By Dr. Jody Muelaner
A nut is a mechanical fastener with a threaded hole. It is attached to a bolt, screw or stud to provide a clamping force and prevent axial movement. For example, a bolt is inserted through two parts, with aligned holes, and a nut is then screwed onto the bolt. The shaft of the bolt prevents radial movement of the parts, while the head of the bolt and the nut together prevent axial movement.
The most common type of nut is the hex nut. This has a hexagonal outer shape for the faces of tools to apply torque when fastening. To provide a secure fastening, this type of nut typically relies on torque being applied to generate an axial force. This axial force causes sufficient friction at the threads so as to prevent the nut from unscrewing. The force may act between the bolt head and the nut, causing compression of the parts being fastened. Alternatively, it may act between a pair of adjacent nuts, allowing parts to be fastened more loosely.
Thread-locking adhesive may also be applied to the threads of a nut to prevent it from working loose. Special locking nuts may also be used which work with devices such as pins, lockwire and nylon inserts to prevent loosening.
Older nuts were often manufactured with a simpler square head and this design is still sometimes used to enable increased torque to be applied using a spanner.
There are many more specialized types of nut such as:
- Cap nut: The hole is only accessible from one side with a smooth domed surface on the other side to prevent injury to persons.
- Barrel nut: A cylindrical shape which can be inserted into a hole running perpendicular to the axis of the bolt.
- Cage nut: A form of captive nut which can be attached to parts such as sheet metal, allowing bolts to be fastened in inaccessible spaces.
Coupling nut: An elongated nut which allows two threaded shafts to be joined coaxially.
- Flange nut: A hex nut with an increased diameter around the base, removing the need for a washer.
- Knurled nut: A nut with a large textured diameter, designed to be hand tightened.
- Swage nut: A nut which attaches itself to a sheet metal part when tightened, by swaging the material around the hole.