Threaded fasteners and powers screws introduction

1. Fasteners, Powers Screws

2. • Helical thread screw was an important invention.
• Power Screw, transmit angular motion to liner
motion
• Transmit large or produce large axial force
• It is always desired to reduce number of screws
Threaded Fasteners

3. Introduction
• Fasteners are the thousands of bolts, nuts, screws, clips
and adhesives that hold a vehicle together
• Must be able to identify and use fasteners properly
• Each fastener is engineered for a specific application
• Always replace a fastener with exactly the same type
that was removed from the original equipment
manufacture (OEM) assembly

4. Bolts
• Bolt is a shaft with a head on one end and threads on the other
• Cap screw is a high-strength bolt
• Bolts and cap screws are named after the body part they hold
– Fender bolt, hood hinge bolt, etc.
• Also named after the shape and head drive configuration
• Bolt head is used to tighten the bolt
• Socket wrench fits over the bolt head
• Some metric/SAE sockets are very close in size
• Improper wrench or socket size could strip or round off the both you
are working on
– This could damage your tools or even cause an injury if the tool
slips off

5. Bolts

6. Thread pitch gauge.
The thread pitch gauge is fit against threads. Threads that match
the gauge equal the pitch number printed on the gauge.
Gauges can be used to tell thread, bolt or nut sizes.

7. Bolt Strengths or Grades
• Bolt strength is the amount of tightening force that should be applied
• Bolts are made from different materials
– Have degrees of hardness for different situations
• Bolt grade markings are lines or numbers on top of the head to
identify hardness and strength
• Hardness or strength of metric bolts is indicated by a property class
indicator on bolt head
• Bolt strength markings are given as lines, more lines means more
strength
• Metric bolt strength markings are numeric
– Higher number means more strength
• Tensile strength is the amount of pressure bolt can withstand before
breaking when pulled apart
• The harder the bolt, the greater tensile strength

8. Nuts (continued)
• Wing nuts have two arms for turning by
hand
– Used when a part must be removed
frequently for service or maintenance, such as
for air cleaners
• Acorn nuts are closed on one end for
appearance, to keep water/debris off the
threads
• Body nut has a washer formed onto the
nut to distribute the clamping force of the
thin body panel or trim piece to prevent
warpage

9. Body nuts are specially designed for specific holding applications.

10. Washers
• Washers prevent damage to surface of parts and provide better
holding power
• Flat washers prevent smaller bolt heads from pulling through sheet
metal and plastic
• Wave washers add a spring action to keep parts from rattling and
loosening
• Body or fender washers have large outside diameter for the size
hole in them
– Have better holding power on thin metal and plastic parts
• Copper or brass washers prevent fluid leakage

11. Washers
• Spacer washers allow for the adjustment of parts
• Fiber washers prevent vibration or leakage but cannot be
tightened to a great extent
• Finishing washers have a curved shape for appearance
• Split lock washers are used under nuts to prevent
loosening by vibration
• Shakeproof or teeth lock washers have teeth or bent
lugs that grip both the work and the nut

12. Screws
• Screws hold nonstructural parts on a vehicle
• Machine screws are threaded their full length and are
relatively weak
• Set screws have an internal drive head for an Allen
wrench and hold parts onto shafts
• Sheet metal screws and self-tapping screws have
pointed or tapered tips
• Trim screws have a washer attached
• Headlight aiming screws have a plastic adapter

13. Keys and set screws are both used to align parts on shafts. (A) Key
and keyway. (B) Set screw application

14. Definition of important Terminologies
Major diameter d,
Minor diameter dr
Mean dia or pitch diameter dp
Lead l, distance the nut moves for one turn rotation

15. Single and Double threaded screws
Double threaded screws are stronger and moves faster

16. Screw Designations
• United National Standard UNS
• International Standard Organization
Roots and crest can be either flat or round
Pitch diameter produce same width in the thread and space,

17. Coarse thread Designated by UNC
• Fine Thread UNF, is more resistance to loosening,
because of its small helix angle.
• They are used when Vibration is present
• Class of screw, defines its fit, Class 1 fits have widest
tolerances, Class 2 is the most commonly used
• Class three for very precision application
• Example:1in-12 UNRF-2A-LH, A for Ext. Thread and B for
Internal, R root radius
• Metric M10x1.5 10 diameter mm major diameter,1.5 pitch

18. Square and Acme Threads are
used for the power screw
Preferred pitch for Acme Thread
d, in 1/4 5/16 3/8 1/2 5/8 3/4 7/8 1 1 1/4
p,in 1/16 1/14 1/12 1/10 1/8 1/6 1/6 1/5 1/5

19. Mechanics of Power Screws

20. Used in design to change the angular motion to linear motion, Could you recall
recent failure of power screw leading to significant causalities

21. What is the relationship between the applied
torque on power screw and lifting force F

22. Torque for single flat thread
)
sec
sec
(
2 



fl
d
fd
l
Fd
T
m
m
m
R



)
(
2
)
(
2
fl
d
l
fd
Fd
T
fl
d
fd
l
Fd
T
m
m
m
L
m
m
m
R










If the thread as an angle α,
the torque will be
Wedging action, it
increases friction

23. Stresses in the power Screw
p
n
d
F
A
V
p
n
d
F
p
n
d
F
d
T
t
r
t
r
b
t
m
B








3
2
3
6
2
/
16
3






Shear stress in the base
of the screw
Bearing stress
Bending stress at the root
of the screw
Shear stress in the thread
nt number of engaged
thread

24. Loading to the fasteners and their Failure
considerations

25. Bolts are used to clamp two or more parts
It causes pre tension in the bolt
Grip length is the total thickness of parts and washers

26. L’ effective grip= h+t2 if t2<d
=h=d/2 for t2 d


27. Failure of bolted or riveted joints

28. Type of Joints
• Lap Joint (single Joint) But Joint