A locknut, also known as a lock nut, locking nut, prevailing torque nut, stiff nut or elastic stop nut, is a nut that resists loosening under vibrations and torque. Elastic stop nuts and prevailing torque nuts are of the particular type where some portion of the nut deforms elastically to provide a locking action. The first type used fiber instead of nylon and was invented in 1931.
There are various kinds of specialised lock nuts, including:
- Castellated nut
- Distorted thread locknut
- Interfering thread nut
- Jam nut
- Jet nut (K-nut)
- Keps nut (K-nut or washer nut) with a free-spinning washer. In the locknut form, this is a star-type lock washer.
- Plate nut
- Polymer insert nut (Nyloc nut)
- Serrated face nut
- Serrated flange nut
- Speed nut (sheet metal nut or Tinnerman nut)
- Split beam nut
A locknut's static breakaway torque value is how much torque it takes to turn when it is not tightened against anything, but simply traveling over the mating threads with no impediment except that imparted by the locking feature itself. For example, on a nylon-insert nut, it is the torque needed to overcome the resistance of the nylon dragging across the mating thread. This torque value is usually not very high. Tolerance ranges for it are specified in some specifications, such as for military catalog hardware (AN-, MS-, NAS- NASM-).
- Glossary of Terminology Related to Nuts and Bolts, retrieved 2008-11-30.
- Smith, p. 104.
- "Nut With Elastic Ring Can't Work Loose", April 1931, Popular Science bottom of page 67 drawing of how lock nuts work
- K-NUTS.com, retrieved 2012-03-01.
- Smith, Carroll (1990), Carroll Smith's Nuts, Bolts, Fasteners, and Plumbing Handbook, MotorBooks/MBI Publishing Company, ISBN 0-87938-406-9.
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- "Hold Everything", February 1946, Popular Science page on lock nuts and washer technology developed during World War Two