Types of Threads
The following are Different Types of threads:
- British Standard Whitworth (B.S.W.)
- British Association (B.A.) thread
- American national standard thread
- Unified standard thread
- Square thread
- Acme thread
- Knuckle thread
- Buttress thread
- Metric thread
1. British Standard Whitworth (B.S.W.)
British Standard Whitworth (B.S.W.) thread. This is a British standard thread profile and
has coarse pitches. It’s asymmetrical V-thread in which the angle between the flanks, measured in an axial plane, is 55°.
These threads are found on bolts and screwed fastenings for special purposes. The various proportions of B.S.W.
The British standard threads with fine pitches (B.S.F.) used where great strength at the root
required. These threads are also used for line adjustments and where the connected parts are
subjected to increased vibrations as in aero and automobile work.
The British standard pipe (B.S.P.) threads with fine pitches are used for steel and iron pipes and
tubes carrying fluids. In external pipe threading, the threads are specified by the bore of the pipe.
2. British association (B.A.) thread
This is a B.S.W. thread with fine pitches. The proportions of the B.A. thread. These threads are used for instruments and other precision works.
3. American national standard thread
The American national standard or U.S. or Seller’s thread has flat crests and roots. The flat crest can withstand more rough usage than sharp V-threads. These threads are used for general purposes e.g. on bolts, nuts, screws, and tapped holes.
4. Unified standard thread
The three countries i.e., Great Britain, Canada, and United States came to an agreement for a common screw thread system with the included angle of 60°, in order to facilitate the exchange of machinery. The thread has rounded crests and roots.
5. Square thread
The square threads, because of their high efficiency, are widely used for the transmission of power in either direction. Such types of threads are usually found on the feed mechanisms 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. The pitch of the square thread is often taken twice that of a B.S.W. thread of the same diameter.
6. Acme thread
It is a modification of the square thread. It is much stronger than square thread and can be easily produced. These threads are frequently used on screw-cutting lathes, brass valves, cocks, and bench vices.
When used in conjunction with a split nut, as on the lead screw of a lathe, the tapered sides of the thread facilitate ready engagement and disengagement of the halves of the nut when required.
7. Knuckle thread
It is also a modification of the square thread. And it has a rounded top and bottom. It can be cast or rolled easily and can not economically make on a machine. These threads are used for rough
and ready work.
They are usually found on railway carriage couplings, hydrants, necks of glass bottles, and large molded insulators used in the electrical trade.
8. Buttress thread
It is used for the transmission of power in one direction only. The force is transmitted almost parallel to the axis. This thread unit has the advantage of both square and V-threads.
It has a low frictional resistance characteristic of the square thread and has the same strength as that of V-thread. The spindles of bench vices are usually provided with buttress threads.
9. Metric thread
It is an Indian standard thread and is similar to B.S.W. threads. It has an included angle of 60° instead of 55°.