Washer (Hardware)

Washer (hardware) 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 pre- cision is NOT critical. Type B is a series of flat washers with tighter tolerances where outside diameters are cate- gorized as 'Narrow', 'Regular' or 'Wide' for specific bolt sizes.[2] Assorted washers: flat, split, star and insulated 'Type' is not to be confused with 'form' (but often is). A washer is a thin plate (typically disk-shaped) with a hole The British Standard for Metric Series Metal Washers (typically in the middle) that is normally used to distribute (BS4320) written in 1968 coined the term 'form'. The the load of a threaded fastener, such as a screw or nut. forms go from A to D for Bright Metal and denote out- Other uses are as a spacer, spring (belleville washer, wave side diameter and thickness. They can be summarised as washer), wear pad, preload indicating device, locking de- - vice, and to reduce vibration (rubber washer). Wash- ers usually have an outer diameter (OD) about twice the • Form A: Normal diameter, normal thickness width of their inner diameter (ID). • Form B: Normal diameter, light thickness Washers are usually metal or plastic. High quality bolted joints require hardened steel washers to prevent the loss • Form C: Large diameter, normal thickness of pre-load due to Brinelling after the torque is applied. • Form D: Large diameter, light thickness Rubber or fiber gaskets used in taps (or faucets, or valves) to stop the flow of water are sometimes referred to col- • Forms E to G relate to black metal washers. loquially as washers; but, while they may look similar, washers and gaskets are usually designed for different functions and made differently. 1.1 Plain washers Washers are also important for preventing galvanic cor- A plain washer (or 'flat washer') is a flat annulus or ring, rosion, particularly by insulating steel screws from alu- often of metal, used to spread the load of a screwed fas- minium surfaces. tening. Additionally, a plain washer may be used when The origin of the word is unknown; the first recorded use the hole is a larger diameter than the fixing nut.[3][4] of the word was in 1346, however the first time its defi- A fender washer is a flat washer with a particularly large [1] nition was recorded was in 1611. outer diameter in proportion to its central hole. They are commonly used to spread the load on thin sheet metal, and are named after their use on automobile fenders. 1 Type and form They can also be used to make a connection to a hole that has been enlarged by rust or wear. Washers can be categorised into three types; A penny washer is a flat washer with a large outer diameter, in the UK. The name originally comes from the size • Plain washers, which spread a load, and prevent of the old British penny. In the UK, most industries refer damage to the surface being fixed, or provide some to all large OD washers as penny washers, even when the 1 2 1 TYPE AND FORM OD is as much as twice the size of the old penny. They in debate of late, with some publications advising against are used in the same applications as fender washers. their use on the grounds that, when tight, the washer is flat A spherical washer is part of a self-aligning nut; it is a against the substrate and gives no more resistance to rota- washer with one radiused surface, which is designed to be tion than a normal washer at the same torque. NASA re- searchers have gone as far as to say “In summary, a lock- used in conjunction with a mating nut in order to correct [10][11] for up to several degrees of misalignment between parts. washer of this type is useless for locking.” How- ever, a spring washer will continue to hold the bolt against An anchor plate or wall washer is a large plate or washer the substrate and maintain friction when loosened slightly, connected to a tie rod or bolt. Anchor plates are used on whereas a plain washer will not.[lower-alpha 2] exterior walls of masonry buildings, for structural rein- A toothed lock washer, also known as a serrated forcement. Being visible, many anchor plates are made [8] in a style that is decorative.[5] washer or star washer, has serrations that extend ra- dially inward or outward to bite into the bearing sur- A torque washer is used in woodworking in combination face. This type of washer is especially effective as a with a carriage bolt; it has a square hole in the centre into lock washer when used with a soft substrate, such as alu- which the carriage bolt square fits. Teeth or prongs on minium or plastic,[8] and can resist rotation more than the washer bite into the wood preventing the bolt from a plain washer on hard surfaces, as the tension between spinning freely when a nut is being tightened.[6] washer and the surface is applied over a much smaller area (the teeth). There are four types: internal, exter- nal, combination, and countersunk. The internal style has 1.2 Spring and locking washers the serrations along the inner edge of the washer, which makes them more aesthetically pleasing.[12] The exter- nal style has the serrations around the outer edge, which Belleville washers, also known as cupped spring wash- provides better holding power, because of the greater ers or conical washers, have a slight conical shape, which circumference.[13] The combination style has serrations provides an axial force when deformed. about both edges, for maximum holding power.[14] The A curved disc spring is similar to a Belleville, except countersunk style is designed to be used with flat-head the washer is curved in only one direction, therefore there screws.[15] are only four points of contact. Unlike Belleville washers, Tooth lockwashers are also used for ground bonding they exert only light pressures.[7] where a metal lug or object must be electrically bonded Wave washers have a “wave” in the axial direction, which to a surface. The teeth of the washer cut through sur- provides spring pressure when compressed. Wave wash- face oxides, paints or other finishes and provide a gas- ers, of comparable size, do not produce as much force as tight conductive path. In these applications the washer Belleville washers. In Germany, they are sometimes used is not placed under the head of the screw (or under the as lock washers, however they are less effective than other nut), it is placed between the surfaces to be bonded. In choices.[lower-alpha 1][8] such applications, the tooth washer does not provide any [16] A split washer or a spring lock washer is a ring split anti-rotation locking features. at one point and bent into a helical shape. This causes Lock washers, locknuts, jam nuts, and thread-locking the washer to exert a spring force between the fastener’s fluid are ways to prevent vibration from loosening a bolted head and the substrate, which maintains the washer hard joint. against the substrate and the bolt thread hard against the nut or substrate thread, creating more friction and resistance to rotation. Applicable standards are ASME B18.21.1, DIN 127B, and United States Military Stan- dard NASM 35338 (formerly MS 35338 and AN-935).[9] Spring washers are a left hand helix and allow the thread to be tightened in a right hand direction only, i.e. a 1.3 Gaskets clockwise direction. When a left hand turning motion is applied, the raised edge bites into the underside of the bolt or nut and the part that it is bolted to, thus resist- The term washer is often applied to various gasket types ing turning. Therefore spring washers are ineffective on such as those used to seal the control valve in taps. Crush left hand threads and hardened surfaces. Also, they are washers are made of a soft metal such as aluminium or not to be used in conjunction with a flat washer under the copper and are used to seal fluid or gas connections such spring washer, as this isolates the spring washer from bit- as those found in an internal combustion engine. ing into the component that will resist turning. Where a A shoulder washer is a plain washer type with integral flat washer is required to span a large hole in a component, cylindrical sleeve; they are used to keep separate different a nyloc nut (nylon insert) must be used. metal types, and as seals.[17] This term is also used for The use and effectiveness of spring lock washers has been electrically insulating grommets.[18] 3 1.4 Specialised types 4 See also A Keps nut or K-lock nut is a nut with an integral free • Bit guard spinning washer; assembly is easier because the washer is captive. • Dowel A top hat washer is a shoulder washer type used in plumb- • Unified Thread Standard ing for tap fitting. An insulating shoulder washer is used to electrically isolate a mounting screw from the surface it secures. Of- 5 Notes ten made of nylon, these are also made of teflon, PEEK or other plastics to withstand higher temperatures.

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