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 length 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 aluminum 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.[1]
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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; and locking washers which prevent fastening loosening by preventing unscrewing rotation of the fastening device; locking washers are usually also spring washers. The term washer is also often used for disc shaped devices used as grommets.
A plain washer (or 'flat washer') is a flat annulus or ring, often of metal, used to spread the load of a screwed fastening. Additionally a plain washer may be used when the hole is a larger diameter than the fixing nut.[2][3]
A fender washer is a flat washer with a particularly large outer diameter in propertion 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. They can also be used to make a connection to a hole that has been enlarged by rust or wear. Generally a washer with an outer diameter more than three times the inner diameter is called a fender washer.
A spherical washer is part of a self-aligning nut; it is a washer with one radiused surface, which is designed to be used in conjunction with a mating nut in order to correct for up to several degrees of misalignment between parts.
An anchor plate or wall washer is a large plate or washer connected to a tie rod or bolt. Anchor plates are used on exterior walls of masonry buildings, for structural reinforcement. Being visible, many anchor plates are made in a style that is decorative.[4]
A flange nut is a nut with an integral fixed washer.
A torque washer is used in woodworking in combination with a carriage bolt; it has a square hole in the center where the carriage bolt square fits into. Teeth or prongs on the washer bite into the wood preventing the bolt from spinning freely when a nut is being tightened.[5]
Belleville washers, also known as a cupped spring washer or conical washer, has a slight conical shape, which provides an axial force when deformed.
Curved disc spring is similar to a Belleville except the washer is curved in only one direction, therefore there are only four points of contact. Unlike Belleville washers, they only exert light pressures.[6]
Wave washers have a "wave" in the axial direction, which provides spring pressure when compressed. Wave washers, of comparable size, do not produce as much force as Belleville washers. In Germany, they are used as a lock washer, however they are ineffective.[7]
A split washer or a spring lock washer is a ring split at one point and bent into a helical shape. This causes the washer to exert a spring force between the fastener's head and the substrate. Applicable standards are ASME B18.21.1, DIN 127B, and United States Military Standard NASM 35338 (formerly MS 35338 and AN-935).[8]
The use and effectiveness of spring lock washers has been in debate of late, with some publications advising against their usage. Sources in reference are IS:3063,[9] and NASA Fastener Design Manual.[10]
A toothed lock washer, also known as a serrated washer or star washer,[7] has serrations that extend radially inward and/or outward to bite into the bearing surface. This type of washer is effective as a lock washer when used with a soft substrate, such as aluminum or plastic.[7] There are four types: internal, external, combination, and countersunk. The internal style have the serrations along the inner diameter of the washer, which makes them more aesthetically pleasing.[11] The external style have the serrations around the outer diameter, which provides better holding power, because of the greater surface area.[12] The combination style has serrations about both diameters, for maximum holding power.[13] The countersunk style is designed to be used with flat-head screws.[14]
The term washer is often applied to various gasket types such as those used to seal the control valve in taps. Crush washers are made of a soft metal such as aluminium or copper and are used to seal fluid or gas connections such as those found in an internal combustion engine.
A shoulder washer is a plain washer type with integral cylindrical sleeve; they are used to keep separate different metal types, and a seals.[15] This term is also used for electrically insulating grommets.[16]
A Keps nut or K-lock nut is a nut with an integral free spinning washer, assembly is easier because the washer is captive.
A top hat washer is a shoulder washer type used in plumbing for tap fitting.
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Common materials include steel, stainless steel, and plastic. Hardened washers are steel washers that have been heat treated. Other materials include aluminum, aramids, bimetals, bronze, brass, ceramics, copper, felt, fibers, hastelloy, iron, leather, mica, Inconel, Monel, rubber, silicon bronze, zinc, and titanium.[17]
| Nominal screw sizes M (mm) |
Screw pitch thread P (mm) |
Internal (Inside) diameter hole d1 (mm) |
External (Outside) diameter d2 (mm) |
thickness H (mm) |
||
|---|---|---|---|---|---|---|
| 1st choice |
2nd choice |
coarse | fine | |||
| 1 | 0.25 | 1.1 | 3 | 0.3 | ||
| 1.2 | 0.25 | 1.3 | 3.5 | 0.3 | ||
| 1.4 | 0.3 | 1.5 | 4 | 0.3 | ||
| 1.6 | 0.35 | 1.7 | 4 | 0.3 | ||
| 1.7 | 1.8 | 4.5 | 0.3 | |||
| 1.8 | 0.35 | |||||
| 2 | 0.4 | 2.2 | 5 | 0.3 | ||
| 2.5 | 0.45 | 2.7 | 6 | 0.5 | ||
| 2.6 | 2.8 | 7 | 0.5 | |||
| 3 | 0.5 | 3.2 | 7 | 0.5 | ||
| 3.5 | 0.6 | 3.7 | 8 | 0.5 | ||
| 4 | 0.7 | 4.3 | 9 | 0.8 | ||
| 5 | 0.8 | 5.3 | 10 | 1 | ||
| 6 | 1 | 0.75 | 6.4 | 12 | 1.6 | |
| 7 | 1 | 7.4 | 14 | 1.6 | ||
| 8 | 1.25 | 1 | 8.4 | 16 | 1.6 | |
| 10 | 1.5 | 1.25 or 1 | 10.5 | 20 | 2 | |
| 12 | 1.75 | 1.5 or 1.25 | 13 | 24 | 2.5 | |
| 14 | 2 | 1.5 | 15 | 28 | 2.5 | |
| 16 | 2 | 1.5 | 17 | 30 | 3 | |
| 18 | 2.5 | 2 or 1.5 | 19 | 34 | 3 | |
| 20 | 2.5 | 2 or 1.5 | 21 | 37 | 3 | |
| 22 | 2.5 | 2 or 1.5 | 23 | 39 | 3 | |
| 24 | 3 | 2 | 25 | 44 | 4 | |
| 27 | 3 | 2 | 28 | 50 | 4 | |
| 30 | 3.5 | 2 | 31 | 56 | 4 | |
| 33 | 3.5 | 2 | 34 | 60 | 5 | |
| 36 | 4 | 3 | 37 | 66 | 5 | |
| 39 | 4 | 3 | 40 | 72 | 6 | |
| 42 | 4.5 | 3 | 43 | 78 | 7 | |
| 45 | 4.5 | 3 | 46 | 85 | 7 | |
| 48 | 5 | 3 | 50 | 92 | 8 | |
| 52 | 5 | 4 | 54 | 98 | 8 | |
| 56 | 5.5 | 4 | 58 | 105 | 9 | |
| 60 | 5.5 | 4 | 62 | 110 | 9 | |
| 64 | 6 | 4 | 66 | 115 | 9 | |
| 68 | 70 | 120 | 10 | |||
| 72 | 74 | 125 | 10 | |||
| 76 | 78 | 135 | 10 | |||
| 80 | 82 | 140 | 12 | |||
| 85 | 87 | 145 | 12 | |||
| 90 | 93 | 160 | 12 | |||
| 100 | 104 | 175 | 14 | |||