Types of Screw Threads

The following are various forms of screw threads:
1. British standard whitworth (B.S.W.) threads: 

It is a symmetrical V-thread in which the angle between the flanks, measured in an axial plane, is 55o. These threads are found on bolts and screwed fastenings for special purposes.

2. British association (B.A.) thread: 

 It is a B.S.W. thread with fine pitches. The included angle for this thread is 47½°.These threads are used for instruments and other precision works.

 3. American national standard thread: 

 It is also known as U.S. or Seller's thread and has flat crests and roots. The included angle is 60o.These threads are used for general prupooses e.g. on bolts, nuts, screws and tapped holess. 

4. Unified standard thread: 

This thread has rounded crests and roots, with the included angle of 60o.

5. Square thread: 

 The square threads, because of their high efficiency, are widely used for transmission of power in either direction. Such type of threads are usually found on the feed mechanism of machine tools, valves, spindles, screw jacks etc. The square threads are not so strong as V-threads, but they offer less frictional resistance to motion than whitworth threads. 

6. Acme thread: 

It is a modification of square thread. It is much stronger than square thread. these threads are frequently used on screw cutting lathes, brass valves, cocks and bench vices. 

7. Knuckle thread: 

It is also a modification of square thread. It has rounded top and bottom. These threads are usually found on railway carriage couplings, hydrants, neck of glass bottles etc. 

8. Buttress thread: 

It is used for transmission of power in one direction only. It has a low frictional resistance characteristics of the square thread and have the same strength as that of V-thread. The spindles of bench vices are usually provided with buttress thread.

9. Metric thread:

It is an Indian standard thread and is similar to B.S.W. threads. It has an included angle of 60o instead of 55o.

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