A thin nut is a type of fastener that's thinner than a normal-height nut. Thin nuts are used in a number of applications. They can be used in tight spaces where a standard-height nut won't fit, and they can also be used to prevent loosening of bolts. Thin nuts are often used to lock other fasteners. In some cases, they're used with shoulder bolts, which are used in mechanical equipment. The two nuts can press against each other to keep the bolt from loosening, even when the equipment is subjected to vibrations or other stress.
The use of two plain nuts to achieve a locking effect goes back 150 years or more, based upon observations of historic machinery. One of the most common places to observe this is in column attachments, where a thin nut is placed next to the joint, and then a normal height nut is tightened on top. This is not an ideal configuration if you want to get the full strength of the threads of each nut.
If you tighten the thin nut first, and then the normal nut on top of it, the bolt threads may be damaged. This happens because the thick nut lifts the load on the thread flanks of the thin nut as it's being tightened, causing them to stretch. The result is that the threads can't take the full load of the bolt, and they can become weak and unstable.
In other words, the technique is flawed in that it doesn't achieve the locking effect you would expect it to. To do it properly, you need to use a specific procedure. Here's an article explaining how to do it right: Tightening a Thin Nut on Top of a Thick One
Jam nuts are also commonly used as a way of preventing loosening. They're thin, hexagonal nuts that are designed to be pressed or jammed against another nut in order to keep it from loosening. Jam nuts can be used in situations where there isn't enough room for a standard-height nut, or where you need to hold the bolt securely but you still want it to be able to rotate freely.
The key is that you need to tighten the jam nut against the existing nut before you start to tighten the other nut on top of it. This way, you don't damage the threads of the existing nut by applying too much force. When the nut is tightened, it will apply the necessary force to keep the jam nut against the existing nut, and the resulting friction between them will ensure that the bolt can't come loose. That's why you need to be careful when using this method of locking a fastener - it requires some skill, and it's not something you want to try in an emergency situation! You should also check out these other comments that have received high ratings:
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