Types of Washers and When to Use Them Explained

The word washer points to two different tools. One is the small disk of metal, rubber, or plastic that sits under a bolt head or nut and spreads clamping force over a wider area. The other is the power cleaning machine that blasts grime off siding and driveways. If the cleaning machine is what you came for, start with how pressure washers work, which covers psi, flow rate, and cleaning technology. This article covers the fastener: the ring that keeps bolted joints tight.

Fastener washers look simple, but they solve real mechanical problems. They stop a nut from digging into soft wood, keep a bolt from spinning during assembly, seal a joint against water, and resist vibration that would otherwise back a fastener loose. Builders reach for them on nearly every bolted connection.

What Is a Washer and What Does It Do?

A washer is a thin plate with a hole in the center, made from metal, rubber, or plastic. It sits between a fastener and the clamped surface, or between the nut and the surface. Its primary job is to distribute the load from the bolt or nut across a wider area so the surface underneath does not deform or crack. It can also act as a spacer, a seal, or an insulator.

Load distribution is why most washers exist. A bolt head or nut concentrates the entire clamping force on a small ring of contact. On soft materials such as wood, aluminum, or plastic, that pressure leaves a permanent dent and the joint loosens. A washer spreads the same force over several times the area.

The Four Jobs of a Washer

• Load distribution: spreads clamping force across a wider bearing area.
• Surface protection: prevents marring, galling, and corrosion at the contact point.
• Spacing and alignment: positions the fastener or shims a gap.
• Sealing: blocks water, oil, or air from passing through the hole.

The Math Behind Load Distribution

The effect shows up in numbers. A 3/8 inch bolt carrying 4,500 pounds of clamping force bears on about 0.10 square inches of bare hex head, producing roughly 45,000 psi. That pressure crushes soft steel, wood, or aluminum. Add a standard 3/8 inch SAE flat washer and the contact area grows to about 0.47 square inches, cutting stress to roughly 9,500 psi. A 1.5 inch fender washer drops it to about 2,700 psi.

The name confusion shows up in hardware stores too. Ask for washers and the clerk may point to the fastener aisle or the cleaning aisle, where electric pressure washers for home cleaning line the shelves. The machines share only a name with the rings in this article, but they matter when you plan exterior maintenance.

The Three Families of Washers

All fastener washers fall into three families: plain, spring, and locking. Plain washers spread load and protect surfaces. Spring washers keep tension. Locking washers bite into mating surfaces to resist rotation.

Plain Washers

Plain washers are the largest family, with several shapes for specific jobs.

• Torque washer: a square hole grips the bolt so it cannot spin during adjustment; common in carpentry.
• Flat washer: the standard ring shape for general load distribution and for supporting small-head screws.
• Fender washer: a large outside diameter with a small center hole; spreads load over thin panels in automotive, plumbing, and electrical work, plus sign board and banner fixing.
• Finishing or countersunk washer: available in 90 and 120 degree versions that match countersunk screws so the head sits flush.
• Shoulder washer: a cylindrical shoulder insulates wires and separates components; made in nylon, fiberglass, PCTFE, and PTFE.
• C-washer: a slot cut from the center hole forms a C so it slides over a shaft or bolt without removing the fastener, handy when adjustments happen in place.

Insulation is a serious job, not a convenience. When dissimilar metals touch, galvanic corrosion attacks the joint, and an insulating washer breaks the electrical path. Installing insulating washers between steel bolts and aluminum plates is standard in marine, electrical, and structural work because it stops the corrosion at the source.

Spring Washers

Spring washers add axial flexibility. They compress under clamping load and keep pushing back, so the fastener stays tight even when vibration or shock tries to shake it loose.

• Belleville or conical washer: a dished cone delivering high load in a small space; stacking changes deflection and load.
• Dome spring washer: a smooth convex shape that maintains tension while limiting stress on the fastener.
• Wave spring washer: a wavy ring with low load and moderate deflection, common in bearings and electrical contacts.
• Finger spring washer: fingers press against the mating part where flexibility and low load are needed.
• Crescent or curved spring washer: a curved ring supplying light axial load in compact assemblies.

Locking Washers

• Helical or split lock washer: a split ring that acts like a spring and digs into both surfaces.
• External tooth lock washer: teeth on the outer edge bite into the part; best for soft materials.
• Internal tooth lock washer: teeth on the inner edge suit small screws and countersunk heads.

When to Use Washers

Use a washer whenever a fastener meets a soft, thin, or oversized surface, when the joint will see vibration, or when the hole must seal. Skip it only where the design calls for a bare head, such as precision assemblies where extra parts reduce clamp load.

Application-by-Application Guidance

• Wood framing: use flat washers under bolts and nuts on load-bearing connections so the wood does not crush.
• Metal structures: hardened flat washers under head and nut, with lock washers where vibration is expected.
• Automotive and machinery: spring or locking washers on mounts, brackets, and anything subject to shock.
• Electrical work: shoulder and insulating washers protect wires and separate conductors.
• Plumbing: rubber or fiber washers seal hose couplings and drain plugs.

The same word follows crews around a jobsite. The washers that protect bolted connections ride in the toolbox, while the machines that clean concrete and equipment after a renovation burn fuel. For heavy-duty cleaning on sites without outlets, gas pressure washers are the standard choice.

How to Use a Washer Correctly

1. Choose the washer type for the job: plain for load spread, spring for tension, locking for rotation resistance.
2. Match the inner diameter to the bolt or screw shank; it should slip on without binding, and the head or nut should bear on the washer.
3. Place the washer under the bolt head, under the nut, or both when either surface is soft.
4. Orient spring and locking washers so the split, curve, or teeth engage the mating surface.
5. Torque the fastener to its specified value; a washer is not a cure for over-tightening.

Orientation and Stacking Rules

A split lock washer goes under the nut, split facing the nut. Tooth lock washers bite into the softer of the two surfaces. When stacking, put the flat washer closest to the surface and the lock washer under the fastener. Never stack multiple lock washers: they act as springs and reduce clamp load.

When One Washer Is Not Enough

Soft or uneven surfaces may need a larger fender washer or a second flat washer. Structural connections often specify washer diameter from code tables, so check the design before changing parts.

On jobsites the word shows up yet again on battery-powered gear. Crews carry cordless pressure washers for quick cleanup between trades, and like fastener washers these machines are chosen by matching specs to the job: flow, pressure, and portability.

Washer Shapes and Materials

Materials at a Glance

• Steel with zinc or galvanized finish: the everyday choice for dry interiors and light outdoor duty.
• Stainless steel: resists rust and handles marine, food, and chemical environments.
• Brass: non-sparking and corrosion resistant, used in plumbing and electrical fittings.
• Nylon and plastic: insulating and lightweight, used in electronics and where isolation matters.
• Rubber and fiber: flexible seals for hoses and drain plugs rather than load carriers.

Heat changes the rules. Standard rubber washers soften and swell in hot water, which is why equipment that handles hot water, such as hot water pressure washers, uses high-temperature seals.

Washer Shapes Beyond the Ring

Most washers are round, but the exceptions solve real problems. The C-washer opens sideways for in-place service. The torque washer locks onto a square bolt shank so the bolt cannot spin. Countersunk washers match screw head angles of 90 or 120 degrees for a flush finish. Shoulder washers add a precision step that positions parts and insulates them.

Sizing Standards and a Selection Checklist

Washer sizing comes down to three numbers: inner diameter, outer diameter, and thickness. The inner diameter must clear the bolt shank, the outer diameter must bear on solid material, and the thickness must survive the clamping load.

SAE Versus USS Sizing

Two sizing systems dominate in the United States. SAE washers are smaller and thinner, sized for a given bolt diameter, and suit structural and precision work. USS washers are larger and thicker, with a bigger outside diameter, and appear in general construction where more bearing area helps. Metric washers follow ISO standards with the inner diameter matching the bolt size.

Selection Checklist

1. What surface is the fastener bearing on? Soft or thin material needs a larger washer.
2. Will the joint vibrate or shock load? Add a spring or locking washer.
3. Does the hole need to seal? Pick rubber, fiber, or a sealing washer.
4. Will the joint see moisture or chemicals? Match the material to the environment, stainless or brass where rust is a risk.
5. Does the assembly get serviced in place? A C-washer may save hours.

Garden hoses prove the point. The flat rubber ring inside a hose coupling is a washer too, and most leaks trace back to a worn one. The guidance on choosing washers and preventing leaks, plus selecting durable materials, applies to that small part as much as to a structural washer.

The same cost-versus-spec thinking runs through construction purchases. Buyers compare electric pressure washers by psi, gpm, and what 200 really buys before upgrading. The same discipline applies to a box of washers: match the part to the load, the environment, and the maintenance plan, and the joint will outlast the project.