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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). 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.〔http://www.straightdope.com/columns/read/1713/whats-the-purpose-of-washers-the-round-metal-things〕 ==Type and form== Washers can be categorised into three types; * ''Plain washers'', which spread a load, and prevent damage to the surface being fixed, or provide some sort of insulation such as electrical * ''Spring washers'', which have axial flexibility and are used to prevent fastening loosening due to vibrations * ''Locking washers'', which prevent fastening loosening by preventing unscrewing rotation of the fastening device; locking washers are usually also spring washers. The American National Standards Institute (ANSI) provides standards for general use flat washers. Type A is a series of steel washers with 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.〔(bokers.com )〕 '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. They can be summarised as - * Form A: Normal diameter, normal thickness * Form B: Normal diameter, light thickness * Form C: Large diameter, normal thickness * Form D: Large diameter, light thickness * Forms E to G relate to black metal washers. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Washer (hardware)」の詳細全文を読む スポンサード リンク
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