HYDRAULIC
PROPERTIES AND SYSTEMS
Hydraulic Fluid
The efficient operation of any hydraulic system depends as much on the
liquid, which transmits the power as the mechanical components.
Basic requirements of a hydraulic fluid.
 Low compressibility.
 Adequate fluidity to permit efficient transmission of power.
Primary function of a hydraulic fluid.
 Transmit power.
 Lubricate the moving parts.
Additional desirable qualities of a hydraulic fluid.
 Provision of seal between moving parts.
 Minimise wear.
 Protect the system from corrosion.
 Low freezing point.
 High boiling point.
 Will not damage seals and flexible tubes.
Mineral Oil
Petroleum base hydraulic oil is the most commonly used oil for industrial and
mobile applications as it has the following desirable qualities:
 Good lubrication.
 Prevents rust.
 Good sealing ability.
Note: Mineral oil is not suitable for industrial applications where fire resistance
is required.
Fire Resistant Fluids
A fire resistance fluid is one which is difficult to ignite. It is used in hydraulic
applications where there is a danger of a ruptured line or leaking fitting
spraying the hydraulic liquid onto a source of ignition.
Basic types:
 Emulsions, both water in oil and oil in water.
 Glycol water base.
 Synthetics.
 Phosphate esters.
 Halogenated compounds
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 Silicone.
Emulsions
Emulsions, particularly water in oil and Glycol in water, obtain their fire
resistance from the snuffing and cooling action of the steam which evolves when
the fluid contacts an ignition source.
Phosphate Esters and Silicon types have inherent fire resistant characteristics
because of their chemical composition.
Hydraulic Fluid Comparison Chart
Petroleum
WaterEmulsion
WaterGlycol
Phosphate
Ester
Poor
Fair
Very
Good
Good
Poor
Fair
Good
Very Good
Cost compared
to Petroleum
1
1
Stability
Excellent
Good
Excellent
Excellent
Excellent
Good
Excellent
Excellent
Excellent
Very
Good
Excellent
Fire
Resistance
Bare Flames
Hot Surfaces
Lubricity in
Pump
Balance Vane.
Gear.
2 to 2.5
cost of
3.5
petroleum
oil
Excellent
Temperature
Limits
-180 to540C 100 to
490C
00 to 490C
00 to 540C
Corrosion
Protection
Very Good
Good
Good
Very Good
Compatibility
Excellent
Very
Good
(except
paint)
Very
Good
(except
paint)
Good
(except
paint,
rubber,
plastic)
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HYDRAULIC COMPONENTS
Hydraulic Pumps
Hydraulic pumps convert mechanical energy into hydraulic energy.
They provide the driving force to move liquid under pressure thus transmitting
power (Power = Pressure x Flow Rate).
A major advantage of the hydraulic pumps is that they can deliver power in a
package of small size and weight, unmatched by any other power-transmitting
device.
The hydraulic pump performs two functions during its operation:

Creates a negative pressure (vacuum) at its inlet allowing
atmospheric pressure to force liquid from the reservoir into the
inlet port of the pump.

It delivers (forces) the liquid out of the discharge port into the hydraulic
system.
Note: Resistance to the delivery of the liquid from the pump causes pressure in
the system.
Categories ;
1.
Positive Displacement Pumps
(Hydro-static pumps)
Pumps are classified into one of two groups.
These pumps have a positive mechanical seal between the inlet and discharge
ports.
There are two basic categories of Positive Pumps:

Fixed Displacement.

Variable Displacement
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2.
Non-Positive Displacement Pumps
(Hydro-Dynamic)
These pumps do not have a mechanical seal between the inlet and discharge
ports; therefore the discharge flow rate is greatly influenced by the
pressure at the discharge port.
Types of Hydro-static;
1.
External gear pumps
(Hydro-static pump)
Gear pumps have a positive and fixed displacement, and are often used for the
following reasons:
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
They cost less to manufacture than most other pumps.

Simple in construction, in most pumps the gears are the only moving
parts.

They can operate against pressures up to 21MPa and sometimes higher.
The pump consists of drive and a driven gear enclosed in a closely fitting
housing.
As the teeth of the two rotating gears come out of mesh, a low-pressure void is
formed at the inlet of the pump. Oil from the reservoir is forced into this lowpressure void by either atmospheric pressure and/or gravity.
The oil at the pump inlet port is trapped between the gear teeth and pump
housing. It is carried around to the discharge port of the pump and forced out
through the discharge port into the hydraulic system.
Note:
Frictional and load resistance on the discharge flow from the pump will cause
pressure.
2.
Axial In-Line Piston (Plunger) Pump
(Hydro-static pump)
These pumps have a cylinder block, which is mounted on the drive shaft and
rotates with the shaft.
The pistons stroke in the bores of the cylinder block, which are parallel to
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the axis of the block. The heads of the pistons are in contact with a tilted plate
called a swashplate.
The swashplate does not rotate but it can be tilted back and forth. It is
mounted on a pivot and is controlled either manually or by an automatic
servo device.
If the angle of the swashplate were fixed, the pump would operate as a fixed
displacement pump, delivering the same amount of oil with each revolution.
If the swashplate is tilted the angle between it and the pistons causes the
pump to have a displacement, proportional to the angle of the swashplate.
The angle of the swashplate controls the distance the pistons stroke their bores.
The greater the angle the further the pistons stroke, and more oil is discharged
from the pump with each revolution.
3.
Un-Balanced Vane Pumps
(Hydro-static pump)
The unbalanced vane pump uses the same basic principle of a turning
rotor with vanes working inside a fixed rotor ring.
However, the operating cycle only happens once each revolution.
So this pump has only one inlet and one outlet port.
The slotted rotor is offset in a circular ring, resulting in the discharge
pressure “induced force” acting against the pump shaft and its bearing,
resulting in increased wear and potential pump failure.
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ANCILLARY EQUIPMENT
Hydraulic Reservoirs
Capacity
A hydraulic reservoir should contain fluid so that its working level is always
maintained high enough to prevent a “whirlpool” effect at the pump inlet.
It should also have enough capacity to hold the system’s fluid when the
equipment is in the shut down state.
As a general rule the reservoir should contain at least three (3) times the
pump capacity per minute.
Construction
1. Welded steel with large clean out / inspection cover.
2. Internal surface should be protected from rust.
3. Breather or vented cap is used on most reservoirs with the inclusion
of an air filter screen.
4. Drain plug.
5. Baffle plate to separate return line fluid from the suction line.
The baffle plate prevents:
 Turbulence.
 Foreign material, air etc returning into the inlet.
6. Sight glass to provide a visual check on fluid level.
7. Drain line should be constructed so that it is always below the fluid
surface.
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Accumulators
Purpose:
An accumulator is primarily a device for storing pressurized hydraulic fluid.
Function:
1.
2.
3.
4.
5.
As a shock absorber.
To provide oil make - up in a closed circuit.
To compensate for leakage in a system.
To provide a source of power supply in emergency.
To maintain steady delivery pressure over a period of time without
keeping the pump operating.
Types:
1. Weight loaded
2. Spring loaded
3. Air or Gas Accumulators
Accumulator
Weight Loaded or Gravity Type
The weight-loaded type consists of a movable piston and a weight. As
hydraulic oil is pumped into the cylinder, the piston pushes the weight higher,
increasing the potential or stored energy of the weight. The potential energy
is released with the downward motion of the weight.
An accumulator of this type is custom built for a particular installation. A single
large accumulator may provide service for a number of different machines.
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Accumulator
Spring Loaded Type
The spring-loaded type consists of piston loaded with a spring. Adjustment
of the spring is sometimes provided.
As the oil is pumped into the accumulator the piston compresses the spring.
This energy is stored in the spring and is released when required. The
pressure on the oil is not constant for all positions of the piston because
the spring force depends on the compressed length of the spring. Usually
this type of accumulator delivers only a small amount of oil at low pressure.
Accumulator - Air or Gas Type
Hydraulic oil is nearly incompressible. This means that a large increase in oil
pressure results in a small decrease in the volume of air or gas. Oil cannot
therefore be used to store useable energy. But gas can be compressed to store
energy. The gas acts in a similar manner to the spring
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Categories;
Non-separator types consist of a fully closed cylinder, air nitrogen or inert
gas if forced into the cylinder to pre-charge the accumulator. As a greater
quantity of oil is pumped into the accumulator the gas above the oil is compressed
still further storing the energy in the compressed gas.
This type of accumulator should be mounted in a vertical position because
the gas must remain at the top of the accumulator. Aeration can occur with
this type of accumulator. To prevent the gas being exhausted into the oil, only
about two thirds of the accumulator volume can be used for the air or gas
volume.
Heat Exchangers
Heat in hydraulic circuits is generated through the dissipation of pressure energy
that produces work.
Hydraulic components, pumps, valves, etc contribute to heat generation by
internal friction.
Heat build-up results in decreased efficiency and shortened system life.
Heat build-up can cause:
1. Deterioration of hydraulic fluid.
2. Shortened seal life.
3. Accelerated wear of moving parts.
4. Safety hazards.
5. Power loss.
6. Increased cost due to viscosity change.
7. Loss of lubricity.
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Pressure Gauges
Pressure gauges are used in fluid power equipment to provide:
1. An indication of the operating pressure, especially where the pressure
must be chosen by the operator.
2. An indication or alarm of abnormal pressure within the system.
Pressure and pressure change within a system must be correct for proper
operation of hydraulically powered or controlled equipment. The pressure
gauge indicates this pressure and helps to prevent malfunctions.
Gauges can also be calibrated in values proportional to pressure, such as total
force exerted by a hydraulic cylinder.
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Types:
1. Bourdon tube type
2. Spring loaded piston
Bourdon Tube Gauge
Bourdon tube type gauges are used to measure from vacuum to above 140Mpa
(20,000 psi)
Advantages of Bourdon
Tube Gauge:
1.
2.
3.
4.
5.
Accuracy
Ruggedness
Reliability
Simplicity
Low cost
Operation:
One end of a tube usually formed into a segment of a circle is fastened to a
socket, which connects to a pressure source. The tube is flat on opposite sides.
When pressure is applied inside the tube, the walls deflect and tend to assume
a round cross section.
This sets up stresses that increase the coiling radius and the free end moves a
small amount. This movement is translated into rotary motion of an indicating
pointer by linkage and or gear arrangements.
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Spring Loaded Piston Gauge
Spring loaded piston gauges are less likely to be damaged than the Bourdon
type because they do not have levers, gears, cams or bearings. They are not as
accurate as Bourdon gauges but are most suitable for fluctuating pressures.
Operation: The fluid acts on a piston, which moves in a cylinder against the
resistance of a spring. A carrying bar or indicator moves with the piston along
a calibrated scale.
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a calibrated scale.
Spring Loaded Pressure Gauge
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Gauge Calibration
Calibration is the process of ensuring that the quantity indicated by a measuring
instrument is an accurate indication of the actual quantity being measured.
Gauge calibration is a sensitive and exact procedure, which requires ski
instrument shop.
Fluid Conductors
Pipe Size
Outside diameter of pipe conforms to the standard thread sizes and remains
constant regardless of wall thickness. Pipe sizes are designated by a dimension
(this size was originally the inside diameter of the pipe).
Pipe Threads
NPT NPTF BSPT BSP -
National Pipe Taper
National Pipe Taper (Dry Seal)
British Standard Pipe (Dry Seal)
British Standard Pipe
The NPT and BSP threads seal by flank contact.
The NPTF and BSPT (Dry Seal). Threads seal by destructive interference fit
along the thread crest.
A thread sealant (pipe dope or Teflon tape) must be used in assembling NPT
and BSP pipe threaded joints and is recommended for NPTF and BSP threads.
Flow Meter
The flow meter shown is a device used to measure the rate of fluid flow of a
fluid.
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It consists of a vertically mounted tapered glass tube through which the fluid
flows. The fluid enters at the bottom (small end) flowing to the top (large end),
causing the indicator to rise upwards in the tube to indicate the flow rate.
Since the tube is tapered the space between the wall and the indicator increases
as the indicator rises, allowing more flow through.
The indicator will rise to a height corresponding to the flow rate; the flow rate
is read from the graduations on the meter.
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Tubing
Fluid conductors
All types of tubing are made of relatively malleable materials. Thus, tubing can
be bent easily to reduce the number of fittings necessary for fabrication.
Sizes
Tubing is manufactured in standard size and is classified by the outside
diameter. Wall thickness is usually expressed in mm, as a decimal of inch
or as a gauge number.
Tubing Type
Steel tubing is the only tubing material permitted by J.I.C. (Joint Industry
Conference) standard without restriction. There are two types - seamless and
electric welded.
Seamless tube is produced by: the cold drawing of pierced or hot extruded
billets. Welded tube is made by shaping a cold rolled strip of steel into a
tube and then welding and drawing it to size.
Copper Tube
The use of copper tubing is restricted because it acts as an oil-oxidation
catalyst and tends to work harden when flared.
In addition, copper tubing has poor resistance to vibration. Vibration will also
cause the copper to work harden, making it brittle and likely to fracture.
The use of copper tubing is limited to stationary applications at low pressure
and to air circuits.
Aluminium Tubing
Aluminium tubing of seamless quality has good bending and flaring properties
and is suitable for low pressure applications.
Plastic Tubing
Plastic lines are made from three basic materials - PVC, Polyethylene and
Nylon.
Plastic lines are limited in their pressure rating.
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Pipe Application
Since bend radii affect pressure loss in lines, a minimum of 3½ diameters is
recommended for bends. Tubes are joined by flared or flare-less fittings.
Advantages of tubing include its adaptability to bending and flaring, vibration
resistance and heat conductivity.
Pipe Fittings
Pipe Fitting Table
Figure M164.2.01
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Tube Fittings
370 Flare
The 370flare provides excellent
results for connections when
tubing is flexible.
450Flare
The 450 flare may be used with
flexible tubing and will withstand
pressure up to 5,000PSI
The 450-inverted flare provides
protection for the seat and thread
This design provides
excellent results for highpressure hydraulic
applications
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Tube Fittings
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Tube Fittings
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Hose
The use of hydraulic hose permits relative motion between components.
Advantages
1. Use where there is severe vibration.
2. Compensation for manufacturing tolerances in piping.
3. Absorption of hydraulic shocks.
4. Where connections and disconnections are frequently made.
Reinforcement Construction
Materials used include natural or synthetic yarns or fibres, metal wires or
combination of these materials. The reinforcement may be braided, spiral wound
or both. Each group of wires is termed a Plait and each wire an End. The number
of ends in a Plait varies. A bonding material is applied between each component
of the hoses.
Size
Flexible hose for fluid power application range in standard sizes measured in
internal diameter.
Hose Fittings
A hose fitting couples the hose to pipe or tubing or accessories. A fitting consists
of two major parts: the portion that provides a means for attaching or connecting
to an accessory or to other fluid lines.
Types of Fittings
1. Non-Reusable
2. Reusable
The non-reusable fitting is crimped or swaged and is squeezed onto the hose and
in the even of failure the fitting cannot be reused.
Reusable fittings can be removed from a failed hose and installed on a new length
of hose.
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Hose
The information on this chart was sourced from Parker Hose and Fitting Catalog # 4400 March 1990 p.15
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Actuators
Hydraulic Actuators (Linear)
Hydraulic actuators perform the exact opposite function from pumps; they take
energy out of a hydraulic fluid and convert it into extension of a shaft
(movement) with the ability to overcome resistive force (load).
The force developed is a product of the piston area and the maximum pressure.
Force= Area x Pressure
e.g. 0.2m2 x 1 MPa
0.2 x 1 x 106
200000 newtons or 200kN
Operating Principal
Fluid is applied to one side of the Piston and the opposite side of the piston is
exhausted.
When calculating the force developed on the reaction stroke (Rod End) the
pressure does not act on the total piston area, the area of the rod must be
subtracted from the piston area.
Travel Speed
Cylinder travel is controlled by the quantity of fluid pumped into the cylinder.
Cushions
Hydraulic cylinders may be supplied with cushions on the rod end, blind end or
both ends. The cushion consists of a closed chamber close to the end of the
stroke; the fluid is trapped and metered out slowly in order to slow the cylinders
movement.
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Cylinder Types
Standard Double Acting
Provides a power stroke in both directions. This is the standard type used for the
majority of applications.
Single Acting
Where thrust is needed in only one direction, a double acting cylinder may be
used with the active end vented to atmosphere through a breather in the case of an
air cylinder, or vented to the reservoir below the oil level, in the case of an oil
hydraulic cylinder.
Double Rod
Are used where equal displacement is needed on both sides of the cylinder.
Sometimes the extra end issued to mount cams for machine tool applications.
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Cylinder Types
Ram Type Single Acting
This type has only one fluid chamber, and is usually mounted in a vertical
position. Used on lifting cranes.
Telescopic Type
Are used where collapsed length must be shorter than could be obtained with a
standard cylinder. They are available with up to 5 sleeves. Commonly used for
tray elevation on tip trucks.
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Rotary Actuators
SEMI-ROTARY ACTUATOR
Fixed
Vane
Housing
Moving Vane
Rotor
Rotary actuators produce oscillating power by rotating an output shaft through a
fixed arc. They produce high instantaneous torque in either direction and require
only small space and simple mounting.
The actuators consist of a chamber or chambers for containing the working fluid
and a moveable surface against which the fluid acts.
The moveable surface is connected to a shaft to produce the output motion.
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Types of Rotary Actuators
1. Linear Cylinder - with crank arm. 85 to 110 degrees.
2. Rack and Pinion - rotation available. 90, 180, 360 degrees.
3. Scotch Yoke - 90 degrees rotation.
4. Vane - Up to 280 degree rotation.
5. Helix - Rotation 100 to 370 degrees.
6. Sprocket - this type of unit is available with shaft rotation up to five (5)
complete turns. 1800 degrees.
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Fluid Motors
A fluid motor is a device, which converts fluid power into mechanical force and
motion.
Fluid motors are similar in basic construction to a hydraulic pump.
Basic Types
1. Fixed displacement
2. Variable displacement
In a fixed displacement motor, a fixed quantity of fluid is used for each
revolution; the speed will remain constant as speed is controlled by the quantity
of fluid introduced into the motor.
In a variable displacement motor the quantity of fluid can be varied by different
methods to control the speed of rotation.
Fluid motors can be applied directly to work applications, they provide excellent
control for acceleration, operating speed, deceleration, and smooth reversals and
positioning.
The use of fluid motors in operating units is called: Hydrostatic Transmission.
Design types of hydraulic motors
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Piston Motors
These follow a variety of configurations, the majority are multi cylinder units,
which may be in-line, vee, H, Flat 4 or Radial.
Other types include conventional reciprocating engine design such as crank-less
motor using a rotating piston cylinder assembly with slotted piston ends
traversing an elliptic cam ring. The most popular configuration is the "vee” for
four cylinder motors and Radial for three cylinder and upwards.
The number of cylinders can be odd or even and 3, 4, 5 and 6 cylinder radials are
all common.
Seals and Sealing
Static Seals
A seal that is clamped between two mating parts is called a static seal.
The clamped parts do not move, but the seal may move due to increases and
decreases in pressure.
Examples:
1.
2.
3.
Pipe thread connections
Mounting Gaskets
Seal rings on cylinder ends
Dynamic Seals
These are installed where movement takes place between two mating parts.
Examples:
1.
2.
3.
Spool Seals
Gland Packing
Piston seals
Seal Types
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O-Ring
This is the most commonly used fluid seal. It is moulded in synthetic rubber and
has a round cross section. O-Rings can be used in static and dynamic
applications.
T- Ring Seals
These seals are used in dynamic applications, i.e. reciprocating cylinders and
pistons. They are moulded in synthetic rubber. They act in a similar manner to Orings but because of shape does not have the tendency to roll in their grooves. H.
Section seals are similar.
Lip Seals
These are dynamic application seals and are used on rotating parts, ie. hydraulic
motors. They have positive sealing properties, which are assisted by increase in
pressure. These seals are made in rubber or leather supported by a pressed steel
housing. Single and double seal construction are available.
Cup Seals
A positive seal used on cylinder pistons. The pressure forces the lips against the
cylinder wall. This type of seal is backed up with backing plate and can be used
for very high pressure.
Piston Rings
These are of square section and are made from cast iron or Teflon. Used on
cylinder pistons, in sets of two or more and when plated offer less resistance to
motion than leather or synthetic seals.
Compression Packing
Compression packings are usually of the U or V form and are formed or moulded
to shape. They are made of leather or synthetic rubber and are generally used in
multiple packs. They are compressed into a gland by a ring that is tightened
against a female gland support ring. Excessive tightening of the gland ring or nut
will accelerate wear.
Some designs of packing are adjusted by a series of springs instead of the gland
nut. This permits for correct tension on the packing and increases the seal life also
decreasing the problem of over tightening of the packing.
Seal Materials
Synthetic rubber elastomer are used extensively for the manufacture of
O-rings and other types of seals. They are compatible with most oils.
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Nitrile (buna N) - is the standard for most service. It performs satisfactorily in
petroleum lubricants and hydraulic oils, petrol, water, alcohol and many other
fluids.
Temperature range: - 40 to 125 degree C.
Neoprene - provides excellent performance in refrigeration fluids and has good
resistance to oxygen, weathering and many combinations of chemicals plus
moderate resistance to petroleum oils.
Temperature range : -55 to 140 degree C.
Ethylene Propylene
Water, steam, brake fluids, acids, alkalis and phosphate eaters, all may be sealed
with O-rings of this compound. Oxygen and weathering resistance is also
excellent.
Temperature range: -55 to 150 degree C.
Silicone
This type of material is limited to static service. It is compatible with a variety of
fluids.
Temperature range: - 65 to 250 degree C.
Fluorocarbon (viton fluorel)
Compatibility with nearly all fluid types and high temperature stability are
attributes of this compound.
Temperature range: - 31 to 225 degree C.
Where seals are used, a good surface finish on the contacting metal surfaces is
required. Groove finish should be 32 micro inches or better and where the ring
rubs over a cylinder or rod, the finish needs to be 16 micro inches or better.
Back up rings are usually made of leather, rubber (hard) or teflon.
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Directional Control Valves
Directional Control Valves (DCV) provide a means of controlling when and
where fluid is required. The valves start, stop, accelerate, decelerate and control
direction of motion of actuators.
They are used to:
1. Hold an actuator in a fixed position and then direct fluid to move it in
either direction.
2. Permit free flow of pump output to tank at low pressure when high
pressure is not needed.
3. Vent relief valves by electrical or mechanical control.
4. Isolate branches of a circuit.
5. Select pressures by opening appropriate lines to relief pressure control or
to various pumps.
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Classification:
A.
Directional valves may be classified according to the type of internal
control element: sliding spool, rotary spool, rotary plate, poppet and ball.
B.
Directional valves may also be classified as two-way, three-way or
four-way, to indicate the type of circuit for which they are applicable.
The term one-way valve can be applied to a check valve, which has two ports but
permits flow in one direction.
Poppet and Ball Control Valves
These valves are two way or on-off valves; they are used as simple shut-off
valves.
Operating Pressures Range: 550 P.S.I. (4 MPa) to 5000 P.S.I. (35 MPa)
A four-way valve controls a double acting cylinder or hydraulic motor in both
directions by admitting flow to either side of the actuator.
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Spool Valve
Purpose:
To direct or block the flow of oil to a required circuit.
Types:
1. Two positions (forward, reverse)
2. Three position (raise, hold, lower)
3. Four position (raise, hold, lower float)
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Configuration: The major types of sliding spools are:
These configurations refer to the spool in the centre position.
Although a particular spool may be desirable for a specific operation, its effect in
the circuit must be considered. For example, an open centre valve may be desired
to open the pressure and cylinder ports to tank, but if other valves are to be
actuated while the valve is centred, the open centred type cannot be used because
the fluid will be lost to tank. Thus a closed centre or partially closed centre spool
is needed.
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Construction:
1. Hardened, ground and lapped steel spool accurately fitted to a cast iron or
aluminium alloy body.
2. Throttling slots machined in the lands to allow partial flow.
3. Machined grooves around the valve land to assist in lubrication, sealing
and centring to the spool in the bore.
Actuation:
1. Manual
Manual operation of a valve is used when the time element is not precise, and an
operator must initiate the required action. Because of the relatively slow action of
manual valves, flow forces become more critical and maximum capacity of the
valve may be lower than for pilot or electrically operated valves.
2. Mechanical
In a mechanical operated valve, a roller or other suitable connector replaces the
operating lever or knob (e.g. table control on surface grinding machine).
3. Air Actuated
Air actuated valves can exert higher forces than other means and can control
higher hydraulic flows with lower pressure air and without excessively large
actuators.
4. Electric
As electric solenoid is mounted on one or both ends of the valve. When an
electric current is passed through the coil windings, the spool is actuated. Because
of the ease of running electric wires, this type of control can be actuated from a
distance and is ideal when control is required in a hazardous situation.
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DCV Actuation
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Pressure Control Valves
Function:
 Protect the system from damage of excessive pressures.
 Set the upper limit of force exerted by a linear or rotary actuator.
 Precise control of system pressure permits hydraulic clamping of delicate
products, control of straightening parts after heat treatment.
Wide ranges of pressure control valves are available and will be either normally
open or normally closed valves.
The valves are named by the function and control they perform in a hydraulic
circuit.
The normally closed type blocks flow between two ports until an established
pressure is reached.
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The normally opened type is used for reducing, priority valves and pressure
switches. These valves permit flow through the valve until an established pressure
is reached; then the flow is restricted or blocked completely.
Relief Valves
System Protection
A "Safety Relief" valve should be built into all hydraulic circuits; control of
maximum pressure level is the first consideration. Dependability outweighs
considerations such as noise, chatter, etc.
Safety relief valves are not expected to operate unless there is a malfunction in
the system. They may be non-adjustable or have the adjustment protected from
tampering.
Operation:
A relief valve crates an orifice between the pressurized supply line and secondary
lower pressure area. Normally the relief valve is closed until the pressure level
reaches the pre-set value. As the system pressure rises, flow through the valve
increases until the entire pump output volume passes through the valve. When
system pressure drops the valve should close smoothly and quietly.
Valve Loading
Relief valves may be loaded by a spring or weight, which makes the spool or
poppet to a close position. Fluid under pressure moves the valve member against
the spring or weight to provide the fluid flow through the valve.
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Pilot Operated Relief Valve
Pilot operated relief valves use pressurized fluid to assist the main spool spring or
weight to hold the passage through the valve closed.
Fluid is directed through a restricted passage from the input supply to a control
chamber. The pressurized fluid acts against a piston and adds to the spring force.
A small capacity relief valve that is usually a poppet or piston type limits the
resultant force available. This piloted relief permits the assist fluid to pass back to
the low-pressure area at a pre-determined pressure.
A pilot operated relief valve can also be combined with an unloading function, by
diversion of the pilot pressure fluid back to tank. The relief valve control is
reduced to the value established by the main spool spring.
Some directional control valves may be combined with the relief mechanism to
provide either of two pressure levels.
A piloted operated relief valve may be provided with a separate drain if
restrictions are expected in the major return tank line.
Page 43 of 66
Balanced Piston Relief Valve
The balanced piston or spool (shown on diagram) is named because in normal
operations it is in hydraulic balance.
Pressure at the inlet port acting
under the piston is also sensed on
its top by means of an orifice
drilled through the large land. The
piston is held on its tank seat by a
light spring if the inlet pressure
less than the valve setting
When the pressure reaches
the setting of the adjustment
spring, the poppet is forced
off its seat limiting pressure
in the upper chamber.
The restricted flow through the orifice into the chamber results in an increase of
pressure in the lower chamber. This unbalances the hydraulic force and tends to
raise the piston off its seat. When the difference in pressure between the upper
and lower chambers is sufficient to overcome the force of the light spring (approx
150Kpa) the larger piston unseats permitting flow directly to tank. Increase flow
through the valve causes the piston to lift further off its seat but since this
compresses only the light spring very little over-ride is encountered.
Page 44 of 66
Pressure Reducing Valves
Pressure reducing valves control pressure downstream by limiting its outlet
pressure. These valves are used to control clamping pressures and variations in
system pressures.
Operation:
A normally open valve, it provides a free flow passage through valve from inlet to
secondary port, until a signal from outlet (downstream) side tends to throttle the
passage through the valve.
Pilot pressure from downstream acts against an adjustable spring holding the
valve open.
Application:
1. To control light clamping pressures for fragile objects.
2. To vary operating pressures in branch circuits, from normal downstream
pressure.
Page 45 of 66
Pressure Sequence Valve
Function:
A sequence valve is used to cause an operation to take place in a circuit in a
definite order or sequence. It is also used to maintain a minimum pressure in the
primary line while other operations take place.
Operation:
The fluid flows through the primary passage to operate the first phase until the
pressure setting of the sequence valve is reached. At this stage flow is diverted to
the secondary port to operate a second phase.
Application:
To clamp a work piece with the first stage, then hydraulically control the cutting
tool as the second stage.
These valves are drained externally since the secondary port is under pressure
when the valve sequences. If the pressure were allowed in the drain passage it
would add to the spring force and raise the pressure required to open the valve.
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Pressure Switches
Use:
Pressure switches are used to make or break (open or close) electrical circuits at
selected pressures to actuate solenoid operated valves or other devices used in the
system.
Operation:
An electrical switch is operated by a push rod which bears against a plunger
whose position is controlled by hydraulic and spring force. The pressure at which
the switches operate is selected by turning the adjusting screw to increase or
decrease the spring force.
When the preset pressure is reached the plunger will compress the spring and
allow the push rod to move down causing the snap action switches to revert to
their normal condition.
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Hydraulic Filters
A hydraulic filter is used as a device to remove soluble contaminants from a
liquid. These contaminant particles are trapped by porous material within the
filter.
When properly matched to a hydraulic system, a filter not only serves as
insurance but can also significantly reduce down time due to abrasive wear
caused by unfiltered particles.
Filter Rating
Filters are rated on their ability to retain contaminants of certain size.
Absolute Filtration Rating
The diameters of the largest hard spherical particle that will pass through a filter
under test conditions. This is an indication of the largest opening in the filter
element.
Filtration Ratio
The ratio of the number of particles greater than a given size (m) in the influent
(still) fluid to the number of particles greater than the same size in the effluent
fluid.
Mean Filtration Rating
A measurement of the average size of the pores of the filter element.
Note: Filtration ratings are in units of meters.
1 micron = 1 m = 10-6 metre
Nominal Filter Rating
An arbitrary value indicated by filter manufacturers.
How abrasive Wear Occurs
Particles the same size as or slightly smaller than the clearances interact with
surfaces to cause abrasive wear.
Large particles as shown on the sketch cannot get into the critical clearance areas,
very small particles less than 1 micron usually flow through without abasing
either surface.
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Filter Types
Strainers
Strainers are constructed of fine mesh wire screens, or of screening elements
consisting of specially processed wire of varying thickness wrapped around metal
frames. Strainers offer small resistance to flow and are used in pump inlet lines
when pressure drop must be kept to a minimum.
Strainers can be installed singularly or in parallel depending on the supply
demand of the pump. Any exposed inlet fitting must be airtight; strainers require
periodic cleaning, they prevent catastrophic failures caused by chips and other
large contaminants. This is the principal reason that suction strainers are used on
the inlet of many hydraulic pumps.
Filter Material
Filters
Filter elements are made of various materials such as paper, cellulose felt, glass,
and sintered powders of metal, ceramics and plastics.
These elements usually screen much finer than strainers and can remove particles
as small as 2 microns. Filters offer protection against abrasive wear, which in
most components is caused by silt (1 to 5 microns in size).
This type of filter should be used with strainers capable of chip control.
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RATING  m
Filter Material
100 mesh screen
200 mesh screen
Sintered woven
wire mesh
Resin
impregnated
paper
Ultra-fine
Paper
Cellulose
Felt
Glass
Sintered metal
Ceramics
Plastic
Chip
Control
Cleanable
Cleanable
Cleanable
Nominal
Mean
Absolute
135
70
10
140
74
17
220
105
25
Disposable
10
18
30
Silt Control
Disposable
0.45
0.9
3
Disposable
0.5 - 100
2 - 50
Disposable
2-65
13-100
Page 50 of 66
Filter Location
Location depends on the function for which the filter is to be used.
Filters used for chip control should be located as close as possible to the
components to be protected.
Example of such filters includes suction strainers protecting pumps; screens
mounted ahead of orifice and filters and screens mounted ahead of control valves.
Filters with the primary function of silt control can be located almost anywhere in
the system.
Examples include filters mounted in the pressure lines ahead of critical
components such as servo valves or hydraulic motors, return lines to tanks and
filters located in a separate cleaning loop in conjunction with a pump providing
continuous cleaning up of the hydraulic system.
For the best silt removal filtration, the filters should be full flow filters in a
location that passes all of the system flow.
Classification:
Filters can be classified as Full Flow or Proportional. In a full flow filter, all of
the fluid enters the unit, passes through the filtering element. Although the full
flow type provides a more positive filtering action it offers greater resistance to
flow particularly when it become dirty. For this reason a full flow filter often
incorporates a valve to automatically by-pass the element (return line filter).
Page 51 of 66
Air Service Unit
Air Preparation and Conditioning
The quality of compressed air used in today's industry varies greatly from the
extremes of totally oil free and sterile air as required by the food industry, to the
well lubricated dry air used in manufacturing machines.
To produce these wide variations of compressed air conditions, various
equipment must be employed, these being 1. Filters to remove unwanted contaminants.
2. Pressure regulators to control pressure.
3. Lubricators to supply correct amounts of lubricants.
Page 52 of 66
Air Service Unit Filter
All compressed air systems will contain contamination of some type, these being:




Moisture
Solids
Gases and odours
Oil
Moisture is the most common contaminate in either droplet or vapour form.
Vapour is very difficult to remove as once the droplets size is below one (1)
micrometer, mechanical separators of the cyclonic type are unable to remove the
vapour and other means must be employed, eg. air dryers.
Operation:
Dirty moist air enters the filter via the main body and is directed down into the
bowl as it passes from main body to the bowl, the air passes through a set of
directional vanes these impart a swirling motion to the air, this cyclonic motion
tends to cause heavy contamination to be flung by centrifugal force out to the
bowl where it gradually slides down the wall and into a quiet zone at the bottom
of the filter. This quiet zone exists below the splash shield, heavy contamination
in this area is unable to be picked up by the airflow and carried onto the filter
element.
The heavy contamination is made up of water and oil droplets larger than one (1)
micrometer and solids such as dust and rust particles.
Page 53 of 66
The air then proceeds through the filter element that may have a pore size from 5
to 30 micrometers; the filter element removes the remaining solids larger than the
pore size employed.
Solids -Solid pollution exists in compressed air systems in the following forms:
 Intake dust.
 Compressor wear products.
 Rust and scale from supply network.
Contaminates of this type are adequately removed by the selection of appropriate
filters of the conventional type. It is important to note that intake filters fitted
before the compressor remove between 96% and 99% of air bound solids from
intake air to compressor.
Liquid Oil and Aerosols - Highly efficient coalescent filters are commonly
employed for the removal of liquid oil and aerosols.
Operation:
Air is passed into the centre of the filter element, which is made up of microfibres enclosed in a shell of metal or plastic, this is then surrounded by open pore
plastic foam. The air passed though the boro-silicate glass fibres and the droplets
of aerosol impinge onto the fibres and run down the fibre to a crossover point,
when many droplets run together they coalesce into a large drop, which is forced
through the filter and appears on the outer surface of the filter as more and more
drops appear their weight causes them to run down the surface and finally collect
in the porous form from where it can be easily collected and removed from the
system. Coalescing filters are prone to be contaminated by solids and therefore
should be preceeded by a pre-filter.
Vapours and Gases - Micro-fibre filters cannot remove vapours and odours,
Vapours and gases must be removed by catalytic conversion by absorption onto
activated surfaces.
The most common activated surface is ‘activated carbon’, though it is not too
effective with carbon monoxide, carbon dioxide, acetylene, light hydrocarbons
and sulphur dioxide. These require catalytic conversion.
Note: The catalysts are adversely affected water vapours.
Page 54 of 66
Air Service Unit Pressure Regulator
Pressure Regulators (Pressure Reducing valves)
Air regulators are an essential part of correct air preparation within a compressed
air system, their purpose is to control the pressure being supplied to equipment,
be it a hand tool requiring full system pressure or a machine control circuit
requiring very low pressure.
The advantages of using regulators within a pneumatic system are:






Supply air at constant pressure regardless of flow.
Operate the system more economically by minimizing the amount of
pressurized air wasted.
Promotes safety by operating the tool or actuator at reduced pressure.
Extends component life, by decreasing wear rates.
Increase operating efficiency.
Able to have controlled pressures within a single system.
Balanced Poppet Self Venting Regulators operation
The poppet as the name implies, is pressure balanced. The poppet is exposed to
the same reduced pressure on both the top and bottom surfaces, therefore the
effects produced by reduced pressure fluctuations are cancelled out, and the
sensitivity and response are greatly improved. This type of regulator has a very
large flow capacity and good pressure regulation.
Page 55 of 66
Venting excess system pressure
As downstream pressure is applied to the diaphragm, it causes the diaphragm to
move down sufficiently to cause the top poppet to close thus preventing further
flow, and a bottom poppet valve to open allowing the increased pressure to be
dumped to atmosphere.
Compressed Air Lubrication
Most moving parts in an air system require some form of lubrication for efficient
operation, eg. cylinders, valves, motors. Some pneumatic components are
lubricated using an airline lubricator; this introduces oil into the system in an
atomised state.
Note:
 Correct lubrication decreases friction, prevents corrosion.
 Most new pneumatic automation equipment components do not require
lubrication. (refer to manufacture specifications)
Operation:
Air flowing through an automatic airline lubricator creates a differential between
air line pressure and pressure to the lubricator reservoir. This pressure differential
causes oil to be lifted from the reservoir into a sight feed dome and dripped
through a metering tube into a body section where the oil atomises and mixes
with the air flowing through the lubricator.
With an oil-mist lubricator, the air passes a deflector to atomise the oil. Both
methods depend on an increase in air velocity to atomise the oil. The lubricate air
then carries the oil-mist to the component.
Oil Fog - coarse droplet
Oil Mist - fine mist.
Lubricators are mounted downstream of the pressure regulator.
Page 56 of 66

Oil Fog mounted as close as possible to the equipment they are to serve.

Oil Mist can be installed above or below WORK STATION.
Air Service Unit servicing
Maintenance:
DO NOT allow the collected condensate to fill the filter bowl.
Cleaning Plastic Bowls
 Remove all air pressure from the air service unit before servicing.
 Remove the bowls taking care not to damage seals.
 Clean the bowls with a soft cloth and warm soapy water.
 NEVER use solvents to clean plastic (polycarbonate) bowls.
Cleaning Filter Elements
 Blow out with compressed air in the opposite direction to the normal
airflow.
 Replace with a new element if required.
Lubricator
 Use only a light mineral oil.
 Take care not to overfill the lubricator.
 Ensure the correct drip rate is set for the application.
Page 57 of 66
Review questions for
Learning
activity
1.
Section 2
– Fluid Power
What fluid is used in a standard Industrial Pneumatic system?
……………………………………………………………………………
2.
What fluid is used in an industrial hydraulic system?
……………………………………………………………………………
3.
List three basic types of fire resistance fluids.
…………………………………………………………………………….
…………………………………………………………………………….
…………………………………………………………………………….
4.
List the two categories of air compressors.
………………………….………………………………………………….
………………………….………………………………………………….
5.
List a compressor to represent each of the two categories.
………………………….………………………………………………….
……………………………………………..………………………………
6. Give another description for each of the following hydraulic pump
classifications.
Positive Displacement Pump or? ……………..……………………
Non- Positive Displacement Pump or?……………..………………
Page 58 of 66
7. Identify the positive displacement pump category from the given symbols.
Symbol
8.
Description
Label the five missing items of the hydraulic reservoir shown.
………………………………………………………………………..
………………………………………………………………………..
………………………………………………………………………..
………………………………………………………………………..
………………………………………………………………………..
9.
State the main purpose of an accumulator.
………………………………………………………………………
………………………………………………………………………
Page 59 of 66
10. List three accumulator types.
1. …………………………………………………………
2. …………………………………………………………
3. …………………………………………………………
11. Complete the following accumulator function statement.
To maintain steady delivery…………………………………..
……………………………………………………………...
……………………………………………………………...
12. Air receivers are used to:

…………………………………………

…………………………………………

…………………………………………

…………………………………………

…………………………………………

…………………………………………
13. Air Receiver Size is determined by:
…………………..…………………………………………
…………………..…………………………………………
Page 60 of 66
14. Heat build-up in hydraulic systems
can cause:
15.

…………………………………………

…………………………………………

…………………………………………

…………………………………………

…………………………………………

…………………………………………

…………………………………………
After-coolers are used to? (Tick your response/s)
.
Cool Hydraulic Systems
Cool Pneumatic systems
16.
Name two types of pressure gauges.
1. ……………………………………………………….
2. ……………………………………………………….
17.
What is the angle of JIC flare fittings?
…………………………………………………………..
18.
What is the angle of SAE flare fittings?
……………………………………………………………
Page 61 of 66
19.
What function do hydraulic
linear actuators perform?
……………………………………………………………………..
.……………………………………………………..……………...
……………………………………………………………………..
……………………………………………………………………..
20.
Describe a single acting hydraulic Ram.
…………………………………………………………….………..
………………………………………………………..…………….
……………………………………………………………………...
21.
What would be a typical application for a telescopic hydraulic actuator?
…………………………………………………………………….
………………………………………………………..……………..
22.
Complete the following statement.
A fluid motor is a device which ………………………………….
.……………………………………………………………………
Page 62 of 66
23. Name three design types of fluid motors.
1. …………………………………………………………..
2. …………………………………………………………..
3. …………………………………………………………..
24.
What does the fluid-power term DCV represent?
…………………………………………………………………
25.
Label the 4 DCV centre configurations shown.
26.
What is a major function of a pressure
relief valve in a hydraulic circuit?

…………………………………………..

…………………………………………..
Page 63 of 66
27. List two applications for a pressure-reducing valve.

……………………………………………………………….
………………………………………………………………

……………………………………………………………….
………………………………………………………………
28. A pressure-reducing valve is: (Tick your response/s)
.
Normally open valve.
Normally closed valve.
29. State an application for a pressure sequence valve.

…………………………………………………………………...
……………………………………………………………………….
30. List three locations for filters in hydraulic systems.

……………………………………………………..

……………………………………………………..

……………………………………………………..
Page 64 of 66
31. Identify the symbol shown.

……………………………………………….
32. Label the three components of an air service unit.
33. What is the most common contaminate of compressed air?
 …………………………………………………………
34. How is this contaminate removed from the compressed air?

…………………………………………………………
…………………………………………………………
…………………………………………………………
Page 65 of 66
35. Complete the following statement.
When cleaning the plastic bowl of an air service unit, you should NEVER.
………………………………………………………………………………
36. Complete the following. (Tick your response/s)
.
Oil fog compressed lubricators produce:
Produce coarse oil droplets.
Produce a fine oil mist.
Note:
Teacher will re-direct student to the relevant section to locate the correct responses and if
necessary will initiate group decision on any of the areas covered by these review questions.
Page 66 of 66