A fastener is a machine component that holds or connects two or more elements of a machine or structure. Fastening is the process of putting the pieces together. There are two sorts of fasteners: permanent and detachable (temporary). Permanent fastening is accomplished by the use of riveting and welding procedures. Bolts, studs, and nuts in combination, machine screws, set screws, and keys, cotters, couplings, and other screwed fasteners are used to fasten components that require frequent assembly and disassembly.
Among the removable fasteners, screwed fasteners are the most common. Screwed fasteners are commonly used:
- To hold parts together
- To adjust parts with reference to each other
- To transmit power.
Screw Thread Nomenclature
Cutting a continuous helical groove on a cylindrical surface produces a screw thread (external thread). A screwed fastener is formed when the threaded section connects with a corresponding threaded hole (internal thread). The terminology connected with screw threads are listed below.
Major diameter is the largest diameter of a screw thread, touching the crests on an external thread or the roots of an internal thread.
2. Minor (core) diameter
This is the smallest diameter of a screw thread that touches the external thread's roots or core (root or core diameter) or the internal thread's crests.
3. Pitch diameter
This is the diameter of an imaginary cylinder that passes between the threads at the points where the thread width equals the thread spacing.
4. Pitch
Pitch is the distance measured parallel to the axis, between corresponding points on adjacent screw threads.
5. Lead
Lead is the distance a screw advances axially in one turn.
6. Flank
The straight section of the surface on either side of the screw thread is known as the flank.
7. Crest
Crest is the peak edge of a screw thread, that connects the adjacent flanks at the top.
8. Root
Root is the bottom edge of the thread that connects the adjacent flanks at the bottom.
9. Thread angle
This is the angle formed by the thread's flanks when measured in an axial plane.
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