Washers serve several critical functions in a bolted joint, but their primary job is spreading the bolt’s clamping force over a wider area so it doesn’t damage the material underneath. Without a washer, the small contact area under a bolt head or nut concentrates enormous pressure on a single point, which can crush softer materials, pull through thin sheet metal, or gradually deform the joint surface until the bolt loses its grip.
How Washers Distribute Clamping Force
When you tighten a bolt, all the clamping force funnels through the small ring of metal where the bolt head contacts the surface. That concentrated stress can exceed what the material can handle, especially with high-strength bolts designed for heavy preloads. A flat washer sits between the bolt head (or nut) and the workpiece, spreading that same force across a much larger circle of contact.
This matters most with softer materials like wood, aluminum, plastic, and mild steel. In these cases, the bolt head can easily dig into or crush the surface without a washer. On thin sheet metal or when holes are oversized, a washer prevents the fastener head from pulling straight through. Even on harder materials, the stress distribution a washer provides helps the joint maintain its clamping force over time, because the surface underneath is less likely to deform and allow the bolt to gradually relax. Finite element studies confirm that as washer thickness and stiffness increase, stress distribution becomes more uniform and the joint retains more of its original clamping load.
Surface Protection During Tightening
As you torque a bolt or nut, the rotating fastener grinds against the surface it sits on. This scratches, gouges, or galls the workpiece, damaging both its structural integrity and its finish. A washer acts as a sacrificial barrier, absorbing that rotational friction so the part underneath stays intact.
This protective role also improves how consistently you can tighten bolts. When a fastener drags directly against a rough or damaged surface, friction becomes unpredictable. The same amount of torque on the wrench might produce very different clamping forces from one bolt to the next. Placing a washer between the turning component and the joint surface reduces and stabilizes that friction, giving you more consistent clamp force across every bolt in the assembly.
Preventing Corrosion Between Metals
When two different metals touch in the presence of moisture, an electrochemical reaction called galvanic corrosion can eat away at one of them. A steel bolt threaded into an aluminum housing, for example, will accelerate corrosion of the aluminum. Washers made from electrically inert materials (nylon, rubber, Teflon, or glass-reinforced epoxy) can insulate the bolt from the workpiece, breaking the electrical path that drives the reaction.
In structural applications, dielectric washers paired with insulating bolt sleeves achieve full electrical isolation of the fastener. The key is choosing a washer material that won’t absorb moisture, since any conductive path between the metals restarts the corrosion process. This is especially common in marine environments, outdoor structures, and anywhere dissimilar metals must be bolted together.
Resisting Vibration and Loosening
Vibration is the biggest threat to a bolted joint’s long-term reliability. Repeated shaking can gradually rotate a nut loose, bleeding off clamping force until the joint fails. Several washer types are specifically designed to fight this.
Spring lock washers (the split-ring type you see in hardware stores) compress when tightened, storing energy like a small spring. That stored force pushes back against the nut, maintaining tension even as vibrations try to rotate it loose. Their elastic memory lets them return toward their original shape under load, keeping pressure on the fastener. These are common in machinery, automotive assemblies, and construction where moderate vibration is expected.
For more demanding applications, wedge-locking washers use a completely different principle. Instead of relying on friction to resist loosening, they use angled cams between paired washer faces. Any attempt to rotate the nut actually stretches the bolt further, increasing tension rather than releasing it. This makes them effective in critical joints facing heavy vibration and dynamic loads. They cost more upfront but can be reused, which lowers long-term costs in industrial settings where joints are regularly serviced.
Common Washer Types and When to Use Them
- Flat washers are the general-purpose option. They distribute load, protect surfaces, and can compensate for slightly oversized holes. They come in varying thicknesses to match different load requirements.
- Fender washers have a small center hole and a much larger outer diameter. Originally named for their use on car fenders, they spread force over a large area and work well on thin metal or soft materials where a standard flat washer wouldn’t cover enough surface.
- Spring (lock) washers provide vibration resistance through their built-in flexibility. They’re a good default choice for any joint exposed to movement or repeated loading.
- Hardened washers are heat-treated to resist deformation under extreme compressive loads. In structural steel work, they prevent “embedment,” where a softer washer would slowly crush and allow the bolt to lose tension. The ASTM F436 specification covers hardened steel washers for use with high-strength structural bolts in sizes from 1/4 inch through 4 inches.
- Wedge-locking washers are the premium choice for critical joints in high-vibration environments like bridges, wind turbines, and heavy equipment.
Where Washers Matter Most
Not every bolt needs a washer. A steel bolt threaded into a tapped steel block with a machined surface may do fine on its own. But several situations make washers essential rather than optional.
Soft materials top the list. Wood, composites, plastics, and softer metals like aluminum will deform under bolt heads without washers, and that deformation means lost clamping force over time. Oversized or slotted holes are another clear case, since the bolt head needs a washer to bridge the gap and maintain full bearing contact. Any joint exposed to vibration benefits from a spring or wedge-locking washer. And painted, coated, or finished surfaces need the protective barrier a washer provides to avoid scratching during assembly.
In structural steel construction, hardened washers are typically required by code when using high-strength bolts. The washer must be at least as hard as the bolt to avoid becoming the weak point in the joint. Extra-thick versions with a nominal thickness of 5/16 inch are specified for oversized holes in structural applications, ensuring the washer itself won’t dish or deform under load.
The takeaway is straightforward: a washer is cheap insurance for a bolted joint. It protects the surface, spreads the load, stabilizes friction during tightening, and in specialized forms, actively fights loosening. Skipping one to save a few cents can mean a joint that slowly loses its grip, damages the material it’s supposed to hold, or corrodes from the inside out.