A locknut, also known as a lock nut, locking nut, prevailing torque nut,^[1] stiff nut^[1] or elastic stop nut,^[2] 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.^[2] The first type used fiber instead of nylon and was invented in 1931.^[3]

Locknuts, jam nuts, lock washers, and thread-locking fluid are ways to prevent vibration from loosening a bolted joint.

Types

There are various kinds of specialised lock nuts, including:

• Castellated nut
• Distorted thread locknut
  • Centerlock nut
  • Elliptical offset locknut
  • Toplock nut
  • Philidas nut
• Interfering thread nut
  • Tapered thread nut
• Jam nut
• Jet nut (K-nut)^[4]
• 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)
• Security Lock Nut All steel reusable nut for high vibration and harsh environments.
• Serrated face nut
• Serrated flange nut
• Speed nut (sheet metal nut or Tinnerman nut)
• Split beam nut
• Palnut

Prevailing torque values

Prevailing torque^[5] differentiates a locknut from a free spinning nut based on a value of how much torque is required during installation before clamp loading. 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 relative to final installation torque. Tolerance ranges for torque are specified in some standards such as (ISO, DIN, IFI, ASME, SAE, AN-, MS-, NAS- NASM-).

See also

• Safety wire
• Thread-locking fluid