A jam nut is used to lock a bolt in place. By threading a second nut onto a bolt and then tightening the two nuts against each other, the bolt becomes locked in place and is unable to loosen on its own. In typical installation, the second nut or jam nut is the nut tasked with keeping the bolt secured. By placing a wrench on each nut, the jam nut is tightened by holding the first nut steady and tightening the second nut tightly against the first.
The typical type of situation where a jam nut is used is when a bolt may encounter severe vibration or when the bolt is used to adjust and hold tension on another component. There are installations such as the rear-wheel adjustment and a chain tensioner on a motorcycle, where a jam nut is used to prevent movement of the wheel once tightened. In this instance, the wheel is adjusted by tightening the adjustment bolt until the chain is tight and the wheel is straight in the swing arm. Once the adjustments have been made, the jam nut is run down to the adjustment nut and then tightened against it.
While the typical jam nut is nothing more than a common nut, there are times when a special lock nut or nylon-lined nut may be used as the jam nut. These special nuts are often used in extremely tight areas where normal maintenance and observation is not an easy task. This is used to ensure that no movement of a critical component can take place. Automobile tie rod ends are one such example of this method. A locking nut is used to jam the threaded tie rod adjustment sleeve and prevent it from loosening or moving.
Any machinery component that is subject to adjustment, such as a drill press or saw, can have a positive lock or stop that uses a jam-type lock system. In this situation, the nut is jammed to prevent the tool from passing a specific point. When drilling a series of holes that must only be a specific depth, the measurement is placed on the drill press's adjustment and jammed with a nut to lock it at that depth. This same form of locking is used to set the depth of some power planers and radial arm saws. A jam nut can often make the control of an apparatus as well as the quality control of many machines easier to manage.