Intro and Jam Nuts

Nov. 15, 2002
A variety of these common fasteners can be used.

A variety of these common fasteners can be used.

Hex and square nuts are the most commonly used varieties. Square nuts are normally used for lighter duty than are hex nuts. Flanged nuts have integral washers that simplify handling and may bridge oversized holes.

Several methods are used to prevent nuts from working loose:

  • Peening the bolt end which extends through the nut.
  • Staking or deforming the nut threads.
  • Using lockwashers.
  • Using a jam nut with a regular nut.
  • Providing special threads on both nut and bolt.
  • Doping the mating threads with adhesives, lacquers, or special sealants.

Locknuts should be considered when:

  • The joint is subject to vibratory or cyclic motion.
  • Accurate preloading of assembly is difficult because of resilience of parts.
  • Joint members are too fragile to withstand preload.
  • Locking is preferred because of unknown service conditions.
  • Accurate positioning of the nut along a threaded element is required, such as in spacer applications where parts must rotate without end play.

The four principal types of nuts which provide these locking features are jam nuts, castle or slotted nuts, free-spinning locknuts, and prevailing-torque locknuts.

Jam Nuts

The jam nut is a thin nut, normally used under a full nut to develop locking action. Recommended practice is to torque the jam nut to seat only, then assemble a full nut on top of the jam nut and torque to full preload value while the jam nut is held stationary. The same effect can also be achieved with two full nuts if preload must be developed when the first nut is tightened into position.

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