A jam nut is a low profile type of nut

Nut (hardware)

A nut is a type of hardware fastener with a threaded hole. Nuts are almost always used opposite a mating bolt to fasten a stack of parts together. The two partners are kept together by a combination of their threads' friction, a slight stretch of the bolt, and compression of the parts...

, typically half as tall as a standard nut. It is commonly used as a type of locknut

Locknut

A locknut, also known as a lock nut, locking nut, prevailing torque nut, stiff nut or elastic stop nut, is a nut that resists loosening under vibrations and torque. Elastic stop nuts and prevailing torque nuts are of the particular type where some portion of the nut deforms elastically to provide a...

, where it is "jammed" up against a standard nut to lock the two in place. It is also used in situations where a standard nut would not fit.

A jam nut can also refer to any nut that is used in the same function.

Use

A jam nut is often used when a nut needs to be locked in place without clamping against another object. The jam nut essentially acts as the "other object", as the two nuts are tightened against each other. They can also be used to secure an item on a fastener without applying force to that object. This is achieved by first tightening one of the nuts onto the item. Then the other nut is screwed down on top of the first nut. The inner nut is then slackened back and tightened against the outer nut.

Note that jam nuts can be unreliable under significant loads. If the inner nut is torque

Torque

Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....

d more than the outer nut, the outer nut may yield

Yield (engineering)

The yield strength or yield point of a material is defined in engineering and materials science as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed...

. If the outer nut is torqued more than the inner nut, the inner nut may loosen up.