A washer is a thin disk with a hole, usually in the middle. It is normally used to support the load of a threaded fastener. Other uses are as a spacer, spring (belleville, wave washers), wear pad, pre-load indicating device, and locking device.
Washers are usually metal or plastic. High quality capscrews require hardened steel washers to prevent the loss of pre-load after the tourque is applied (due to Brinnelling). Rubber washers are used in taps as a gasket to stop to flow of water. A common home-repair chore is to repace the washer to stop leaky faucets.
Types of washers