Locknuts are fasteners, usually metal nuts, that screw onto a threaded bolt to lock it in place. A locknut may have a collar that spins freely or may be seated against the workpiece. When tightened, the locknut presses against the workpiece and the bolt head, jamming them together to prevent loosening. Locknuts are used in many applications, such as automotive engines and machinery, where it is important to keep bolts from loosening due to vibration.