Washer (hardware)

A washer is a thin plate (typically disk-shaped, but sometimes square) with a hole (typically in the middle) that is normally used to distribute the load of a threaded fastener, such as a bolt 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 often have an outer diameter (OD) about twice their inner diameter (ID), but this can vary quite widely.
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. Washers are also important for preventing galvanic corrosion, particularly by insulating steel screws from aluminium surfaces. They may also be used in rotating applications, as a bearing. A "thrust washer" is used when a rolling element bearing is not needed either from a cost-performance perspective or due to space restraints. Coatings can be used in attempt to reduce wear and friction either by hardening their surface, or providing a solid lubricant (i.e. a self-lubricating surface).
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.
Rubber or fiber gaskets used in taps (or faucets, or valves) as seal against water leaks 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.
Washer types.
Most washers can be categorized into three broad types; 
Spring and locking washers.
Lock washers, locknuts, jam nuts, and thread-locking fluid are ways to prevent vibration from loosening a bolted joint.
Gaskets.
The term washer is often applied to various gasket types such as those used to seal the control valve in taps.
Specialised types.
The DIN 125 metric washer standard refers to subtypes A and B. ISO 7089 calls these Form A and ISO 7090 calls them Form B. They are all the same overall size, but Form B is chamfered on one side.
Materials.
Washers can be fabricated from a variety of materials including, but not limited to:
Corrosion resistance.
A number of techniques are used to enhance the corrosion resistant properties of certain washer materials:
Type and form.
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 G and dictate the outside diameter and thickness of the flat washers.
Standard metric flat washers sizes.
Washers of standard metric sizes equivalent to BS4320 Form A are listed in the table below. Measurements in the table refer to the dimensions of the washers as described by the drawing. Specifications for standard metric flat washers were known as DIN 125 (withdrawn) and replaced with ISO 7089. DIN (Deutsches Institut für Normung - German Institute for Standardization) standards are issued for a variety of components including industrial fasteners as Metric DIN 125 Flat Washers. The DIN standards remain common in Germany, Europe and globally even though the transition to ISO standards is taking place. DIN standards continue to be used for parts which do not have ISO equivalents or for which there is no need for standardization. 