A machine element used for holding or joining two or more parts of a machine or structure is known as a fastener. The process of joining the parts is called fastening. The fasteners are of two types : permanent and removable (temporary). Riveting and welding processes are used for fastening permanently. Screwed fasteners such as bolts, studs and nuts in combination, machine screws, set screws, etc., and keys, cotters, couplings, etc., are used for fastening components that require frequent assembly and dissembly.
Screwed fasteners occupy the most prominent place among the removable fasteners. In general, screwed fasteners are used : (i) to hold parts together, (ii) to adjust parts with reference to each other and (iii) to transmit power.
A screw thread is obtained by cutting a continuous helical groove on a cylindrical surface (external thread). The threaded portion engages with a corresponding threaded hole (internal thread); forming a screwed fastener. Following are the terms that are associated with screw threads.
- Major (nominal) diameter – This is the largest diameter of a screw thread, touching the crests on an external thread or the roots of an internal thread.
- Minor (core) diameter – This is the smallest diameter of a screw thread, touching the roots or core of an external thread (root or core diameter) or the crests of an internal thread.
- Pitch diameter – This is the diameter of an imaginary cylinder, passing through the threads at the points where the thread width is equal to the space between the threads.
- Pitch – It is the distance measured parallel to the axis, between corresponding points on adjacent screw threads.
- Lead – It is the distance a screw advances axially in one turn.
- Flank – Flank is the straight portion of the surface, on either side of the screw thread.
- Crest – It is the peak edge of a screw thread, that connects the adjacent flanks at the top.
- Root – It is the bottom edge of the thread that connects the adjacent flanks at the bottom.
- Thread angle – This is the angle included between the flanks of the thread, measured in an axial plane.