Washers are primarily used as a seat to distribute load, but they may also provide spring tension, span oversize holes, insulate, seal, or provide electrical connection. Various kinds include flat, conical, and helical-spring washers, tooth or ribbed lockwashers, and special-purpose washers.
Flat washers, also known as plain washers, provide a bearing surface for a nut or screw head, cover large clearance holes, and distribute fastener loads over a large area, particularly on soft materials. This, in turn, reduces contact stresses and can thus reduce relaxation.
Two types of washers are defined for general use. Type A is a series of steel washers with broad tolerances, where design refinement is not important. They range from No. 6 to 3-in. hole size and from 3/16 to 5-in. OD. Type A washers are satisfactory for most assemblies. Type B washers are of higher quality and are specified in narrow, regular, and wide diameters for each screw size from No. 0 to 3 in. However, many other standard sizes are available.
Conical spring washers are used with screws to add spring take-up to screw elongation. Type L conical washers are designed for use with unhardened screws. Type H series is for hardened screws. Conical washers are designed to flatten at approximately one-half the ultimate load of the screw.
These washers are used with screws or bolts to increase the elastic properties of the joint. Screw and bolt connections loosen in different ways. Because the initial tightening is done by applying torque, loosening is most often thought to be caused by the bolt or screw backing off. However, all metal and nonmetal components under a steady stress will show some amount of relaxation (creep, cold flow) -- less for hard, smooth surfaces and more for softer and rougher surfaces. A similar effect results from differential thermal expansion/contraction, such as in many piping systems. The typical elastic elongation of a properly tightened screw or bolt is on the order of a few thousandths of an inch, and elastic compression of the clamped parts is usually even less. If the total relaxation approaches the initial elastic deformation, the clamping load will be almost lost. The joint loosens, although no reverse torque or backing off has occurred.
Properly selected spring washers may increase the elastic reserve of a joint by a factor of 10 or more. This means that relaxation that would cause 90% loss of preload without a spring washer will cause a loss of less than 10%.
"Belleville washer" is a term commonly, but incorrectly, applied to conical washers. The Belleville washer has precise dimensional relationships which give it greater spring action with load-bearing strength comparable to a conical washer of the same size.
Some conical washers have an off-center circular plane, creating a secondary system which reinforces and increases the spring action of the basic coned washer.
Another conical washer with a flexible conical rim for initial loads and an arched secondary square cone for secondary loads gives more consistent clamp loads under a variety of conditions. A flat washer used in conjunction with a spring washer may often be replaced with this type of conical washer.
Helical spring washers are made of slightly trapezoidal wire formed into a one-coil helix so that the free height is almost twice the thickness of the cross section. They are usually made of hardened carbon steel, but are also fabricated from aluminum, silicon bronze, phosphor bronze, stainless steel, and K-Monel.
The most common regular helical spring washer acts as a spring when bolt tension is reduced. Bolt tension is maintained by the washer's expansion. When flattened, this washer becomes equivalent to a flat washer. However, these washers can flatten at as little as 10% of proper bolt load.
Tooth lockwashers are used with screws and nuts to add spring take-up to the screw elongation and to increase frictional resistance under the screw head or nut face. They bite into both the head of the screw and the work surface to provide an interference lock. Even at zero tension, the tooth lockwasher provides frictional resistance to loosening.
Rib washers (serrated safety washers) combine the significant spring properties of conical spring washers with the increased frictional resistance of tooth lockwashers. They are available in a variety of materials.
Special-purpose washers, such as the finishing washer, eliminate the need for a countersunk hole and are used extensively for attaching fabric coverings. The outer rim grips the material over a large area.
The fairing washer, an aircraft development, is used with flat-head screws on aluminum sinks. Holding pressure is spread over a large area, eliminating localized strains around the screws. The shape of this washer allows for flush surfaces.