Bureau of Indian Standards (BIS) adapts ISO (International Organisation for Standards) metric threads which are adapted by a number of countries apart from India.
The design profiles of external and internal threads are shown in below figure. The following are the relations between the various parameters marked in the figure:
It may be noted from the figure that in order to avoid sharp corners, the basic profile is rounded at the root (minor diameter) of the design profile of an external thread. Similarly, in the case of internal thread, rounding is done at the root (major diameter) of the design profile.
Other Thread Profiles
Apart from ISO metric screw thread profile, there are other profiles in use to meet various applications. These profiles are shown in ‘Types of thread profiles’ figure below, the characteristics and applications of which are discussed below :
- V-Thread (sharp) – This thread profile has a larger contact area, providing more frictional resistance to motion. Hence, it is used where effective positioning is required. It is also used in brass pipe work.
- British Standard Whitworth (B.W) Thread – This thread form is adopted in Britain in inch units. The profile has rounded ends, making it less liable to damage than sharp V-thread.
- Buttress Thread – This thread is a combination of V-and square threads. It exhibits the advantages of square thread, like the ability to transmit power and low frictional resistance, with the strength of the V-thread. It is used where power transmission takes place in one direction only such as screw press, quick acting carpenter’s vice, etc.
- Square Thread – Square thread is an ideal thread form for power transmission. In this, as the thread flank is at right angle to the axis, the normal force between the threads, acts parallel to the axis, with zero radial component. This enables the nut to transmit very high pressures, as in the case of a screw jack and other similar applications.
- ACME Thread – It is a modified form of square thread. It is much stronger than square thread because of the wider base and it is easy to cut. The inclined sides of the thread facilitate quick and easy engagement and disengagement as for example, the split nut with the lead screw of a lathe.
- Worm Thread – Worm thread is similar to the ACME thread, but is deeper. It is used on shafts to carry power to worm wheels.