Engineering Reference

Prevailing torque

Friction built into a nut or bolt that resists loosening without a separate locking device.

Definition

A prevailing-torque fastener — nylon insert nut, deformed thread nut, or patch-lock bolt — resists rotation from first engagement. The torque needed to turn it is higher than the torque to keep it turning, and that difference is what resists vibration loosening.

It is one of the few locking methods that actually works. Split lock washers do not: testing shows they provide very little resistance and can reduce clamp load by flattening.

Common misuse

Relying on a split lock washer for vibration resistance. Use a prevailing-torque nut, a threadlocker, or correct preload instead.

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