A fiber type lock nut is held firmly on the threads of a bolt by pressure caused by?

• A. The use of an unthreaded fiber locking insert
• B. Friction from the threads
• C. A spring washer
• D. Lock wire

Answer: (A)

Locking happens because the nut contains a nonthreaded fiber insert that is pressed against the bolt threads when the nut is tightened. That insert deforms and grips the threads, creating friction that resists loosening from vibration or dynamic loads. The fiber insert is the key element that provides the holding force, rather than relying on the bare thread friction alone.

The other mechanisms aren’t what provides the locking action here: a spring washer relies on spring tension to maintain preload, a lock wire physically secures pieces together, and plain thread friction without the fiber insert wouldn’t reliably prevent loosening under vibration.