to 10 times the diameter of the shaft in inches.
MILLWRIGHT-SHAFTS AND ATTACHMENTS
Safety when using pullers
Safety cannot be stressed enough, since every task presents a new set of
problems.
A
Caution!
Be carefol when using a puller.
When using any puller, do the following:
•
Plan the task.
•
Make sure that you know how the parts come apart.
•
Select a puller large enough and suited to the job.
•
Set up the puller correctly so that it is in line and firmly grips the part.
•
Wear protective equipment such as approved safety glasses.
•
Restrain the work.
•
Apply forces gradually.
•
Maintain clean work areas.
MILLWRIGHT-SHAFTS AND ATTACHMENTS
8-51
MIlLWRlGHT MANUAL: CHAPfER 9
Bearings
Friction bearings ........... .......................................................... ........ 9: 1
Bearing housings ................. ....................................................... 9: 1
Dimensions of friction bearings ....................................................... 9:2
Styles of friction bearings ................................................................. 9:2
Joint design ....................................................................................... 9:4
Liner materials ........................................................................... 9:5
Babbitt .............................................................................................. 9:6
Fitting a babbitt bearing for contact ............................................. 9: 10
Clearance in a bearing ...................................................................... 9: 13
Preformed bearing liners .................................................................. 9: 17
Thrust control ............................................................................ 9:19
Anti-friction bearings ..................................................................... 9:22
Anti-friction bearing parts ........................................................... 9:22
Worlring conditions ........................................................................... 9:25
Bearing size and design .................................................................... 9:26
Installing and removing anti-friction bearings ............................... 9:32
Shaft and housing checks ................................................................. 9:32
Push fit .............................................................................................. 9:32
Interference fit .................................................................................. 9:33
Installing bearing outer rings ............................................................ 9:37
Axial positioning ........................................................................ 9:38
Installing tapered-bore bearings ....................................................... 9:43
Bearing removal ......................................................................... 9 :45
Pillow block installation and removal .............................................. 9:50
Mounting other bearings ................................................................... 9:51
Maintaining anti-friction bearings .................................................. 9:53
Keeping bearings clean ..................................................................... 9:53
Keeping bearings in good condition ................................................. 9:55
Special cautions ................................................................................ 9:56
Bearings
Two basic types of bearings are used-friction and anti-friction bearings:
MWM #1a
•
Friction bearings have a sliding contact between a shaft and the bearing.
A special, low-friction material lines a rigid housing. The lining directly
contacts and supports the shaft. In use, the shaft slides over the liner
material, separated by a thin film of lubricant. The area of contact is
relatively large and pressure on the bearing material is usually low.
Friction bearings are also referred to as plain bearings, sleeve bearings,
and journal bearings. The journal is the part of a machine's shaft that is
inside the housing of a bearing.
MWM #1b
•
Anti1riction bearings have a rolling contact between the shaft and
bearing using balls, needles, or rollers. In this type, the area of contact is
very small and the pressure on the rings and balls or rollers is quite high.
Friction bearings
Bearing housings
MWM #2
The housing (sometimes called a pillow block) of a bearing is its outer
casing. The principal parts of the housing of a friction bearing are the base,
the base bolt .slots, the cap, and the cap bolt holes. See Figure 1.
Cap
Cap bolt
Liner
Figure 1 Parts of a friction bearing
Housings are made of one of the following materials:
MWM #3
•
cast iron for genera) use with light to medium loads
•
cast steel for gcneral use. It is stronger than cast iron and can be used for
light to heavy loads.
•
fabricated steel for bearing housings for special jobs.
The choice of a bearing housing depends on the load. the rotational (or
rubbing) speed. the direction or pull. and the support design.
Dimensions of friction bearings
MWM #4
The bearing has a nominal size such as "27116" bearing." This is the size of
the bore ofthe bearing (ID in Figure 3). Note this is not called a 2.4375"
bearing---{)ther dimensions are also fractional, not decimal.
MWM #5
The dimension from the base to the shaft centre (called the eye of the
hearing) is important for installation and aligmnent. Bearings of tbe same
size, design. and service weight from the same supplier have the same shaft
centre dimension. Bearings from different suppliers may have different
values for this and other dimensions.
Styles of friction bearings
MWM #6
The choice of a bearing depends on the load, the rotational (or rubbing)
speed, the direction of pull, and the support design.
Regular or flat bearings
The two kinds of flat bearings differ in their housings. They are solid
hearings and split hearings.
Solid bearings
MWM #7a
Solid hearings are used
when speed and load are
low. They are designed
to be used in locations
where the load is applied
to the top part of the
bearing. Solid bearings
are frequently mounted
in an inverted position.
They must be slid on or
off the shaft, which
means that the bearings
must he put on as an
assernb led unit.
Solid housing
called
Dolly Box
Liner
Figure 2 Solid bearing
Figure 3 shows the important dimensions for a solid bearing.
Figure 3 Nominal dimensions of solid bearing
Split bearings
Split bearing designs vary from one manufacturer to another.
•
A two-bolt bearing is used for light to medium loads.
•
A four-bolt bearing is used for medium to heavy loads.
r - - _ ......_
.... ··--,Cap bolts
Base
bolt
slot
Figure 4 Two-bolt split bearing
.~...A~rl_ap-::~cap bolts
Figure 5 Four-bolt split bearing
Angle bearings
Angle bearings are used for drives in either of the following situations:
•
when the load is applied parallel to, or at a slight angle to, the horizontal
•
when the bearing is mounted on a vertical support
Cap bolls
Base
Figure 6 Angle bearing
Integrated bearings
Some heavy-duty machines in industrial plants have the bearing base cast as
an integral part of the machine. These bearings are either babbitted or fitted
with shells or liners like other journal bearings.
Joint design
You must consider the design of the bearing joint when choosing a bearing
for a specific job Bearing joints are designed in three general styles:
•
with flat joints
•
with a gib in the joint
•
with an angle joint
Flat joints
When the joints are flat as shown in Figures 5 and 6, the bolts hold the cap in
alignment. The opposite sections of the joint may be completely level with
each other (calledjlat joint bearings) or be at an angle (calledjlat-joint angle
bearings) as shown in Figure 7.
Gibjoints
When the joints have a little "step" in them as shown in Figure 8, they keep
the cap from moving sideways. A gib bearing combines the rigidity of a
solid block with the advantages of split construction, and is suitable for
limited side loading. If the bearing has gib joints. it need not be loaded only
on its bottom half.
.,...:;;=:::::;:-.,,5;:- ~r-··········Lube entry
lrr~i-C:iib joint
Load area
Load area
Figure 7 Flat-joint angle bearing
Figure 8 Gib-joint bearing
Liner materials
Journal bearings and pillow block bearings with anti-friction material liners
are commonly used for low to medium speed and for light to heavy loads.
They can be used for ultra-high speeds with air-jct lubrication only under
exact conditions-for example the main bearings on a turbine.
When selecting liner material, it should be:
•
softer than the shaft material, so it deforms slightly under heavy loads
•
oflow coefficient of friction
•
wear-resistant
•
a good heat conductor and remain relatively stable with heat changes
•
readily available.
Metallic liner materIals
Metallic bearing materials must have a low coefficient of expansion. They
are also dimensionally stable in the presence of water. They may react
chemically with water, mild acids, alkalis, salts, or other materials.
Metallic liner materials are:
•
babbitt for general use
•
brass (copperlzinc alloy) for higher speeds and heavier loading than
babbitt
•
bronze (copper/tin alloy) for higher speeds and heavier loading than
babbitt
•
aluminum (common in hydraulic pumps)
•
sintered bronze (used for self-lubricating chain). This is a porous bronze
with a built-in oil supply (oilite™); oil to bronze ratio is 1:2 by volume.
Non-metallic liner materials
Non-metallic liner materials are:
• nylon
• polyurethanes
• phenol laminates-such as Celoron™ and Micarta™
Nylons and polyurethanes
Nylons, polyurethanes and other synthetic bearing materials are sometimes
called plastic bearing materials. The advantage of plastic bearings is that
they are generally inert to most mild acids and alkalis. The lubricant for
nylon or plastic bearings can be oil or grease, but water is also used. With
low rubbing speeds and low operating temperatures, some grades of nylon
will run with no lubrication.
They have varying coefficients of heat expansion, all higher than those of
metallic bearing materials. In addition, some synthetic materials expand
when saturated with water. Before doing any critical fitting, check the
specifications of the material. Basic nylon has a thermal expansion rate
roughly ten times that of steel, and a fully saturated water expansion rate of
0.0256" per inch of material. Heat expansion is fairly constant through the
grades. This is important when dry fitting a nylon bearing which will run
with water lubrication.
Phenol laminates
Phenol laminates have layers of cotton or other natural fillers bonded with
phenolic resin. They are strong, shock resistant, and compatible with most
fluids.
Babbitt
Babbitt is a common anti-friction lining material for bearings. It melts at a
temperature of about 288°C (550°F) or less. In a liquid state, it fills all
cracks, voids, and irregularities in a casting, giving a smooth surface to
match the shaft surface. Babbitt can be used in badly worn bearing housings,
thus elinTinating the cost of a new housing. Figure 9 opposite shows a babbitt
lining in a new and a worn bearing.
Babbi! or
other liner
material
Babbitt fills
up the worn
area
- - - - ..
New bearing
~
..
~~
....
Worn bearing
Figure 9 Babbitt or other liner in a new bearing and in a worn bearing
Babbitt includes several alloys that contain various proportions of tin,
copper, antimony and lead:
•
Tin-based babbitts (copper or antimony, with up to 90% tin) are the
hardest and the toughest.
•
Tin-based babbitt with a very high tin content is sometimes called nickel
babbitt. It is used in conditions of heavy service and extreme pressures.
•
The introduction of a small percentage of lead to a tin-based babbitt
gives a slightly softer material.
•
Lead-based babbitts are those in which the tin has been largely replaeed
by lead (up to 10 percent tin and 75 percent lead). They are cheaper and
can be used for light loads at low speeds.
When pouring babbitts the three temperatures to consider are:
A
•
complete melting point-from 275 to 285°C (495 to 545°F)
•
pour point-from 343 to 371°C (650 to 700°F)
•
complete solidification point~from 25 to 28"C (40 to 50"F) below the
complete melting point
Do not overheat the babbitt.
If babbitt is overheated, the service life of the material is greatly reduced.
Overheating babbitt is equivalent to overheating an anti-friction bearing to
install it on a shaft; the physical shape is not altered, but the normal service
life is reduced.
Keys
Babbitt is held in position in a bearing casting by keys which are grooves or
slots in the casting. Any bearing liner to be re-babbitted must have the keys
cleaned out. Figure lOon the next page shows babbitt holes and slots.
~
0
0
a
0
0
0
0
0
0
0
0
0
0
0
0
0
t
Holes'
Slots
Figure 10 Babbitt holes and slols
Using mandrels to pour babbitt
A mandrel (dummy shaft) is
used to shape and size
bearings. It is a short, smooth
piece of shaft of the required
diameter and several inches
longer than the widest bearing
to be poured. When a lot of
babbitting is being done, the
most common sizes of
mandrels are fitted with side
pieces to help positioning.
Mandrels come in the
following styles:
.Mandrel
• plain mandrel
• mandrel with side pieces;
this can be quickly centred
in the bearing
When the bearing base is set up and
the mandrel is in position, the poured
babbitt is held in place by a ring held
in position with a hacking of babbitt
putty-a commereial product manufactured specially for this job.
rlL.-...,~- Wings
or
side
pieces
Figure 11 Plain mandrel and
mandrel with side pieces
Mandrel parallel to base A & Beven
- ---- - -
J
Spacer
.Spacer
I
(
I
~
Figure 12 Position of mandrel
Pouring babbitt
A
i
Pouring babbitt can be quite dangerous and is usually done by a designated
millwright in the plant.
Always observe the following safety measures when babbitting:
•
When you pour into a bearing and mandrel assembly make sure that it
has been heated to 94°C (about 200 0 P) fIrst to remove any free water or
surface moisture. Free water causes a "blow back" or explosion.
•
Use all protective equipment as called for by:
- Workers' Compensation Board (WCB) rules and regulations
company safety policy.
i
Preparation for babbitting is critical: the shaft must be brought up to level or
aligued to other parts of the machine. Since you cannot adjust the position of
the bearing, the vertical and horizontal shaft positions are most important.
Emergency pouring
A situation sometimes arises when a bearing bums out and has to be poured
in a rush in order to let the operation run to the end of a shift.
The following procedure for emergency pouring is effective for temporary
work on bearings with low-speed shafts.
A
Keep afire extinguisher handy, as the oil on the outside of the bearing will
often catch fi re from the torch.
1. Gather the following equipment:
fIre extinguisher
oxyacetylene torch
putty
pair of pliers
cake of babbitt
2. Fit the torch with a large tip and melt out the remaining babbitt in the
casting.
3. SlTIDke the shaft with a straight acetylene flame.
4. Put a putty dam around the shaft and bearing.
5. Hold the babbitt with the pair of pliers.
6. Melt it into the bearing with the torch.
Tinning babbitt bearings
When a thin skin of babbitt must be finnly attached to a metallic backing, a
tinning process is used. The process is similar to that used in brazing a brass
coating. It requires a heat source such as an oxyacetylene or propane torch.
Fitting a babbitt bearing for contact
As the babbitt is poured, it fit~ perfectly to the
shaft and allows no clearance for lubricant Any
bearing (regardless of shape or friction material)
must be prepared carefully . You must check:
•
surface finish and area of contact
•
•
•
running clearance
lubrieation entry (for setting up the oil wedge)
groove(s) (for distributing the lubricant)
Hand tools for fttttng bearings
Hand tools, such as scrapers, used for
fitting bearings may be commercially
produced or made in the plant to suit
the millwright's preference. Figure 13
shows some babbitt fmishing tools.
Hollow
ground
Scraper cutting surfaces are
curved or straight:
• Curved scrapers are
usually commercially
manufactured and tend to
produce a wavy finish.
• Straight scrapers are usually
home-made and produce a flat
finish. Home-made scrapers wi th
smooth, sharp edges can be made
from various kinds of files: mill
bastard, half-round, machinist's
or triangular-ground files. They
should be ground slowly and
carefull y to prevent burning or
local hot spots that will change
the temper of the steel.
Groove cutter
cape chisel
Figure 13 Babbitt finishing tools
The fitting process
1. After the bearing has been babbitted, the corners of the bottom half of
the bearing must be relieved to keep the shaft from binding. This is
shown in Figure 14.
O
Relieved
learner
Figure 14 Bearings with relieved corners (exaggerated for illustration)
2. The top edge of the
bearing must be
chamfered almost to the
corners as shown in
Figure 15. This channels
the lubricant to the shaft.
The amount of chamfer is
usually up to the
individual, but the
chamfer is often greater
for grease lubrication
than for oil. For heavy
loads, the bottom part of
the bearing is chamfered
also.
Chamfer for oil
I~--'Do not cut to end
Bearing \ \
end
J7
~
Figure 15 Bearing with chamfered corners
To obtain a better lubricant entry, the chamfer can be extended on the
entry side, almost down to the area of contact between the shaft and
bearing material.
3. Another alternative is to cut an oil groove in the centre of the bearing to
help maintain an effective oil wedge. This method is used mainly for
casual, marginal lubrication. See Figure 16.
Figure 16 Chamfering and grooving a bearing
A
Always cut grooves in the unloaded section. Do NOT extend grooving or
chamfering into the load area or the high-pressure film area of the bearing.
• Usually, regardless of the
load put on a split bearing,
the groove is cut in the centre
ofthe bearing cap. Sometimes, the groove must be
specially located to
accommodate unusual loads.
• The bearing in Figure 17
has a load applied
horizontally to the side of
the bearing. It is shown
with the oil supply and
groove in the
recommended location.
Chamfer
=
•
\
Load
Figure 17 Special grooving for horizontal loading
• A one-piece or solid bearing used in an inverted position may need to
be lubricated through the base of the bearing rather than through the
bearing cap. The grooving is in the unloaded section.
4. Mter the chamfers and/or oil grooves have been cut in the load-bearing
surfaces, check the bearing for contact. To do this, lightly coat a mandrel
or shaft with mechanic's bluing, and rotate it in the bearing. For a good
impression, apply the bluing in a light, smooth, even coat. The bluing
wipes off at the points of contact and transfers to the bearing surface to
show the high spots. Do NOT USE LAYOUT INK.
An untouched bearing
has high points along
the edge. Remove
these by scraping to
allow the shaft to
make contact with the
bottom of the liner.
Figure 18 shows
contact points on
bearings at various
stages.
Contact points
o
"" co
o
""
00
o
o
=0
o
oo
Figure 18 Fitting bearing for contact
Clearance in a bearing
When a bearing is assembled, there must be a small amount of clearance
between the shaft and the bearing cap. This prevents a binding or clamping
action on the shaft.
To adjust clearance:
1. Put between the bearing halves the original shim used for babbitting the
cap (or a shim of the same thickness) and an additional thin shim.
2. Make sure that the shims do not touch the shaft. Shape them to clear thc
chamfer cut in the babbitt.
3. Tightly boltlogelher the cap and base and then try to rotate the shaft or
mandrel.
4. If the shaft does not rotate:
• For a gib-style pillow block, add more shims.
• For a flat split bearing, pull the cap slightly to one side by the cap
bolts and align the cap with a few sharp blows with a ball-peen
hammer. If this does not work, add more shims until the shaft turns
freely.
Figure 19 shows a fitted bearing with shims in place and clearance in the
cap.
~
..
I
I
Shim+0f
I
I
~ Shim
I I
I
I
I I
-' - 1_.
,
--
-]
I
Figure 19 Bearing shims in place
Shims
Shims are made from material that will not compress and is not affected by
oil. For example, they can be sheet packing, tin plate, or brass shim stock.
Slip-in shims are used in gib-style blocks or any split block. This type of
slip-in shim may be lost if the bearing cap becomes loose. The advantage of
a slip-in shim is that it can be inserted or pulled out after slacking off the
bearing cap.
Shims made to fit over the cap bolts will not get lost, but the bearing cap
must be taken off to adjust the amount of shim (as shown in Figure 20 on the
next page).
Shim for 2-bolt bearing
~
L~J
mm_~)
Shaft mmmml
(
Shim for 4-bolt bearing
Narrow shim
style bearing
Shim--i~~~
Figure 20 Shim styles
Amount of clearance
The amount of clearance set into a friction bearing depends on machine
design and company policy. Some machines with a constant load toward the
base of the bearing do not have a fitted cap. (The cap merely keeps out
foreign material and supplies the lubricant.)
As a general fule, for any shaftJbearing assembly;
•
with constant one-direction load and rotation, bearing clearance can be
from medium to loose.
o
with reversing fotation and fluctuating load, bearing clearance can be
from medium to tight (see Table I).
o
with reciprocating action, the clearance must be tight (sec Table 1).
When fitting small bearings in the shop, clearanee is often determined by
working a feeler gauge between the shaft and the s!rimmed cap.
In large bearings that have been poured into place, it may be impossible to
tum the shaft, and steps must be taken to check thc clearance under the cap.
Dial
indicator 1F=;E~+-"'7'-.,--,-J:::~=t
For some, the cap must be removed; for others, it is
not necessary:
•
o
If the bearing is open at either end, you can
insert long pieces of feeler stock of varying
thickness to check for clearance under the cap.
If the bearing is shielded at both ends by gears
or pulleys, the simplest way to check is to:
1. Mount a dial indicator
2. Pry or jack up the shaft in small increments,
w!rile watching the reading on the diaL See
Figure 21.
Figure 21 Dial indicator for
checking bearing clearance
Table 1: Recommended clearances for lubricated bearings
Journal diameter
114
11
1/2"
3/4 u
1"
11/4"
11/2"
13/4"
2"
21/4"
21/2"
2314"
3"
3 1/4 11
4"
41/2"
5"
6"
7"
8'
Tightest fit
0,0004
0.0006
0.0007
0.0009
0.0010
0.0012
0.0013
0,0014
0.0015
0.0017
0.0018
0.0019
0.0021
0.0023
0.0025
0.0026
0.0030
0.0033
0.0036
Loosest fit
0,0014
0.0018
0,0021
0,0025
0,0028
0.0030
0.0033
0.0034
0.0035
0.0039
0.0041
0.0043
0.0045
0.0049
0.0051
0.0054
0.0060
0.0063
0,0068
Tightest fit
0.0006
0,0009
0,0012
0.0014
0.0016
0.0018
0.0020
0.0022
0.0024
0.0026
0.0028
0.0029
0.0032
0.0035
0.0038
0.0041
0.0046
0.0051
0.0056
Loosest fit
0.0022
0,0029
0.0036
0.0040
0.0044
0.0047
0.0052
0.0054
0.0058
0.0062
0.0065
0.0067
0.0072
0.0077
0.0080
0.0085
0.0094
0.0101
0,0108
To check clearance by reading the dial:
I. Mount jacks at both ends of the shaft to lift the shaft evenly for an
accurate reading. If a jack is mounted at only one end of the shaft, the
shaft will tilt in the bearing, and give a false reading,
2. Note that the dial indicator reading gives the total clearance in a bearing.
It does not indicate the high and low spots. Check the wear by visual
inspection.
To check clearance without an indicator dial, using Plasugage™:
1. Choose plastic gauge steck (called Plastigage™) with diameter about
equal to the clearance. Use cormnon sense in choosing the diameter. If
the expected clearance is in the 0,010" to 0,015" range, pick a diameter
slightly larger than this. For some bearings, you can use lead wire such
as 50150 solder rather than plastic gauge stock.
2. Take off the bearing cap and place lengths of the Plastigage™ across the
shaft in several places, They should reach from one edge of the base to
the other. Numher each piece of Plastigage™ on the shaft.
CONTINUED
3.
Have a record sheet handy, with space to record all readings.
4.
Check that the Plastigage™ is not pinched at the corners of the top cap
of the bearing.
5.
Tighten down the cap, then take the cap off carefully because some
Plastigage™ pieces may stick to it.
6.
Compare the width of the crushed Plastigage™ with the chart on the
side of the packet.
To check clearance using lead wire
1-5. Use the same procedure as for Plastigage™.
6.
Measure the thickness of the wire with a micrometer. Start from the
same end of each wire and take the wires in sequence.
7.
Record the thickness of each compressed wire.
This process takes time and may have to be repeated several times. The
routine should give an accurate readings of the high and low spots in the
bearing. The routine can also be used to check a worn bearing for excessive
clearance.
Lead wires (Flattens
, _ _ _ _ _ _ _---,=,,;u;;,n:,;;dff'e pressure)
~
o
Figure 22 Wire check for bearing clearance
Figure 23a represents a new bearing installation with a chamfer cut to allow
oil to reach the point of contact. Figure 23b represents a worn bearing, with a
small, inadequate chamfer and too much bearing contact. You can scrape the
worn bearing to improve the lubricant entry and surface finish. For proper
fitting, the bearing (b) should be scraped until it resembles bearing (a).
Worn
a
b
Figure 23 Bearing wear
Preformed bearing liners
Shells
Shells are two-piece liners installed in a bearing housing. They are usually
held in position by pins or dowels, screws, special bearing designs,
compression, or combinations of these. They are made from any common
metallic or non-metallic bearing material. Fitting shells for surface finish,
lubrication entry, clearance and lubrication is done by the same procedures
as for poured bearings.
Crush
f
Bearing
housing
Figure 24 Shell crush-clearance
When shells are first installed, they frequently extend past the face of the
bearing by a small amount, which is known as the "crush" or the "crush
allowance." This is shown in Figure 24. When the cap and other shell half is
installed and the fasteners are torqued to the correct tension, the shells are
forced into full contact with the housing. If the shells are machined for the
bearing, the crush allowance should not be changed.
Shells can be installed and used in two cases:
•
where there is no adjustment for wear-that is, when shells are worn a
certain amount, they are replaced (compressor connecting rods are a
good example)
•
where adjustment for wear is obtained by shims between the cap and the
base. To obtain effective crush or clamping action, the shims must be
between the shell halves as well as between the cap and base.
_-1-1-
I
LI________________
Figure 25 Bearing shims in place between shell halves
One advantage of shell liners is that. in some machine designs. the worn
liner may be removed by taking the weight of the shaft off the liner (by
removing the cap) and rolling out the worn liner.
Hold base liner against
shaft. Rotate liner and
remove. Roll in new liner
in same manner,
Figure 26 Roiling out a worn shell liner
Bushings
Bushings are one-piece liners and can be made from any of the common
bearing materials. Fit and clearance is usually set by the dimensions
machined into the housing and bushing. It is held in position by a press fit. a
press fit and dowels. or by dowels with the clamping action between cap and
the base.
Small bushings can be reamed to bring the bore to the correct diameter.
Large diameter bushings often require honing or scraping.
If the bushing is supplied with an oil hole. you must align the oil hole with
the supply line when pressing in a bushing. With some bushings. the oil hole
may be drilled after installation and should therefore be scraped to remove
any high spots caused by the drill pushing through the surface.
When the bushing is worn it must be replaced as there is no adjustment for
clearance on a bushing.
LUbrica;'tio;n~"'---_ _ _"':S~::::=::::::
entry'"
Figure 27 Bushing
Thrust control
Friction (plain) bearings are usually considered radial load bearings with
limited thrust capacity. Thrust (axial force) is mainly controlled by the use of
other components.
The thrust in a plain bearing is controlled by a shaft collar fastened on the
shaft, or by a thrust washer backed by a sprocket or any other hub. See
Figure 28. Such a design is good for low speeds, light to medium
intermittent thrust loading, and simple lubrication.
o
o
Figure 28 Thrust control in a plain bearing
Thrust of a shaft is controlled by the shaft shoulder (see Figure 29) and a
collar that adjusts the amount of end float. (End float is the amount the shaft
moves axially in a bearing. It is also called end play, axial float, or axial
displacement.) The shaft shoulder may not be high enough to provide
enough area of contact. If this is the case, you may add a collar or a
machined component in contact with the bearing surface. This style of thrust
control is good for high-speed applications.
~
.
...
_ _~..
~'haf'~~o~er
: ...•
~:::::=~m~
0
Figure 29 End thrust in a shell bearing
The liner material that covers the ends of the bearings can be any of the
following:
•
solid babbitt poured in the bearing
•
shells made of brass or bronze
•
brass shell with a tinned-quality babbitt wear face
Some bearings are designed to carry both radial and thrust forces without the
use of other components. The shaft has a number of parallel grooves which
match grooves in the liner material. Liner material can either be babbitt cast
in place in the housing or shell liners that have been machined to suit the
shaft profile. There is no adjustment for end thrust control, and lubrication
can be a problem.
Fangsbury thrust bearings
The Kingsbury bearing is a special thrust bearing designed to take high
speeds and heavy loads. It comes in a variety of styles.
The basic design consists of:
•
a revolving ring with a flat contact surface, usually hardened and ground
•
a stationary ring with a number of flat coutact surfaces of a low-friction
rnaterial-either brass, bronze, or a tinned-on, high-capacity babbitt on a
brass backing. These surfaces are separated by lubricant grooves and
have chamfers or slopes on the leading edges to set up oil wedges. (In
Figure 30, there are six contact surfaces.)
/
Load
\
--~=====}~-I}
I~.===:::;--;=========::::;---;::::::-,,:±-
Aatrevolving
ring
~
0
~DiVided
:...!_
v _ _ _ _ _--=-___
~
___::J.stallonary ring
Revolving
ring
Figure 30 Fixed-pad Kingsbury thrust bearing
In the bearing shown, oil flows from the centre of the ring to the outside, due
to the centrifugal force of the bushing ring throwing oil outward. Lubricant
must be supplied to the centre of the bearing.
Kingsbury bearings come in the following styles:
•
fixed pad-a backing which has a convex shape and fits into a mating
housing to correct for minor misalignment. Figure 30 shows this type of
Kingsbury bearing.
•
floaling or lilting pad-individual load blocks are pivoted on the fixed
ring to adjust for thrust and load as shown in Fignre 31.
Figure 31 Kingsbury tilting-pad bearing
Guide bearings
A guide bearing is used as a positioning device or guide for linear motion.
It is used for such things as machine tools and gas compressors.
It often has a thin layer of bearing
material on its surface to reduce
friction. but normally operates with
lubrication. Figure 32 shows one
example of a guide bearing.
Guide bearing
,.....:;:.----,
Slide
Bed
Figure 32 One style of guide bearing
Pivoted shoe bearings
The pivoted shoe bearing is a split journal bearing. used mostly for high
peripheral speeds and shaft stabilization. The shoes are machined
cylindrically to fit freely in grooves in a retaining ring (see Figure 33). Its
operation is like a flat. tilting pad. Kingsbury bearing.
,Shoe retaining plate
- . .JA'-Aligning ring
Figure 33 Pivoted shoe bearing
The surfaces of the shoes and rings are corrosion resistant and low-friction.
The bearing is dowelled sO that the upper and lower halves Can be joined in
only one way. The lower half of the aligning ring also has a dowel to
position the bearing axially and prevent rotation. Each shoe has a separate oil
inlet, which helps to lower the operating temperature and keep it uniform.
The plates are bored so that spent oil discharged from the bearing is
regulated without shaft contact.
When using and maintaining pivoted shoe bearings, carefully inspect the
babbitted surfaces, pivoting surface and seat, and the diametral clearance
between shoe and journal for signs of wear.
Anti-friction ....."'.... ,
The advantages of anti-friction bearings are versatility, low-friction
operation and ability to be packed to avoid frequent lubrication.
Anti-friction bearing parts
The basic parts of anti-friction bearings are two hardened steel rings, the
hardetted balls or rollers, and the separator. A number of variations are in
use. Some types, such as needle roller bearings, may be used with or without
an inner ring, outer ring, or separaturs. If there is no inner ring, the rollers fit
directly onto the hardened shaft. For speeific applications. bearings may
have other parts such as a snap ring used to set axial location.
Figures 34 to 41 show some common bearings and bearing parts. This
manual uses the names shown in Figures 34 to 38 for the parts common to
all standard ball and roller bearings.
i
Bearing
~width
Cup I
length,
1
Outside
diameter
-::~::::;:===l-Inner ring
ballraee
r.-
Outside
diameter
Bore
~""'f-- Separator
Outer ring
ball race
Figure 34 Radial ball bearing parts
Figune 35 Tapered rolier bearing parts
Outer··· .~~==(
r i n g _ Cylindrical rolier
1-0---- Outside diameter --~~-I
I.-~~ Bore ~ --.J
!
Outside
diameter
r&~~::&*. . lnner ring
race
Separator
Face
Housing
------r-~S] washer
f
Width
.___-_. r~_~
tBore corner
___.....:-,-o--~""-Shaft
Separator & ball
washer
(Rolling assembly)
Figure 36 Cylindrical rolier bearing parts
Figure 37 Ball thrust bearing parts
Outer
• ··-ring
~
Bore
Needles
l
Inner
ring
IWidthI.
Cylindrtcal roller
Figure 38 Needle bearing parts
Figure 39 Rolier thrust bearing
Spherical rolier
Separator
~3-i---~ Outer ring
Inner ring
Figure 40 Spherical roller bearing
Figure 41 Standard ball bushings
RolHng elements
The rolling elements of anti-friction bearings are classified by three basic
bearing styles:
•
ball bearings
•
roller bearings
cylindrical
tapered
spherical
•
needle bearings
In spherical roller bearings, the surfaces of the rollers are curved across their
width and run in concave channels.
Ball bushings
Ball bearings can be used as a guide for axial shaft measurement using
recirculating ball bushings. See Figure 41. The balls cireulate through
radial grooves. Each carries only a small amount of the total load at anyone
time.
Shields and seals
Many bearings use shields and seals to prevent unwanted material from
entering the housing. The locations of the shields and seals vary. This affects
the choice of bearing type.
•
Open bearings are used where:
foreign material is kept out of the housing by shaft lip seals or the
equivalent
- the lubricant is expected to work through the bearing from one side to
the other
•
Shielded bearings (one or both sides) are used where:
- the bearing is exposed and the shield will keep out solids but not
fluids
- the lubricant is metered out of the housing into the bearing
•
Sealed bearings (one or both sides) are used where:
the bearing is exposed and the seal will keep out solids and fluids
- the lubricant is to be kept in the bearing
•
Sealed and shielded bearings-such as pillow block cartridges-are
used where:
- conditions are very dirty and abrasive
When replacing a bearing with a single shield or seal, take care that the new
bearing has the seal or shield in the same position as the original installation.
For example, a scaled bearing on the input shaft of a small hydraulic pump is
lubricated by the hydraulic fluid. The seal prevents fluid loss to the outside.
If the seal is reversed, the bearing will run dry.
Working conditions
Types of load
Ball bearings have a low to medium load capacity while roller bearings
usually have a low to high load capacity. Load is applied to bearings in
various directions and anti-friction bearings are classified according to load
conditions:
•
radial load-Maximum or total radial load means that all forces on the
bearing are in a radial direction, with little or no sideways thrust load.
Examples are needle roller bearings and single-row ball bearings (see
Figure 42).
•
thrust load-A pure thrust (axial) load means there is no radial load. All
forces run parallel to the shaft axis. Examples are ball and needle thrust
bearings (see Figures 37,39, and 43).
•
combination thrust and radial load --also called angular load.
Examples are:
tapered roller bearings (Figures 35 and 44)
- ball bearings with angular contact (Figure 44)
- spherical roller, self-aligning bearings (Figure 44)
Loadt
t
Figure 42 Radial load
Figure 43 Thrust
(axial) load
Figure 44 Combined
(angular) load
Speed
Ball bearings are generally suited to high-speed applications (up to
40 000 rpm) and roller bearings to low-speed applications. The speed of a
bearing is expressed as either rpm, or as "rubbing speed." Some makes of
roller bearings are good for speeds up to 20 000 rpm. The normal range of
speeds is 2000 to 20 000 rpm.
Service or degree of loading
As the rated load capacity is increased, the bore stays the same but the
diameter of the rolling unit and the width and thickness of the rings are
increased. Service weight or service use is a means of classifying bearings
with the same types of rolling element and bore for their load capacity. The
service weight is shown in the code number of the bearings.
Series 00
Service weight 0
100
200
300
1
2
3
400
4
Series
Service weight
Figure 45 Bearings with same inner diameter, but larger
outer diameter and load capacity
Common groups are ultra light (00 series), extra light (100 series), light (200
series), medium (300 series), and heavy-duty (400 series); other grades are
also available.
Loading
notches
Conrad type
Figure 46 Maximum capacity type and Conrad type of bearing
Bearing size and design
Dimensions and shape
Roller bearings may have either a straight or a tapered bore. Practically all
bearings are made to metric dimensions. The dimensions outside diameter
(OD), bore (ID), and bearing width (W) are indicated in millimetres. These
are matched to the standard code markings on the bearing ring.
Determining bore
The bore of a bearing is indicated by the last two digits of the bearing
number.
•
Up to 20 mm, the bore of a bearing is designated as:
00= lOmm
01=12mm
02= 15 mm
03=17mm
Example: A 6200 ball bearing has a bore of 10 mm.
•
From 20 mm to 480 mm, to find the bore, multiply the last two figures of
the bearing number by five.
Example: A 6204 ball bearing has a bore of 04 x 5 = 20 mm.
•
Above 480 mm (number 6296), the bearing size is directly included in
the bearing number.
Example: A 62/500 (or 62500) ball bearing has a bore of 500 mm.
Construction
Anti-friction bearings may also be classified
according to their construction:
•
non-separable bearings-are
designed as a single piece. They
cannot be separated easily.
Examples are a single-groove,
radial ball bearing or a double
spherical roller bearing (see
Figure 47).
Figure 47 A non-separable spherical bearing
•
separable bearings--<:!an be easily taken apart. They consist of one of
the following:
- three separate parts: a housing washer, rolling assembly, and shaft
washer. Examples are ball or needle thrust bearings. (Dual-direction
thrust bearings use two housing washers, two rolling assemblies and a
shaft washer.)
- two separate parts: one separate ring with the rolling element, and a
separator combined with the other ring. An example is a taper roller
bearing.
•
non-aligning bearings-used in machines where the bearings are held in
alignment by the bores in the housing
•
self-aligning bearings-may be intemal or extemal. In an internal selfaligning bearing, the inside of the outer ring is concave. In an external
self-aligning bearing, the outside of the outer ring may be convex. See
Figure 48.
Internal
External
Figure 48 Self-aligning bearings
Examples of self-aligning bearings are:
pillow blocks for independent mounting; these are external selfaligning
double ball or roller types for heavy-duty loading; these are internal
self-aligning
single barrel roller types for light loading; these are internal selfaligning
Self-aligning bearings can be identified by the curved surface of the
outer ring, which gives a choice of positions for the balls or rollers. The
degree of allowable misalignment is slight, as the balls or rollers must be
in contact with the outer ring at all times.
Bearing codes
This section covers the basic ISO codes for standard ball bearings and
spherical roller bearings with metric dimensions. For information about the
long list of prefixes, suffixes, and codes for special bearings, consult a
bearing manufacturer's catalogue. Tapered roller bearings have a standard
code designation giving the ID of the cone and the OD of the cup.
Most bearing codes give three pieces of information:
•
the type of rolling element
•
the service weight
•
the bore in millimetres.
Usually a sequence ofletters at the beginning indicates the name of the
manufacturer. Except for special cases (such as matched-face bearings), the
orientation of the bearing code makes no difference to the service life of a
bearing. However, if the code numbers face the end of the shaft, it helps the
millwright to quickly identify the bearing.
Basic codes for most standard ball bearings are four-figure or five-figure
numbers following the manufacturer's identifying letters.
Four-figure code
In a four-figure code:
•
First figure indicates the type of rolling element.
•
Second figure indicates the service weight, outside diameter and width.
This figure ranges from 0 to 4, indicating 00 series, ultralight to 400
series, heavy duty.
•
Third and fourth figures indicate bore size.
Example 1:
SKF-6208
SKF indicates the manufacturer
6 =Conrad
= single-row, deep groove ball bearing
2 = service weight (light)
08 =08x5 =40 mm bore
Example 2:
SKF-6308
The 6308 bas a different OD and a different width
than the 6208.
6 =Conrad
3 = heavier service weight than Example I
(medium)
08 =the same bore as Example 1.
Five-figure code
In a five-figure code:
•
First figure indicates the type of rolling element.
•
Second figure indicates 0 D
•
Third figure indicates the service weight, outside diameter and width.
•
Fourth and fifth figures indicate bore size.
Example 3:
NTN-22208
NTN indicates the manufacturer
22 = double spherical roller, self-aligning
2 = service weight (light)
08 = 08 x 5 = 40 mm bore
Compared to the 6208 bearing, the 22208 has:
• different rolling elements indicated by 22
• the same OD indicated by the third 2
• the same bore indicated by 08
• different widths-single vs. double
Example 4:
NTN-23208
2 = rolling element
3 = series diameter (0)
2 = service weight (light)
08= 08 x 5 =40mm bore
Codes for tapered-bore bearings
Basic codes for tapered-bore bearings are like the standard bearings but are
followed by a K:
Example 5:
FAG-620SK
FAG indicates the lUlUlufacturer
6= Conrad
2 = service weight (light)
08 = bore size 40 mm
K = tapered bore
Note that this pillow block bearing will fit on an
adapter sleeve that has a code number ending in 8,
and will fit into the same bore housing. (An
adapter sleeve is a cone-shaped device used to
locate and secure a taper-bore bearing to a parallel
shaft seat.)
Note that K designates the common l-in-12 taper. K30 designates the l-in30 taper used for larger bearings. The position of the K and K30 is
important-it must be at the end of the code.
Other letters and numbers may appear in the bearing code. To interpret them,
refer to the lUlUlufacturer's manual. For example:
•
A shield is indicated by the letter Z at the end of the code.
•
A rubbing seal is indicated by RS at the end of the code. (2RS indicates
rubbing seals on both sides)
•
P numbers indicate tolerances: P2 indicates close (small) tolerance; P6
indicates large tolerance.
Clearance
Clearance is the total internal clearance between the balls or rollers in a
bearing and their raceways. This clearance has several functions:
•
It compensates for expansion of tbe inner ring or for contraction of the
outer ring when interference fits are used.
•
It compensates for differential expansion of the two rings when the inner
ring of the bearing operates at a higher temperature than the outer ring.
•
It affects the end play of ball journal bearings and their capacity for
carrying axialload~. The greater the clearance, the greater the capacity
for supporting axial load.
When using bearings with small clearances. pay select seating dbnensions
very carefully. Once ball and roller bearings are mounted and running, a
small running clearance is usually desirable. Bearings under radial load run
more quietly when this clearance is as close to zero as practicable.
The definitions of internal clearance under various conditions are as follows:
•
Initial clearance, or uninstalled clearance is the clearance in the
bearing as it comes from the box.
•
Installed clearance is the elearanee left in the bearing after installing it
on a shaft and in a housing; or the clearanee left in a taper-bore bearing
that has been forced up the taper.
•
Running clearance is the clearance in the bearing when the machine is
up to operating temperature and lubrication has been supplied.
Internal
clearance
Figure 49 Internal clearance
Clearances for ball bearings have been standardized by international (ISO)
agreement. Ball bearings are made with six different specific ranges of
clearance. ISO bearings use "C" clearance numbers that are usually etched
onto the bearing after final inspection. Manufacturers leave unmarked those
bearings that have normal (the most widely used) bearing clearance
The clearance markings are C 1 through C s:
Cl
less than C2 -these have the smallest clearance. Use them only
when freedom from all vibration is required and when there is
no possibility of their clearance being ellruinated by external
causes.
C2
less than nonnal-these are used where a uormal bearing would
allow tuo much deflection in the shaft.
NO MARK
this is the standard clearance (normal)
C3
more than normal-tllese are used wllere running temperature of
a normal bearing would eliminate all the clearance.
C4
more than C3-these are used where there are press fits on botll
rings (housing and sllaft).
Cs
more than C4-tllese are used for tile same job conditions as ~
bearings when there will be a marked rise in temperature.
P numbers
P numbers give further subdivisions of the C clearance markings. They
classify the bearing's tolerance. P numbers range fromP2 (close tolerance)
to P6 (large tolerance).
Installing and removing
ant i-fri cti on beari ngs !li~,!Jjl.~\Ilm1Miiii~ii~!liiiiiig~~~~~~?&i!!~~~~~J!!l~·~:J!!l!tlJ!!lliill~[({illltjli!jf~;jil~~ii!ll!lI!
Bearings are installed (fitted) square on the shaft. They are installed in
several ways. To determine appropriate fitting methods the housing and shaft
must be checked. Bearings may have a push fit or an interference fit.
Shaft and housing checks
Before installing any bearing, check the shaft and housing for:
•
diameter
•
roundness (out of round)
•
shaft deflection (bending)
•
any surface damage that will cause high spots on the bearing seat
•
general finish
•
any damage to the shoulder supporting the inner ring
•
cracks in the housing
Be careful when using an emery cloth to improve the shaft surface-do not
remove the interference that is deliberately built into the shaft diameter.
•
After checJcing the shaft, wipe it clean and coat it with light oil. With a
press fit, you may need to use an anti-seize compound.
•
If tbe housing is split, check the comers for damage and correct them if
necessary.
Push fit
Push fit means that a bearing is installed by hand without mechanical aids.
This method may be used whenever the bearing can be held in place by
mechanical aids such as snap rings.
Creep
Creep is the very slow rotation of a push-fit bearing race. One rotation of the
ring by creep requires several million revolutions of the rest of the bearing.
Creep changes the contact area on the raceway and extends the life of the
bearing.
Interference fit
Interference fit is a description of how tightly a bearing fits on the shaft.
Inteiference is expressed as a measurement. The amount of interference in
the shaft and bearing ring is exactly equal to the clearance reduction in the
bearings after installation. The purpose of the interference fit is to hold the
bearing ring firmly in position and reduce the clearance to a specified
amount.
To achieve an interference fit, bearings are installed using press or shrink fits
or by using a taper adapter sleeve. In a press fit, a mechanical means of
applying force is used. Shrink fits use expansion and contraction of metal
during temperature changes to firmly position the bearing.
Examples offit and clearance
For information on shaft and bearing dimensions, see Shaft and Housing Fits
to ISO Standards. Fits and clearances are available in most bearing
manufacturers' catalogues.
In general:
o
For any parallel bore bearing, the tighter the bearing is on the shaft, the
less clearance is left in the bearing. The usual bearing installation has
one ring with an interference fit, and another ring with a push fit.
o
In mounting a straight bore bearing, the rotating ring is a press fit and the
non-rotating ring is a push fit.
o
Some installations (for example, a separable bearing) use a press fit on
both rings.
o
Most bearing applications that millwrights deal with require an
interference fit between the shaft and bearing inner ring.
Press fits
Press
Force
Press fits are used with small bore
bearings. To press fit a bearing, you
use a tube to distribute the force
evenly.
A bearing can be pressed onto the
shaft, or the shaft pressed through the
bearing, depending on the shaft
design and weight, and on the
components in place on the shaft. For
example, when replacing bearings in
a three-shaft reduction unit, the
bearings are usually pressed onto the
intermediate and final shafts, but it
may be easier to press the input shaft
through its bearing.
Bearing
Figure 50 Using a tube to
press a bearing onto a shaft
Figure 50 (previous page) illustrates a typical design of tube or press for
pressing a bearing onto a sbaft. The press ram bears down on a tube which is
in contact with the bearing's inner ring.
With large diameter bearings, a plate is needed between the ram and the tube
to distribute the press force evenly.
Use eye protection when press fitting bearings.
To press a bearing onto a shaft:
I. Apply a coating of light oil to the bearing bore and to the shaft to
reduce friction.
2. Put fhe bearing on the shaft, making sure that it is square to fhe shaft.
3. Place a clean pipe or mounting tube slightly larger than the shaft
diameter on the inner ring of the bearing.
4.
Apply pressure by hand to start the bearing moving into position.
5. Check fhe position of the bearing to see that it is still squarely on.
6. Press it to the final position.
7. As the bearing moves down the shaft, rotate fhe outer ring by hand to
check for drag or loss of clearance. If the bearing clearance is lost
partway down fhe shaft, do not press the bearing the rest of the way.
To press a shaft into a bearing:
When using an arbor or hydraulic
press, do fhe following:
1. Use a groove or slot in the press
bed that just clears fhe shaft. The
support blocks must be flat.
smoofh. and of fhe same
fhickness.
Press ram
•Force
'=r==T='
1
Spacer
Shaft
2. Make sure that fhe support blocks
touch bofh the inner and outer
rings of fhe bearing as shown in
Figure 51.
Figure 51 Pressing a shaft
through the bearing
Shrink fits
In a shrink fit, the dimensions of a bearing are changed before assembly.
This simplifies the initial bearing-to-shaft or beariog-to-housing assembly by
allowing the parts to slide together. The fit tightens as the temperatures of
the parts equalize.
The dimensions can be changed in two ways in each type of assembly:
•
In a bearing-to-shaft assembly:
- Heat the bearing, or
- Chill the shaft.
•
In a bearing-Io-housing assembly:
- Chin the bearing, or
- Heat the housing.
Chilling the bearing
To shrink fit a bearing, chill the bearing or the shaft in a mixture of dry ice
and alcohol, or in liquid alr. These methods are preferred because they are
easier than heating the housing. Dry ice in alcohol has a temperature of
-79'C (-IIO'F). Liquid air boils at-190·C (-310°F).
A
Take extreme care to avoid frostbite when /reezing a bearing or shaft.
Use approved protective equipment.
Heating the bearing
The recommended maximum temperature for heating a bearing varies from
manufacturer to manufacturer. Heating beyond the maximum temperature
may soften the bearing metal. The bearing is usually heated 45-50°C
(80-90°F )above the ambient temperature. Above 120'C (250'F) the bearing
structure changcs may be permanent.
A
When heating a bearing, always stay close enough to check the
position and temperature of the bearing throughout the process.
Do not exceed 120·C (250°F).
When heating the bearing, the heat source must be clean, gradual, and
indirect. Various heat sources are used:
•
heat lamps or irifra-red lamps can be used for small bearings. This is a
slow process, but has the advantage of allowing the bearing to be heated
either wrapped or unwrapped.
•
an oven can be set up tu heat bearings. This is a relatively safe way of
heating a bearing, as the temperature can be controlled. Before using the
oven, check the heat settings with an accurate thermometer.
•
a good induction heater is quick and quiet. Early induction heaters or
one-stage heaters left residual magnetism in the bearing. This attracted
any ferrous particles in the oil lubrication system. Modern two-cycle
induction heaters have a demagnetizing cycle to produce a non-magnetic
bearing.
This method has no visible way
to control beat, so expansion
must be checked by:
marking the outer ring using a
wax crayon (called temp stick)
or other material which
changes state at a known
temperature, or
measuring the bore of the
bearing to check for the
required expansion.
Figure 52 Induction heater
•
an oil bath is used to heat large bearings using an oil with a high
flash point. To do this:
- Keep the bearings out of contact with the bottom of the pot, as it is
important to keep the bearings away from any localized heat source.
Use wire handles as shown in Figure 53.
Keep the temperature below 121 'C (250'F).
During the heating, check the ID of the bearing against the OD of the
shaft using telescopic gauges or other comparator tools, and a
micrometer. There should be sufficient expansion for a slip fit.
To do this:
1. Measure the shaft with
an outside micrometer.
2. Set the telescopic gauge
3. Use the telescopic gauge
as a standard to check
the bearing. Once the
gauge can slip through
the bearing ID, the
bearing is ready for
installation.
Max
Bearings
to the outside
micrometer setting plus
the necessary expansion.
I
--,;,r------------ _1<-0 illevel
I1:Ii!!ma .....
-.~
!IISIl'
I
.,
f--
Grill
I
-
Figure 53 Oil bath
Installing a hot bearing
When installing a hot bearing on a shaft, move it quickly to its final position
to keep it from seizing. If any resistance is felt, remove it as quickly as
possib Ie, and reheat it.
Press or shrink fitting stretches the inner ring of the bearing and reduces the
internal clearance. If too tight a press Or shrink fit is used, the bearing's
internal clearance will be eliminated or reduced so much that the bearing will
drag and seize when it warms up. See the section on interference fitting.
Internal
r-_-,clearance
LD,"""",-ot .::;::':': :::f-
Reduced
r - - - , internal
L../____--'
clearance
Figure 54 Loss of clearance
A replacement bearing such as a
tapered roller or an angular contact
has the end float controlled by shims.
When these bearings have cooled,
check them for a possible gap
between the shaft shoulder and the
cone or inner ring (see Figure 55). It
may be necessary to press the bearing
ring tight against the shoulder.
".---.... Bearing
·radius
Shaft
/ 7......... L
llliel
Correct lit
Figure 55 Shoulder check
Installing bearing outer rings
An outer ring can be installed in either a bored (one-piece, solid) housing or
a split housing.
Split housing
To install bearings in a split housing;
1. Install the bearings on the shaft.
2. Put the mechanical assembly into the split housing.
3. Install the caps.
4. Torque the bolts.
Bored housing
A push-fit bearing must be
started square with the housing.
If the ring is misaligned at the
start, the ring will not self-align
as it is forced into the bore.
.
-
.""
\MiSaligned
rin g
Figure 56 Incorrect starting
Axial positioning
If radial load bearings arc installed with a minimum of interference, it is easy
to assemble and remove them. A press or shrink fit is suitable for holding a
bearing in position on the shaft but positive keeping (that is, using a device to
hold the bearing in place) may also be needed. The device may give a
definite position to the inner ring, the outer ring, or the cap.
The bearing may have a straight bore on a straight shaft; or it may have a
tapered bore on a tapered shaft. The threaded portion is straight or "parallel."
Positive position of the Inner race
Bearings may be held by:
•
The snap ring holding the bearing
in place is in a groove cut in the
shaft or housing.
'"~~
Figure 57 Positive position
using a snap ring
•
A large washer having the same
diameter as the inner ring is flush
with, or slightly protrudes over
the end of the shaft. The bearing,
locknut, and washer have:
- a straight fit on the shaft
- a tapered fit in the bearing
Washer
Locknut
Screw i
I~
Washer I
Figure 58 Positive position using a large locknut
Taper adapter sleeve
r--~-lj
i
I
~.
If a withdrawal sleeve is
used, a nut gives positive
positioning as shown in
Figure 59.
K::
L,.
I
Locknut
d:
""
1 Id
I
Lockwasher
Figure 59 Using a withdrawal sleeve
Positive position of the outer race
In tapered roller bearings the inner
ring is called the cone and the outer
ring is called a cup.
•
The end caps of the bearings are
used for positive positioning
(axial) with or without shims.
•
For small bearings, the snap ring
is in the housing.
Figure 60 Positive position of the cup
Bearing
()
Shaft-+_-+-_
Snap ring (for
positive positioning
of outer ring)
Figure 61 Bearing held by snap ring in housing
Floating and fIxed bearings
To prevent end float, only one bearing on any shaft assembly should be fixed
axially in the housing. This is called the "held" or "fixed" bearing. It is
generally better to hold the bearing at the drive end. All other bearings on the
shaft should have axial clearance in the housing to allow movement. They
are called "free" or "floating" bearings. This allows the shaft to expand and
contract.
Stabilizing rings to locate bearing axially
a
Clearance to permit axial motion
b
Figure 62 Fixed and floating bearing assemblies
Plant policy and the availability of space for rings determine whether fixing
is done with one or two stabilizing rings. Either assembly method holds the
bearing in position as shown in Figure 62a.
The floating bearing is usually in the centre of the pillow block-never on
the shoulder. If the installation uses a relatively long shaft, and the shaft is
subject to a marked heat rise when in operation, a better position for the
floating bearing is shown in Figure 62b.
Fixed/floating assemblies do not include axial-control bearings (such as
tapered roller) and angular contact bearings.
Axial clearance or thrust adjustment
Tapered roller and angular contact ball bearings are mounted so that the end
float of a shaft between two bearings is held to a required minimum.
Thrust adjustment may be made in two ways:
•
Adjust the inner raee or cone when the outer race or cup is fixed in the
housing.
•
Adjust the outer race or cup when the inner race or cone is fixed on the
sbaft.
In most industrial applications, the outer race is adjusted by means of an end
cover and shims. The amount of clearance to be obtained is determined by
the manufacturer's specifications for the machine.
Figure 63 shows two places where shims can be IIsed to aehieve end
adjustment:
•
•
At point (a) the ring shims
are between the bearing cup
and the end cover. Adding
shims decreases axial
clearance; removing shims
increases axial clearance.
-BearirlQ cap
At point (b) the shims are
between the end cover and
the machine housing.
Adding shims increases
axial clearance; removing
shims decreases axial
clearance.
jc..lEi2~i-SPigot
Figure 63 Two shim positions in a
high-speed shaft-seal assembly
End
cap
In Figure 64, adding shims increases the
amount of end float in the shaft;
removing shims reduces the amount of
end play. The most common shim
position is shown in the upper left comer.
Figure 64 Using shims to adjust end float
Thrust control can also be obtained by using a single taper roller bearing or a
single angular contact bearing on one end of the shaft, and allowing the other
end to float.
Arrangement of angular-contact bearings
These bearings are bought and installed as a matched pair (matched duplex).
There are three basic arrangements:
•
back-to-back
•
face-to-face
•
tandem
The arrangements selected depends on the load, speed, direction and amount
of thrust, and which ring is the press or push fit on the shaft or in the
housing. See Figures 65 to 67.
•
Back-to-back arrangements are used when the outer ring is push-fit.
Figure 65 shows a back-to-back matched pair; notice that the outer rings
are tight up to each other in the housing but there is a gap between the
inner rings on the shaft. When the locknut on the shaft is tightened, the
inner ring closes up leaving no gap. The movement puts a definite
amount of pre-load into the bearing set. Note that this amount of preload is not adjustable.
•
Face-to-face arrangements are used when the inner ring is push-fit.
Figure 66 shows a face-to-face matched pair. In this case, the inner rings
touch and the outer rings have a gap. When the bearing end cap or cover
plate is tightened, the gap closes and this pre-loads the bearing set.
•
Tandem arrangements are used when both rings are press-fit. The inner
or the outer ring is ground down a given amount by the manufacturer.
Figure 67 shows a tandem pair. They are also called face-to-back
matched pairs. In this arrangement The back surfaces of the outer rings
are supported against the housing shoulder. They carry thrust in one
direction only. This provides maximum thrust capacity for
unidirectional, axial loads applied along the shaft.
jSupport here
Support here
Figure 65 Back-to-back
angular-contact bearing
Figure 66 Face-to-face
angular-contact bearing
Figure 67 Tandem
angular-contact bearing
Installing tapered-bore bearings
Using sleeve adapters
The use of a tapered adapter sleeve allows you to install (mount) a bearing at
any place on a standard shaft. The sleeve has the following characteristics:
•
The outer face of the sleeve is tapered to match the tapered bore of the
bearing's inner ring.
•
The sleeve is slotted to permit expansion and contraction.
•
The sleeve is threaded at the small end to take a locknut.
•
When the sleeve is drawn up tight, a press fit is made with both the shaft
and the inner ring.
When adapter mountings are used, take care that the bearing is not tight
internally, or clearance will be eliminated.
Mounting ball bearings using a tapered sleeve adapter
To mount ball bearings, the general procedure is to:
1. Rotate the outer race and swivel it slowly while tightening the locknut.
2.
You will feel a light drag on the bearing when the clearance is reduced.
3. At this point, stop tightening and lock the nut in position with the tabwasher.
Mounting tapered-bore bearings using a hydraulic nut
When mounting tapered-bore bearings using a hydraulic nut:
1. Check the reduction in radial clearance of the bearing to ensure proper
installed clearance exists.
2. Dismounting the bearing hydraulically gives an abrupt release. Make
sure a stop, such as the shaft or sleeve nut, is provided to act as a stop
during dismounting.
Mounting spherical roller bearings
Check the clearance:
1. Check the initial clearance by inserting a feeler gauge between the top
rollers and the outer ring as shown in Figure 68 on the next page.
2. Press the two top rollers inward to assure their contact with the centre
guide flange as well as the inner-ring raceway.
3. Check the clearance by starting with the thinnest feeler (preferably
0.0015"), using progressively heavier blades until one fails to go
through. The blade thickness before "not go" is the measure of the
clearance before installation.
Figure 68 Checking clearance in spherical roller bearings
Mount the bearings:
1. Apply light oil, or other suitable lubricant, to the outside, and to the
threads of the tapered sleeve and to the threads and face of the locknut.
2. Assemble the parts, locate their position on the shaft, and check for
initial clearance.
3. If unloaded rollers are at tbe bottom, make sure tbey are raised to the
seat on the inner race and set against the guide flange.
4. • Assemble small bearings with the tab-washer in position.
• Mount large bearings for final clearance with the tab-washer left out.
Then install the washer and tighten and lock the nut. This prevents the
forces involved from shearing off the inside tab of the washer
5. Tighten the adapter nut with a spanner wrench until it is snug. The
adapter sleeve may have a tendency to tum, but it can be held in position
until it binds on the shaft.
6. Drive against the nut, using a hammer and soft steel bar to release the
pressure on the threads.
7. Tighten the nut with a wrench, then check for clearance.
8. Repeat this driving and tightening until the final clearance is reached.
9. Secure the locking tab-washer.
Reduce the clearance
You must reduce the amount of initial clearance to suit manufacturers'
recommendations.
Example 6:
Initial clearance = 0.004"
Recommended reduction = 0.0015 - 0.0025"
Average reduction = 0.002"
Final clearance = 0.002"
Example 7:
Initial clearance = 0.007"
Recommended reduction = 0.0026 - 0.0025"
Average reduction = 0.003" (rounded off)
Final clearance = 0.004"
•
A reduction of 0.0026" would leave an installed clearance of
(0.007" 0.0026" = 0.0044"). This would be suitable where the load is
uniformly from one direction; or when the shaft heats during operation,
but the housing stays cool.
•
A reduction of 0.0032" would leave an installed clearance of 0.0038"
(0.007" 0.0032" = 0.0038"). This would be suitable where the
temperature of the shaft and housing remains relatively even, but the
bearing is subject to a reciprocating load.
•
Therefore, 0.004" is an adequate estimate of final clearance.
Any hearing installation involving high temperature changes or heavy
reciprocating loads should be installed according to specifications.
For standard-clearance, spherical roller bearings only, when no tables are
available, a rule of thumb is to reduce the initial clearance by roughly
50 percent. Another rule of thumb is to convert the last two figures of the
bearing number into thousandths of an inch: for example, 2224->0.0024".
Reduce the initial clearance by this amount to get installed clearance.
Bearing removal
Non-destructive removal
The best way to pull off a bearing with the
minimum amount of damage to the bearing shaft
is with a set of pullers or a press.
When using a press, make sure that:
•
All the strain is applied to the
inner race and the strain is even
on the opposite sides.
•
The parallel bars are under the
inner race.
No load
through
balls -rl~-"+1
Figure 69 Press removal of a bearing
You can use a bearing splitter and a two-jaw puller to remove a cone from a
shaft. See Figure 70 (next page). With this setup, make sure that the jaws of
the puller do not bear on the threaded section of the ruds holding the puller.
Puller plate I
Bearing puller
-=1
Figure 70 Bearing splitter and two·jaw puller
In some installations, it is not possible to pull or press on the inner ring;
force must be applied to the outer ring only. This increases the binding
action of the inner ring on the shaft. It can also fracture the outer ring. When
pulling or pressing on the outer ring, wrap or guard the outer ring to prevent
injury from flying fragments in case the ouler ring breaks. This wrapping is
important when using a hydraulic power source.
Push puller
Cup puller
You can remove bearing
cups from a housing with a
cup puller (also called an
inside puller) provided
Cup
that tbe jaws can be
inserted behind tbe
shoulder. See Figure 71.
Figure 71 Inside puller for removing bearing cups
After tbe bearing is removed, future problems can be prevented by drilling
and tapping for two setscrews in the bearing cap. During operation, these
tapped holes can be filled with hollow head setscrews.
Housing drilled
and tapped
--I7":l- Capscrew
LL~~UL~~
hole
'!hrerujed hole
for a setscrew
Figure 72 Drilling and tapping after the bearing is removed
Withdrawal and adapter sleeves
When the outer ring is mounted in a solid housing, a withdrawal sleeve
assembly may be used with a standard tapered-bore bearing to permit easy
removal of the sleeve and bearing. The differences between a withdrawal
sleeve and an adapter sleeve are as follows:
Withdrawal sleeve:
•
solid bored housing
•
shaft is machined as needed
•
sleeve slides under the bearing
•
driving nut is threaded onto the shaft
•
bearing is held by a shaft shoulder or a machine part against the
inner ring
•
installation procedure: the bearing f1rst, then the sleeve
Adapter sleeve:
•
split bored housing
•
no shaft changes
•
bearing slides over the sleeve
•
driving nut is threaded onto the sleeve
•
bearing does not need a backing or shoulder
•
installation procedure: the sleeve first, then the bearing
Both styles of mounting use the Same routines to check the initial clearance
and reduce the required installation clearance.
Removal of bearings mounted with adapter sleeves
Method J:
1. Pry the tab-washer tab out of the slot in the locknut.
2. Remove the locknut and washer.
3. Drive or pull the bearing off the wedge on the sleeve;
uSe force only on the inner ring. See Figure 73method 1.
4. Remove the sleeve and the bearing from the shaft.
Method 2:
I. Pry the tab-washer tab out of the slot in the locknut.
2. Back the locknut to the cnd of the sleeve.
3. Use a tubular drift against the locknut to drive the
sleeve through the bearing. See Figure 73-method 2.
4. When the bearing is loose, unscrew the locknut and
remove it. Also remove the lockwasher, the sleeve and
the bearing.
All parts may be used again after they are thoroughly cleaned and examined.
Adapter
, locknut
Drive this way_.
(method 2)
+
Drive this way
-----4i---' (method
1)
c::_,",_::-o_=-=cc_=-c_="i
"'" =[e,
sleeve
Figure 73 Removal of bearings mounted with adapter sleeves
Removal of a withdrawal sleeve
Figure 74 shows an installed bearing withdrawal sleeve.
1. First remove the bearing nut and washer on the shaft.
2. Then install the withdrawal nut on the threaded end of the sleeve.
3. Tighten it with the correct-sized wrench to poll out the sleeve (see
Figure 75).
4. Tag the withdrawal nut and keep it where it can be found easily.
Withdrawal
sleeve
Fi~ure 74
Installed
Withdrawal sleeve
Figure 75 Removal of
withdrawal sleeve
Destructive removal of a bearing
Destructive removal is used for bearings on a shaft when the bearings are
definitely not to be re-used. An oxyacetylene torcb is used to cut away the
bearing rings. The procedure is as follows:
1. Cut through the outer ring in two places 1800 apart.
2. Cut away the roller assembly in two places 1800 apart.
3. Carefully cut away the inner ring as follows:
a. Hold the torch along the surface of the ring.
b. Move the torch slowly back and forth across the surface to "wash
away" the metal of the bearing ring.
c. Stop just before you break through the metal.
d. Tap lightly with a hammer if necessary to break away the ring.
e. Pull the ring off.
Welding a bead
Bearing cups are frequently set in seats with no room to use a set of pullers.
In such a case, the cup can be removed by welding a bead on the face of the
raceway. This will shrink the bearing diameter.
Bearing
~
i I
~~~ring-F~
Bead of weld all
around the cup
Figure 76 Welding a bead onto the face of the cup raceway
Hydraulic (oil injection) removal
Hydraulic dismounting (removal) uses the force developed by oil or grease
under pressure to expand the inner ring of the bearing.
Figure 77 shows a shaft
modified for oil injection by
cutting a semicircular groove
at the approximate centre of
the bearing, and drilling a
supply line to a fitting at the
end of the shaft.
The oil injected into the shaft
and groove is under pressure.
It expands the inner ring of
the bearing, making it easy to
remove.
Figure 77 Hydraulic removal
Oil injection equipment can be either of the following:
•
a grease gun capable of developing over 1000 psi
•
a hand-operated pump for a portable jack, using SAE 40 or heavier oil
Hydraulic dismounting of tapered-bore bearings
To hydraulically dismount bearings mounted on a tapered journal, oil is
pumped between the contact surfaces of the sleeve and shaft. This
immediately and abruptly releases the press fit.
Make sure a stop such as a shaft nut is in place.
Pillow block installation and removal
Pillow blocks are housings used to independently mount anti-friction
bearings. They come in two main groups: split and one-piece.
Split housings
The most common bearing housing
has a base and a cap separated by a
borizontal split. See Figure 78.
The housing sections are put together
so that their mate marks match. If
there are no mate marks on a housing,
make your own before disassembling
the housing.
\
:$:=::::jttJ marks
Male
I-+j-;'
I
...
Figure 78 Horizontal split housing
Horizontally split
A horizontally split housing, has the following five components with code
numbers. If they are standard, their last figures are the same.
•
the housing-a two-bolt, four-bolt, or plain case with no drilled holes
•
the bearing-self-aligning style
•
adapter ~tyle-one OD to suit the stock bearing; two more possible ID to
suit shaft diameters
•
tab-washer
•
bearing nut
The assembly also has seals, blanking discs, and locking or fixing rings for a
held assembly.
Vertically split
A vertically split bearing housing has a one-piece ring and a base with
separate end covers.
One-piece housings
Pillow blocks with one-piece housings are pre-assembled. The unit can be
slipped over a smooth shaft and bolted into position. Unlike a standard
bearing, the inner ring is extra long to distribute the load over the shaft.
Common designs are:
•
single-groove ball bearings-for high radial, low axial thrust capacity
•
double-taper roller bearings-for high radial and axial thrust loading
•
spherical or concave rollers-for high radial and axial thrust loading
These bearings are externally self-aligning because of the shape of the outer
ring which rocks or pivots in the housing. This self-alignment allows more
misalignment than a self-aligning split bearing. Refer back to Figure 48.
The inner ring is held in position on the shaft in various ways depending on
the manufacturer. Two common methods used are:
•
Thread two setscrews intu a collar and pass them through matching holes
in the inner ring to engage the shaft.
•
Use a self-locking collar (''Camlock™'') held in place by one or two
setscrew(s). This is only used on urn-directional drives because it is
eccentric.
Mounting
A pillow block is mounted with its base parallel to the shaft. When mounting
(installing) pillow blocks with one-piece housings, tighten them on the base
before locking the inner ring in position on the shaft. This allows the bearing
to align to the shaft.
Removal
When the bearing must be frequently removed, file a flat spot on the shaft at
the setscrew location. For easy removal of one-piece bearings, mount them
with the screws or setscrews on the side away from the shaft end. If the shaft
is damaged by the setscrews, you will not need to move the bearing very far
to clear the rough surface and there will be less contact between the
bearing's long inner ring and any roughness on the shaft.
Mounting other bearings
Flange bearings
A flange bearing has its base at 90' to the shaft. See Figure 79. Flange
bearings are available in two-. three-. and four-bolt styles.
80lt
hole
a
b
Figure 79 Four-, three-, and two-bolt flange bearings
c
The inner ring, rolling element, and outer riug of flange bearings are
supplied as a unit called a cartridge. To replace a cartridge:
1. Pivot the worn bearing at the loading notches.
2. Pull out the cartridge.
3. Slide in the new cartridge.
4. Pivot the new cartridge to the correct position.
Needle bearings
One common type of needle bearing has no separators or inner ring, but only
an outer ring and the rolling elements. To keep the needles in place before
mounting, the bearing is fitted with a temporary sleeve or inner ring of shaft
size. During mounting, the bearing (with its sleeve) is pushed over the shaft,
so that the shaft pushes the sleeve out. See Figure 80 (and also Figure 38).
[j{
jJ
Temporary -F~""";z;;z;z;;;:z:;:;~ r-~~~~~~~~"""'"
sleeve
>0
)IJ
Shaft
Figure 80 Mounting a needle bearing with no inner ring
It is possible to coat smaller needle bearings with light grease so that they
stay in position during mounting.
Thrust bearings
Thrust bearings (Figure 81) are designed to carry thrust (axial) loads.
Because of centrifugal force, these bearings are limited in the speed at which
they can be used. A dual-direction thrust bearing has two housing washers
and one shaft washer. Single and double thrust bearings are mounted in the
same way. To mount thrust bearings, fit one riug to the housing and the other
to the shaft
~---> Self·aiigning
Figure 81 Thrust bearings
Maintaining antifriction bearings
Keeping bearings clean
If bearings are in good condition, dip them in oil or grease before installing
them. If they are not, clean them. Do NOT wash bearings with seals on both
sides. Wipe them off with a clean rag to keep the dirt from working inside.
Treat bearings with one side shielded like bearings without a shield.
Cleaning bearings without shields
To clean a bearing without shields, do the following;
1. Use recommended solvents in a well-ventilated area.
2. Soak the bearing in the solvent long enough to loosen the grease and
dirt.
3. Rotate the races slowly to work out the old lubricant.
4. When using an air hose, don't spin the bearing. Hold both races and let
the air jet blowout the particles.
5. Finish washing the bearing in a container with clean solvent.
•
The use of so/vents creates a fire hazard. Do not use fire-fighting
equipment to hold solvents.
•
Some commercial solvents remove all traces of lubricant from bearing
surfaces. This leaves a DRY bearing with metal.to·metai contact. This
may give a false reading when checking bearing wear.
Storing bearings
Cover the clean bearings with a light coat of oil. Wrap the bearings in a
special, acid-free bearing wrapper (oil-impregnated paper) and store them in
a clean, dry place until needed. Partial assemblies should also be cleaned and
covered until needed. Attach a tag or label to the wrapping with the bearing
code number and the machine for w hieh it is used.
Preventing bearing contamination
Avoid bearing contamination by keeping surrounding things clean as
follows:
I. Work in clean surroundings on a smooth bench.
2. Use clean tools in good repair.
3. Handle bearings with clean hands.
4. Remove all outside dirt from the housing before exposing bearings.
5. Lay bearings out on clean paper.
6. Wipe bearings with clean, lint-free rags.
Seals
Seals are used to keep the lubricant in the bearing housing and contamination
out. They may be mounted on the machine or as part of the housing. Various
types of seals used for anti-friction bearings are shown in Figure 82.
Felt lip
Annulus
Labyrinth and flinger
Rubber lip
Annulus and flinger
Figure 82 Seals
Lip contact seals
Lip contact seals come in two forms:
•
Conunercial seals-these seals have a contact lip of synthetic rubber or
leather, with a spring backing for more positive sealing action. They are
used where a shaft is totally or partially inunersed in oil.
•
Felt seals-these seals are good with grease at low speeds. They are not
suitable for high speed, high temperature, or abrasive conditions.
Annulus seals
Annulus seals are a series of grooves in the housing or end cover with a
drain hole at the bottom. These are non-contact seals-there is a very slight
clearance between the rotating and stationary parts. They are not effective if
the drain plugs.
Labyrinth andflinger seals
Labyrinth and flinger seals depend on centrifugal force to throw material
away from the housing.
Annulus andflinger seals
Annulus and flinger seals keep oil in and foreign material out.
For more details, see Chapter 14: Seals.
Keeping bearings in good condition
To keep bearings in good condition you must continually check them for
wear and other signs of failure. All bearings need regular lubrication with the
correct lubricant.
Check for bearing wear
Check as follows:
I.
Be sure that the bearing is flat or horizontal; this ensures that there is
contact all around the ring.
2. Hold one ring stationary and turn the other ring slowly.
3. Turn the bearing over and repeat
4. If the bearing is self-aligning (double ball or double spherical), misalign
it and check all around the outer ring.
Check for bearing failure
Continually check for the following symptoms of bearing failure:
•
temperature rise
•
increased vibration
•
unusual noise
The most common cause of anti-friction bearing failure is contamination.
Dirt or foreign material gets into the bearing during assembly or works past
the seals during operation.
Lubrication
You must use the grade of grease or oil recommended for the particular
bearing and its application.
For a double row of self-aligning bearings
1. Misalign the outer race and pack grease between the balls or rollers.
2. If the bearings is in position in the bottom half of the pillow box, force
the grease through the openings on one side. Do this until grease shows
on the other side.
continued
3. Fill the case one-third to one-half full of grease. For oil lubrication, fill
the case to a point halfway up the lower ball or roller.
For deep-groove ball and cylindrical roller bearings
Pack the grease well between the balls or rollers, covering both sides of the
bearing.
Grease
Figure 83 Double-row, self-aligning
ball and roller bearings
Figure 84 Deep-groove and
cylindrical roller bearings
Maintaining Cooper (split) bearings
Most anti-friction bearings are considered to be one-piece units. However,
where installation and removal of bearings is not practical because of other
components mounted on the shaft, the Cooper bearing may be used. It is a
split bearing with all its components in halves as shown in Figure 85.
To maintain these bearings, you must pay attention to the whole assembly.
Separate it and then maintain each part.
Special cautions
Problems in adapter sleeves
A
•
The inner ring may be forced too far up on the taper. This expands the
inner ring enough to bring the clearance reduction below standard.
Check the clearance with feelers and back the bearing off if clearance
has been reduced too much.
•
If the inner ring is not tight enough on the sleeve, allowing the ring to
turn on the sleeve or the sleeve to turn on the shaft, or both.
•
During installation, the sleeve needs to be free to turn on the shaft and
the inner ring of the bearing needs to turn on the sleeve. If the tabwasher is not set on the locknut, allowing the nut to back off, the sleeve
and ring may not be free to turn.
See the next page for some special working precautions.
Figure 85 Cooper split bearing (essembly view)
A
ALWAYS cover an exposed bearing after working on it.
Also:
•
Do NOT mount radial bearings or self-aligning roller bearings with
heavy thrust loading.
•
Do NOT mount angular-contact ball bearings backwards; this causes
heavy thrust on the shallow shoulder.
•
Do NOT use too few or too many shims when mounting thrust bearings
in casting; always use the correct clearance for end play.
•
Do NOT mount self-aligning bearings at too extreme an angle; they
are self-aligning to a small degree, but should be mounted as evenly
and as level as possible.
•
Do NO]' "hold" more than one bearing on the shaft; allow roomfor
exponsion.
These actians may cause overloading and fallure of the assembly.
MILLWRIGHT MANUAL: CHAPTER 10
Belt Drives
Belt drive prineiples ........................................................................ l0:l
Area of contact ........................................................................... 10: 1
Materials of belts and pulleys ........................................................... 10: 1
Belt tension ....................................................................................... 10:2
Slip and creep in belts ....................................................................... 10:5
Flat belts ......................................................................................... 10:6
Flat belt materials ............................................................................. 10:6
Joining flat belts ............................................................................... 10;7
V-belts ............................................................................................. 10;9
Advantages ....................................................................................... 10;9
V-belt construction .......................................................................... 10:10
V-belt types, sizes. and codes ......................................................... 10; 12
Belt drive assemblies .................................................................... 10:19
Pulleys and sheaves ........................................................................ 10; 19
Other drive components ................................................................. 10;20
Drives and pulleys for flat belts ...................................................... 10;22
Drives and sheaves for V-belts ....................................................... 10:23
Maintaining belt drives ................................................................. 10:26
Troubleshooting belt drives ............................................................ 10:27
Belt Drives
This chapter deals with power transmission by means of belts. A belt drive
offers a wide range of shaft centres (the distance between the axes of two
parallel shafts) with versatility of sheave or pulley diameters.
Belt drives arefriction drives. They transmit power by means of a belt
pressed tightly onto a pulley. The power of the prime mover is transmitted to
the pulley by the belt.
The belt and pulley may be of various shapes and materials to suit the
specific application. The amount of power that a belt drive can transmit is
directly related to the grip of the belt on the pulley. This grip depends on:
•
area of contact
•
materials of the belt and pulley
•
tension in the belt
Area of contact
The belt wraps around a pulley wheel so that their surfaces are in contact.
Frictional grip oeeurs between the two surfaces, preventing the belt from
slipping and allowing power to be transmitted
The arca of contact between the belt and pulley surfaces depends on the belt
width and the arc of contact, The arc of contact depends on pulley diameter,
centre-to-centre distance, and the take-up devices used. To take-up means to
tension the belt by moving the prime mover and/or using an idler.
Drives are designed to use specific sizes to obtain maximum efficiency for a
given rpm. The profile or cross-sectional shape of the belt may be circular.
square, rectangular, V-shaped, or other special shapes for particular
applications. The pulley surface may be flat, crowned or grooved, When they
are grooved they are called shcaves. A crowned pulley surface is convex.
with the highest point at the centre,
Materials of belts and pulleys
Belts are constructed from various strung and flexible materials. The most
common are listed on the next page.
MILLWRIGHT-BELT DRIVES
10
Common belt materials are:
• cotton
• leather
• rubber
• nylon or other synthetic materials
The materials industrial pulleys are most commonly made from are:
• cast iron
• pressed steel
• die-cast alloys
Coefficient of friction
The frictional grip varies with the coefficient of friction between the belt and
pulley. It is determined by the belt and pulley materials and their surface
struetnre.
Belt tension
Belt tension is the amount of stretch applied to a belt. When a belt is slack on
a drive, the driver is moved to tighten the belt to the correct operating
tension.
Installed tension
The installed tension is the tension on the belt when the drive is not running.
For flat and V-section belts, this is expressed as percent elongation or
percent tension.
When the drive is not running. the belt's top and bottom strands are then
under equal tension.
Effective tension
Effective tension is the tension needed On the belt to transmit power without
slipping. When the drive is running, the pull on the belt increases the tension
and stretch on the tight side as it overcomes the resistance of the load. See
Figure 1. The slack side has no tension increase because it simply returns the
belt to the driven pulley.
Slack side
Driven
(
Driver
Tight side
Figure 1 Drive assembly in operation showing
tight and slack sides of the belt
MILLWRIGHT-BELT DRIVES
Deflection method for correct tensioning
Use manufacturer's specifications for the belt being tensioned. V-belts are
tensioned in the same way as flat belts but to specific forces. Table I shows
the reconunended forces to produce required deflections in a V-belt.
Table 1: Recommended V-belt deflection forces In pounds
Ellillt type
Normal
Maximum
New belts
A
2
4
8
12
21
4
9
20
3
6
12
22
35
7
12
30
4
8
14
26
B
C
D
E
3V
5V
8V
40
9
15
40
For example, a new B-section installation requires 8 pounds offorce to get
the required deflection. A new belt is mOre rigid than a run-in (in-service)
belt, so more force is required to deflect it.
A
Note: A new belt should have a 12- to 24-hour run-in period. 1t should then
be re-tensioned to manufacturer's specifications.
Halfway between the two shafts, at point C in Figure 2, enough force is
applied to deflect the belt 1/64" for each inch of distance between the shaft
centres.
If the span (shaft centres) = 32"
Example 1
Proper amount of force gives deflection
= 32/64"
Amount of deflection
from point A
Shaft centres
I
I
I
A
Figure 2 Belt deflection method
MILLWRIGHT-BELT DRIVES
10-3
Using a deflection
gauge (see Figure 3),
measure to check
that you have the
recommended
deflection.
Sliding
rubber
~ O·rings
If it is too tight,
adjust the tension by
moving the centres
closer together. If
too loose, move
them apart.
Deflection
force scale
(read down)
Figure 3 Deflection gauge
Approximate belt length
You can calculate the approximate belt length by using the following
formula:
D+d
Belt length = -2-1t + 2C
where:
D = large pulley diameter
d = smaIl pulley diameter
C =shaft centres
1t = 3.1416
Example 2
If D = 20"
d = 10"
C = 38"
Approximate belt length =
20+10
2
1t + (2 x 38)
= 123"
~~llir!1Pxhl~~$;%&.%'1;.¥$imil&.~&®ru'''i~M'~i'gIii;X:iw;rt''ii'iie!iB;''i!00iii0i0itiWj%W!i!~;$'ii0ib~iMUHii!0~'«ii0j;ji'§!PBD~e(10&W5li!jN;U@mm.1iwm"W'_~~""_",
~,
10-4
MILLWRIGHT-BELT DRIVES
Slip and creep in belts
When a belt slips or creeps, the driven pulley speed is decreased. Both slip
and creep cause a loss of transmitted power.
Slip
Slip is caused by the surface characteristics of the belt and pulleys. Severe
slippage burns the belt quickly, destroying its usefulness. It also polishes the
belt and pulley surfaces, reducing the friction grip between the belt and the
pUlley. The pulley should look smooth and rather dull, not shiny.
Excessive slip is caused by poorly designed drives, where:
•
the driving pulley is too small
•
the load is too great
•
the belt is running too loose
•
the belt is not being properly cleaned and tensioned
You can control slip by:
•
using smaller pulley ratios (such as 3:1 or 4:1)
•
aligning the pulleys correctly
•
ensuring adequate take-up
Creep
Creep is a physical characteristic of a belt. It affect.~ all power transmissions
that use belts. Creep is caused by the elasticity of the belt. Figures 4a and 4b
show a belt at rest and transmitting power. The thickness of the belt is
enlarged for clarity.
(b)
(a)
Figure 4 Belt drive at rest and in motion
1. The belt on the slack side is delivered to the pulley slower than the rate
at wlllch it comes off the tight side.
2. The belt creeps ahead on the pulley and the belt surface runs slower than
the belt as a whole.
MILLWRIGHT-BELT DRIVES
-5
Flat belts
Modem, high-power machines do not use flat belts, but they are still found
in some older machine tools, sawmills and grain elevators. They are also
used in conveyer and drip belts, where the belt itself carries the load.
In a flat belt, the drive members (power plies) carry the forces to transmit
power. They may be embedded in substances such as rubber and encased in
covers to provide protection and traction.
If the top side is not easily recognizable, it will be marked. If the belt must
be used in one direction only, that direction will also be marked.
Flat belts may be manufactured endless (in a continuous loop) or as reel
stock (open-ended) that must be fastened together.
Flat belt materials
Leather
Leather flat belts have single or multiple layers. The leather may be
combined with other materials such as cords and polymers. Nowadays,
leather belts are being replaced by rubber or synthetic belts.
Rubber
Flat rubber belts are made from fabric or cord impregnated with natural or
synthetic rubber compounds. They provide various degrees of strength,
stretch, pulley grip, and protection against abrasion, oil, and moisture.
Fabric
Flat fabric belts are made from cotton or synthetic fibres, with or without
rubber impregnation. They are made in layers and are 3 to 12 ply, depending
on their width. The direction of the weave varies, but alternate layers have
their fibres running in different directions for added strength. See Figure 5.
Fabric belts are used for moderate loads and speeds.
Figure 5 Fabric ply flat belt
Cord
Flat cord belts have drive members made from twisted cotton or synthetic
cords embedded in rubber. The rubber has a fabric cover to protect against
wear. Usually, they are heavy-duty belts used for high-speed, small-pulley,
shock-load applications. Compass belting contains a single row of cords.
10- 6
MILLWRIGHT-BELT DRIVES
Figure 6 Compass belting (folded or rolled edges)
Steel-cable
Steel-cable flat belts are similar in construction to cord belts. They have
higher capacity and lower stretch than cord flat belts.
Nylon and other synthetics
Nylon belts are flexible to permit use
on small-diameter pulleys. Figure 7
shows their construction, with the
nylon drive member sandwiched
between the outside cover and the
pulley-side cover.
Top rover
side
Figure 7 Synthetic belt construction
Joining flat belts
When endless belts are not supplied, you must join the ends of flat belts. The
joins may be chemical, vulcanized, or mechanical. Vulcanized splices and
chemically bonded splices are recommended. These bonds havc several
advantages over mechanical fasteners. They are:
MILLWRIGHT-BELT DRIVES
•
long-lasting and dependable
•
smooth and free from noise and vibration
•
stronger, allowing a possible reduction in the number of plies needed
Vulcanized splices
Vulcanized splices use heat and pressure to create a bond, These splices are
the strongest and most efficient belt joints. See also the sectiou on
vulcanized splicing of conveyer belts in Chapter 19: Material Handling
Systems.
Chemical splices on synthetic ply belts
Chemical spliccs use adhesives. The type of belt determines the adhesive to
be used. Old and new belts may be chemically spliced together. A belt
containing more than onc synthetic may need two different chemicals,
A
With a two-chemical splice, take care thot the correct chemical adhesives
are used in appropriate quantities according to the mamifacturer's
specifications.
Mechanical flat-belt fasteners
Connecting
pin
In a mechanically fastened joint,
it is important to cut the belt so
that the ends align properly
before being fastened. The types
of fasteners used are wire
lacing, steel hinges, and plate
fasteners. Figure 8 shows wire
lacing used to join a flat helt.
Wire
CliPS!
Figure 8 Wire lacing
Figure 9a is called alligator
lacing, These fasteners are
assembled using a steel
hinge pin,
(b)
Figure 9b uses threaded
fasteners to bond with the
belt.
--Hinge pin
Figure 9 Steel hinges
tt{~Y'2,.'!lFh<$h"'rD{Ifi,;;,,,;pg;'!iM!£%iffl2%m£¥"N8i0~-liI!"Y"Gr?,,~0SiW,)§;gf&fJXi;!?;;~~~WKi&~-&#Xp/ffSSJhY({fifj(jf'UjCt"""'i({};:i;i'0ix,'8iii'i?i!!@~Jf~r&,t{'~:$tWH.""'~~
10 - S
MILLWRIGHT-BELT DRIVES
Excess threads
removed
Threaded
Double
plate
(a)
Figure lOa shows a plate tastener
consisting of matching pairs of plates
which ara bolted to the belt ends. The
bolts ara tightened enough to compress
the belt between the plates and give
maximum holding power. After the belt
is tightened, exCess threads are
removed flush with the nut
(b)
Figure lOb shows another type of plate
fastener. This Is a steel-pronged plate
used on the top side, away from the
pulley.
Figure 10 Plate fasteners
V-belts
V-belt drives are the most common way of driving loads between shortrange pulleys or sheaves.
Advantages
The main advantage of a V-belt is !be simple wedging action of tbe belt
against the sides of the sheave groove. A greater pull or load results in a
tighter belt grip.
Advantages of V -belts are:
MILLWRIGHT-BELT DRIVES
•
Wedging action pennits lower arc of contact on small pulley and a
large speed ratio.
•
Shorter centre distances can be used to achieve a compact drive.
•
They absorb shocks to cushion motors and bearings against load
fluctuations.
•
Vibration and noise levels are low.
•
Maintenance and replacements are quick and easy.
•
Power transmission efficiency Can be as high as 93% after run-in.
10-9
V-belt construction
V-belts can have a variety of sizes and cross sections, each with its own
particular function. Some of the several types of V-belts are shown in
Figure 11. Each has four sections:
•
The top section of the belt is the extension section. It is rubber and
stretches as the belt wraps around the sheave.
•
The bottom section is the compression section. It compresses when
wedged into, and shaped around, the sheave.
•
The neutral section of the belt neither compresses nor stretches. The
driving members in this area give the belt its tensile strength.
•
The cover section protects the inner parts of the belt from wear. The
cover is the only part in direct contact with the sheave.
Strength section
(pulling cords)
Compression
section
Standard duty
Heavy duty
Double angle
Figure 11 Construction of common V-belts
Notice in Figure 11 that the heavy-duty belt uses a different material for the
cords and may use a heavier fabric cover than the standard-duty type.
Figures 12 to 15 show some typical V -belt construction styles.
Single-layer, compass
construction is used mainly on
short-centre drives with small
diameter pulleys. Each strand
is separated by, and bonded
in. rubber.
Figure 12 Single-layer compass construction
10-10
MILLWRIGHT-BELT DRIVES
Multi-layered cord construction is used
for long, heavy drives. Each strand is
separated by, and bonded In, rubber.
This type has two main bands of blocks
or cords. These V-belts are designed to
flex in only one direction and should not
be used with reverse bending.
Figure 13 Multi-layer cord construction with two bands
The layer or layers of small
cords can be replaced by
two grammers or ropes to
carry the load. This style
has both high load capacity
and great flexibility.
Figure 14 Grommet-style construction
Several plies of fabric can be
bonded together to make a
driving section. Like cord belts,
they can be supplled with one
of (he following:
•
a single band on the
pitch line
•
one band on the pitch
line and another near
(he bottom of the belt.
Figure 15 Ply construction
There are also additional design factors such as:
MILLWRIGHT-BELT DRIVES
•
straight (flat) or concave side-walls
•
notched (corrugated) or plain undersides
10 - 11
Straight and concave side-walls
Figure 16 shows some side-wall designs.
Straight
unloaded
Straight
loaded
Concave
loaded
Concave
unloaded
Figure 16 Flat and concave side-walls 01 belts
Notched V-belts
Figure 17 shows a plain V-belt and one with notches cut into its underside.
These notches give the belt increased flexibility and heat-dissipation
capabilities. They are usually used on small-sheave, short-centre, and highspeed drives.
Notched V-beH
Plain V-belt
Figure 17 Notched and plain V-belts
Endless and joined belts
Most V-belts are
manufactured
endless. Reel-stock
types are used where
an endless belt
cannot be installed.
The ends have metal
fasteners joined by a
pin or by a link and
two pins.
Figure 18 shows a
mechanically joined
V-belt
Mechanically joined
V-belts should not be
used with flat idlers
because of metal-!Ometal contact.
Link
..
~"~-
o
Figure 18 Mechanically joined V-belts
MILLWRIGHT-BELT DRIVES
V-belt types, sizes, and codes
V-belts are not manufactured with the same close tolerances as gears or
roller chain. Regardless of who manufactures the belt, the dimensions are
similar.
V-belts are made to an industry standard of section sizes and angles so that
belts from different companies can be used on the same sheaves. The size
lllay be indicated by a standard code molded into the belt, a company code
stamped on the belt, or a catalogue number attached to the belt.
The code is a combination of letters and numbers such as 5V300. It indicates
the type and nominal length of the belt. To interpret a code, refer to the
manufacturer's specifications.
Sometimes you need to replace a belt with a different type. Figure 19 shows
how some V-belts may be substituted.
Figure 19 Cross-sections of common V-belts
Conventional V-belts
These are also called standard cross section. They may have straight or
concave side walls. The lengths are measured around the inside in inches.
There are five cross sections: A, B, C, D, and E. See Figure 19.
High-capacity V-belts
High-capacity V-belts are also called wedge, hi-torque or narrow V-belts.
The three sizes are designated 3V, 5V, and 8V, meaning that they have 3/8",
5/8", and s/s" (1") of top width. See Figure 19.
They are available in various stock lengths and side-wall configurations.
They can transmit up to three times the horsepower of standard V-belts in
the same drive space. This is due to sheave groove design and increased
contact area.
MILLWRIGHT-BELT DAIVES
10-
The codes of these belts are read along the pitch line.
Example 3
3V250 indicates;
3/8" width and 25" pitch-line length;
8V500 indicates;
s/s" = 1" width and 50" pitch-line length
Light-duty V-belts
Light-duty belts are also referred to as fractional horsepower (FHP) belts.
These have cross sections the same as conventional V-belts but coded 2L,
31." 41." and 5L. These codes indicate width increments of l/s". For example
3L = 3/8".
Modern applications put increased demands on FlIP V-belts. Tbc drives are
more compact and the belt covers are usually made of neoprene for oil and
heat resistance. Neoprene covers also give better protection against the
cffects ofwcather, heat, detergents, and other chemicals.
Double-angle V -belts
Doubl<>-angle V-belts are also called
double V-belts or hex belts. See
Figure 20. In these belts, the strength
cords are in the centre of the belt.
Both halves absorb compression and
tension forces.
Figure 20 Double-angle V-belt (hex belt)
Double-angle V-belts
are used for drives
where the sheaves are
driven by both the top
and the bottom of the
belt. This is so in
serpentine drives, see
Figure 21.
·····-~Driven
Driven
Driver
o
Idler
Figure 21 Serpentine drive using double-angle V-belt
:et.n1"'~!p-Ay:;,*MJ;~,fdmJ;f'fW=~~'W;rJ/tS';7&l}Ni;g00:>V:$,*hhli0:i'!'K:nr':mr:0,v:\Y!,1P!:".:1i(".(i0LlfSWJ!0!J'J!i0'&:;,!:"'0'Ai"0Yt0'~"'00'~~"'r?!;;;;BGiif3Sm!S_,=""Y",f!'ii#b
10-14
MILLWRIGHT-BELT DRIVES
Wide V-belts
Wide V -belts used in variable-speed drives have cord drive members in a
neoprene filler enclosed by a fabric cover. The belt is reinforced by cross
ribs on the underside of the belt for stiffness. See Figure 22.
Figure 22 A wide V-belt for a variable-speed drive
Markings on wide V-belts can be a problem. Wide V-belts usually have
company codes without standard matching numbers. You need conversion
tables to frod out how these belts fit into machines made by other companies.
Positive-drive belts
Positive-drive belts are also called timing belts or gear belts. Thesc belts
drive by using molded teeth on the inside of the belt. These teeth mesh with
similarly shaped teeth cut into the driver and driven pulleys. See Figure 23.
These belts have the advantages of gears and chains. but are quieter. The
disadvantages of positive-drive belts compared to chains and gears are that
they do not last as long and cannot handle heavy loads .
.Neoprene teeth
Nylon facing
Tension member.
Flange
Neoprene backing
Figure 23 Positlve-drive belt
MILLWRIGHT-BELT DRIVES
10 -15
Pitch in positive-drive belts
Pitch is a fundamental consideration for positive-drive belts. See Figure 24.
The pitch line of a positive-drive belt is located within the drive members.
The pitch circle of a positive-drive pulley coincides with the pitch line of the
belt mating with it. All positive-drive belts must be run with pulleys of the
same pitch.
Belt
\..../:r-;I-l,
"
pitch
" , line
,
,,
,,
,,
I,
I
Figure 24 Circular pilch measured on pilch circle
Positive-drive belts use the code system shown in Table 2 to indicate the
pitch of the system;
Table 2: Codes for positive-drive belts
Code
Code meaning
XL
L
H
XH
XXH
Extra light
Ught
Heavy
Extra heavy
Double extra heavy
Pitch
1/5" (0,2")
3/a
ll
1/2u
7/8"
1115" (1.2")
The belt number for a standard, positive-drive belt is made up of three parts:
•
the pitch length of the belt expressed as the actual pitch length x 10
•
the code fur the pitch of the drive:
•
the belt's width x 100
Example 4
Belt number: 390 L 100
Pitch length
=39.0"
Pitch of drive = light; 3/8"
Belt width
I"
MILLWRIGHT-BELT DRIVES
Because of a slight side thrust of positive-drive belts in motion, at least one
puUey in a drive must be flanged. See Figure 24. Both pulleys should be
flanged when:
•
centre distance between shafts is eight or more times the diameter of the
small pulley
•
the dri ve is operating on vertical shafts
Double positive-drive belts
Double positive-drive belts have teeth on both sides as shown in Figure 25.
They provide a wide variety of
options for changing shaft
rotations and serpentine drives.
Double positive-drive belts have
the same range of sizes as single
positive-drive belts and the
pulleys are the same.
Figure 25 Double positive-drive belt
Linked V-belts
In linked V-belts. individual links can be added or removed to get the exact
length needed for a particular drive (see Figure 26). This is especially useful
on large fixed-centre drives.
The links are shaped so
that the sides of the belt fit
a standard sheave groove.
The links are connected by
removable metal stods.
Flexible
links of
rubber and
fabric
Linked V-belts can only
transmit light loads.
Figure 26 linked V-belt
Poly V-belts
Poly V-belts, as shown in Figure 27. are also referred to as multi-ribbed or
V-ribbed belts. Poly V-belts can be used for compact drives. Unlike joined or
group V-belts. a poly V-belt has the driving member or pitch line located
above the sheave. These belts are endless and available with rib spacing of
3/32 11 ,3/1611 , and 3/8 11 ,
~P;;G"Q'>I"'~i'0f$t1¥1~0,*~':'?i*UlflW~~';7m~R!;;;l";'-~:'~("~\\H_~~7W'o!1~:)lt,;~","-44!¥Ul11111'"'ili1i;l1':;';;;i;-;1(\k{·i;ti1i,\"Jt~;1i,@\\l0f.\J'JXJ'J};1'ft1hl~Y1h\(1lr(i%~~JG0m'h)~~_*:C?X+)
MILLWRIGHT-BELT DRIVES
10- 17
Driving member
Figure 27 Poly V-baits
During operation, poly V-belt drives:
•
operate at high ratios on short centres
•
retain contact between sheaves and belts under extreme conditions of
misalignment and tension.
•
reduce belt thickness to pennit use of smaller diameter sheaves
•
have low vibration
Poly V-belts are identified using codes. The size code on the belt gives the:
•
pitch length x 10
•
cross-section proportions: J indicates 0.16" depth, 0.092" between ribs
L indicates 0.38" depth, 0.185" between ribs
M indicates 0.66" depth, 0.37" between ribs
number of ribs (Vs)
•
Example 5
Belt size code (675L6)
675= 67.5" pitch length
L = L cross section
6 = 6 ribs or Vs
Poly V-belt sheaves are identified using codes. The sheave size code gives
•
the number of grooves (Vs) in the sheave
•
the cross section of the V s
•
the pitch circle diameter x 10
Example 6
Pulley size code (61.48)
6
= 6 grooves or Vs
= L cross section of grooves
48 = 4.8" pitch circle diameter
L
Power band V -belts
Power band V-belts are also referred to as joined V-belts, group V-belts or
grip-bands. The power band is made by adding a common backing to the top
10-
MILLWRIGHT-BELT DRIVES
oftwo or more endless V -belts. The V-belts may be of any standard cross
section and are oil and heat resistant. The wedging action in the sheave
groove is the same as for other type of V-belts. The backing increases the
rigidity of the drive belts. These belts are ideal for pulsating loads and long
centres. Power band V-belts are used in situations where matched belts were
previously used.
Figure 28 Power band V-belt
Matched belts
Today. belts are manufactured much more consistently than in the past.
When matched belts are required. ensure that:
•
belts are from the same manufacturer
•
belt codes (sizes) are the same
•
none of the belts have been used before installation
In the past, manufacturers used an addition to the code to specify the exact
length of a belt. These code additions had two possible scales:
•
From 105 to 95. with 100 being standard length. Each number above or
below 100 indicates a 1110" deviation from the standard.
•
From 53 to 47, with 50 being the standard length. Each number above or
below 50 indicates a 1/32" deviation from the standard.
Belt drive
assemblies
Pulleys and sheaves
Pulley and sheave diameter
Too small a pulley or sheave diameter puts exira strain on the outer plies of
the belt. For maximum belt life, the pulley or sheave has a specified
minimum diameter. This minimum si7£ depends on the grade of belt
material, the number of plies, and the speed.
MILLWRIGHT-BEL T DRIVES
-19
Rim speed
The speed and distance travelled by a belt is determined by the speed of
rotation (rpm) of the drive pulley. This rpm converts tu a linear speed at the
rim of the pulley or sheave, called rim speed. Rim speed is often expressed
in metres per minute (mlmin) or feet per minute (ftfmin).
Drives can be designed for 3000 mlmin (about 10 000 ftfmin) or more.
Manufacturers recommend 1200 - 1800 mlmin (4000 - 6000 ftfmin) as the
most efficient speed.
Drives are dynamically balanced to control vibration when they:
•
are in sensitive situations (such as computer-controlled machinery)
•
run at over 1500 mlmin (5000 ftfmin) causing vibration due to
centrifugal force
(See also Chapter 20: Preventive Mainten(lllce.)
When pulley dIameter (0) is given in millimetres the rim speed in mlmin is
calculated using the formula:
.
Rim speed
~
= 0 X rpm X 1000
When pulley diameter (0) is given in inches, the rim speed in ftfmin is
calculated using the formula:
~
Rim speed = 0xrpmx 12
The following examples show how to calculate the rim speed and belt speed
from the rpm of a pulley or a sheave.
Example 7
If 0 = 200 mm and rotational speed = 1750 rpm
Rimspeed
0xrpmx~
1000
=200xI750xO.00314
=1099 ml min
Example 8
If 0 = 16" and rotational speed = 1750 rpm
Rim speed
=0 x rpm x ~
12
=16xI750xO.262
= 7336 ftf min
Other drive components
Idler pulleys
Idler pulleys are not directly involved in power transmission, but are part of
the drive assembly. They may be used on fIat or V-belt drives. Idler pulleys
-20
MILLWRIGHT-BELT DRIVES
cause a reverse bend in the belt, tending to shorten the belt's life. They have
two main purposes:
•
to increase the arc of contact on the drive pulley
•
to act as a belt take-up adjustment for belt drives without a
movable unit
Figure 29 shows inside and outside idlers in preferred and acceptable
orientations:
•
An outside idler (Figures 29a and 29c) is a flat, uncrowned pulley. It
should be at least oue third larger than the smallest drive pulley. It is
used to increase arc of contact. Whenever possible, it should be used on
the slack side near the drive pulley. ff used on the tight side, the idler
should be placed near the driven pulley.
•
An inside idler (Figures 29b and 29d) can be a flat, uncrowned pulley or
a grooved sheave. It reduces the arc of contact but the amount of take-up
is unlimited. It is as large as, or slightly larger than, the smallest drive
pulley. Its best location is on the slack side, close to the drive pulley. IT
used on the tight side, the idler should be placed near the driven pulley.
Outside idler
r:::/
Slack side
"ioO!o -I
)
Driver
(b)
(
+
t····Tlght side
Driven
Preferred
Inside
idler
"-'----~.../
(c)
('-.+.~-r""
I
Acceptable
(d)
(
+
+
Figure 29 Idler pulleys
MILLWRIGHT-BELT DRIVES
-21
Pulley and sheave hubs
The type of hub in a pulley or sheave dictates the way it is mounted onto tbe
shaft. The hubs may be:
•
plain parallel bore
•
plain taper bushing
•
flanged taper bushing
There are many designs of taper bushings on the market. Their main function
is to facilitate rapid mounting and dismounting at the machine shafts. Use
tbe same bolts or setscrews to install and to disassemble taper bushings.
Some typical styles of hubs are shown in Figures 30 to 32.
Setscrews
Tapered fit
__ Tapered fit
Keyway
Parallel
bore
Parallel
bore
Parallel
bore
Figure 30
Plain parallel bore
Figure 31
Plain taper bushing
Figure 32
Flanged taper bushing
Drives and pulleys for flat belts
Continuous drives need automatic tension on tbe flat belt. The drives are
often mounted witb large take-ups or idler pulleys. The sizes of pulleys are
determined by the grade of belting, the number of plies, and the speed.
Pulley width
The width of a new flat belt is
determined by tbe width of the
narrowest pulley. In general, tbe
pulley face should be:
•
one inch (about 25 mm)
wider tban the belt for
pulleys up to six inches
(150 mm) wide
•
two inches (about 50 mm)
wider tban the belt for
pulleys over six inches
(150 mm) wide
r-:-
--:-1
I L Belt width J
I
Pulley width
Figure 33 Comparison of belt and pulley widths
10 - 22
MILLWRIGHT-BELT DRIVES
Crowned pulleys
When a pulley's diameter varies, a belt tends to move to the largest diameter
of the pulley. Crowned pulleys use this action to centre the belt and prevent
it wandering (see Figure 34). The standard erown is very slight-for
example, lIs" for each 12" (or I mm in 96 mm) of nominal pulley face width,
which is a ratio of 1:96. You may need to use a straight edge against the
pulley surface to ensure that it has a crown.
Crown
Figure 34 Crowned pulleys
The higher the belt speed, the smaller the required crown. Too much pulley
crown puts excessive tension on the centre of the belt, causing rapid wear.
Flanged pulleys
Flanged pulleys are used to keepa
flat belt on a drive when the pulleys
are small and speeds are high . They
are also used when there are sudden
starts under heavy loads.
"'" E
Belt /"
I
Flange
'-
p
C.
""
0-
A
Figure 35 Flanged pulley
Drives and sheaves for V-belts
Groove(s) are machined into the pulley to suit the style of belt to be used.
V-belts have a pitch line which is located approximately 1/3 of the distance
from the top of the belt. This pitch line corresponds to the pitch line of the
sheave. The V -belt sheave also has a pitch diamcter which is not to be
confused with the outside diameter of the sheave, see Figure 36. All pullcy
speeds and ratios are calculated from this pitch diameter of the sheave and
the pitch line of the belt.
MILLWRIGHT-BELT DRIVES
10-23
Groove
angle
7
---"..;
f
Pitch
Outside diameter
diameter
I
I
•
Figure 36 Groove angle and pitch diameter in V-belt sheaves
The groove angle of the belt varies with the section of the belt and the
sheave pitch diameter. The angle is between 34' and 42·. To measure these
angles, you need a set of sheave-groove and belt-section gauges (see
Figure 37). The proper belt should sit flush with or slightly above the outside
diameter of the sheave.
BELTT.W.
Figure 37 Sheave-groove gauge & belt-section gauge
Combination-groove sheave
A number of special sheaves called combination-groove sheave or simply
combination sheaves are available. With these sheaves, the design of the
grooves allows different sized belts to be used on the same sheave. The user
has the advantages of upgrading the drive simply by increasing the belt size.
A typical NB combination sheave is shown in Figure 38. Either A, B, or 5V
belts may be used in this type. There are other similar combinations.
Pitch
diameter
8 belts
Pitch
diameter r:;...o,-,~,,-c-c-<
A beits
Figure 38 Combination-groove sheaves
f!{;;w&'"'fff!Ji?3.'P®4t'i0_'i#~1-'"tJ'0,');;iff'JiJSf;:0;j'0!i~'<i.r<.:n'in'l'f;'KJI5di';j£f00;;rfJ!i7!5im;::;f;t?Wd,.~¢.0.jlS"itF0X2Jih'¥0:;;4;M@i.S!W1~'!i))$)>>:~0;.r0¥1Ji92;;;¥!?%"'}:!.W~j¥';;d&K;;k*a:4jf.%.y~~",,*~,%
10 - 24
MILLWRIGHT-BELT DRIVES
V-flat belt drives
With this drive the driver pulley is a V sheave and the driven pulley is a flat
pulley with a small amount of crown. The V-belt is gripped on the sides by
the driver sheave but drives the driven pulley on the bottom of the belt, See
Figure 39,
Figure 39 V-flat belt drive
Variable-speed belt drives
Variable speed drives can be obtained by using:
•
conventional V -belts and mechanically or manually adjusted sheaves
•
a wide V-belt with spring-loaded sheaves
Wide V. variable-speed drives give a wide range of speeds due to the amount
of belt travel in the adjustable sheave.
Adjustable
motor base
Driven
sheave
nl
=-__
Variable
speed ~.-:!:::::;==;==-_____
pulley
-
Figure 40 Drive assembly with adjustable centres
There are two general styles of drive assembly for variable speed:
•
Adjustable centres-these consist of an adjustable motor base with a
variable sheave on the driving shaft and a fixed sheave on the driven
shaft.
•
Fixed centres--these consist of an adjustable sheave on both the driver
and the driven shafts. Adjusting the belt position on the driving sheave
moves the belt position on the driven sheave in the opposite direction.
~;r0~1tt%tf'i>;\t$f'J,~,t;:w-no/~xmm0;:~;~t-;(*'ITtF"@%W;1!&-£\~~"!;_~~",19.'fG'iflFA%_~~%)'~_~W$("flX,?,0¥0%1t(j14j7!0)~tl'X~1)*,,'0n'f:s.4V'.A0'JJ/n)(Binn:K¥wm:)(%1i.ffidtN1$N\1;~\0?;3i!f
MILLWRIGHT-BELT DRIVES
10 - 25
In both styles, the spring force tends to move the flanges together and push
the belt to the outside.
A
Make all speed adjustments while the drive is running.
Variable sheave action
When using a spring-loaded, variable sheave, the act of tightening the belt
by the adjusters causes the side flanges of the pulley to move further apart.
This action permits the V-helt to sink further toward the centre of the hub.
This changes the pitch diameter of thc sheave in very small increments,
changing the speed.
Drive unit
A
B
Figure 41 Variable pilch sheave
In Figure 41, point A is the belt position giving increased speed; B is the
position giving deereased speed. Tightening the belt causes the belt to move
from position A to position B. Loosening the belt causes the reverse action.
Beeause the variable-pitch sheave is on the motor, the rpm of the sheave
remains the same. The driven machine will change speed because of the
changed sheave ratio.
Variable sheave maintenance
Single- and multi-groove variable sheaves have various designs and
configurations, each with its own maintenance requirements.
A
When installing or adjusting a multi-groove variable sheave, read the
mamifacturer's specifications.
Cheek the base and slides for the following possible problems:
•
slides and guides rusted
•
slides and guides lubricated but loaded with grit
2,"-W'",'3'Z~tt¥1}ffi~,,'ZJi14''ft6':(!ffiP:%~Xt;'&~,7'*,1I§i1tq0t'7)jFi:zq;t;pmflS'i!S0'''¥ii'£0HJ0flJFiEHFiilSGS0K!L'!iMJiH%'6!£12:?8J?!!3Ii¥iihlii\BiL"B?4Jl!2J&si7~07.x:r*JQi'lf'£lflfit0'ji!3J:fX00mXigdtSA.410f&lif01,DI
10-26
MILLWRIGHT-BELT DRIVES
•
screw and nut rusted or corroded
•
collars frozen
•
guides stuck---on some makes, a shear pin is installed in the adjustment
wheel and can be sheared
•
guide rods are painted
Maintaining
belt drives
Correct installation and alignment will ensure maximum belt life.
Environmental factors such as abrasion, temperature, and moisture are also
important.
A
ALWAYS use proper lockout procedures when working on drive assemblies.
Installation and alignment
Machinery cannot work efficiently if it is not aligned properly. The various
methods used to align belt drives are described in detail in Chapter 22:
Installation and Levelling. The two main purposes of alignment are to:
•
align the centre lines of the pulleys
•
ensure that all shafts are parallel to the driven shaft.
After V-belts are installed, tensioned, and aligned, you must be sure that
shafts remain parallel and sheaves maintain proper alignment. If possible,
check these points after several hours of operation (run-in). Use the checks
described in Chapter 22.
Storage
Store rubber belts in a cool, dry place. Store synthetic belts in a cool and
humid place, but NOT in contact with water.
Hang all endless belts in loops to prevent the belts from kinking.
If possible, store reel stock upright on racks. To do this, put a bar through the
centre of the reel and hang the bar on the rack. If there is no rack, store the
reels off the floor with good air circulation.
Troubleshooting belt drives
Regular visual inspection is simple and costs only time. Inspect the drive
during operation if possible. Figures 42 to 51 show the various types of wear
that can be found by carefully looking at the belt drive.
Table 3 on the following page is a brief troubleshooting guide. Refer to
manufacturers' troubleshooting guides for more detail.
MILLWRIGHT-BELT DRIVES
10-27
All graphics on this page courtesy of THE GATES CORPORATION.
Figure 47 Ribs run out of sheave
Figure 42 Belt failure
Figure 48 Tensile break
Figure 43 Abnormal wear on top surface
Figure 49 Premature tooth wear
Figure 44 Abnormal wear on bottom corner
Figure 50 Tooth shear
Figure 45 Separated tie band
Figure 46 Frayed or worn tie band
Figure 51 Cracked inside ply
&0X»:;&;l;BG.?'/J:?2iJ¥f:'$;"ti',S(,;'::+p/X0,fWsYfBiP};fH;PJ?,)\'J;X\Z'i"V£\<>JUt,";/Ptt7AtNttt't'l:%1~kw~~)1"'~"iBB_"'..HBhtiH;~;1i[iil!itP,,,"';_W(,:WNNW_)%.!>'J_0i;_m'15;;,",,~'j:¥Wtt!gi;)\t&':;P;P~~1J1J~v(l\!r¥\Fil$,~;:W::
10 - 28
MILLWRIGHT-BELT DRIVES
Table 3: Troubleshooting tips for belting
Trouble
Slips and squeals
Excessive stretch
Runs crooked
Corrections
Causes
8elt too loose
1 Increase belt tension
2 Insufficient capacity
2 Use thicker or wider bell
3 Pulley crown too high, causing increased
wear of narrow centre section of belt
3 Decrease crown taper to
1/8" per foot
4 Insufficient arc of contact on pulley
4 Increase the arc of contact
8elt capacity too low
1 Stretched on one side by forcing over
pulley
2 Ends not squared when joining
3 Unevenly stretched by running on
misaligned pulleys
4 Loose belt unevenly stretched by running
Use stronger, thicker, or wider belt
1, 2, 3, & 4-ln each case:
•
Repair the damaged belt section
or replace the belt.
• Eliminate the physical cause
of the problem when re-Installing
the belt.
up on flanged or stap-cone pulley
Runs off pulleys
1 Misalignment of pulleys or shafting
(If belt continues 10 run off the same
side when belt Is turned end-Io-end)
2 Crooked belt
:3 Pulley crown too small
4 Improper storage
Correct alignment
2 Replace the belt
3 Increase crown tepsr to
1/8" per foot
4 Correcl problem and store
properly
Whips and flaps
Pulsating load or power source
2 Shaft, motor, Or machine not rigidly
supported
3 Lopsided pulley
4 Bent shaft
5 Too little ben tension
Weaves back and
forth across the
pulley
Cracked outside ply
Wobbly pu Iley
2 High spot on pulley
1, 2, 3, & 4-ln each case, eliminate
the cause where possible
1 Try change of speed or addition
to flywheel to smooth out load
5 Tighten the belt
1 & 2-ln each case, correct the
faulty condition
3 Bett extremely crooked
3 Repair or replace belt
1 Excessive ben tension
1 Reduce tension
2 8eH diameter too small
:3 Oil Or grease On beH surface
2 Provlae proper pulley for bell
4 Severe reverse bend from idler pulley
3 Eliminate surplus oil or grease
thickness
4 Increase Idler pulley diameter
or take up more bell slack
Cracked inside ply
8urnlng caused by excessive slip
2 Pulley diameter 100 small
3 Oil or grease contamination
1 Treat as for slips or squeals
2 Replace with larger-diameter
pulley
:3 Eliminate oil or grease
MILLWRIGHT-BELT DRIVES
~~~_.;J!;_tlfI,1IlI__
MILLWRIGHT MANUAL: CHAPTER 11
Chain Drives
Chain drive components .... ............ ....... .... ..................... ....... ...•.... 11: 1
Links ....................................................................................... 11:1
Roller chain .................................................................................... 11:4
Types of roller chain.............. ..... .......................... ......................... 11:6
Sprockets ........................................................................................ 11: 10
Roller-chain drive assemblies ....................................................... 11:12
Drive design .................................................................................... 11: 13
Aligning shafts and sprockets ......................................................... 11: 18
Roller chain maintenance ...... ...... ......................... ........ ................ 11: 19
Roller chain lubrication ............................................................ 11:19
Routine maintenance ...................................................................... 11:21
Replacing a roller chain .................................................................. 11 :23
Troubleshooting roller-chain drives ............................................... 11:26
CHAPrERll
Chain Drives
The chain drive is used to transmit the force and motion of a drive unit to a
driven machine. A roller chain drive combines the positive action of a gear
drive with the wide choice of shaft centres of a belt drive.
Chain drive
components
Links
A roller chain assembly can contain several types of links:
)
•
roller links
•
pin links
•
connecting links
•
offset links
Roller links
A roller link consists of:
•
two bushings which are press-fit into two roller link plates
•
two rollers which revolve freely on the bushings
Links with oversized. oil-impregnated bushings that take the place of the
rollers are called rollerless.
r- Roller link plates ----.
(0\ CD CO ((O)~
\OJ CD CO \OJ
i
Rollers
BUShing)
Figure 1 Roller links
Pin Jinks
A pin link consists of:
•
two link plates which hold the pins
•
roller links which pivot on the pins.
Assembled
pin link
i
Figure 2 Pin links
Standard roller chain
A standard roller chain (Figure 3) is an assembly of alternating roller links
and pin links.
+ROlierlink
Figure 3 Section of standard rolier chain
Connecting links
A connecting link is used to join a roller chain a.~sembly, making it endless.
A connecting link consists of:
•
two pins press-fitted and riveted pennanently to a link plate
•
a link plate which may have either a slip fit or a press fit on the pins
•
a securing method (spring clips, cotter pins, spring pins, or rivets)
These connecting links are either slip-fit or press-fit connectors.
o
•
Slip-fit connectors ensure an efficient, simple connection of two roller
links. They are most commonly used for efficient chain removal and are
easily recognized. Their side plates are secured by spring clips, cotter
pins, or spring pins. See Figure 4.
Press fit connectors are riveted for permanent connections and heavy
applications.
Spring ollp
Cotter pin
Spring pin
type
type
type
Figure 4 Connecting links
Offset links
Ideally, roller chains should he installed using standard connecting links to
obtain a chain length with an even number of pitches. Because this may not
aiways be possible, an offset link is used to take up any variations.
One-pitch
An offset link is used with one connecting link, to permit the lengthening or
shortening of a chain length one pitch at a time. A one-pitch offset link is a
combination pin-and-roller link. It consists of:
•
two offset link plates
•
a single bushing with or without a roller
•
a single removable pin with a
head on one end and a flat on
the other to prevent rotation
•
a cottcr pin
__ Assembled 1- pitch
offset link
-Link plate
-Roller
Pin
-Bushing
~_ Link plate
......... Cotter pin
Figure 5 One-pitch offset links
The link plates are offset to accommodate the difference between the widths
of the pin links and the roller link. Holes in the plate correspond to the head
and flat on the pin and are sized for a slip fit on the pin, which is secured in
position by a cotter pin.
Two-pitch
A two-pitch offset link is designed for high-speed or heavy-service
application (Figure 6). The pin is riveted into the offset link plate,
permanently joining the two sections. Two press fit connectors are normally
used with a two-pitch offset link to join the chain.
Rivetted
Figure 6 Two-pitch offset link
Roller chain
Roller chain dimensions
A standard roller chain has five principal dimensions (see Figure 7):
•
pitch-the distance, in inches or millimetres, between the centres of
adjacent joint members. Pitch determines the other dimensions for
standard chain.
•
chain width (w)-the minimum distance between the link plates. Chain
width is approximately 5/S of the pitch (0.625 x pitch). This is not the
overall width of the chain.
•
roller diameter (D)-the outside diameter of the roller, approximately
5,S of the pitch (0.625 x pitch).
•
pin diameter (c)-approximately 5h6 of the pitch (0.3125 x pitch)
•
link plate thickness (D-approximately l/S x pitch.
Figure 7 Chain link dimensions
1 - 4
These dimensions determine the profile and width of the engaging sprocket.
The sizes and fonnulas used for calculations in this section apply to only two
chain standards:
•
American National Standards Institute (ANSI). This was previously
American Standards Association (ASA). All ANSI dimensions are in
inches.
•
International Standards Organization (ISO) Type A (American). In this
standard, all ANSI dimensions have been converted into millimetres to
two decimal places.
Standard chain designations allow chains from one manufacturer to be
replaced with chains from another. Each standard has its own code. For
example, Table 1 describes an identical chain in ANSI and ISO Type A.
Table 1: ANSI and ISO Type A standards
Code
Pitch
Pin diameter (C)
Roller diameter (D)
Roller width ()N)
ANSI
ISO Type A
80
1.000"
0.312"
0.625"
0.625"
16A·1
25.40mm
7.94mm
15.88 mm
15.86 mm
Roller chain pitch variation
The number of different pitches available varies with the rough size of the
chain. Chain pitch variations are:
•
lIs" variation '/4, 3/S, 'Iz, 5/8, 3/4
Above 3", chains are specially ordered.
Roller chain widths
Roller chains in the ANSI or ISO Type A series are single strand (simplex)
or multi-strand. Multi-strands are commonly up to eight strands wide.
Roller chain code numbers
ANSI roller chain code
ANSI code numbers have two parts. The digit on the right specifies the type
of chain:
•
0 =usual, or regular proportion
•
1 = lightweight chain
•
5 = rollerless bushing chain
11
5
The digit on the left (or the two leftmost digits in a three-digit code) is called
the pitch number. It specifies the number of 118 inches in the pitch:
•
35 chain = 3/S" pitch; rollerless
•
41 chain = 4/8" or 1/2" pitch; lightweight
•
100 chain = IO/S" or 11/4" pitch; regular.
If the letter H follows the chain number. it denotes a heavy series chain.
•
50-H = 5/S" pitch: heavy series
If a hyphenated number follows the chain number, it specifies the number
of strands. If there is no hyphenated number. the chain has only one strand.
•
l00-2H= 11/4" pitch; regular; 2-strand, heavy series
ISO Type A roller chain code
In an ISO code number for roller chain, the size is indicated by the two
numbers which always appear in the code. Refer to a catalogue for size
specifications. Note that the ISO Type A pitch number is always double that
of the corresponding ANSI number. For example, ANSI code 35 has pitch
number 3 and the corresponding ISO Type A pitch number is 06. Table 2
shows some equivalent ANSI and ISO codes.
Table 2: ANSI and ISO Type A codes for Single-strand chain
ANSI
Pitch
Code
ISO Type A
Pitch
Code
3/S
35
112
40
9.525mm
12.70mm
OeG-1
08A-1
1
80
25.40mm
16A-1
Construction style is indicated in the ISO code by the letter C or A:
•
C = rollerless construction
•
A = standard construction
Types of roller chain
Multiple-strand roller chain
Multiple-strand roller chains are used to transfer maximum power with the
smallest pitch size. High sprocket speed combined with a long pitch creates a
noisy and destructive drive due to the impact of the chain on the sprocket.
This impact increases in proportion to the weight of the chain. A multiple
strand chain with a small pitch at high speed gives the drive a smoother
operation and a longer life.
11
For power transmission, a multiple-strand chain permits the use of a smaller
pitch chain and smaller sprockets where space is at a premium. See Figures 8
and 9.
Figure 8 Multiple-strand rolier chain
A multiple-strand chain has a power delivery rating slightly less than the
number of strands. For example:
•
Single strand (simplex) = I
•
Double strand (duplex) = 1.9
•
Triple strand (triplex) = 2.8
•
Quadruple strand (quadruplex) = 3.7
1" Multi-strand chain drive ---,,<-...~
2" Single-strand chain drive
--___to"
Figure 9 Using multiple-strand chain for power transmission
For a given pitch, the minimum ultimate tensile strength (UTS) is
proportional to the number of strands:
PITCH
MooMUMUTS
1" single
12500 lbf
1" multiple
M x 12500 lbf
(M = number of strands
in multiples. See the
following note. ")
*NOTE: Multiple-chain drives require very accurate alignment in order to
spread the load evenly over every strand. Where alignment cannot be kept,
two single chains on special sprockets may be more efficient than a duplex
cbain. Two duplex chains can be used instead of a quadruplex chain.
DOUble-pitch (extended-pitch) roller chains
Double-pitch drive chain can be used in place of standard roller chain when
speeds are low and loads relatively light, or on low-speed applications with
long spans to reduce the chain weight.
DOUble-pitch chain drive is similar to standard chain, except that the link
plates have twice the pitch as shown in Figure 10. Roller diameter and chain
width stay the same. Double-pitch chains are not made in multiple widths.
Figure 10 Extended or double pilch
Ansi codes
There are ANSI and ISO side-plate numbers for various pitches:
SIDE-PLATE NO.
PITCH
ANSI
ISO
ANSI
ISO
2040
20BA
1"
25.40
2050
2l0A
11/4"
31.75
2060
212A
11/2"
38.10
The Ansi code 2040 is interpreted as follows:
The first figure (2) indicates double pitch.
The second and third figures (04) indicate the pitch in (x 1/4") increments
(in this case, 4 x 1/4" = 1")
The last figure (0) indicates the roller size: 0 = standard; 2 = oversized
1
8
snentchain
Silent chain consists of a series of toothed link-plates assembled on pin
connectOIl!. See Figure 11. In operation, the chain passes over the face of the
sprocket like a timing belt (see Chapter 10: Belt Drives). The sprocket teeth
do not protrude through it. The chain meshes with the sprocket by means of
teeth extending across the width of the undeIl!ide. The links have no sliding
action, either on or off the teeth, and create a smooth, quiet action. Silent
chain is designed for use on high-speed drives.
Figure 11 Silent chain engagement in the sprocket
The notches or projections on the chain prevent rotation of the pin and
bushing and therefore resemble a gear more than a roller chain sprocket. The
chain is held on the sprocket by one of the following (see Figure 12):
•
a flanged sprocket
•
a row of centre guide plates fitting into a groove in the sprocket
•
side guide plates on each side of the chain, straddling the sides of
the sprocket.
Flanged
sprocket
Centre
guide
plates
Side
guide
plates
Figure 12 Three ways of retaining silent chains
Sprockets
Sprocket types
Sprockets are classified in four groups: Types A, B, C, and D, based on the
style of construction. See Figure 13. Depending on their use, all four types
may be made from cast iron, cast steel, steel plate, carbon fibre, or plastics.
o
Type A sprockets, also called plain or disk sprockets, are designed
without hubs. They are mounted on existing hubs either by bolting or by
machining and welding. These sprockets can be fitted to any length or
shape of hub.
o
Type B sprockets are units with a hub on one side only. They can be used
with single-strand and multiple-strand chains.
o
Type C sprockets have hub projections on both sides. (Note: In some
catalogues, Type A sprockets with a hub attached to both sides are listed
as Type C.)
o
Type D sprockets are considered detachable. They may have the
following characteristics:
- be split in half
- have an A sprocket bolted to a hub
- have a split A sprocket bolted to a hub
- have an A sprocket bolted to a split hub
Type D sprockets are manufactured for specific applications.
Type B
Type A
~TypeD/
TypeC
Figure 13 Types of sprockets
Sprocket mounting
Type B and Type C sprockets have straight bores or taper bushings. Straight
bore sprockets are supplied with either:
10
•
a minimum straight bore that allows the user to machine the bore, cut a
keyway, and drill and tap for setscrews to suit the particular job
requirements
•
a fInished bore, where the bore, keyway and setscrews are made to
standard dimensions.
Taper bore sprockets are either plain or flanged. Their main function is to
facilitate rapid mounting and dismounting at the machine shafts. Their
design is the same as for pulleys and sheaves (see Chapter 10: Belt Drives).
Sprocket sizing
Sprockets arc sized in a common manner, but each manufacturer may use
variations according to the special features of their sprockets. For correct
catalogue numbers and specifications, refer to manufacturers' catalogues.
An example of a sprocket specification is:
D40B 15
where
D =double-indicates the number of strdIlds in the chain to be used
40 = chain number
B = hub type
15 =number of teeth on the sprocket
)
Sprockets tor double-pitch chlrln
Single-cut sprockets
Single-cut sprockets are cut su that each tooth is effective in delivering
power to the links in contact with the sprocket. Sec Figure 14.
Figure 14 Single-cui sprocket for double-pitch chain
Double-cut sprockets
Double-cut sprockets are cut so that every other tuoth is effective in
delivering power to the links. See Figure 15. When sprockets have an even
number of teeth, wear will be on every other tooth in a regular pattern.
When these teeth are worn. the sprocket can be rotated by one tooth to
provide a new set of teeth for the chain. Sprockets with an odd number of
teeth wear evenly on all teeth.
Standard sprockets with 36 teeth or more can be used with double-pitch
chain. Sprockets requiring less than 36 teeth are cut specifically for doublepitch chains.
Figure 15 Double-out sprocket for double-pitch ohain
Roller-chain drive
assemblies
Some advantages of roller-chain-drive assemblies are:
11
•
Power is transmitted at 98% to 99% efficiency.
•
There is no slipping on overloads or during continual heavy loads.
•
No special take-up devices are needed to maintain tension. Within limits,
the chain will work efficiently while running slack. This is not true for
reversing drives.
•
One length of chain will drive several sprockets without slipping. This
applies to straight line or serpentine arrangement.
•
The chain can be shortened quickly by any number oflinks using offset
links.
•
The replacement chain is accurate to lengths within 1.25 mm per metre
(0.015" per foot).
•
The mechanism will tolerate high temperatures.
•
They accommodate reversing drives.
•
They are readily available in a variety of pitches.
Drive design
A sprocket with a greater number of teeth and a finer pitch gives longer
service than one with fewer teeth and a coarse pitch. When changing from a
coarse pitch drive to a finer pitch drive, a multiple-strand chain is needed to
deliver the same horsepower.
Centre distances of shaft
In deciding centre distances of shafts, you must ensure that the sprocket
teeth do not touch and that at least a third of the teeth engage the chain
(120° wrap).
As a general rule:
•
A centre distance of 30 to 50 pitches of chain is adequate
•
For pulsating loads, centres as short as 20 pitches of chain can be used.
•
If extra-long distances between shafts are unavoidable, a countershaft
(jackshaft) may be needed at the centre of the span. A countershaft is an
intermediate shaft used to break a drive into two or more stages. See
Figure 16. It can also be used to either maintain the same final speed
ratio or increase or decrease the final speed ratio.
Countershaft
)
~-_ _ _ _~~.--' Driven
~---~-
Figure 16 Using a countershaft
Drive sprocket
A hardened (heat treated) sprocket should be used when:
•
the operating speed is slow, the drive is heavily loaded, and the chain has
been selected for tensile strength
•
the operating speed is moderate, and sprockets have 17 teeth or less
•
the operating speed is high and sprockets have 25 teeth or less
•
speed ratios exceed 4: 1
•
operating conditions expose the drive to dirt, dust or abrasives
Driven sprocket
The driven sprocket should not exceed 120 teeth. The speed ratio is the
critical factor for this sprocket. It is expressed as:
11
13
. .::hi",,·g",h_-..::.sp",eed:.=:..::sh.::a..::ft..::.rpo.:m..::
Speed ratIo =
low - speed shaft rpm
or
.
_n_umb-:_er_o_f-::te..;.e..;.t-:h_o....n_d-::n-::·v_en-"sp'-roc----:-k_e_t
Speed ratio=
number of teeth on drive sprocket
Note the following:
•
Speed ratios up to 12: 1 are allowed if the speed drive is very low (not
more than 100 rpm).
•
Do not use a single chain drive where the speed ratio exceeds 7:1.
•
Where the speed ratio exceeds 5:1. use a double-reduction drive or
countershaft (see Figure 16) to obtain maximum service life.
Slack
To perform efficiently, a chain must have the right amount of slack. Correct
slack is usually expressed as percent slack. It is based on the amount of
deflection in the slack side of a chain dri ve:
•
For horizontal, or nearly horizontal. one-direction drives, the
recommended amount of slack is 2%.
•
For reversing drives or shock-loaded drives, the amount is less.
•
For vertical drives the amount of deflection should be approximately
one-quarter the chain piteh.
I--~~---~
I
I
~~-I
I
I
!
~~_~_f_~f
----~-i-------
I
Figure 17 Measuring chain slack
In Figure 17, the tight side is on the botIom and the slack side is on the top.
The deflection D is found by measuring from a straightedge across the chain
on the sprockets. If S represents the span, or shaft-centre distance:
D = desired percentage X S
Horizontal drives
On horizontal drives, the slack strand is nearly always on the lower side. For
moderate distances, the slack may be on either the upper or lower side of the
drive, but is preferred on the lower side. See Figure 18.
I
I
(~f~
~+~
Slack side on the bottom
I ..
-+_
(~ ~
I
Slack side on the lOp
Figure 18 Slack in horizontal drives
If the slack strand is on the upper side:
•
on drives with long centre distances, there is
a danger of it rubbing against the lower
strand after wear occurs
•
on drives with short centre distances the
upper strand may push itself out of proper
engagement with the teeth of the smaller
sprocket.
Vertical drives
When you need to use vertical drives, run them
taut. This prevents the chain from sagging and
disengaging from the teeth of the lower
sprocket. Whenever possible. place the chain
drive slightly off the vertical position as shown
in Figure 19.
Figure 19 Vertical drives
Idler sprockets
Idler sprockets are used to take up chain slack where it is not possible to
adjust the shaft centres. They may be frxed or adjustahle. depending on their
pm:pose. Adjustable idlers have the advantage that chain tension can be
controlled.
Idler sprockets are applied against the slack side of the chain as shown in
Figure 20:
•
When running outside the chain drive. idlers are located toward the
smaller sprocket. On drives with short centres, the idler on the outside of
the chain should be as close as possible to the smaller sprocket.
•
When running inside the chain drive. idlers are located in the middle of
the drive. slightly toward the larger sprocket.
Outside idler
Figure 20 Two-point drive with fh(ed centres showing idler sprockets
outside and inside the chain drive
Note that on reversing drives idlers can be used on both sides of the chain,
allowing for the different position of the slack. Figure 21 shows drives with
two idlers. Figure 22 shows the idler supporting the chain. This keeps chain
sag at a minimum. It is recommended for slow to medium chain speeds up to
150 mlmin (500 ft per min).
Chain support using guide rails
When chain speeds are less than 90 mlmin (300 ft per min), guide rails may
be used to support the top and bottom parts of the chain. See Figure 23.
11
1
\
I
I
\
:~
,,
,
Inside
idler
~I,
I
I
\
Outside
Idler
-
I
r ,f,' 'I
~"I
I
•
~. .
I
~
Figure 21 Two-point drives, each with idlers on both sides of the chain
Adjustable
supporting idler
Figure 22 Adjustable supporting idler
Section
A-A
Guide rails
Figure 23 Supporting guide rails
11
17
Aligning shafts and sprockets
Sprockets and shafts must be properly aligned 10 prolong their useful llfe.
Rapid wear is commonly caused by misalignment. Misalignment causes
chain parts to rub against the sides of sprocket teeth, causing excessive
friction.
Use the following procedure to align the various parts:
1. Ensure the shafts ron level. Use a precision level directly on the shafts
(Figure 24). With a multiple-width sprocket, a precision level may be
applied across the sprocket teeth. If necessary, elevate the shaft( s) to the
desired height by placing shims of the same required thickness under the
bearing bases.
~I
~I
Figure 24 Levelling shafts using a level
2. Align the shafts using a feeler bar to check that they are parallel
(Figure 25).
----------- --
----~
---,
t)
Figure 25 Aligning shafts using a feeler bar
3. Recheck the level adjustment. Tighten all securing bolts and nuts to hold
alignment.
4.
Align the sprockets axially on the shafts, using a straightedge
(Figure 26). Take care to apply the straightedge to a finished surface on
the side of the sprocket. For long centre distances, use a stretched piano
wire, string or laser heam instead of a straightedge. Rorate the sprocket
1800 and check the alignment again. This procedure ensures that the
sprocket runs true.
Figure 26 Aligning sprockets using a straightedge
With a shaft having some endplay (for example, the shaft of an electric
motor), align the sprockets with the shaft in its running position. To
determine the running position, chalk the shaft, run the motor at
operating speed, and scribe a line in the chalk opposite a convenient
fixed point. Adjust the alignment with the shaft blocked in this position.
5. Secure the sprockets against axial movement by the use of setscrews
either through the sprocket hubs or through separate collars fastened to
the shaft.
6. Prevent the sprockets from turning on the shaft. Do not depend on
setscrews to do this. Use the appropriate key for the keyseat in the hub
and shaft.
Roller chain
maintenance
Roller chain lubrication
Lubrication is necessary to minimize metal-to-meral contact of the pin and
bushing joints in the chain. Access to pin and busbing areas is through the
clearance between the plates. Therefore, oil should be applied to the outside
and inside plate edges. See Figure 27. Lubrication is explained more
thoroughly in Chapter 6: Lubrication.
r-
- '""'66
J r
PI"
'-
U
'II
LRolier
"'l J
Bushing
Figure 27 Lubricating pin and bushing joints
Oil grades
Use a good grade of mineral oil, free flowing at the prevailing temperature.
A fme-pitch chain needs a lighter grade of oil such as SAE 10 or 20. A
coarse-pitch chain need~ SAE 40 or 50. In general, oil viscosities should be
the same as for machine engines: Use heavier oils in hotter weather and
lighter oils in colder weather.
Lubrication methods
The lubrication method is determined by the speed of the chain and the
amouut of power transmitted. Chains are lubricated manually, semiautomatically, or automatically. Manual lubrication is fine for simple drives.
More complex drives with higher speeds and loads require more frequent
lubrication-some constantly by automatic devices.
Remember these six points about chain lubrication:
•
Lubricate at regular intervals.
•
At higher speeds, lubricate more frequently.
•
Remove excess dirt and dust from chains before lubricating.
•
Lubricant must penetrate the chain joints.
•
Protect the chain from contaminants wherever possible.
•
Follow manufacturer's instructions on lubrication.
Manual
A brush or oil can is used for simple drives (Figure 28a). Ensure that a
posted lubrication schedule is followed regularly (for example, every eight
hours). Apply oil just ahead of where the chain goes around Ihe sprocket.
a Brush and oil can
Semi-automatic
The semi-automatic method uses a drip cup for lubricating drives of low
horsepower and speed (Figure 28b). Regularly inspect the filling or oiler
cups and Ihe rate of feed. Ensure that the feed pipes are not clogged.
Automatic
Automatic methods use one of Ihe following:
b Drip cup
- - -/-"
... -::: '"
I
\
.., - /
I
•
oil hath for low to moderate horsepower and speed (Figure 28c)
•
oil disk for moderate to high horsepower and speed (Figure 28d)
•
oil stream or mist for high horsepower and speed (Figure 28e)
I
For oil bath and clisk systems, inspect the oil level and check that there is
no sludge. For oil stream systems, inspect the oil level in the reservoir,
check Ihe pump drive and the delivery pressure so that there is no clogging
of the piping or nozzles. With reservoir systems, drain and refill at least
once a year.
c Oil bath
Oil disk
Routine maintenance
Every chain drive should be checked
perioclica1ly for misalignment, chain
wear, sprocket wear, and excessive slack.
dOil disk
Misaligrunent
Misalignment is inclicated when the
sides of the sprocket teeth Or the
inside surfaces of the roller link plate
show wear. See Figure 29.
Figure 29 Sprocket tooth
showing wear
e Oil stream
Figure 28 Methods of
lubricating chain drives
Chain wear due to stretch and pin wear
Chain wear caused by stretch and by pin wear is inclicated when the chain is
running close to the tips of the teeth on the larger sprocket. When the drive is
down, try lifting the chain away from the large sprocket as shown in
Figure 30. Make sure that the chain is in mesh at the leading and leaving
teeth. Wear is inclicated if a gap appears between the roller and the bottom of
the tooth.
,....y .........~~ap
;ltl~~:;:::;~~ 0 "-' Leaving tooth
Figure 30 Checking for wear
If the chain is slack. support the slack with a board and secure one end. By
pushing and pulling several links. you can see if any pin wear is developing.
When the chain is pushed together. it should be the original length. Then.
when the chain is pulled. the difference in length indicates the amount of pin
wear.
Tbe preceding checks for chain wear show that the chain is worn, but not by
how much it is worn. A more accurate way of measuring chain wear is to
compare it to new chain:
1. Stretch the old chain using a specific force and measure its length. (The
force applied can be a spring scale for horizontal measurement or dead
weight for vettical measurement.)
2. Stretch a new chain with same number of pitches using the same force.
Measure it.
3. Compare the lengths.
L
New chain
~~~~~-,-~~~~,-~~~~~~
Stretched chain
compressed
Figure 31 Testing for pin wear by push and pull method
Stretch is determined by the increase in length (S) expressed as a percentage
of the original chain length (L) (see Figure 31):
S
stretch =- x 100%
L
Chain companies recommend that the chain should be replaced when the
extension be I % to 2%, depending on the speed and operating conditions of
the drive, and the production importance of the machine.
Sprocket wear
This is indicated by a change in tooth fonn. Nonnal wear is hard to detect,
but excessive wear from misalignment or overloading is easy to see. See
Figure 32.
Tooth wear
Figure 32 A worn sprocket
Excessive slack
This may be caused by alan:
•
stretched chain
•
idler becoming loose
•
prime mover moving on its base
•
machine moving on its base
Replacing a roller chain
Sprocket condition
When a new chain is installed on an existing drive, examine the sprockets.
If new chain is used on worn sprockets, chain life will be shortened and
there may be maintenance problems due to the new chain climbing the
sprocket teeth.
To prevent this, take the sprocket off the shaft, reverse it, and then remount
it. Ensure that there is clearance for any obstructions on the sprocket. If this
is not possible, replace the sprocket. Then recheck the alignment and Secure
the sprockets to the shaft.
New chain length
The amount of new chain needed to replace an old chain is based either on
the total length or On the number of pitches.
•
If the drive chain has been shortened during service life, the new chain
will have the same length but more pitches.
•
If the roller chain is involved with timing or sequence of operation, the
new chain will be shorter and have the same number of pitches.
Breaking the chain
A chain with a cotter pin connecting the links can be broken at any place
using a pair of pliers, a punch and a hammer. The connecting link may be
salvaged.
A
You must wear eye protection when grinding and hammering.
To discOimec.'t a riveted chaln on the job, grind off the rivet heads. This
connecting link must be replaced. Screw-operatcd extractors or chain
breakers are often used for removing the outer plate of riveted chain. See
Figure 33. Before pushing the pin through the pin plate on a large chain,
grind away the rivet head to avoid distorting the thread on the screw. New
chaln is cut to its proper length using either of these methods.
Joining the chain
When possible, use the sprocket to support the chain while inserting the
connecting link. If this is not possible, using a chain lace is very successful.
See Figures 34 and 35.
To join roller chain ends with a chain lace:
I. Pull together the ends of chain to be joined between sprockets.
2. Using a leather lace about 1/4 inch wide and 3 to 4 feet long, wrap the
lace around a roller at each end of the chain. See Figure 35. The best
location is three or four rollers back.
3. Hold the chain flat between the para11el wraps of lace.
4. Hold it in position to insert a connecting link.
To join a tight or heavy chain, SOme form of mechanical puller is needed.
Various types of commercial mechanical pullers are available from chaln
sales outlets. A handy puller can easily be made from a small turnbuckle by
cutting the turnbuckle and bending the ends as shown in Figure 36.
-24
Figure 33 Chain-detaching
tools for roller chain
Figure 34 Inserting a connecting link
Lace pulled through slit in broad end of lace
Figure 35 Pulling chain with a lace
Fit to accept
small chains
~
Turnbuckle
Nut welded
to RH & LH threads
~J;."S,-_ r,S;T.T.T.-F.Jrn··. . .~~:=tr.Ti-TiT.i.1-~
LH thread
1 ............./j'.L..J../
Fillo accepl
large chains
Figure 36 A fabricated mechanical puller
After the chain is installed, adjust the take-up to ensure that the slack in the
chain is correct.
Troubleshooting roller-chain drives
Table 3 on the following pages is a brief troubleshooting guide. Refer also to
manufacturer's maintenance manuals.
11
Table 3: Tips for troubleshooting chain drives
Trouble
Noisy drive
Rapid wear
Chain climbs
sprockets
Stiff chain
•
Cause
Correction
Moving parts rub stationary
parts
• Tighten and align the supports,
casing, and chain. Remove dirt
and other matter.
• Casing or cover rattles
• Tighten the casing, cover, and
supports.
• Chain doesn't lit sprocket
•
•
Loose chain
• Tighten the chain, keeping a
small amount of slack. Long
chains need Idlers, rollers, or
guide shoes.
•
Faulty lubrication
• Lubricate properly.
Replace with correct parts.
• Misalignment or Improper
assembly
• Correct the alignment and
assembly of the drive.
• Worn parts
• Replace worn chain or
bearings. Reverse worn
sprockets before replacing.
•
•
Faulty lubrication
Lubricate properly.
• No slack
• Add extra pitch or adjust idler.
• Loose or misaligned parts
• Align and tighten the entire
drive.
• Abrasives
• Clean the chain and install a
dust-proof guard.
• Chain doesn't fit sprockets
•
Replace the chain or sprockets
• Worn chain or worn sprockets
•
Replace worn chain. Reverse or
replace worn sprockets.
•
Loose chain
• Tighten the chain.
• Faulty lubrication
• Lubricate properly.
•
• Clean and lubricate.
Rust or corrosion
• Misalignment or Irnproper
assembly
• Correct the alignment and
assembly of the drive.
• Worn chain or worn sprockets
• Replace worn chain. Reverse or
replace worn sprockets.
CONl'/NUED
Table 3 continued
Broken chain or
sprockets
•
Shock or overload
• Avoid shock and overload, or
isolate the chain or sprockets
through the couplings.
• Wrong size chain; chain that
doesn't fit sprockets
• Replace with correct size parts.
• Worn chain or worn sprockets
•
Replace wom chain. Reverse or
replace worn sprockets.
•
•
Replace the faulty parts.
Correct the corrosive
conditions,
Rust or corrosion
• Misalignment
• Correct the alignment.
•
•
Interferences
Make sure no solids obstruct
the chain and sprocket teeth.
Loosen the chain if necessary
for proper clearance of the
sprocket teeth.
n
-...
::::I"
Do)
"CS
CD
.....
N
Gear Drives
Gear design ................................................................................... 12:1
Gear terminology ............................................................................ 12:1
Gear materials ................................................................................. 12:6
Shaft arrangements ................................................................... 12:7
Gear types ............................................................................... 12:8
Overdrive and reduction units ...................................................... 12:20
Overdrive ........................................................................................ 12:20
Reduction .................. ,..................................................................... 12:22
Wonn gear reduction units ............................................................. 12:22
Helical and herringbone gear reduction units ................................. 12:26
Bevel, mitre, and hypoid gear reduction units ................................ 12:28
Planetary gear reduction units ........................................................ 12:29
Installing and maintaining gear drives .......................................... 12:30
Installation ...................................................................................... 12:30
Mounting styles ....................................................................... 12:32
Lubrication ..................................................................................... 12:35
Troubleshooting gears .................................................................... 12:35
Gear Drives
Gear drives are used to transmit force and motion (rotary or linear). Gears do
the following:
•
eonneet shafts by using a gear train
•
reduce speed and increase torque by using a reduction unit
•
increase speed by using an overdrive unit
Gear design
Two smooth cylinders pressed together can transmit power and motion along
the line of eontact, but slippage occurs when the load is greater than the
frictional force between the two cylinders. Slippage is prevented if the
cylinders have teeth that mesh as the cylinders rotate.
Gear terminology
Gears can be considered as a development of the friction wheel. The
circumference of the cylinder represents the pitch circle of the gear. The line
of contact between the two cylinders is the pitch line which extends across
the face of the teeth. See Figure 1.
Pitch circle
/~
Figure 1 Pitch line on a gear
The amount of the gear tooth above the pitch circle is called the addendum.
The amount below is called the dedendum which is equal to the addendum
plus the clearance.
MILLWRIGHT-GEAR DRIVES
12-1
Whole depth
Thickness
of tooth
Addendum
/
--
~
/
/
v.;:oth
face
/
/
\
/
Clearance
Pitch
circle
Working depth
Dedendum
1 - - - Pitch diameter
1---- Outside diameter - - - - I
Figure 2 Parts of a gear and gear teeth
The most usual names for gear parts are listed here with the formulas used to
calculate them (refer to Figure 2):
Addendum (ADD)
The part of a tooth that lies between the pitch
circle and the outside diameter of a gear.
ADD= 1 +DP
Dedendum (DED)
That part of a tooth that lies between the pitch
circle and the root diameter (see Figure 2) of a
gear.
DED = 1.157 + DP
Clearance (CL)
The gap between the working depth and the whole
depth.
CL
= DED-ADD
= 0.157 + DP
Circular pitch (CP)
The distance between two corresponding points on
adjacent teeth, measured on the pitch circle.
CP
Outside diameter (OD)
= 3.1416 + DP
The diameter of the cylinder into which the gears
are cut.
OD = (N + 2) + DP
where N is the number of teeth.
Diametral pitch (DP)
The total number of teeth on the gear divided by
the pitch diameter in inches.
DP
12-2
= N+PD
MILLWRIGHT-GEAR DRIVES
Pitch diameter (PD)
The diameter of the pitch circle.
PD
= (N X OD) + (N + 2)
=N+DP
Working depth (WKD)
The depth of a tooth engagement of two meshing
gear teeth.
WKD=2xADD
Whole depth (WHD)
The total height of a tooth or the total depth of a
tooth space.
WHD = ADD + DBD
Chordal thickness (T)
(or tooth thickness)
The thickness of the tooth (at the pitch circle) on
the face of the gear.
T= 1.57+DP
Conditions for gear meshing
Mesh is the correct fitting and operation of two gears in contact. This is
achieved only when the gears have the same:
•
•
•
diametral pitch
pressure angle
tooth profile.
)
Dillmetral pitch
In modem gear design the circular pitch (CP) is used very little. When the
pitch of a gear is mentioned it usually refers to the diametral pitch (DP)
Note that the diametral pitch is usually a whole number.
Pressure angIe
The pressure angle is easily
recognized in tenns of a rack. A
rack is defined as a spur gear
having a straight pitch line and
straight-sided teeth. The angle
that the side of the tooth makes
with a line drawn square to the
pitch line is the pressure angle.
A line square to the pressure
angle at the pitch line is the line
of action. See Figure 3.
Line of
action
Pressure
angle
Rack
Pitch
line
Pinion
Figure 3 Pressure angle
Most spur gears have a pressure angle of 14.5° or 20°. Gears with a pressure
angle of 20° are preferred because of their greater strength and wear
resistance and because they pennit the use of fewer teeth on pinions.
MILLWRIGHT-GEAR DRIVES
12-3
It is usually quite easy to tell them apart by comparing the bottom width of
the tooth to the top width.
•
If the two widths are about the same, this indicates a 14.5° pressure
angle.
•
If the bottom width is definitely larger than the top width, this indicates a
20° pressure angle.
Gear gauges
Sets of gear gauges are available from suppliers for checking diarnetral pitch
at both 14lh0 and 20° pressure angles. They range from 3 to 48 DP. See
Figure 4.
Figure 4 Using a gear gauge
Tooth profile and action
The teeth on a gear rack are straightsided. See Figure 3. The teeth on a 10
inch diameter gear with 40 teeth have
an obvious curve on their faces and
flanks. Most gear teeth have an
involute profile. An involute curve is
shaped like part of the spiral that
would be traced by unwinding a
string from a cylinder. See Figure 5.
Meshing gears may have the same
diametral pitch and pressure angle but
if the number of teeth on the gears are
different, then the amount of curve on
their teeth will be different. This is
often the case, but does not harm the
meshing action of the two gears. It
enhances the rolling action of the
meshing teeth.
Figure 5 Inl/oluta curve
12-4
MILLWRIGHT-GEAR DRIVES
Machining limitations, cost factors, and slight imperfections prevent a pure
rolling action. In practice, there is a small amount of sliding.
Backlash
Backlash is the amount of play between mating teeth. It is the difference
between tooth space and tooth width measured in thousandths of an inch
(hundredths of a mm). See Figure 6. The American Gear Manufacturers'
Association (AGMA) produces tables of recommended backlash for gears.
The purpose of backlash is to:
a
Trapped oil
--
Driver
--....
)
•
allow for tooth expansion due to heat
•
prevent interference with incoming and
outgoing teeth
•
allow lubrication to escape
Lack of backlash eliminates the path for
trapped lubrication to escape. It also causes
extreme noise and wear. Backlash does not
include the effects of incorrect shaft centre
distance or clearances in bearings or
mountings.
Figure 6a shows lubrication being trapped in
the tooth space as the gear rotates. The space is
reduced as the gear teeth align with the shaft
centres.
b
--------..
Reduced space
Driver
At this point, hydraulic pressure is created and
forces the gears apart (see Figure 6b). This
action causes overloading and failure of gears
or support bearings. This situation is most
evident with herringbone gear sets where the
lubricant is forced into the centre and has no
place to go.
c
Backlas:~ l
--
Driver
Correct backlash allows any excess lubrication
to escape, relieving pressure (see Figure 6c).
--........
Figure 6 Hydraulic effect and backlash of meshing gears
MILLWRIGHT-GEAR DRIVES
12 - 5
Gear materials
Materials used for gears fall into three main groups:
•
ferrous metals
•
non-ferrous metals
•
non-metallic materials
Choosing which one to use depends on the application. The following factors
must be considered:
•
load
•
speed
•
•
method of lubrication
tooth form
•
noise levels
•
environment
•
temperature
Ferrous metals
Steel and cast iron are the most used materials for this group. Steel is by far
the most common.
•
Steel gears, when heat treated, can carry the greatest loads and torques.
Alloy steel gears can be hardened and tempered to precise specifications.
Also, the composition of the steel can be controlled by modern steel
making processes.
•
Cast iron is easy to machine. It is inexpensive and has good wear
resistance characteristics. Cast iron gears run quieter than steel gears but
are more brittle.
•
Other metals such as stainless steel may also be used for gears due to
their corrosion resistance. These are used only in specialized
applications because of their high cost.
Non-ferrous metals
Some non-ferrous gear materials are used where corrosion resistance, cost,
or low weight is important. Bronze and aluminum are the most common
non-ferrous materials.
•
Bronze is a very tough and wear-resistant material, easy to cast and
machine.
•
Aluminum is commonly used for lightweight gears. Wear resistance can
be improved by anodizing the gear teeth.
Low cost, light weight gears can be die-cast from zinc-based, magnesiumbased, and copper-based alloys.
12-6
MILLWRIGHT-GEAR DRIVES
For certain applications, ferrous gears are run with non-ferrous gears. An
example of this is hardened worm gears meshing with bronze worm wheels.
Because of the properties of the metals involved, friction is reduced,
eliminating many lubrication problems.
Non-metallic materials
Non-metallic gears are used primarily because of their quiet operation at
high speeds. These materials may be natural or synthetic. Historically, wood,
leather were used. Now many man-made materials are used such as plastic,
laminated phenolic, and carbon-fibre composites.
These non-metallic materials display excellent wear and corrosion-resistance
properties and need very little lubrication. In some cases they run dry with
no ill effects.
When plastic gears arc used in gear trains, excessive heat must be avoided.
Distortions occur quite rapidly if plastic is overbeated.
Shaft arrangements
Gears are used to connect shafts in various arrangements as shown in
Figure 7.
)
1
2
4
Figure 7 Shaft arrangements that can be connected by gears
MILLWRIGHT-GEAR DRIVES
12-7
The shaft arrangements in Figure 7 are:
1. shafts parallel to each other
2. centre lines intersecting at right angles (90°)
3. centre lines crossing at right angles (90°)
4. special cases when intersection or crossing of shafts is not 90°.
Gear types
Spur gears
In spur gears, the teeth are cut parallel tc the bore. Figure 8 shows a pair of
mating external spur gears. The smaller gear is called the pinion, while the
larger gear is called the gear. These gears are used to connect shafts that are
parallel tc each other. They do not generate axial thrust loads tc the shaft
bearings.
External spur gears
The teeth of external spur gears are on the outside of both the pinion and the
gear. The connected shafts of external spur gears rotate in opposite
direetions. If these shafts were required to run in the same direction an idler
gear would have tc be used.
Figure 8 External spur gears
Internal spur gears
Internal spur gears have their teeth developed on an internal pitch circle
inside a ring. The teeth mesh inside the larger gear. The pinion is the same as
the external spur gear pinion. Figure 9 shows a pair of meshed internal spur
gears in single reduction.
12-8
MILLWRIGHT-GEAR DRIVES
In internal spur gears, the connected shafts rotate in the same direction. The
shaft centre distance is one half the sum of their pitch diameters, thus making
a compact drive. Internal spur gears can carry the same loads and run at the
same speeds as external Ones. But they run much more quietly because more
teeth are engaged at the same time.
Pinior
Figure 9 Internal spur gears
)
Rack and pinion spur gears
Rack and pinion spur gears are used to convert rotary motion to a linear
motion and vice-versa. The rack is a flat section of gear teeth with straight
sides at the required pressure angle. They are used extensively to open and
close chute doors, large air valves, etc. See Figure lO.
Figure 10 Rack and pinion spur gears
Helical gears
Helical gears are like spur gears except that the gear teeth are at an angle to
the bore of the gear. (See Figure II.) This angle is known as the helix angle
and can vary from a few degrees to the standard 45°. In a pair of helical
gears, two or more teeth mesh at the same time, depending on the helix
angle. This action creates a smooth, quieter drive.
MILLWRIGHT-GEAR DRIVES
12- 9
~~
~He~
angle
Figure 11 Helix angle on a left-hand helical gear
WM~Lh
~elixi­
angle
Figure 12 Helix angle on a right-hand helical gear
Single helical gears connecting parallel shafts
A pair of mating helical gears must:
•
have the same helix angle
•
have the same diametral pitch
•
have the same pressure angle
•
have the correct centre distance
•
be of opposite hand (direction of tooth slope).
To determine the hand of a helical gear (see Figures II and 12), place the
bore of the gear vertically. Then:
•
Left hand gears have teeth that slope downward from left to right.
•
Right-hand gears have teeth that slope downward from right to left.
When parallel shafts are connected by a single set of helical gears, the shafts
develop axial thrust due to the helix angle of the gears. The direction of the
shaft thrust depends on:
•
whether the shaft is a drive or a driven shaft
•
the direction of rotation
•
the hand of the gear on the drive shaft.
This thrust must be compensated for by axial thrust bearings.
12-10
MILLWRIGHT-GEAR DRIVES
Assessing direction of thrust for gears with angled teeth
For an types of gears with
angled teeth, the leading faces
of the teeth of the rotating drive
gear push away from the
direction of axial thrust.
The thrust of the driven shaft
is always opposite to the
direction of thrust of the drive
shaft. Figure 13 shows the
thrust direction of a pair of
helical gears.
Figure 13 Helical gear showing directions of thrust
Figure 14 is a set of thrust diagrams for helical gears used on parallel shafts.
Thrust
bearings
Drive
shafts_+-!
Figure 14 Thrust diagrams for helical gears, shafts parallel
Single helical gears connecting crossing shaft
If two single helical gears of the same hand are meshed together, they can be
used to connect crossing shafts. This gear arrangement is not used for high
speed or heavy load applicatiollS but usually on controls for small louvres,
doors, etc.
Figure 15 on the next page shows a meshed pair of single helical gears of the
same hand, shafts crossing.
MILLWRIGHT-GEAR DRIVES
12 -11
Figure 15 Single helical gears with the same hand, shafts crossing
Double (opposed) helical gears connecting parallel shafts
A double helical gear drive consists of a two helical gear pairs of opposite
hand mounted on the same shaft. See Figure 16. The thrust generated in one
of the gears is counteracted by the thrust set up in the other gear mounted on
the same shaft. Note that left-hand and right-hand gears mesh with each
other.
Figure 16 Double helical gears
12 -12
MILLWRIGHT-GEAR DRIVES
Herringbone gears
Sometimes the two helical gears are abutted, to act as a single unit. These are
called herringbone gears. A channel may be cut at the join to drain
lubricants. Figure 17 shows a pair of parallel shafts connected by
herringbone gears. These act like a double helical gear collapsed into a
single unit. The thrust set up in one side of the gear tooth is counteracted by
the opposite thrust set up in the other side of the gear tooth. Herringbone
gears are often found on slow-toming, heavy loads.
Figure 17 Herringbone gears
Bevel gears
In spur, helical and herringbone gears, the teeth are cut on the surface of a
cylinder (drum shape), In bevel gears, the teeth are cut on the surface of a
cone. See Figure 18. The smaller gear is called the pinion and the large gear
is called the ring gear. Bevel gears are used to connect shafts that intersect,
usually at right angles.
Figure 18 Straight bevel gears
Bevel gears are developed from two cones which rotate upon each other. The
point on which the cones pivot is called the apex (see Figure 19). The line of
contact between the two cones which extends across the face of the teeth is
called the pitch line, These cones are called pitch cones.
MILLWRIGHT-GEAR DRIVES
12-13
Pinion
Pitch line
Ring
gear
Pitch diameter
Mounting distance
Figure 19 Pitch cones of bevel gears
Some terms and definitions used in bevel gears are:
12-14
Pitch dinmeter
diameter of the base of a pitch cone
Addendum
amount of the tooth that projects above the pitch
line at the pitch diameter
Dedendum
amount of the tooth that projects below the pitch
line at the pitch diameter
Pitch COne angle
angle that the pitch line makes with the axis of the
gear
Shaft angle
angle between the axes of the meshed gears
MII.LWRIGHT-GEAR DRIVES
l,ear ratio
ratio of the pitch diameters of the meshed gears
Mounting distance
distance from the back of the gear hub to the
apex-gears mounted to this distance will mesh
correctly. The mounting distance is either stamped
directly on the gear or can be found in the
manufacturer's specifications.
Because the piteh-cone angles of the meshing gears are related to the shaft
angle, most bevel gears must be bought as a matched set. The gear ratio of a
bevel gear umt cannot be changed by simply changing one of the gears. If a
bevel gear has a pitch-cone angle of 90° it is called a crown gear. Crown
gears are disk shaped. If the pitch-cone angle is greater than 90° the gear is
called an internal bevel gear. An intcrnal bevel gear is cup shaped.
Straight and Zerol™ bevel gears
Bevel gears that have their teeth cut parallel to the axis of their bore are
called straight bevel gears (see Figure 18). Like straight spur gears, these
gears lUn noisily. ZeroFM bevel gears have curved teeth at a zero angle. This
makes them lUn more quietly and smoothly then straight bevel gears. Both
types of bevel gears create little radial and axial thrust.
Spiral bevel gears
Figure 20 shows a pair of meshed
spiral bevel gears. In these, several
teeth are meshed at the same time.
Spiral bevel gears lUn quieter than
straight bevel gears.
Spiral bevel gears are either right or
left handed. The hand is deterrrtined
the same way as for helical gears.
These gears develop axial thrust. The
bevel gear's direction is determined as
clockwise or counterclockwise when
viewed from the back looking toward
the apex.
Figure 20 Spiral bevel gears
MILLWRIGHT-GEAR DRIVES
•
When the drive gear (usually the pinion) has a right-hand spiral and is
rotating clockwise, its thrust is toward the apex. If any end play exists,
this will eliminate any backlash and bind or break the drive.
•
When the drive gear has a left-hand spiral and is rotating
counterclockwise, its thrust is away from the apex. Any end play would
increase the amount of backlash and create a less hannful situation.
12 -15
Angular bevel gears
Straight and spiral bevel gears may be used to connect shafts that intersect at
other than 90 0 • These gears are called angular bevel gears.
Mitre gears
Mitre gears are bevel gears which
have a pitch cone angle of 45 0 and
generate a 1: I ratio.
These may have straight or spiral
teeth and can be bought singly
because both gears are the same size.
See Figure 21.
They are used to change shaft
direction without altering the drive
ratio.
Figure 21 Spiral mitre gears
Hypoid gears
Hypoid gears (see Figure 22) are a modification of the spiral bevel gear.
They are used for transmitting power between shafts that cross but do not
intersect. The shaft angle is usually 90 0 • Because hypoid gears have hand,
the direction of thrust on spiral bevel gears also apply to these.
Figure 22 Hypoid gears
The advantages of hypoid gears over spiral bevel gears are that:
•
for the same ratio, their pinion is increased in diameter
•
higher ratios can be achieved with the same strength
•
they are extremely smooth running
•
they connect shafts that do not intersect.
One of the disadvantages of the hypoid gear is that significant sliding action
takes place between the teeth. This causes a lubrication problem. It is
essential to use extreme-pressure (EP) lubricants to reduce this problem.
12 -16
MILLWRIGHT-GEAR DRIVES
Hypoid gears must be carefully adjusted for cleamnce and backlash. You
must follow manufacturers' specifications at all times. Use mechanics' blue
to indicate contact points. Use dial indicators to ensure the correct amount of
backlash.
Worms and worm gears (worm wheels)
Worms and worm gears are used to connect shafts that cross (Figure 23).
The worm is a special form of helical gear that resembles a screw. The
helical teeth around the worm shaft are called threads. There may be more
than one start to a thread on a worm. The number of threads on the worm are
often called the number of starts. The gear ratio of a worm and wheel set is
calculated by dividing the number of teeth on the worm gear by the number
of starts on the worm.
Figure 23 Worm and worm gears
Do not coufuse the number of starts with the number of threads per inch
which describes how closely each thread wraps around the shaft. The
distance that one thread advances during one revolution is called the lead.
This is also the distance that the gear advances during one revolution of the
worm. The centre to centre distance between similar points on adjacent teeth
is called the pitch. See Figure 24.
MILLWRIGHT-GEAR DRIVES
12-17
-Helix angle
Figure 24 Parts of a worm
Worm gearing sets can have three different contact patterns when the worm
and worm gear are in their correct positions. These types of contact are
referred to as non-throated, single-throated and double-throated. See
Figures 25 and 26.
Non-throated contact
In a non-throated worm-gear set, the teeth on the worm all have the same
depth (diameter) and linear pitch. Figure 25 shows how meshing contact is
limited to only one or two points on the teeth. Contact over such a small area
concentrates the load, which leads to early tooth wear and eventual failure.
Figure 25 Non-throated
Throated contact
In throated contact, the tooth forms of the worm and worm gear are modified
to give better contact between the worm and worm gear.
•
12-18
In a single-throated worm-and-worm-gear set, the outline of the worm
gear teeth is curved to match the shape of the worm. This increases the
area of tooth contact, spreading the load over a greater area to reduce
pressure.
MILLWRIGHT-GEAR DRIVES
•
In a double-throated set. the profile of the worm is also modified to
increase contact between the worm and the curve of the worm gear.
Figure 26 Double-throated
Worm and worm gears have either right or left hand. This creates end thrust
which varies with the location of the worm and direction of rotation. See
Figure 27.
•
To determine the hand of a worm. use the same rules as for screw
threads.
•
To determine the hand of a worm gear, the same rules apply as to helical
gears.
•
For a worm and worm gear set. the hands must be the same.
I~~
'/~
'+(1'\) ..
Thrust
bearings
~
"~:; 7
-a~.
{~.~
...--
~;.
~
r;
-
fi
L:.[ .... V
-\::::
-
.-
Figure 27 Thrust charts for worm gear assemblies
Gear types and shaft arrangements
Gear types and shaft arrangements used in mechanical devices are shown in
Table 1 on the next page.
In addition to these general purpose gears, a number of specialized gears are
used to perform particular operations.
MILLWRIGHT-GEAR DRIVES
12-19
Table 1: Common gears and shaft arrangements
Name of gear
Shaft arrangement
1.
External spur
Parallel
2.
3.
4.
Internal spur
Parallel
Rack & pinion
One shaft only
Single helical
Parallel or crossing
5.
6.
Double helical
Parallel
Herringbone
Parallel
7.
Straight bevel
Intersecting
8.
9.
Spiral bevel
Intersecting
Straight mitre
Intersecting
10.
Spiral mitre
Intersecting
11.
HYPoid
Straight worm & wheel
Crossing
Throated worm & wheel
Crossing
12.
13.
Crossing
Overdrive and
reduction units
Overdrive
The purpose of an overdrive unit is to increase speed and, as a result,
decrease torque. Overdrives can be spur, helical, herringbone, or planetary
gear drives.
PIVTM drives
A variable speed transmission or PIVTM (positive infinitely variable) drive is
a type of overdrive. It can reduce or increase the speed of the output shaft. It
is similar to a variable-speed belt drive (see Chapter 10: Belt Drives) except
that it uses an all-metal, laminated (modified leaf), chain. The chain moves
against conical pulley flanges with radial grooves (see Figure 28). To ensure
normal service life, you must avoid overloads, and sudden load changes or
reversals.
PIVTM drives and variable-speed belt drives are commonly used together
with reduction units to change or finely adjust output shaft speed (see
Figure 29).
12-20
MILLWRIGHT-GEAR DRIVES
Figure28 Variable speed transmission
Figure29 Variable speed transmission attached to a reducer
MILLWRIGHT-GEAR DRIVES
12-21
Reduction
A reduction unit consists of a pair of gears in a rigid, strong, movable
housing. Its purpose is to reduce the speed and increase the torque supplied
to the driven shaft. A reduction unit may be:
•
single, with two gears and two shafts
•
multiple, with four or more gears on three or more shafts.
Gear arrangement in reduction units can be any of the following:
•
worm and worm gear with shafts at right angles
•
spur, helical, or herringbone gears with parallel shafts
•
bevel gears with shafts at an angle
•
planetary gears with parallel shafts
•
any combination of the above groups.
Worm gear reduction units
Working parts
The working parts of the unit are the worm, the worm gear (wheel) and
bearing assemblies:
•
The worm shaft is one piece of alloy steel with the worm ground to a
precise shape and smooth finish.
•
The gear shaft may be made of standard or high-tensile steel for heavy
loading. The worm gear is keyed and pressed onto this shaft.
•
The worm gear can be constructed in any of the following ways:
- a cast bronze gear-for small sizes only
a bronze ring cast onto a cast iron disk; extra holding power is
obtained by welding or pinning the ring to the disc
a bronze ring dowelled and bolted to a cast iron disk-for large
units only.
•
The bearings must have axial thrust capacity to allow for the thrust
loading of the gear action. Tapered roller bearings or angular contact
bearings are frequently used. Note also that:
On small units designed for light loading, the bearings can be deepgroove or maximum-capacity, radial ball bearings.
For North American demand, some imperial worm gear reduction
units use tapered roller bearings instead of angular contact bearings.
Imported reduction units may use imperial or metric bearings.
Note: Before removing bearings, make sure that replacement bearings
are available.
12-22
MILLWRIGHT-GEAR DRIVES
Fitting
In order to transmit maximum force with minimum wear, it is crucial that the
worm and worm gear mesh correctly.
Figure 30 Worm gear and bearing assembly
To ensure correct fitting of the reduction unit, you must:
1. Assemble the worm shaft with the bearings and place the assembly
into the housing.
2. Use the correct pack of shim gaskets to get the correct bearing preload.
3. Mount the worm gear with its bearings and place then into the
housing.
4. Determine the shim pack thickness required for correct bearing
clearance. (See axial clearance or thrust adjustment in Chapter 9:
Bearings.) This shim pack is normally between the retaining cap and
the housing (see Figure 30).
5. Assemble the retaining caps with half of the shim pack under each cap.
6. Wipe the worm free from oil.
7. Wipe a thin coating of bluing onto the worm.
8. Rotate the worm in the desired direction while applying some
resistance to the worm gear.
9. Check the worm gear to sec where the worm contacts its teeth.
10. Adjust the worm gear by transferring some shim gaskets from one cap
to another. Do this until it shows contact as central, favouring the
leaving side (see Figure 31).
Note: Because of the varying configurations and brands of reduction units,
refer to the individual manufacturer for correct replacement parts and
specifications.
MILLWRIGHT-43EAR DRIVES
12-23
Figure 31 Worm gears-<:hecklng the mesh
Input shaft
left-hahd
(a)
left-hand
Right-hand
(b)
(e)
Figure 32 Single-reduction worm gear units showing orientation
12-24
MILLWRIGHT-GEAR DRIVES
Single reduction
A single reduction worm and worm gear unit has its shafts at right angles.
See Figures 32a. b. and c. In Figure 32. the solid arrows show units with
clockwise rotation and the dotted arrows show units that rotate
counterclockwise. Most units are c1ockwise-counterclockwise units need to
be specially ordered. The worm may be mounted:
•
below the worm gear with both shafts horiwntal (a)
•
above the worm gear with both shafts horizontal (b)
•
vertically with the worm-gear shaft horizontal (c)
Hand of drive
The hand of drive of a reduction unit refers to the position of the output shaft
as viewed from the input shaft. Figure 32 shows some examples. The output
shaft of a reduction unit may extend out from both sides of the housing.
These are called left-handlright-hand units, or simply LR units. Before
ordering, refer to the manufacturers catalogue for parts numbers and for hand
of drive codes.
Some units have their hand reversed by reversing the worm gear assembly
and reassembling it so that it extends out of the other side of the housing.
Speed range and size
Single reduction units have a speed range of approximately 5: 1 to 70: 1. The
size of the worm gear unit is based on the distance between the centres of the
input/output shafts.
Multiple reduction
Multiple reduction units have two or more stages of reduction. These units
can use the same or a variety of different reduction methods within a single
unit. The type of reduction method used is determined by orientation of the
shafts and the amount of reduction required. Figures 33 to 35 show samples
of multiple reduction units.
Figure 33 Double reduotion unit with helical
and worm gears (shafts crossing)
MILLWRIGHT-GEAR DRIVES
Figure 34 Double reduction unit with
worm gears (shafts parallel)
12 -25
Figure 35 Triple reduction unit with worm gears
(shafts crossing)
Speed range
A double reduction, cOIL~i~ting of:
•
a primary helical reduction combined with a worm and worm gear
reduction-can havc ratios of approximately 20: 1 to 280: 1
•
a primary worm and worm gear combined with a secondary worm and
worm gear-can have ratios of approximately 25:1 to 4900:1
•
a triple reduction consisting of all worm and worm gears-can have
ratios of approximately 1000: 1 to 180 000: 1.
Different manufacturers' catalogues show slightly different ranges.
Helical and herringbone gear reduction units
Reduction units using helical or herringbone gears have their shafts parallel.
These reduction units can have ratios of approximately:
•
single reduction of3:1 to 12:1
•
double reduction of 7:1 to 30:1
•
triple reduction of 20: 1 to 70: 1
•
quadruple reduction of 80: 1 to 280: 1.
Refer to the manufacturers catalogues for specific ratios and number of
reductions.
When helical gears are used. axial thrust is created and tapered roller
bearings are commonly used. When herringbone gears or double. opposed
helical gears are used. axial thrust is not created and radial bearings are used.
Figure 36 shows a single reduction unit with herringbone gears using
cylindrical roller bearings.
12-26
MILLWRIGHT-GEAR DRIVES
Figure 36 Herringbone reduction unit
Figure 37 shows a double reduction unit with helical gears. In this case the
intermediate shaft has double opposing helical gears and radial ball bearings
are used. The high and low speed shafts use tapered roller bearing to handle
the axial thrust.
Figure 37 Double reduction unit with single helical gears
Fitting
When replacing gears into the reduction unit, a good rule of thumb is to fit
the largest gear first, the intermediate gear or gears next, and the pinion last.
When fitting shaft assemblies that use radial bearings, the axial float must be
controlled. Refer to the manufacturers specifications.
MILLWRIGHT-GEAR DRIVES
12-27
.
When tapered roller bearing are used, a shim pack is necessary to set the
correct clearance (preload). By moving the shims from one end to the other,
the position for correct gear meshing is established. Helical gears have much
more sideways allowance then do herringbone gears.
Herringbone gears in mesh have virtually no sideways movement. The high
speed (pinion) shaft's end float is controlled by tandem tapered roller
bearings at the outer side of the shaft (see Figure 36). The gear floats in the
cy lindrical roller bearings and is held in position by the gear mesh only. The
correct backlash is established by the centre distance of the shafts. This is set
during manufacture.
Due to the sliding action of the gear mesh, the teeth can develop razor-sharp
edges on the leaving side. Be careful when taking an old unit apart, as the
sharp edges may not show through the lubricant.
Bevel, mitre, and hypoiGt gear reduction units
Bevel gear reduction units are primarily used for right-angle drives whose
shafts may intersect. They are often found in conjunction with helical gear
sets to create a right-angle drive from a parallel one (see Figure 38).
Helical
gear
Bevel
gears
sat
Figure 38 Changing shaft direction using a sel of bevel gears
Their ratios are low compared to previously mentioned reduction units. They
range from I: 1 to approximately 10: 1. Refer to the manufacturer's catalogue
for the specific ratio. Units with a 1:1 ratio are called mitre gear boxes.
Hypoid gear reduction units are used in similar applications to bevel gear
reduction units except where the shafts cross. They are commoniy used in
the rear drive of automobiles. Their ratios range from about 2: 1 to 8: 1.
12-28
MILLWRIGHT-GEAR DRIVES
Fitting
Tapered roller bearings are commonly used with these reduction units
because of the axial tbrust created by the bevel, mitre and hypoid gears. To
achieve the correct mesh of the gears, shims are used between the beariug
retainer and the housing (see Figure 39). The apex of the two gears must
meet. To achieve this, the mounting distance must be known (see Figure 19
in the discussion of bevel gears). To determiue the correct mounting
distance, refer to the manufacturer's specifications.
Shim pack
pack
Figure 39 Shims used between the bearing retainer and housing for
correct gear mesh
Planetary gear reduction units
Planetary gears allow shafts to be in-line. They may be reduction, overdrive,
reversing, or direct drive. Planetary gear reduction unitq may have high
reduction ratios in a compact unit (see Figure 40). They are available in
siugle or multiple reductions. Their ratios range from approximately 1.1: 1 to
50000:1.
The planetary gear set is made up of three drive members. They are the sun
gear, the ring gear, and the carrier which holds pinion gear(s). See Figure 40.
Any of these drive members could be the input or the output drive. In a
direct drive, two of the drive members are connected to become the input.
See Table 2 fur possible combinations.
MILLWRIGHT-GEAR DRIVES
12-29
Figure 40 A planetary gear
Table 2: Operation chart for planetary gears
Reduction
Sun gear
H
Ring gear
Carrier
I
0
Note: I = Input
0= Output
H
Overdrive
0
H
H
Reverse
0
0
0
H
I
0
H
Direct drive
2 3
I
I
0
0
0
H= Held
Installing and maintaining
gear drives
~~.~1lI~l:!!~I!l:!!1l~~~:!!l~~',l!~~~~~'*~:R1}3m~~.~B!!t~r&1l!j!j!j~tti!j!jt\1l[!@t1l:j{~i~t£l!!t~l!
Installation
The base on which the reduction unit and the power source is to be mounted
should be fumly fastened to a solid foundation capable of withstanding the
load imposed on it. Reduction units are joined to the power source in a
variety of ways.
12-30
MILLWRIGHT-GEAR DRIVES
A reducer and power source can be joined in the following ways:
•
direct drive
- using a flexible coupling, both units must be bolted to a strong, rigid
base (see Figure 41)
- combination units have the power souree (gear motor) attached
directly to the reducer by a flange mounting (see Figure 42)
•
indirect drive
using a V-belt or chain drive, the units can be on separate bases (see
Figure 43).
Figure 41 Using a flexible coupling
Gear motorl
Flange mounting
Reducer
Figure 42 Power source attached directly to reducer
MILLWRIGHT-GEAR DRIVES
12
31
Power
source
I·__--~ Base
V-belt
1:
Reducer
Figure 43 Using a V-belt
Positioning
The unit and motor should be in a position where they can be easily serviced
by routine checks of oil, seals, hold-down bolts and other minor work. They
should also be in a position where major overhaul work, such as removing
gears and shafts can be done without having to dismantle surrounding
equipment.
Mounting styles
Reduction units are available with
many different mounting styles.
,,,,,flo. ."
<...
..•• J
,~
Base-mounted
Base-mounted units can be mounted
in various positions (see Figure 44).
Every unit is designed to be mounted
in a specific orientation. For example
a floor mounted housing must not be
mounted on the wall or vice versa
because the lubrication system is not
designed for this purpose. Refer to
the manufacturer's specifications for
the preferred orientation.
Figure 44 Mounting pOSitions
Shaft-mounted reduction units
Shaft-mounted reduction units have a hollow output shaft which mounts
directly onto the shaft of the equipment its meant to drive. These units have
parallel or crossing shafts and come in a variety of different types reductions.
They have single or double reduction with either helical or worm gears.
Figures 45 and 46 show samples of the available combinations. Refer to
manufacturers' catalogues for all available combinations.
12-32
MILLWRIGHT-BEAR DRIVES
Torque
arm~,/>""
Figure 45 Shaft-mounted, single, helical reduction unit with a torque arm
)
Flange
Figure 46 Shaft-mounted, double, worm reduction unit with a flange mount
MILLWRIGHT-GEAR DRIVES
12-33
Using a torque arm
To prevent rotation of the unit, the housing is held onto the equipment by a
flange or supported by a torque arm, See Figure 47.
--::~~
::::~~-
----~
--
,
Figure 47 Shaft-mounted, single, helical reduction unit driven by a V-belt
held in position by a torque arm
When torque arms are used, ensure that:
•
the torque arm is as close as
possible to a right angle with a
line running from the output shaft
to the point of attachment (see
Figure 48)
•
the V-belt drive is at right angles
to a line between the output and
input shafts (see Figure 49 on the
next page).
Refer to manufacturers' specifications
for preferred mounting positions.
,
$
o
When a V-belt drive is used, extra
variations in speed can be obtained by
changing the relative size of V-belt
sheaves.
o
Figure 48 Orientation 01 torque arm
12-34
MlllWRIGHT-GEAR DRIVES
o
Figure 49 V-belt drive orientation
Lubrication
Lubrication of most reduction units is by the splash method. Some units use
a channel in the housing to catch the splashed oil from the top of the casing.
The oil is then funnelled to the shaft bearings. Some units have special
arrangements:
•
Under-driven wonn gear units use close-fitting wipers on the worm gear
to scrape oil off the gear and convey it by channels to the gear shaft
bearings.
•
Some reduction units use aU integral pumping system to lubricate the
bearings.
•
Reduction units with vertically mounted shafts have special provisions
for lubricating the top bearings of the vertical shafts.
Oil grades
The grade of oil to use for lubrication is specified by the manufacturer of the
unit. If not, major oil companies recommend suitable grades. Most
operations use several makes of reduction units. It is more efficient to use a
multi-purpose oil for them all, rather than using a different grade of oil for
each unit.
Troubleshooting gears
Table 3 (on the next three pages) is a troubleshooting chart for gears.
MILLWRIGHT-GEAR DRIVES
12-35
Table 3: Troubleshooting chart for gears
TROUBLE
CAUSE
CORRECTION
NOISE ANO TEMPERATURE•••
Excessive noise and
vibration
1. Worn bearings (usually on
Input shaft)
1. Change the bearings.
2. Worn gears or poor mesh
2. Note the gear wear; a gear change is
the last resort.
3. Unit loose or out of line
3. Align and tighten the uni!.
(coupling noise)
4. Motor bearings worn (a
4. Check or have the motor checked.
difficult noise to pin down)
5. Low oil level (no muffling
5. Bring the oil up to level.
effect)
Unit runs hot
1. Oil too low or overflowing
(heated from churning or
wrong grade)
1. Correct the oil level and use the
correct grade of oil.
2. Worn bearings
3. No air flow around the
2. Change the bearings.
3. Check to see if the fan is working;
housing
4. Unit overloaded
clean or blow off the outside of the
unit If It is covered with 011, grease or
foreign material.
4. Check for conditions causing
temporary overload such as non
turning pulleys, Idlers, etc. If the
overload is due to trying to force a
5 hp rated unit to do 10 hp work,
change to a heavier weight or an
additive type of oil. Ask for a larger
unit and motor.
5. Unit loose, or out of line
5. Align and bolt down the unit.
CONTINUED...
12-36
MILLWRIGHT-GEAR DRIVES
••• CONTINUED
Table 3: Troubleshooting chart for gears
TROUBLE
CAUSE
CORRECTION
OIL CHECK•••
Oil seals leak
1 . Leather or synthetic seal
perished. hardened or
cracked
1. Change the oil seals.
2. Foreign matter under lip of
2. Remove foreign matter; check the
oil seal
3.
seal to see it is cut - replace if
necessary.
Shaft worn or rough at point 3. Smooth the shaft with a fine emery
of seal contact
cloth; move the seal - a 1/16-inch
change of position in the seal is
usually sufficient.
4. Bearings worn (shaft
vibration preventing seal)
4. Change the bearings.
5. Shaft loose and vibrating in 5. Knurl or build up the shaft and change
bearings
011 level drops but
the oil is clean
the bearings.
1. Worn or damaged oil seals
1. Replace the oil seals.
2. Loose end plates
2. Check and tighten all bolts. If the unit
continues to leak, take the top half off
and check the gasket, if used, or
check for foreign matter caught
between the faces of the top and
bottom sections.
)
Possible crack in the unit
base
3. Clean off the unit and check the feet.
4. Incorrect assembly of end
covers or plates
4. Some reduction units use end covers
with annular grooves and a drain hole,
to prevent oil from leaking out past the
shaft. If the end cover is put on
without the drain at the bottom, oil will
leak out.
1. General deterioration
through wear and
oxidization
1. Oil turns darker with general
deterioration. Change the oil
2. Water in oil through seals,
2. Oil turns lighter when water is present.
3.
Oil is dirty with even
discoloration
filter plugs or condensation
Oil is dirty with colour
streaks or ringsj pour
test will show a
distinct wave effect
or streak effect if
allowed to run slowly
from the drain plug.
1. Bearings or gears worn
Change the oil and check for water
entry.
1. Replace the bearings; check the geal
condition; clean out suspended
metallic particles from the housing anc
change the oil
CONTINUED •••
MILLWRIGHT-GEAR DRIVES
12-37
••• CONTINUED
Table 3: Troubleshooting chart for gears
TROUBLE
CAUSE
CORRECTION
BEARINGS •••
Bearing failure
1. Improper lubrication; foreign 1. Replace bearings, change the oil, and
check the oil level and grade of oil
material in oil
2. Unit running too hot over
long periods; overload
3. Unit out of line
2. Check for overload causes; correct
where possible. Clean off the unit.
3. Correct the alignment. This is the
greatest cause of failure because the
term "flexible coupling" is taken to
mean allowing too much
misalignment. Flexible or not, the
closer to perfect, the longer the
coupling and bearing life.
4. Thrust bearings to tight or
too loose
4. Check for proper end play after
mounting the new bearings.
5. Improper bearing preload
5. Adjust bearings to proper preload
6. Excessive overhung load
6. Reduce overhang
7. Bearings worn
7. Change the bearings
B. Base too light, allowing
B. Rebuild or stiffen base.
flexing.
12-38
MILLWRIGHT-GEAR DRIVES
MILLWRIGHT MANUAL: CHAPTER 13
Couplings and Clutches
Rigid couplings .........•.....................•............................................... 13:2
Sleeve couplings ............................................................................. 13:2
Flanged couplings ........................................................................... 13:2
Clamp couplings ............................................................................. 13:4
Flexible couplings ......................................................................... 13:5
Mechanically flexible couplings ..................................................... 13:5
Elastomeric couplings .................................................................... 13:9
Failure of flexible couplings ........................................................... 13: 13
Universal joints ............................................................................. 13:14
Centrifugal couplings ............................................................................. 13:16
Thermal cutout ................................................................................ 13: 17
Clutch-style couplings .................................................................... 13:17
Fluid couplings ............................................................................... 13:18
Dry fluid (shot) couplings .............................................................. 13:21
Clutches and brakes ...................................................................... 13:22
Mechanical clutches ....................................................................... 13:22
Electromagnetic clutches and brakes .............................................. 13:29
Actoation methods for clutches and brakes .................................... 13:31
Couplings and Clutches
Couplings are used to join two shafts together and transmit power from a
dri ving source to a dri ven machine. The various types of couplings are:
•
rigid couplings which require positive positioning of the shafts
•
flexible couplings which permit slight misalignment and end play of the
shaft
•
universal-joints (U-joints) which connect shafts with angular and/or
offset misalignment
•
centrifugal couplings which are used when the drive is required to reach
a certain speed before engaging. These are also considered to be
clutches.
Couplings are manufactured from ferrous and non-ferrous materials.
Stainless steels and non-ferrous materials are used in corrosive atmospheres.
The misalignment conditions which arise wheu coupling two shafts are
illustrated in Figure 1. When choosing a coupling, you must consider the
amount and type of misalignment that can be tolerated, and how much is
likely to occur.
Angular misalignment
Parallel misalignment
Axial movement
Figure 1 Angular misalignment, parallel (offset) misalignment and axial misalignment (end play)
Clutches are used to engage and disengage the power of the two shafts. In
rotating drive systems, clutches and brakes are the key to effective control
and transmission of drive torque, speed and power. Clutches and brakes may
be separate components or combined into a single unit called a clutch-brake.
Their functions are:
•
clutching-transfers torque from an input shaft to an output shaft
•
braking-stops and holds a load.
This chapter fID! discusses couplings and then clutches and brakes.
Rigid couplings
Rigid couplings are designed to connect two shafts in a fixed position. They
may only be used when:
•
offset misalignment is less than the bearing clearance
•
there is no angular misalignment
•
the machines do not heat up while operating (creating axial shaft
movement)
Rigid couplings may be sleeve, flange, or clamped couplings.
Sleeve couplings
Sleeve couplings are long, heavy-walled tubes with a precision bore. They
have a keyway (keyseat) cut along the length of the bore (see Figure 2).
They are used where assembly and disassembly is not often done. To ensure
accurate alignment, the fit between the coupling bore and the shaft must be
close (snug).
Setscrews
Keyway
Figure 2 Sleeve coupling
Flanged couplings
Flanged couplings are made in pairs, each with a flanged hub. The flanges
have a series of matching holes through which they are bolted together. A
rim around the flanges shields the bolt heads and the nuts so that foreign
objects do not become entangled around the coupling (see Figures 3 and 4).
Some of these couplings have spigots and recesses, others have a tapered
bore and tapered compression sleeves.
Flanged coupling with spigot and recess
In this type of coupling, one flange has a recess and the other has a spigot.
The fit between the spigot and the recess and bore must be held to close
tolerances to accurately line up the shafts. The hubs of the two halves are
r!.&£gH@~M.\t~Em'E;W!'z"w:!:k"e_S"?74N~:8S:;;;~Rg;;:~_Wcq'KW'1;;-~?~,@;::%!.8X_%''8r''T;a;#'4'~TJ"J3%X',&8W;"'8;H:mr,::t~8:'-;$J:Hj"(((~~(#~(i~jhiji&v,.2jiJ3H;1ii"2Hm;~~~~"©©i;Wfixjjii1d;UM1'i;iWMl~':iW$M_~-;;:!i'0'ii!i&t
13 - 2
MILLWRIGHT-COUPLINGS AND CLUTCHES
keyed and set-screwed to the shaft and the flanges bolted together. These
flanged couplings may be used to connect two shafts of any diameter, but
they are usually used for shafts of equal diameter.
Flange rim
Figure 3 Flanged coupling with spigot and recess
Compression coupling
Compression couplings are two flanged hubs which have a tapered bore.
Split, tapered sleeves fit inside these hubs, see Figure 4.
flange
Tapered sleeve
Figure 4 Flanged compression coupling
The shafts to be coupled fit into the straight bore of the sleeves. As the
flanged hubs are drawn together the sleeves compress onto the shafts. These
couplings use no other driving means than the compression fit onto the
shafts. Due to this requirement they are usually used in light load and light
torque situations. Before installing these couplings. ensure that bores. sleeves
and shafts are the correct size and have an adequate surface finish.
13
Mounting flanged couplings
Before mounting flanged couplings on the shaft, do the following:
•
Check that the halves have correct bore, keyways, and setscrews.
•
Examine the fit of the spigot into the recess; remove high spots on the
corners and ensure the depth is correct using a depth gauge
•
Align the fastener holes as follows:
- If the coupling comes from the supplier bolted together, place
alignment marks on the rims of the halves to help assembly.
- If the coupling halves are separate, fit the fasteners to fmd the
matching holes and place marks on the rims.
Problems may arise with snug fitting or tap-fitted fasteners if the hole
centres are off by only a few thousandths of an inch.
•
Ensure that the fasteners fit correctly in the holes.
Clamp couplings
Rigid, ribbed compression couplings like the one shown in Figure 5 are
called clamp or mUff couplings. Clamp couplings use keys to transmit the
torque from one shaft to the other. Because of their unbalanced weight
distribution, they are used mainly for low-speed drives. Properly engineered
muff couplings can be well balaneed and used for higher speeds, but, in this
case, shaft alignment is critical.
Figure 5 Clamp (mull) coupling
Because they are split axially, they are easily assembled and disassembled.
When assembling a clamp coupling, there should be a slight gap between the
two halves after the bolts are snugged up. This ensures that the coupling is
seated all around the shafts.
The bolt torquing pattern starts in the centre of the coupling and extends
outwards in both directions to the ends. To ensure consistent bolting
pressure, tighten the bolts progressively from hand tight to their specified
torque values.
Flexible couplings
In low-speed, low-power applications shafts may be connected using flexible
metal shafts and plastic or rubber sleeves. Low-rigidity composite shafts
may also be used. In power transmission, flexible couplings are used to join
two shafts when there is limited lateral displacement or unavoidable
misalignment between them. They should NOT be used when there is major
misalignment.
During use, a flexible coupling allows for the effects of slight axial
misalignment, thermal growth, and vibration between members. However,
the stresses introduced by any misalignment transfers directly to the drive
and driven machines. This results in premature equipment failure. When
using flexible couplings, ensure that equipment is aligned as accurately as
possible. Refer to Chapter 23: Alignment.
The three most common categories of flexible couplings are:
•
mechanically flexible (Figures 6 to 11)
•
elastomeric (Figures 12 to 17)
•
universal joints (Figures 19 to 21).
Mechanically flexible couplings
Couplings of this type get their flexibility from the sliding or rolling of
mating parts. The parts within these couplings usually require lubrication.
These couplings create a positive transmission of power and torque from the
drive to driven machines. That is, there is no rotational play.
Some of the more common types of these couplings are:
•
•
jaw and slider
•
•
•
chain
gear, double engagement
metallic grid
meta11ic disk.
Jaw and slide.. coupUng
Jaw and slider couplings are composed of three units: two jawed hubs and a
slider block between them. See Figure 6 on the next page. These couplings
allow for angular and offset misalignment. They are designed for low speed
and high torque situations. Large couplings have replaceable wear surfaces
on the jaws.
5
Figure 6 Jaw and slider coupling
Gear-type, double engagement couplings
Gear-type, double engagement couplings may be unlubricated (metal and
nylon) or lubricated (metal). This coupling has two hubs with curved
external teeth which compensate for up to 1 Y2. of angular misalignment.
The hubs are joined by an outer member with internal teeth. See Figure 7.
_-r--.
O-ring seal,.......
Lubrication fill
Figure 7 Gear-type, double-engagement coupling
A sealing device such as an O-ring seals the coupling teeth from the
environment and retains the lubricant within the coupling. They are
expensive, but if properly eared for, have a long life span. They can operate
at high speeds.
Chain couplings
Chain couplings have two sprockets and a length of chain which mate to
give overall flexibility. These couplings are available with roller, silent, or
synthetic chain. See Figures 8 and 9. Synthetic chain is II non-metallic
coupling.
Figure 8 Roller chain coupling
Figure 9 Silent chain coupling
The sprocket teeth are hardened to give high torque-carrying capacity for
their size. Roller chain couplings also have their chain rollers hardened.
Metallic grid couplings
In these couplings, multi-grooved flanges hold a flat steel grid that weaves in
and out through grooves as shown in Figures 10 and 11. The grooves are
machined to allow room for the grid to bend during starting and peak load
conditions. For their size, they have higher torque-carrying capacity and
stiffness than elastomeric types. They are available with horizontally or
vertically split casings. They are usually packed with grease to lubricate the
elements and reduce wear.
7
Figure 10 Metallic grid coupling (horizontally spilt)
Steel grid
Connecting
Figure 11 Metallic grid coupling (vertically split)
Metallic disk couplings
Metallic disk couplings need no lubrication. In this type of coupling, a flat
centre member is connected between metallic disks which are bolted to the
hubs. See Figure 12. This is a double-engagement coupling. The
transmission of power is from hub to disk to centre member to other disk to
other hub.
Metallic disk couplings allow highspeed operation and are well balanced.
Torsional characteristics are very stiff and allow this coupling to be used
where no backlash is permitted. Medium lateral and axial stiffness rates are
obtained by using two sets of disks, one at each end of the coupling.
Metallic disk
Hub
Figure 12 Metallic disk coupling
Elastomeric couplings
An elastomer is an elastic substance like natural rubber or synthetic, rubberlike plastics (polymers). Various types of elastomeric elements are used in
these couplings. Blastomeric couplings give low torsional stiffness and
reduce the lateral force due to misalignment.
Jaw elastomeric couplings
Couplings of this type use elastomers in flexing compression. The
elastomeric elements are called spiders. Spiders have various degrees of
hardness to suit various load-carrying and torsional requirements.
Increasing or decreasing the number, width, or diameters of the jaws alters
torsional stiffness, torque-carrying capacity, and overall dimensions. For
low-power applications, the spider has a one-piece design. See Figure 13 on
the next page. For medium- and high-power applications, the spiders have
more load cushions and may be split.
Load cushions
Spider
Figure 13 Jaw elastomeric coupling with spider (one-piece)
Undamped doughnut couplings
Unclamped doughnut couplings transmit torque through shear loading of the
elastomer. That is, the force acts across the teeth, which shear off if the load
is too large. The coupling is not fixed to the hubs with fasteners. For a given
coupling size, torsional and lateral stiffness increase as load carrying
capacity increases. Figure 14 shows the toothed design of the elastomer and
hubs in an undamped doughnut coupling.
Hub
Elastomer
Hub
Figure 14 Elastomeric doughnut coupling, undamped
Clamped doughnut couplings
These couplings have an elastomeric, split doughnut with metal inserts
permanently moulded into them. See Figure 15. The coupling is clamped
onto the hubs by fasteners. As the doughnut is clamped onto the hubs, it
precompresses each leg increasing its torsional strength. This design allows
for the disassembly and assembly of the doughnut without altering the hubs
or the alignment ofthe equipment.
Hubs
Inserts
!
\
Fasteners - - -
t
Figure 15 Elastomeric doughnut coupling, clamped or restrained
Clamped tire couplings
These couplings have a flexing element with reinforcement at the outermost
radii. See Figure 16 on the next page. This reduces the overall length for a
given torque capacity. Internal reinforcement and external clamping
increases the tire's torque capacity and overall stiffness.
iii!ii~'&"gtW","ko;@rtill(.JtiM0i~~~tA'3u~@~j'!i!iW"'j,\'iiiW!ii!iiiiii!A'Ii}ii:'iii-;;'!!_;:ro"HJ_S"'-W"'_;;;@;'iR"ii;;;2ii&ii!;;CF!r,;~;o~4;:;;_P;;;_8tW,:F!;';;;;~R?4;;Q;e,-G.:;8:;8-'_W8"""'-''-PNW;;CF,;;;o;;Q,\f,\"Fii,\H<iSB'iiHMifmlli)#,,%'B
MILLWRIGHT-COUPLINGS AND CLUTCHES
13 - 11
Flexing element (tire)
Figure 16 Elastomeric tire coupling. clamped
Bushed pin couplings
These couplings are used to connect high-inertia drives to low-inertia driven
members. An example is the coupling of a diesel engine to a hydrostatic
pump.
Figure 17 shows how the pins on the hubs are inserted into the coupling. The
elastomer is chosen to gi ve torsionally stiff coupling and good thermal
stability. It can operate at high rotational speeds and accommodate slight
misalignment.
Reinforced bushing In flexible element
Pins
Figure 17 Elastomerlc. bushed, pin coupling
13
12
Offset coupling
Offset couplings are designed to accommodate a nmch larger amount of
offset misalignment then other flexible couplings. They do so without
creating any side load on the shafts. The couplings consist of two hubs or
flanges, a centre plate and a series of pinned link arms. One set of link arms
connect the hubs to the centre plate and another set connect the centre plate
to the other hub. See Figure 18.
Mounting holes
Figure 18 Offset coupling
To aceommodate slight angular misalignment the needle or straight roller
bearings between the pins and the link arms can be replaced by spherical
roller bearings.
Failure of flexible couplings
All rotating drive shafts operate better when they are accurately aligned.
Using very flexible couplings rather than assembling drive-shafts carefully is
not a good idea. Excessive misalignment can destroy flexible couplings as
well as the eoupled equipment.
The flexible coupling can act like a fuse: when there is a problem, they fail
before more expensive and permanent parts of the drive line. You should not
rely on this if failure is dangerous as in a lift or overhead crane. In these
cases, the brake should be between the coupling and the load.
Troubleshooting coupling failure
When investigating coupling failure, check the hardware and the flexing
member carefully and stody the hubs closely. Table 1 On the next page
shows some symptoms and possible causes. If these symptoms are not
present, analyze the flexing member for other signs of overload or
misalignment. It may be necessary to return the broken coupling to the
manufacturer with its performance history to find the cause.
il'.t'Ji0:l~$m;:J~l"d:">."i:;~%~k'1'""",,)E~S.t:>,-,,,,:t*:;""':i.l'?S'JlGJ!K;0t*,,,m\Vt!0;Z0W"8n:8U1TtW:ttnnZ;9J\;$'ii0:'lJ\':t;\:1;tJ'MnM"tJ'J«,';\ftif!tie:iilu,f1"-q~ii!l&'L"';:'i&:rii:'K'"J.'1''';;;;;_i2o/&0l.?i?,t;tm_~t;%\*,qWl\tlil(':~mi¢
MILLWRIGHT--COUPLINGS AND CLUTCHES
13
13
Table 1: Troubleshooting flexible coupling failure
!'JI1I111 symptom
Secondary symptl)lTI.::cs~_ _ _ _-'p'-o:..s:...:sc.:.lb'-'I.:.e-'c.:.a..:.:u;:.se-=-_____~
Bore of fractured hub
shows galling marks
• Force marks on exterior
• Bore was undersized for shaft &
was forced on
• Rolled-up metal flakes in the bore
& exterior hammer marks
• Hub was cocked when positioned
• Keyway extended radially in only
one direction of rotation
• Overload or frequent high
starting torque
• Keyway extended in both directions
of rotation
• Bad key fit for a reversing load
• Only part of the length of the keyway
is deformed
• A short key or slipped key
• Rings of grease, dirt, rust or fret
• The hub was not engaged with
shaft over its whole length
Hub keyway Is deformed
Internal markings on the bore
Rippled appearance of bore,
keyway or connecting bolt holes
• Torsional vibration
Elongation or wear of peripheral
jaws, bolt-holes, & point of
engagement
• Excessive misalignment or
torsional vibration
• Heavy, uncontrolled loading
(vibration loading)
Universal joints
Universal joints are more commonly called U-joints. They allow positive
transmission of rotating power. U-joints are used to accommodate shafts
which have misalignment greater than flexible coupling allows. Units for
industrial applications are made from carbon steel, heat-treated alloy steel,
bronze, or staInless steel. A single U-joint is used to connect shafts with
angular misalignment. Two U-joints are used to connect shafts with angular
andlor offset misalignment.
U-joints cannot accommodate axial misalignment (end play). Where this is
present they should be specialiy mounted on a splined connection. The
recommended maximum shaft angle is 15· even though at slow revolutions
(below 100 rpm), up to 45° can be successful.
0*'tt0%t7A'Sl?~'t"'''t-'%;1?-$8WJt'r,t%%';;,W'h;;ut;:~,<t\;;\;''''?''i;'lit4_iiU.j%~iit_m((Wtt1;At""~"'iI',"',;'"~_\W~_tNi11ii,'t~\'i'M'f'1R*Jr«'.f.01~t.~J%'IJJ!:tn'!'ii{ITi:'0S,Fl'tW_'2k!it'¥_I_M«'1S<$X,!i¥Pii5i8;thW:S$'~~
13 - 14
MILLWRIGHT-COUPLINGS AND CLUTCHES
When installing two U -joints on a common shaft or splined connection,
ensure that the arms of the yokes are in line with each other (see Figure 19).
This will result in a uniform rotational velocity of the drive. While there are
many different designs of U-joints, the simplest types are block-and-pin and
cross-and-bearing U-joints.
Figure 19 Correct installation of two U-joints on a common shaft
Block-and-pin V-joint
Block-and-pin U-joints consist of two U-shaped yoke hubs and a centre
block through which a large and a small pin fit. The small pin also fits
through the large pin. See Figure 20. A snap ring or screws holds the small
pin in place. The centre block and pins are made from hardened steel and
require lubrication on a constant manner.
,, ,
Large pin -----...
Small pin
-----...t
Hole for
small pin
g
Hub
Osnap ring
Hub
Centre block
Figure 20 Block-and-pin U-joint
j;®j))Wj~WtJio"i~"",,'~'fEffli!ijM!j8j@iio,,§!!i;~fSiidjR!£''''iiiij%'iiio'0t§~ifj~#ti~B~$11MMfj1@j@'H~~UW'?Jt;'h%~%?ooOj~ITN;M1lQ)/*;"'B:!!_QWK'b%!_8mt~j~,,-,_8~;;8'!8_FPW~'i':"''!iXiB"'i,.'".{:"+0)\Pti''W~fiM0BifJ}f''''1130~X;;-iW_'!_;_:;;.11'!.8)!"'t-01i
MILLWRIGHT-COUPLINGS AND CLUTCHES
13 - 15
Cross-and-bearing V-joint
These units have two U-shaped yoke hubs joined by a centre cross-shaped
unit which has a trunnion on each end. See Figure 21.
Trunnion cap
/
Needle bearings
~~
_" Centre unil
Figure 21 Cross-and-bearing U-Joint
Trunnion caps with needle bearings are installed between the yoke arm and
the trunnion. Some trunnion caps are pressed into the yoke arms while others
are held in place by trunnion clamps.
A
Caution! While assembling bearing caps, take care that the needle bearing
stays in position. Use grease to hold needle bearings in the caps.
Centrifugal couplings rely on centrifugal forces to act upon the coupling
element to create the drive. Their mechanism may he:
•
mechanical (clutch-style couplings)
•
hydraulic (fluid couplings)
•
dry fluid coupling.
These couplings allow the prime mover to reach a set rotational speed before
applying the load of the driven equipment. This is known as a soft start.
These couplings conserve the energy of the prime mover by allowing it to
reach near-operating speeds rapidly.
Thermal cutout
Centrifugal couplings also serve as an overload protection by slipping if an
excessive load is applied to the driven equipment. Slippage creates heat and
this heat must be monitored. Sensors are installed on the couplings which
extend when the coupling reaches a critical temperature. When the sensor is
extended it trips a switch which shuts down the prime mover or drive (see
Figure 22). This safety device is called a thermal cutout.
Figure 22 Thermal cutout
Clutch-style couplings
When these couplings allow the drive shaft to rotate independently from the
driven shaft (soft start), and allow the two shafts to slip under excessive
loads, they act as clutches. Even though this action takes place periodically,
they are still meant to couple shafts together.
Centrifugal or c1uteh-style couplings allow the motor to come up to partial
speed before the load is engaged. TIris is useful with drives that are
frequently started and stopped under heavy loads. If the drive is not under
direct control, a heat-sensing device should be installed to shut off the power
when the drive slips.
In this type of coupling, centrifugal force causes weights to press friction
material against a drum to transmit power. As soon as the motor starts to
revolve, the weights are set free to engage the friction material. See
Figure 23 on the next page. Spring-loaded weights exert force after a certain
speed is reached. Oil or grease on the friction material can cause grabbing or
slipping problems.
Oil flow
Impeller
Power
output
Oil flow
Figure 24 Fluid coupling
The shape and rotation of the impeller and the shape and rotation of the
turbine produce a flow path called a vortex (see Figure 25):
•
A high vortex occurs at start-up when the impeller is turning and the
turbine is not moving.
•
This vortex action decreases as the speed of the turbine approaches the
speed of the impeller.
•
A low vortex occurs when the components run at almost equal speeds.
Impeller
Turbine
\
Direction of
oil flow (vortex)
Rotation of
crankshaft
Figure 25 Vortex action of a fluid coupling
There are several types of fluid-couplings:
•
constant fill
•
•
delayed fill
•
centrifugal (lockup).
variable fill (scoop)
Constant-fill couplings
The most common fluid coupling is the constant-fill coupling. The general
description above was of the constant-fill coupling. Torque development in
this coupling depends on the amount of fluid available in the working section
and the speed of rotation.
Delayed-fill couplings
In this type of fluid coupling, some of the fluid is stored in a reservoir while
the motor is starting. Because less torque develops in the coupling, the load
starts more slowly and less current is drawn. As motor output shaft speed
increases, centrifugal force slowly expels the fluid from the reservoir so that
the coupling can develop full torque.
This type of coupling offers excellent overload protection if a conveyer belt
jams for a short time.
Variable-fill (scoop) couplings
In a variable-fill coupling, the drive motor can be started and run without any
load until fluid is introduced to the coupling. This is controlled by using an
extemallever. The coupling operates like a frictionless clutch and an
adjustable-speed drive with smooth, gradual acceleration or deceleration.
The extemallever may be manually or automatically controlled.
Output speeds are adjustable over a:
•
5:1 range for centrifugal equipment such as industrial pumps, fans, and
blowers
•
3: 1 range for constant-torque equipment such as crushers or positive
displacement pumps.
Centrifugal lockup couplings
Centrifugal lockup couplings help to solve the problem of heat generation in
a fluid coupling. They combine a fluid coupling with an internal clutch
operated by centrifugal force.
At about 850 rpm, input and output are mechanically linked 1: 1. This is
called lockup. During lockup, triangular wedges cause three weights in the
clutch to press against a friction lockup ring. In lockup mode, it can carry a
higher load than a conventional fluid coupling of the same size.
-20
If the load exceeds the unit's rated value, rollers on the lockup weights roll
onto the lockup ring. This causes the turbine's speed to drop and the unit
operates as a regular fluid coupling.
Dry fluid (shot) couplings
Dry fluid couplings have in their housing a single rotor and a predetermined
amount of heat-treated metal shot. The shot is called the flow charge. The
metal housing is keyed to the drive shaft and the rotor is attached to the
driven member. When the driven member is a rigid shaft of a machine, a
flexible coupling should be used.
When the drive starts, centrifugal force flings the flow charge to the outer
edge of the housing where it packs between the housing and the rotor and
transmits power. See Figure 26. The amount of power delivered Can be
varied by changing the quantity of shot. These couplings also supply some
protection against overloading the drive.
When speed is established, there is no slippage and it provides lockup.
Power is then transmitted without heat loss. When the speed drops, the
centrifugal force decreases, allowing the rotor to slip. Continued slippage
generates heat which tends to shorten the operating life of the coupling.
Housing
fII----Flow charge
Output sheave
Rotor
In motion
At rest
Figure 26 Dry fluid coupling
13 - 21
Figure 27 Two-jaw, square-jaw clutch
Figure 28 Spiral-jaw clutch
Multi-tooth
This type combines the advantages of rnechanicallockup clutches with those
of mechanical, electric, pneumatic, or hydraulic actuation. They have two
mating rings with a large number of small teeth or serrations. Because of the
angle of the teeth, when actuated, they engage quickly and smoothly.
Running engagement speeds up to 300 rpm are possible.
Radial teeth
/
Figure 29 MUlti-tooth clutch
-23
Friction clutches and brakes
This type uSeS the friction developed between two mating surfaces to engage
or disengage and stop the load.
•
One surface is metal, usually cast iron. It is shaped as a disk, band, or
drum.
•
The other surface has a friction facing of moulded organic material
bound by heat-cured resin. This organic material sometimes includes
metal chips to increase life and dissipate heat. This surface is shaped as a
plate, pad, or shoe.
Disk clutches and brakes
•
A simple disk clutch or brake has a single plate and disk. Figure 30a
shows a cluteh in its engaged position. Both shafts are connected by
friction through the cluteh plate. Figure 30b shows them disengaged so
that the shafts are able to rotate independently. Various mounting
arrangements and actuation methods are possible.
Engaged
Disengaged
Figure 30 Single disk clutch
•
A torque limiter is a protective device which limits the torque
transmitted to the equipment. It consists of friction disks on both sides of
a driven element. An adjusting nut forces a spring washer against a
pressure plate which sandwiches the driven element between the friction
disks.
The amount of torque transmitted to the drive is controlled by the
adjusting nut. Figure 31 shows a sprocket used as the driven element.
When the load on the sprocket becomes greater then the torque limiter
setting. it is allowed to slip.
• Fri,~lon disks
/ ,..,,,,,.,ure plate
Spring washer
Adjusting
nut
Figure 31 Cross-section of a torque limiter
Fixed
housing
Input shaft
Output shaft
Air to clutch
Figure 32 Pneumatic (disk) clutch-brake
•
A clutch-brake ha.~ a friction plate and disk pair for each function. Each
function can be actuated mechanically, pneumatically, hydraulically or a
combination of these. Figure 32 shows a pneumatically operated clutchbrake unit. The upper half shows it in the brake position and the lower
half shows it in the clutch position.
•
More complex designs increase the working friction surfaces by using
multiple disks and friction plates. Submersion in oil increases cooling
efficiency and extends the life of friction components.
•
Some disk brakes use calipers rather than a friction plate as shown in
Figure 33. Advantages of this method are that:
- Additional calipers can be used to increase braking torque.
- Brake pads are easily serviced.
Figure 33 Caliper disk brake
Drum clutches and brakes
Drum clutches and brakes have cylindrical friction surfaces. Drum clutches
and brakes wear evenly because they contact across the cylindrical surface
where the velocity is constant. They also are able to transmit high torque.
There are two types of drum units:
•
Constricting types have the drum contacted on its outside diameter to
force engagement. This type responds very fast because centrifugal force
helps withdraw the shoes rapidly, See Figure 34a.
•
Expanding types have the drum contacted on the inside diameter to force
engagement. See Figure 34b.
Drum clutches and brakes can be actuated in various ways. Figure 34 shows
units actuated by air.
Constricting type
Expanding type
)
Figure 34 Air actuated drum clutch
Cone clutches and brakes
Cone clutches and brakes have conical friction surface.~. They are coaxial
with the shaft and engaged axially. These clutches are easy to engage and
give good power transmission, but are difficult to disengage. They are not
widely used in modern power transmission systems.
Friction surface
Inner core and hub
. Friction surface
Figure 35 Cone brake
13-27
Over-running clutches
Over-running clutches are uuits which allow unidirectional power
transmission. In one direction the clutches allow the shafts to free wheel and
in the other direction they lock them together. These clutches have various
designs. Some common designs are:
•
sprag
•
wrap spring
•
roller ramp.
Sprag clutches
A sprag clutch has cylindrical inner and outer races, with sprags filling the
space between. The sprags are composed of two non-concentric curves of
different radius. A retaining device such as a spring or a cage holds the
sprags in contact with the races.
Figure 36a shows the clutch in its locked position, transmitting power. The
sprags have rotated into their larger position which locks the two races
together. Figure 36b shows the clutch in its free wheeling position. A portion
of the retaining spring has been removed to show the non-concentric curves.
a. Transmitting power
b. Free wheeling position
Figure 36 Sprag clutch
Wrap spring clutches
In these clutches, the input and output shafts are connected by a coiled spring
whose inside diameter is slightly smaller than the outside diameter of the two
shaft hubs. Rotation in one direction tightens the spring tu transmit torque
(see Hgure 37). Rotation in the opposite direction loosens the spring and
disengages the unit. These are used primarily in light applications.
Wrap spring
Figure 37 Wrap spring clutch
Roller ramp clutches
These clutches transmit torque through rollers that ride on the ramped or
cammed surface. These surfaces are either on the outer race or on the inner
race. A spring is used to keep the rollers in contact with the races. Figure 38
shows an outer cam type clutch.
•
When the clutch is engaged, the rollers are forced into the ramps where
they transmit torque from the drive race to the driven race.
•
When the clutch disengages by stopping or reversing direction, the
rollers apply pressure against the springs and away from the races.
Ramp (cam)
Roiler
Spring
retainer
Figure 38 Roller ramp clutch
Electromagnetic clutches and brakes
These clutches and brakes use electromagnetic attraction rather than friction.
They are used primarily where variable slip is needed. There are three basic
types:
•
magnetic particle
•
eddy current
•
hysteresis.
Magnetic particle clutches and brakes
The operating principle of this type is shown in Figure 39 a&b. Dry iron
particles fill the space between input and output members. When the coil is
energized, the magnetic ficld lines up these particles along magnetic flux
lines (see Figure 39b). This locks the input and output members so that they
rotate as a single unit.
You can vary the amount of torque transmitted because it is directly
proportional to the current flowing to the rotor. These clutehes and brakes
are useful where continuous changes of speed are needed.
Coil
Magnetic particles
Drive shaft
a.
Disengaged
MagnetiC particles
energized
b.
Engaged
Figure 39 Magnetic particle clutch
Eddy current clutches and brakes
These clutches and brakes have an input drum, a stationary field coil, and an
output rotor. When the drum rotates, small currents called eddy currents
develop in the metal and create a new magnetic field that interacts with the
field in the pole assembly. This creates a coupling torque proportional to the
coil current.
These clutches and brakes cannot be operated at zero slip because they have
no torque then. Therefore, they are not used to hold a load. They are used
mostly for variable-speed devices. They are useful" for providing drag loads
where needed, as in tensioning.
Hysteresis clutches and brakes
Hysteresis loss is the energy lost in the process of magnetization. Hysteresis
losses transmit torque in this type of clutch. A coil generates a magnetic field
in the input rotor and in the output element called a drag cup.
Hysteresis losses cause the field to change slower through the cup than the
rotor. Therefore. the drag cup transmits torque. At regular speeds, these
brakes provide constant torque for a given control current. They are used in
fractional horsepower applications. They wear very well and have a long
service life.
Actuation methods for clutches and brakes
To optimize selection of clutches and brakes, you must consider the
actuation method. Clutches and brakes are acwated using several different
methods:
•
•
•
•
•
mechanical
electrical
pneumatic
hydraulic
self-activating.
Mechanical
This is the simplest and cheapest way to engage clutehes and brakes. It uses
a series of rods. cables, levers. or carns. Mechanical actuation depends on
human strength and mechanical advantage. This limits torque transmission,
response times, and cycling rates. This type is usually used for vehicles and
small industrial equipment.
The advantages of this method of actuation are:
•
low cost
•
the physical "feel" of engagement received by the operator through the
pedal or lever
•
the ability to judge the amount of braking force or slip needed.
Electrical
Electrically actuated clutches have limited torque range, but allow very fast
cyclillg rates. They are expensive but convenient for electrically controlled
machinery and remote applications. They lack the "feel" of engagement
given by otber types of clutches.
An electromagnetic clutch or brake has two basic parts: an annular
electromagnet (coil) and an armature. When voltage is applied to tbe coil of
wire ill tbe electromagnet, a magnetic field forms and locks the two friction
surfaces so tbat tbey rotate together. If the armature is free to rotate and the
electromagnet held steady, the unit fum.,tions as a brake.
The simplest types are plug-ill modules tbat convert AC liue voltage to DC and
provide for ON-OFF switching. More sophisticated types include time-delay
and torque-adjustment features.
Pneumatic
In illdustrial equipment, air pressure is the most frequently used method of
actuation for clutches and brakes. Pressures to 200 psi are used to expand
inflatable tubes or act on pistons. These engage or disengage friction
surfaces, sometimes in combination witb spring pressure. They are usually
operated at fairly low cycling rates. Pneumatic controls may be local,
through hand-operated throttles (giving "feel"), or remote using electrical
controls.
The main advantage of this type is its low heat generation. Once tbe piston
chamber is full, static pressure is constant. This results in low power
requirement for sustained torque transmission. Also, the actuators are simple
to maintain.
One disadvantage of pneumatic clutches is the cost of needed support
components. To operate efficiently, pneumatic clutches require tbe following
components:
•
•
filters
•
control valves
•
pressure regulators
•
quick exhaust valves
•
exhaust mufflers.
lubricators
Hydraulic
Hydraulically actuated clutches are similar to pneumatic units. Oil pressure
is used in combined witb spring pressure tu engage and disengage friction
surfaces. This type provides fast response and smooth engagement. The main
disadvantage is the cost of supporting equipment
C')
:::r
C»
-=CD
........
~
Seals
Static seals ... ............................................................ ............ ......... 14: 1
Gaskets ........................................................................................... 14:1
O-rings ............................................................................................ 14:8
Sealants ........................................................................................... 14:10
Dynamic seals. ....... ...... ...... ........ ...... .............. ............... ................ 14: 11
Contact seals ................................................................................... 14: 12
Clearance seals ............................................................................... 14:37
Seals
Seals may be used between two stationary parts (static seals), or between a
moving and a stationary part (dynamic seals). They prevent or control
leakage of a fluid (liquid or gas). They also prevent contaminants from
entering.
Seals which prevent any fluid from leaking from one side of a system to the
other are called positive seals. Seals which control the amount of such
leakage are called non-positive seals.
Static seals
Static seals seal two surfaces which have little or no movement. All static
seals are positive seals because they prevent any fluid movement between
areas. A tapered fit and an interference (shrink) fit are ways to seal two
mating surfaces without the use of additional material. These methods use
accurately machined surfaces to ensure correct sealing. In most cases an
additional material, such as a gasket, an O-ring, or sealant is used to assist
the sealing.
Gaskets
Gaskets are installed at a joint between stationary surfaces to :
•
seal against a pressure load-in hydraulic or pneumatic systems, gas or
diesel engines, etc.
•
seal against leakage-in a reduction unit where no pressure is involved
•
control position-by means of shim pack gaskets (See Chapter 12: Gear
Drives)
In pressure sealing, the gasket is subject to the force of compression exerted
by the bolts. This compression force must be greater than:
•
the internal pressure, which tends to move the gasket sideways
•
the hydrostatic end force due to the internal pressure, which tends to
push the joint apart (see Figure 1).
Bolt
load
Figure 1 Forces acting on the gasket
Using gaskets to seal housings
When using gaskets to seal housings. it is critical that the origioal gasket
thickness be known and the same thickness of gasket replaced. 'This avoids
increasing or decreasing any internal clearance. For example. if the gasket
between the top and bottom casings of a reduction unit is 0.006" thick but is
replaced by a gasket 0.015" thick. little or nO pressure will be exerted against
the outside bearing rings. allowing them to turn in the housing. Always
consider gasket thickness when replacing gaskets in an axially split pump
with wearing rings.
Gasket and flange arrangements
Flat (full) face
Raised face
Tongue and groove
Figure 2 Common flange arrangements
Spigot and recess
Gasket joints may be flat-face (full face). or raised-face (ring). tongue and
groove, spigot and recess, or a combination of these. See Figure 2. The
surfaces must be parallel and regular in fInish, whether they are smooth or
grooved.
Gasket compression
Free
Compressed
Figure 3 Conformation of the gasket to the flange
The gasket material is designed to conform to the machined surfaces or to
flow slightly to fill small irregularities (see Figure 3). The compression nf
the gasket is greatest at the bolt locations and is least halfway between the
bolts. The closer the bolts, the more even this compression becomes.
The thickness of the material of the joint also detennines the number of bolts
required. Thin flanges require more bolts to hold an even pressure than
heavy flanges of the same configuration. Figure 4 shows how a thin flange
has a tendency to distort under bolting forces while a heavy flange does not.
Heavy flange
Thin flange
Figure 4 A thin flange bolted to a heavy flange
Gasket materials
Gaskets may be made from metals or non-metals. Gasket material comes in
sheets for on-the-job cutting or in special forms for specific applications.
Gasket material should:
•
withstand the bolt load without being crushed
•
withstand the necessary temperature and pressure extremes
•
be compatible with the product being contained.
14-3
Non-metallic gaskets
Non-metallic gaskets are made of paper, cork, natural or synthetic rubber, or
plastics. These are soft and used in relatively low-pressure applications
compared with metallic gaskets.
A
•
Paper is inexpensive. It is often impregnated with fillers such as waX
and seals well against water, oil, and petroleum fuel. Its upper
temperature limit is 120"C (23O"P).
•
Cork is used in low-load applications. It can be impregnated with an
elastomer such as neoprene or nitrile rubber and seals well against water,
oil, and solvent. Its usable temperature range is from -30°C (_22°P) to
150"C (300"F) depending on its composition. Refer to manufacturers'
specifications.
•
Natural rubber has excellent mechanical properties and resists water and
air. Its upper temperature limit is 121 "C (250°F).
•
Synthetic rubber includes a variety of materials such as buna-S, buna-N,
butyl, neoprene, nitrile, Viton, and silicone. All have specific
characteristics and temperature limits. Refer to manufacturers'
specifications.
•
Asbestos is used tuday only for special applications such as in high
temperatures and stearn turbines.
Caution!
Because it is a health hazard, take extreme care when working with asbestos.
For example, when gasket sUifaces using asbestos are cleaned, scrape them;
do not grind them. Handle and dispose of materials containing asbestos in
accordance with the procedures specified in the Workers' Compensation
Board's Industrial Health and Safety Regulations.
•
Plqstics such as acrylic and Teflon (PfFE) are commonly Jl_s~d.,Ac!1'li~_
resists heat, oil, and oxidation but does not perform well at low
temperatures. Its upper temperature limit is 232°C (4500 P). Teflon is
nearly chemically inert and performs well in extremely low
temperatures. Its usable temperature range is from -190°C (_3100 P) to
250°C (480°F).
These gaskets are often coloured to show the thickness of the stock. A
colour-thickness chart is used to determine their thicknesses.
Metallic gaskets
Metallic gaskets come in a variety of different materials and configurations.
They are harder than non-metallic gaskets and are normally used ill highpressure and high-temperature applications. A few of the various materials
are lead, brass, and stainless steel (of various alloys such as InconeJTM and
HastelloyTM). Their temperature limit ranges from 100°C (212°F) for lead to
1094"C (2000°F) for Inconel™. See suppliers' charts for specifications.
4
Metallic gaskets can be solid or a composite of several materials. Some of
the possible cross sections are shown in Figure 5:
a
tlat
b
v- or U-shaped (light section)
c
rectangular, triangular, or octagonal (heavy section)
d
round or oval
e
spiral-wound
f
corrugated or embossed
g
jacketed with a filler material core (jacket may be corrugated)
h
flexi-metallic has a metallic inner ring and non-metallic outer ring.
a
b
•
I
d
c
h
Figure 5 Designs of metallic gaskets
Metallic gaskets seal by the flow or malleability of the gasket when the bolts
compress the two surfaces together. The surface finish of the gasket and the
sealing surface is very important. The smoother the surface fmish the better
the seal.
5
Gasket styles
A circular gasket can have two styles. A ring style covers only the area
inside the bolt cirele. A full-face style covers the total flange area. See
Figure 6.
Full-face gasket
Ring gasket
Figure 6 Full-face and ring gaskets
The ring gasket is preferred over the full-face gasket for two reasons. It takes
less time to make and it is easily installed because all the bolts do not have to
be removed to allow the gasket to enter. An extension or handle left on the
gasket allows even easier handling and positioning during installation. See
Figure 6.
6
Making gaskets
Metallic gaskets are nonnally supplied pre-stamped and formed for a
particular application. Making gaskets from non-metallic material is
reasonably simple:
•
Either draw the gasket from measurements:
1. Get the OD, ID, bolt circle diameter, and any other needed
infonnation from drawings or actual equipment.
2. Draw the gasket shape onto the gasket material.
•
Or mark the material directly from the mating parts using one of the
following methods:
1. Smear the part with a marking ink or graphite.
2. Lay the gasket stock on the material to get an impression.
Or
1. Hold the gasket stock against the face.
2. Draw the outline and bolt holes.
Most of the cutting is done with a pair of scissors or tin snips, depending on
the gasket material. The holes are nonnally made by using a gasket hole
punch of the required size. Always use a gasket hole punch against the end
grain of a block of wood to protect its cutting edge.
Installing new gaskets
Before installing a new gasket:
1. Remove all old gasket stock.
2.
Check the metal faces for irregularities.
3. Smooth the faces where necessary.
4. Smear a light film of lubricant on the contact surfaces. This helps the
gasket material to flow between the rigid mating parts and makes it
easier to remove.
Bolt tightening procedures
The bolt tightening procedure is important for the successful installation and
sealing of a gasket. Regardless of the shape of the mating parts or the gasket
material, the bolts are first pulled snug around the casing, then tightened
gradually, working across the body. It is more efficient to take two or more
passes around the bolt sequence than to try to reach full tension the first time
around. Figure 7 shows the tightening sequence for various configurations.
14-7
3@
@2
@4
@
@
1
J
4
2
1@
@
@
Figure 7 Bolting pattern
O-rings
An O-ring is circular with a round cross section like a doughnut. It is usually
made from synthetic rubber. It can be used as a dynamic or a static seal (see
Figure 8).
Dynamic seal
inside packed
Static seal
Piston rod
Figure 8 O-rings used as dynamic and static seals
8
An O-ring used for a static seal has
standard dimensions. The groove
which holds the O-ring in place has
the correet allowance for
compression. See Figure 9.
The O-ring docs not usually make a
difference to the relative position of
the machine parts. The joint is
designed so that the correct
compression of the O-ring is
accomplished when the two machine
surfaces are in contact with each
other. See Figure to.
Figure 9 Correct O-ring groove design
Figure 10 O-ring groove designs
O-ring materials
The material used for an O-ring is chosen to resist chemicals and suit the
temperature. O-rings made of materials other then synthetic robher have
manufacturer's markings on them. Refer to manufacturers' information
sheets to ensure that the markings comply with the type of O-ring needed.
Incorrect application of an O-ring may prevent it operating properly.
Metallic O-rings are commonly used where extreme conditions of
temperature, pressure, or corrosion exist. These O-rings are formed to the
required shape, butt welded, and ground smooth. They may be open (see
Figure II) or closed. They compress slightly when installed, forming an
effective seal.
9
Figure 11 Metallic a-ring
O-rings in hydraulic equipment
Multi-lobed, square, or rectangular section rings are often used as gaskets in
hydraulic equipment. See Figure 12. The material is the same as that used in
standard O-rings.
a
b
c
Figure 12 Four-lobed (quacf} ring, square ring, and rectangular ring
Sealants
A sealant is a gasket-forming or sealing compound often used in zero- or
low-pressure applications. They are found in tape. paste or liquid form.
These sealants are frequently used with a flat gasket to fill up minor
depressions in surfaces that are slightly corroded or pitted. They are
classified as non-hardening, hardening-flexible, hardening-rigid, or tape.
Flang~
~~~'''';I+ Sealant
Flange I
Figure 13 Sealants filling depressions In surfaces
14
10
Non-hardening sealants
Non-hardening sealants are mastic (resin) materials which are soft and
adhesive. They contain plasticizer which allows them to remain soft. They
are usually brushed, extruded, or thumbed into place.
Hardening sealants
Flexible
Hardening-flexible sealants are available in a variety of materials such as
butyl, acrylic, silicone and polyurethane. These compounds contain curing or
setting agents which also allow them to remain flexible. Some of these are
true rubbers and others are adhesive. They are all resistant to various
environments.
Rigid
Hardening-rigid sealants are based on epoxies, polyesters, acrylics,
polyamides. or polyvinyl acetates (PVA). Because these cure to a rigid state.
they tend to crack and result in joints that are difficult to remove. Some of
these sealants can be used to join aq well as seal.
Tapes
Tapes are available in different shapes for a variety of different applications
such as edge, sandWich. and lap sealing. Teflon packing in string form iq
frequently used to replace flat gaskets. The ends are twisted together three or
four times to form a ring gasket. Tapes have various adhering methods such
as pressure sensitive, solvent activated. and self-sticking. Teflon tape is
commonly used to seal pipe-threaded connections.
)
Dynamic seals
Dynamic seals are grouped into two main categories: contact and clearance:
•
Contact seaIs make contact with the two surfaces to be sealed. Examples
of contact seals are packing. and lip, diaphragm. and mechanical seals
(piston rings).
•
Clearance seals seal without touching each other. Examples of clearance
seals are bushings. and annulus. slinger, and labyrinth seals.
Dynamic seals may be positive or non-positive.
14-
Contact seals
Packing
Packing used as a non-positive seal, controls leakage between stationary and
moving parts. The action of the moving part may be reciprocating, rotary,
helical, or swinging-rotary. See Figure 14. One or more of these actions are
found in equipment such as pumps, compressors, presses, and blowers.
f . . .- ... ~
ReCiprocating
Rotary
Helical
Swinging-rotary
Figure 14 Various motions
Packing used as a positive seal prevents any leakage between the stationary
and moving parts. These are fouud in situations such as gate valve stern.s.
Packing is divided into three categories: compression, automatic and
floating. These categories are described on the next few pages.
Inside- and outside-packed
Packing installed in the moving unit is called inside-packed. Examples are
cylinder pistons and air valve spools.
Packing installed in the stationary unit is called outside-packed. A stuffing
box falls into this category.
Inside packed
Outside packed
Figure 15 Inside packed, outside packed
Hardness of packing
Packing must withstand temperature, pressure, various fluids, and shaft
misalignment and movement The hardness of packing or a seal helps to
detennine its action under pressure and movement. Hard packing withstands
extrusion better than soft packing, but it also creates more friction. Where
clearances are excessive, hard packing, special packing, or packing aids
should be used.
Compression packing
Compression packing is so named because it relies on its compression to
develop a seal. The amount of sealing or leakage is contrulled by the amount
the packing is compressed. Compression packing is commonly found in
centrifugal pumps and reciprocating water pumps. (Sec Chapter 15: Pumps.)
Compression packing is made of three classes of material: fabric, metallic
and plastic.
•
Fabric packing is made of strong, pliable materials such as braided
asbestos, metal, graphite, cotton, flax, wool, aramid. carbon, or
polytetrafluoroethylene (PTFB) filament. PTFE is better known as
Teflon. (Note: Asbestos is used less now due to its association with
health problems and the difficulty of disposal.)
•
Metallic packing is made from materials such as lead, copper, or
aluminum. It is either machined or pressed into rings to fit specific
stuffing boxes and shafts.
•
Plastic packing is moulded to fit specific needs.
Compression packing's cross section may be round, square or rectangular.
The choice of packing shape and material depends on the stuffing box
design, temperature, pressure, and the type of fluid it is in contact with.
Some packing is impregnated with lubricant, such as graphite or Teflon, so
that it is self-lubricating at start-up. It is supplied ill continuous coils of the
desired cross-section or ill pre-fonned, die-moulded rings.
Lantern rings
A lantern ring (seal cage) is often used with compression packing. It is a ring
with channels in its inner and outer perimeters. Figure 16 shows that its cross
section is H-shaped. The channels are connected by radial holes.
Packing
Radial holes
Figure 16 Lantern ring
The lantern ring distributes sealing liquid under pressure to the packing. This
prevents air infiltration and provides lubrication, cooling and cleaning to the
area. The sealing liquid is commonly delivered from the high-pressure
section of the pump (internally supplied). When semi-fluids such as slurry
are pumped, an external souree of sealing liquid is used (externally
supplied).
Compression packing installation
Compression packing is replaced periodically because it deteriorates from
wear and also dries out. How often it is replaced depends on operating
conditions, quality of the packing, and quality of the packing's installation.
The procedure for replacing packing is as follows:
A. Prepare the pump for the replacement work.
B. Remove the old packing and identify its sequence, amount, and size.
C. Check the conditions of the lantern ring and stuffing box.
D. Cut the new packing.
E. Install the new packing.
F. Adjust the gland and check the leakage.
The details are as follows:
A. PREPARE TIlE PUMP FOR TIlE REPLACEMENT WORK
I. Ensure that the pump is locked out and drained to relieve any pressure
behind the packing.
2. If externally supplied sealing liquid is used, sbut off the supply and
drain the line.
B.
REMOVE TIlE OLD PACKING AND IDENTIFY ITS
SEQUBNCB, AMOU.NT, AND SIZE
1. Remove the gland.
2. Remove all the old
packing using a
packing puller as
shown in Figure 17.
3. Remove the lantern
ring if used.
Figure 17 Using a packing puller
4. Count the number of rings
removed and record the position of the lantern ring, if used.
5. Confirm this information with its stuffing box code. For example
"2-L-3-G" indicates that the packing sequence is: 2 rings of packing,
a lantern ring (L), 3 rings of packing, then the gland (G).
If the stuffing box code is unavailable, determine the correct amount
of packing by measuring the depth of the stuffing box, then:
-Subtract the width of the lantern ring if used.
-Divide it by the width of the new packing.
6. Determine the correct size and type of packing to be used by referring
tu equipment manuals:
-The correct type is determined by matching the categories of the
pumped product with the manufactorers' recommendations.
-If manuals are not available, find the correct size by subtracting the
diameter (10) of the shaft or sleeve from the bore (00) of the
stuffing box. See Figure 18. Divide the difference by two.
C.
CHECK TIlE CONDITIONS or TIlE LANTERN RING AND STUFFING BOX
I. If a lantern ring is used, ensure that the sealing water inlet to the
stuffing box is clear.
2. Check the condition and cleanliness of the stuffing box and shaft or
shaft sleeve.
t
~r
OD
ID
~~- Figure18 Determining the correct cross section
D. CUT THE NEW PACKING
1. Cut the new packing to size. This is best done on a wooden mandrel
with the same diameter as the pump shaft or shaft sleeve. Wrap the
packing around it in the same direction as it comes off the roll.
• Cut rings straight with the axis of the mandrel (butt cut) as shown
in Figure 19. This reduces the chance of fraying and sloppy joints.
Figure 19 Square end cut on a mandrel
• You may also cut the rings on a diagonal. To do this, draw two
lines, the width of the packing apart. Then, cut each ring of packing
diagonally between the lines as shown in Figure 20.
Figure 20 Cutting diagonally on a mandrel
2. If it is necessary to cut rigid (rolled, pressed or die formed) packing,
niake a diagonal cut (a skived cut).
E.
INSTALL TIIB NEW PACKING
1. Install packing onto the shaft or shaft sleeve.
• Wrap mandrel-cut paeking around the shaft.
• For rigid paeking, use an S twist to plaee it over the shaft. See
Figure 21. Do not pull it open.
Figure 21 Sliding rings sideways over the shaft
2. Where compatible, lightly lubricate the ID and the OD of the paeking.
3. Install one ring into the stuffing box at a time. If stuffing is slightly
bulged, it may be difficult to install. If this is the case, the packing
may be slightly flattened with a roller. Ensure that this is done on a
clean surfaee.
4. Use a split bushing or tamping tool to plaee each ring in position.
Ensure that the ends of the packing line up to each other. Then, work
it in around the circumference in both directions. Ensure that each
ring of packing is seated at the bottom.
5. Ensure the ring joints are staggered approximately 90° from each
other as shown in Figure 22.
Figure 22 Staggered ring joints
6. If a lantern ring is used, ensure that it lines up with the seal water
inlet. See Figure 23.
Seal water
inlet
Correct
- - - .---{Incorrect
Figure 23 Proper positioning of the lantern ring
F.
ADJUST THE GLAND AND CHllCK THE LEAKAGE
1. After the last ring is installed. allow the gland to enter the stuffing box
by I/S" to 3116"
2. Tighten the gland nuts with a wrench to seat and form the packing to
the stuffing box and the shaft.
3. Ensure that the gland enters the stuffing box square with the shaft.
4. Undo the nuts and allow the packing to expand.
S. Finger-tighten the nuts again.
6. Start the pump allowing the packing to leak freely.
7. Gradually tighten the nuts equally (a sixth of a tum or one flat) until
leakage is reduced to the recommended lubrication level.
Note that the packing in stoffing boxes should not be compressed too tightly.
A slight leakage is allowed to provide lubrication between packing and shaft.
If leakage is completely stopped the resulting friction causes excessive heat
to build up. This burns the packing and damages the shaft.
Note also that inside the stuffing box, the individual rings of packing do not
compress equally. The rings closest to the gland compress the most. See
Figure 24.
Figure 24 Unequal compression of packing
Automatic packing
Automatic packing is so called because it uses system pressure to improve its
sealing action. It does this by forcing the flexible lips of the packing onto its
contact surfaces. These consist of V-ring and U-ring packing, O-rings, and
cup packing.
•
V-ring and U-ring packing and O-rings are considered balanced packing
as they seal on both the ID and OD. Pressure is balanced at both
diameters.
•
Cup packing is considered unbalanced packing as it seals on one
diameter only.
V-ring packing (chevron packing)
V-ring packing (also known as chevron packing) is a circular, one-piece
moulded ring with a V-shaped cross section. It is used in sets of three or
more rings, depending on the working pressure.
-19
V -ring packing is used as an inside or outside packed seal. V -ring packing
must be used with shaped internal and external adapter rings (see Figure 25).
,Internal adapter ring
V-ring packing
Figure 25 V-ring packing with internal and external adapter rings
The internal adapter ring should be hard enough to prevent extrusion, yet
soft enough to compress slightly under pressure. The external adapter ring
merely shapes the V -ring and is not suhject to wear.
Where possihle, V -ring packing sets should be installed as endless rings.
When V -ring packing must be cut for installation, stagger the joints about
90° on successive packing.
V -ring packing requires both diameters to contact mating surfaces to ohtain
initial sealing. An adjustable gland is a common means of ensuring this and
to maintain this seal due to wear.
At the initial installation, the rings are tightened just enough to prevent
leakage. This can be done in two ways:
•
hy operating the unit and tightening the gland until any leaks cease
•
or, if the packing is frequently replaced, a spacer can be made the
thickness of the necessary gap between the gland and the housing.
Tightening the gland to the spacer ensures correct loading.
As leaks develop, the gland can be tightened further. Excessive tightening
during first installation causes friction and rapid wearing of the packing.
Spacer
Internal
adapter
ring
External
adapter
ring
Figure 26 Even spacing used to ensure consistent loading
a
One coil
b
Series of individual
springs
Figure 27 Methods 01 adjusting V-ring packing
Some applications use spring-loaded external adapter rings to maintain
constant pressure. In this case, the gland is tightened down to the housing.
This leaves no chance of overloading the packing set (provided it has the
correct number of rings and has been properly installed). Figure 27a shows
one continuous spring supporting the external adapter ring. Figure 27b
shows a series of small springs supporting the external adapter ring.
V-ring packing sets used on double-acting pistons should face away from
each other. See Figure 28. The internal adapter ring should be supported by a
solid backing, not by the opposing series of V-rings. In this case the adapter
ring is often a non-metallic material and acts as a bearing to guide the piston.
Figure 28 V-ring packing set for double-acting pistons
Materials in the V-ring packing sets can all be the same or be dissimilar.
Dissimilar materials consist of hard and soft rings arranged in a specific
sequence. The soft rings do most of the sealing while the hard rings act
mainly as spacers and bearing rings. When installing a set of mixed rings
(for a special order), always keep the rings in the same sequence as they
were supplied.
22
U-ring packing
AU-ring (U-cup) packing is a circular, one-piece moulded ring with a
U-cross section usually made from one of the synthetic elastomers, This
endless design is meant to be used singly per seat, unlike compression or
V-ring packing,
U-ring packing can be used as an inside- or outside-packed seal. See
Figures 29 and 30, Due to its low friction, it is primarily used in systems
with pressures of less then 10 340 kPa (1500 psi), Pressures above that
require the addition of an anti-extrusion washer (support ring),
U-ring packing
Support ring
III
.............----~
Figure 29 U-rlng used as an outside packed assembly
a
b
Figure 30 U-rlng used in a single-acting piston
For a double-acting piston, two separate seats must be used. Two U-rlngs
should not be placed against each other in the same groove. Figure 31 shows
two different options for mounting U-ring packing for double acting pistons.
Support
rings
Figure 31 U-rings used in a double-acting piston
Cup packing
Cup packing has only one lip and is considered an unbalanced packing. At
low pressures, the cup may seal at the lip, but it normally does not seal until
the heel diameter has been expanded to the diameter of the cylinder wall.
The assembly must be tight enough to prevent leakage along the piston shaft.
It is standard to fasten the cup against the piston by means of a tightly
clamped plate. U-ring packing often replaces cup packing because it requires
less maintenance time.
Cup packing with double-acting pistons requires a rigid separator between
the two cups. In some assemblies the separator is made of synthetic or
laminated bearing material and acts as a guide as well as a separator. When
installing new cups, there should be a gap between the follower and the
back-up plate (see Figure 32).
Cup material can be leather, synthetic rubber or elastomers, or impregnated
fabric, depending on air or oil used and the maximum pressure.
14
24
Clamp
plate
Figure 32 Cup packing configurations (single- and double-acting pistons)
O-rings
O-rings are used as dynamic seals on slow turning shafts or reciprocating
parts such as valve spools. An D-ring sits in a groove whose width is
generally 135% to 150% ofthe D-ring's cross section. This allows for
deformation due to squeeze, swelling due to fluid contact, and slight rolling
of the D-ring. The rolling provides some surface lubrication during
reciprocating motion.
Diametral squeeze is necessary to maintain a seal. If there is no squeeze,
leaks start at low pressure. Too much squeeze rapidly wears out the rings.
Generally, the squeeze is equal to 10% of ring diameter, but the exact
amount of squeeze is shown on suppliers tables.
The clearance between the carrier and the mating part should be held to a
minimum to prevent extrusion into this gap. Standard D-rings are rated to
10 340 kPa (1500 psi) with properly designed grooves, and up to 20 680 kPa
(3000 psi) using anti-extrusion rings or back-up washers.
Zero pressure
1000 psi
3000 psi Extrusion
a
b
Back up
washers
Figure 33 O-rings under pressure
Figure 33a shows the action of an a-ring under pressure up to and over the
rated maximum. Figure 33b shows the action of the same a-ring with backup washers under pressures up to 20 680 kPa (3000 psi).
Back-up washers are often made of leather, Teflon or moulded nylon. Their
purpose is to:
•
eliminate extrusion of the a-ring
•
permit wider clearance between moving parts
•
form a seal across a clearance gap
•
act as a dirt wiper
Note: When back-up washers are used with O-rings, the grooves are
required to be wider. Do not use a back-up washer and O-ring in a groove
designed for O-rings only.
To install a-rings:
1.
Lubricate the a-ring with a light oil before seating.
2. Roll the a-ring into its seat. Stretch it only as much as needed for
installation.
3. Do not cut back-up washers. Teflon washers are supplied as spiralwound split washers to permit stretching. Leather back-up washers can
be stretched by soaking them in hot water for a prescribed time before
installing. Air-dry the washers at about 200°F (93°C) to shrink the
leather. Put the flesh side next to the a-ring.
4. Protect the a-ring from cuts and from nicks from sharp metal corners.
14
Floating packing
Floating packing is a seal that is retained in a groove but not held in a static
position. Split rings and piston rings are examples of these seals. They are
able to float within the groove and seal by means of spring tension. See
Figure 34.
Sealing position
Figure 34 Piston rings
Automotive or steam piston rings are often used when rapid travel and
frequent cycling are needed. Material in rings can be cast iron or steel, either
plain fmished or chrome plated. Piston rings are not a positive seal because
they leak slightly.
Some advantages for using floating packing rather than synthetic seals are
that floating packing:
•
(.-reates less friction
•
can take high pressure
•
is less affected by temperature extremes
Some disadvantages are that floating packing:
•
confonns less to the mating surfaces
•
has a longer run-in period
Handle cast iron rings with care. Due to their brittleness, they often break
when they expand too much as they are slipped onto a piston.
Lip seals
Lip seals are used in low-pressure areas (2 to 10 psi). They are primarily
used on rotating shafts which are partially or totally immersed in lubricant. A
hydrodynamic film of lubricant is formed at the point of contact to prevent
friction and wear. This film of lubricant should be about 0.025 mm (0.001 ")
thick. If it is any thicker, leakage may occur. If it is much thinner, friction
and heat occur, rapidly wearing the seal down.
To maintain this hydrodynamic film, the shaft's sutface must be controlled.
For speeds up to 8 mls (26 ftls), the shaft should have a ground finish
(16 micro-inches = 0.4 microns). For speeds above this, the shaft should be
hardened and ground (4 micro-inches = 0.1 microns).
Note: 1 micro-inch (,.un) = 0.0254 micron
Materials used for lip seals are felt, leather or synthetic rubber. A simple lip
seal consists of a lightweight pressed metal housing with felt or leather
secured into it or synthetic rubber bonded onto it (see Figure 35). For the lip
to maintain contact with the mating surface, it relies on the elasticity of the
material used or on a garter spring behind this material.
Housing
(
Pressed
~---1
metal
housing
Romting
---------------~~r.--~ shaft
Figure 35 Single synthetic lip seal
Single Up seals
A single lip seal consists of one sealing edge. It is usually installed with the
lip inward to include or contain the lubricant in the housing (inclusion seal
or seal in), assuming that the atmospheric side is relatively clean.
When the atmospheric side is dirty, it can be msmlled with the lip facing out,
to exclude foreign material (exclusion seal or seal out). The small amount of
lubricant that leaks out acts as a flushing medium to keep the conmct
sutfaces clean. See Figure 36.
Double Up seals
A double lip seal consists of two sealing edges. They can have their lips back
to back or facing the same direction. See Figure 37. These are used where
additional sealing is required. Examples are where there is higher housing
pressure or as inclusion and exclusion seals in dirty environments.
14
Inclusion
Exclusion
Figure 36 Up seal facing inward (inclusion) and outward (exclusion)
)
a Back to back
b Facing same directions
c
Seal with lips
on front and rear
Figure 37 Double lip seal configuration
-29
Installation
The seals are manufactured with the required press fit. providing the housing
is machined to standard dimensions.
•
SEAL INTO HOUSING
I. Lubricate the shaft. the housing. and the sealing edges. Leather seals
should have the leather saturated with oil before they are installed.
2. Determine whether the seal is to be used as an inclusion or exclusion
seal.
3. Remove any sharp edges around the bore ofthe housing.
4. Align the seal with the bore.
S. Press the seal into the housing squarely and smoothly. Ensure that the
pressing device contacts the seal on its outer rim only, not to the
unsupported inner section.
6. If the seal is to rest below the machine face. ensure that the pressing
device is slightly smaller then the bore of the housing. See Figure 38.
Seal seat
Housing
Figure 38 The method of installing lip seals
•
SHAFf INTO SEAL
1. Ensure that the sealing edge does not slide over sharp edges such as
key-seats or threads.
2. Use plastic, shim stock, or evcn paper to cover any sharp edges over
which the seal must pass. See Figure 39.
3. Unless the shaft has a chamfer with a long taper, use a proper leader
or light-gauge shim stock. When using srum stock, make sure the seal
twists away from the sharp edge of the shim stock.
A
Caution! Do not use a screwdriver to pry the seal onto the shaft because this
can dnmage the sealing edge.
Mounting sleeve
Keyway
Figure 39 Installing lip seal onto shaft
Emergency repairs
A shaft wears under the seal contact point when fme abrasives or dirt are
held by the seal. When the amount of wear interferes with proper sealing, the
shaft should either be refurrushed or replaced. These repairs are tirneconsuming and short-term solutions may be needed.
The following are common short-term solutions:
•
Change the seal to a wider seal to move the location of the lips.
•
Reverse the seal if the lip has a pronounced offset.
•
Instal! a spacer behind the new seal of original size to move the lip
contact to a new area (see Figure 40a).
•
Install a thin sleeve over the worn spot. In some cases these sleeves can
be purchased (see Figure 4Ob).
a
New seal
I
b
Figure 40 Short-term repair methods
Ring seals
A ring seal is a non-positive seal which is mounted into a groove in the
housing. They are made of cork, felt, leather, or synthetic rubber. Figure 41
shows a felt ring seal.
Figure 41 Felt ring seal
Cork, felt and leather ring seals are precision-cut washers with a sealing edge
broader than that of the lip seals. Over an extended period, they tend to
polish the shaft. Synthetic rubber ring seals are moulded into shapes required
for the job.
Ring seals are used as follows:
•
Cork ring seals are used in slow speed, light duty applications where its
operating temperature does not exeeed 65°C (1500 P).
•
Felt and leather ring seals are also used in slow speed, light duty
applications but where their operating temperatures won't exceed 105°C
(220°F). These seals absorb the lubricant and redistributes it as a film
between the seal and the shaft.
•
Synthetic ring seals are used in high speed applications and can
withstand temperatures as high as 250°C (480°F). Synthetic seals can
retain lubricant without absorbing it and resist acids and alkalis.
Even though these seals are simple and inexpensive, they are being replaced
by other types of seals.
Wipers
A wiper (scraper) is a non-positive seal mounted on the stationary housing. It
flexes against axially moving parts (see Figure 42). Wipers expel foreign
material away from the seals, packing or bearings. They are commonly
found on hydraulic and pneumatic cylinders in dusty or dirty locations. They
are also used on guides and slides on which equipment rides.
r:::=::::t---:.r-Mc,Ving part
)
Figure 42 Wiper used around a shaft
•
Synthetic rubher or plastic wipers protect against dust and fme materials.
•
Metallic wipers protect against solid abrasive materials.
Their lip shape allows lubricant from the equipment to remain on the rod or
guide but restricts any entry from the atmospheric side.
Boots and bellows
Boots and bellows are positive seals which have one end secured to the
housing and the other to the moving part. Boots are used for short
movements (strokes) and bellows for long ones. They are made of rubber or
some other flexible material. See Figure 43. They replace wipers when it is
vital that no atmospheric contaminants reach the working parts (for example,
on shock absorbers).
t
Figure 43 A boot around a reciprocating shalt
Diaphragm seals
A diaphragm seal is a positive seal which has a flexible membrane which
spans between the stationary and moving parts. It is clamped between the
two stationary housing halves and is clamped to the moving part with plates.
This seal does not allow any fluid to transfer from one chamber to another.
There are two styles of diaphragm seals:
a
•
Aflat diaphragm seal is designed for a small amount of movement. It is
cut from a flat membrdlle and the elasticity in the material is enough to
handle the movement. See Figure 44a.
•
A rolling diaphragm seal is a moulded membrane and rolls on the two
mating surfaces as it moves. See Figure 44b.
t
Figure 44 Flat and rolling diaphragm seals
14
Mechanical seals
A mechanical seal is an extremely efficient axial seaL It is much more
effective than any other dynamic seals in preventing leakage. The leakage is
so minimal that the naked eye cannot detect it. Any fluid thatleab past
tends to evaporate immediately.
A mechauical seal has primary and secondary sealing:
•
The primary seal is dynamic and is composed of two lapped (polished)
faces in contact. One face is stationary and the other rotates. The mating
faces are at right angles to the shaft (see Figure 45). A spring loaded
device kecps these faces together. The contact faces are made of various
comhinations of materials such as carbon-graphite, ceramic, tungsten
carbide, stellite, and stainless steeL
•
The secondary seal is static and is between the contact face and the shaft.
and between the contact face and !be housing. The seal between these
parts can be done in a variety of different ways, but the most common is
by means of a-rings.
Mating face
(primary
Seat
(stationary seal
ring)
141-- Housing
Spring
)
Retainer~~_-=::::t~~
Shaft
(rotating)
O-rings
(secondary seals)
Figure 45 Mechanical seal components
Flushing
The mechauical seal must always run in fluid. The flushing ports feed the
fluid into the product side of the seal. The fluid can be used directly from the
product discharge or from an external sOUrce. If the product discharge is
used, this fluid may have to go through a cye/one strainer (separator) to
remove any solids. Or it may pass through a heat exchanger to remove any
excess heat.
The advantages of flushing are to:
•
maintain a consistent operating temperature
•
prohibit any vaperization of volatile fluids around the contact faces
•
prevent crystallization of fluids near the contact faces
•
prevent solids from accumulating around thc seal
•
prevent the product from entering the stuffing box (if the flushing fluid
is at a higher pressure then the system),
Quenching
Quenching is done through quenching ports on the atmnspheric side of the
seal. The fluid comes from an external source and must be compatible with
the seal faces, the product being sealed, and the atmosphere. Steam is
commonly used. A throttliug bushing is used to keep the fluid at the contact
face.
The advantages of quenching are to:
•
prevent leakage of corrosive or toxic fluids into the atmosphere
•
prevent crystallization around the contact face
•
maintain a constant operating temperature and minimize any heat
transfer along the shaft.
Mechanical seal maintenance
If either face is damaged it is best to replace the complete seal than to try and
adjust a seal face or mix seal parts. Seals may be sent to the manufacturer for
factory overhaul.
If the proper equipment is available, the seal can be hand lapped in the
following procedure:
I. Charge the lapping (polishing) plate with a diamond lapping paste
2. Lightly press the seal facc against the lapping plate and move it in a
figure eight motion.
3. Continue this until the surface appears flat.
4. Wash the face with solvent.
5. Check the surface with a mouochromatic light and optical flats.
6. Repeat the procedure if necessary.
A
Caution! Mechanical seals are precision products. Handle them carefully.
Do not touch their mating faces.
Clearance seals
Bushings
A bushing seal is a non-positive seal which is mouuted in the stationary
housing. The minimal clearance between it and the rotating shaft restricts the
amount of leakage from the high to low pressure sides. It is considered the
simplest of seals. See Figure 46.
Figure 46 Bushing used to seal a shaft
Annulus seals
An annulus seal is a non-positive seal with a series of concentric or helical
grooves in the housing bore (see Figure 47). A drain hole is placed at the
bottom of the seal to make the seal effective. In a concentrically grooved
seal. lubricant fills the grooves to prevent the entry of contaminants. When
helical grooves are used. the oil is returned to the bearings by this action.
)
,
,
, "
,"
i
!
i
!
I III
III f
j
Concentric
grooves
Helical
grooves
Figure 47 Annulus seals with concentric and helical grooves
Slinger (flinger) seals
A slinger seal is a non-positive seal which has a cupped washer-like flange
mounted to the rotating shaft. It is placed just outside the seal housing to
assist the sealing of a ring seal. See Figure 48. As any lubricant passes the
ring seal. the lubricant is flung out by centrifugal force. This prevents
contaminants from entering the seal.
Figure 48 Slinger seal used with a ring seal
Labyrinth seals
A labyrinth seal is a non-positive seal which has a tongue and groove design.
A series of internal and external passages form a maze-like path for foreign
material to follow. A labyrinth seal can be made to seal axially or radially,
depending on application. See Figures 49 and 50. Radial labyrinth seals
require split housings.
Figure 49 Radial labyrinth seal
Because their surfaces are not in contact, labyrinth seals are used in high
speed applications. They are also very effective where the environment
includes abrasive contaminants or semi-solid liquids. This seal is usually
lubricated and the lubricant fills the passages and forms a seal to trap
contaminants. As the seal is re-lubricated, the lubricant forces trapped
contaminants out.
14
Tortuous
(stationary)
clearance
path
Figure 50 Axial labyrinth seal
MILLWRIGHT MANUAL: CHAPTER 15
Pumps
Dynamic pumps... ...... ............... .................... ....... ......................... 15: 1
Terms and deftnitions ..................................................................... 15:1
Radial flow pumps .......................................................................... 15:4
Axial-flow pumps ........................................................................... 15:6
Mixed-flow pumps ......................................................................... 15:7
Peripheral pumps.... .......... .............................. ......... ..... ...... ..... ....... 15: 8
Pump casings .................................................................................. 15:9
Pump impellers ............................................................................... 15: 13
Choosing a centrifugal pump .......................................................... 15: 13
Pump components .......................................................................... 15:15
Installing centrifugal pumps ........................................................... 15:22
Maintaining centrifugal pumps ....................................................... 15:25
Troubleshooting centrifugal pumps ................................................ 15: 27
Positive displacement pumps ........................................................ 15:30
Volumetric efficiency ...................................................................... 15:30
Reciprocating pumps ...................................................................... 15: 30
Rotary pumps .................................................................................. 15:36
Regulating positive-displacement pumps ....................................... 15:48
Maintaining positive-displacement pumps ..................................... 15:49
Pumps
A pump i~ a machine that moves fluids (liquids or gases) or semi-fluids
(wood pulp or slurries). It moves them iuto, through, or out of a system,
against the action of gravity Ill1d other forees such as friction. It does this by
converting mechanical energy into fluid energy, using suction or
compression. When it is designed to compress air or other gases it's called a
compressor.
Pumps play Ill1 important part in power pllll1ts Ill1d industry generally. They
are used to feed water to boilers, to conveyor meter fuel and chemicals, and
to circulate coollll1ts Ill1d condensates. There are two basic classifications of
pumps: dynamic Ill1d positive displacement.
Dynamic pumps
)
A dynamic pump is a machine that moves fluid by using centrifugal force to
spin it outwards. It is better known as a centrifugal pump. The pump casing
contains the impeller which gives kinetic energy to the fluid, also giving it
velocity. The casing guides its motion and produces a smooth, continuous
flow. They are generally used to convey large volumes of fluid.
Centrifugal pumps are also known as non-positive displacement pumps.
They allow the fluid to slip after the required pressure is built up. Slip is
caused when the centrifugal force ereated by the impeller is the same as the
resistance in the discharge pipe.
The four categories of centrifugal pumps are based on flow; radial, mixed,
axial, and peripheral flow.
Terms and definitions
Pump bead
Pump head is the pressure that a pump has to overcome to be able to move
liquid through the system. This pressure is expressed in one of the following:
•
pounds per square inch (psi)
•
kilopascals (kPa)
•
bars (1 bar equals 100 kPa)
The pressure is then converted to a height of a column in which the pumped
liquid can maintain. This height is measured in feet or meters and is simply
called head.
15
1
With different liquids the pump pressure must change to maintain the
required head. For example it takes more pressure to maintain a required
head of wood pulp than water.
Static
•
:
Static :
discharge
head
discharge ...-+-.;
head
i ,
,
I- Pump
..1.._
~,
.
- r''Pump
.L
, centre-line
,
I
: centre-line
r I j,
""
: !~tt:
'Total
static
'head
,, ",I
, I
!is \'ct Siatic suction lift
\
I
,,,
centre-line
Static suction lift
a
b
c
Figure 1 Total static head
Several factors affect pump head and a description of these factors follows,
t5
2
Static suction lift
The vertical distance from the liquid supply level
to the pump centre line when the liquid supply
level is below the pump centre-line (see Figures la .
and Ib).
Static suction head
The vertical distance from the liquid supply level
to the pump centre line when the liquid supply
level is above the pump centre-line (see
Figure Ic),
Static discharge head
The vertical distance from the pump centre line to
either the surface of the liquid in the discharge
tank (see Figure la) or to the point of discharge
(see Figure Ib and Ic).
Total static head
The vertical distance from the liquid supply level
to the surface of the liquid in the discharge tank or
to the point of discharge.
Frictional loss
The pressure needed by the fluid to overcome
friction as it moves through piping, valves and
fittings in the system.
Velocity head
Velocity head is the force causing the fluid to flow
through a chamber which is due to the velocity of
the fluid.
Dynamic suction lift
This is equivalent to the static suction lift plus the
velocity head minus all the frictional losses in the
suction pipes and fittings.
Dynamic suction head
This is equivalent to the static suction head minus
the velocity head and all the frictional losses in the
suction pipes and fittings.
Dynamic discharge head This is equivalent to the static discharge head plus
the velocity head and all the frictional losses in the
discharge pipes and fittings.
Total dynamic head
Total dynamic head is the term most used in
industry to describe a pump's head.
• For pumping systems which have suction lift
(see Figures la and Ib), the total dynamic head
equals the dynamic suction lift plus the
dynamic discharge head.
• For pumping systems which have suction head
(see Figure Ic), the total dynamic head equals
the dynamic discharge head minus the dynamic
suction head.
Vapour pressure and net positive suction head (NPSH)
Vapour binding
)
If pressure on the suction side of a pump drops below the vapour pressure of
the liquid, vapour forms. This could partially or completely stop liquid flow
into the pump. The pump is then said to be vapour-bound. This pressure drop
could happen because of insufficient suction head, high suction lift,
excessive friction head, or high liquid temperatore.
Cavitation
If pressure inside a centrifugal pump drops below the vapour pressure,
vapour bubbles form. The bubbles flow with the liquid until they reach an
area of higher pressure (normally at the outer area of the impeller). They
then collapse, producing a shock wave. This process is called cavitation.
During cavitation, the bubbles collapse (implode). liquid then suddenly fills
the space, hitting and eroding the surface. This action causes vibration and
noisy operation. If continued, this erosion causes the impeller to become
imbalanced, which mechanically destroys the pump (hearing failure).
Net positive suction head
To prevcnt cavitation and vapour binding, and to ensure maximum flow
through the pump, suction pressure must be greater than the vapour pressure
of the pumped liquid. This required pressure at pump suction is called the net
positive suction head (NPSIl).
Capacity, rating, and size
The pump's capacity is determined by the volume of liquid delivered per
unit of time. For example US gallons per minute (USgpm) or Hires per
minute (lImin).
Slip/actor
This is the difference between the actual volume of liquid discharged by a
pump and its theoretical capacity. It is expressed as a percentage of
theoretical capacity.
Volumetric efficiency
Volumetric efficiency is the ratio of the volume of fluid discharged to the
theoretical capacity of the pump. It is also expressed as a percentage of
theoretical capacity
Rating and size
Pumps are rated according tu the capacity at a given head and speed (rpm).
For example, a pump might be rated at;
•
900 USgpm at 80 ft and 1150 rpm
•
3400 IImin at 25 m and 1150 rpm.
The pump's size is expressed as the diameter of the discharge nozzle, suction
nozzle and impeller in inches or millimetres. For example:
•
3" X 4" x 10"
•
BOmm x l00mm x 260mm.
Radial flow pumps
Radial flow pumps move the liquid out radially from the shaft's axis. See
Figure 2. The most usual of these is the volute pump.
Volute pumps
In volute pumps, the rotating impeller discharges the fluid into a spiralshaped cavity called a volute. This is the most commonly used design (see
Figure 3).
Action
1. Fluid is fed into the centre (eye) of the impeller (see Figure 2) and flows
into the blades.
2. The rotating impeller causes fluid to discharged at its circumference at
an increased velocity.
3. As the fluid leaves the impeller, it moves outward at right angles to the
shaft's axes. (That is, it has tangential velocity.) It is flung into the
volute at high velocity.
4. Because the chamber widens, the velocity of the fluid decreases and part
of the velocity head is transformed into pressure.
Discharge
Inlet
Figure 2 Centrifugal pump action
Impeller
eye ---1--:--"
I
Volute
casing --.~....
Figure 3 Volute pump
1 -
Diffuser pumps
Diffusers are used in pump casings to create multiple volutes. They are
stationary, curved vanes in the casing which redirect the flow of the liquid.
The vanes direct the flow outwards thus forcing the liquid toward the
discharge nozzle (see Figure 4). This type of pump is called a diffuser pump.
Its chamber casing may be symmetrical.
Impeller eye
r- -t---Impeller
1-1-,'--- Stationary diffuser
vanes
Figure 4 Diffuser pump
Axial-flow pumps
In an axial-flow pump, tbe impeller moves liquid through the casing without
changing its direction. The impeller has vanes like a ship's propeller. The
pump develops its head by the lifting action of the vanes on the liquid. The
liquid moves through the casing parallel to the shaft. See Figure 5. These
pumps are usually mounted vertically.
Suction
end
Figure 5 Axial-flow pump
They have little suction power and are usually mounted below the surface of
the liquid being pumped. They have a large volume output flow, but
relatively low discharge head.
Mixed-flow pumps
Figure 6 shows a mixed-flow pump. It combines characteristics of radial
flow and axial flow pumps. It develops its discharge head by using both
centrifugal force and lifting action of the vanes on the liquid. This pump is
mounted vertically or horizontally. It is used for low-head, high-capacity
applications.
Discharge lIanas •.
)
Impeller
t
Suction end
Figure 6 Mixed-flow pump
Multiple staging
The pressure developed by a centrifugal pump with a single impeller is
limited. Usually pumps must develop much higher discharge pressures than
this. To do this, pumps are equipped with several impellers connected in
series, these are called multi-stage pumps. The discharge of one impeller is
connected to the suction of the next impeller and so on.
Figure 7 Three-stage, mixed-flow pump
For example, Figure 7 shows a cross-sectional view of a three-stage, mixed
flow pump. The liquid enters the suction of the first stage on the bottom.
Uquid discharges from this stage through the chamber into the suction of the
next impeller and so on until the liquid reaches the discharge outlc\. This
action increases the velocity of the liquid each time it passes through a stage
thus pressure is increased at the discharge outlet.
Peripheral pumps
Peripheral pumps have the liquid entering at the periphery of the impeller.
The pump energizes it and discharges it out again at its peripheral.
Regenerative turbine pump
A regenerative turbine pump is an example of a peripheral pump. The
impe\ler of this type of pump has a double row of vanes cut in its rim (see
Figure 8). The impeller is then centred in a machined groove in the casing.
Discharge
Suction
port
Figure 8 Turbine pump
8
These pumps create high pressure for their size, so they are used in compact
areas. They are suited for high-pressure, low-eapacity service. They are often
used as feedwater pumps for small boilers, as condensate return pumps, and
as hot-water circulation pumps.
Action
L The liquid enters at the outer edge of the impeller
2. As the impeller rotates, the vanes travel in a machined channel in the
casing. This gives the liquid a forward motion.
3. As the liquid speeds up, centrifugal force throws it into the channel.
4. Because of the channel's shape, the liquid returns between the vanes.
5. This process is repeated several times. As a result, the liquid follows a
spiral path around the outer wall of the casing. See Figure 9. It travels
almost 360· around the casing to the discharge outlet.
6. Each time the liquid re-enters a vane, it receives an impulse. This series
of impulses inereases the pressure gradually from suction to discharge.
Figure 9 Spiral flow path in a turbine pump
The regenerative turbine pump ean develop a discharge pressure several
times larger than a radial flow pump can with the same impeller diameter
and speed.
Pump casings
Split casings
•
Axially split casings are split along the axis of the shaft. The suction and
discharge nozzles are usually in the lower half of the casing. The upper
half is easily lifted for inspection.
Shaft
a Spill horizontally
b Split diagonally
Figure 10 Axially split casings
9
•
Radially split casings are split at right angles to the shaft.
Figure 11 Radially split casing
Barrel casings
In a multi-stage, high-pressure, centrifugal pump, it is difficult to maintain a
tight joint between the halves of a axial split casing or the sections of a
radially split casing. Therefore, the inner casing is fitted into an outer casing
called a barrel casing. See Figure 12, The barrel casing has no axial joints.
The space between the two casings is subjected to the high discharge
pressure, This tends to hold the sections of the inner casing together,
Figure 12 Barrel casing
Pump casing materials
The materials used in pump casings depends on the liquid that will be
pumped, Table 1 shows the choices of casing materials for various pumped
liquids,
-10
Table 1: Pump casing materials
FLUID
PUMP MATERIAL
Water, gasoline and other
chemically neutral liquids
• cast Iron casings with bronze fittings
Acids
• mild
• moderate
• concentrated
• bronze
• stainless steel
• 1. special stainless steel
e.g., Hastelloy
2. monel
3. nickel
4. rubber
5. ceramic
Alkalis
• mild
• moderate
• concentrated
)
• cast iron
• 1. Ni-resist
2. stainless steel
• special stainless steel
e.g., Hastelloy
Slurries (chemically neutral)
•
up to 1/8" diameter solids
• rubber linings
(with temperature limitations)
•
low % solids
• 1. cast Iron
2. NI-reslst
3. hard Iron
• moderate % solids
• 1. hard Iron
2. chrome steel
• concentrated solids
• 1. chrome steel
2. NI-hard
3. manganese steel
Shroud
b
a
c
d
e
f
9
Figure 13 Impeller designs
Pump impellers
Impeller design
Impeller blades are curved to push the fluid efficiently. Their size, shape and
amount of enclosure varies as shown in Figure 13.
a
open impeller have vanes attached to a partial shroud on one side.
b
semi-open, single inlet impeller. A full shroud closes off one side.
c
closed, single-inlet impeller. These have an inlet on one side only.
Shrouds close off both sides. These produces axial thrust which must be
overcome by using appropriate bearings or balancing holes.
d
closed. double-inlet impeller. These have inlets on both sides. This type
bas very little axial thros!.
e
screw-style impeller used in paper-stock pumps for suspended solids.
The screw acts as a mechanical draw to the impeller.
faxial-flow impeller used in axial flow pumps
g
open mixed flow impeller used in mixed flow pumps.
Impeller mounting
JmpeUer(s) are mounted either on the end of the shaft or mid-shaft and is
held in place in a variety of ways.
When it is mounted on the end of the shaft the impeller:
•
is screwed onto a threaded shaft and butting against a shoulder. The hand
of the thread must have a tightening action as the shaft rotates.
•
has a parallel bore with a sliding fit on the shaft. It is held in position by
a shoulder or sleeve on one side, and a keeper (retainer) plate and nut
(with a locking device) on the inlet side. A key is needed for a positive
drive.
•
is installed on a tapered shaft. A key is also used for driving. A nut (with
a locking device) and a keeper plate is used to hold the impeller on the
taper.
When it is mounted mid-shaft the impeller is driven by a key and held in
place by:
•
a sliding fit against a shoulder retained by sleeves and a nut
•
a sliding fit against sleeves and retained by nuts on both ends of the shaft
Choosing a centrifugal pump
Centrifugal pumps are used to pump a variety of liquids including slurries
such as sewage, pulpy solids, grit, or gravel.
Table 2 shows the pumps used for various fluids.
Table 2: Choosing a centrifugal pump
FLUID
PUMP TYPE
IMPELLER TYPE
Clear, non-corrosive
liquids at low or
moderate temperatures
• single or double suction
Closed except for very
small capacities
Liquids above 120'C
(250'F)
• single or double suction
• multiple stage pumps (usually
boiler-feed service at high
pressure)
Closed except for very
small capacities
Hydrocarbons, hot
• single suction; often special
refinery pumps designed for
high temperatures
Closed with large inlets
• single or double suction
Closed except for very small
capacities or where fluid forms
scales on surfaces of moving
parts
Corrosives (acid or alkali)
•
mild
• strong
·hot
• single or double suction
(single cheaper if available for
the required rating)
• single suction; many refinery
pumps are used to withstand
high temperatures and suction
pressures
Liquid slurries
• fine abrasives (particles
pass through 1/8" mesh)
• single suction with end
clearance wearing fits; if there
are no corrosives or high
temperatures, use rubber
linings on metal pumps; use
special rubber compounds for
resistance to some chemicals
• coarse abrasives
• -
Pulpy solids
such as paper stock
15
14
single suction (small
capacities hard to get)
Open to allow beUer application 0
rubber linings except in large
sizes; sometimes also
made in closed type
Closed
for large rocks (above 1"
diameter) use dredge
pumps with large impellers
operating at slow speed
• single suction:
use double suction with
special end-clearance wearing
fits only on very light
concentrations of solids
Closed (open was the standard
until end-clearance
wearing fits changed)
Pump components
Wear rings and plates
To produce maximum pressure, the clearance between the casing and the
suction passage of the impeller must be kept to a minimum. This prevents
the circulating liquid from moving back to the suction side. This seal is
provided by the fit formed by the rim around the impeller eye and the casing.
The required clearance is specified by the manufacturer.
Duriog operation, continuous leakage through this fit slowly wears away the
surfaces. Some wear is allowable, but if it exceeds three times the initial
clearance, the pump loses loo much efficiency. Replaceable surfaces called
wear rings or wearing rings are incorporated into the design of the pump
casing andlor impeller (see Fignre 14). Some smaller pumps have wear rings
in the casing only.
Impeller
wear
casing
Casing
wear
ring
Figure 14 Flat wearing rings on impeller and casing
Wear rings may be radial, axial or a combination of the two. They come in a
variety of different configurations: flat, stepped (L-shaped), or labyrinth (see
Figures 14 and 15).
Casing
Impeller
Impeller
ring
Impeller
Impeller
ring
a Stepped
b Labyrinth
Figure 15 Other configurations of wear rings
15
15
Wear rings are held in position using one of the following methods:
•
a shrink fit
•
a spigot, a groove or pins
•
screw threads
Figures 16a and 16b show two ways of mounting impeller wear rings.
Impeller
Impeller
wear
ring
Screw
a
b
Figure 16 Ways of mounting wear rings
Impeller
Figure 17 Wear plate in position
5
16
Corrosion and lubrication of wear rings
Wear rings are made of materials, chosen to suit the pumped liquid. Often
bronze or cast iron are used. When only one surface is fitted with a wear
ring, the wear ring is made of a softer material than the mating surface. This
allows most of the erosion to take place on the wear ring.
Wear rings are lubricated only by the pumped liquid, so their life is extended
if their are always run in liquid.
Wear (cheek) plates
With open impellers in radially split casings, wear plates (cheek plates) are
used instead of rings. Wear plates arc easily replaced plates which protect
the front andlor back faces of the casing.
Clearance between the plate and impeller can be adjusted by moving the
shaft axially. This is usually done by using shims.
Hydraulic baJancing devices
Axial hydraulic thrust is produced when the two faces of an impeller have
different pressures. This may be eliminated by using opposing impellers as
shown in Figure 18.
Inlet
Figure 18 USing opposed impellers
Half the impellers have inlets facing in the opposite direction to the other
half. If the inlets all face in the same direction, the axial thrust must be
compensated. This is done with hydraulic balancing devices such as
balancing holes, a balancing drum or a balancing disk.
In single stage pumps balancing holes may be used to counteract the pressure
differences. Figure 19 shows how the excess pressure that builds up in the
back face of the impeller can pass through these holes into the suction side.
15
17
Suction
------------>t-
Figure 19 Balancing holes
On multi-stage pumps a balancing drum or disk or combination of the two
may he used. In Figure 20, any excess pressure built up in the back face of
the impeller is allowed to pass through the radial clearance between the
balancing drum and the balancing drum head. This pressure then returns to
suction side of the pump.
Balancing
chamber
Balancing
drum head
Balancing
drum
Figure 20 Balancing drum
Balancing disks work on the same principle and the excess pressure passes
through the axial clearance (see Figure 21)_ As the pressure fluctuates, the
axial movement of the shaft changes the axial clearance which acts as a
valve.
Figure 22 shows a combination balancing drum and disk. This method
combines the constant radial clearance of the balancing drum and the valving
effect of the balancing disk.
18
To pump
suction
_ ____ BalancinQ
disk head
Impeller
Figure 21 Balancing disk
To pJ1mp
suctIon
Balancing
___ chamber
Combinalion
_ _ _ belancing
Impeller
head
- - - - Combination
balancing
unit
Figure 22 Balancing drum and disk combination
Sealing
Sealing may be done using mechanical seals or compression packing. They
are assembled into a stuffing box whlch is a cylindrical recess in the casing
around the rotating shaft of the pump. This minimizes leakage between the
casing and the shaft.
Mechanical seals
Leakage from stuffing boxes is not wanted when fluids such as gasoline,
acid, or ammonia are pumped. Also, compression packing does not work
well at high pressures. In these conditions, pumps use mechanical seals to
minimize leakage.
15
19
Figure 23 shows a mechanical seal mounted into a stuffing box. Setscrews
are used to hold it in place. O-rings are used as a secondary seal to prevent
leakage from the mechanical seal tu the housing and shaft. See Chapter 14:
Seals.
O-Rings
Figure 23 Mechanical seal
Compression packing and lantern rings
Compression packing or simply packing is an effective method of sealing the
bearing from the pump cavity. See Chapter 14: Seals. Figure 24 shows a
stuffing box holding multiple rings of compression packing and a lantern
ring. They are held in place by a gland. The gland is adjustable by tighteniug
nuts to compress the rings for the desired fit.
When a pump operates at negative suction pressure, air can be drawn into the
casing. This stops the leakage needed for lubrication. To prevent this, the
stuffing box is fitted with a lantern ring (also called seal cage) and a sealing
liquid connection. See Figure 24. This connection may be external piping or
an internal passage consisting of passage holes drilled in the casing.
Lantern rings are also used on pumps handling abrasive fluids. The clean
sealing liquid keeps the gritty substances out of the stuffmg box.
Figure 24 Stuffing box with lour rings 01 packing and a lantern ring
Shaft sleeves
Shafts can corrode and wear at the stuffing boxes. This weakens them and
makes effective sealing with packing rings difficult. Therefore, shafts of
smaller pumps are usually made from corrosion- and wear-resistant
materials. Larger pump shafts are usually protected by renewable sleeves as
shown in Figure 25. These are made of stellite, stainless steel or chrome
plated brasslbronze.
Cooling
liquid
Shaft
sleeve
Figure 25 Renewable shaft sleeves
Hand rotary
Shaft sleeves are normally a slide fit onto the shaft and are held in place
between the impeller and the shoulder on the shaft. Some sleeves are also
keyed to the shaft (see Figure 25).
In time the sleeves may become seized to the shaft which makes it difficult
to remove. An effective method of removal is to:
1. Grind two grooves with a small hand grinder (see Figure 26). The
grooves should be ground 1800 apart, axially along the sleeve.
2. Split the sleeve with a cold chisel, being careful not to damage the shaft.
Figure 26 Removal of sleeves
Pump bearings
The functions of bearings in a centrifugal pump are:
•
to support the shaft carrying one or more impellers
•
to allow the shaft to rotate with minimum friction
•
to keep the rotating shaft and impellers in correct position within the
stationary parts of the pump.
The two basic kinds of bearings used in pumps are
•
friction bearings
•
anti-friction (rolling element) bearings.
Friction bearings
In a small pump, the bearings are usually a bronze bushing. In other pumps,
babbitt bearings may be used. These bearings allow the shaft to move
axially, for adjusting impeller clearances.
These bearings are oil-lubricated:
•
Horizontal pumps use either drip-feed oiling, or if the housing has an oil
reservoir, an endless chain or a ring riding on the shaft can supply oil to
the bearings.
•
Vertical pumps use shaft-driven pumps to supply oil under pressure to
the bearings.
Anti-friction (rolling element) bearings
Ball and roller bearings are more commonly used in today's pumps. (Refer
to Chapter 9: Bearings.)
•
On axially split pumps radial load bearing are used at both ends of the
shaft. This is due to the axial thrust of the shaft being theoretically
balanced.
•
On radially split pumps radial load bearing are used at the impeller end
and combination (both radial and axial load) bearings are used at the
drive end.
Installing centrifugal pumps
Pump mounting
Details about installation and alignment of equipment are given in
Chapter 22: Installation and Levelling and Chapter 23: Alignment. Both the
pump and the drive source must sit on a secure base. This base may be made
of cast iron or fabricated steel. The base is bolted to a solid foundation to
absorb vibration. Fabricated steel bases can be easily modified to suit minor
design changes.
Outline Dimension Type~CL
..
..... _ - p - - - - - - - - - - - 1
A
~
{}
N
~
a Rotation
{}
I
Motor and spacer coupling are extra equij;Hnent,
AU dimensions are in jnches, 00 nol use lor construction unless celliliedo
.-.
-"""
Brack·
.1
SIze
Pump Size
)
1'i,X3X9
2 X 3 X 10'k
2 X3 X 121)
3 X4 X 9
3X4Xl0\,
3X4X12';,
4X 6 X9
7
7
7
7
7
9
4 X 6 X 10 1/1
4 X 6 X 121/J
5X6X9
9
9
9
6 X 6 X lOti}
9
6X8Xl1',
9
Pump Size
Brack·
el
Size
9
7
7
7
l'hX3X9
2 X3 X 10'"
2X3X12'h
3X4X9
3 X 4 X 10~',
:
3 X 4 X 12\\
4X6X9
4X6XlO'"
4 X6 X 12'/,
6X6X9
6 X6 X 10'1,
:
6X8Xl1 1o;'
I
Hlllzl Siz••
Dis·
Sue lion
charge
3
3
3
4
4
4
6
!
6
5
5
6
B
I
,
A
8
F
G
K
1%
:il
4';'
10
2
2
3
3
3
4
4
4
54
54
15
15
15
15
15
15
15
15
15
15
15
18
19 718
6
54
54
50
56
60
50
64
6
60
6
54
!
:
,
31/l
31J.
311~
,
4¥.
lH':
4%
4'h
4 lit
13
51h
21 ?1132
121h
SY2
12'h
13 1; '
13'.
13
5'1>
6
S'h
5
22
21 314
21 716
22 118
13'17
6\\
51Jt
1tv?
4'.
12v?
4'.
111J:
4:1.4:
4%
4%
5'A
P
R
2Olf.
233A
23 1/4
23¥,
47 114
51 13118
5'"
3!i
9
9
9
31j~
4
4
23¥.
28
25'/4
26'h
9
4
30'!~
9
9
9
4
4
4
26'/}
"
31l.
29'4
3OlJ.
51 118
5111'
51 314
59 ii4
55 llilll
573/4
61 1/8
62 51B
:
I
1911116
19718
:
!
21 7J8
22 lIB
T
Jill
5'/'
9
71~
10
9
6]J~
57314
62 II'
19 13116
I
H
7
7
19 3i4
4
5
41)
4 ~i
5
,
M
i
l
7
9
7J/~
71/4
10
BY,
7'"
BI!.
7112
a
BY,
10
10
9
12
12
I
,
~
•
Figure 27 Typical general outline dimensions
15
Pump manufacturers may supply general base drawings to suit the various
sizes available, or they supply pump-specific installation drawings.
Figure 27 shows general outline dimensions for a Type-CL pump from
Bingham Williamette. These can be used to design a suitable base.
Base mounting
Base mounted pumps have mounting lugs on the bottom of the pump casing.
See Figure 27. Any change in the temperature of the pump while operating
can cause the pump to lift and become misaligned with the coupling.
Centre-line mounting
In centre-line mounting, the pump body is carried on lugs extending from the
centre of the pump casing which rest on the base. See Figure 28. Because the
pump casing expands in both directions from the shaft centre line during
operating temperature, there is very little effect on coupling alignment.
Figure 28 Centre-line
mounting
Close-coupled mounting
In a close-coupled pump the motor shaft extends through the pump casing.
The impeller is then mounted directly onto this shaft. The pump ca~ing is
nonnally mounted directly onto the motor casing. See Figure 29.
Motor
Motor
shaft
Suction
end
Figure 29 Close-coupled pump
Pump start-up
After the pump and drive are properly mounted and aligned, precautions
must be taken before start-up. Check to ensure that:
A
•
the unit will tum over freely by hand
•
the pump is driven in the direction shown on the pump casing
•
all bearings are properly lubricated
•
the pump is primed
•
all air from the seal housing is vented
•
plenty of quenching fluid is flowing to the seals
Caution!
When starting a pump for the first time, follow the manufacturer's
instructions.
Safety
Safety is the first consideration when any maintenance is done. When
working on pumps, the following precautions must be taken:
1. Always lock out and tag all electrical controls.
2. Shut off, lock and tag all main valves to and from the pump, including
the seal-fluid supply.
3. Drain the pumps. This is important with suction-head mounting.
4. Flush the pump if it has been pumping acids or any other substance that
could injure workers.
Maintaining centrifugal pumps
Preventive maintenance
During preventive pump maintenance, the following are inspected:
•
volume and temperature of liquid leaking past the packing
•
oil level (see Oi/lubrication systems in Chapter 6: Lubrication)
•
bearing temperature and noise (vibration analysis)
•
joints and seals for leaks
•
flow rates at operating conditions
Routine maintenance
Routine maintenance consists of changing worn parts without major
downtime.
Bingham Type CL
Opon Imperlor Process Pump - Adjustable Bea(ing
165
117
154
102-G
123
106
110
142
'---142
'---115
208
144
140-6
134
164-6
'---118
Unnumbered parts In the illustrations are {he same as the corresponding parts shown with numbers
~r
DESCRIPTION
PART
NO,
102,A
102,8
102C
102·0
106
107
110
113
114
1>5
Stud
116
123
132 A
132·8
134
140·A
140,8
142
144
154
1:7
Glanrl
163
Gasket-Volute
Gasket-Sleeve
Gaskl:'t-8earlllg Cover
"0" 8,"g
Thrust Bearing
Radial Bearing
Locknut-Beariflg
Impeller
Volute
PAijT
~
DESCRIPTIO N
NO,
DESCRIPTION
Retainer Ring
Shaft
Lockwasher-Bea ring
Lockwasher ~Imp'):f!er
Slufling Box
O.lleetor Disc-Outboard
De1lector Oi6c-lntloard
Nul-Adjusllng
Capscrew-Impeller
Inspection Cover
HOllsing-Searlng
164-A
164-8
165
166
Cover-Thrust Bearing
Cover-Radial Bearirg
Lantern Ring
Impelle! Retainer
Sight Glass
Shaft Sleeve
PackllljJ
Shims
Key·lmpellel
Key-Shalt Extension
11,8
171
196
206
213-A
213-8
..........
I
.. -----~------'
Figure 30 Sectional print of a pump
Where needed, do the following:
•
replace the compression packing
•
inspect and, if necessary, replace the shaft sleeve
•
change the oil
Pump overhaul
A pump overhaul generally consists of:
•
inspecting/replacing the impeller
•
inspecting/replacing the wear rings or plates
•
inspecting/replacing the shaft and shaft sleeve
•
inspecting the stuffmg box and replacing the compression packing or
mechanical seal
•
replacing all bearings and seals
Pay attention to the condition and size of fit in things such as spigots,
sleeves, impellers, and bearing housings. Also, check the shaft and wear-ring
run-out before and after tightening the impeller.
Pump manufacturers supply:
)
•
a drawing showing the positions of all parts. For example, Figure 30 is a
sectional or cutaway print of a pump designed by Bingham Williamette.
•
an information sheet showing details such as pump rating, model,
packing seal, and bearing details (see Figure 30)
•
an installation and service manual giving step-by-step routine for general
overhaul
Troubleshooting centrifugal pumps
Table 3 on the next two pages contains tips for troubleshooting centrifugal
pumps.
Table 3: Troubleshooting centrifugal pumps
Trouble
Fall ure to deliver
liquid
Cause
1. pump is not primed
2. speed is too low
3. dynamic discharge head
Correction
1. prime Ihe pump
2. increase speed by
changing drive ratio
3 raise the pump
is too high
15
4. worn wear ring
4. replace wear ring and
5. worn impeller
6. plugged intake line
7. wrong impeller direction
adjust clearance to the
required amount
5. replace impeller
6. clear intake line
7. ensure direction matches
the direction arrow on the
pump
Reduced capacity or
pressure
1. air leakage in the inlet line 1. check for cracks in the
inlet plumbing and
tighten or replace all faul!\,
joints
2. air leakage through the
2. replace packing
stuffing box
3. insufficient inlet pressure 3. increase the suction head
for the vapour pressure of
or decrease the suction
the liquid
11ft
4. excessive air in the
4. reduce turbulence or
supply tank
install baffles
Pump overloads the
driver
1. speed is too high
2. viscosity of the liquid
different then
recommended for the
pump
3. mechanical resistance in
the pump
1 . reduce the speed
2. check the specification
sheet for the pump and
make the required
changes
3. disconnect the drive and
rotate pump by hand to
find area of resistance.
repair necessary areas
Table 3 continued: Troubleshooting centrifugal pumps
Cause
Trouble
Pump vibrates
1. misaligned coupling
2. insecure foundation
3. unbalanced impeller due
to a chipped blade
4. unbalanced impeller due
to excessive cavitation
5. bent shaft
6. worn bearings
Correction
1. realign drive to pump
2. secure mounting bolts
and if necessary, re-grout
3. repair and replace
impeller, and screening at
the inlet line
4. repair and replace
Impeller, and increase
suction pressure
5. replace shaft
6, replace bearings
1,2 repair casing by means of
Pump casing shows
wear
1, abraSive slurry being
pumped
2. abrasive chemicals being
pumped
Excesalve packing
wear
1. incorrect grade of packing 1- replace with proper grade
2, shaft is scored or rough 2. • reduce shaft diameter
3. shaft sleeve is scored or
rough
4. abrasives between
welding or installing an
insert. replace the pump
casing with a more
suitable material for the
pumped liquid
then use appropriate
packing size
• replace shaft
3. replace shaft sleeve
4. check and correct the
packing and shaft due to
faulty sealing
5. overh eatin g
5.
Pump casing damage
1. Impact
2. Uneven base
3. Freezing
faulty sealing.
• ensure cooling supply
to the stuffing box is
operational
• ensure gland nuts are
not too tight
1. examine Internal parts lor
damage or interference
and repair or replace
necessary items
2. repair damaged lugs and
remount pump correctly
3, examine caSing for crackl
and repair. overhaul
pump before putting it
back in service
Positive displacement pumps
Positive displacement pumps (or simply displacement pumps) are used
where a constant flow or a measured amount of flow is critical. The two
classifications of displacement pumps are:
•
reciprocating
•
rotary
Volumetric efficiency
In theory, a displacement pump expels the same amount of fluid (liquid or
gas) as it takes in. But, due to intemalleakage, this is not always true. As the
system pressure increases, so does this leakage. The degree to which this
happens is called volumetric efficiency. It is expressed as a percentage:
Volumetric
ffi'
Actual output x 100%
e IClency = Theoretical output
Reciprocating pumps
A reciprocating pump uscs a back and forth motion to pump fluid. This
motion gives the fluid a pulsating flow. Two types of reciprocating pumps
are piston Iplunger and diaphragm. They both increase and decrease the area
of the cavity, which pumps the fluid. As this area changes, valves are used to
give the fluid one direction. The movement of the piston or diaphragm in one
direction is called the stroke. The distance it moves is called the stroke
length.
The amount of liquid pumped per stroke of a reciprocating pump depends on
the cross-sectional area of the piston, plunger or diaphragm, and on the
length of the stroke. That is, it depends on the volume of liquid displaced per
stroke.
Plunger and piston pumps
Both plunger and piston pumps are connected to a crankshaft or eccentric
which gives it a reciprocating movement. The difference between these types
of pumps, is:
•
the plunger pump has its plunger sliding inside packing
•
the piston pump has its packing sliding with its piston.
These pumps are best for low flow rates and high-suction lifts but are not
suitable for dirty Or viscous fluids.
Plunger pumps
A plunger pump has a plunger which displaces the cross-sectional area of a
chamber. See Figure 31. With the use of check valves, the liquid is permitted
to enter one side and exit the other. These pumps are often found in metering
situations, where a measured amount of liquid is to be expelled.
)
Figure 31 Single-acting plunger pump
Sing Ie-acting piston pumps
A piston pump has a piston which slides inside a cylinder (see Figure 32).
The cylinder acts as a chamber and as the piston moves in the cylinder it
displaces most all the fluid in the cylinder.
Single-acting refers to only one side of the plunger/piston doing the
pumping. It has a single set of inlet and outlet valves. The pump discharges
every other stroke. A pump system may contain two or three single-acting
pumps linked together.
The action of a single-acting plunger/piston type is:
1. Plunger/piston extends:
• outlet valve opens
• inlet valve closes
• fluid is forced out of the chamber
15
31
2. Plunger/piston retracts:
• outlet valve closes
• inlet valve opens
• fluid is sucked into the chamber
3. 1 and 2 repeat.
system
Outlet
Inlet
Figure 32 Single-acting piston pump
Double-acting piston pumps
In double-acting pumps, both sides of the piston moves fluid. The piston
discharges at each stroke.
The action of a double-acting plunger/piston type is:
1. piston extends (see Figure 33a): The liquid is:
• drawn in the left side of the cylinder
• forced out the right side
2. piston retracts (see Figure 33b): The liquid is:
• drawn in the right side of the cylinder
• forced out of the left side
15-
a
b
Figure 33 Double-acting piston pump
Packing
For efficient pumping, it is important to prevent leakage between the plunger
and the chamber or between the piston and the cylinder wall. (Chapter 14:
Seals describes various types of packing.)
•
In plunger pumps, V-or U-ring packing is used. See Figure 34.
•
In piston pumps this is done in one of two ways:
with a series of cast iron rings which are fitted into grooves in the
perimeter of the piston
- with cup or U-ring packing material held in position by retainers. See
Figure 34.
Split ring
packing
Figure 34 Piston packing
Diaphragm pumps
The action of these pumps is similar to the action of plunger/piston pumps. A
diaphragm (flexible membrane) moves instead of a piston. The diaphragm
separates the pumped fluid from the mechanism, preventing leakage and
corrosion. A fuel pump operates in this way, because it is important that fuel
pumps do not leak.
Suction
Figure 35 Diaphragm pump
In Figure 35, the diapbragm is attached to the piston by the retainer. The
reciprocating movement is caused by the eccentric shaft. This gives the
diaphragm its pumping action.
Reciprocating pump vahes
Check valves
The check valve for the liquid end of a reciprocating pump is opened and
closed by the pressure difference above and below it. This pressure
difference is caused by the pumping action.
15
34
There are many different check valve designs. The type used is detennined
by the operating pressure and the properties of the pumped liquid.
•
ball check valves are used where free opening of suction and discharge is
needed, See Figures 36,
Flow
Figure 36 Ball check valve
•
stem-guided check valves are used for low pressures. See Figure 37, The
disc seat and stem are alloy, usually bronze,
Figure 37 Stem-guided check valves
•
wing-guided check valves are used for moderate or high pressures,
Figure 38 shows a wing-guided valve with bevelled face. It is used for
high pressure, clear liquids.
..
Spring
p
Flow
Figure 38 Wing-guided check valves
•
flap check valves are used for low pressures and free flow of semi-solids
(see Figure 39). They are primarily found in diapbragm pumps.
Figure 39 Flap check valve
Rotary pumps
In rotary pumps, fluid (liquid or gas) is positively displaced at a constant rate
by rotating parts. The flow is continuous and the discharge smooth. They are
used for fuel, lubricants, hydraulic oil, and other liquids of various
viscosities, including gases and liquified gases.
Rotary pumps have a closed casing. Gears, vanes, lobes, screws, or rotary
pistons rotate with a minimum of clearance in this casing.
Rotary pump systems may have fixed or variable delivery. This means:
•
fixed del/very-always delivering the same volume at a given speed.
Gear, vane, and piston types are all used in these systems.
•
variable delivery-----delivering volumes varyiug from zero to a given
maximum. Only vane and piston types are used in these systoms.
Gear pumps
Lubricating systems often use small gear pumps. This pump has a pair of
meshed gears enclosed in a casing. One of the pair drives the other. The
meshing teeth prevent the liquid from flowing back to the inlet (suction)
side. There are internal and external types:
•
ExterMl gear pumps have two oppositely-rotated, externally-cut gears
inside one casing (see Figure 40).
Discharge
Suction
port
part
Figure 40 External gear pump
•
Internal gear pumps have one internally cut gear and one externally cut
gear. They are separated on one side by a crescent-shaped partition (see
Figure 41).
Discharge
part
External
gear
-
Suction _
port
Figure 41 Inlernal gear pump
15
37
Action of gear pumps
1.
Fluid enters the pump at the inlet port into the space between the gear
teeth.
2. It is then carried around the casing to the discharge port.
3. At the discharge port the meshing of the teeth restrict the oil from
returning to the inlet port thus forcing it out.
Sliding-vane pumps
A sliding-vane pump has a rotor set slightly off centre in its casing. Vanes in
the rotor are free to slide in channels, pushed outwards by centrifugal force
as the rotor rotates. See Figure 42.
Suction
Discharge_"'--'. .
port
Figure 42 Sliding vane pump (unbalanced type)
Because of the offset, the vanes create chambers of different sizes as it
rotates around the casing. They are bigger near the intake and smaller near
the outlet.
Action of sliding-vane pumps
1. The fluid is drawn into the chambers at the inlet and is carried around by
the vanes.
2. As the chamber passes over the outlet, the chamber reduces in size and
the fluid is forced out.
Unbalanced vane pumps
Because the pumping action is on only one side of the pump, the basic vane
pump is said to be unbalanced (see Figure 42). This means that the highpressure fluid, exerts a force in one direction. High-capacity bearings are
needed to accommodate this force.
Balanced vane pumps
A balanced vane pump is made by mounting the rotor in the centre of an
elliptical case with two inlets and two outlets. The fluid is drawn in and
discharged during each half-revolution. See Figure 43.
15
38
Rotor
Suction
port
Discharge
port
Sliding
vanes
)
Figure 43 Balanced vane pump
In this action, the opposite forces produced by the pumping action balance
each other, reducing stress on the bearings. Light- or medium-service
bearings can be used. Also, the volume delivered is larger.
Variable-capacity vane pumps
In these pumps, the rotor may be centred or off-centre in various directions.
This is done by moving the pressure-chamber ring as shown in Figure 44.
Figure 44a shows the rotor centred. The spaces between the rotor and ring
are even and there is no pumping action.
Figure 44b shows the ring at its highest point within the housing. This is a
point of maximum discharge. The left side is suction and the right side is
discharge.
Figure 44c shows the ring at its lowest point within the housing. This also is
a point of maximum discharge. The left side is discharge and the right side is
suction.
a
Suction
Discharge
b
Discharge
c
Figure 44 Moving the pressure-chamber ring
Automatic controls with a spring-loaded governor are built into the pump.
They vary the flow to meet demand; levels are pre-set. If the discharge rate
is seldom changed, it is adjusted manually. These systems sometimes rely on
pump action rather than relief valves to limit pressure.
External vane pumps
An external vane pump has an oval rotor rotating centrally in the housing.
One sliding vane separates the inlet and discharge ports. See Figure 45. As
the rotor rotates, it carries fluid from the inlet port to the discharge port. The
vane maintains a seal between the rotor and the housing.
Discharge
port
Suction
port
)
Figure 45 External vane pump
Flexible member pumps
A flexible member pump uses a flexible tube, liner, or vane to carry fluid
from the inlet to the discharge port.
Flexible tube pump
A flexible tube pump uses a rotor with two lugs mounted on the periphery,
1800 apart. See Figure 46. These lugs compress the tube against the housing.
This fonn.~ a seals that prevents the liquid from returning to the inlet port. As
the rotor rotates it forces the liquid around the housing to the discharge port.
Rotation is relatively slow (up to about 200 rpm) with pressures up to
100 psi. It discharges liquid every half a revolution. This pump is used for
measured flow of liquids such as food products Guiee), paint, chemicals,
slurry, sludge, and pulp.
tube
Suction
port
Discharge
port
Figure 46 Flexible tube pump
Flexible liner pump
A flexible liner pump uses an eccentric rotor to force the internal walls of a
flexible liner outwards against the housing. It discharges liquid once per
revolution. The liner has a divider between the discharge and inlet ports.
This divider prevents the liquid from returning to the inlet port. Its operation
and usage are similar to the flexible tube pump.
Flexible
liner
Figure 47 Flexible liner pump
15
42
Flexible vane pump
A flexible vane pump has a rotor with flexible vanes. The vanes carry the
liquid around the housing from the inlet to the discharge ports. An insert is
mounted between the discharge and inlet ports, The insert compresses the
vanes and forces the liquid out. See Figure 48,
These pumps rotate at moderate speeds (up to approximately 6000 rpm).
They are used to transfer liquids such as coolants.
Suction
Discharge
port
port
Rotor with
flexible vanes
Figure 48 Flexible vane pump
Lobe pumps
A lobe pump has two rotors, each with one, two, or three lobes. See
Figures 49a, b, and c, These rotors are placed in a casing with a set of
external timing gears which synchronize the lobes. The liquid is trapped in
the pockets formed by the lobes and the casing. The motion of the lobes
carries the liquid around the casing to the outlet.
If the pump is taken apart, check the timing gears for mate marks, These
marks are often put on at the factory to aid re-assembly.
A
Caution!
If you cannot see any mate marks, put on a set before taking the gears apart,
Make sure that only one set of mate marks is visible.
Screw pumps
A screw pump uses a screw to move the fluid from the input to the outlet.
They may have, one, two, or multiple screws,
Single screw pump
A single screw (progressive cavity) pump has one rotor (screw) inside a
stationary lobed casing called a stator (see Figure 50). This pump acts much
like a lobe pump. The cavity is produced between the inside surface of the
rotor with the stator, while the outside surface of the rotor remains in contact
with the stator to prevent the fluid from moving back 10 the inlet.
)
Suction
Discharge
port
port
a
b
Suction
port
Discharge
port
-.;;..~
c
Figure 49 Single, two-lobe, and three-lobe pumps
Discharge
port
Figure 50 Single-screw pump
Two-screw pump
The two-screw pump has two rotors each with two opposing screw threads
(one left and one right hand). See Figure 51. One rotor is the drive and stays
in proper mesh by means of timing gears.
Rotor with
opposing screws <C:
Timing
gears
Suction
port
Figure 51 Two screw pump
Multiple-screw pump
The multiple-screw pump has multiple rotors. The centre (drive) screw
thread has one hand and the other screw threads have the opposite hand. As
the screws rotate, the liquid is carried between the threads and the casing,
axially towards the outlet. The threads are in constant mesh with each other,
restricting the fluid from returning to the inlet.
Axial piston pump
An axial piston pump has its pistons positioned axially in the pump. The
pistons connect to a drive shaft which rotates. A cylinder block houses the
pistons. The cylinder block rotates against a stationary valve plate which
houses the intake and outlet ports. See Figure 52.
Drive shaft
shaft flange (swash plate)
Piston rod
Piston
Valve plate slot
(stationary)
Rotation
Cylinder block bore
Outlet port
Inlet port
Figure 52 Axial piston pump
The drive shaft is at an angle to the cylinders. This angle is called the
housing angle.
Action of an axial piston pump
I. The drive shaft rotates the cylinder block and the pistons.
2. When the housing angle is other than zero degrees, the distance between
the piston and valve surface continually changes,
3. Each piston moves away from the valve surface during one half of the
revolution, causing suction and inflow.
4. Each piston moves toward the valve surface during the second half of
the revolution. causing compression and discharge.
Fixed and variable capacity
If the housing angle is fixed. the pump delivers a constant volume and is a
fixed-displacement pump. If the housing angle is variable as shown in
Figure 53, the pump's volume is also variable.
•
in Figure 53a housing angle is zero. piston stroke length is zero, and
there is no pumping action.
•
in Figure 53b there is a small housing angle with some stroke length and
some fluid is pumped.
•
in Figure 53e there are maximum housing angle, stroke length, and
pumping capacity.
Piston stroke length
a
No pumping action
Piston stroke length
"
Housing
angle
b
Increased pumping action
Piston stroke length
\
Housing
angle
c
Maximum pumping capacity
Figure 53 Varying housing angle to affect pumping
Radial piston pump
A constant displacement, radial piston pump has a rotating, eccentric shaft
rather than a cylinder block. The piston cylinders are contained in the fixed
housing of the pump. See Figure 54. Each piston has a check valve for fluid
to enter and another for fluid to exit.
Suction
port
Figure 54 Radial piston pump
Action
1. As the eccentric shaft rotates (180°) from the high to low point, the
piston cavity fills with fluid.
2. As the eccentric shaft rotates the other 180°, the piston cavity discharges
the fluid.
3. This cycle aets on every piston in sequence.
Regulating positive-displacement pumps
The capacity of displacement pumps is usually regulated by varying their
speed. Their output is only slightly affected by pressure variations, dropping
gradually as pressure rises. Excessive pressure is relieved by a pressure
relie/valve that redirects fluid after it reaches a set pressure limit. At the
preset pressure limit, the valve opens and dumps excess fluid back to the
supply. Without a pressure relief valve. a blocked line may cause the motor
to stall or the pump to break.
15
Maintaining positive-displacement pumps
Pumps must be installed and maintained properly. A pump should be
mounted on a strong. rigid base and aligned with the power source. See
Chapter 22: Installation and Levelling for details.
Repairs
The amount of allowahle wear for a pump depends on the demands put on it.
If it operates at 60% to 70% capacity. a small amount of wear can be
tolerated. If it works at 100% capacity. there must be no wear.
Other factors in judging acceptable wear are size. make, and style of pump,
cost of repairs, and company policy.
A worn pump can be:
•
discarded
•
sent to a commercial overhaul depot
•
repaired in the plant
Troubleshooting
Table 4 on the next two pages contains troubleshooting tips for positivedisplacement pumps.
Table 4: Troubleshooting positive-displacement pumps
Trouble
Cause
External leakage
around the shaft
1. Shaft seal worn
1 . Inpect the bearings and replace if
necessary and replace the shaft
seal as specified.
Pump doesn't deliver
fluid
1. Drain line plugged
2 Closed shut-off valve in
suction line; pump
mounted below fluid
level.
3 Pump did not prime
1. Clear drain line
2. Inspect valve and open fully.
Pump shaft rotates in
wrong direction
5. Oil level too low in tank
4.
6. Oil intake pipe or suction
filter clogged
7. Air leak in suction line.
This prevents priming or
causes noise and
irregular pump action
8. Oil viscosity too high for
the pump to pick up and
prime
9. Broken pump shaft or
rotor
15
Correction
50
3. Bleed air from the pump; prime or
lubricate. Inspect suction line for air
entry and repair
4. Reverse immediately to prevent
seizure and breakage
5. Add recommended oil; check level
on both sides of tank baffle to be
sure line is submerged
6. Clean or replace filters. Filter
capacity should be at least twice thE
pump capacity.
7. Tighten and seal the connection
8a. Use thinner oil and follow
recommendations for temperature
and service
8b. Heat oil for cold-weather startup
9. Replace the pump
Table 4 continued: Troubleshooting positive-displacement pumps
Trouble
Pump does not
develop pressure
Cause
Correction
1. Relief valve setting 100
1. Block Ihe machine travel or stop oil
low
2.
Relief valve is sticking
open
circulation. test with pressure
gauge. Set relief valve to
recommended pressure.
2. Remove relief valve clean and
inspect thoroughly
3. Leak in hydraulic control
3. Test this independently by
system (cylinders or
progressively blocking off the
valves)
circuit.
4. Free re-circulation o! oil to 4a Check the direction-control valves
the tank through the
4b The valve return line may be open
system
5. Pump shalt sheared due 5. nspect and replace necessary parts
to rotor seizure.
or entire pump
a. Relie! valve venting
6. Test the venting circuit by blocking
the vent line near the relief valve
Pump makes unusual
noise
1. Small air leak at pump
suction piping joints
1. Test by pouring pumped fluid on
3. Relief valve chattering
the Joints while listening for a
change in the sound of operation
2. Test by pouring pumped fluid
around the shaft while listening for It
change in the sound of operation
3. Check as for 1 & 2
due to air leak at pump
Intake or shaft packing
4. Coupling misalignment
4. Replace shalt seal and realign the
2. Air leak at pump shaft
packing
5. Partially clogged Intake
6.
line or filter, or restricted
intake pipe.
Debris pulled into intake
pump
5. Clean the lines and filter to prevent
cavitation
6. Disassemble and clean the lines
and pump
7. Air bubbles in intake oil
7. Check that return lines are below oil
S. Tank air vent plugged
S. Open tank air vent through a
level and separated from intake
9. Pump running too fast
10. Filter too small
breather opening or air filter
9. Check for recommended maximum
speed and reduce accordingly
10. Capacity may be correct only when
clean. The filter should be twice
pump capacity
15-52
)
MILLWRIGHT MANUAL: CHAPTER 16
Hydraulic Systems
Hydraulic theory and laws ..........................................................
Force and pressure ........................................................................
Work, power, and energy ..............................................................
Properties of liquids ......................................................................
Fluid flow .....................................................................................
16: 1
16: 1
16:2
16:2
16:4
Hydraulic actuators .....................................................................
16:7
Cylinders ....................................................................................... 16:7
Motors ........................................................................................... 16:14
Direction control valves .............................................................. 16:20
Symbols ........................................................................................ 16:20
Classification ................................................................................ 16:21
Activation and control.................................................................. 16:23
'lYpes of DCVs ............................................................................. 16:24
Pressure control valves ............................................................... 16:30
Types of valves ............................................................................. 16:31
Flow control valves ................ .......... ........................................... 16:36
Throttle valves .............................................................................. 16;37
Flow dividers ................................................................................ 16:40
Eleetro-hydraulie controls ................................... ....................... 16:40
Solenoids ...................................................................................... 16;40
Hydraulic pumps ................. ........................ ............... ................. 16:42
Fixed and variable capacity.......................................................... 16:43
Gear pumps ................................................................................... 16:43
Vane pumps ........................................................................... 16:44
Piston pumps ................................................................................ 16:44
Pump rating .................................................................................. 16:45
Pump mounting .................................................. .......................... 16;46
Hydraulic filtration ..................................................................... 16:46
Magnetic plugs ............................................................................. 16:46
Strainers and filters ....................................................................... 16:46
Filtering elements ......................................................................... 16;48
Filter location ................................................................................ 16;49
Reservoirs ..... ............... ........................... .............................. ...... 16: 50
Capacity ........................................................................................ 16:50
Construction ................................................................................. 16:51
Heat exchangers .......................................................................... 16:52
Coolers.......................................................................................... 16:52
Heaters .......................................................................................... 16:53
Accumulators ........................... ................................................... 16:53
Maintenance precautions ..................................................... 16:54
Classifications ...................................................................... 16:54
Hydraulic accessories ................................................................. 16:58
Pressure intensifiers ...................................................................... 16:58
Measuring Instrument~ ................................................................. 16:59
Pressure switehes .......................................................................... 16:61
Hydraulic fluids .......................................................................... 16:61
Seleeting a fluid ............................................................................ 16:62
Fire-resistant fluids ....................................................................... 16:63
Storing hydraulic fluid ............................................................ 16:64
Hydraulic fluid conductors.. ..................... .................................. 16:65
Pipe ............................................................................................... 16:65
Tubing ........................................................................................... 16:67
Hose .............................................................................................. 16:71
Fittings and couplers for tube and hose ........................................ 16:74
Symbols used in hydraulic circuits ............................................. 16:77
Hydraulic circuits ....................................................................... 16:89
Automatic bleed-down circuit for accumulators .......................... 16:90
High-low circuits .......................................................................... 16:91
Hydrostatic drive cincuits .............................................................. 16:94
Sequencing circuits ....................................................................... 16:97
Metering circuits ........................................................................... 16:98
Counterbalancing circuit .............................................................. 16:99
Multiple actuator circuits .............................................................. 16: 100
Troubleshooting hydraulic systems ............................................ 16:102
Communication ............................................................................ 16: 102
Isolation and identification of the problem ................................... 16: 102
Shut-down ..................................................................................... 16: 102
Removal and repair ....................................................................... 16: 103
Start-up ......................................................................................... 16:103
Follow-up ...................................................................................... 16:103
Troubleshooting tips ............................................................... 16:103
Troubleshooting cylinders ............................................................ 16:109
Troubleshooting pumps and motors ............................................. 16: 110
Troubleshooting valves ........................................................... 16:110
CHAPTER 16
Hydraulic Systems
A hydraulic system uses liquids in pipes of various sizes to transfer power.
The word hydraulic comes from Greek words "hydro" meaning water and
"aulos" meaning pipe. Water was the firstjluid used in hydraulic systems hut
now has heen replaced hy petroleum and synthetic oils. The term fluid
includes liquids and gases. This chapter deals with the uses and
characteristics of hydraulic systems.
Hydraulic theory
and laws
Many theories and laws apply to all fluid power systems (hydraulic and
pneumatic). Those used specifically in hydraulic systems are covered in this
chapter. Pneumatic systems are covered in Chapter 17.
)
Force and pressure
Force is required to move a stationary object or to change how fast an object
moves. The amount of force required depends on the object's inertia or
immobility. More massive ohjects are harder to move. Force is commonly
expressed in newtons (N) or pounds (lb). More correctly, the imperial unit of
force is pounds-jorce (lbf).
Pnessure is defined as the force per unit area. It is calculated by dividing the
force by the area over which the force is distributed. This is also stated as:
Force (F) = Pnessure (P) x Area (A)
F=PxA
p=F
A
Pressure may be expressed in newtons per square metre (N/m2), in pounds
per square inch (psi), or in bars. One newton per square metre is also known
as a pascal (1 N/m2 = 1 Pa). A pascal is such a small unit that pressure is
normally expressed in kilopascals (!cPa) or bars.
1N/m2 =1Pa
1 kPa
= lOOO Pa
lbar
= lOOkPa
= 14.5 psi
Atmospheric pressure
Earth's atmosphere exerts force and pressure. The blanket of air around the
earlb has weight. Imagine a tube 1 square inch in cross section, extending
from the earlb at sea level to the outskirts of the atmosphere. The weight of
air within this tube creates a force of 14.7 Ibf. This weight is pushing on One
square inch of the earlb at sea level. That is, the pressure of the atmosphere
at sea level is 14.7 psi.
14.7 psi = 101.35 kPa = 1.01 bar (usually rounded off to 1 bar).
Work, power, and energy
In mechanics, work is done whenever a foree moves an object. The amount
of work is equal to the distance moved (in metres or feet) times a force
exerted (in newtons or pounds-force). It is usually expressed in joules (1) or
foot-pounds (ft-Ibt).
=1 N x 1 m
1 ft-Ibf = Ilbf X 1 ft
1 joule
Power is the rate or speed at which the work is done. It is the work divided
by the time. The units used to express power are the watt (yi) and
horsepower (hp).
I watt is 1 newton lifted 1 metre in 1 second.
One horsepower = 33 000 pounds lifted I foot in 1 minute.
746 watts
1 horsepower
Whenever work is done, energy is used.
Law of Conservation of Energy
Energy has many forms-heat, light, mechanical, electrical, and so on. There
is a fundamental principle that energy cannot be lost. When it seems to
disappear in a system, it is because it has been changed into another form.
This principle is known as The Law of Conservation of Energy:
Energy cannot be destroyed, but it may be converted
from one form to another.
Whenever energy is converted from one form to another, work is done.
Properties of liquids
A liquid has no definite form, but takes the shape of its container. Unlike a
gas, it is virtually incompressible. When force is applied to a fully contained
liquid, the liquid shows the same resistance to compression as a solid. It is
extremely flexible, yet as unyielding as steel.
16
Hydraulic fluid compresses about ,Y,. % at 1000 psi. This loss of volume is
not big enough to he considered for most practical calculations.
A liquid suhjected to a force seeks the patb of least resistance. If the force is
gravity, the liquid seeks its own level. If it is subjected to a pressure
differential, it flows from the high pressure area to the low pressure area.
Pascal's Law
In the 17th century, Blaise Pascal formulated one of the basic laws of fluids.
See Figure 1. Pascal's Law states that:
Pressure applied to a confined fluid at rest; force is
transmitted equally in every direction and always at
right angles to the containing surface.
. . -t--
Forces act in
all directions
)
Figure 1 Forces resulting from pressure on a confined fluid at rest
When the fluid flows, the pressure no longer remains the same, as force is
necessary to overcome friction.
Multiplication offorces
By using Pascal's Law, liquid can he used to gain a mechanical advantage. It
does this by acting as aforce multiplier. In Figure 2 on the next page, piston
A has an area of 1 square inch (1 sq in), and piston B has an area of 50 sq in.
A force of 2 pounds-force (2 lbf) applied to A transmits a pressure of 2 psi to
the liquid. This pressure of 2 psi is also applied to the under-surface of piston
B. Because the area is 50 times as large, this results in an upward force of
1001hf.
100lbf
21bf
Figure 2 Downward force of 21bf converted to 100 Ibf of upward force
Pressure (P) )( Area (A)
Force (F)
= 2 psi)( 50 sq in = 100 Ibf
Force is gained at the expense of distance moved. If piston A is moved down
50 inches. 50 cubic inches of fluid are moved into the larger cylinder. Piston
B, with an area of 50 sq in is moved only 1 inch to make room for this fluid.
Fluid flow
The pump in a hydraulic system creates flow. Fluid flow rate is measured by
the volume of fluid passing a given point in a uuit of time.
Flow rate is expressed as Iitres per minute (Umin) or gallons per minute
(gpm). US gallons are most commonly used in North American industry, but
occasionally imperiai gallons are used in Canada.
One imperial gallon = 277 cu in.
One US gailon = 231 eu in.
Smaller rates can be expressed as cubic centimetres per second (cm3/s) or
cubic inches per minute (eu in/min),
Velocity
Fluid velocity refers to the average speed (in a particular direction) ofthe
hydraulic fluid passing a given point. This is the distance travelled by the
fluid in a lIIilt of time.
16
.
distance
VelocIty = .:..-':-."'-'~
bme
It is commonly expressed in metres pcr second (mls) or feet per second (ftls).
It can also be found in inches or feet per minute.
There is a simple relationship between flow rate and velocity:
•
Using metric units, flow rate is measured in likes per minute (Umin),
velocity in metres per second (mls), and cross-sectional area of the
conductor in square millimetres (mm2).
·
flow rate x 1666.7 (
. )
Ve IOCIty =
metnc
area
•
Using imperial units, flow rate is measured in gallons per minute
(gpm), velocity in feet per second (ftls), and cross-sectional area of the
conductor in square inches (in2).
flow rate (.
. I)
·
Ve IoCIty =
ImperIa
3.117 x area
Note that velocity varies inversely as the square of the inside diameter of the
conductor. Doubling the effective (inside) diameter of a conductor increases
the cross-sectional area four times. Tripling the diameter, increases the area
nine times:
Example:
'.
,1
I" diameter conductor has an area of 0.785 sq in
3" diantetcr conductor has an area of 7.065 sq in
(,aminal' and turbulent flow
In ideal flow situations, the fluid (liquid or gas) moves in layers, parallel to
the walls of the conductor. See Figure 3. The fluid at the outer walls of the
conductor moves more slowly than the fluid in the centre of the conductor.
This is known as laminar flow.
Figure 3 Laminar and turbulent flow
When smooth flow is disrupted, the fluid particles move in a random pattern
rather than parallel to the direction of flow. This random patteru is called
lurbulentflow. Turbulent flow can be caused by high velocity, obstructions
or projections in the fluid stream, sharp bends andlor roughness in the
conductor, a large number of bends in the system, or a combination of these.
-5
Friction
Friction is the resistance to relative motion betwcen two bodies. When a
fluid flows through a conductor by means of kinetic and potential energy,
friction is created. Friction occurs between the liquid and the walls of the
conductor, and between the layers of the liquid itself (viscous shear action).
Friction transfonns some of the kinetic and potential energy into heat energy
which is lost through the conductors and component's walls.
The correct diameter conductor is crucial to minimi7..e heat resulting from
friction. The friction in laminar flow is usually proportional to the velocity.
The friction in turbulent flow is proportional to the square of the velocity.
The main causes of excessive friction in hydraulic lines are:
•
excessive length of lines
•
excessive velocity (because the lines are too small)
•
excessive number of bends or fittings, or unsuitable bends or fittings
•
sustained flow at high pressure.
Energy in fluid flow
Fluid moving in the lines contains kinetic energy from the weight and
velocity of the fluid. It also contains potential energy from the pressure, and
heat energy caused by friction.
Fluid leaving a pump has kinetic energy and pressure energy. When fluid
goes through a restriction, the kinetic energy is increased and the pressure
energy is decreased.
Benwulli's Principle
Bernoulli's Principle states that if the flow rate is constant, the sum of the
kinetic energy and the potential energy at various points in the system is
constant. Therefore, whenever the velocity (kinetic energy) of a fluid
increases, the pressure (potential energy) decreases.
In Figure 4, the pump delivery is constant and flow is as follows:
L The fluid is forced through an orifice in the conductor.
2. This, increases its velocity. The pressure gauge (B) shows a
corresponding pressure drop.
3. After the fluid leaves the restriction, the velocity decreases, and the
pressure increases.
4. As the fluid passes through an increased pipe size, the velocity decreases
and the pressure increases. as shown in gauge D.
S. After the fluid leaves the pipe enlargement the velocity increases and the
pressure decreases.
o
Constant
flow
®
@
@
®
_----'--
\
Velocity
increases
Velocity
decreases
Figure 4 Pressure and velocity in fluid flow
In actnal use, gauge C and E show a progressive drop from gauge A. This is
due to heat loss caused by friction at the orifice. Disregarding friction losses,
gauges C and E should read the same as gauge A.
Hydraulic actuators
Hydraulic actnators are the components of hydraulic systems that convert
hydraulic energy into mechanical energy. They are either cylinders or
motors:
•
Cylinders are used to create linear motion.
•
Motors are used to create rotary motion.
Hydraulie actnators combine with mechanical devices to create the action
that does the work. In hydraulic system drawings, these actnators are
represented by symbols. All the graphic symbols in this Chapter are in
accordance with standard number ISO 1219 set up by the International
Standards Organization.
Cylinders
A cylinder is a linear actnator. It is available with either a single- or doubleacting motion. It consists of a tnbular housing containing a piston, with
required sealing, and one or more rods attached to the piston. The housing is
sealed with a cap on one end and the head on the other end. The head is
bored to hold the necessary sealing gland through which the rod passes. The
housing has the necessary ports to allow the hydraulic fluid to enter and exit.
16
7
Length
'I
Piston
diameter
Figure 5 Cylinder sizing
Cylinders are usually sized by the piston diameter, piston rod diameter and
stroke length. See Figure 5. They can be classed according to mounting style
and method of attachment to the driven part. They are also available with
non-rotating piston rods (see Figure 9). These use a rod, offset in the
cylinder, which passes through the piston to restrict rotation.
Single-acting cylinders
A single-acting cylinder applies force, or is pressurized, in one direction
only. The return action is accomplished by an external force such as gravity,
a spring, or a small-diameter auxiliary piston.
DE:
Cylinder rod returned by an unspecified force
Rods and rams
In a cylinder, the cross-sectional area of a piston rod is less than hal/the
piston face area. If the cross-sectional'areais more than hal/the piston face
area,.it is considered a ram (or plunger).
Figure 6 A hydraulic ram
Spring return cylinders
~
A
Cylinder rod returned by a spring
Caution!
Any spring-return single-acting cylinder should be taken apart carefully
because the spring can be dangerous.
Port
Figure 7 Single-acting cylinder with spring return
Diaphragm spring-return cylinders
Diaphragm cylinders such as the pancake type are used to provide short
strokes with large forces. Large forces are available due to its large piston
area. Rolling diaphragms are used for longer strokes. Both types have very
little friction to overcome.whim beginning to move. They have zero leakage.
Inlet
)
8. Single-acting flat diaphragm
b. Roiling diaphragm
Figure 8 Diaphragm cylinders
. Double-acting cyUnders
Double-acting cylinders have the
hydraulic fluid delivered under .
pressure to both sides of the piston.
This produces force in either direction.
Figure 9 is a non-rotating, doubleacting cylinder. It has one piston rod
and a piston guide pin which restricts
piston rotation. In this type of cylinder,
the force exerted by the piston is
different from one side to the other.
Ports
Figure g Non-rotating, doubleacting cylinder
If the same fluid supply exists on each side, the speed of piston travel is also
different. This is because the piston rod takes away some of the surface area
from the piston and also creates a smaller cavity for the fluid to fill.
Figure 10 shows the different surface areas of the two sides of the piston.
These differences can be overcome by using valves.
Piston
Less surface area
More surface area
Figure 10 Different surface areas of the piston faces on a Single-rod
double-acting cylinder
•
Single piston rod-the force exerted by the piston is less on the side with
the piston rod. This is because of the smaller surface area (see Figure
10). As the cylinder extends. more fluid is displaced. This creates more
force at a slower rate than when it retracts.
(g: Double-acting cylinder with single piston rod
•
Double-ended piston rod-in a double-rod cylinder. where the rods are
the same diameter. the forces 011 the piston can be the same. Because
equal amounts of fluid are displaced, the force and the rate of travel is
the same. See Figure 11.
:~=n:::i= Double-acting cylinder with double-ended piston rod
16
Figure 11 A double-rod cylinder
Cylinders with cushions
A double-acting cylinder is often designed with a cushioning device at one
or both ends of the stroke. This slows down the piston as it approaches the
end of its travel.
(]g: DAC with single fixed cushion
rng
DAC with double fixed cushion
~ DAC with single adjustable cushion
~ DAC with double adjustable cushion
)
The cushioning is created by slowing the flow of oil being discharged. As
the piston enters the cushioning area, the normal discharge is blocked off by
the cushion sleeve. See Figure 12. The remaining volume of oil is forced
through one of the following:
•
the clearance between the sleeve and cushion area (fixed)
•
a small orifice controlled by an adjustable needle valve (variable).
The slow rate of fluid discharge slows the piston. The cushion of oil tends to
absorb any shock.
Figure 12 Variable and fixed cushioning action
-
1
Tandem cylinders
Tandem cylinders have two pistons attached tu one rod. This design allows
for higher forces at the rod end without an increase in fluid pressure or
cylinder diameter. This is achieyed because the pistons' total surface area is
almost doubled. Tandem cylinders require more linear mounting space.
Pistons
Tandem
cylinders
Figure 13 Tandem cylinders
Telescopic cylinders
~ Single-acting telescopic cylinder
~ Double-acting telescopic cylinder
TelescopiC cylinders have a series of tubular rod segments called sleeves
. which fit inside each other. This permits a working stroke much longer than
its retracted length. They are .availahle as single- or double-acting cylinders.
Figure 14 Telescopic cylinder extended
Cylinder mounting
Cylinders can .be oriented in any position and will work successfully,
provided they are mounted on a strong and rigid base and aligned with the
part they are to move. There are many different mounting styles, which the
millwright must be familiar with. To examine specific mounting styles, refer
to manufacturers ' reference material.
Calculations for cylinders
When designing or troubleshooting hydraulic circuits, millwrights must
calculate cylinder forces and speeds.
Cylinder force
To calculate the force (F) that the rod end delivers, you must multiply the
pressure (P) delivered to the cylinder by the eff<X.'tive piston area (A).
I' = force of the rod end (N Or lb)
P = pressure delivered (Pa or psi)
A = area of the effective piston surface (m2 or in2)
F=PxA
P= F
A
)
A F
P
Millwrigbts commonly use this metric formula for force:
The force in newtons equals pressure in atmospheres
(bars) times area in square millimetres divided by to.
N=barxmm2 + to
Cylinder speed
To calculate the speed (S) at which the cylinder travels, you must dividc the
flow rate or volume per time (V) by the effective piston area (A).
S = speed of the rod end (mls or inlmin)
V", volume delivered (m3/s or in3/min)
A = area of the effective piston surface (m2 or in2)
S= V
A
V=SxA
A=V
S
Because the volume (V) is often given in litres per minute (Umin) or gallons
per minute (gpm), and millimetres are used instead of metres, conversion
constants are used. In a cylinder the piston speed is the same as the fluid
velocity. These types of formulas were discussed for fluid flow velocities
earlier in this Chapter.
. mIs were
h
. .In mm2 ( metrIc
.)
' der speed = jlowrate x 1666.7 m
area IS
CylIn
area
This formula is the same as is used for metric fluid flow velocity.
h
. . (.
'al)
rmn were
area .
IS In sq In Impen
CYli nder speed = flowratex231.In in!'
area
This is different from the formula used for fluid flow velocity. The speed is
measured in in/min rather thanft/sec.
Motors
The constrnction and design of hydraulic motors are very similar to that of
hydraulic pumps. For an added description of the constrnction and action of
these, see "Positive Displacement Pumps" in Chapter 15: Pumps. The
precautions taken with pumps should also be taken with motors. The general
differences are that pumps drive the hydraulic fluid and motors are driven by
the hydraulic fluid.
Design, capacity and now direction
Like pumps, motors may be of gear, screw, vane, or piston design. They may
have fixed or variable capacity. They may be designed to run in one
direction only (uni-directional), both directions (hi-directional), or
oscillating. A variable oil supply to the motor allows the motor to be run
over a wide range of speeds:
o
o
If any variation must be contrelled independently of the motor, then the
motor is considered to have a fixed capacity, Fixed capacity motors run
at a constant rate according to the predetermined flow rate.
When the variation is done in the motor, then the motor is considered to
have variable capacity. Regardless of the flow rate, the speed of variable
capacity motors can be altered.
¢=
¢=
Fixed capacity with one direction of flow
Fixed capacity with two directions of flow
~ Variable capacity with one direction of flow
~ Variable capacity with two directions of flow
16
Gear motors
In a gear motor the fluid flow from the system enters the inlet port and
travels in either direction around the casing. forcing the gears to tum in the
direction shown by the arrows in Figure 15. Both gears are driven by the
fluid. but only one is connected to the output shaft. Gear motors are fixed
capacity hydraulic motors.
Inlet
Outlet
port
port
Figure 15 Gear motor
)
Screw motors
A screw motor (see Figure 16) uses the force of the fluid against the face of
the screw threads to generate motion. There are two or more screws in the
housing but only one is attached to the drive. The others are idlers which act
as a seal between the helical chambers. This prevents reVerse fluid flow in
the bousing. Screw motors have a fixed capacity and operate quietly and free
of vibration.
Outlel
port
Rotor with
opposing screws
Timing
gears
Inlel
port
Figure 16 Screw motor
-15
Vane motors
In a vane motor the fluid flow from the system enters the inlet port(s) and
exerts force against the vanes and rotor. The maximum amount of force is
exerted against the vane with the largest area exposed to the fluid. As a
result, the rotor turns in the direction indicated in Figure 17.
•
Inlet
port
~_
Outlet
port
. Figure 17 Vane motor
No centrifugal force exists until the rotor is put into motion. Therefore,
springs, or some other meehanical means, are needed to hold the vanes
against the casing, Like vane pumps, vane motors may be balanced or
unbalanced and have fixed or variable capacity.
Rotary piston motors
Rotary piston motors h!lve either radial or axial pistons.
Radial
In radial piston motors, pressurized fluid enters the cylinder block in the
centre forcing the pistons outwards against the reaction ring. The rotor and
cylinder block centre-lines are offset causing the pistons to move to the
farthest point. See Figure 18. The offset between the rotor and the cylinder
block may be fixed or variable, altering its capacity.
Fluid flow is as follows:
.•
During the half of the cycle that the pistons extend, the pressurized fluid
enters the motori
•
Du ring the other haIf they contract and the fluid is exhausted into the
reservoir line.
The cylinder block is connected to the output or drive shaft and the casing is
held stationary.
16
Pintle is
non-rotating
Figure 18 Radial piston motor
Axial
In an axial piston motor the pressurized fluid enters the valve plate, forcing
the pistons towards the swash plate. See Figure 19. The angle of the swash
plate causes the housing to rotate as the piston is forced to the furthest most
point 'of travel. This angle determines the capacity of the motor.
•
During one half of the cycle, pressurized fluid enters the motor as the
pistons lr;lvel to their full extent.
•
During the other' half of a cycle, fluid is exhausted into the reservoir line.
Axial piston pumps are available with a fixed or variable housing angle.
Piston
subassembly
Outlet port
Swash
plate
Inlet port
Figure 19 AKial piston motor
Oscillating motors (rotary actuators)
:::f>= Oscillating hydraulic motor
Oscillating motors or rotary actuators are designed to give a shaft rotation of
less than 360°. Actuators commonly consist of a single, moveable vanc
mounted in the rotor. Fluid enters one port, forcing against the vane and
rotating the rotor in one direction. When the fluid enters the other port, the
rotation is reversed.
Oscillating motors can develop high torque at low speeds. Their speed and
torque can be adjusted in the same way as in staudard motors or cylinders.
A
CauJion!
Excessive travel of the driven component should be controlled by external
mechanical stops, not by the vane hitting the housing.
Port
Moveable
vane
Body
Limit of rotation
Figure 20 Oscillating vane motor
Calculations for motors
When designing and troubleshooting circuits, millwrights must be aware of
or calculate displacement, pressure, torque, mechanical efficiency, and rotary
speed of motors. Motors are rated by their displacement (siz.e), pressure,
torque, and speed.
Displacement
Displacement is the amount of liquid handled in one rotation of the motor. It
is usually expressed in cubic centimetres per revolution (cm3/rev) or cubic
inches per revolution (in3/rev).
Pressure
Pressure requirements for motors vary with the size of the motor's
displacement. The larger the displacement of the motor the less pressure
required to produce a given torque, and vice-versa.
Torque
Torque output is a function of the system pressure and the motor
displacement.
It is expressed in newton-meters (N.m) or inch-pounds (in.lb):
N.m = bar X cm3/rev + 2011:
in.lb = psi X in3/rev + 211:
(where 11: is 3.142)
Torque rating is used to find the size of the motor required for the job. It is
expressed in newton-metres per bar (N.mlbar) or inch-pounds per 100 psi
(in.lblloo psi).
Mechanical efficiency
Torque values are theoretieal, assuming the motor is 100% efficient. Each
motor has some slippage. which is fluid that moves through the motor
without doing any work. This robs the motor of some torque.
The mechanical efficiency of any machiue is usually expressed as a
percentage:
. I e ff!lC1CllCY
.
actual torque delivered x 100%
M ecb alliea
=
theoretical torque
Speed
Speed (S) is a rotary speed, usually expressed in rpm. It is determined by the
flow rate or volume per unit time delivered (V) divided by the motor's
displacement Or area (A).
s=-Av
V=SxA
A=V
S
V =flow rate =Umin or gpm
A = area (displacement) = cm3/rev Or in3/rev
•
In the metric system, volume delivered per unit time (flow rate) is
measured in litres per minute (Umin) and the area is measured in
cm3/rev. Then:
Rotary spee d =
flow rate X 1000
rpm ( metric)
area
•
In the imperial (US) system, flow rate is measured in gallons per
minute (gpm) and the area is measured in in3/rev. Then:
t.~,
Ro~J spe
ed = flowratex231 rpm
. . US'Impena
. I
area
Table 1 shows the effects on the system pressure, speed and torque of
changing the pressure, flow rate or displacement size. This assumes that the
load on the motor is constant.
Table 1: Effects of changes on a hydraulic motor
S},stem pressure
ghange
Speed
Torgue
Increase pressure setting
No change
No change
Increases
Decrease pressure setting
No change
Noehange
Decreases
Increase flow rate
No change
Increases
No change
Decrease flow rate
No change
Decreases
No change
Increase displacement
Decreases
Decreases
Increases
Decrease displaCl'ment
Increases
Increases
Decreases
.
Direction control
valves
Direction control valves (Deys) are used to control the direction of
hydraulic fluid flow. Dey. may have single or multiple positions:
•
A single position Dey maintains a uni-directional flow pattern
.(check valve).
•
Multiple position Dey. start, stop, or change direction of flow to or
from hydraulic actuators.
Symbols
+
Single position DCVs
Free check valve
Multiple position DCVs
[II
$
16~
Two or more squares (envelopes)
In these symbols, eaeh square or envelope represents a position and the flow
pattern inside the valve body. Two or more envelopes joined together
indicate that the valve can be used with two or more flow patterns through it.
All the port connections (ports) are shown on one envelope:
•
Two-position valves indicate the ports on the preferred Or starting
position, either nonnally open or normally closed.
• Three-position valves indicate the ports on the central (neutral) position.
- The lower two ports on the symbol are labelled pressure (P) and tank (T).
- The top port(s) are lettered A, B.
Transitory condition
I--!._..I...........JI Transitory condition
An envelope with dashed ends indicates a transitory (in transit) but
significant condition between two distinct positions. ThL, means that the
valve passes through this position, but does not stop in it. It is optional to
represent this position when symbolizing a DCY.
Flowpatbs
An arrow indicates the flow path of the fluid through the valve. The T
indicates a blocked flow path or port. Arrows and Ts can be combined in an
envelope as necessary.
ill One flow path
IilI Two flow paths
CJ Two closed ports
!Ell Two flow paths with cross connection
A dot at the intersection of erossing lines indicate flow paths which are
connected. The above symbol shows the flow paths connected in the centre
as they pass through the valve. This allows the fluid to flow in any direction.
Flow paths which cross without being connected do not show a dot.
Classification
Dey s are identified numerically:
•
The first figure indicates the number of ways the fluid can flow. This is
usually also the number of ports (excluding pilot ports).
•
The second figure indicates the number of distinct positions.
For example 3/2 designates a 3-way valve (3 ports) with 2 positions.
21
Dev s valves are classified by their:
•
spool type-sliding or rotary
•
nominal size
- usually nominal pipe connection size
- the recommended maximum volume through the valve
•
maximum allowable pressure
•
port connection
- threaded: national pipe taper (NPT) or national pipe straight (NPS)
flanged or flat face.
Spools
Devs have close-fitting, movable spools that either open or block off
various passages to control the direction of flow. Valves of this type are
classed as two-way, three-way, or fuur-way valves. DeVs seal by means of a
close fit between the spool and the body.
Spools may be rotary or sliding. Figure 21 shows rotary and sliding spool
valves in various positions. They are 4-way, 3-position DeVs. The symbols
for the positions are also shown.
A
T
p
TAP
c
B
A
B
TAP
B
A
B
TAP
Figure 21 Rotary and sliding spools and their symbols
B
Activation and control
These valves may be activated manually, mechanically, hydraulically,
pneumatically, electrically or a combination of these.
Manual control
Push button control
Lever control
Pedal control
Mechanical control
Plunger or tracer control
Spring control
Roller control
r
Roller control in one direction only
Hydraulic and pneumatic control
T Hydraulic flow (solid)
\l Pneumatic flow (outline only)
Direct acting
By application of pressure
By release of pressure
-- -n:=P--- By different control areas
Indirect acting
---IE[
- - -oc::[
:"".1.
'"L
By application of pressure
By release of pressure
Interior control paths
Electrical control
Controlled by a reversing electric motor
Detents
After being activated the spools may be held in position by means of a
detent. This is a spring-loaded mechanism which fits into grooves on the
periphery of the spool extensions.
-+--.. Detent
Types of DCVs
DCYs may be non-throttling or throttling:
•
Throttling DCYs allow the spool to pass through from one envelope to
another at any given rate. Because of this, they are also called infinite
positioning.
•
Non-throttling DCY s "snap" into only one of the envelopes at a time.
These types include, two-, thrce-, and four-way valves, and closed-,
open-, tandem-, float-, and regenerative-centre envelopes.
Throttling DCVs (infinite positloning)
Throttling DCYs have an infinite number of intermediate conditions. They
are indicated by parallel lines above and below the envelopes.
OJ
I I I I Throttling directional control
An example of a throttling valve is a tracer valve, activated by a plunger
against a spring.
Non-throttling DCVs
Two-way valves
A two-way DCY (212) has two ports (excluding pilot lines) and two
positions. The flow in the valve is either through or blocked. Its preferred
position may be either normally open or normally closed. See Figure 22.
Normally open valves allow fluid to pass through until the valve is activated.
Normally closed valves block the flow of fluid until the valve is activated.
-24
Normally open
Check
valve
Oil in
Normally closed
Figure 22 Normally open and normally
closed Iwo-way valves
Figure 23 Normally closed
two-way valve in a circuit
Figure 23 shows the schematic drawing of a circuit which may be used in
equipment such as a simple floor jack. A normally closed, two-way valve is
used to control a single-acting cylinder. A. the fluid flows past the check
valve, the piston is forced outwards. The fluid flow can be stopped at any
time and the check valve and the DeV lock the piston in place. When the
DeV is activated the fluid is allowed to flow back to the tank and gravity or
weight of the load retracts the piston.
Three-way valves
A three-way DeV has three ports (excluding pilot lines) and two or more
positions (312, 313, etc.). This valve has two possible passages, one from the
pressure port to the actoator and another from the actuator to the tank.
Normally open
Normally closed
Figure 24 Three-way, two-posilion valve (312)
Four-way valves
A four-way DCV has four ports (excluding pilot lines) and two or more
positions (412,413, elc.).
An actuator can move in either direction under power using a four-way
DeV. Figure 25 shows how the position on the spool determiues flow paths
in a circuit.
a
b
c
Figure 25 Spool position In a four-way DCV determines flow paths
•
In Figure 25a the valve is in the extreme left position. Flow from P
enters cylinder port A and moves the piston to the right. The return flow
leaves cylinder port B and returns through T to the tank or reservoir.
•
In Figure 25b the valve is in the opposite position. Flow P enters
cylinder port B, moving the piston to the left. Return flow leaves
cylinder port A and returns to the reservoir through port T.
•
In Figure 25c, the centre position locks the piston in place by blocking
all the ports, this valve is said to have a closed centre.
Centre envelope flow patterns
There is a wide choice of centre envelope flow patterns. The most common
are closed, open, tandem, float and regenerative centres. Each centre has a
defInite effect on:
-26
•
the actuator control or position
•
the fluid flow from the pump (heat build-up and hp demand).
Closed centre-t
ttl! !I XI
All ports are blocked off in neutral. The actuator is "locked" so that it cannot
move out of its position. Over time some movement occurs due to minor
internal leaks. The fluid from the pump must go through the relief valve at
maximum pressure, generating heat. This requires maximum power and
wastes energy through heat.
Open centre-1
UIHIX I
All ports are connected in neutral. The actuator moves in the direction of any
external forces. The flow from the pump is back to the tank with minimum
power demand. There is no pressure and only a low heat rise.
Tandem
centre-~
The actuator ports are blocked off, and the flow from the pump is back to the
tanl" This centre provides a hydraulic lock to hold the actuator in position.
Fluid is allowed to flow from the pump back to the tank. This requires
miuimum power and generates a small amount of heat.
Float centre
-i niH IX I
Ports A and B are connected to the tank, and the pump is blocked off. This
centre allows the actuator to coast to a stop or be moved manually without
disconnecting the circuit. As the actuator's movement slows down, oil flows
from one side of the actuator, through the centre, and then back to the other
side. The fluid from the pump must go through a relief valve at maximum
pressure, generating heat and requiring maximum power.
Regenerative c e n t r e - U
Ports A and B are ~'Onnected to the pump. This centre maintains constant
pressure to both ports of the actuator. This design allows for a very rapid
start of the actuator. The fluid from the pump must go through the relief
valve at maximum pressure, generating heat and requiring maximum power.
Check valves
Check valves are single-position valves which allow the free flow of fluid in
one direction only. Some check valves have a restricted flow in the reverse
direction. Their common valve styles are ball or poppet. Check valves are
available for in-line or right-angle mounting (see Figure 26).
a. In line
Figure 268 Free check valve (in-line)
Poppet
Closed
Open
Figure 2Gb Spring-loaded check valves
28
Free and spring-loaded types
i
f
Free check valve
Spring-loaded check valve
Free check valves use gravity to hold the valve closed and must be mounted
in the correct orientation. Spring-loaded check valves use a light spring force
of approximately 34 kPa (5 psi) to hold the valve seated, regardless of its
mounting position.
Restricted flow types
When some reverse flow is required in a circuit, restricted flow or pilotoperated check valves can be installed.
>=:: Fixed flow restriction in the line
m
Check valve with a restricted flow
Figure 27 Right-angle chock valve with restricted flow through the poppet
Check valves with restricted flow have an orifice in the centre of the valve.
This allows a set amount of fluid to flow in the reverse direction. These
check valves are typically used to control the rate of decompression before
shifting (activating) the main DCV. These arc found in equipment such as
large presses.
Pilot-controlled types
Pilot-controlled check valves permit free flow of fluid in one direction. They
either prevent or control flow in the reverse direction. A line with long
dashes indicates a pilot or control line.
_ .... __ Pilot or control line
I~L_ Pilot-controlled check valve to open
$---
Pilot-controlled check valve to close
•
A pilot-controlled check valve to open (pilot-to-open) uses a pilot
pressure to allow the check valve to open when sufficient pilot pressure
is applied. These are commonly used to lock cylinders in place until the
main Dey shifts.
•
A pilot-controlled check valve to close (pilot-Io-close) uses a pilot
pressure to hold the valve closed until sufficient pilot pressure is applied.
These valves are used in safety situations-for example, automatic
unloading of accumulators when equipment is shut down.
Pressure control
valves
Pressure control valves control the hydraulic pressure in all or part of the
circuit. They may he either normally closed or normally open valves:
•
Normally closed valves open when pressure reaches a set limit.
Examples are relief or sequence valves.
•
Normally open valves close when pressure reaches a set limit. Examples
are pressure reducing valves.
Dc=JITJ
One single square
One single square (envelope) indicates a unit for controlling pressure or
flow. It may have a variety of different symhols inside it.
16
Types of valves
Direct-acting relief valves
L~
Pressure relief valve
A direet-acting relief valve is a normally closed valve. It uses a bailor
poppet held on a seat by a spring similar to a check valve. Some valves may
use a guided piston instead of a poppet. Its spring is either set to a
predetermined foree or adjusted to meet a range of requirements. Directacting relief valves are frequently used as safety valves to prevent damage
from high surge pressure.
Spring
Tank
port
Ballor
poppet
Pressure
port
Figure 28 Direct-acting pressure relief valve (safety valve)
The port on the spring side of the valve seat retorns fluid 10 the reservoir.
The other side is attached to the pressure line. It works as follows:
1. While the system pressure is less than the spring force. the valve remains
closed.
2. When the pressure exceeds the spring foree. the valve opens and the
fluid is allowed to flow back to the reservoir.
3. As the pressure decreases below spring force. the valve closes again
stopping the flow.
The pressure at which the valve first starts to open is called the cracking
pressure. A higher pressure is needed to fully open the valve. The difference
between cracking pressure and full flow pressure is called the pressure
override or pressure differential.
Pilot-operated relief valves
[~ Pilot-operated pressure relief valve
If a large amount of fluid is to be relieved under a small pressure differential,
then a pilot-operated pressure relief valve is used. This is a normally closed
valve. Figure 29 shows a pilot-operated relief valve with vent port blocked
off and the valve acting as a straight relief valve.
Main
releave
valve
Vent port
Pilot valve
System
pressure----'
Drain
port
Figure 29 Pilot-operated relief valve
The main relief valve has an orifice in it. This allows the system pressure to
act equally against both sides of the main valve and the pilot valve. The pilot
valve operates as follows:
I. Due to equal pressure on both sides of the main valve, only a light spring
force is needed to keep the valve closed.
2. Because the pilot valve has a light spring force, its cracking pressure is
low. When system pressure rises, it overcomes the pilot valve spring
force.
3. The pilot valve opens up, allowing fluid to flow out of the pilot chamber.
4. The fluid flows out of the pilot chamber faster than it can be replaced.
5. The pressure on the spring side of the main valve beeomes less, allowing
it to open.
6. The valve remains open until the pressure in the system drops, allowing
the main valve to close.
7. When this happens, the pilot valve closes to equalize pressure on both
sides of the main val ve.
Unloading valves
In many systems, accumulators (covered later in this chapter) are used and a
continuous flow of fluid may not be needed. An unloading valve returns
pump output (at low pressure) to the reservoir after the required system
pressure has been reached. An internal or remote check valve is used to
maintain pressure in the system.
Unloading valves are normally-closed valves aod are usually installed in the
pump outlet line with a tee connection. They use a pilot valve to activate the
main valve. They have a sensing or pilot line downstream of the unloading
valve.
To system
,....---I>--......- - { ) - - - 1 > - - - - - - - - From pump
Control piston
Figure 30 Unloading valve and its symbol
Pressure.reducing valves
ct __"""'. ,,J~
When a secondary circuit operates efficiently at a pressure lower than the
relief valve setting, a pressure-reducing valve is used to reduce this pressure.
This valve is a normally open valve and is held open by an adjustable
spring. It has a pilot line downstream of the valve. It works as follows:
1. Fluid from the main circuit enters at the inlet port.
2. It flows past the valve and through the outlet port to the secondary
circuit.
3. Pressure on the secondary or outlet side acts on the bottom of the spool
through the pilot line.
4. When the pressure on the outlet side and against the spool exceeds the
spring thrust, the valve partially closes.
5. This increases the valve's resistance to flow and reduces the pressure at
the outlet port regardless of pressure fluctoations at the inlet port.
Figure 31 on the next page shows a pressure-reducing valve and its symbol.
Pressure
sensing
passage
l
Reduced pressure
(secondary circuit)
Figure 31 Pressure reducing vaive
The spring chamber is always drained to the reservoir to prevent fluid
pressure from building up and holding the valve open. Back flow will
completely close off the valve.
Sequence valves
In hydraulic circuits with more than one cylinder, it is often necessary to
work the cylinders in a specific order. This can be done by hand, by
electrical control, or by sequence valves.
A sequence valve is a normally closed, two-way valve. It has a pilot line that
senses the pressure of the inlet port and a line that drains the spring chamber
back to the reservoir (external drain). It works as follows:
1. The valve remains closed until pressure of the primary circuit increases
to its set limit. This happens when the priority actuator completes and
satisfies its function.
2. At this time the pressure through the pilot line forces the valve open and
allows fluid to flow through it to the secondary circuit.
3. When the secondary aetuator completes and satisfies its function, the
fluid is redirected to the next circuit or back to the reservoir by means of
a relief valve.
To primary circuli
~
Pressure
sensing
passage
··• .
... ......... t
To secondary
circuli
Figure 32 Sequence valve
A sequence valve uses a check valve to allow the reverse flow to bypass the
normally closed centre and return freely to the reservoir.
Counterbalance valves
A counterbalance valve is a normally closed, pressure control valve. A pilot
line senses the pressure of the inlet port and an internal drain. This valve is
used to maintain a set pressure in part of a circuit. This controlled pressure is
required to keep a weight such as the platen on a press from falling or to
keep a rotating load from running away. This valve is attached in the exhaust
side of the actuator. An internal check valve is used to allow free return flow
to the reservoir.
.
~
)
:...........:
Figure 33 Counterbalance valve
Brake valves
A brake valve is commonly found with a motur. It stops its rotation as the
Dey is shifts to its centre position. This valve is a normally closed, pressure
control valve, with both a direct and remote pilot connected to the circuit for
simultaneous operation. The direct pilot line acts on a piston area smaller
than the area acted on by the remote pilot line (which requires more pressure
to move the spool).
The brake valve is (''Onnected to the exhaust side of the motor. It works as
follows:
1. The direct pilot line is connected to the inlet side of the valve, sensing
the pressure from the motor.
2. The remote pilot line is connected to inlet side of the motor, sensing the
pressure from the pump.
3. As the DCY shifts out of its centre position, the pressure from the pump
(through the remote pilot line) holds the valve open.
4. If any external force on the motor increases its rpm, the pressure from
the pump decreases. The valve then closes until sufficient back pressure
is built up.
S. When the DCY shifts to its centre position, the pressure from the pump
stops and creates no pilot pressure in the remote line.
6. The direct pilot line is now under pressure due to the inertia of the load
on the motor. Due to the higher pressure required to move the spool, the
valve closes more rapidly.
Figure 34 Brake valve
Flow control valves
In many hydraulic systems, the speed of a motor or the rate of travel of
cylinder must he regulated. This is done by controlling the volume of
hydraulic fluid entering or leaving the actoator. In systems using a fixed
capacity pump, the regulation is hy flow control or flow metering valves.
To control the amount of flow, these valves reduce the opening of the
conductor by:
•
an orifice in the line for fixed control
•
a throttle valve for adjustable control.
X Fixed flow restriction in the line
-36
Figure 35 A fixed-orifice flow control
Throttle valves
-¥-
Throttle valve
--l><l-- Shut-off valve
In a throttle valve, fluid flows through an orifice. As the valve is adjusted,
the area uf the orifice changes. Gate, globe, plug, ball and needle valves are
all throttling valves. The simplest and most finely adjustable is the needle
valve. Throttle valves may also be used as shut-off valves where flow needs
to be stopped.
)
Figure 36 Throttle valve
Non-compensating flow-control valves
Throttle valves are non-compensating because they do not compensate for
any varintion in pressure or flow in the system. As the pressure increases, the
flow through the valve increases.
Fixed (non-adjustable), non-compensating valves
Fixed, non-compensating flow control valves have a pre-set amount of flow
in one direction and a free flow in the other. A very simple and effective way
to achieve this i< to use a poppet check valve with a correctly sized orifice
drilled into it.
Drilled
orifica
Free-flow
direction - - I . ~'I;f\,VII'~MrA.Yj~W
1
T
,.....
Figure 37 Fixed, non-compensating flow control valve (with symbol)
Adjustable, non-compensating valves
Adjustable, non-compensating flow control valves have an adjustable
amount of flow in one direction and a free flow in the other direction by
means of a separate check valve.
Controllec
Free
flow
,.I
.,. ....
.J•
.....,_flow
i
..,
A
...
"
Figure 38 Adjustable, non-compensating flow control valve (with symbol)
Pressure-compensatlng Dow-control valves
A pressure-compensating flow control valve maintains a constant rate of
flow through the valve, regardless of down-side pressure. It uses a spring
loaded poppet or pressure-compensating spool with a control orifice and a
throttling orifice to monitor the flow rate through the valve.
Fixed, pressure-compensating valves
A fixed, pressure-compensating flow control valve has a pre-set orifice sized
for its specific application. As the pressure increases at the control orifice:
•
•
the poppet moves against the spring tension, reducing the opening at the
throttling orifice and, therefore. the flow (Figure 39)
or
opening the relief port and allowing the excess fluid to return to the
reservoir (Figure 40).
Controlled
direclion
oillow
Inlet
Ouijal
Figure 39 Fixed, pressure.compensating flow control valve
Inlel
Figure 40 Fixed, pressure-compensating flow control valve with relief port
Adjustable, pressure-compensating valves
An adjustable, pressure-compensating flow control valve has an orifice
)
which can be adjusted to the required flow rate. TIlls valve acts in a similar
manner as the fixed pressure-compensating flow control valve.
Figure 41 Adjustable, pressure-compensating flow control valve
Pressure- and temperature-compensating flow control valve
As the temperature of hydraulic fluid changes, so does its viscosity. When
the fluid warms up, more goes through an orifice. A common method of
controlling the amount of fluid passing through an orifice as the temperature
changes, is by means of throttle attached to an aluminum alloy or bi-metal
rod (temperature-compensating rod). The temperature-compensating rod
expands and contracts with the changing temperatures, and moves the
throttle to decrease and increase the orifice.
Flow dividers
I)f1( I Flow dividing valve
A hydraulic system may have more than one circuit. Flow dividing valves
control the amount of fluid to each circuit. They are usually located between
the pump and the direction control valves. The flow divider is shown in the
circuit as two fixed or variable restrictions. The valve can deliver equal flow
rates or a preset ratio of flow rates into two separate circuits.
Electro-hydraulic
controls
Electro-hydraulic controls are an increasing part of the fluid technology.
They use electronic means to activate hydraulic circuits. This technology
functions in three possible stages:
•
electrical signal converting to mechanical movement.
•
mechanical movement to a piloting stage
•
piloting stage to the main valve
In small controls, the piloting stage may be omitted and the mechanical
movement activates the main valve directly.
Solenoids
Solenoid with one winding
Solenoid with two windings
A magnet in which the magnetic lines of force are produced by an electric
current is called an electromagnet. A solenoid is a simple form of
electromagnet. It is a coil of insulated copper wire or other suitable
conductor. Within the coil lies an armature or plunger made of iron or iron
alloys. When the coil is electrically energized, a magnetic field is produced
which attracts the plunger and draws it up into the core of solenoid.
This electromagnetically induced movement can be used to open or close
valve ports. Solenoids are available as either pull or push types. Springs are
used to return the plunger to its original position.
Manual override
Solenoid control valves usually have a manual override built into the
solenoid assembly which allows the millwright to see whether the trouble is
electrical or hydmulic.
A
Caution!
Do not use manual override unles8 the SY8tem machine, and operator.,
are in the clear.
By solenoid and pilot directional valve
By solenoid or pilot directional valve
Solenoid.controlled, pilot-operated valves
Solenoid-controlled, pilot-operated valves are a combination of a small
solenoid-controlled pilot valve and the pilot-operated main valve. The
solenoid-controlled pilot valve is called the master valve. It directs flow to
either end of the pilot-operated main valve which is called the slave valve.
The pilot valve is normally mounted on top of the larger main valve.
Figure 42 shows the basic opemtion of a master and slave valve. This is a
simple ON/OFF type of solenoid valve.
Nit !IXp
P T
Figure 42 Single-solenoid, two-position DeV (with symbol)
IN ITS NORMAL POSmON:
1.
The fluid enters pressure (P) port in the main valve and splits into two
paths.
2.
One path goes through the main valve into the pilot valve and then back
to one side of the main valve, holding its spool in one direction.
3. The other path goes to port A and carries on to the actoator.
4.
As the fluid returns from the actuator, it enters port B and flows through
the valve back to the tank (T).
5. The master valve has its own drain back to the tank.
WHBN THE SOLENOID IS ACTIVATEO:
1. It forces the pilot spool against the spring, thus changing the flow pattern
through the pilot valve.
2. The fluid then flows to the other side of the main valve, forcing its spool
in the other direction.
3. This changes the flow from port P to port B then on to the actuator.
4. From the actuator the fluid enters port A and then back to the tank
through port T.
5. When the solenoid is deactivated the pilot valve returns to its normal
position which forces the main valve back to its normal position.
Proportional solenoids
Solenoid with two windings with variable control
Conventional solenoids have a simple ON/OFF action, allowing flow or shutting
it off. A proportional solenoid allows the operator to vary the position of the
plunger by varying the amount of current going to the solenoid. This varies
the amount of flow. These are used in conjunction with throttling DCVs,
pressure control valves and flow control valves. With the use of proportional
solenoids, valves can be placed near the actuators and controlled by a remote
microprocessor control unit.
Hydraulic pumps
Hydraulic pumps convert mechanical energy into hydraulic energy as follows:
1. The mechanical action of the pump first creates a partial vacuum at its
inlet side.
2. This vacuum allows atmospheric pressure in the reservoir to force the
hydranlic fluid through the inlet line to the pump.
3. The pump's mechanical action then forces the hydraulic fluid to the
pump's outlet and into the system.
The basic pumps used in a hydraulic system arc positive displacement
pumps. These pumps and their action are described in Chapter 15: Pumps.
Fixed and variable capacity
Positive displacement pumps can be classified as fixed or variable capacity
according to their performance:
•
Fixed capacity-these run at a given speed, delivering a constant flow
rate. They may be gear, vane, or piston pumps.
•
Variable capacity-these run at a given speed, deliveriug a variety of
flow rates from maximum to zero in one or both direction(s). They may
be vane or piston pumps.
¢=
Fixed capacity with one direction of flow
¢=
Fixed capacity with two directions of flow
~ Variable capacity with one direction of flow
~ Variable capacity with two directions of flow
Gear pumps
A gear pump is a fixed capacity pump which has two or morc rotors. There
are external and internal gear pumps.
External gear pumps
A conventional external gear pump has its gears meshing on their periphery
(outer edges). One gear is driven by the other. The gears carry the liquid
from the suction port to the discharge port, around the inner walls of the
casing.
wbepump
A lobe pump has larger spaces between its teeth than conventional gear
pumps do. Their rotors must be driven by suitable drive gears mounted
outside the casing. These pumps deliver a more pulsating flow than
conventional gear pumps.
Screw pump
A screw pump is an axial flow pump. It may have one, two or three screws
which carry the liquid from the suction port to the discharge port. This pump
also needs to be driven by external drive gears. These pumps move the liquid
linearly through the pump, which eliminates pulsations. There is no metal to
metal contact within the pump, which makes its operation very quiet.
Internal gear pumps
An internal gear pump has one external gear rotating within an internal gear.
The rotation of the external gear is off-centre to that of the internal gear. This
arrangement is compact. The crescent seal and the geroror pump (see Figure
43) are two commonly used internal gear pumps.
Figure 43 Gerotor pump
Vane pumps
A hydraulic vane pump may be unbalanced or balanced, and have fixed or
variable capacity. See Chapter 15: Pumps. The variable vane pumps may he
able to reverse flow through the system.
Piston pumps
Hand pumps
The most basic of piston pumps is the hand pump. It is found in equipment
such as hydraulic jacks, floor jacks, and floor cranes. Their design is similar
to hydraulic cylinders and are either single.- or double.-acting. See Figure 44.
Check valves are used to maintain correct flow direction. A release valve is
used to return the liquid to the reservoir.
Figure 44 Double-acting hand pump
Axial and radial pumps
Axial and radial piston pumps are used in powered hydraulic systems. See
Chapter 15: Pumps. Both may have fixed or variable capacity. Some also
reverse flow through the system.
Pump rating
Hydraulic pumps are often rated according to their capacity and pressure.
Capacity and displacemcnt
The pump's capacity is equal to its flow rate at a given speed (rpm). Pump
speed changes the flow rate. Therefore, pumps are sometimes rated
according to their displacement. Displacement is the amount of liquid the
pump delivers per cycle.
Pressure
Pressure in a hydraulic system is created by resistance to flow. A pump can
produce the flow of liquid necessary to develop pressure, but cannot itself
produce pressure. Pressure is caused by the workload on the system from the
actuator(s). Pressure is regulated by a pressure-regulating valve.
Pressure in the circuit affects the flow rate of the pump. As the pressure
increaqes, the flow rate decreases slightly. This drop in flow rate is caused by
the increase in intemalleakage within the pump. Intcmalleakage or slippage
is common to all pressure pumps.
The pressure rating of a pump is stated by the manufacturer. Equipment is
rated to work safely at pressures up to a specified maximum under specified
conditions.
Pump mounting
Hydraulic pumps may be mounted above or below the fluid reservoir.
Above the reservoir
When mounted above the reservoir, the pump must be able to create enough
vacuum or pressure drop to overcome
•
the weight and friction of the liquid
•
the height from the liquid level to the pump's centre-line (suction lift).
The pump's inlet line should be as short and large as possible. If the vacuum
is too hlgh, any air dissolved in the liquid vaporizes. causing cavitation (see
Chapter 15: Pumps).
Below the reservoir
When the pump is located below the reservoir, atmospheric pressure helps to
push the liquid into the suction side of the pump. This gives the pump the
added advantage of being charged, (or pressure-fed) by the suction head of
liquid.
Hydraulic filtration
In a hydraulic system, contamination is a major factor in component failure.
Magnetic plugs, strainers, and filters are used to contain or separate
contaminants from the hydraulic fluid.
Magnetic plugs
Magnetic plugs are used to attract steel or iron particles and are normally
mounted in the reservoir.
Strainers and filters
Strainers and filters are similar in function.
-<l>-
Filter or strainer
Strainers
Strainers can be considered as coarse filters. They remove larger solids from
fluids travelling in a straight path. They are mounted in the reservoir, on the
inlet line of the pump. They consist of either a fine wire mesh screen or a
screening element wrapped around a metal frame. Strainers remove the
larger contaminants from the hydraulic fluid before it enters the pump. They
offer less resistance to flow than filters. More than one strainer can be used
to supply the demands of the pump.
46
Filters
Filters remove fine C()ntarninant~ from hydraulic fluid which must travel in a
tortuous path. They are confined in a slllllli container and can be mounted in
various places along the circuit. Filters can be classified as proportional or
full-flow types.
Proportional filters
A proportional filter has only a portion of the oil passing through the
filtering element. The rest flows directly to the reservoir. With continuous
recirculation, all of the oil eventually passes through the filter.
Figure 45 shows hydraulic fluid passing through the venturi (constriction or
throat) of a proportional filter. The pressure drop at the venturi draws the
hydraulic fluid up through the filtering element into the line.
Venturi throat
/
Body , /
Filter
element
Figure 45 Proportional Ii Iter
Fullllow filters
In a full-flow filter, all the oil passes through the filtering element. This
design gives more filtering action but builds up resistance to flow as the
filter becomes dirty. For this reason the filter housing often has a bypass
valve. See Figure 46.
Filter
Bowl
Figure 46 Full-flow filter with a bypass valve (and symbol)
Hydraulic fluid flows between the case on the outside of the cartridge,
through the filtering element, and up the centre of the cartridge to the outlet
port. When the filter will not handle the flow of oil, the valve poppet or ball
unseats and some of the oil is allowed to bypass it.
Filtering elements
Mechanical (metal) filters
Mechanical filters are considered coarse filters or strainers, and consist of
layers of wire screens or discs of perforated metal. They remove the larger
solid particles but do not remove water or very fine solids.
Absorbent filters
Absorbent (inactive) filters contain materials such as paper, wood pulp,
fabric waste, or wool. They remove fine particles, as well as water and
water-soluble impurities.
Adsorbent filters
Adsorbent (aeti ve) filters remove impurities by both mechanical and
chemical means. Bone-black, charcoal, fuller's earth. and other active clays
are examples of these filtering materials. These filters remove all solid
particles and insoluble sludge. plus nearly all water and soluble, oxidized
material.
A
Caution!:
Adsorbent filters may also remove most additives used in inhibited
hydraulic fluids.
Sizes of filtered particles and filter ratings
The size of the solids removed by a filter is rated in microns. One micron
equals one millionth of a metre, or 39 millionths of an inch (0.000039 inch).
The smallest particle that can be seen with 20-20 vision is about 40 microns.
Wire mesh strainers are graduated by mesh size or standard sieve number.
For example, a nominal lOO-mesh strainer has openings of 0,0059 inch or
149 microns. Filters are rated directly in microns.
Nominal and absolute filter ratings
The nominal rating of any filter indicates that the filter will remove most
particles of that size or larger. For example, a 20 micron nominal filter
removes most solids of 20 microns or more in any dimension, However,
depending on how it enters the filter, some particles of 20 microns and larger
can pass through. If a particle hits the filter with its narrow end, it might go
through; if it hits it broadside it is kept out.
16
Absolute rating means that a filter will stop all particles of that size or larger.
A 20 micron nominal filter may have an absolute rating of 35 microns.
Warning indicators
A large number of filter housings have indicators that show the condition of
the filter unit. These indicators act on the pressure needed for the hydraulic
fluid to go through the filter. The two most cornman styles are:
•
gauges with green, yellow and red divisions of the dial
•
"tell-tales" with green, yellow and red bands.
One example is a dial attached to a helical rod on which a poppet rides. As
the pressure in the filter increases, the poppet moves up the rod, rotating the
dial. See Figure 47.
Dial
Indicator
Filler
Is
Filler
needs
Filter
Bypassing
clean
Figure 47 Warning indicator action
Filter location
A filter can be located in three positions in the system; on the inlet side, on
the pressure side, or on the return line side.
Inlet-side filters
Inlet-side filters filter all hydraulic fluid going to the system. They also
protect the pump and the relief, or unloading, valves. This position requires a
ftIter that is large enough 10 not produce much pressure drop for the pump as
the filter becomes dirty. Often the volume of hydraulic fluid going into the
system is extremely large and a suitable size in-line filter is not a stock item
or is very expensive.
-49
An isolated, or independent filtering system is then used. This consists of a
small pump used to circulate the fluid from the tank through a filter and back
to the tank.
Pressure-side nIters
A filter on the pressure side protects the valves but does not protect the pump
or main pressure valves. Because of their position. the ftIter element and the
filter housing must be able to withstand the maximum pressure allowed in
the system.
Return-side nIters
The return-side position is often considered best for a fme filter as the fluid
is at its highest temperature and therefore at its lowest viscosity. The filter in
this position removes all solids resulting from wear on parts in the system. It
does not protect parts from contaminants such as scale and rust. These may
form in the tank or may be added to the tank by a careless filling routine.
Reservoirs
The reservoir or tank provides an adequate supply of hydraulic fluid to the
system. It also allows air in the hydraulic fluid to escape, dirt and water to
settle out, and heat to dissipate.
The reservoir should be set in an area away from any heat souree. If legs are
not built on the reservoir. it should be mounted on brackets or a stand to
allow air circulation around the bottom.
L....J Reservoir open to the atmosphere
~ Reservoir with inlet pipe above fluid level
J.,; Reservoir with inlet pipe below fluid level
T Reservoir ,vith header line
Capacity
The volume of fluid in the reservoir is normally equal to two to three times
the rated pump delivery for one minute. The capacity of the reservoir should
be large enough to keep the fluid level several centimetres above the intake
when the system is using the maximum volume of fluid. There should also
be an air space above the fluid level when the system is at its minimum use.
16
Construction
The reservoir tank is nonnally made from steel plate, with all joints welded.
It must meet general industrial specifications. Some reservoirs have a
sloping bottom and a drain plug at the lowest point. This allows
contaminants to drain out easily. The reservoir normally has removable end
caps for easy cleaning of the inside. Figure 48 shows typical reservoir
construction.
'Ld,f;JLh.:diii~iii~
Gasket
;1'
""'~I'" Suction
Level
guege
line
Drop--llI'-1
line
Suction
strainer
Drain
Figure 48 Reservoir construction
The baffle plate prevents a direct flow of fluid from the return line to the
suction line. Slowing the oil movement allows trapped air to escapo and
foreign material to settle on the bcttom. The recommended height of the
baffle is about two-thirds the height of the minimum fluid level.
The recommended height of the return and suction lines from the bottom is
about 1 Yzto 2 times the pipo diameter. The return line is on one side of the
baffle and extends below the minimum fluid level to prevent foaming.
The suction line is located on the other side of the baffle. close to the bottom.
Vortexing of the liquid (resembling the whirlpool swirl of water going down
a drain) allows air to enter the suction line if the fluid level is near to the
intake.
Vents and filler holes
Most reservoirs are open to the atmosphere. The opening is large enough to
let the air move as fast as the fluid is either removed or returned to the
reservoir. An air filter is recommended on any air vent to keep out dust and
foreign material. The air vent and filler hole are frequently combined.
A fine mesh wire screen is installed in the filler hole to trap any foreign
material that might fall in while hydraulic fluid is being added. The filler
screen slows the flow of fluid into the reservoir and should not be removed
merely to increase the flow.
Indicators and gauges
A temperature gauge is often installed in the reservoir to help monitor any
excessive heat build up in the system.
An oil level indicator (sight glass) is located where it can be easily read.
Caution!
Ensure that lhe sight glass Lv cleaned regularly.
Heat exchangers
-+
Temperature controller
The purpose of heat exchangers (temperature controllers) is to ensure correct
operating temperature of the hydraulic fluid. Heat exchangers are frequently
used to cool the oil in a hydraulic system. They may also be needed to heat
the oil for cold weather start-ups. This is because, in cold areas, it may be
necessary to heat the oil to reduce the oil viscosity.
Figure 49 shows various configurations allowing the fluid to pass through
the heat exchanger.
Shell
Shell
J
I BaRles I Uiow
L
side ~~:=;*#l*=::~~
flow l 00000 i
Tube
side
I I117ow:.:,...,:::-::-...J
side
TUb""..J~~~~
Shell
L;;.
fI~ ]
I Irrrnm
ft~~ ] l¢frill!!!;
Two pass
Single pass
'IZui
Tube..J
Four pass
Figure 49 Multiple pass heat exchanger
Coolers
The basic symbol for a cooler shows the arrows pointing outwards,
indicating that heat is leaving the system.
+-
Cooler
~ Cooler with coolant lines
Water coolers
Water coolers usually consist of a nest of tubes in a shell (cylindrical
container). Oil flow is in one direction and cooling water flow is in the
opposite direction. Parts must always be checked for correct connections.
Air coolers
In air coolers, the tubes are vertically mounted and have fins for heat
removal. Oil flows through the tubes and a fan drives cooling air over the
tubes and fins to remove heat.
Heaters
The basic symbol for a heater shows the arrows pointing inward, indicating
that heat is added to the system.
-$-
Heater
Heat may be added to the fluid in several ways:
•
by using electric immersion heaters with thermostat control
•
by passing steam or hot water through a coil or length of pipe submerged
in the tank
•
by starting up the hydraulic system and pumping oil over the relief valve
at maximum pressure to create heat from fluid friction.
Some operations keep the hydraulic pump running through a dump valve
during mill down time, so that the fluid will not have a chance to cool.
Accumulators
Accumulators store fluid under pressure for future use as a source of
potential energy. They also absorb shock waves or dampen pulsations and
maintain constant pressure in the system.
o
Accumulator
When using accumulators, two important values must be known:
•
the amount of oil to be added before the gas charge
•
the recommended pressure of the gas charge
(pre-charge is a percentage of tbe operating system's maJ<imum
pressure).
Caution!
Because they store potential energy, accumulators are a hazard. Before
putting a new accumulator in service, read the instructions carefully.
Hydraulic circuits which use accumulators should have an automatic bleed
down or some way to isolate the aceumulator from the system. See Circuits.
Maintenance precautions
Before doing any maintenance work on an aceumulator-loaded system do
one of the following:
• Isolate the aceumulator with a shut-off valve.
or
• Discharge or drain the aceumulator back to the tank.
A
Caution!
A charged accumulator is a hazard. Relieve pressure before servicing.
Classifications
Accumulators may be:
•
wcight-loaded
•
spring-loaded
•
pneumatic or gas-charged.
Weight-loaded accumulators
A weight-loaded accumulator is a vertical cylinder fitted with a piston. A
packing gland or similar oil-retaining device keeps the fluid in the cylinder
as the piston moves. A platform on the piston is loaded with scrap iron,
concrete blocks, or other heavy material. The force of gravity provides the
energy to keep the fluid under constant pressure. See Figure 50.
Figure 50 Weight-loaded accumulator
This style of accumulator can deliver a very large volume of fluid at constant
pressure. The pressure is constant through the entire stroke of the piston.
Friction created by the cylinder packing tends to slow the movements of the
piston. Due to moveable parts being exposed to the atmosphere, the
accumulator should be installed in a clean environment.
Spring-Joaded accumulators
Spring-loaded accumulators use compression springs instead of gravity to
supply resistance. Springs must be evenly loaded to allow even travel of the
piston through the cylinder. See Figure 51.
Figure 51 Spring-loaded accumulator
This type of accumulator does not produce constant pressure through the
entire stroke. The springs exert minimum pressure when the accumulator is
at a low volume. They exert maximum pressure at high volume. These
accumulators must also he installed in a clean environment.
Gas-charged accumulators
Gas-charged accumulators depend on the compressibility of a gas (such as
nitrogen or air) to produce the necessary pressure and delivery. They use the
principle of Boyle's Law. This states that:
At constant temperatare, the volume (V) of a gas varies
inversely to the absolute pressure (P).
The mathematical formula is P, VI = P2 V..
At a particular temperature, as gas pressure goes up, its volume goes down.
This principle is discussed further in Chapter 17: Pneumatic Systems.
Gas-charged accumulators are available as non-separated, piston, bladder
and diaphragm types.
The gases used
•
Compressed air is commonly used in low-pressure systems. When air is
compressed, any water vapour in the air condenses. This may cause rust
and contamination.
•
Dry nitrogen is commonly used in medium- to high-pressure systems.
Nitrogen is used beeause it is inert (will not react) to oil.
•
Pure oxygen is never used with petroleum oil because an explosion or
fire may result.
Non-separated accumulators
Non-separated accumulators have no physical barrier between the gas and
liquid. Therefore, they are used mainly on low-pressure systems. The low
pressure limits the amount of gas that dissolves in the liquid. (The higher the
pressure, the more gas dissolves in the liquid.)
These ac~wnulators are usually vertically mounted cylinders. See Figure 52.
The liquid line connection is at the bottom end. The pneumatic (gas) line
connection is at the top end. High and low liquid level switches are required
to prevent air from getting into the circuit.
High level
switch
Low level
switch
Figure 52 Non-separated accumulator
Piston accumulators
Piston accumulators are much like cylinders without piston rods. A simple
piston accumulator has a free-floating piston between the liquid and the gas.
The piston has two sets of required packing. These seal the two chambers
and eentralize the piston to prevent metal-Io-metal contact. A bleed hole is
used to eliminate any build-up of pressure between the seals.
Bladder accumulators
Bladder accumulators have a natural or synthetic rubber bag mounted inside
a chamber. This bag separates the liquid from the gas. The bag is moulded
around the gas valve and !ll(lunted through the top of the chamber. The liquid
conneetion is mounted through the bottom of the chamber. See Figure 53.
Protective devices are in place to prevent the bladder from being drawn into
the liquid connection and rupturing.
Gas port
Gas
Bag
~~)"'I--iiil- Valve
Figure 53 Bladder accumulator
It works as follows:
1. As the bladder is charged, it fills the space in the chamber.
2. When the system pressure builds. the liquid enters the accumulator and
compresses the bladder.
3. As the bladder compresses, its pressure also increases.
4. When the pressure in the system falls below that in the accumulator, the
fluid is forced out by the expanding gas.
Diaphragm accumulators
Diaphragm accumulators are similar in operation to bladder accumulators.
They use a natural or synthetic rubber diaphragm mounted in the centre of
the chamber to separate the liquid from the gas. See Figure 54 on the next
page.
, High-pressure
air valve
To hydraulic _ ~
system
Figure 54 Diaphragm accumulator
Hydraulic
accessories
Various accessories are used in hydraulic systems to perform specific
functions. Examples are pressure intensifiers, measuring instruments, and
pressure switches.
Pressure intensifiers
In some situations an extremely high pressure is needed. If it is not possible
for the system to create such a pressure, pressure intensifiers are used.
W
Pressure intensifier for one type of fluid
~
Pressure intensifier for two types of fluids
.
Intensifiers for one type of fluid use hydraulic fluid in both sides. Those with
two types use compressed air or other gas on one side and hydraulic fluid on
the other.
They are often cylindrical as shown in Figure 55. The cylinder has two
different size pistons connected by a piston rod, or one piston and a plunger.
It operates as a force multiplier. The lower pressure act on the large piston
area and forces the small piston forward, creating much higher pressure.
Inlet
area piston
Large area piston
Figure 55 Pressure intansifler
Pressure intensifiers are often used for clamping or piercing plate, or as
riveters.
Measuring Instruments
In hydraulic systems, three conditions are measured: pressure, flow and
temperature.
Pressure gauges
f--:
' ( Pressure gauge
Pressure gauges are calibrated in bar, kPa or psi. Two types of pressure
gauges used in hydraulic systems are the Bourdon tube pressure gauge and
the Schrader (plunger) pressure gauge.
Bourdon gauges
The Bourdon tube pressure gauge consists of a calibrated dial face and a
pointer attached through linkage to a flexible metal tube, called a Bourdon
tube. The Bourdon tube is connected to the system pressure. As the system
pressure increases the Bourdon tube tends to straighten slightly. This is due
to the difference in area between its inside and outside surfaces. This action
causes the pointer to movc around its dial face and indicate the pressure.
\
/
_
Section A • A
Cp)
Bourdon tube
t
Pressure inlet
Figure 56 Operation of a Bourdon tube gauge
1
Schrader gauges
The Schrader (plunger) pressure gauge has a calibrated dial face with a
pointer attached through linkage to a plunger and bias spring. The system
pressure is connected to the gauge and acts on the plunger. As the pressure
increases the plunger is forced against the bias spring. This moves and
rotates the pointer around the dial face, indicating the pressure.
Connecting Dnk
__""*_
Sleeve moves linkage
when pressure
is applied
Pivot
Figure 57 Operation of a Schrader gauge
Flow meters
-§- Flow meter
Flow meters are generally portable
testing devices. They are rarely
permanently attached to the
equipment. They are used to monitor
the flow in a line and to determine the
efficiency of pumps and motors.
A typical flow meter consists of a
weighted device (ball Or cylinder)
inside a calibrated, tapered tube (see
Figure 58). This type of flow meter
must be mounted vertically because
the weighted device relies on gravity
to operate properly.
port
Graduations_
Ball
Inlet
port
Figure 58 Cross section of a
typical flow meter
Another style has a turbine in-line with the flow. A sensing device converts
the pulses of the turbine to flow rate information. The sensing device is
connected to an electronic readout display.
Temperature gauges (thermometers)
cP
Thermometer
Thermometers are used to measure the temperature of the hydraulic fluid in
the line. It indicates whether the heaters or coolers are set correctly or
forewarn its malfunction.
Pressure switches
--
~ Pressure activated electric switch
Pressure activated switches are likely the most common electrical interface
device found in a hydraulic system. They protect operators, equipment and
work in progress by sounding alarms and shutting off equipment when the
pressure gets too high.
A mechanical device such as a diaphragm, piston or Bourdon tube is used to
sense pressure changes This device is attached to a switch which opens or
closes an electrical circuit if the pressure goes beyond the predetermined
level. The predetermined level may be factory-set or field-adjusted.
Hydraulic fluids
Even though fluids include liquids and gases the tenn hydraulic fluid
describes only a liquid. Some of the hydraulic fluids available today are:
•
petroleum oil-most commonly used
•
fluids with higil water content-used as a fire-resistant hydraulic fluid
•
invert emulsion fluids-petroleum-based liquid with 40% water content
which acts as an oil
•
glycol fluids-mixed with water and used in extreme cold conditions
•
synthesized hydrocarbon hydraulic fluids-used in areas of extreme high
and low temperature ranges
•
vegetable or grain oils.
Selecting a fluid
The type of hydraulic fluid to choose is determined by its quality and ability
to perform its required task. The most commonly used hydraulic fluid is
petroleum oil, but, because of its flammability, it should only be used where
there is no serious fire hazard. The following qualities should be considered
when selecting a hydraulic fluid:
A
62
•
viscosity-its rate of pour at a given temperature. The recommended
viscosity is determined by the pump manufacturer.
•
viscosity index-its change in viscosity over a given temperature range
•
pour point-the lowest temperature at which oil will flow. The pour
point should be about 20°F below the lowest expected temperature
•
thermal stability-the ability of an oil to resist chemical or physical
change at high temperatures
•
resistance to oxidation-hydraulic oil at a high temperature and
pressure can oxidize rapidly to form soluble and insoluble products such
as acids and sludge. Inhibitors are added to minimize oxidization
•
resistance to rusting-rust is formed on ferrous parts by water in the
system. The water is formed by condensation of air entering the
reservoir. Inhibitors are added to reduce rusting from the oil by natural
means
•
resistance to air foaming-the oil in the system contains air in solution
and absorbs more under pressure. Air develops heat as it is compressed,
and oil with air bubbles in solution generates heat while the air is being
compressed during the pumping action. When the pressure is reduced,
air comes out of the fluid, producing foam. Foaming is reduced by
additives which allow air to separate quickly from the oil and bubbles to
break away
•
fire resistance--see below.
•
lubricating qualities-the oil itself lubricates the moving parts of the
system and must maintain an oil film between all contact surfaces
regardless of temperature and pressure. Extreme pressure (EP) additives
are used where there is an increase of temperature, pressure, or metal-tometal contact
•
long life--the length a hydraulic oil will last between system oil
changes.
•
cost-oil with long life may cost more than lower quality oil but can
save money in the long run. The cost of changing oil and the
inconvenience of shutting down machines should therefore be
considered carefully when choosing an oil.
•
disposability.
Caution!
Ensure that only hydraulic fluid is used in hydraulic systems.
Fire-resistant fluids
Where hydraulic systems are located near high-temperature equipment or
open flames, fire-resistant fluids must be used. The four basic types of fireresistant fluids are:
Fluid
ISO designation
High water content
HFA
Water in oil
HFB
Water glycol
HFC
Synthetic
lIFO
High water content (BFA) fluids
HFA fluids are a soluble oils or syuthetic chemical emulsions (oils-in-water).
They generally eontain 5% to 10% oil. The soluble oil has a milky
appearance and the synthetic solutions have a clear appearance. These fluids
have excellent cooling ability.
To avoid evaporation and deterioration of the fluid, they must operate at low
temperatures-maximum 49°C (1200 P). They should not be allowed to
freeze because the water and oil will separate. Additives improve rust eontrol
and lubrication and keep the oil and water from separating.
Seals, metals and protective coating are not necessarily compatible with all
HFA fluids. To be sure, refer to the manufacturer's information sheets.
Water in oil (lIFB) fluids
HFB fluids are invert emulsions which conraiu more oil then water. They
generally contain only 40% water. HFB fluids should be checked regularly
to maintain the viscosity and the water-oil ratio. Water Or oil can be added to
maintain the required ratio.
HFB fluids are similar to HFA fluids in their operating temperatures and
additives.
Any seal material used with a petroleum oil system is usually eonsidered
safe to use with HFB fluids.
Water glycol (lIFC) fluids
HFC fluids consists of 30% to 40% water dissolved into glycol. HFC fluids
should be checked regularly to maintain the correct water-glycol ratio.
Glycol is of the same family as permanent antifreeze ethylene. Therefore,
HFC fluids can endure temperatures below freezing. They contain additives
to improve viscosity, lubricating qualities, and foam control.
These fluids are heavier than oil and should be used with either a very short
suction lift, a special inlet design, or in situations where the fluid level is
above the pump inlet.
Many of the new synthetic seal materials such as neoprene or Buna-N which
are suitable for petroleum oils are also suitable for fIFe fluids. However,
ffi'C fluids attack galvanized (zinc), cadmium-coated (bright finish) parts,
and most common paints used around an oil hydraulic system. They also
attack some magnesium, aluminum and die cast alloys.
A
Caution! When using a glycol mix, get the supplier's specifications for
restrictions on its use as to seals, materials, or paints.
Synthetic (lIFD) fiuids
HFD fluids are special chemical compounds which do not support
combustion. They have no water content and can therefore be used at high
temperatures with no evaporation problem. These are the heaviest of the
hydraulic fluids and require special pump inlet designs or special pump
mounting positions.
These fluids attack most common seal materials used in a petroleum oil
system. A new hydraulic system with synthetic fluids should have all the
component seals, gaskets, pipe sealant, and paint selected to suit the fluid. If
an existing oil or water mix system is changed to a synthetic system. the old
system must be completely flushed out. All seals and gaskets must be
changed and the tank interior cleaned and painted with a special epoxy paint.
A
Caution! 'When changing to a synthetic fluid, remember that even the spare
valves and components must have the seals changed. Filters and strainers
must also be compatible.
There are several types of synthetic fluids on the market, such as phosphate
esters, polyol esters, and halogenated hydrocarbons. Each has its own
characteristics. To avoid confusion. check the manufacturer's specifications
for compatibility. operating conditions. and handling safety.
Storing hydraulic fluid
It is very important to maintain clew hydmulic fluid in the system. Hydraulic
fluid is delivered clew and free from contaminants. The pails or barrels have a
spout or bung in their lids. When these containers are stored, keep the spout or
bung clear from moisture and other contaminants. When the container cools, a
vacuum is created inside. If any moisture collects around the openings of the
container, it lends to be drawn in. If the containers are to be stored outside, tilt
them to prevent water from collecting around any bungs. See Figure 59.
Figure 59 Preferred storage of a container exposed to the weather
Hydraulic fluid
conductors
Hydraulic fluid is conducted to and from components and through the
system in pipe, tubing, or hose. Some manufacturers claim that the plumbing
in a hydraulic circuit causes a large amount of the trouble in the system.
Manufactured components are built to meet rigid standards, but the way in
which they are connected may prevent them from operating efficiently.
The inside diameter (ID) of a line determines the rate of flow that can pass
without excessive friction, heat, and power loss. Velocity for given flow is
less through a large opening than through a smaJl opening. Velocity varies
inversely as the square of the inside diameter of the line. Also, as the inside
diameter decreases, turbulence and friction increase, causing increased
power loss.
The waJl thickness and the inside diameter determine the bursting pressure
of a line. The greater the waJl thickness for a given inside diameter, the
higher the bursting pressure. Conversely, the greater the inside diameter for a
given wall thickness, the lower the bursting pressure.
(Force = pressure X area.)
Pipe
Pipe is selected for economy and for its ability to carry large flows in the
larger sizes of pipe. It is best suited for long, permanent lines. Seamless steel
(black) pipe, with its interior free from rust, scale and dirt, is recommended
for hydrauiic systems.
A
Caution! Galvanized pipe should not be used because scales or flakes of zinc
may enter the system.
Dimensions
Black pipe for hydraulic use come in a variety of wall thicknesses. For any
nominal size the outside diameter (aD) remains the same, and the ID
decreases as the wall thickness increases. Pipe is threaded on the aD only.
Therefore, the aD must remain constant.
Table 2 on the next page shows the dimensions of black pipe.
Table 2: Dimensions of black pipe
Nominal size
PI~OD
1/8
0.0405
1/4
0.540
3/8
0.675
1/2
0.840
3/4
1.050
1
1.315
11/4
1.660
11/2
1.900
2
2.375
21/2
2.875
Schedule If Wall thickness
40
80
40
80
40
80
40
80
160
40
80
160
40
80
160
40
80
160
40
80
160
40
80
160
40
80
160
0.068
0.095
0.088
0.119
0.091
0.126
0.109
0.147
0.188
0.113
0.154
0.219
0.133
0.179
0.250
0.140
0.191
0.250
0.145
0.200
0.281
0.154
0.128
0.344
0.203
0.276
0.375
PI~e ID
0.269
0.215
0.364
0.302
0.493
0.423
0.622
0.546
0.464
0.824
0.742
0.612
1.049
0.957
0.815
1.380
1.278
1.160
1.610
1.500
1.338
2.067
1.939
1.687
2.469
2.323
2.125
Threads
The most usual types of pipe threads are American National Pipe Taper
(NP'I). American National Pipe Straight (NPS) and Dryseal Pipe Taper
(NPTF).
NPT and NPTF threads
The NPT and NPTF threads are self sealing. They seal more with every
revolution. Over-rotation may stretch or strip the threads. Reverse rotation
may lessen the seal. They are used where rotational orientation is not
important
NPTF threads require the use of special taps and dies for thread cutting. The
crest and roots engage before the thread flanks, and are crushed as the
threaded parts tightened to full eontact on the flanks. See Figure 60. There is
no leak path between the crest and roots of the mating parts.
Spiral clearance
Figure 60 Fits of NPT and NPTF threads
NPS threads
NPS threads require an O-ring to seal. If rotational orientation is important,
NPS threads are used. See Fittings and Couplers later in this section.
Using sealing compounds
Pipe threads should be treated with a sealing eompound on the male end
before being threaded into afitting and tightened. See Figure 61. This
prevents threads from gaMing (welding) to each other. It also prevents fluid
from leaking through the spiral clearance at the roots of the thread.
Any sealing device should start about two threads from the end of the pipe.
If Teflon tape is used, it should be wound in the same direction as the pipe or
fitting is to be tightened. It should be wrapped only twice around the pipe.
Any burrs or sharp edges should be removed from the ID and OD of the ends
of the pipe. All this is done to prevent contamination of the system.
Thread sealer
on this area
~I
&_
Remove burr
after cuttin91
r
[.. j.Y [];.1
ill i
Figure 61 Proper preparation of pipe ends
Tubing
Seamless steel tubing is most commonly used for hydraulic lines because it
can be easily bent and flared. Other tubing available is heavy-wall, hardtemper steel, copper, aluminum, stainless steel, or plastic depending on
pressure and other conditions.
16-
Dimensions
Tubing sizes are taken from the outside diameter (aD) and are beld to close
tolerance. The nominal dimensions are given in fractions of an inch or dash
numbers. The dash number represents the outside diameter in sixteentbs of
an inch. For example, a tube with a dash number of 8, indicates:
aD
= %6 Or X inch.
Fittings and tube ends
Tube fittings connected to the tubing allow the tubing to remain stationary
while the fitting is tightened. The tube ends can be prepared flared for flared
joints or flareless or straight for bite and compression joints. The tubing is
cut square with the axis of the tube. This is best done by a rotary (pipe)
cutter. After the tubing is cut, any inside burrs are removed.
Flared joints
Flared joints rely on the compression of the tubing material between the
inner and outer walls of the flare. Tube flares are made with a flaring tool
kit. The tube can be made single or double flared. See Figure 62. After the
flare is made it should be inspected for cracks, finish and any imperfections.
Single
flare
Figure 62 Single and double flared tubing
Flare angles for tobing are:
•
45° (90° included angle)-SAE standard.
•
37° (74° included angle)-Joint Industrial Council (TIC) standard
•
30° (60° included angle)-British Standard Pipe (BSP)
•
24° and 60· included angle-Metric.
SAE and TIC standards are most commonly used in Canada.
Flareless joints
F1areless joints (see Figure 63) use an intermediate product to grip the
tubing. As the nut is tightened onto the end fitting, the intermediate product
is compressed. This causes it to bite into and press onto the tubing to create a
seal and hold it in place.
16
Figure 63 Flareless joints
Tube bending
Tubes for hydraulic systems must be bent carefully and precisely. When
bending tubes, consider the following factors:
•
To ensure a smooth bend, use proper bending tools and an appropriate
bend radius for the tubing size. Generally, the correct bend radius is
3 to 4 times the tube OD.
•
Use the bend radius to calculate the required cutoff length of tubing. See
Figure 64.
•
Bend the tube carefully to avoid distortion. See Figure 65. Any
flattening, kinks or wrinkles in the bends cause turbulence in the flow.
•
Leave a straight length of at least twice the nut length between a bend
and a fitting. This allows the connecting nut to slide away from the
filting when necessary.
•
Form bends properly so that the filtings are in alignment. Misaligned
tubes cause stress to the parts to be connected.
•
Allow for expansion or contraction due to temperature when short
lengths of tubing are used.
,,-t--
Radius of bend (R) is
measured from
centreline of tube
TubeO.D.
Nominal
Minimum
Bend
Radius R
318"
7116"
112"
518"
314"
718"
1"
1 118"
1 1/4"
1 114"
1 1/4"
1 1/2"
1 3/4"
2"
1 318"
3"
J 112"
3314"
5"
1 1/2"
5"
1 1/4"
Figure 64 Minimum bending radii
Figure 65 Tube bending
Tube installation
Basic tubing installation consists of the following procedure:
16-
o
Star! from a fixed poiut.
o
Use proper hydraulic fittings.
o
Use as few fittings as possible by making bends iu the tubing. (Every
fitting is a source of turbulence as well as a potential leak.)
o
Locate the simplest route with the least number of bends.
o
Make sure all joints or fittings can be easily reached for maiutenance.
o
Put the tube line where it will not be a hazard to workers.
•
Put the line where it does not iuterfere with any other equipment
Right
~Il
Wrong
•
Wrong
Wrong
(Tube cannot be removed easily)
Figure 66 Tube rouling
Hose
Hose is used to connect parts which move in relation to each other. or which
are subject to vibration. A hose has an accumulator action as it bulges
slightly with pressure surges.
Hose sizes are specified by ID, OD and dash number, corresponding to a
standard tube size. In most cases, the dash number also corresponds to the
nominal ID of the hose in sixteenths of an inch.
The main parts of a hose are the:
•
inner tube
•
reinforcement
•
outer protective cover.
Inner tube
The inner tube is usually synthetic rubber which can withstand temperatures
up to 133°C (275°P) for short intervals. Hose for higher temperatures is
available such as Teflon with a stainless steel braid reinforcement.
Reinforcement and pressure ratings
Various types of hydraulic hose are shown in Figure 67. Reinforcement
determines the pressure rating. Low-pressure hose has two or more fahric
braid reinforcements. It can withstand pressures from 250 psi to 500 psi
depending on its ID.
Hose with one metal wire hraid for reinforcement is called single-wire braid
and is good for working pressures up to about 1500 psi. Two-wire hraid hose
has a pressure rating of up to about 3000 psi. Substituting spiral wrap wire
for the wire braid brings the pressure rating still bigher-up to 6000 psi for
hoses with smaller ID.
-71
Nylon hose with nylon tubing, braided nylon reinforcement, and nylon cover
is rated between the two-fabric braid and the single-wire braid hosc. It has
high flex, fatigue and abrasion resistance.
SAE 100R3
Fabric braid
Single-wi re braid
Two-wire braid
'AEFIIII
@
Spiral wrap braid
Braided nylon
Fabric covers
Figure 67 Hydraulic hoses
Covers
Hose covers are usually neoprene. which has high resistance to oil, abrasion.
and weathering. Some hoses are supplied with oil-resistant fabric covers.
Hose end fittings
Hose end fittings are fastened to the hose in two ways:
•
with the cover left on-no skive
•
with the cover removed-skive
They are either permanent or reusable.
Permanent
Permanent fittings are crimped or swaged on the hose end and are discarded
with the hose. A hose crimping maehine is required to assemble the fittings.
Caution!
Ensure that the hose, jitting, and crimper are all compatible. Mixing parts
from different manufacturers may Cause connections to blow apart.
Teeth grip wire
No skive
!,",usnlcm grip
Figure 68 Permanent hose fittings
Reusable
Reusable fittings are screwed or clamped to the hose ends and salvaged
when the hose is discarded. Hose for reusable fittings can be purchased in
bulk and each section assembled as needed.
, screwed
Reusable, clamped
Figure 69 Reusable hose fittings
Hose installation
Install hose as follows:
•
Allow enough slack to avoid kinking the hose at a ridge connection.
•
Do not use a taut hose: pressure tends to bulge the hose and shorten it.
•
Do not twist the hose: this can be checked by markings on the cover (use
fittings to avoid long loops).
•
Follow specifications for minimum bend radius.
•
Install hose lines so parts can be easily reached for maintenance. In
Figure 70a, it is almost impossible to work a wrench between the two
fixed objects to tighten or loosen the nut. In Figure 70b the line is easier
to connect or disconnect.
•
Keep hoses from rubbing on fixed objects and keep moving objects from
rubbing on them. (This can be done by clamping or tying the hoses out
of the way, or by using hose guards.
•
Keep the hose away from high hcat sources. If the hose cannot be
moved. insulate it.
Figure 70 Recommended hose connections
Fittings and couplers for tube and hose
Fittings for tube and hose come in a wide variety of sizes and designs. See
manufacturers' catalogues for all available types. They are made to meet the
standards of the:
74
•
American National Standards Institute (ANSI), which is the hydraulic
industry standard
•
Society of Automotive Engineers (SAE), which is the automotive
industry standard
•
International Standards Organization (ISO), which is the international
standard.
Threaded fittings
Threaded ends for port connections can be NPT (tapered), NPTF (tapered
dryseal) Or NPS (straight). Due to the wrench size, threaded fittings are most
practical on tubing sizes up to %" diameter.
NPS threads are used when rotational orientation is important. See
Figure 71. A locknut and washer allows the fitting to be positioned in any
orientation and locked. An O-ring between the fitting and the housing creates
the seal.
Figure 71 Accurate alignment with NPS threads
NPS threads can also be used on fittings that do not need to be oriented in
any particular position. These have the O-ring under the hex or in a groove
on the face of the fitting. See Figure 72.
Figure 72 a-ring positions
Flanged fittings
For the same nominal size, both flange and split-flange fittings have the
same bolt-hole pattern. This makes them interchangeable.
For tubing sizes above %" diameter, split-flange fittings are used (see
Figure 73 on the next page). These fittings have 4 smaller fasteners to secure
the fitting to the housing and an O-ring on its face to seal it. When using a
split-flange fitting, the tubing is permanently attached to the flange member.
When a flanged fitting (Figure 74, next page) is used for pipe. it is not split.
The pipe is either welded to the flange or threaded into the flange.
-75
Bolt
(2 of 4 shown)
Tubing permanently
connected to flange
Clamp
half
Flange member
Figure 73 Split-flange fittings
Weld
Pipe --'~_ _.....
washer (optional)
Figure 74 Flanged fittings
%Bf\'!"'_H;rK2i";x-{m-_"l1;%8_n,'t~"g'bW'!.("'$,,~T%'%'2!.1@o~"'1%~~'h'1l1'do1m--,m1%,%1§jg"'lig:H:'J";I;mg:'ooOOd:'d:H%%%%n,lM%%-',*;3,'%'d%'1W#~W;~W;;X$oa",'%'oa.!!'-M'\Wd:'Mgm,g;;n,~a;m"w:W';;"''%''G*''{,'%'l$1.~*}'%.\',';l;h,,*%'%t\t1
16 - 76
MILLWRIGHT-HYDRAULIC SYSTEMS
Quick disconnect couplers
Quick disconnect couplers are often used where hoses are meant to be
removed periodically. Various styles are available. Couplers from one
manufacturer may not necessarily mate with those from another
manufacturer. Maintain consistency throughout the plant for best results.
All couplers function in basically the same way. They seal hose ends when
they are not coupled and allow hydraulic fluid to pass through them when
coupled. Figure 75 shows an example of the operation of a quick disconnect
coupler.
Figure 75 Quick disconnect coupler
Symbols used in
hydraulic circuits
Drawings of hydraulic circuits use standard symbols that are shown
throughout this Chapter. The following tables display them together.
Table 3: Basic hydraulic and pneumatic symbols
ISO symbols
Definitions
Main line conductor, outline, and shaft
Pilot line
Drain line
Mechanical connection
Enclosure outline
o
Energy conversion unit (pump, motor, etc.)
D
Semi-rotary actuator
DCJ[IJ
I I I I
Control valves (except non-return valves)
I
<>
Conditioning apparatus (filter, lubricator, etc.)
Flow lines or conductors connected
Row lines or conductors crossing but not connected
Spring
Restriction affected by viscosity
V
f\
Restriction unaffected by viscosity
Hydraulic flow
Pneumatic flow
tt !
Arrows indicating direction
((
Arrows Indicating rotation
! 1J t
Arrows indicating path and direction through valves
/
Indication of variability
78
Table 4: Symbols for pumps and compressors
ISO symbols
Definitions
Unidirectional, fixed-capacity, hydraulic pumps
Bidirectional, fixed-capacity, hydraulic pumps
Unidirectional, variable-capacity, hydraulic pumps
Bidirectional, variable-capacity, hydraulic pumps
Fixed capacity compressor
Table 5: Symbols for motors
ISO symbols
Definitions
Unidirectional, fixed-capacity, hydraulic motor
Bidirectional, fixed-capacity, hydraulic motor
Unidirectional, variable-capacity, hydraulic motor
Bidirectional, variable-capacity, hydraulic motor
Unidirectional, fixed-capacity, pneumatic motor
Bidirectional, fixed-capacity, pneumatic motor
Unidirectional, variable-capacity, pneumatic motor
Bidirectional, variable-capacity, pneumatic motor
Hydraulic, oscillating motor
Pneumatic, oscillating motor
79
Table 6: Svmbols for pump/motor units
ISO symbols
Definitions
Fixed-capacity unit with reversible flow direction
Fixed-capacity unit with single flow direction
Fixed-capacity unit with reversible flow in two directions
Variable-capacity unit with reversible flow direction
Variable-capacity unit with single flow direction
Variable-capacity unit with reversible flow in two directions
Table 7: Symbols for cylinders in hydraulic or pneumatic systems
ISO SYMBOLS
Simplified
Detailed
l!§
1'1-
~
~
g:
tEt
:§§
1'11-\
Definitions
tE-
Single-acting cylinder, returned by unspecified force
Single-acting cylinder, returned by spring
~
~
Double-acting cylinder
Double-acting cylinder with double-ended piston rod
I
II
rg
e
tyg
~
Old symbol
~
~
~
~
E-
,
DO
Cylinder with Single, fixed cushion
Cylinder with double, fixed cushions
Cylinder with single, adjustable cushion
Cylinder with double, adjustable cushions
Single-acting telescopic cylinder
Double-acting telescopic cylinder
~
~
~
Differential cylinder
!
I I :
I t- Press\lre
intensifier for one type of fluid
(shOWIng as pneumatic)
Pressure Intensifier for two types of fluid
Air-oil actuator
Table 8: Symbols for control valves
ISO sylllbo=ls=----_ _ _ _ _c:D:::ef""'I""'n:.::ltl:;:o"'n"'s_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
Simplified symbol for valves in cases of multiple repetition
One flow path through valve
Two closed ports
Two flow paths
Two flow paths and one closed port
Two flow paths with cross-connection
One flow path in a bypass position and two closed ports
OJ
I I
Two-position, non-throttling directional control valve (DCV)
Three-position, non-throttling DCV
Non-throttling DCV with transitory intermediate conditions
IT]
II I I
Throttling DCV
Free, non-return valve (check valve)
Spring-loaded check valve
Check valve, pilot operated to open
Check valve, pilot operated to close
Check valve with restriction
Shuttle valve
Rapid exhaust valve
16 82
Table 9: Symbols for pressure-control valves
ISO symbols
Definitions
~
~
One throttling orifice, normally closed
$
QJ
One th rottling orifice, normally open
QJ
Two throttling orifices, normally closed
f~
'.
Hydraulic pressure relief valve (safety valve)
f~
Pneumatic pressure relief valve (safety valve)
'-
f~-
Pilot-operated, hydraulic pressure relief valve
f~-
Pilot-operated, pneumatic pressure relief valve
'-
)
'-
Table 10: Symbols for flow-control valves
ISO symbols
Definitions
Throttle valve
Manually controlled throttle valve
Mechanically controlled throttle valve
Flow dividing valve
Old
QTIOff
Shut-off valve
crfJon
16-
Table 11: Symbols for energy sources and connections
==-_____--'De=f:_::inltlona
ISO sYlllbola
0-+-
Hydraulic pressure source
@-l>-
Pneumatic pressure source
Old
®=
~
'-...)'
®
OJ
Electric motor
Heat engine
Flexible hose, usually connecting moving parts
1.
Electric line
y
Exhaust port with no provisions for connection
~
Exhaust port, th readed for connection
~
-7<~
Power take-off with a plugged port
Power take-off with a take-off line
)I(
Quick-release coupling without non-retum valve (connected)
~
----7i
Quick-release coupling with non-return valve (connected)
Quick-release coupling without non-return valve (disconnected)
-9+-
Quick-release coupling with non-return valve (disconnected)
0
One-way rotary connection
Three-way rotary connection
Silencer
Table 12: Symbols for reservoirs
ISOsy~=b=o~ls~
__________-=D~e~fj~n~lti~o~n~s__________________________________________~
Vented reservoir
Reservoir with inlet pipe above fluid level
Reservoir with inlet pipe below fluid level
Reservoir with a header line
Pressurized reservoir
o
Accumulator with Its fluid maintained under pressure:
Old
~
• spring loaded
Q
• gas charged
~
• weighted
Table 13: Symbols for miscellaneous apparatus
ISO symbols
Definitions
Filter or strainer
Manually drained water trap
Automatically drained water trap
Filter with manually drained water trap
Filter with automatically drained water trap
Air dryer
Lubricator
,------------1
i,
(9,
,I
i'
:
:
'
,
Conditioning unit (detailed)
,_ _ _ _ _ _ '--:' _ _ _ _ 1
-1 :(S) l-
-+
-+
-+
16
86
Conditioning unit (simplified)
Temperature controller
Cooler
Heater
Table 14: Symbols for control mechanisms
ISO symbols
Definitions
Controlled by operator pushing a button
Controlled by operator using a lever
Controlled by operator using a pedal
Mechanically controlled by a plunger or tracer
Mechanically controlled by a spring
Mechanically controlled by a roller
Mechanically controlled by a roller operating in only one direction
Electrically controlled by a solenoid with one winding
Electrically controlled by a solenoid with two opposed windings
As above, with variable progression
Old
~
~
Controlled by a reversing electric motor
- -»- -[
Direct control applied by hydraulic pressure
- -I>- -[
Direct control applied by pneumatic pressure
--.-[
Direct control released by hydraulic pressure
- -<I- -[
Direct control released by pneumatic pressure
---IC}--Di rect control by different control areas
- --lE[
Indirect control (pilot actuated) applied by hydraulic pressure
- --1E[
Indirect control (pilot actuated) applied by pneumatic pressure
... continues
Table 14: Symbols for control mechanisms
ISQsymbols .
- --a:::[
... continued
08flnltlons
Indirect control (pilot actuated) released by hydraulic pressure
- --a::[
Indirect control (pilot actuated) released by pneumatic pressure
:··1
'·L
Interior control paths
Combined control by solenoid and hydraulic pilot directional valve
Combined control by solenoid and pneumatic pilot directional valve
Combined control by solenoid or hydraulic pilot directional valve
Combined control by solenoid or pneumatic pilot directional valve
Table 15: Symbols for supplementary equipment
ISO sym
......
bo...ls"""----_ _ _ _ _0::.8
...1...ln..II..lo...n,..8____
Pressure gauge
Thermometer
Flow meter
Integrating flow meter
Pressure electric switch
Hydraulic circuits
To be able to design, build andlor troubleshoot hydraulic systems, it is vital
for the millwright to read and understand hydraulic circuit drawings.
Circuits are a combination of components, to do a particular task. A
complete circuit shows what each component will do when the pump is
started.
When designing a system, the starting point is usually the work to be done.
The decisions that must be made are:
I. which actuator is to be used, and the valves to control it.
2. the size and type of pump which will supply the required flow and
handle the necessary pressure
3. the size of the reservoir
4. any other component needed for the system.
Circuit reading means following the flow of fluid from the pump to the
actuator. This allows you to see what is open to fluid flow and what is closed
to fluid flow.
By shifting the DCV, you can examine which components function and
determine any changes in fluid flow or pressure. A bill ofma/erial gives the
make, model, size and other details of each part. The symbols representing
the parts are generic (not specific).
A solid triangle in the line indicates the direction that the hydraulic fluid is
flowing:
.... hydraulic flow
(An outlined triangle indicates pneumatic flow.)
When a portion of the circuit is to be shown, a symbol representing the
pressure source is used:
0-+- hydraulic pressure source
In a circuit, the reservoir symbol may be repeated throughout to clarify the
flow path back to the main reservoir.
Caution!
No matter how many reservoir symbols are used, there is normally only one
reservoir in the system.
On the following pages, some common circuits are described.
Automatic bleed-down circuit for accumulators
Automatic bleed-down cireuits ensure that the system can accumulate
pressure only to a set limit. An unloading valve then feeds the flow back to
the reservoir at low pressure.
, - - - -....- - - - - - - - -..... To system
Figure 76 Automatic bleed-down circuit
1. This circuit has an unloading valve between the pump and the
accumulator. The unloading valve has a built in check valve and a
remote pilot line which senses down stream pressure.
2. As the pressure exceeds the recommended limit, the pilot line activates
the unloading valve and allows the flow to be directed back to the
reservoir.
3. The check valve keeps the flow towards the accumulator.
4. When the accumulator reaches its charged pressure, the unloading valve
allows the continuous flow to return to the reservoir at a low pressure.
The unloading valve is a solenoid-activated, normally-open, 212 valve.
5. The 2/2 valve allows the system to drain when the pump is turned off.
When the power to the pump is on, this valve closes and allows the
system to build up pressure.
90
High-low circuits
High-low circuits are used when rapid advance of the actuator and constant
high pressure are required. Several methods may be used.
Method!
1200
PSI
~I
(b)
5gpm
Figure 77 High-low circuit using two pumps on the same drive
Pump (a) in Figure 77 delivers 20 gpm. It has an unloading valve which is
set at 500 psi. Pump (b) delivers 5 gpm and has a pilot-operated relief valve
which is set at 1200 psi. A 4/3 DeV controls a dOUble-acting cylinder.
1. When the cylinder rod advances or retracts under no load, both pumps
supply it with a large volume of flow at low pressure.
2. When the cylinder rod meets resistance, pressure builds.
3. The unloading valve allows all the flow over 500 psi, from pump (a) to
dump back to the reservoir.
4. Pump (b) then builds and maintains pressure of 1200 psi with a volume
of 5 gpm in the system.
5. When the system reaches 1200 psi, the excess fluid dumps over the
maximum pressure relief valve until the Dev is shifted.
Method 2
Another option to achieve a high-low system is by using two pumps with
two separate motors and a pressure switch connected to the ON/OFF control of
motor (a). See Figure 78.
To system
•
....... I-----~
(a)
20gpm
(b)
5gpm
Figure 78 High-low Circuit using separate drives
6
92
Method 3
A third option for a high-low system is by using a pressure-compensated,
variable-capacity pump. See Figure 79. Hydraulic presses often use this type
of high-low system.
To system
Figure 79 High-low circuit using a pressure-compensated, variable-capacity pump
Hydrostatic drive circuits
Hydraulic motors used with hydraulic pumps are called hydrostatic drives. A
hydrostatic drive has either an open or closed circuit.
Open circuit-Method 1
An open circuit returns the flow that moves through the motor back to the
reservoir. Figure 80 shows a circuit for a hydrostatic drive with an open
system.
... .........
,
Figure 80 Open circuit for a hydrostatic drive with tandem-centre DCV
This circuit has a unidirectional, fixed-capacity pump with a pilot-operated
pressure relief valve supplying the system. This circuit creates motor output
with fixed speed and torque.
A 413 Dey is used to change direction and stop the motor. The Dey has a
tandem centre to hold the motor and to return the flow to the reservoir when
the valve is in neutral. [n this system the reservoir must be large enough to
handle the continuous recirculation of the hydraulic fluid. The reservoir also
acts as a cooler for the hydraulic fluid.
With closed or tandem centre DeV s, high pressure surges can result when
the valve is suddenly shifted or if a large external force is applied to the
motor output shaft. This problem is dealt with by the use of check valves and
pilot-operated pressure relief valves. These are set slightly higher then the
main (pump's) relief valve, on both sides ofthe motor. Any excess pressure
can then be transferred to the other side of the motor.
Open circuit-Method 2
An alternate design to create a variable drive, uses a variable capacity pump
or motor. It uses an open eentre DeV to relieve any pressure build up. A
brake valve is shown in the exhaust lines on both sides of the motor to
prevent unintentional rotation. See Figure 81. The brake valve is piloted
from the supply and the exhaust lines .
..,
'..
(
Cross line
relief vslves
•
. .. ..··
......... .i:
U~ ..............
......
.:
rl=~=t---r .
'
~
'
~~
r-----
Bmke
valves
Figure 81 Hydrostatic drive using an open·centre DeV
Closed circuit-Method 1
A closed circuit 8end~ the flow that moves through the motor directly back
into the inlet of the pump. Figure 82 shows a closed circuit, hydrostatic drive
which operates without a DCV.
Replenishing
check valve
Reversible
pump
Replenishing
pump
Fixed
displacement
motor
,,
~
Replenishing
relief valve
Overload
relief valve
Figure 82 Closed circuit for a hydrostatic drive
A reversing, variable-capacity pump regenerates this system. Pilot-operated
pressure relief valves are used to relieve any excess pressure in the system.
Check valves are used to ensure the correct direction of flow and to feed the
pressure relief valves.
Because pumps and motors are not 100% efficient. some hydraulic fluid
leaks past the pump and motor(s) and back to the reservoir. A replenishing
pump is required to maintain a fully supplied system. The replenishing pump
supplies the fluid to the low-pressure side of the system.
Due to the small amount of hydraulic fluid that returns to the reservoir, a
much smaller reservoir is required than in an open circuit. Due to the smaller
reservoir, there is no means of dissipating heat through the tank. Usually a
heat exchanger is added to the circuit to keep the fluid temperature at the
proper level.
Closed circuit-Method 2
An alternate desigu of this circuit uses a unidirectional, fIxed-capacity pump
and a reversing, variable capacity motor.
Closed circuit-Method 3
Closed circuits can also usc a double-acting cylinder with a double-ended
piston rod. These allow for reversing, linear movement. They are found in
equipment such as planer beds.
Sequencing circuits
Sequencing valves are used to control the order of operation of two or more
actuators. Figure 83 shows a sequencing valve controlling two cylinders.
600 psi
r---'-'
!
!
.I
I
._-----,
I,
-',, I,
..,, I
.. ...... '
I,
.--~
1'-----'1
if,·····:
,,
,,
i,
Figure 83 SequenCing circuit
97
1. The sequencing valve is set at 600 psi so that the flow through the valve
is blocked until the pressure reaches 600 psi.
2. When the DCV is shifted to extend the cylinders, the first cylinder
moves until it reaches a resistance of 600 psi.
3. Then the sequencing valve opens to allow the flow to extend the second
cylinder.
4. The first cylinder maintains pressure on the work while the second
cylinder moves. In this circuit, the retractions of the cylinders are at the
same rate. (Flow control valves used for metering out may be used to
control the retraction of the cylinders.)
Metering circuits
The rate of one or more actuators may be controlled by means of meter-in,
meter-out, or bleed-off circuits. The most accurate are the meter-in and
meter-out circuits.
Meter-in and meter-out circuits
•
A meter-in circuit consists of a flow control valve with a check valve
(for free return flow). They are located in the pressure line to the
actuator. See Figure 84. This method is recommended where the force is
always against the actuator. The disadvantage of a meter-in circuit is that
it could run away in the event of an overhauling load-that is, the inertia
of the machine could overcome the actuator.
•
A meter-out circuit (Figure 85) consists of the same valving, but located
in the discharge line from the actuator. This circuit is commonly used
where there is a tendency of the load to "run away."
Figure 84 Meter-in circuit
Figure 85 MeIer-out circuit
Bleed-otT circuits
A bleed-off circuit (Figure 86) consists of a flow control valve located off a
tee in the pressure line to the actuator. This method removes a set volume
from the pressure line regardless of the flow in the system. Because it
controls the flow returning to the reservoir, any fluctuation in flow or
leakage witbin the system varies the perfonnance of the actuator(s). This
circuit is used where the load on the actuator(s) is relatively consistent.
Figure 86 Bleed-off circuit
Counterbalancing circuit
When excessive forces act on actuators to pull or override their function,
counterbalancing valves are used to control their movement. Figure 87
shows an example of a weighted platen hanging from a cylinder.
Counterbalance
valve
Platen
Figure 87 Counterbalance valve in a circuit
Gravity works to extend the piston. If this is allowed to happen the cylinder
acts as a pump, drawing fluid into it. The counterbalance valve is placed in
the exhaust line of the cylinder and controls the pressure leaving the
cylinder.
Multiple actuator circuits
Circuits using multiple actuators can be controlled in a variety of ways
depending on the task to be done.
•
When the actuators must be controlled independently from each other,
flow dividers are used.
•
When one DCY is used to control multiple actuators, the actuators are
arranged in series or in parallel.
Flow dividers
Flow dividers are normally located between the pump and the DCYs. Their
flow may be fixed or variable. Figure 88 shows a three-port valve with two
internal functions. The flow to either DCY is adjusted separately. Any
excess pressure is returned to the reservoir through the main relief valve .
.-------,,
Flow dividers
Figure 88 Variable flow divider
16-100
Series circuit
Figure 89 shows two identical motors, arranged in series after the Dey. In a
series cireuit the speeds of the motors dare the same, regardless of their
illdividualloads. The pressure available to each motor is proportional to the
load on each. The available torque is equal to that obtained by a single
motor.
Figure 89 Two motors in a series circuit
Parallel circuit
Figure 90 shows two identical motors, arranged in parallel after the Dey. In
a parallel circuit the speed of the motors is divided. The speed varies
depending on the load. If the load on either motor is the same, then they
rotate at half the speed of a single motor. The available torque to each motor
is double that of a single motor.
P
T
Figure 90 Two motors in a paraliel circuit
16
101
Troubleshooting
hydraulic systems
Troubleshooting hydraulic systems requires a logical approach. Due to the
high potential energy in the system, safety is the top priority. Successful
troubleshooting begins with creative conununication with the operator
(human or automated). It continues with identification and isolation of the
problem, safe shut-down of the equipment, clean and orderly removal and
repair of the problem, and follow up on the repair.
Communication
Discuss the problem with the operator. The operator is able to tell you how,
when, and where the problem began. The operator may also be able to show
you what is or is not happening. Then try the operation yourself.
Isolation and identification of the problem
Isolate the problem by taking pressure and flow readings along the system.
Remember that:
•
The pump creates flow. The loss of speed is a reduction in volume.
•
Resistance creates pressure. The loss of force is a reduction in pressure.
Refer to the schematic or blueprint of the system to help analyze the
problem. Use your senses-for example, mechanical problems are often
detected by sound such as a noisy pump. Use troubleshooting charts to
identify the problem.
A
Caution!
Never pass your hand over a suspected leak. The force with which hydraulic
fluid may leave a system Is great enough to pierce your skin.
Shut-down
Shut down the equipment safely by doing the following:
•
Lower or mechanically secure all suspended loads.
•
Release all pressure in the system.
•
Discharge all accumulators and intensifiers.
•
Isolate the electrical control system and power supply using correct
lockout procedures.
Table 16: Excessive noise
Trouble
Cause
Correction
Cavitation in the pump
Clogged strainer
Clean inlet strainer
Dirty filter
Replace filter
An obstruction in the inlet line
Eliminate obstructions
Fluid viscosity too high
Replace system fluid
Operating temperature too low Warm up system before operation
Excessive pump speed
Change pump drive motor speed to
manufacturers specifications
Air in the fluid
Fluid level too low
Fill reservoir to proper level
Leaky or damaged inlet line
Tighten or replace inlet line
Damaged shaft seals
Replace seals
Check shaft for damage
Remove & repair shaft
Damaged pump parts
Damaged motor parts
Air trapped in the system
Bleed system
Worn out due to extended use
Overhaul pump
Coupling misalignment
Align drive with pump
Cavitation
Examine seals, bearings, coupling
Wom out due to extended use
Overhaul motor
Coupling misalignment
Align motor with driven equipment
Examine seals, bearings, coupling
Relief valve noise
16
104
Setting too close to another
valve setting
Adjust pressure setting
Worn poppet and seat
Overhaul or replace component
Table 17: Excessive heat
Trouble
Cause
Cavitation in the pump
Same as Table 16
Same as Table 16
Air in the fluid
Same as Table 16
Same as Table 16
Excessive load on pump
Relief or unloading valve
Decrease pressure setting
or motor
set too high
Rise in fluid temperature
Correction
Worn bearing
Replace bearings & seals
Mechanical binding
Locate & correct mechanical binding
Pump coupling misaligned
Realign drive with pump
Motor coupling misaligned
Realign motor with driven equipment
System pressure too high
Decrease relief valve setting
Dirty fluid or low fluid level
Replace dirty filters
Clean strainers
Change system fluid
Fill to required level
Incorrect fluid viscosity
Change filters & check fluid viscosity
Change if necessary
Faulty cooling system
Clean cooler lines
Check &lor replace control valve
105
Table 18: Incorrect flow
Trouble
Cause
Correction
No flow
Pump not receiving fluid
Replace dirty filters
Clean strainer & Inlet line
Overhaul or replace drive motor
Pump drive motor faulty
Pump to drive coupling sheared
Pump turning in wrong direction
DCV in the wrong position
Low flow rate
Check for damaged pump or drive
Replace and realign coupling
Reverse direction
Check position & move accordingly
Adjust selting on relief valve
All flow passing over relief valve
Damaged pump
Overhaul or replace pump & realign
Flow control set too low
Adjust flow control valve
Relief or unloading valve set 100 low Adjust relief or unloading valve
Flow bypassing thru' partly open valve Overhaul or replace valve
EXcessive flow
External leak In the system
Tighten leaky connections
Variable capacity pump Inoperative
Bleed air out of the line
Overhaul or replace pump
Incorrect pump speed
Internal leakage due to worn parts
Determine correcl speed & adjust
Overhaul or replace un~
Flow control valve set too high
Variable capacity pump Inoperative
Decrease flow setting
Overhaul or replace pump
Incorrect pump speed
Incorrect pump size
Determine correct speed & adjusl
Replace pump
Realign with drive
Table 19: Incorrect pressure
Trouble
Cause
Correction
No pressure
No flow
Replace di rty filters
Clean strainer & inlet line
low pressure
Erratic pressure
Pressure reducing (PRJ valve
set too low
Adjust valve setting
Pressure reducing valve damaged
Overhaul or replace valve
Damaged pump or actuator
Overhaul or replace pump or actuator
Air in the fluid
Tighten leaky connections
Worn relief valve
Overhaul or replace valve
Contamination in the fluid
Replace dirty filters
Bleed air out of the line
Change system fluid
Defective accumulator
Overhaul or replace accumulator
loss of charge in accumulator
Check gas valve for leakage.
Recharge to correct pressure
Excessive pressure
Pressure reducing, relief, or
unloading valve mlsadjusted
Adjust to the correct setting
Pressure reducing, relief or
unloading valve worn or damaged
Overhaul or replace valve
Variable capacity pump inoperative
Overhaul or replace pump
16
Table 20: Faulty operation of the actuator
Trouble
Cause
Correction
No movement
No flow or pressure
Replace dirty filters
Clean strainer & inlet line
Limit or sequence device inoperative
Mechanical bind
Overhaul or replace components
Slow movement
No command signal to servo-
Locate bind and repair
Repair command console or
amplifier
interconnecting wires
Inoperative servo valve
Wom or damaged actuator
Overhaul or replace servo valve
Low flow rate or pressure
Fluid viscosity too high
Overhaul or replace actuator
See Table 18
Check fluid temperature.
Check sys1em's fluid viscosity
Erratic movement
Excessive speed
or movement
No lubrication on moving parts
Change fluid if necessary
Lubricate moving parts
Sticking servo valve
Wom or damaged actuator
Clean & adjust or replaca valve
Overhaul or replace actuator
Erratic pressure
Air in the fluid
See Table 19
No lubrication on moving parts
See Table 16
Lubricate moving parts
Erratic command signal
Repair command console or
Repair interconnecting wires
Misadjusted or malfunctioning
servo amplifier
Adjust, repair or replace servo amplifier
Malfunctioning feedback transducer
Overhaul or replace transducer
Sticking servo valve
Worn or damaged actuator
Clean and adjust or replace valve
Overhaul or replace actuator
Excessive flow
Malfunctioning feedback transducer
See Table 18
Overhaul or replace transducer
Misadjus1ed or malfunctioning
servo amplifier
Adjust, repair or replace servo amplifier
Runaway or overhauling
Adjust, repair or replace
counterbalance valve
Troubleshooting cylinders
The most common problems with cylinders are leakage and rod damage.
Leakage
A cylinder has various areas of leakage that may he visible or invisible.
The visible leaks may be:
•
at the packing gland-This may be caused by worn packing (due to old'
age), lack of luhrication, over tightening. damage due to a damaged rod,
or to solids sticking to the packing.
•
between cylinder walls and the cap or base-This may be caused by the
improper or uneven torquing of the tie rods, or by damaged or missing
gaskets or seals.
The invisible leaks may be:
•
between the piston and the cylinder walls-This may be caused by
damaged piston seals or by scored or grooved cylinder walls.
•
between the piston rod and the piston (not very common).
Invisible leaks can be checked by breaking the fluid line from one side of the
piston and pressurizing the other side. Take the following precautions:
•
Use a container to collect the leaking fluid
•
Use the correct valve position
•
Keep the broken line and part clean
•
Connect the line properly after the test is finished.
Rod damage
Rod damage may be in the form of surfaee damage or bending.
Surface damage may be caused by:
•
impact or hammering on the rod
•
rough spots due to rusting - usually from exposure to the atmosphere,
such as after extended shutdown or outside use
•
the chrome plate breaking or peeling off
•
pipe or chain wrench marks on the surfaee due to sloppy maintenanee.
The bending of a cylinder rod may be caused by:
•
an overload in compression
•
an accident with moving equipment
•
the use of a too small rod diameter,
16
109
Troubleshooting pumps and motors
Pumps and motors of the same style have similar problems. The common
problems are:
•
intcmalleakage past their rotors
•
damaged shaft bearings and seals due to misaligrunent
•
extemalleakage at the housing or fittings.
When maintaining a pump or motor refer to the manufacturer's service
manual and maintain a clean work environment.
Caution!
Handle any spring-loaded part carefully. It may fly apart if the spring
control is lost.
Troubleshooting valves
Common valve problems are:
•
Spool sticking due to
foreign material lodged between the spool and the valve body
- broken spring (if spring-positioned)
detents have come out of position
•
Spool not shifting duc to a burned-out solenoid
•
Excessive leakage due to
- defective O-rings
- scored spool or valve body.
Operational maintenance is not easily done on four-way valves. If a valve is
giving trouble, replace it and overhaul it later.
Valve mounting
The way valves are mounted affects the ease with which they may be
rep1aced. Valves are usually mounted in one of three ways:
10
•
an individual valve with foot mounting-The valve is fastened to a flrm,
flat base and lines are connected to the valve bedy. This makes the valve
difflcult to replace.
•
an individual valve with mounting plate or sub-plate mounting-Tbe
rigid sub-plate is bolted to a rigid support,. It has tapped holes or the
equivalent for connecting to the fluid lines. Sealing between the subplate and valve body is done by O-rings or equivalent seals. Replacing a
valve can be done without touching any of the line connections.
•
stacked valves-A series of special valve bodies clamped together side
by side with a common internal supply and a drain to the tank. These are
used mainly on mobile equipment.
Troubleshooting solenoid pilot valves
Potential problem areas are:
•
sticking spool
•
loose coil
•
broken armature
•
broken spring
•
worn pin between the armature and the spool
•
speed control adjustment on pilot-operated valves.
Overhauling valves
Before working on valves, try to obtain the manufaetorer's service manual
and/or parts list. This is important for pilot-operated or solenoid pilotoperated control valve.
Valves are usually disassembled by removing the end caps and extracting the
spool and other parts from the body.
A
Caution!
Check springs to see if they are loaded before removing the spring keeper.
•
If no service manual is available, pay careful attention to the position of
the spool. It works properly only when installed in one direction.
•
Replace a-rings and seals at every overhaul. Coat them with hydraulic
fluid before installing.
•
If the valve is put away in storage for future use:
- Lubricate all internal components to prevent rusting (check on rustproofing material when using synthetic fluids).
- Seal off all ports or openings to keep out contaminants.
C"')
::::r
-.......
E»
'CI
CD
......
MILLWRIGHT MANUAL: CHAPTER 17
Pneumatic Systems
Pneumatic theory and laws ............................................................. 17: 1
Pressure scales and measurement ...•..... ........... ......... ........... ........... 17: 1
Properties of compressed air ..... ......... .................... ........... ............. 17:5
Vacuum pumps ............................................................................. 17:6
Pneumatic compressors ................................................................ 17:7
Maximum-pressure control............................................................ 17:8
Reciprocating compressors ............................................................. 17:9
Rotary compressors ........................................................................ 17: 11
Air treatment ................................................................................. 17:15
Intake filters .................................................................................... 17: 16
Intercoolers ..................................................................................... 17: 16
Conditioning compressed air ...... ....... ...... ................. .................... 17 :20
Airdryers ........................................................................................ 17:21
Air filters ........................................................................................ 17 :22
Pressure regulators .......................................................................... 17 :24
Air lubricators ................................................................................. 17:24
FRL (filter, regulator, lubricator) units ........................................... 17:26
Pneumatic valves and accessories ................................................ 17 :27
Pneumatic DCVs (directional control valves) ................................ 17:27
Flow control valves ........................................................................ 17:30
Mufflers (silencers) ......................................................................... 17:32
Pneumatic actuators ...................................................................... 17:33
Pneumatic conductors and fittings ................................................ 17:33
Pneumatic hose ............................................................................... 17: 34
Routing and installation of pneumatic lines ................................. 17:36
Pneumatic line routing .................................................................... 17:36
Pneumatic line installations ............................................................ 17:38
Pneumatic symbols and circuit~ ................................................... 17:39
Symbols .......................................................................................... 17:39
Pneumatic circuits .......................................................................... 17:40
Maintaining and troubleshooting pneumatic systems .................. 17:43
Maintenance ................................................................................... 17:43
Pneumatic Systems
Systems that use gas for transmitting force are called pneumatic systems.
Their design is similar to that of hydraulic systems. The general differences
are that the fluid medium used is commonly air (which can be compressed)
and the supply is the atmosphere.
The main eomponent of a pneumatic system is the pneumatic compressor.
Pneumatic compressors are used to compress atmospheric air, reducing its
volume and increasing its pressure. This pressure can be transmitted through
the system and used to do work such as driving equipment.
Pneumatic theory
and laws
Many of the theories and laws applied in hydraulic systems are aiso applied
in pneumatic systems. These laws apply to fluids which include gases as well
as liquids. For example Pascai's Law, Bernoulli's Principles, and the Law of
Conservation of Energy all apply to air as well as to liquids. For information
on these laws and other related matters, see Chapter 16: Hydraulic Systems.
Pressure scales and measurement
Absolute temperature and pressure
There are absolute scales of pressure and temperature. Absolute scales start
at absolute zero. At absolute zero of temperature, all molecular action in a
gas eeases and a perfect vacuum occurs.
Temperature
In regard to temperature, absolute zero is where no molecular movement
occurs. The absolute scale that uses degrees Celsius is the kelvin scale. The
absolute scale that uses degrees Fahrenheit is the Rankine scale. Zero
degrees kelvin and zero degrees Rankine are the same, equalling -273°C and
-459°F respectively.
Pressure
At absolute zero, there is no pressure-pressure is zero pounds per square
inch absolute (0 psia). At sea level, and under normal conditions of
temperature (I5.5°C or 600F) and humidity, the atmospheric pressure
(caused by the weight ofthe air) is approximately 1 bar or 14.7 psia (see
"Pressure" in Chapter 16: Hydraulic Systems).
Pressure is also sometimes measured in millimetres or inches of mercury
(mm Hg or "Hg). Note that 2.04"Hg equals I psia. See the description of
barometers below.
Gauge pressure
Under atmospheric pressure at sea level, pressure gauges read zero (0 psig).
In gauge pressure, atmospheric pressure is disregarded. Adding 14.7 to a
psig reading gives you the psia value.
Vacuum
Vacuum occurs when the pressure in a container beccmes less than the
pressure surrounding it (normally atmospheric pressure). On a pressure
gauge this occurs when the gauge pressure becomes less than 0 psig (less
than 14.7 psia). A perfect vacuum is equal to 0 bar (0 psia). When there is a
perfect vacuum in a system, the maximum force in the system caused by the
vacuum is 1 bar (14.7 psi).
Barometers
Vacuum is often measured with a barometer. It is measured in mm Hg or
"Hg. A simple barometer consists of a tube (of any diameter) which is sealed
at one end. The tube is completely filled with mercury and stands vertically
with the open end placed in a pool of mercury. See Figure I. As the pressure
on the pool of mercury changes so does the height of men:ury.
Vacuum
"Hg
Atmospheric
.pressure.
Figure 1 Principle of a barometer
1
At sea level, the tube must be more than 765 mm or 30" high. This is
because the atmospheric pressure at sea level can support a column of
mercury with height of 760 mm or 29.92".
In areas where the atmospheric pressure is less then I bar (14.7 psia), the
achievable vacuum is also less. It is harder to make the pressure less than
atmospheric pressure. A general rule of thumb is that, for every 1000 ft of
altitude above sea level, the atmospheric pressure drops 1 "Hg.
Comparison of scales and calibration
Figure 2 shows the comparison of the gauge scale, the absolute scale, the
barometer scale, and the vacuum scale.
PSIA
(pounds per
square IliCh)
(bar)
(kPa)
Perfect vacuum
)
Feet of oil
absolute
(poundS per
scale
(inches of mercury) (metres)
square Inch gauge) (inches af mer('lUry (mbar)
(bar)
abSOlute)
(kPa)
(mbar)
0
(0.00)
(0.00)
-15
5
(0.34)
(34.47)
10
(0.6S)
-10
(-MS)
(-00.94)
-5
(-0.34)
1(-34.47)
(00.94)
14,7
1 atmosphere absolute
(atmospheric pressure)
PSI gauge
(1.01)
(101.34)
IN.HGABS.
barometer
scale
(~1 ,OS)
•• •
:
HOS.(1) I
0
(0.00)
(0.0)
0
0
0
0
,
•
0
0
•
•0
0
0
0
,, •
,
,
,
0
0
0
•
0
0
0
0
0
0
0
0
0
0
0
(0.00)
29.92
(1013.21)
0
(0.00)
10
(338.64)
20
12
(677.21)
(3.65)
20
I (677.29)
10
(336.64)
24
(7.31)
29.92(30)
(1013.21)
(1015.92)
0
(0.00)
29,4
44.1
2 atmospheres absolute (3.04)
(304.06)
2 atmospheres gauge
14.7
(1.01)
(10\.34)
Feet of water
absolute
(bar)
37
(11.26)
,,
,
••
0
0
0
0
0
•0 00
0
0
0
0
0
0
,
,, ,
0
(0.00)
11112
(0.34)
,•
,,
,
,• ,
22213
(0.67)
,
34
(1.01)
0
0
0
0
•• ,•
0
0
•
(60)
0
•0 •0
0
0
•• 0,•
•0• 00
0 •
0
29.4
(2.02)
(90)
(202.6B)
{3047.75) :
0
0
0
0
0
0
0
0
0
0
0
, ,
,•
0
•
0
0
0
0
0
,• ,•
•
•
0
0
,
r0
0
0
0
0
0
0
,, ,,
(2031.63} :
0
0
0
0
, •
•
,,•
0
0
2 atmospheres absolute (2.02)
(202.66)
1 atmosphere gauge
IN.HG
vacuum scale
,
74
(22.55)
6.
111
(33.63)
10.
(3.04)
(2.03)
f--
-
indicates that the scale in not used
in this range. Values are shown
lor comparison only.
Figure 2 Comparison of pressure scales
Gauges are calibrated in various ways to measure pressure and/or vacuum.
Figure 3 on the next page shows a few different types of gauges and their
face dials. The gauges are designed to move in a clockwise direction as the
pressure increases.
Vacuum scale
Absolute pressure
scale
Gouge pressure
scale
CombInatIon
vacuumfgauge
pressure scale
Figure 3 Various gauges
17
Properties of compressed air
Air under pressure (compressed air) has the following properties:
•
It can be compressed or reduced in volume.
•
Air will expand to fill any container.
•
Pressure in a confined, static (at rest) fluid acts the same and equally in
every direction. It always acts at right angles to the containing surface.
(This is Pascal's Law.) See Clwpter 16: Hydraulic Systems.
•
There must be a pressure change to create air flow.
•
Air flows from high- to low-pressure areas.
Ways to increases pressure
In a scaled container, pressure is created on the walls of the container. It is
formed by rapidly moving molecules of air striking the walls of the container
and creating a force. The pressure can be increased by:
•
reducing the volume to create more impact ou a smaller wall area
•
introducing more air into the confmed space
•
heating the air so the molecules travel faster and increase the intensity
and amount of impact on the same wall area.
Air flow and pressure
Air flows through conductors in much the same manner as hydraulic fluid.
See "Laminar and turbulent flow" in Chapter 16: Hydraulic Systems.
As the conductor's cross-sectional area changes, so does the pressure in the
line (assuming that the flow rate is constant). This is known as Bernoulli's
Principle. see Chapter 16: Hydraulic Systems.
Boyle's Law
Boyle's Law states that:
At constant temperature, absolute pressure (P)
varies inversely to the volume (V).
This is expressed in the formula:
PJ x V, '" P, X V,.
In using this formula. all pressure calculations should be based on absolute
pressures (psia).
For example. a cylinder with a volume (V,) of 200 em> has a pressure (PI) of
15 psia. When volume is reduced to V2 '" 100 cm3 , the pressure increases to
P 2 • To calculate Pz:
P1 x V 1
'" Pz X V 2
15 x 200 '" P z x 100
1 -
Rearranging this formula,
P2 = 15x 200 + 100
30psia
Charles' Law
Charles' Law states that:
At a constant pressure, the volume (V) of a gas varies
proportionately to its absolute temperate (T).
This statement is also known as Gay-Lussac's Law. It is expressed in the
formula:
~
Yo
T,
1;
Charles' Law also states that:
For a constant volume of a gas, the pressure (P)
varies proportionately to its absolute temperature
(T).
It is expressed in the formula:
Ideal Gas Law
Because gas does not compress without a change in temperature, Boyle's
and Charles' laws are combined to create the Ideal Gas Law. It is expressed
in the formula:
Other considerations (such as humidity, heat of friction, and efficiency
losses) come into effect when calculating for pneumatic systems. Even so,
this law is still used for design calculations.
Vacuum pumps
Vacuum power is a form of fluid power. A vacuum pump removes the air
rather than compressing it. Vacuum pumps are available in a variety of
different styles. See manufacturers' product lists for available pumps and
read Chapter 15: Pumps for a description of the different types of pumps.
The table below shows the vacuum rating of some of the more common
types of vacuum pumps.
7
Table 1: Vacuum pump ratings
Type of vacuum pump
Maximum continuous
vacuum rating ("Hg)
------------------~
Piston (multi-stage)
Rotary screw
28.5
29.5
28.0
26.0
15.0
28.5
Centrifugal (regenerative peripheral)
7.0
Rotary vane (multi-stage)
Rotary vane (oil-lubricated)
Rotary vane (dry)
Lobed rotor
Pneumatic
compressors
Before air can be used in a pneumatic system, it must be compressed to
create pressure. A pneumatic compressor is a pump designed to increase the
pressure of air by reducing its volume. Full descriptions of the following
compressors are available in Chapter 15: Pumps.
¢=
Fixed capacity compressor
Classifications of compressors
Compressors may be classified by their principle operation:
•
Dynamic action compressors deliver large volumes of air at relatively
low pressures.
•
Positive-displacement action compressors deliver moderate volumes of
air at high pressures.
Compressors may also be classified by their motion:
•
Reciprocating compressors have a positive displacement action.
•
Rotary compressors may have either a positive displacement or a
dynamic action.
- Positive displacement, rotary compressors are vane, lobe and screw
compressors.
Dynamic, rotary compressors are centrifugal compressors.
17
7
Maximum-pressure control
Compressors need some way to prevent further compression once the
required maximum is reached. One way to do this is to build an unloading
device into the compressor. This allows the drive to run with the least hp
demand when it reaches maximum air pressure.
Unloading devices (fingers)
A common unloading device consists of a set of fingers controlled by a pilot
line from the system's air pressure. These fingers hold the intake valves open
when the maximum system pressure is reached. See Figure 4.
Pilolline
(pressure below max)
Pilot line
(pressure at max)
.--------.
Unloading
finger
_,!::---==
Air
Inlel
Check
valve
~....--__r
-4___
/r::------"'7
Compression
. chamber
Piston
a. Compressing stage
b. UnloadIng stage
Figure 4 Unloading fingers
When air pressure is helow maximum, the valve is allowed to seal (see
Figure 4a). At maximum pressure, the pilot pressure activates the fingers
which hold the inlet valve open (Figure 4b). This allows the inlet air to flow
in and out of the chamber without compression.
17
8
Other methods
Other methods of controlling the maximum air pressure of the compressor:
•
throttling the air intake, thus allowing less free air to enter the
compressor.
•
using a variable speed drive, such as a gas or diesel engine pewer unit.
(At maximum pressure setting, the engine rpm is reduced and the
centrifugal clutch disengages. This allows the engine to run at low rpm
with nO load.)
•
using a pressure switch to start and stop an electric drive motor. (A
pressure switch with a preset high-low range stops and starts the electric
motor.
Reciprocating compressors
Reciprocating means the forward and backward, alternating movement of
the piston. In a reciprocating compressor, a cylinder contains the air. A
sliding piston increases and decreases the volume of the cylinder and valves
control the flow of air tbrough the compressor. A maximum-pressure control
valve keeps the air pressure below a set limit. Spring-loaded check valves
are mounted in either direction of the inlet and discharge areas. The piston
does not touch the head, but leaves a clearance gap.
It works as follows:
1. When the suction stroke begins, the movement of the piston creates a
partial vacuum.
2. Atmospheric pressure then unseats the inlet check valve, allowing air to
fill the chamber.
3. The discharge check valve stays closed due to spring pressure and air
pressure,
4. On the compression stroke, the inlet check valve closes and the piston
advances, compressing the air in the cylinder.
5. It does this until the air pressure is greater than the combined line
pressure and spring pressure of the check valve.
6. At that point, the discharge check valve opens,
7. This allows the compressed air to leave the cylinder.
S. The suction stroke begins and the cycle is then repeated.
Types of reciprocating compressors
Reciprocating compressors may be single-acting or double-acting. They may
also be single-stage or multi-stage. Multi-stage compressors consume less
power than single-stage compressors. Two-stage, double-acting compressors
give more compressed air per energy dollar than any other compressor.
-9
Intercooler
Outlet
Inlet
/valve
Outlet
~valve
Second
stage
First
stage
Two-stage compressor
Figure 5 Multi-stage reciprocating compressor
17
10
The differences are as follows:
i
•
Single-acting compressors compress air on one side of the piston only.
•
Double-acting compressors compress air on both sides of the piston.
They are commonly used for large, heavy-duty applications. See
Chapter 15: Pumps.
•
Single-stage compressors reach final pressure with one compression
stroke.
•
Multiple-stage (two or more stages) compressors compress air to a fixed
pressure in the large, low-pressure cylinder. The air is then discharged to
a smaller, higher-pressure cylinder where it is compressed to the rated
pressure and discharged to the receiver. See Figure 5. An intercooler
between the cylinders reduces the temperature of the air. (See Air
Treatment: Intercooler, later in this Chapter.)
Rotary compressors
Rotary compressors are generally smaller than piston compressors, have less
vibration, and need less mass in a foundation when delivering the same
volume of usable air. Some high-speed rotary compressors are very noisy
and must be used with silencers and enclosed in a sound-deadening room.
There are various types of rotary compressors.
Vane compre.'iSors
The sliding vane compressor is composed of a rotor with sliding vanes
mounted in an eccentric (off-centre) housing. The vanes move in and out
under centrifugal force and form a seal against the housing. Air is picked up
through intake ports as the compartments expand. The air is then compressed
as the compartments decrease in size, and discharged through exhaust ports.
See Figure 6 (next page).
Vane compressors are available in single- or multi-stage units. Multi-stage
units require intercoolers between stages.
Lubrication and dry air
Lubrication is required where the vanes make contact with the housing. This
is done by injecting oil or supplying an oil mist into the air stream. Oil
separators arc used to remove the oil from the air downstream of the
compressor. The oil absorbs some of the heat of compression and must be
cooled after it is removed from the air stream.
Dry air (without oil) can be obtained from a vane compressor by using vanes
with special wearing surfaces that don't require oil.
-
1
Exhaust port
Bending
stress
Maximum
volume
Intake
Figure 6 Compression action of a vane compressor
Lobe compressors
Lobe compressors work through the action of two closely meshed lobed
rotors driven by timing gears. The pressure range is usually low, but the
volume delivered is high.
Air is trapped between the lobes and the casing and carried around without a
mechanical reduction in volume. Pressure is built up from restrictions at the
discharge port and from the system's resistance to flow.
There is no metal-to-metal contact. Therefore, no surface lubrication Or air
stream lubrication is required.
Mate marks on the timing gears
If the machine is to be taken apart, check the timing gears for mate marks.
These marks are often put on atthe factory, but if none are evident, put a set
on. If the gears are already marked, do not add another set. Accurate
reassembly requires mate marks.
Screw compressors
A screw compressor eonsists of two screWS or lobes on a helix. Air is
trapped between the meshing units and reduced in volume as it moves
axially to the discharge port.
The matching rotors do not have the same number of lobes. The 4 + 6
assembly shown in Figure 7 is for general use. It has four lobes on the drive
and six lobes on the driven rotor.
Figure 7 A 4+6 assembly of rotors for a screw compressor
Other possible lobe combinations are;
•
6 + 8 for high-pressure, low-volume conditions
•
3 + 4 for low-pressure, high-volume conditions.
Dry screw compressors
Dry screw compressors use two timed gears to prevent contact between the
rotors. Gear backlash and bearing wear must be held to a minimum due to
the very small clearances between the rotors.
A
Caution!
Ensure that timing gears have matching mate marks before
disassembly is done.
Wet screw compressors
Wet screw compressors have one rotor driving the other. This allows the
timing gears to be eliminated. These rotors require oil to reduce wear
between mating parts. As with vane compressors, the oil must be removed
from the airstream after it leaves the compressor. This oil also reduces the
amount of air slip and removes some of the heat of compression.
-13
Air slip
Air slip is air returning to the inlet side past the sealing surfaces in a screw
compressor. The amount of air slip in all screw compressor can be reduced
by increasing the rpm of the rotors. However, a higher rpm also increases the
noise of the machine to the point where the compressor must be isolated
from the work area or installed in a sound-reducing enclosure.
Centrifugal compressors
Centrifugal compressors use a dynamic action to build up pressure. The
dynamic action is explained in Chapter 15: Pumps. Centrifugal compressors
do not have the passage from the inlet to the exhaust ports sealed. When the
compressor is not operating, the air is able to flow from one port to the other
in either direction. When it is in operation the dynamic action resists the
reversal of flow. Centrifugal compressors operate at high speed and deliver
large volumes of air. Multiple-stage units are used to increase the working
pressure delivcred to the system.
Figure 8 shows a cross section of a multi-stage, mixed-flow compressor. It
has a series of impellers mounted on a single shaft. The air flow enters the
eye of the impeller and is discharged at its periphery. The air is then directed
into the eye of the next impeller and so on until the air reaches the discharge
port.
Suetion
Discharge
Figure 8 Multi-stage, mixed-flow, centrifugal compressor
Figure 9 shows a cross section of an axial-flow compressor with a
photograph of its rotor. The air is forced axially along the rotor by a series of
fins and directed by the stationary vanes. The impeller cavity decreases from
one stage to another. This compresses the air as it moves towards the
discharge port.
Suction
A Discharge
Figure 9 Multiple-stage, axial·flow, centrifugal compressor
Air treatment
It is important to treat air against contamination and heat before it is used in
a pneumatic system. Treating the air ensures that the air is not harmful to the
system. Free air (atmospheric air) contains some degree of airborne
contamination and moistore. Also, its temperature varies. These factors must
be controlled by some kind of air treatment before free air enters a
pneumatic system. Air treatment consists of fIltering, cooling, removing
moisture and oil, and storage.
Figure lOon the next page shows a compressor system with a combination
of all the components described in this section. As the compressed air is in
the receiver, it is under pressure and available for specific applications.
/
Air-intake and
silencer
Flowo!
air
To plant
air distributior.
Receiver
~
Intercooler
/
(
..
Aftercooler
/
--------~
Q~ ~::::=:i'=~" ~i"t(2~.1J
~~
First o r /
low-pressure
stage
\
'second or
high-pressure
stage
"
Automatic-...,.....-drain ''''-..
Two-stage
compressor
Figure 10 Typical compressor system
Intake filters
--<l>-- Filter or strsiner
All air has some level of contamination in it. Depending on the environment,
the level of contamination varies. The intake (breather) filters control the
amount of airborne contaminant.~ allowed to enter the compressor. The
amount of contamination removal depends on the requirement of the
compressor. Filters are usually supplied as an integral part of the
compressor. They should be serviced regularly because a dirty filter tbrottles
the intake line.
Many dry or oil-bath intake filters are available. Most are of a felt or cotton
material, held in shape by a rigid. open mesh or screen. They remove most of
the dirt and other solid contaminants from the air.
Intercoolers
-+
-1t-
Cooler without representation of the flow lines of the coolant
Cooler indicating the flow lines of the coolant
..
As the air is compressed, heat develops in the air. This excess heat causes the
air to expand. For efficient compression, this expansion should be kept to a
minimum. As the air leaves the flrst stage of a multi-stage compressor, the
air must be cooled to reduce its volume before il enters the second stage and
so on. Intercoolers are used to cool the air between stages.
Intercoolers can be air cooled or liquid cooled.
•
Air-cooled intercoolers use a fan. It is driven from the compressor to
force air over finned tubes through which compressed air passes. The
fins and tubing must be kept clean for efficient heat dissipation. (See the
multi-stage reciprocating compressor shown in Figure 5.)
•
liquid-cooled intercoolers use a nest of tubes immersed in recirculating
liquid. The liquid cools the compressed air which flows through the
tubes.
Dew point and water traps
The dew point is the temperature at which water condenses as it is cooled.
Water vapour in the compressed air settles oul as free water if the intercooler
temperature falls below the dew point. To avoid this, one of the following is
done:
•
The intercooler has a water trap to gather and remove the excess water.
•
The cooling temperature is kept above the dew point.
-<>-<>-
Water trap with manual control
Water trap, automatically drained
Water traps are vital components in the removal of water from the system.
They can be placed in many locations to expel large amounts of water from
the lines. Wherever the compressed air is cooled and may fall below its dew
point, a water trap is installed. Common areas are after the intercooler, aftercoolers, and air dryers.
Some water traps have a moisture separator within the unit to erthance the
water removal process. Figure lion the next page shows this type of water
trap with an automatic drain.
The float rises with the increase of water until the valve unseats. The water is
then expelled out of the water trap until the water level allows the valve to
reseat and so on.
Figure 11 Water trap with a moisture separator and an automatic drain
Aftercoolers
Mtercoolers are heat exchangers which cool the air after it leaves the
compressor and before it enters the receiver. The receiver can also act as an
aftercooler. The aftercooler pennits the removal of most of the moisture and
other entrained liquids which may be in the compressed air during the
compression stages.
Aftereoolers can be air- or water-cooled
17
•
Air-cooled aftereoolers act similarly to automotive radiators, except that
compressed air flows through the tubes instead of liquid.
•
Water-cooled aftereoolers have a coil of tubing within a chamber. Water
passes through the tubing in one direction while compressed air flows
over it in the other direction. As the air cools it condenses and the
moisture collects within the chamber. The water drains into a water trap
which can expel the water out into the atmosphere automatically or
manually. Figure 12 shows a water-cooled aftereooler with a manually
drained water trap.
Coolinlg water
Air
inlet
Figure 12 Cross-sectional view of a water-cooled aftercooler
Reusing compressor heat
Most of the energy put into a eompressor is wasted as heat through this
process. Plant policy has usually been to waste this heat into the atmosphere.
but more companies are now using compressor heat to warm buildings or
domestic water.
Air reservoirs (receivers)
o
Pressurized reservoir
An air receiver is a storage tank for the compressed air before it enters the
system. Air receivers should be located as close to the compressor as
possible. The receiver has two functions:
•
It acts as a reservoir to accommodate any fluctuations in the system's
supply. This dampens pulsations from the compressor and provides a
steady pressure to the system.
•
It also reduces the velocity of the air. This allows any moistore carried
over from the aftercooler to settle out. If an aftercooler is not used, the
receiver acts as the aftercooler.
The air receiver is classified as an unfired pressure vessel. That is, it is
potentially explosive and dangerous. It is built to specific standards. Safe
operation requires that the receiver is equipped with a relief or safety valve,
a drain valve, a fusible plug, and an air pressure gauge.
Caution!
Safety devices must be installed on the equipment, Read the instruction book
to find out which safety devices are used and where they are located,
Safety valve
The safety valve releases any excessive pressure, This excess may be due to
the failure of the unloading valve or to a pressure surge transmitted back
from the system, The safety val ve is usually set 5% to 10% above the
system's maximum pressure. The usual maximum working air pressure is
about 690 kPa (100 psi g). This is set by the compressor, and is never higher
than the maximum working pressure stamped on the receiver.
The maximum pressure control for an air system is located in, or on, the
compressor, not on the air receiver, The safety valve on the air receiver is
used in case of any failure in the system.
Drain valve
The drain valve must be installed at tbe lowest point of the receiver. Due to
the range of different shapes and mounting positions of the receivers, careful
attention must be taken to ensure the drain valve can drain the tank
completely, If the receiver's position is altered and the existing drain valve is
not in the lowest point of the tank, a new drain valve must be installed in the
new lowest point. Water and oil emulsions which settle out should be
drained off by this valve regularly.
Fusible plug
A fusible plug will melt and release all the pressure in the receiver if the air
temperature becomes dangerously high, Ensure the path in which the
pressure would vent will not endanger workers nearby.
Air pressure gauge
The air pressure gauge monitors the pressure inside the receiver.
Conditioning
compressed air
Compressed air must be free from unwanted contaminants or conditioned to
perform well throughout the system. Some systems require lubricated air to
prevent equipment from seizing while others require extremely dry air for
instruments to operate accurately. Industrial compressed air can be divided
into two groups:
•
instrument air-must be clean, dry and free of oil.
•
plant air--should be clean and have a low moisture and oil content.
Air dryers
~ AirDryer
Air dryers are used to remove water from the compressed air. This reduces
the dew point. They can lower the dew point to as low as -400°C (-400°F).
The required dew point is based on the operating conditions of the system.
There are four basic types of industrial air dryers: deliquescent. regenerative
desiccant, refrigeration. and membrane.
Deliquescent dryers
Deliquescent dryers contain chemical desiccants. which absorb moisture.
These desiccants are consumed in the drying process. This means that the
chemicals must be replenished periodically and their disposal may be a
problem.
Regenerative desiccant dryers
Regenerative desiccant dryers use a solid desiccant which absorbs water onto
its surface. This solid desiccant is usually silica gel. activated alumina. or a
molecular sieve. These dryers use two identical chambers in which the air is
dried. As the desiccants of the first chamber become saturated with moisture,
this chamber needs to dry out. Valving redirects the flow into the second
chamber while the saturated chamber dries out. Heat may be used to speed
the drying process, but the chamber must be cooled before it is ready to use
again. When heat is used 75% of the time is used for heating and 25% is
used for cooling.
Refrigeration dryers
Refrigeration dryers condense moisture from compressed alr by cooling the
air in heat exchangers chilled by refrigerants. The moisture collects into a
moisture trap and periodically drains into the environment. The compressed
air is then reheated to its operating temperature before it enters the system.
This prevents condensation from forming on the exterior of the air line
downstream from the dryer.
Membrane dryers
Membrane drycrs are gas separation devices. They consist of permeable
membrane surfaces that block nitrogen and oxygen molecules (air), but
allow water vapour molecules to pass through. Typically. the membrane is
constructed of thousands of fibre tubes through which the water vapours
pass. Because the membrane vents gas (water vapour), not condensate, there
is no need for regeneration Or fear of freezing. These dryers have no moving
parts to wear out. They are non-electric and snitable for most hazardous
locations. The shell (housing) is constructed from plastic and aluminum to
prevent corrosion.
Air filters
--¢-
Filter with a manually drained water trap
V
Filter with an automatically drained water trap
Foreign material in the system's air is extremely harmful to the components
in the system. Filters within the system are used to remove the necessary
contaminants from the air before the air enters the components. Even though
filters are installed before the compressor to clean the air entering the
system, foreign material can still be present in the system. Contamination is
produced in the system by:
•
construction, assembly, and maintenance debris
•
oil carried over from the compressor
•
operational wear particles, pipe scale, and rust generated within the line.
The minimum size of the particles removed is determined by the size of the
rating of the filtering element. The particle size is measured in micrometers
<!.Im) which is one millionth of a meter or 0.000039 of an inch. See
Chapter 16: Hydraulic Systems (Filtration).
Filtering action
The compressed air enters a polycarbonate (high-strength plastic) or an allmetal filter bowl. The metal bowl mayor may not have a sight glass. It
works as follows:
I. A deflector plate swirls the air around the filter bowl.
2. A shroud ensures that the swirling action occurs around the filter bowl
and not around the filtering element.
3. This causes the larger particles and excess moisture to be thrown out to
the side of the filter bowl.
4. A baffle below the filtering element creates a quiet zone which allows
the particles and moisture to collect and prevents them from becoming
entrained in the air flow again.
5. The air is then forced through the filtering element which removes the
smaller particles.
6. Clean air then exits the filter and enters the system.
17
22
Inlet port
-+
Deflector plate
Baffle
Filtering
element
Filter bowl
Depth filter element
Edge filter element
Figure 13 Typical air filter
Coalescing filters
Standard filters filter solid particles and collect any excessive moisture that
happens to condensate at that time. Coalescing filters are designed to remove
all solid particles as small as 0.3 1Jm, together with nearly 100% of the oil
and water vapours, from the air. They use a desiccant similar to the desiccant
used in air dryers.
Pressure regulators
,.~
~
i..f'" and 'I Pressure regulator with gauge
A pressure regulator reduces the line pressure. 'The working elements of a
pressure regulator consist of a main piston or diaphragm which controls a
poppet or val ve by means of a connecting pin. Figure 11 shows the parts of a
pressure regulator.
Adjustable
screw
Spring
Vent
Outlet port
)to
Body
Poppet
valve
Figure 14 Pressure regulator
It works as follows:
1. An adjusting serew preloads a spring on top of the piston.
2. The pilot passage is open to the outlet port. This allows the down· side
pressure to control the piston.
3. As the down-side pressure inereases. the piston rises, along with the
poppet, and the flow decreases.
4. A pressure gauge is mounted on the down-side of the regulator so that
any adjustment of the regulator can be read directly.
For additional infonnation see Chapter 16: Hydraulic Systems.
Air lubricators
-<>-
Lubricator
To function correctly, many pneumatic components require lubrication. Air
lubricators supply clean oil into the line to be carried to the components. 'The
lubricator must be located downstream from any device that might be
contaminated by lubricating oil (such as paint sprayers). The air passing
through the lubricator must be clean, dry and at operating pressures.
24
If the oil droplets carried downstream are relatively large, they are often
termed afog. Smaller droplets are often termed a mist.
•
Fog lubricators serve best when the flow to the component is straight
and short.
•
Mist lubricators serve best when the oil must be carried for long
distances, so that the oil will not settle in the line.
Lubrication should be applied according to manufactorers' specification
sheets. The wrong oil or excessive lubrication form carbon deposits,
pennitting the components to leak.
The discharge valves, ports and cylinders of pneumatic systems are subject
to extreme heat and are cooled by lubrication.
Feed-rate
Sight
glass
Inlet port -·~--"'Ilil
I plug
Venturi
... ----- Outlet port
Bypass
valve
Capillary
II.;;:=",,:::::-tr- tube
Oil
Figure 15 Cross section of a mist lubricator
Lubricators work as follows:
1. As the air enters the lubricator, a predetennined amount is allowed
through the venturi.
2. The remaining air is let through a bypass valve into the bowl and out the
outlet port.
3. The air pressure in the bowl forces the oil up the capillary tube which
supplies oil to the venturi. As the air passes through the venturi it creates
a low-pressure area which ailows the oil to enter the airstream.
4. A feed-rate adjustment allows for the desired amount of oil to enter the
airstream. Only a few drips of oil a minute are needed for most air
lubrication systems.
5. As oil enters the airstream, it is atomized into an airborne oil mist or fog
which is carried to the pneumatic device.
6. A sight glass allows for a visual inspection of the flow-rate of the oil.
FRL (filter, regulator, lubricator) units
r------------]
.i
~,
.
I
.
: >--1-1-+'-<
i'
:
.
. Conditioning unit--dctailed symbol
.------=~---.1
-l :(S) fI
C ond'"
.
. lif-Ied symbol
ItlOrung urut--simp
Air filters, regulators and lubricators are frequently preassembled and called
FRLs. In the past, these were often joined using short pipe nipples. To
replace or repair one component, the whole assembly had to be removed.
Now they are available as a modular system. Any individual component can
be removed without disturbing adjacent ones.
-26
Pressure
regulator
/
011 metering
control
Inlet connection
Outlet connection
Filter
Lubricator
Pressure
gauge
~/
Figure 16 Modular FRL unit
Pneumatic valves
and accessories
Valves used in a pneumatic system function similarly to those in hydraulic
systems. See Chapter 16: Hydraulic Systems. Pneumatic systems operate at
around 6.89 bar (lOa psi) as opposed to the much higher pressures of
hydraulic systems. Therefore, the valves may be made from different
materials. Pneumatic valve bodies are frequently aluminum or some other
lightweight alloy.
Pressure is controlled in the compressor by unloading. Reducing valves
(pressure regulators) reduce the pressure of branch lines for specific
applications. Directional control valves (DeVs) control the movement of the
actuator(s). Flow control valves control the volume (and therefore speed) of
air to the actuator.
Pneumatic DCVs (directional control valves)
Devs in pneumatic systems differ from those in a hydraulic system by:
•
the method in which the spools are sealed
•
the release of exhaust air into the atmosphere. The exhaust air is released
at the valve, at the actuator, or, in a remote area, through a conductor.
Pneumatic DeVs can be either rotary or spool types, as are hydraulic valves.
Rotary valves are primarily shifted manually. Spool valves can be shifted
manually, mechanically, hydraulically, pneumatically, or electrically.
Spool sealing
The spool in a pneumatic valve is sealed by the use of dynamic seals such as
O-rings or U-sections in contact with the bore and the spool. The seats are
held in position by spacers and the spool has rounded earners to allow easy
engagement with the seal.
Figure 17 on the next page shows two methods of using O-rings to seal the
spool to the body:
•
In the packed bore design. the O-rings are fixed in grooves in the body
through which the spool slides.
•
In the packed spool design the O-rings are fixed in the spool.
The lapped spool design is another method which has the mating surfaces
lapped to precise tolerances. This design relies on the lubrication between
the mating surfaces to do the sealing.
Ports
Dev s can be used as a simple 2-way shut-off valve, or as a 3- or 4-way
valve, depending on the porting. In Chapter 16: Hydraulic Systems all these
DeV s are covered in greater detail.
3-position, 5-port, 4-way DCVs
In pneumatic Devs some of the 4-way valves are equipped with 5 ports.
They may be 2- or 3-position vaives and are designed for mUlti-purpose
applications. The 3-position, 5-port DCVs are available with three types of
centres: exhaust, pressure, and blocked. See Figure 18. Normally the
pressure port is the centre port (I) and the outer two ports (3 and 5) are
exhaust ports.
These DeV s may have their pressure and exhaust ports reversed. This is
used to control an actuator at different pressures in each direction. Fignre 19
shows an example of a cylinder that can extend at a regulated pressure and
retract at full pressure. This situation Can be used when a cylinder is used to
open and close a gate valve. The gate valve has a slight taper and takes little
force to wedge the mating surfaces together tu make a tight seal. It requires 3
to 5 time.~ as much force to open or unseat the val ve.
A
17- 28
Caution!
Not all4-way DCVs may be reversed-their packing seals may be designed
to seal air in only one direction. To prevent internal damage, refer to the
manufacturer's specifications.
Connected to
circuit
Packed bore - normal position
\,\
Connected to
power source
Connected to
atmosphere
Packed bore - shifted po$ition
Seals
Packed spool
2
Figure 17 Packed bore and packed spool DCVs
1"\ IUlltl
Exhaust Centre
1"\ IS.! ltl
Pressure Centre
Blocked Centre
Figure 18 Exhaust centre, pressure centre, and
blocked centre 4-way valve
Figure 19 Double-acting cylinder operated by a 3-position, S-por! DeV
Solenoid-controlled DCVs
Dey s can be remotely controlled by pilot pressure, direct solenoids, or
solenoid and pilot valves. Some solenoid-controlled valves may be activated
manually. This is known as a manual override and permits the valve to
operate when the electricity has been disconnected.
A
Caution! Before using a manual override, ensure that:
1. The equipment is safe to operate.
2. Everyone associated with the equipment is aware that the machine may
operate. They should stand clear of any moving parts.
Solenoid-controlled Dey s permit very little control as they are either wide
open or off. Due to the extremely fast action of air, manually controlled or
pilot-operated Deys are frequently used to control cylinders that must be
operated at speeds from creep to wide open. They permit a varying flow of
compressed air to the actuator(s).
Flow control valves
Flow control valves restrict the flow rate in a leg of a pneumatic system.
They act and are symbolized in the same manner as hydraulic flow control
valves. See Chapter 16: Hydraulic Systems for detailed descriptions.
Figure 20 shows a typical flow control valve with metered flow in one
direction and free flow in the other. Flow meters are used to measure the
amount of flow through the line.
17
30
Metered flow
Adjustable
orifice
Check valve remains
held against seat
Free flow
)
Light spring is
compressed
Figure 20 Operation of a flow control valve
Quick exhaust and shuttle valves
When a cylinder is required to return rapidly, a quick exhaust valve is
installed in the retract line of the cylinder. This allows the air to exhaust right
at the cylinder instead ofreturning through the DeV to exhaust. Figure 21 on
the next page shows the action of the valve as the air flows tu the cylinder
and as it exhausts.
Outlet port
To
actuator
Inlet port
Figure 21 Quick-exhaust valve
A shuttle (self-activated) valve is used when it is necessary to have two
DCV s operating a single actuator. This valve shifts to allow either of the
DCV to activate the actuator but only one at a time. Figure 22 shows the
action of the shuttle valve when pressure is applied.
Outlet port
Inlet port
Inlet po~
Shuttle
Figure 22 Shuttle valve
Both the quick-exhaust and shuttle valves are usually used with 3- or 4-way
DCVs. Many quick-exhaust valves may also be used as shuttle valves by
changing the line to their port connections.
Mufflers (silencers)
~
Silencer
Compressed air exiting an open conductor may generate high intensity
sound. A muffler breaks up the sound waves and smooths them out, reducing
their energy. Sound is measured in decibels (db). Mufflers reduce the decibel
level. Every manufacturer may have a different method of muffling the
sound such as sintered bronze, felt, styrofoam, etc. See manufacturers'
17
32
catalogues for types and styles. Figure 20 shows an example of a series of
passageways through which the air must travel in order to exit through a
pneumatic muffler.
)
Figure 23 Pneumatic muffler
Pneumatic actuators (cylinders and motors) are of the same design, style, and
action as hydraulic actuators. The difference between the pneumatic and
hydraulic actuators is that many pneumatic actuators use corrosion-resistant
materials (aluminum, brass and stainless steel) in their construction. Due to
the lower pressures, they may use seals with lower capacities. A further
description of the actuators is in Chapter 16: Hydraulic Systems.
Pneumatic
conductors and fittings
Pneumatic systems use conductors in the same manner as hydraulic systems.
Due to the lower pressure (below 200 psi) and potential moisture in the lines,
pneumatic conductors are often made of brass, copper, aluminum, plastic
and other corrosion-resistant materials. Seamless steel (black) pipe is also
used because of its low cost but corrosion can occur more rapidly. The end
fittings, pipe, and tubing dimensions for pneumatic conductors are the same
as those for hydraulic conductors. See Chapter 16: Hydraulic Systems.
17
Pneumatic hose
The type of material and the size of pneumatic hose are different from
hydraulic hose. So is the method in which hose is attached to its end fittings.
Pneumatic hose is nmch lighter and more versatile then hydraulic hose. It is
available in materials such as nylon, polyethylene, vinyl, Teflon, etc. It
comes in straight lengths or retractable coils. Straight lengths are normally
used to connect components which are permanently placed. Coils arc used
with remote equipment. See Figure 24 .
....~
~--. End fittings ~
~---"
Figure 24 Coil hose with end fittings
Swivel end fittings
Coiled hoses are often supplied with swivel end fittings which allow the hose
to rotate in use. Because of the variety of different fittings available, refer to
the respective manufactorer's fastening procedure.
A
Caution!
Ensure all parts of the fitting are compatible with each other.
Barbs
The hose is connected to the fittings by means of barbs. As the hose is
pushed onto the fitting the barbs bite into the inside of the hose. These barbs
tighton their grip as the hose tries to pull off.
Spring guards
A spring guard is placed onto the fitting to protect the hose from kinking.
A
Caution!
Wherever possible, use a clamp to secure the hose to the fitting.
Figure 25 shows how the fitting, hose and spring guard are assembled.
Spring
/
Hose
LLLlLLLlcLl.l;u..u..l.l.ll.1..lll..L.U.JLU.LLLl..LLLU."'--_ _-¥~
~
-
- _.
V
End fitting
Figure 25 Hose end assembly
Flareless jomts
Hose can also be attached to fittings by flareless joints (compression
fittings). When these fittings are required, ensure that an internal tube
support is used to prevent the hose from collapsing., See Figure 26.
Compression sleeve
Tube support
Hose-··· ..
End fllting
Compression nut
Figure 26 Compression filtlng with support tube
Quick-disconnect couplers
Pneumatic quick-disconnect couplers have a shut-off in the recessed half and
an open passage in the protruding half. These are single shut-off as opposed
to the double shut-off of a hydraulic quick-disconnect coupler. Always
ensure that fittings are clean to make a positive fit. Figure 27 on the next
page shows a typical pneumatic quick-disconnect coupler, connected and
disconnected.
~
Connected
Disconnected
Seal
Seal
Valve
Valve
Figure 27 Pneumatic quick-disconnect coupler
Routing and
installation of pneumatic lines
Correct installation and routing of pneumatic lines throughout a plant are
vital to ensure successful, smooth operation of the pneumatic system. Using
the correct conductor size ensures consistent volume at peak demand.
Pneumatic line routing
Pneumatic lines can be routed through a plant in various ways. The three
most common systems are the grid. decentralized. and loop systems.
Grid system
The grid (dead end) system (see Figure 28) is the simplest and least
expensive system. It consists of a main line from the compressor which
begins large and becomes progressively smaller in diameter as it reaches the
end. Feeder lines of uniform size provide outlets at convenient locations.
The problem which arises with this system is that the work stations at the
end of the system may have insufficient air supply (air starvation) when
demands are heavy.
36
Feeder lines
..
~
compresso~ [=~==~=::=~==:;~=:=~===*==::::J
~ Work stations
Figure 28 Grid system
Decentralized system
)
The decentralized (unit) system consists of two or more grids within the
main system, each with its own compressor. See Figure 29. This
arrangement allows fOf shortef supply lines which results in more unifonn
air supply and system pressure. This system is also more versatile and adapts
ea.~i1y to changing requirements.
o
Figure 29 Decentralized system
Loop system
The loop system (see Figure 30) consists of two or more compressors around
a continuous loop. This arrangement provides a parallel path to all work
stalions. This system allows the air to move continuously around the system,
in either direction, to supply the work stations. This tends to be the preferred
system.
t
..
~
I ..
~
..
Figure 30 Loop system
Pneumatic line installations
Permanent pneumatic lines are installed in such a manner that any moisture
accumulating in the line flows toward a drain. Air-drop lines can be placed
anywhere along the main line to feed air to working compnnents. An airdrop line is installed witii a tee junction (a tee-off) that goes up and then
down to its work area. This causes any moisture to flow past the tee to the
drain and not into the air-drop line. A water leg must be installed at the end
of the main line to accumulate and drain any excess moisture. Figure 28
shows a recommended tee-off arrangement, the slope of the main line, and
the water leg.
Drop taken from
top of conductor
/
Slope
2.5 - 5 cm per 3 m
(1 - 2 in. per 10 fl.)
-Air-drop line
Moislure
flow
Shut-oil vallie
Sludge
collection
Lubricator
~
Filter
drain
Automa1ic
Hose
Figure 31 Typical pneumatic line arrangement
Symbols
The construction of pneumatic components differs from that of hydraulic
components, but the symbols for their functions are the samc. The symbols
which define whether a circuit is pneumatic or hydraulic are the arrows
showing the direction of flow. The arrow showing a pneumatic flow is an
outlined triangle (see symbol below) whereas the hydraulic symbol is a solid
triangle.
V Pneumatic flow
The symbols used in drawings of pneumatic systems are shown throughout
this chapter. Refer to the tables of symbols in Chapter 16: Hydraulic
Systems.
Pneumatic circuits
A pneumatic circuit is read in the same way as a hydraulic circuit. Air
circuits do not usually show the compressor or the maximum pressure
control, but start off instead with a pneumatic pressure source symbol.
0--t>- Pneumatic pressure source
The following are some examples of the common pneumatic circuits.
Exhaust lines are shown leading to the atmosphere and may include chokes
or mufflers as extra equipment. Pilot air supply may be at main line pressure,
but for precision work, air is usually taken from a source that has a pressurereducing valve to maintain a constant pressure less than the supply air
pressure.
Two-hand or safety circuit
In a 2-hand circuit, the Dey has the air blocked at both the supply and
exhaust ports. This keeps the actuator in a locked position. Figure 32 shows
a cylinder being controlled by 4 Deys.
A
B
A
B
Figure 32 Two-hand circuit
•
To advance the piston, the operator must shift both Deys (A in
Figure 32). This allows air to enter the cap end and exit the head end.
•
To retract the piston, the operator must shift both Deys (B) to create a
reverse air flow. If only one Dey is shifted no action will occur.
A two-handed circuit is often used on equipment to protect the operator's
hands while work is in progress.
Dual air exhaust control circuit
This circuit shows an actuator controlled by a 4-way valve with 5 ports and 2
distinct positions (412). See Figure 33. Both exhaust ports have variable
output, flow control valves to control the speed at which the air escapes from
the actuator. This controls the speed at which the actuator moves.
.o::::r.•
t
Flow
Figure 33 Dual air exhaust control
Quick-exhaust circuit
Quick exhaust circuits are used where very rapid actuator speed is required.
Figure 34 shows the cyliuder of an impact tool being controlled by a 4/2
valve. A quick-exhaust valve is coupled between the cyliuder's head end and
the DeV. Because the pistun must extend very rapidly, the exhaust air is
allowed to escape near the cylinder instead of returning through the DeV.
The retraction stroke is not required to move as quickly, so the path through
the DeV is adequate.
r - --,
I
II
I
II
I
/
L -
--
Quick-exhaust valve
close coupled to cylinder
Figure 34 Quick-exhaust cineuit
Circuits with multiple remote control positions
When a single piece of equipment needs to be operated from various
positions, multiple pilot valves must be used. Figures 35a and 35b on the
next page show two methods of achieving this.
~""--r-
----I
I
I
t
I
Retract
I~I--t>
-<1----1--.1_ _ _ _ _ _
~
I
I
I
I
I
I--~
Advance
1
2
~_-r-----I
8. Counterclockwise
b. Clockwise
Figure 35 Multiple remote control positions
The circuit in Figure 35a uses two pilot valves on opposite sides of a shuttle
valve to provide pilot pressure to the main Dey. The Dey moves the
actuator in a counterclockwise direction. The operating positions could be
extended to three or more positions by the use of additional pilot and shuttle
valves. The shuttle valve allows only one valve to activate the actuator at any
given time.
The circuit in Figure 35b uses two pilot valves with valve 2 cOlUlected to the
exhaust port of valve l.
•
When valve 2 is shifted, the air flows through valve 1 to the main Dey.
•
When valve 1 is shifted, it blocks the flow path from valve 2 and
maintains pressure to the main Dey.
This circuit could be extended to more positions by adding additional valves
in the same configuration. Again, only One position is allowed to activate the
actuator at any given time.
Maintaining and
troubleshooting pneumatic systems
Maintenance precautions and troubleshooting procedures are similar for
pneumatic and hydraulic systems. See Chapter 16: Hydraulic Systems. Bach
system has some unique situations and methods. For example, a pneumatic
leak is detected by sound as opposed to sight, and pneumatic systems have a
tendency to freeze up easily in cold weather.
Maintenance
Maintenance and overhaul of parts should be done to meet tolerances or
elearances suggested in manufacturers' specifications. The working
environment must be clean and orderly. Service manuals describing
maintenance procedures are supplied with the equipment. See also
Chapter 16: Hydraulic Systems.
Cold weather handling
When work area temperatures go below freezing. valves tend to freeze up.
This is due to the cooling action created by air expanding in a pressure drop
through the ports. Methods of preventing freezing depend on local
conditions. Some common ways to prevent freezing are:
•
installing an infra-red lamp or heat source on the valve
•
introducing a small amount of permanent anti-freeze into the
lubricator-this takes care of the whole system
•
using an anti-freezer to put alcohol into the airlines-this also takes care
ofthe whole system
•
heating the air supply-usually done in a steam mill by running a steam
line next to or even through the air line
•
maintaining the building or work area temperature above freezing.
Thaw a frozen valve gradually. This allows the seals to soften slowly and the
valve body and components to expand at an even rate. Rapid heating of the
valve body can result in cracks or fracrures.
Caution!
Do not thaw a valve suddenly with concentrared hear such as an
oxyacetylene flame.
Tables 1 and 2 on the following pages give troubleshooting tips for
compressor units and for pneumatic circuits.
Table 1: Troubleshooting tips for compressor unHs
Trouble
Cause
Will not start
1. Overload blown
1. Reset overload
2. Fuse blown
2. Replace fuse
3. Defective safety switch
3. Replace switch
4. Magnetic coli damaged
4. Replace coli
5. Inoperative electrical system 5. Repair electrical system
Excessive nOise
1. Worn bearing
='------
Correction
2. Loose flywheel
3. Loose or worn piston
4. Insufficient head clearance
Excessive vibration
1. Overhaul compressor & ensure adequate
oil supply
2. Remove flywheel,
Inspect mounting shaft diameter,
Inspect flywheel bore,
Replace damaged items, & reassemble
3. Overhaul compressor
4. Measure clearances & adjust to
specifications
1. Misalignment between
compressor & drive una
2. Loose mounting bolts
3. Damaged flywheal
1. Realign compressor & drive unit
2. Torque mounting bolts & check alignment
3. Remove, re-balance, & replace
Low 011 pressure
1. Damaged oil pick-up
2. Low oil level
3. Plugged suction strainer
4. Defective pump
5. Worn-out bearings
1. Replace oil line
2. AU to correct oil level
3. Clean suction strainer
4. Repair or replace oil pump
5. Repair or replace bearings
High 011 pressure
1. Blocked 011 passage
2. Dirty filter
1. Clear all oil passages
2. Change the filter
-----
- - - -
--------
----
Low air pressure In the receiver 1. Leaky compressor valves
2. Stuck piston rings
3. Faulty unloading valve
4. Break In the line
1. Overhaul compressor valves
2. Overhaul compressor
3. Repair or replace unloading valve
4. Repair line
Excess air pressure in receiver 1. Faulty unloading valve
2. Faulty safety valve
1. Repair or replace unloading valve
2. Replace safety valve
Compressor overheating
1. Maintain recommended oil level
2. Maintain recommended coolingwater or air flow
3. Repair or replace damaged components
4. Change intake filter
5. Replace thermal valve
--------
- - _.... _ - -
Air overheating
1. Improper 011 level
2. Inadequate circulation of
cooling water or air flow
3. Damaged cooling system
4. Dirty Intake filter
5. Faulty thermal valve
------
-----
----
1. Excess moisture in Intercooler 1. Drain moisture from water trap
2. Inadequate cooling agent
2. Maintain recommended flow of cooling
agent
3. Dirty intercooler or after cooler 3. Clean the cooler
Table 2: Troubleshooting tips tor pneumatic circuits
Trouble
Cause
Low air pressure
1. Pressure regulator sat too low
2. Damaged line
~~
...
Correction
1. Increase pressure
2. Replace damaged seelion
~~~--~~-.~~~--~~---~
--~
--~
Excessive pressure
1. Pressure regulator set too high
1. Reduce pressure
Sticky valves
1. Excessive carbon deposits on
the valve components
2. Insufficient lubrication
3. Valve packing too tight
1. Overhaul valve
--~
2. Increase 011 flow in the lubricator
3. Readjust packing gland
Incorrect delivery of lubrication
1. Excessive oil discharge
2. No oil discharge
1. Reduce 011 flow
2. Check oil level in the lubricator; if level
OK, increase oil flow
Cylinder moving inconsistently
1. Scored piston or liner
2. Bent piston rod
1 . Change filters & overhaul cylinder
2. Replace piston rod & seals
(Overhaul cylinder)
3. Replace piston seals
(Overhaul cylinder)
4. Increase pressure setting on regulator
3. Damaged piston seals
4. Insufficient air pressure
Motor moving inconsistently
1. Scored rotor casi ng
2. Worn bearings
3. Insufficient air pressure
1. Change filters & overhaul motor
2. Replace bearings & seals
(Overhaul motor)
3. Increase pressure setting on regulator
n
:::r
=
...-.....
'1:S
CD
CO
MILLWRIGHT MANUAL: CHAPTER 18
Prime Movers
Internal combustion engines ......................................................... 18:1
Diesel engines ................................................................................. 18: 1
Gasoline engines............................................................................. 18:3
High-compression, gas-burning engines ........................................ 18:4
Principles of operation.................................................................... 18:5
Construction and components of four-stroke engines .................... 18: 11
Construction and components of two-stroke engines ..................... 18: 17
Routine preventive maintenance .................................................... 18:19
Steam turbines .............................................................................. 18: 19
Definitions and construction ................ ............ ......... ..... ..... ............ 18: 20
Principles of operation .................................................................... 18: 21
Casings and flow ............................................................................ 18:22
Back-pressure turbines ................................................................... 18:24
Condensing turbines ....................................................................... 18:27
Rotating elements ........................................................................... 18:28
Steam chests and nozzle blocks ...................................................... 18:32
Diaphragms and steam nozzles ...................................................... 18:33
Shaft seals ....................................................................................... 18:36
Thrbine condenser and hot well ...................................................... 18:38
Thrbine control devices .................................................................. 18:39
Thrbine auxiliary systems ............................................................... 18:46
Pre-start systems for a turbine-generator ........................................ 18:49
Gas turbines .................................................................................. 18:49
Principles of operation .................................................................... 18:49
Types of gas turbines ...................................................................... 18:50
Gas turbine components ................................................................. 18:54
Gas turbine controls and auxiliary systems .................................... 18:64
Pre-start checks for gas turbines ..................................................... 18:66
Electric motors .............................................................................. 18:66
Motor frames .................................................................................. 18:66
DC motors ...................................................................................... 18:67
AC motors ...................................................................................... 18:70
Advantages and disadvantages of various electric motors ............. 18:73
Name plate information .................................................................. 18:73
Prime Movers
Prime movers are any devices designed to drive other machines. They may
be powered by a variety of energy sources such as gasoline, diesel oil, highpressure steam, gas, and electricity. The millwright installs, uses, and
maintains prime movers. This chapter describes internal combustion engines,
steam and gas turbines, and electric motors.
In tern aI-combust ion !il[!!lJfJ!!!i!k!lJililll!!wiiiit!lJiliU!ltti11!!i~:!IJil'f:1!!1!!21:li~Eji12~it:U!ig:lm!lllit1l~Q~Ri!iRii!i!rt;ji::ijl;l:i:EilljGi:l:iiii!:1iE:l:1l!lJil!!ii%111!!i!!~1i
engines
Internal-combustion engines derive their energy from fuel which is burned
within the engine itself. This is unlike, say, the steam turbine which requires
a separate boiler to provide steam to drive the turbine. Internal-combustion
engines require less space and are usually less expensive to run.
)
Internal combustion engines can use a variety of fuels depending upon
application, availability, and cost. The two fuels most commonly used are
diesel fuel and gasoline, although propane and natural gas fuels are used to a
lesser extent.
The major difference between diesel and gasoline engines is in their method
of ignition. The gasoline engine uses a spark plug to ignite the fuel mixture.
In the diesel engine, compression raises the fuel's temperature to ignition
point.
Diesel engines:
What sets diesel engines apart from other internal combustion engines is:
•
Diesel engines use air alone to fill the cylinders during the intake stroke.
Therefore, they have no carburetor system but, instead, use a fuelinjection system.
•
The fuel is ignited solely by compression. Because ofthe increased
temperature and compression these engines must have heavier
construction to withstand the stress.
•
All diesels are fuel injected. That is, they have a metered flow of fuel
into the combustion chamber.
Diesel engines may be divided into two basic types: four-stroke and twostroke. (These are discussed in more detail under Principles of operation.)
18 -1
Four-stroke engines are either naturally-aspirated or supercharged:
•
In naturally aspirated engines, fresh air is drawn into the cylinder by a
vacuum. The vacuum is created when the piston moves down the
cylinder away from the head and combustion area.
•
In supercharged engines air is forced into the cylinder at higher than
atmospheric pressure. This is done by means of a pump or blower
similar to those used on two-stroke engines.
Compression ratios
Compression ratio refers to how may times the air is compressed from its
original volume during the compression stroke. For example if a diesel
engine has a compression ratio of 18 to I, the air at the top of the
compression stroke has been compressed to one-eighteenth of its original
volume. Diesel engines run at significantly higher compression ratios than
most other engines. The range of compression ratios for diesel engines is
from 16: I to 23: I. Compare this with the range for gasoline engines, which
is from 6:1 to 13:1.
Compression-combustion
When air is compressed rapidly to a significantly smaller volume, the
temperature of the air increases dramatically. When fuel is sprayed into the
cylinder(s), the temperature and compression of the air cause the fuel-air
mixture to ignite and burn. The rapidly expanding mixture pushes the piston
down the cylinder.
Diesel engines may have glow plugs to preheat the combustion chamber for
start-up. They are not required for continuous running. Although there are
different styles of diesel engines, each one operates on the principle of
compression-combustion. Figures 1 and 2 show the various ways that diesel
engine cylinders may be oriented.
Figure 1 In-line cylinder action
18
2
\
\~
~A~___
I
-----'" I
\
\
I
I
Piston
rod
Figure 2 V-arrangement of cylinders
Fuel injection
Fuel is supplied to the cylinders of a diesel engine by fuel injection_ There
are two types of fuel-injection system:
•
In air injection, an external source of air forces a measured amount of
fuel into the cylinders.
•
In mechanical injection, an injector pump forces fuel into the cylinders
by applying a hydraulic force to the fuel.
The mechanical injection system is the most used.
Gasoline engines
As with diesel engines, gasoline engines can be divided into two basic
classifications of four-stroke and two-stroke. Similarly, gasoline engines can
be naturally aspirated or supercharged, and may be fuel injected. Other than
appearance, this is where the similarities end.
Spark plug Ignition
Gasoline engines require tbat fuel and air be mixed together before ignition
can take place. Once the piston reaches the top of its stroke on the
combustion cycle, a spark plug fires. This causes the air-fuel mixture to
bum. As a result compression is not used for ignition. Therefore, gasoline
engines are not subject to the same stress and temperatures as diesel engines.
Compression ratios
Generally compression ratios for gasoline engines are much lower-usually
in a range of 6:1 to 9:1, although they may be up to 13:1. Compare this with
the range for diesel engines, which is from 16: 1 to 23: 1. As a result then
gasoline engines can be made of lighter construction.
As with diesel engines, gasoline engines may be built many different styles;
however, each style is built on the same principle of spark-ignition and an
air-fuel mixture before ignition.
Carburetors and fuel injection
Some gasoline engines use a carburetor to supply fuel to the cylinder. In the
carburctor, the fuel is atomized. It is then drawn into the cylinder where the
spark plug fires, causing the fuel to burn. Gasoline engines now often use
fuel-injection systems to supply fuel to the cylinders. The gasoline still
requires a spark plug for ignition.
High-compression, gas-burning engines
These engines use either compression firing or spark ignition. They are
designed to burn different fuels or a combination of fuels such as diesel and
gas. The gases burned are natural gas, propane, methane, and other such fuel
gases.
These engines are very similar to gasoline and diesel engines:
•
They can be divided into four-stroke and two-stroke.
•
They may be naturally aspirated or supercharged.
•
They may differ in outward appearance but their basic principles remain
constant.
Unlike diesel and gasoline engines (which serve multiple purposes) highcompression gas engines serve a specific purpose within a given industry.
One example is their use for driving compressors for gas transmission
pipelines. Because gas is readily available and relatively cheap, sparkignited, high-compression, gas-burning engines are ideal for this type of
work.
Compression firing with two fuels
When two fuels are burnt, compression firing takes place in one of two
ways:
•
Either the two fuels are injected at the top of the stroke, where
combustion takes place.
•
Or gas is mixed in with air, compressed and then the second fuel is
injected at the top of the compression stroke, where combustion takes
place.
Spark plug ignition with one fuel
When one fuel is burnt, bigh-compression engines have a spark plug ignition
system. For that reason, they use a very lean mixture (small ratio) of gas to
air so as not to detonate by compression. This air-fuel mixture will ignite
only when an electrical current arcs across the spark plug's poles.
Principles of operation
Four-stroke action
In a four-stroke cycle, four separate and distinct processes take place in two
complete revolutions of the crankshaft. To understand the process, the
description starts with the intake and follows through the progression of
compression, power, and exhaust. The diesel engine is used as the model for
the explanation. Diagrams of each type of engine previously discussed are
also shown.
Intake:
L
Once the piston has travelled up the cylinder to top dead centre (IDC),
the intake valve opens and air intake begins (see Figure 3a.) The intake
valve can open before the TOe. This depends on manufacturer's timing
specifications.
2. As the piston travels down the cylinder, air is drdwn into the cylinder
(see Figure 3b).
3. When the piston travel reaches bottom dead centre (ROC), the intake
valve closes and the next stroke begins (see Figure 30).
Fresh air
entering
Cylinder full of
fresh air
cylinder
Piston
Connecting ~ .•...•. 1
rod
~~l---'-~ Crankshaft journal
I"'--~ Main bearing
a.
b.
Figure 3 Intake stroke 01 a four-cycle diesel engine
c.
Compression
1. At this point the piston begins its second travel up the cylinder (see
Figure 4a). Both the intake and exhaust valves are closed.
2. As the piston travels toward TOC the air is being compressed many
times (see Fignre 4b).
3. Once the piston reaches TOC the crankshaft has completed one
revolution and the next step in the process begins.
Injection nozzle
delivering oil spray
Air being
compressed
B.
b.
Figure 4 Compression stroke to TDC of a four-cycle diesel engine
Power
The fuel may be injectod into the cylinder before TDC, at TDC, or after
TDC, depending on manufacturer's timing specifications (see Figure 5a).
Remember that, due to the extreme compression ratio in the diesel engine,
the compressed air is hol The fuel ignites when it contacts the hot air. The
hot expanding gases react against the piston. forcing the piston down toward
BDC (see Figure 5b). Once the piston reaches BDC the exhaust valve in the
head begins to open and the last (exhaust) stroke begins.
A flywheel provides a smooth transition between power strokes. Remember,
as the. piston moves up the cylinder on the compression stroke there may be
no power stroke from other pistons. Therefore, the weight of the flywheel
must keep the crankshaft moving smoothly through the transition.
Hot gases expanding
against piston
Spent gases released
from cylinder
Exhaust valve
a.
b.
Figure 5 Fuel injection and power stroke of a four-cycle diesel engine
)
Exhaust
On this stroke the exhaust valve
is open. As the piston travels up
the cylinder, the exhaust gases
are forced out through the
exhaust valve (see Figure 6).
Spentgaseslorced
from cylinder \
Exhaust valve
open
Depending upon timing and
application of engine, as the
piston nears the end of its exhaust
stroke, it is at the TOC and the
intake valve begins to open. At
this point, both intake and
exhaust valves are open. This is
known as valve overlap. The
engine has now completed all
four strokes of the cycle and the
crankshaft has completed two
revolutions.
Figure 6 Exhaust stroke of a four-cycle diesel engine
Figure 7 shows a typical gasoline four-stroke engine. Notice the spark plug
iu the centre of the head.
Intake
Compression
o
Power
Exhaust
o
o
A
Fuel and air mixture
•
Exhaust of spent fuel
Figure 7 A typical gasoline four-stroke engine
Two-stroke action
In a two-stroke cycle, all operations of iutake compression power and
exhaust are completed iu two-strokes and only one revolution of the
crankshaft. There are SOme differences between the diesel two-stroke and the
gasoline two-stroke.
Compression in a two-stroke diesel engine
With the piston at BDC, ports iu the walls of the cylinder are exposed (see
Figure 8). Notice that there are two valves iu the cylinder head-both are
exhaust valves. Note that the exhaust valves are open. Air is pumped iuto the
cylinder by a pump Or blower. Air is pushed iuto the cylinder, replaciug the
spent fuel which iu turn forces the spent fuel out through the exhaust valves.
As the piston moves up the cylinder the first stroke begins. Because exhaust
and intake take place at the same time with the engine at BDC, no stroke
takes place. It is not until the ports are covered and exhaust valves are closed
that the first stroke begins with the piston moving up the cylinder.
Compression takes place during this stroke. When the piston reaches TOC
the se~'()nd and final stroke takes place.
Blower
Intake and exhaust
Compression
Exhaust and intake
Figure 8 Typical two-stroke action in a diesel engine
with exhaust valves and blowers
Power in a two-stroke diesel engine with exhaust valves and blowers
Once the piston reaches TDe, fuel is injected into the cylinder and is ignited
by the heat from compression. The rapidly expanding gases react against the
piston forcing down the cylinder. Once the intake ports are uncovered and
exhaust valves are opened, the cycle begins again. The two-stroke engine has
completed both cycles in one revolution .
... with exhaust ports and scavenging valve
Not all two-stroke diesels use exhaust valves or blowers. Figure 9 shows
how the piston on the power stroke acts as a compressor on the back side. As
the piston moves down, it compresses air in the crankcase. As the piston
moves farther along the cylinder, exhaust ports are uncovered allowing spent
fuel to escape. At this point the intake ports are uncovered and the slightly
higher-pressure air from the crankcase rushes in to replace the spent fuel.
Note the scavenging valve on the crankcase. As the compressed air rushes
into the combustion chamber, the scavenging valve opens to allow more air
into the crankcase. Notice the shape of the top of the piston and how air
coming into the cylinder is forced up and around the cylinder, removing the
spent fuel.
18-9
Exhaust
ports
Transfer
passage
Inlet
ports
Scavenging
valve closed
Scavenging
valve open
l"":::==+=:":::1 Crankcase
Figure 9 Two-stroke action in a diesel scavenging engine with exhaust ports
Rocker arm
Push rod
Valve
Camshaft
Piston
Connecting _ _ _-++-_---'
rod
Crankshaft
Figure 10 Cross-section across the crankshaft of a GMC V-six cylinder gasoline engine
18
Construction and components of four-stroke engines
The internal structure of standard four-stroke internal combustion engines is
very similar whatever the fuel. Each of these engines has:
•
crankshaft with main bearings, crank-throw bearings, and flywheel
•
connecting rod and piston assembly
•
cylinder head and valve assembly
•
camshaft with lifters and push rods.
Figures 10 and 11 show the similarities between gasoline and diesel engines.
Rocker arm
Valve spring
Valve -----~;q;~
)
Valve lifter
Piston
Connecting rod
Figure 11 Cross-section across the crankshaft of a V-type diesel engine
Crankshaft
The crankshaft has two main functions. The fIrst is to support the pistons and
relating parts. The second function is the most important-it transfers energy
of motion. For example, in generators, trucks, and cars, the crankshaft takes
reciprocating motion and changes it to rotary motion. The crankshaft can
also transfer the energy of reciprocating motion from the engine, to the
energy of reciprocating motion for a compressor.
18
11
High-pressure gas engines used to drive compressors on gas transmission
lines are an example of this (see Figure 12).
Exhaust manifold
Fuel gas header
Gas injection valve
Dualvalvas
Gas oomprassor oylinder
oormectlrlg rods
Rotation
motion
c
)t
Linear
motion
Figure 12 Four-stroke spark-ignited, high-compression gas engine and gas compressor
The main journal bearings in the crankshaft along with the bearing caps
have two functions:
•
hold the crankshaft in the block
•
provide a lubrication surface for the crankshaft to turn in.
I hole
Main bearing
journal
Figure 13 A crankshaft
18
12
The connecting rod journals of the crankshaft are offset from the main
journals. This provides the reciprocating motion of the pistons. In Figure 13,
notice the drilled oil holes in the crankshaft. These holes allow pressurized
oil to reach and lubricate the main journal bearings as well as the connecting
rod hearings.
Flywheel
The crankshaft carries the flywheel which has three uses:
•
It provides inertia that carries the crankshaft through periods of no power
stroke and also smooths out the power stroke.
•
In automobile or truck use, the flywheel acts as a mounting surface for
thecluteh.
•
It provides an ideal surface for a ring gear in which to mount a starter for
starting the engine.
Connecting rod and piston assembly
The connecting rod connects between the crankshaft throw and the piston as
shown in Figure 14.
Wrist pin
bearing
Crank pin boll
y
Crank pin bearin~
Figure 14 Connecting rod and crank pin bearing shells
The piston houses the piston rings which seal against the cylinder wall (see
Figure 15 (opposite). In diesel engines especially, the pistons are heavily
constructed. This is because diesel engines run at higher temperatures and
therefore the pistons must dissipate some of the heat that is generated.
Figure 15 Piston and connecting rod in cylinder liner
Figure 17 (on the following page) shows a piston with three rings. The first
two are compression rings which are designed to stop leakage of air or hot
gases into the crankcase. The third ring on the piston is an oil ring which
wipes oil off the cylinder walls so that no crankcase oil is burned and
wasted. The cylinder liner (sleeve) is a removable chamber which acts as a
guide for the piston and as a combustion chamber wall for the power stroke.
See Figures 15 and 16. Note that most gasoline engines do not use cylinder
liners.
Cylinder head and valve assembly
The cylinder head is the top end of the compression-combustion chamber.
The cylinder head assembly consists of the valves, valve springs, rocker
arms, and rocker arm shaft. In some applications, it includes the camshaft.
The cylinder head also houses the intake and exhaust ports of four-stroke
engines, and the exhaust ports of the two-stroke engine.
Flange
Grooves for
sealing rings ~
~
Figure 16 Cylinder liners in diesel and gasoline engines
Camshaft with lifters and push rods
The camshaft is a shaft with eccentric machined lobes called cams. It works
as follows:
1. As the camshaft rotates the lobe lifts a lifter which in turn pushes on the
putrid. The putrid then moves the putrid which in turn opens the valve.
2. As the cam lobe turns the valve spring forces the valve to seat and seal
the compression-combustion chamber (cyliuder).
The camshaft is driven by either chain or by gear from the crankshaft. It
travels at one half the speed of the crankshaft. This is because a four-stroke
engine has exhaust and intake valve~ach valve needs to open only on
alternating second strokes.
The engine block
The engine block is a stationary piece which holds other things in place. It is
also called the cylinder block. The block houses or supports all major and
most auxiliary parts. The basic block is most often cast iron or cast
aluminum. It is machined to accept camshafts. the crankshaft. cylinder
sleeves. bearings. and all other pieces of the engine. Engine blocks are
webbed for strength and heat dissipation. Figure 17 on the next page shows a
basic engine block.
18-
Figure 17 Engine block
Notice that the heads of the engine are not part of the block. The engine
block itself bas no removable parts, it is a one-piece cast.
In larger engines tbe blocks may come as two pieces:
•
the bed plate which supports the main bearings and crankshaft
•
!be upper section in which the cylinders are housed.
Another style of engine block is made from welded pieces of pre-formed
rolled steel and steel plate.
Differences between engines
The main differences between these
engines is in the types of fuel,
ignition system, and auxiliary
systems used. The auxiliary systems
include governors, fuel injection
pumps, and tnrbochargers.
Turbochargers
Turbochargers are blowers that are
powered by the exhaust waste gaSes
of the engine. They are basically air
pumps (compressors). Millwrights
come in frequent contact with
tnrbochargers on stationary diesel
generators and high-compression gas
engines. They need to understand the
basic structnre of a turbocharger.
Figure 18 Turbocharger operation
The turbocharger is not connected by gears, chains or other mechanical
means to the engine's moving parts. Figure 18 shows the path followed by
air as it flows through the turbocharger intu the cylinder and back out
through the exhaust.
Turbochargers can run at speeds from 10 000 rpm to 100 000 rpm.
Turbocharger controls adjust the flow of air or of air and fuel. depending on
the engine type.
Note that:
•
Gasoline and gas engines require a constant fuel mixture to operate
properly. Therefore, turbos on these engines require control~ to adjust
the air flow.
•
Diesel engines compress only air and therefore turbos only need to
deliver the quantity of air required.
•
Turbocharged engines should be allowed to cool before shut-off.
Construction and components of two-stroke engines
The basic components of the standard two-stroke diesel engines are very
similar to the two-stroke gasoline or high-compression engine. Each of these
engines has a:
•
crankshaft with main and crank throw bearings
•
connecting rod and piston assembly
•
head assembly.
As discussed earlier in this seetion not all two-stroke engines require exhaust
valves in the head. Therefore, the style of head to be used depends on
whether the engine has exhaust ports or not. However, in diesel engines,
each cylinder head has a fuel injector. Refer to Figure 19 (gasoline---on the
next page), and contrast it with Figures 8 and 9 (diesel).
In Figures 19 and 9, notice the lack of valves. The gasoline engine has only a
spark plug in the centre of the head and the diesel engine has only the fuel
injector.
The crankshafts and bearings, connccting rods and bearings, and pistons of
four-stroke and two-stroke are basically the same and need not be described
here.
Superchargers
Superchargers are blowers that are found in a significant number of twostroke engines (not all). Figure 18 shows a turbocharger and how it works. A
supercharger perfonns the same function but in a different way. Remember
that turbochargers are driven by exhaust gases. Superchargers are driven
mechanically either from the engine or from a separate source such as an
electric motor.
Leaf valve
Leaf valve
Leaf valve
closed
Fuel and air mixture
l~s';~;~~1 Burning fuel mixture
Exhaust of spent fuel
Figure 19 Operation sequence of a two-cycle gasoline engine
18
18
Another difference is that superchargers are rotary lobe as opposed to
turbine wheel. Note that this blower is more complex than a turbocharger.
However, rotary blowers are very positive when full torque over a wide
range of speed is needed. In other words, regardless of the rpm, the blower
always delivers the same amount of air to the cylinders.
Routine preventive maintenance
Always follow the recommendations and maintenance procedures specified
by the manufacturer in the service manual. Always keep records of tasks
performed so that there is less chance of missing something later on. Routine
maintenance requires:
•
regular oil particle tests
•
regular oil changes
•
regular filter changes (for air, oil, and fuel)
•
daily checks for leaks and missing bolts.
Good routine maintenance is also a good form of preventive maintenance in
that, there is less of a chance of breakdown during operation. Refer to
Chapter 20: Preventive Maintenance. Regular replacement of parts
(including parts not yet worn out) contributes greatly to the longevity of the
engine. Again, the service manuals of each machine provides a good guide
of what needs to be replaced and what number of hours.
)
Shut·down
Another form of preventive maintenance that is crucial to good engine
performance is the shut-down. Manufacturers suggest that after a given time
the engine must be shut down to disassemble the engine and check all
tolerances. The periods between shut-downs depend upou manufacturers'
specifications, need, or a combination of both.
All major parts must be within certain tolerance standardq. Depending upon
costs of rebuilding or replacing parts, meeting tolerances is a major factor in
rebuilding the engine. Certain parts such as piston rings and bearings are
usually changed during shut-down.
Steam turbines
Steam turbines take the high internal (thermal) energy supplied hy
superheated steam, and convert it to useful kinetic energy. The work
supplied by the turbine is available to many applications--the most common
is the generation of electricity. Steam torhines are versatile sources of energy
and are the principal stationary prime movers. They have a very wide range
of capacity and the steam they use may he genemted from any type of fuel.
19
High-pressure steam turbine-generators use superheated steam. The steam's
temperature is usually between 400°C and 560°C (7500 P and 10500 P). Its
pressure ranges from 1.4 MPa to 3.1 MPa (200 psi to 4500 psi). Note that
MPa is the symbol for megapasca/ (a million pascals). Superheated steam is
used because of its very high internal energy and because it is dry. Any
moisture passing through a turbine may cause severe internal damage.
These generators can generate electrical output from 0.5 MW to 1500 MW
depending on their size and application. MW is the symbol for megawatts (a
million watts). In a turbine-generator, electrical power is generated in two steps:
•
In the turbine. internal energy of steam is converted into mechanical
energy by the turbine shaft which is coupled to the generator shaft.
•
In the generator, which is coupled to the turbine, mechanical energy is
converted into electrical energy.
The greater the temperature difference across the turbine, the more efficient
the turbine-generator is and the more electrical energy it can produce for a
given input.
Definitions and construction
Turbine
a rotary engine or motor driven by water, steam, or air
Nozzles
steam inlets
Steam chest
a housing with inlet steam nozzles
Diaphragm
a stationary element (held by turbine casing) housing
the steam nozzles
Rotor
a rotating component attached to the turbine shaft
Blade
rotating elements (attached to the rotor or wheel)
which the steam pushes against to turn the rotor
Stage
a section of a steam turbine made up of a stationary
nozzle and a moving blade.
Governor
a valve controlling the amount of steam admitted to
the turbine
Extraction
bleeding off steam at intennediate pressures
Condenser
the component that condenses steam from the turbine,
producing a vacuum at the exhaust of the turbine.
Figure 20 is a sectional drawing of a small steam turbine sbowing several
stages. Each stage is made up of a stationary nozzle followed by a moving
blade. The purpose of staging is to use the maximum energy of the steam as
it flows through the turbine.
18
Rotor
Steam
inlat
Governor
Exhaust outlet
Oil reservoir
)
Figure 20 A small steam turbine showing its stages
R.produced coul'fBsy of the LeBming Resou"",. Unit, British Columbia Institute of Technology
Principles of operation
As the steam passes through the nozzles, it drives the rotor as it passes over
the blades. The direetion of the sleam is reversed in each stage. This results
in the rotor turning. The force created by the steam jets on the blades
produces the mechanical energy to turn the rotor. This in turn drives the shaft
which may drive a generator.
Steam pressure and temperature are lost as the steam moves through the
turbine. These losses represent the conversion of internal energy to
meehanical energy. The flow through a turbine is as follows (see Figure 20):
1. Steam enters the turbine through the throttle or main shut-off valve.
2. It passes through the over-speed trip valve. (On many turbine-generators
the throttle valve and the trip valve are combined into one valve.)
3. Steam then enters the governor valve. The amount of steam admitted to
the turbine when up to speed is controlled by the governor valve. (The
governor valve and how it works will be dealt with later in the module.)
4. Steam then travels to the steam chest which houses the inlet steam
nozzles. The number of inlet nozzles varies depending on the size and
the eleetrical output of the generator.
-21
5. Steam passes through the nozzles in the steam chest to the first stage
blading of the rotor (or wheel). The rotor is attached to the turbine shaft.
6. Steam then travels onto the next set of nozzles housed in the diaphragm.
The diaphragm is stationary and constructed in two pieces, and is held in
place by the turbine casing.
7. Steam again leaves the nozzles to the blading and this process is
repeated depending on the number of stages in the turbine.
8. In some cases, some steam is bled off at an intermediate pressure, before
the exhaust outlet, to a higher pressure process steam system. This is
called extraction and will be explained in detail later.
9. Steam then passes out of the turbine through the exhaust, to either a
process steam system, or to a condenser.
Casings and flow
The casing of a turbine includes the blades, rotor, shaft, and seals.
Schematics of the different casing layouts are shown in Figure 21. Although
radial flow turbines have become morc common in industry (especially in
pulp and paper) axial flow turbines are still the type most often found. Axial
flow turbines are constructed more simply than radial flow turbines.
A
B
c
o
E
Figure 21 Turbine casings
16-
Axial flow turbines
In axial flow turbines, the steam follows along the axis of the turbine shaft as
it flows through the blading to the exhaust port. Axial flow turhines may be
one-casing turbines, two-casing turbines, etc. A two-casing, tundem
compound turbine is shown in Figure 22.
Turbine
Figure 22 Two-casing, tandem-compound, axial flow turbine
In the low-pressure casing, the steam enters at the centre of the casing and
exhausts from either end of the rotor.
Radial flow turbines
Radial flow turbines are turhines in which the steam flow is perpendicular to
or at a right angle to the turhine shaft as it passes through the turbine blading.
In radial flow turbines the steam flow is reversed in every set of blades.
There are no stationary nozzles other than in the first stage inlet nozzles.
Every second set of blades is driving a turbine rotor in the opposite direction
to its neighboring set of blades. It is like a turbine rotor within a turbine
rotor, one turning clockwise and the other turning counter clockwise.
Casing
Outlello condenl.'"
Figure 23 Radial flow turbine
A typical radial flow turbine is shown in Figure 23. It has one turbine casing
and two interlaced turbine wheels. Each set of blades is attached to a shaft
and drives a separate generator. The generators are coupled to opposite ends
of the turbine.
Radial flow turbines are very useful because they operate with:
•
•
small size-to-output ratios
•
high efficiency
•
•
low-thrust
lighter foundations,
quicker start-up.
Back-pressure turbines
Back-pressure turbines use high-pressure stearn to drive the turbine and
exhaust to a low-pressure stearn system. As the name implies, the exhaust
steam from the turbine is under a back pressure. The low-pressure exhaust
steam system supplies steam required for different mill processes. The stearn
pressure is held constant by a stearn pressure reducing control system.
Figure 24 is a typical back-pressure turbine.
The pressures for the back-pressure systems are normally the lowest pressure
system in a mill, in the range of 345 kPa to 590 kPa (50 psi to 85 psi), The
high-pressure supply steam systems are normally in the range of 4 MFa to
9 MPa (600 psi to 1300 psi).
Back-pressure turbines are normally used on relatively small generators in
the 500 k W to 2500 kW range. The main uses of back-pressure turbines are
for induced draft fans, forced draft fans, feed water pumps, and other types
of large fans and pumps. When used in this type of service they are normally
one-or two-stage turbines of 150 kW (200 hp) to 750 kW (1000 hp).
Back-pressure turbine-generators are used where there is a high-pressure
boiler combined with a need for low-pressure stearn in the process systems.
The greater the pressure difference between the high-pressure steam and the
low-pressure steam, the more economically attractive it is to use this type of
turbine-generator.
18
24
,
I
Actuator
piston
rod
-----$haft
-_.---------
Stationary / k"''''~
nozzles
Moving
blades
Extraction
steam
Exhaust
steam
Figure 24 Back-pressure turbine
Back.pressure/extraction turbines
Back-pressure/extraction turbines also use high-pressure steam to drive the
turbine and exhaust to a low-pressure steam system. In addition, they allow
steam to be extracted at intermediate pressures through extraction valves.
This steam is referred to as extraction steam.
Back-pressure/extraction turbines are very common turbine-generators in
pulp and paper mills. The main reason for this is that the mills always have
at least three different steam pressure systems, of which two are for process
steam. Figure 25 on the next page is a cross-section of a back pressure/
extraction turbine.
Governor valve
I
Extraction valve
Extraction steam
Figure 25 A back-pressure/extraction turbine
The governor valve
A back-pressure/extraction turbine is designed so that it can produce full
horsepower with no steam being extracted and all of the steam passing to the
exhaust. The turbine can then produce full power from the generator. To do
this. the governor valve is fully open.
The extraction valve
As the amount of extraction steam is increased the kilowatt output of the
generator decreases. This is because a portion of the steam being admitted to
the turbine does not travel through all of the turbine blades. This reduces the
amount of work that can be done by the steam.
•
If more extraction steam is required. the extraction valve is closed. This
shuts off the sleam to the lower pressure end of the turbine. This in tum
forces more steam out to the intermediate stoam pressure system.
•
If less extraction steam is required. then the extraction valve is opened.
This allows more steam to flow to the low-pressure exhaust of the
turbine.
The control of the extraction valves can be used to balance the generator
output and the steam load.
Check valve on the extraction steam line
The extraction steam line must be equipped with a check valve to ensure that
no intermediate process steam is allowed to feed back into the turbine.
Condensing turbines
In a condensing turbine, the high-pressure steam is used to drive the turbine.
It exhausts to a vacuum produced by the condenser. The condenser
condenses the steam from the turbine. When the volume of steam reduces, a
vacuum is ereated at the exhaust of the turbine. A condensing turbine
exhaust runs at a vacuum of 95 kPa to 98 kPa (28"Hg to 29"Hg).
Figure 26 shows the flows in a 75000 kW boiler turbine. Condensing
turbines therefore have the largest pressure drop possible across a turbine
making them the most efficient type of turbine. A condensing turbine uses
fewer pounds of steam per kilowatt generated than other types of turbines.
Generator
Steam
Steam
generator
850 PSI 900"F
660,000 Ibe per hour
Load
L]=l
I
1m
Electrical current
92"F steam
Air ' "
113
heater
/L.....I._ _.....1..---I
Condenser
-II-- -....
L--,;-...I......
Fuel
\
)0-1>
92°F water
344"F
172°F
Ig8
=< 0
1.5 ~
ji~
\
Q.
\,,,""
\"'~
,~
\~
T'
~ ;--~='1 \ ' \
Condensate 1
pump
1
1
\...
&00 \;
Pump ~
~-~
~River
1500 HP
Pump house
------
Figure 26 Flow diagram for a 75 000 kW boiler turbine
The steam is condensed by a tube and shell heat exchanger using water to
cool and condense the steam. The only product which can be used in a
process from a condensing turbine is the condensate from the condenser,
which is used for boiler feedwater make-up.
18
Extraction/condensing turbines
The elttraction/condensing turbine is similar to the back-pressure/extraction
turbine-it feeds to botb the intermediate-pressure and low-pressure steam
systems from the extraction stages. The exhaust steam is fed tu a condenser.
An extraction/condensing turbine can be designed to produce the maximum
electrical load when tbe turbine is exhausting to tbe condenser, wilh no
extraction. It can also be designed 10 produce tbe maximum electrical load
witb botb tbe extraction steam systems open and some condensing taking
place. This type of turbine-generator is very good for starting up a mill
where purchased power from an outside utility is limited or very expensive.
Extraction
All of the steam going 10 the turbine can be condensed for the production of
power, or it can be extracted for process as required. Therefure, maximum
power can be produced for use in a mill for start-up beforc the processes
require full process steam. As the mill cOmes up to full production, the
extraction stages can be used to supply a large portion of the intermediatepressure and low-pressure process steam.
As with tbe back-pressure/extraction turbine, the extraction valves are
opened to reduce the amount of extraction, and closed to increase tbe
amount of extraction. This balances generation and steam load.
These types of turbines can be single extraction or dual extraction. That is,
steam may be extracted from more than one extraction valve in order to
supply more than one steam process.
Rotating elements
The rotating element of a steam turbine includes the turbine shaft, the turbine
wheels, the attached turbine blades, the blade shrouding, tbe thrust bearing,
bearings, and the coupling. The rotating element is the primary driver of the
generator which is producing the electricity. As the steam passes through tbe
blades in the rotating element, its internal energy is converted to mechanical
energy in the turbine. The mechanical energy of the turbine passes via the
shaft and coupling and is converted to electrical energy by tbe generator.
Turbine blades
There are two basic types of turbine blading: impulse blading and reaction
blading. These are shown in Figure 27.
Impulse blades
Impulse blades are shaped so that the space between tbe blades does not
allow any pressure drop in the steam as it passes through the turbine. All of
tbe pressure drop occurs in the stationary blading or nozzles. All of the
thrust acting on tbe rotor blades is from tbe change in momentum, or
impulse, of the steam as it changes direction in tbe blades. As can be seen
the inlet opening between the blades is the same size as tbe outlet opening.
y.-.j
~~utl&t_j
:
a.lmpulse
Y"
,
b. Reaction
Figure 27 impulse and reaotion blading
Reproduced courtesy of the Learning ReSQU/'Cf1S Unit, British COlumblsinstitute of Technology
Reaction blades
Reaction blades are sized and shaped to cause the pressure of the steam to
drop as it passes through the blade. This produces an increase in steam
velocity and this increase causes a reaction or back thrust on the blade as the
steam exits the blade. This force is rather like the thrust caused by air
flowing over an airplane wing. Iu these blades, the inlet opening between the
blades is larger than the outlet opening.
Impuise/reaction turbine blades
Some turbines usc only impulse blading, some use only reaction blading.
But, for mUltiple stage turbine-generators a combination of both types is
most common. These are called impulse/reaction turbines. Impulse blading
is used in the first, or higher-pressure stages of the turbine. Reaction blading
is used in the later, or lower-pressure stages of the turbine.
The blade-shaft attachments
The blades of the turbine are attached to the shaft in two ways. One is by a
disk or wheel as shown in Figure 28 on the next page. In this. the wheels are
attached to the shaft and the blades attached to the wheel.
The other is the drum type. The drum is a raised portion of the shaft or an
enlargement of the shaft. In some designs the drums on large turbines are
increased in diameter towards the low-pressure end of the turbine.
Figure 28 Disk blade attachment
Reproduced courlelly of the Lesrnlng RIJ8(JUI'CfIs Unit, British Columbls Institute or Technol"9Y
Shrouding and tie wires
A turbine is more efficient if the blades are rigid and vibrate as little as
possible. To accomplish this, thc end of each blade is mounted securely in
the wheel. The outer rim of the blade is held in place by a thin metal plate.
This metal plate is called the shrouding. It is attached to the blades by an
extension on the end of the blade (sec Figure 29). This extension protrudes
through a slot in the metal plate and is riveted to it. In very large turbinegenerators the exhaust blading may beeome too large to be practical for
shrouding to be effective. Tie wires are then used to join the blades together.
18-
Shroud
Tie wires
Dowel
Blade wheel
)
Shrouded blading
Unshrouded blading
Figure 29 Shrouding and tie wires
Journal bearings and thrust bearings
Compound turbines help to reduce some of the turbine's axial thrust. Journal
bearings and thrust bearings ensure that virtually no movement takes place
radially or axially.
Journal bearings
The bearings at each end of the turbine are calledjoumal bearings. They
allow the rotating element to rotate at high speeds within the casing of the
turbine. These bearings carry tbe entire weight of the rotating element.
Figure 30a shows one type of a journal bearing.
With the low clearances in turbines, the bearing tolerances must be very
exact to ensure moving parta do not come into contact with stationary parts.
The journal bearings shown are a split, babbitt-liner type. They are held
tightly to the shaft by the bearing housing. (Babbitt is a soft alloy metal,
made up of tin or lead, copper, and antimony in various proportions used to
reduce friction.) Lubricating oil is pumped through the bearing to ensure that
the entire beariug is lubricated. The two sections are held tightly to the shaft
to ensure minimum radial movement of the shaft. That is, they prevent any
excess movement vertically or horizontally.
-31
Underside of pad
showing step on
which it tilts \
Radial
bearing
W
b. Thrust
a.Journal
Figure 30 Journal and thrust bearings
Reproduced courtesy of the Lesrnlng Rtnlourcee Unit, Brltleh Columbiainelilute of Technology
Thrust bearings
The thrust created by the steam acting on the rotor is counteracted by the
thrust collar. or thrust bearing. A typical thrust bearing is shown in Figure
30b. The thrust collar can be a machined portion of the shaft which rides on
thrust plates to stop any axial movement of the rotor, or it can be a collar
which is keyed to the shaft.
The shaft, coupling, and gearing
In turbines, both ends of the turbine shaft are utilized. One end of the shaft
supports the coupling. This is the mechanical device used to join the turbine
rotating element to the generator. The other end of the shaft contains gearing
which drives the main oil pump. In some smaller turbines, it also drives the
governor. This is discussed later under lubricating and governor control.
Steam chests and nozzle blocks
Mter the steam passes thruugh the governor valve, the casing of the turbine
houses the inlet steam in a cavity or steam chest. This stearn chest is built
into the turbine casing when the original casting is done.
The nozzle blocks are mounted into the steam chest. These can be seen in
Figure 31. The nozzles admit the steam into the first set of moving blades.
The nozzles may be arranged singly or in multiples (sets). The nozzles for
the subsequent stages are mounted in the diaphragms of each stage.
Governor
driven
cam
Governor
valve
,i~- tt"-"-~\,---~~-jl- Nozzle
block
)
Figure 31 Turbine casing and nozzle blocks
Smaller mechanical drive turbines may have manual valves which allow
steam to be opened or closed to additional nozzles as more or less power is
required. In larger turbine-generators the steam to each set of inlet nozzles
can be opened or closed depending on the load on the generator. This is done
by the governor system which is described in greater detail later in this
chapter.
Diaphragms and steam nozzles
In each stage of the turbine, steam nozzles (known as stationary blading) are
housed in the diaphragms. The nozzles are designed so that they converge
towards the outlet. In other words, the inlet is larger than the outlet. This
type of design causes the pressure to drop as the steam passes through the
nozzles. As the nozzles are stationary, the pressure energy is converted to an
increase in steam velocity as it goes through the nozzles.
The diaphragms are constructed in two halves. One half is mounted in the
lower section of the turbine casing, and the other half in the upper section.
Each half of the diaphragm is securely mounted in the casing of the turbine.
18-
Disk rotor diaphragms
Figure 32a shows a diaphragm and nozzle ammgement used for a disk type
of rotor. The diaphragm for a disk rotor is securely held in place to the
casing by a series of socket-head capscrews. The rotor disk has a seal
mechanism on the casing and the diaphragms have a seal mechanism on the
shaft. These minimize the steam leakage from one stage to the other and stop
steam from short circuiting. This seal mechanism is a simple labyrinth.
a.
b
Figures 32 Diaphragm for disk and drum rotors
Reproduced counesy 01 the Lesrnlng Rssources Unit, Brll/sh Columbia Inalltu/s
0'
Technology
Diaphragm
Drum rotor diaphragms
The diaphragm for a drum type rotor
can be installed the same as a disc
type rotor diaphragm. Alternatively,
it can be built in short curved
segments and these segments
machined to fit into a slot in the
casing of the turbine as shown in
Figure 32b.
The segments are then slid into
position in the two casing halves.
The seal for this type of diaphragm
is similar to the wheel type, but it is
made to the drum rather than the
shaft. Figure 33 shows one type of
seal for the wheels and diaphragms.
Wheel
Figure 33 One type of seal for wheel
and diaphragm
A.
Ret~iner
segment
Carbon-ring
segment
e.
Tension
wire
Figure 34 Carbon seal
Shaft seals
Shaft seals are a very important part of the turbine because they add to the
efficiency of the turbine and to the safety of the area around the turbine. To
do this, they:
•
keep high-pressure steam from leaking into the surrounding area
Any loss of steam due to leakage is a loss in turbine efficiency. Also,
high-pressure, superheated steam is not visible in the immediate area of
a leak. Any noise a steam leak makes is drowned out by the relatively
high noise level in the generator room. Nearby workers can easily be
burnt by the leaking superheated steam.
•
stop air from being admitted into the turbines
In a condensing turbine, any air sucked into the turbine also lowers the
turbine's efficiency by decreasing the vacuum so that maximum vacuum
could not be attained.
The two common type of shaft seals used in turbines are carbon seals and
labyrinth seals. Figures 34 shows carbon seals using different methods of
securing the retainer segments. Figure 35 shows a labyrinth seal.
Carbon seals
Carbon seals are used on most smaller sized turbines using low to
moderately high steam pressures in the range 1.4 MPa to 4 MPa (200 to 600
psi). Carbon seals contain graphite and are self lubricating. They are
constructed in sections and held tightly to the turbine shaft by springs as can
be seen in Figure 34. The shaft, in the area of the carbon seal, is often treated
with MoneJTM metal spray. This hard, corrosion-resistant surface is then
machined smooth. This ensures a tight seal of the shaft for extended periods
of time. Carbon seals are also used in conjunction with labyrinth seals on
mid-sized turbines with pressures up to 6 MPa (900 psi).
Labyrinth seals
Labyrinth seals are used exclusively on large high-pressure turbinegenerators. These seals provide a complex path for the steam to follow in
order to escape from the turbine casing (see Figure 35). This ensures that any
steam escaping is at a very low pressure.
Several types of rings and fingers are used to create a labyrinth seal. The
labyrinth seals are grouped in sets