Washers are disks of metal or non-metallic material placed beneath a nut, an axle bearing, or a joint, to relieve friction, prevent leakage, isolate, prevent loosening, or distribute pressure.
There are many different types of washers for a variety of applications. Lock washers are used to secure fasteners. Spring washers are a load bearing device that provides a preload between two surfaces. Other common types include flat washers, C-washers, D-shaped washers, countersunk or finishing washers, fender washers, SAE washers, torque washers, and shoulder washers.
Spring Washers - Spring washers, sometimes called disc springs, lend their mechanical capabilities to the unique profile of the material: the irregularities of the washer compress with a proportionate resistance to return to their predeflected shape. Spring washers are employed in applications where assemblies need a part to take up play, maintain assembly tension, compensate for expansion or contraction in materials, or to absorb intermittent shock loads and provide a controlled reaction under dynamic loads.
Lock Washers - Lock washers are designed to secure fasteners that have a tendency to rotate or lose friction. Although though there are several different types of lock washers and is each designed for use with a particular fastener or application, they all work under the same principle. The washer is designed to exert a load, partially deform, and lock a fastener in place. A split coil or teeth of the lock washer bite into the head of the fastener and against another flat surface. In some applications they are deployed along with a flat washer in order to distribute the load evenly without deforming the assembly that the fastener is secured to.
Flat Washers - Flat washers, also referred to as Type A plain washers, are thin, flat, and circular general-purpose washers with a centrally located hole. Standardized flat washers may be designed by the use of imperial or metric dimensions. Standard imperial washers include SAE washers, for use with fine threaded nuts and bolts, and USS flat washers, for use with coarse threaded bolts and nuts. Standard metric washers are available in several gages as defined by JIS standards.
Countersunk Washers - Countersunk washers, sometimes referred to as finishing washers, have a countersink that captures the head of the fastener. When secured they provide a flush surface and are available in several shapes including: 90 degree countersunk, angle countersunk, flanged, un-flanged, and rolled flange among others.