A washer is a thin disk with a hole, usually in the middle. It is normally used to
support the load of a threaded fastener. Other uses are as a spacer, spring (belleville,
wave washers), wear pad, preload indicating device, and locking device. Rubber washers
are also used in taps (valves) where they form the seal that shuts off the flow of liquid
or gas.
Washers are usually metal or plastic. High quality capscrews require hardened steel washers
to prevent the loss of pre-load after the torque is applied (due to Brinelling). Rubber or
fiber gaskets used in taps to stop the flow of water are sometimes referred to colloquially
as washers; while they may look similar, however, washers and gaskets are usually made
differently and designed for different functions.
Washers are also important for preventing galvanic corrosion, particularly by insulating
steel screws from aluminum surfaces.
A gasket is a mechanical seal that serves to fill the space between two objects, generally
to prevent leakage between the two objects while under compression. Gaskets are commonly produced
by cutting from sheet materials, such as gasket paper, rubber, silicone, metal, felt, fiberglass,
or a plastic polymer. Gaskets for specific applications may contain asbestos. It is usually desirable
that the gasket be made from a compressible or slightly compressible material such that it tightly fills
the space it is designed for, including any slight irregularities.
One of the more desirable properties of an effective gasket in industrial applications for compressed
fiber gasket material is the ability to withstand high compressive loads. Most industrial gasket
applications involve bolts exerting compression well into the 14 MPa (2000 psi) range or higher.
Generally speaking, there are several truisms that allow for best gasket performance. One of the more
tried and tested is: "The more compressive load exerted on the gasket, the longer it will last". There
are several ways to measure a gasket material's ability to withstand compressive loading. The "hot compression
test" is probably the most accepted of these. Most manufacturers of gasket materials will provide or publish
these results.