Lock washers are installed by putting a bolt or screw through the center of the washer, so that the washer is beneath the bolt or screw head or between the nut and the surface being fastened. Lock washers are used to keep screws and bolts from loosening, especially as a result of vibration or changes in temperature.
There are two main types of lock washers, those with spring action and those that grip the surface. Each type has several varieties.
Split ring washers are spring washers; they are flat rings that are cut and bent slightly. More pressure is needed to tighten the screw or bolt. They are normally used on the nut-end of the joint. The extra pressure of split ring washers is concentrated at the point of the washer that is cut. Belleville washers are conical, spring lock washers that exert pressure equally around the washer. They can be used on both the nut and the head end. Tooth lock washers have sharp points that dig into the screw (or bolt head) or the surface to be joined. They can leave scratch marks. Some lock washers have wedges notched into the surface of one side and teeth on the other. They are used as interlocking pairs.