Spontaneous loosening caused when vibration or cyclic loads make the joint slip sideways. Each slip momentarily relieves friction in the threads and under the head, and the thread helix then lets the nut rotate slightly backward. It is reproduced in the transverse vibration or Junker test, and the main cures are higher preload, longer grip length, threadlockers, prevailing-torque nuts and wedge-locking washers.
Junker test shows the plain nuts self-loosening inside two hundred cycles, the wedge-lock washers held, so that's what we're spec'ing.
Long, slender bolts with a long grip resist self-loosening far better than short, stubby ones at the same preload.
Also heard as
- vibration loosening
- backing off
- rotational loosening
Do not mix it up with
- Embedment: Embedment loses preload by surfaces settling, with no rotation of the nut.
Related terms
Preload
The tension created in a bolt by tightening it, which clamps the joint and keeps it from slipping or opening.
Threadlocker
An anaerobic adhesive applied to threads that hardens between metal surfaces to stop loosening and seal.
Prevailing-torque nut
A nut built to resist turning on the thread before it seats, using a nylon insert or deliberately distorted threads.
Split lock washer
A split, helical spring washer sold to stop loosening, though it does little once a joint is properly tightened.