A type of self-locking nut used on a bolt that is subject to vibration — it cannot, however, be used on a bolt that is subject to rotation. A collar of resilient, or springy, fiber is locked into the end of the threads in the nut. The hole in the fiber is slightly smaller than the major diameter of the threads on the bolt on which it fits.

When the nut is screwed onto the bolt, the threads press, but do not cut, into the fiber and cause so much friction that the nut cannot vibrate loose from the bolt. A fiber locknut can be reused as long as a wrench is needed to turn it onto the bolt.