- Pcpolytechnic

Engineering 22

Threaded

Fasteners

Bruce Mayer, PE

Registered Electrical & Mechanical Engineer

BMayer@ChabotCollege.edu

Engineering 22 – Engineering Design Graphics

1

Bruce Mayer, PE

BMayer@ChabotCollege.edu • ENGR-22_Lec-20__Fasteners-1_Specs.ppt

 Define and Label the Parts of A Screw

Thread

 Identify Various Screw Thread Forms

 Draw Screws in Forms

• Detailed

• Schematic

• Simplified

2

 Properly Apply Screw Thread Technical

Specifications

Engineering 22 – Engineering Design Graphics Bruce Mayer, PE


 Identify Different

Fasteners and

Describe Their

Use

 Draw the Various

Types of Screw

Heads


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Bruce Mayer, PE


 State the Two MOST Important

Threaded Fastener References for

Structural (Mech/Civ/Chem) Design

Engineers

 Use AutoCAD to QUICKLY Draw Most

Threaded Fasteners in Any Size and

Any Representation

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 Invoke the AutoCAD “Design Center” tool to Construct Fastener

Representations



 Helical Threads Have Three Main

Applications

1. Used to HOLD parts together

• The Classic Application

2. Used to ADJUST the Position of parts with reference to one another

• The “Screw Adjustment” knob

3. Used to TRANSMIT Power

• e.g, screw Jack,

Worm Gear Drive


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Bruce Mayer, PE


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 Ref ASME B1.7M-1984 (R2001)

• Nomenclature Definitions, and Letter

Symbols for Screw Threads



Joining

Joining

Pwr Xmission

Lite-Duty (e.g., Light Bulbs)


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Bruce Mayer, PE


 Metric → Pitch in the Thread Designation

 USA → Pitch is Inverse of Thread Count


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Bruce Mayer, PE


 The Unified System (UN) Adopted on 18-

Nov-1948 by the USA, UK , & Canada

• Made fasteners Interchangeable

 The UN Designation (Spec) has Two Main

Elements

• The Major (outside) Diameter

– Specified by Either

 A size No. running from 0-12 (0.06-0.216 inches)

 The Major Dia, in Fractional (ugh!) inches (¼-4 inches)

• The Inverse Pitch in Threads per Inch


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Bruce Mayer, PE


1. COARSE Series (UNC or NC)

• For General Use Where Rapid Assembly is Required

– Threads Engage, or “Start” Easily

2. FINE Series (UNF or NF)

• For Applications Requiring Greater

Strength or Where the Length of

Engagement is Limited

– Used Extensively in Aircraft and

Automobile Manufacturing


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Bruce Mayer, PE


3. EXTRA FINE Series (UNEF or NEF)

• For Highly Stressed Parts

4. 8N Series (8N)

• A Substitute for Coarse-Thread Series for

Diameters larger Than 1”

– All diameters have

8 threads per inch.

Often Used on bolts for high pressure pipe applications.


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Bruce Mayer, PE


5. 12N Series (12 UN or 12N)

• A Continuation of the Fine-Thread Series for Diameters Larger than 1.5”

– All diameters have 12 thds/in. Used in boiler work and in Machine Construction.

12

6. 16N Series (16 UN or 16N)

• A Continuation of the ExtraFine-Thread

Series for Diameters Larger than 2”

– All diameters have 16 thds/in. Used on adjusting collars and other applications where thread must have fine adjustment regardless of diameter.



 Defaults That Need NOT be Part of Callout

• NC or NF implied by Diameter & TPI

• Class → Default is 2A or 2B

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• Hand → Default is RIGHT



 Two Types of Classes

A. Refers to EXTERNAL Threads

(Bolts & Screws)

B. Refers to INTERNAL Threads

(Nuts & “Tapped” holes)

 Class Descriptions

1. Provide Liberal Allowance for Ease of

Assembly Even When Threads are Dirty or Slightly Damaged

• Not Commonly Used


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Bruce Mayer, PE


 Class Descriptions cont.

2. For Commercially Produced bolts, screws, nuts, and other threaded fasteners

• By Far the Most Common

3. Used in Precision Assemblies where a

Close Fit is Required to WithStand Stress

& Vibration

• Typical Use is Aircraft or other Hi-Vibration

Applications


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Bruce Mayer, PE


 Note That Pitch is Stated Explicitly

16

• Compares to Inverse Pitch (TPI) for the Unified Specification



 Why LH Threads???

• To Distinguish a Critical Connection

• To Make One End of a Turn-Buckle

• When Needed to Counter Loosening

Tendency of Rotating Machinery


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Bruce Mayer, PE


 Used when diameter of thread is 1” or larger on plotted or Hand drawing.

 Use ONLY When It is Important to

Show the Function of the Thread

• Not Typical for Hand Drawings


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Bruce Mayer, PE


Simplified & Schematic Forms

Best Overall

 Imply depth of thread with hidden lines for simplified representation

 Use alternating LONG THIN & SHORT THICK lines to represent ROOT & CREST lines in schematic representation

• Spacing is SCHEMATIC; need not Match

Actual Pitch


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Bruce Mayer, PE


 Detailed → Very Laborious to Construct

• Very Infrequently Used on

Engineering Drawings

 Simplified → Fast but Potentially

Confusing

• Hidden Lines can be Mistaken for

Object Features

 Schematic → Best Overall

• Fast To Draw, Clearly ID’s the Threads


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Bruce Mayer, PE


 Normal Practice is to NOT Section the Fastener


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Bruce Mayer, PE


 To Avoid the use of a Thru-Hole & Nut,

The “Parent” Material of An object May be Thread “Tapped”

• That is, Threads are Cut Into the Side of a

Hole Drilled into the Base Material

 Even Thin parts may tapped By Use of

A thru Hole

• Rule of Thumb for the MINIMUM number of threads = 3

22

– For Light-Duty applications, can use 1.5




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Bruce Mayer, PE


 Pipe Threads have An INTERFERENCE

TAPER that Results in PERMANENT Material

DEFOMATION That (Hopefully) Produces a Fluid-Tight Seal at the Joint.


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Bruce Mayer, PE


 Taper Should be Shown

• Need Not be to Scale


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Bruce Mayer, PE


 Only ONE TPI Spec for Pipe Threads

• ASME B1.20.1-1983 = American National

Standard Taper Pipe Thread

 National Pipe Thread (NPT) for Short

• Callout Requires Only the “NPT” Notation,

Along with the “Size”


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Bruce Mayer, PE


 Does Anyone Know the Difference?

 A BOLT is Used with A NUT

 A SCREW is Used with A TAPPED hole

• The Hole May be SELF-Tapped

– e.g., Wood Screws

 A STUD is a TREADED ROD that is

Inserted into a TAPPED HOLE to leave

Exposed a Threaded STEM

• A Stud May Also be WELDED to a Surface


27

Bruce Mayer, PE


 Slot Head Provides More Driving Torque, But Driver is Hard to Center → Not good for Power Driving


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Bruce Mayer, PE


 Use With Soft Parent Materials Such as

Aluminum, Wood, Plastic, etc.


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Bruce Mayer, PE


 Greater clamping pressure in softer materials - provides higher strength, better seals.

 Better user of high-tensile-strength fasteners.

 Resistance to vibration in assembly.

 High-quality standard threads without tapping.

 Permanent, wear-resistant threads.

 Protective of expensive castings and moldings.

 Reduced performance loss from cold flow of plastic.

 Reduce risks in molding cycle.

 Easy to install in a drilled or molded hole.

 Suitable for automatic installation.

 Wide variety of types to fit virtually any application


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Bruce Mayer, PE


 If you design with Threaded Fasteners; you MUST get access to These Books

 ISBN: 0-8311- 2711-2


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 http://www.mcmaster.com/

Bruce Mayer, PE



32

Bruce Mayer, PE


 It has been estimated that CAD-Using

Engineers Spend up to 20 Hrs/Month

REDRAWING Part for their Designs

 Avoid Reinventing the Wheel by

• Calling Part Maker and asking for CAD file

• Consult OnLine Parts DataBases

• Consult your Company’s CAD-Block Library

• Ask your Colleagues

• Check AutoCAD Tools


33

Bruce Mayer, PE


 When I left Watkins-

Johnson Co. in 2000

We had 775 items in our CAD Library

 Let’s Build a A Bolt from a “Forms” File

 The Bolt/Screw

Spec

3/8-16 x 1.25 UNC

SH Cap Screw


34

Bruce Mayer, PE


Fasteners

Make the

World Go

Round

Molded-In

Inserts for

Plastic Materials


35

Bolt-Failure by

Reversing-Load

Mechanical-

Fatigue

→ See ENGR45

Bruce Mayer, PE


Engr/Math/Physics 25

f  2 x 3 

x

7 x 2  9  x  6

Licensed Electrical & Mechanical Engineer

BMayer@ChabotCollege.edu

Bruce Mayer, PE



36

1. Open File New File using Acad.dwg

2. Make Layer “Bolt”

Continuous & GRN

3. Save As 38-

16_125_shcap.dwg

4. Set Snap to 0.1

5. Open Fastener

Forms file

37

• Threaded_Fastener

_Forms_0509.dwg


6. Find SH Cap Form and Copy to ClipBd

7. Paste SHcap Form into new dwg

8. Return to Forms and Copy Detailed

Thread Rep

9. Paste detailed Thd-

Form into new-dwg

10.Chk Major dia at 1”

Bruce Mayer, PE


11.Scale all by Ratio of 1:0.375

12.Move TopV of

Screw Head

13.Rotate Detailed

Thd-form by 90°

14.Explode screw

Side view

15.Make Xline at base of Scr Hd


38

16.Offset xline to rt by

• 1.25 (scr length)

• 0.5 (unthd’s shank)

17.Erase Simplifedform lines

18.Stretch shank line to 0.5 xline

19.Make xline on CL of Screw

Bruce Mayer, PE


 Just before trimming out unneeded thds


39

Bruce Mayer, PE


20.Trim & Erase unused SideV line to Rt of 0.5 xLine

21.Erase Unneeded

Thd-details

22.Connect UnThd’s

Shank to Thd’d

Major Dia. With

Fillet

23.Move TopV of Cap

Scr to Lt of SideV

24.Delete

• xLine

• Detailed Thd-Form

25.Put all objects on

Bolt Layer

26.Adjust LtScale to

0.1


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Bruce Mayer, PE


 Caveat: Screw HEAD may NOT be to-

Scale for 3/8 Socket Head as we scaled down from the 1” Version

• If Clearance is an issue, then Chk Hd

Diameter and Height against Machinery’s

HandBook


41

Bruce Mayer, PE


- Pcpolytechnic

Engineering 22

Threaded

Fasteners

Engr/Math/Physics 25

x

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Engineering 22

Threaded

Fasteners

Engr/Math/Physics 25

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