A washer is a thin plate (typically disk-shaped) with a hole (typically in the middle) that is normally used to distribute the load of a threaded fastener, such as a screw or nut. Other uses are as a spacer, spring (belleville washer, wave washer), wear pad, preload indicating device, locking device, and to reduce vibration (rubber washer). Washers usually have an outer diameter (OD) about twice the width of their inner diameter (ID)
Belleville washers, also known as a cupped spring washer or conical washer, have a slight conical shape, which provide an axial force when deformed.
A split washer or a spring lock washer is a ring split at one point and bent into a helical shape. This causes the washer to exert a spring force between the fastener's head and the substrate.
A toothed lock washer, also known as a serrated washer or star washer, has serrations that extend radially inward and/or outward to bite into the bearing surface. This type of washer is effective as a lock washer when used with a soft substrate, such as aluminium or plastic. There are four types: internal, external, combination, and countersunk. The internal style have the serrations along the inner diameter of the washer, which makes them more aesthetically pleasing. The external style have the serrations around the outer diameter, which provides better holding power, because of the greater surface area. The combination style has serrations about both diameters, for maximum holding power. The countersunk style is designed to be used with flat-head screws