1. 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

2. 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

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

4. 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,

5. 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 M10x diameter mm major diameter,1.5 pitch

6. Some important Data for UNC, UNF and M threads
Lets Look at the Table 8-1 on Page 398

7. 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
1/5

8. Mechanics of Power Screws

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

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

11. Torque for single flat thread
If the thread as an angle α, the torque will be
Wedging action, it increases friction

12. 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
nt number of engaged thread

13. Loading to the fasteners and their Failure considerations

14. 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 d t

15. ld h t2
lt=L’- ld
L’ effective grip= h+t2 if t2<d
=h=d/2 for t2 d

16. Failure of bolted or riveted joints

17. Type of Joints
Lap Joint (single Joint) But Joint

18. Example 1

19. Example 2

20. Example 2

21. Example 3

22. Weld

24. Weld under Bending