Fasteners, Powers Screws, Connections

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

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

Single and Double threaded screws

Double threaded screws are stronger and moves faster

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,

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

Some important Data for UNC, UNF and M threads

• Lets Look at the Table 8-1 on Page 398

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

Mechanics of Power Screws

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

What is the relationship between the applied torque on power screw

and lifting force F

Torque for single flat thread

)sec

sec(

2

fld

fdlFdT

m

mmR

)(2

)(2

fld

lfdFdT

fld

fdlFdT

m

mmL

m

mmR

If the thread as an angle α, the torque will be

Wedging action, it increases friction

Stresses in the power Screw

pnd

F

A

V

pnd

F

pnd

Fd

T

tr

trb

tmB

3

2

3

6

2/

163

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

Loading to the fasteners and their Failure considerations

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

l

ld

l t

t2

ldh

L’ effective grip= h+t2 if t2<d

=h=d/2 for t2 d

lt=L’- ld

Failure of bolted or riveted joints

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

Example 1

Example 2

Example 2

Example 3

Weld

Weld under Bending