Washer (mechanical)

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 larger than their inner diameter (ID).

Washers are usually metal or plastic. High-quality bolted joints require hardened steel washers to prevent the loss of pre-load due to Brinelling after the torque is applied.

Rubber or fiber gaskets used in taps (or faucets, or valves) to stop the flow of water are sometimes referred to colloquially as washers; but, while they may look similar, washers and gaskets are usually designed for different functions and made differently.

Washers are also important for preventing galvanic corrosion, particularly by insulating steel screws from aluminium surfaces.

The origin of the word is unknown; the first recorded use of the word was in 1346, however, the first time its definition was recorded was in 1611.

Washers can be categorised into three types;

The American National Standards Institute (ANSI) provides standards for general use flat washers. Type A is a series of steel washers at broad tolerances, where precision is not critical. Type B is a series of flat washers with tighter tolerances where outside diameters are categorized as "narrow", "regular" or "wide" for specific bolt sizes.

"Type" is not to be confused with "form" (but often is). The British Standard for Metric Series Metal Washers (BS4320) written in 1968 coined the term "form". The forms go from A to D for Bright Metal and denote outside diameter and thickness:

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