Jam nut

It is commonly used as a type of locknut, where it is "jammed" up against a standard nut to lock the two in place.

Jam nuts, other types of locknuts, lock washers, and thread-locking fluid are ways to prevent vibration from loosening a bolted joint.

In normal use, a nut-and-bolt joint holds together because the bolt is under a constant tensile stress called the preload.

If the joint is subjected to vibration, however, the preload increases and decreases with each cycle of movement.

It is then held in place by a wrench while the outer nut is tightened on top using the full torque.

This arrangement causes the two nuts to push against each other, creating a tensile stress in the short section of the bolt that lies between them.