Written by Jody Muelaner
A washer is a plate with a hole that allows a fastener to pass through it. The most common shape is a flat disk with a concentric hole.
Washers are frequently used to distribute the clamping force from threaded fasteners over a larger area. This prevents bolt heads and nuts from indenting into the surfaces of the parts being fastened — which could cause the fastener to loosen.
Washers may be sprung to prevent vibration and loosening or to indicate the correct preload. They may also be serrated or tabbed to prevent unscrewing. Washers may also serve as a spacer or as a plain thrust bearing. Even gaskets that are used in taps are sometimes referred to as washers.
Steel washers are most common although other metals and plastic are also widely used. Several types of washers are available to suit these different purposes in specific applications.
A few of the different types of washers include:
- Plain washers are simple flat discs with a concentric hole for a fastener to pass through. The outer diameter is about twice the size of the hole’s diameter. Plain washers are primarily used to distribute the load in bolted connections.
- Extra-large plain washers are used to spread the load over a larger area when fastening flexible or weak materials, such as thin sheet metal. They may also be used where a hole has been damaged or enlarged and, for this reason, they’re sometimes referred to as repair washers.
- Belleville washers are disc-shaped components that go by several names, including coned-disc springs, conical spring washers, disc springs, Belleville springs, or cupped spring washers. This conical shell can be loaded along its axis, statically or dynamically. And it’s this frusto-conical shape that gives the washer its characteristic spring.
- Toothed lock washers have serrations around their inner or outer edge to prevent rotation.
- Tab washers have a rectangular tab or recess that fits a mating feature on the part being fastened, preventing rotation.
- Thrust washers are flat discs of low-friction material that are typically used as trust bearings. This is a type of plain bearing that’s designed to transmit axial loads in rotating shafts.