Washer (hardware)


A washer is a thin plate with a hole that is normally used to distribute the load of a threaded fastener, such as a bolt or nut. Other uses are as a spacer, spring, wear pad, preload indicating device, locking device, and to reduce vibration. Washers often have an outer diameter about twice their inner diameter, but this can vary quite widely.
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. Washers are also important for preventing galvanic corrosion, particularly by insulating steel screws from aluminium surfaces. They may also be used in rotating applications, as a bearing. A thrust washer is used when a rolling element bearing is not needed either from a cost-performance perspective or due to space restraints. Coatings can be used in attempt to reduce wear and friction either by hardening their surface, or providing a solid lubricant.
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.
Rubber or fiber gaskets used in taps as seal against water leaks 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.

Type and form

Washers can be categorised into three types;
The American National Standards Institute provides standards for general use flat washers. Type A is a series of steel washers at 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.
"Type" is not to be confused with "form". The British Standard for Metric Series Metal Washers, written in 1968, coined the term "form". The forms go from A to G and dictate the outside diameter and thickness of the flat washers.
Please note: Washer 'form' when comparing different washer material types is used quite freely by stockists. In relation to BS4320 specifically, washer forms 'A' to Form 'D' inc. are designated ‘bright metal’ washers supplied self-finish in of various metals – including: various steel alloys, brass, copper etc. Whereas, BS4320 washer form ‘E’ to ‘G’ inc. are designate ‘black’ mild steel washers, which normally are specified with a supplementary protective coating supply condition.

Plain washers

Spring and locking washers

NameImageDescription
Belleville washer, also known as a cupped spring washer or a conical washerA washer with a slight conical shape, which provides an axial force when deformed. The surface may be serrated, which will have improved locking capabilities.
Curved disc springSimilar to a Belleville, except the washer is curved in only one direction, therefore there are only four points of contact. Unlike Belleville washers, they exert only light pressures.
Wave washersA washer with 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 sometimes used as lock washers, however they are less effective than other choices.
Split washer or spring lock washerA 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, which maintains the washer hard 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 B, and United States Military Standard NASM 35338.
Spring washers are a left hand helix and allow the thread to be tightened in a right hand direction only, i.e. a 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 resisting turning. Therefore, spring washers are ineffective on left hand threads and hardened surfaces. Also, they are not to be used in conjunction with a flat washer under the spring washer, as this isolates the spring washer from biting into the component that will resist turning.
The use and effectiveness of spring lock washers has been in debate of late, with some publications advising against their use on the grounds that, when tight, the washer is flat against the substrate and gives no more resistance to rotation than a normal washer at the same torque. NASA researchers have gone as far as to say "In summary, a lockwasher of this type is useless for locking." However, a spring washer will continue to hold the bolt against the substrate and maintain friction when loosened slightly, whereas a plain washer will not.
The benefit of spring lock washers lies in the trapezoidal shape of the washer. When compressed to loads near the proof strength of the bolt, it will twist and flatten. This reduces the spring rate of the bolted joint which allows it to maintain more force under the same vibration levels. This prevents loosening.
Toothed lock washer, serrated washer or star washerA washer with serrations that extend radially inward or outward to bite into the bearing surface. This type of washer is especially effective as a lock washer when used with a soft substrate, such as aluminium or plastic, and can resist rotation more than a plain washer on hard surfaces, as the tension between washer and the surface is applied over a much smaller area. There are four types: internal, external, combination, and countersunk. The internal style has the serrations along the inner edge of the washer, which makes them more aesthetically pleasing. The external style has the serrations around the outer edge, which provides better holding power, because of the greater circumference. The combination style has serrations about both edges, for maximum holding power. The countersunk style is designed to be used with flat-head screws.
Tooth lockwashers are also used for ground bonding where a metal lug or object must be electrically bonded to a surface. The teeth of the washer cut through surface oxides, paints or other finishes and provide a gas-tight conductive path. In these applications, the washer is not placed under the head of the screw, it is placed between the surfaces to be bonded. In such applications, the tooth washer does not provide any anti-rotation locking features.
Tab washerA locking washer that works by having a side tab that can be bent into place against a nut. Metallic or non-metallic, the tab washer notch prevents rotation relative to the shaft or fastener. Variations include single, multiple, formed and flat.
Wedge lock washerA two-piece washer with radial teeth on one side and wedging action of the halves where they join. While generally more expensive per piece, these washers provide the highest amount of vibrational loosening prevention.

Lock washers, locknuts, jam nuts, and thread-locking fluid are ways to prevent vibration from loosening a bolted joint.

Gaskets

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.
NameImageDescription
Shoulder washerA plain washer type with integral cylindrical sleeve; they are used to keep separate different metal types, and as seals. This term is also used for electrically insulating grommets.

Specialised types

Materials

Washers can be fabricated from a variety of materials including, but not limited to:
A number of techniques are used to enhance the corrosion resistant properties of certain washer materials:
Washers of standard metric sizes are listed in the table below. Measurements in the table refer to the dimensions of the washers as described by the drawing. Specifications for standard metric flat washers were known as DIN 125 and replaced with ISO 7089. DIN standards are issued for a variety of components including industrial fasteners as Metric DIN 125 Flat Washers. The DIN standards remain common in Germany, Europe and globally even though the transition to ISO standards is taking place. DIN standards continue to be used for parts which do not have ISO equivalents or for which there is no need for standardization.