Post on 02-Jun-2018
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Fasteners, Powers Screws,
ConnectionsHelical thread screw was an important invention.
Power Screw, transmit angular motion to liner motion
Transmit large or produce large axial forceIt is always desired to reduce number of screws
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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
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Single and Double threaded screws
Double threaded screws are stronger and moves faster
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Bolts, Screws, and Studs
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 leaveExposed a Threaded STEM
A Stud May Also be WELDED to a Surface
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Machine Screw Head Types
Slot Head Provides More Driving Torque, But Driver is Hard to Center
Not good for Power Driving
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Threaded Inserts
Use With Soft Parent Materials Such as
Aluminum, Wood, Plastic, etc.
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Insert Advantages 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 virtuallyany application
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Screw Designations
United National
Standard UNS
International Standard
Organization
Roots and crest can be either flat or roundPitch diameter roduce same width in the thread and s ace
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Common Tread forms
Joining
Joining
Lite-Duty (e.g., Light Bulbs)
Pwr Xmission
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Coarse thread Designated by UNC
Fine Thread UNF, is more resistance toloosening, because of its small helix angle.
They are used when Vibration is present
Class of screw, defines its fit, Class 1 fits havewidest tolerances, Class 2 is the most commonlyused
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
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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
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Mechanics of Power Screws
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Used in design to change the angular motion to linear motion, Could
you recall recent failure of power screw leading to significant
causalities
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What is the relationship between
the applied torque on power screw
and lifting force F
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Torque for single flat thread
)secsec(
2
fldfdlFdT
m
mmR
)(
2
)(2
f ld
lfdFdT
f ldfdlFdT
m
mmL
m
mmR
If the thread as an angle , the torque will be
Wedging action, it
increases friction
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Stresses in the power Screw
pnd
F
A
V
pnd
F
pndF
d
T
tr
tr
b
tm
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
ntnumber of engagedthread
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Loading to the fasteners and their
Failure considerations
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Bolts are used to clamp two or more parts
It causes pre tension in the boltGrip length is the total thickness of parts
and washersl
ld
lt
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t2
ld
h
L effective grip= h+t2 if t2
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Failure of bolted or riveted joints
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Type of Joints
Lap Joint (single Joint) But Joint
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Example 1
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Example 2
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Example 2
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Example 3
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Weld
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Weld under Bending
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