M5 Shakeproof Washer, External, A2/ 304 Stainless Steel, 50 pack.
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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 or 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).
Shakeproof washers are used as a measure to lock nuts in place and withstand movement, vibrations and the environment. Therefore, a shakeproof washer makes the fastening more secure and reliable.
Washers are important for preventing galvanic corrosion, particularly by insulating steel screws from aluminium surfaces and phosphor washers are used for high abrasion and corrosion resistance.
Zinc coated and plain steel shakeproof washers are for indoor use as outside they will corrode and eventually fail. A2 (regular grade) and A4 (marine grade, withstands salt) Stainless Steel are for outdoor applications.