The free-spinning locknut turns freely onto a bolt until seated against a base. Further tightening of the nut produces locking action.

Operating principles of all these nuts are similar. When the locking element engages the base surface, the spring of wedging action produced creates a tight friction grip against the bolt threads.

Unique nut designs are available to develop the locking action including:

  • Two-piece nuts in which the elements distort each other or the bearing surface when tightened.
  • Spring nuts which are notched or grooved to deform when seated.
  • Insert nuts containing soft metal or nylon collars that deform to grip the bolt thread when tightened.
  • Toothed nuts or nuts that have built-in toothed washers that engage and lock against the bearing surface. Some nuts may also cause a wedging action against the bolt threads.
  • Special-thread nuts that obtain their holding power by deforming the bolt thread.

The free-spinning nut is advantageous where long travel under load is required to seat the nut. If pressure on the nut is not sufficient to develop locking action, the nut will turn freely into position without danger of damage to the locking feature.

Preloading is essential for all free-spinning locknuts to develop proper locking action. Except for two-piece nut designs, in which one nut element serves as the base surface for the other, free-spinning locknuts are not recommended for use on assemblies where there is relative motion between bolted parts, or in joints with multiple laminations that may relax bolt tension by embedding or plastic deformation. Use of these nuts should also be avoided with fragile parts.