Along with the bolt and the washer, the common hex nut makes up another indispensable component of the 3-part fastener system. The ubiquitous 6-sided hex nut is a true unsung hero of civilization; just imagine how many times every day you encounter or depend on its holding power. The hex nut provides the holding strength on just about everything from the smallest electrical devices to the propellers that move the largest ships.
Many applications require a very tight connection that is relatively unaffected or immune to the loosening caused by vibrations. In this case a lock nut will be used. Lock nuts have a small nylon insert at the top of the threads to really grip the corresponding threads on the bolt. For this reason, it is difficult to turn a lock nut onto a bolt without using a wrench. Lock nuts are easy to identify as they have a flat bottom and a rounded top.