Washer

A washer is a flat, disc-shaped plate with a hole in the centre, placed under a nut or bolt head to distribute clamping force, protect the surface, or prevent loosening.

How it works

When a nut is tightened, it exerts a rotational force (torque) that can mar or dig into the surface of the part being clamped. A washer spreads that force over a wider area, preventing damage. It also provides a smooth bearing surface for the nut to turn against, making torque more consistent and predictable.

Common types

• Flat washer: The simplest type — a plain disc that distributes load and protects surfaces. Used in almost every bolted joint.
• Lock washer: A split ring or toothed disc that spring-loads against the nut, creating friction that resists loosening from vibration.
• Fender washer: An oversized flat washer with a large outer diameter, used to spread load over thin or soft materials like sheet metal and plastic.
• Belleville washer: A conical disc spring that maintains clamping force even when the bolt stretches or the joint settles.
• Tab washer: Has a tab that bends against the nut or the part surface, mechanically preventing rotation.

Why they matter

Skipping washers is a common shortcut that can lead to loose fasteners, damaged surfaces, and inconsistent torque. In structural and automotive applications, the correct washer is as important as the correct bolt grade.

Flat washer vs. lock washer

The two families do different jobs, and substituting one for the other is a common mistake. A flat washer is a passive spacer: it spreads load and protects the surface, but does nothing against rotation. A lock washer (split ring or toothed) actively resists loosening by biting or spring-loading against the nut and the surface. If the joint vibrates — machinery, engines, anything on a moving vehicle — a lock washer belongs under the nut. If the joint is static and the surface is soft (aluminium, plastic, wood), a flat washer is the right choice and a lock washer’s teeth will only chew the finish.

They can also stack: flat washer against the surface, lock washer between flat washer and nut. That order protects the surface while keeping the anti-rotation action where it belongs.

How to size a washer

Washer sizing follows the fastener, not the hole: the inner diameter should be a close slip-fit on the bolt — a few tenths of a millimetre of clearance, no more. An oversized inner diameter defeats the washer’s purpose, because the nut can still pull through the hole it was meant to cover. Outer diameter is chosen by load: soft or thin materials call for fender washers whose large face bridges the weak zone.