Hex Jam Nuts
A hex jam nut is a low-profile type of nut, typically half as tall as a standard 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. It is also used in situations where a standard nut would not fit. Hex jam nuts are thinner than a standard hex nut but have the same dimension across the flats and corners.
The most important thing about these nuts is if we use a pair of jam nuts to create a point where a wrench may be used. It can be unreliable under significant loads. If the inner nut is torqued more than the outer nut, the outer nut may yield. If the outer nut is torqued more than the inner nut, the inner nut may loosen up.
One more thing we have to know about this nut is that it is very risky to use this fastener against a very soft material and cannot danger hurting the material by fastening against it, but when you need to attach something to the bolt but let it move around on the bolt.
Hex jam nuts have many common applications like a jam nut can be threaded onto a bolt and run tight against the standard nut which will lock the nut in place and prevent it from backing off. Jam nuts can also be used in situations where a threaded rod must be rotated. Jam nuts are sometimes used for leveling purposes as well.