Washers serve one fundamental purpose: they sit between a fastener and a surface to protect the joint. But the specific reasons for using them go well beyond that simple description. Depending on the type, a washer can spread clamping force, prevent loosening, block corrosion, seal out fluids, or compensate for a bolt that’s slightly too long. Understanding which problem each washer solves helps you pick the right one and avoid joint failures down the line.
Spreading the Load
The most common reason to use a washer is load distribution. When you tighten a bolt, all of that clamping force concentrates under the bolt head or nut. On harder materials like structural steel, the surface can handle it. But on softer materials like wood, aluminum, plastic, or thin sheet metal, that concentrated pressure can crush the surface, pull the bolt head into the material, or cause cracking.
A flat washer increases the bearing surface, spreading that force over a wider area. Think of it like snowshoes versus boots: same weight, but distributed so you don’t sink in. This is especially important in any joint where the hole is oversized or the material is relatively soft compared to the fastener. In structural steel applications, hardened washers rated to industrial standards are required when using high-strength bolts with oversized holes, with extra-thick versions available for holes up to 4 inches in diameter.
Preventing Surface Damage
As a nut or bolt rotates during tightening, it grinds against whatever it’s sitting on. On a painted panel, a machined surface, or a decorative finish, that rotation leaves scratches, gouges, or galling marks. A washer acts as a sacrificial barrier, absorbing that abuse so the workpiece stays intact. The washer itself may get scuffed up, but it’s a cheap, replaceable part protecting something far more valuable.
This matters beyond cosmetics. A smooth, hard washer also gives the fastener a consistent surface to bear against, which helps you achieve accurate torque. Without one, an uneven or rough surface creates unpredictable friction, meaning the same torque reading can produce very different levels of actual clamping force.
Stopping Bolts From Loosening
Vibration is the enemy of bolted joints. Repeated shaking, thermal cycling, or dynamic loads can gradually rotate a nut loose, sometimes with catastrophic results. Lock washers are designed specifically to resist this.
Split lock washers (the most common type) dig into both the fastener and the surface to create friction that resists rotation. Star washers, also called tooth lock washers, use small teeth around their perimeter to bite into the mating surfaces. Wedge-locking washers take a different approach entirely, securing the joint with tension rather than friction. They use opposing cams so that any attempt to rotate the nut actually increases the clamping force. In standardized vibration tests, wedge-locking designs consistently outperform split and star washers, which is why they’re specified in critical applications like bridges, wind turbines, and heavy machinery.
Adjusting Grip Length
Bolts come in standard lengths, and the material you’re clamping doesn’t always match up perfectly. If a bolt is slightly too long for the joint, the threads may bottom out in a blind hole before the head clamps down, or the nut may tighten against the threads instead of the assembly. Flat washers solve this by adding thickness to the clamped stack, effectively shortening the bolt’s reach.
This adjustment has limits. Adding too many washers creates a spongy, unreliable joint. The general guideline, and the one used in FAA aircraft fastener standards, caps combined washer thickness at about 1/8 inch for grip length adjustment. If you need more than that, it’s time to find a shorter bolt.
Blocking Corrosion Between Metals
When two different metals touch in the presence of moisture, an electrochemical reaction called galvanic corrosion starts eating away at one of them. A steel bolt through an aluminum panel, for instance, will eventually corrode the aluminum around the hole. The fix is an insulating washer made from a material that doesn’t conduct electricity: nylon, neoprene, rubber, Teflon, or glass-reinforced epoxy.
These dielectric washers (sometimes paired with bolt sleeves for full isolation) break the electrical connection between the two metals, stopping the reaction before it starts. They need to be evaluated for the specific loading conditions of the joint, since plastic and rubber don’t handle heavy compression the same way metal does. But in applications ranging from marine hardware to architectural facades, they’re essential for preventing long-term structural decay.
Sealing Out Fluids
Some washers aren’t about mechanical force at all. Sealing washers create a fluid-tight barrier at threaded connections in hydraulic, pneumatic, and plumbing systems. The simplest version is a fiber or rubber washer compressed between a fitting and a surface, like the rubber washer inside a garden hose connector.
For higher pressures, bonded seal washers combine a metal ring with a rubber element bonded to its inner edge. The metal resists the bursting force of the fluid pressure while the rubber conforms to surface imperfections and creates the seal. These were developed to replace traditional copper crush washers and can handle pressures up to about 14,500 psi across a temperature range from roughly negative 20°F to 390°F. You’ll find them in hydraulic brake lines, fuel systems, and industrial fluid power connections.
Fender Washers for Thin Materials
Standard flat washers work well on reasonably thick, rigid materials. But when you’re fastening thin sheet metal, fabric, fiberglass, or other materials that tear easily, a standard washer’s footprint may not be wide enough. That’s where fender washers come in. They have the same center hole as a standard washer but a much larger outer diameter, giving them a significantly bigger contact area.
Picture bolting a thin metal bracket to a wooden frame. A standard flat washer might let the metal dimple or the wood splinter under clamping pressure. A fender washer distributes that same force across a wider circle, keeping both surfaces intact. They got their name from automotive body work, where they protect thin fender panels from bolt damage, but they’re equally useful in appliance repair, HVAC ductwork, and general construction wherever the material being fastened is thinner or softer than what a standard washer can protect.
What Happens Without the Right Washer
Skipping a washer or using the wrong type doesn’t always cause an immediate failure, which is exactly why it’s so easy to overlook. The problems tend to show up later. The most common issue is preload loss: the clamping force in the bolt gradually drops as surfaces settle into each other. The tiny peaks and valleys on contacting surfaces, invisible to the naked eye, deform and flatten under load. As this happens, the effective thickness of the clamped stack shrinks and the bolt loses tension. Depending on the materials and joint design, this settling can cause a loss of 50% or more of the original clamping force.
Signs that a washer is failing or was wrong for the application include visible cupping or dishing of the washer, surface damage on the workpiece beneath it, and fasteners that feel loose despite being torqued to spec. In assembly settings, a bolt that reaches the correct angle of rotation with unusually low torque is a red flag for material embedding or washer deformation. The washer may look like a minor component, but it’s doing real mechanical work in every joint it sits in.