Lock nuts are designed to prevent nuts from working loose. A loose nut can be caused by vibration, as commonly encountered on a lawnmower, washing machine, garbage disposal and other equipment. It can also occur if a nut is not tightened enough or if it is subjected to extremes in temperature. Lock nuts are readily available from hardware outlets. The most common form has a nylon inset built into one side of the threaded end. When the nut is tightened, the thread on the bolt stretches the nylon, causing the nut to grip the bolt and resist loosening.
Purchase a supply of the most common-sized lock nuts and matching plain washers that you will need to replace problem nuts. These are the nuts that work their way loose from time to time.
Remove any problem nut that's prone to loosening with the correct-sized wrench. Discard the nut.
Place the correct-size plain washer over the threaded end of the bolt or stud. Thread the replacement lock nut onto the bolt or stud by hand until you encounter resistance caused by the nylon insert.
Tighten the lock nut firmly, turning clockwise, with the same wrench you used to remove the original nut. If the head of the bolt is accessible, use a second identical wrench to hold it while tightening the lock nut.