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 d r Mean dia or pitch diameter d p 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 used , 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 • 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 p,in 1/4 1/16 5/16 1/14 3/8 1/12 1/2 1/10 5/8 1/8 3/4 1/6 7/8 1/6 1 1/5 1 1/4 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

T R T L

 

Fd m

2

Fd m

2

l

( 

d



m



fd



m fl



fd

( 

d m m

 

l fl

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

T R



Fd m

2

l

( 

d



m



fd



m fl

sec  sec  ) Wedging action, it increases friction

Stresses in the power Screw

Shear stress in the base of the screw Bearing stress Bending stress at the root of the screw Shear stress in the thread n t number of engaged thread  

B

 16

T



d

3   

d F m n t p

/ 2 

b

 6

F



d r n t p

  3

V

2

A

 3

F



d r n t p

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 l

t

l

d

l d t 2 L’ effective grip= h+t 2 if t 2

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