A washer is a thin plate (usually disk-shaped) with a hole (often 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, wave washer), wear pad, preload signaling device, locking device, and to minimize vibration (rubber washer) (rubber washer). Washers have an outer diameter (OD) that is rough twice as wide as their inner diameter (ID).
Washers are commonly made of metal or plastic. High-grade bolted joints require hardened steel washers to prevent the loss of pre-load once the torque is applied.
Rubber or fiber gaskets used in taps (or faucets, or valves) to halt the flow of water are frequently referred to colloquially as washers; but, while they may seem similar, washers and gaskets are normally designed for separate roles and constructed differently.
Washers are also crucial for preventing galvanic corrosion, notably by isolating steel screws from aluminum surfaces.
Washers can be classed into three types;
Plain washers, which spread a load, and prevent harm to the surface being fastened, or give some type of insulation such as electrical
Spring washers, which have axial flexibility and are used to avoid fastening loosening due to vibrations
Locking washers, which prevent fastening loosening by inhibiting unscrewing rotation of the fastening mechanism; locking washers are frequently also sprung washers.
The American National Specifications Institute (ANSI) establishes standards for general use flat washers. Type A is a series of steel washers with broad tolerances when precision is NOT important. Type B is a series of flat washers with stricter tolerances where outer diameters are classed as ‘Narrow’, ‘Regular’ or ‘Wide’ for certain bolt sizes.
Type’ is not to be confused with ‘form’ (though often is) (but often is). The term ‘form’ was coined in 1968 by the British Standard for Metric Series Metal Washers (BS4320). Outside diameter and thickness are indicated by the letters A to D for Bright Metal. They can be summed up as follows:
Form A has a normal diameter and thickness.
Form B has a normal diameter and a thin thickness.
Form C has a large diameter and is of average thickness.
Form D has a large diameter and a thin thickness.
Black metal washers are represented by Forms E through G.
Washers in their most basic form
A simple washer (sometimes known as a ‘flat washer’) is a flat annulus or ring, usually made of metal, that is used to distribute the load of a screwed fastener. In addition, if the hole is larger than the attaching nut, a basic washer can be utilized.
Fender washers are flat washers having a large outer diameter in comparison to their central hole. They’re named after their use on car fenders since they’re typically employed to spread the load on thin sheet metal. They can also be used to join two holes that have become larger due to rust or wear.
In the United Kingdom, a penny washer is a flat washer with a big outer diameter. The name was inspired by the size of an antique British penny. Even when the OD is twice the size of the old penny, most industries in the UK refer to all large OD washers as penny washers. They’re utilized in the same way as fender washers are.
An aspherical washer is a washer having one radiused surface that is designed to be used in conjunction with a mating nut to compensate for up to several degrees of misalignment between parts as part of a self-aligning nut.
A huge plate or washer linked to a tie rod or bolt is known as an anchor plate or wall washer. Anchor plates are used for structural reinforcement on the outer walls of masonry constructions. Because they are visible, many anchor plates are designed in a decorative form.
In woodworking, a torque washer is used with a carriage bolt; it has a square hole in the center that the carriage bolt square fits into. When a nut is tightened, the washer’s teeth or prongs bite into the wood, preventing the bolt from spinning freely.
Locking washers and spring washers
The modest conical shape of Belleville washers, also known as cupped spring washers or conical washers, exerts an axial force when bent.
A curved disc spring is similar to a Belleville spring, with the exception that the washer is only curved in one direction, resulting in only four points of contact. They simply apply light pressures, unlike Belleville washers.
When compressed, wave washers have a “wave” in the axial direction that creates spring pressure. Wave washers produce less force than Belleville washers of equivalent size. They are sometimes used as lock washers in Germany, but they are less effective than other options.
A split washer, also known as a spring lock washer, is a ring that has been split down the middle and bent into a helical shape. This allows the washer to exert a spring force between the fastener’s head and the substrate, keeping the washer firmly against the substrate and the bolt thread firmly against the nut or substrate thread, resulting in increased friction and rotational resistance. ASME B18.21.1, DIN 127B, and US Military Standard NASM 35338 are the applicable standards (formerly MS 35338 and AN-935). Spring washers have a left-hand helix that allows the thread to be tightened only in the right-hand direction, i.e. clockwise. When turning with the left hand, the raised edge cuts into the bottom of the bolt or nut and the portion to which it is fastened, preventing it from turning. As a result, on left-hand threads and hardened surfaces, spring washers are useless. They should also not be used with a flat washer underneath the spring washer since this prevents the spring washer from biting into the component it is securing. A nylon nut (nylon insert) must be used when a flat washer is required to span a big hole in a component. Spring lock washers have recently been the subject of dispute, with some publications advising against their use on the grounds that, when tightened, the washer is flat against the substrate and provides no more rotational resistance than a regular washer at the same tension. When a spring washer is slightly loosened, it will continue to keep the bolt on the substrate and maintain friction, whereas a simple washer will not.
Serrations that extend radially inside or outward to bite into the bearing surface define a toothed lock washer, also known as a serrated washer or star washer. Because the strain between the washer and the surface is applied across a considerably smaller area, this form of the washer is extremely useful as a lock washer when used with a soft substrate, such as aluminum or plastic, and can withstand rotation more than a plain washer on hard surfaces (the teeth). Internal, exterior, combination, and countersunk are the four types. Serrations run along the inner edge of the washer in the internal style, making it more aesthetically beautiful. Because of the larger circle, the external style features serrations along the outer edge, which gives superior holding power. For optimal holding force, the combination design incorporates serrations along both edges. The flat-head screws are designed to be utilized with the countersunk style.
Tooth lock washers are also used for ground bonding, which involves electrically connecting a metal lug or object to a surface. The washer’s teeth shave through surface oxides, paints, and other finishes to create a gas-tight conductive channel. The washer is placed between the surfaces to be bonded, not under the head of the screw (or under the nut) in these situations. The tooth washer does not have any anti-rotation locking characteristics in these situations.