What Is a Washer - A Complete Guide to Types Materials and Applications

Washers look minor until a joint loosens. A washer spreads pressure, resists loosening, seals and even isolates: choosing the right type protects the bolt head and the clamped surface alike. The same logic applies on flanged joints, where the nut bears down through a washer on the flange face before the load reaches the flange gaskets.

Washer Types and Applications

Six washer families cover most assembly problems. Each one solves one specific job, and using the wrong one is what turns a small part into an expensive failure.

• Flat washer — a plain ring of uniform thickness. Its main task is pressure distribution: it enlarges the contact area so the bolt head does not sink into wood, plastic or sheet metal. The default washer for general connections.
• Spring washer (split lock) — a ring cut and twisted into a helical ramp. The continuous spring force presses back against the fastener and resists loosening under vibration; it is an anti-loosening aid, not a permanent lock.
• Non-standard flat washer — any flat washer whose size, shape, thickness or material falls outside standard tables. Custom made for special hole sizes, extreme temperatures, food-grade media or shaped interfaces.
• Saddle washer — half a flat washer with a flat bottom and an arched top. It clamps pipes and cables to flat surfaces, matching the outer curve so pressure spreads evenly without crushing the surface.
• Square flat washer — a square or rectangular washer with a round centre hole. The corners dig into the material and stop the washer spinning with the bolt, which speeds up assembly on wood and in automated feeding.
• Serrated lock washer — sharp teeth on the outer edge, the inner edge or both. The dual bite into the part and under the fastener head gives strong rotation resistance in high-vibration and safety-critical joints.

Table 1. Six washer types with their primary function and the applications they fit best.

Pressure distribution is the flat washer's core duty; locking is the spring and serrated washers' duty; the saddle and square types exist for one task each. Keeping those roles straight is what selection is about.

Matching the Washer to the Joint

Three questions narrow the field: what must the joint resist, what does the surface allow, and what does the assembly need afterwards.

• Primary need — pressure distribution points to flat washers; anti-loosening points to spring or serrated types; clamping a pipe or cable points to a saddle washer.
• Operating environment — vibration, temperature swings and corrosive media decide the material and whether a lock type is required at all.
• Surface hardness — serrated teeth bite into soft surfaces and ruin them; on painted or machined faces use plain washers or a serrated type sized to sit under the head only.
• Practical constraints — cost, tool access and whether the joint must come apart again. Non-standard washers cost more and take longer to supply, so only specify them when the standard range fails.
• Joint type — bolted flanged connections rely on even clamping around the circumference, so flat or hardened washers under the nuts on pipe flanges protect both the flange face and the gasket seating.

Installation Mistakes to Avoid

Most washer failures come from three habits.

• Wrong size — an oversized hole or undersized outer diameter concentrates pressure again and cancels the washer's purpose.
• Reusing damaged washers — a compressed spring washer or bent serrated washer no longer provides its designed force; replace rather than re-torque.
• Over-tightening — excess torque crushes spring washers flat and digs serrations too deep, which accelerates fatigue in the joint.

Right-sized, correctly applied and replaced when damaged, a washer keeps the joint tight for its full service life — the cheapest insurance in any bolted assembly.

Frequently Asked Questions

Does a spring washer stop loosening completely?

No. It supplies continuous spring force that helps against vibration, but it is not a permanent lock. For safety-critical joints combine it with a serrated washer or a locking nut.

When is a square washer better than a round one?

When the washer must not rotate with the bolt — for example on wood or smooth material in automated assembly. The corners anchor the washer and speed up the work.

Why use a saddle washer for pipes?

Its arched top matches the pipe's outer curve while the flat bottom sits on the mounting surface, so clamping force spreads evenly and the pipe surface is not crushed.

Can I reuse a deformed lock washer?

No. Once a spring or serrated washer has taken a permanent set or bent, its force drops and the joint can loosen under the same conditions the washer was meant to resist.