What a Washer Is For and When It Is Not

Why this matters

Washers get added out of habit, and habit is a poor engineer. Every washer you put in a joint is another interface that can embed, another surface whose friction changes what your torque wrench is measuring, and another thing to creep. A washer that is doing a defined job earns all of that. A washer added because there was one in the bag is a preload leak with a reason nobody can state. Worse, the single most-fitted washer in the trades, the split lock washer, does not do the job its name claims.

Before adding, removing or replacing a washer in an installed joint: you are opening a load path. Rig or block an independent support under whatever the joint carries before the fastener is slackened, and stand clear of the piece rather than beside it. Where the joint clamps a pressurised or spring-loaded part, isolate and relieve to zero at the joint first under 29 CFR 1910.147 in general industry, or 29 CFR 1910.333(b)(2) where the connection is electrical, since 1910.147 excludes exposure to electrical hazards from work on conductors and equipment in electric utilization installations at (a)(1)(ii)(C). Where the connection is structural, meaning it is holding building or equipment loads rather than only sealing something, changing the hardware is an engineering decision and belongs with the engineer of record or the authority having jurisdiction, not with a truck decision.

The four jobs a washer actually does

• Spread bearing pressure. The head or nut has a small bearing area. If the member under it is soft, thin, or both, that pressure crushes it. A washer trades pressure for area.
• Give a consistent friction surface under the turning element. Bearing-face friction is the largest single friction term in torque tightening, so a hard, smooth, uniform washer under the nut makes the torque-to-preload relationship far more repeatable across a set of fasteners than a raw, painted, or as-cast surface does.
• Bridge a hole that is too big. Oversized, slotted and long-slotted holes leave the head with too little to bear on, and a washer, or often a specified plate washer, covers the opening.
• Protect a finished surface. A turning nut scours the coating it sits on. Where that coating is the corrosion protection, the scour is the start of the failure.

There is a fifth case, isolating two metals that should not touch, but the dissimilar-metal joint has its own governing rules and its own sibling article. Route the material decision there rather than treating an isolating washer as an ordinary washer with a different label.

What a washer is not doing

It is not adding clamp force. Nothing between the nut and the member generates tension in the bolt. If the joint is slipping, leaking or loosening because clamp is low, no washer in any material fixes it.

A split lock washer is not locking anything. A split ring is a spring, and its spring force is small compared with the preload of a properly tightened fastener, so it flattens well before the fastener reaches value. Once flat it is a hard, narrow, slightly rough plain washer. It has no remaining travel, so it cannot take up any loss, and it has no mechanism to resist rotation once the faces have started slipping. Where rotation is the real failure, the fix is to stop the transverse slip that causes it, and the sibling article on why joints loosen owns that.

It is not flattening a face. A washer follows the surface it sits on. A bearing face that is not flat, a boss that is not square to the hole, or a burr around a drilled hole all stay exactly as wrong with a washer over them, and the washer will dish to match.

It is not a length adjuster. Stacking washers to take up a bolt that is too long adds interfaces, each with its own embedment, and it usually puts the unthreaded shank across the joint line or leaves the nut running on thread runout. Get the right length.

A soft washer is not cushioning anything useful in a structural joint. Soft material in the grip is a creep source, and creep is preload loss on a timer. There are joints that deliberately want a compliant element, and in those it is specified with a spring rate, not chosen because it felt kind.

The bearing-pressure arithmetic, worked

Equipment mount, 1/2-13 bolt through a wood framing member, installed at a preload of 9,000 lbf. Everything below is in inches and pounds.

Without a washer. The bearing area under a hex head is the ring between the head's bearing circle and the hole. Take the head bearing diameter at about 0.75 inch and a standard 9/16 inch clearance hole at 0.5625 inch:

• Area = 0.7854 x (0.75 squared minus 0.5625 squared) = 0.7854 x (0.5625 - 0.3164) = 0.1933 square inch
• Bearing pressure = 9,000 divided by 0.1933, about 46,560 psi

Compression perpendicular to the grain in common softwood framing runs in the low hundreds of psi. That head is two orders of magnitude past it. It does not "maybe crush"; it buries itself, and every thousandth of an inch it sinks is preload gone.

With a standard 1/2 inch flat washer, outside diameter 1.375 inch, same hole:

• Area = 0.7854 x (1.890625 - 0.316406) = 1.2364 square inch
• Bearing pressure = 9,000 divided by 1.2364, about 7,280 psi
• Improvement factor: 1.2364 divided by 0.1933, about 6.4 times

Read the result honestly, because this is where the habit fails. The washer bought a 6.4x reduction, which is a lot, and the answer is still more than an order of magnitude above what the wood will take. The washer did not fix this joint. It made the arithmetic less absurd.

And the 6.4 figure is optimistic in a way worth naming. That calculation assumes the washer spreads pressure uniformly over its full annulus, which it does only if it is stiff enough not to dish. A thin washer bends under the head and concentrates pressure in a ring near the hole, so the effective area is smaller than the geometric one and the real pressure is higher. Thickness is what makes a washer a distributor rather than a shim, which is exactly why plate washers exist for wood connections and why a specified thickness is part of the specification rather than a detail.

So what is the actual fix. Two terms, and the washer only touches one:

• Area: a much larger plate washer, of specified size and thickness, sized by the engineer or the connection detail rather than picked from a bin.
• Preload: the target itself has to come down. A bolt into a soft member cannot be tightened to a value derived for steel-to-steel, because the limit is the member's bearing capacity and not the bolt's tensile capacity. That is the term people never adjust, and it is why the joint keeps going slack after each visit.

The failure mode of getting it wrong is quiet and slow. The head sinks over weeks under sustained load and moisture cycling, preload bleeds off with the fastener never rotating, and the mount develops movement. The tech re-tightens it, which drives the head in further and buys a shorter interval than last time. Three visits in and the hole is a socket, and the finding written down each time is that the bolts were loose.

When to leave the washer out

• Under the fixed element of a joint where the head does not turn. The washer's friction-consistency job applies to the turning element. On the non-turning side it is one more interface for nothing unless it is spreading load.
• On a machined, spotfaced bearing surface in hard steel, where the head's own bearing area is already adequate and the surface is already flat, smooth and square. Adding a washer there adds an embedment interface and changes the bearing friction the published torque value assumed.
• Where the manufacturer's assembly does not include one. A published torque value was developed for a specific stack. Adding a washer changes the grip length, the bearing friction, and sometimes whether the thread still engages correctly.
• Where a hardened washer is required and only a plain one is on the truck. Under a high-strength fastener, an unhardened washer can be indented by the head, which both loses preload and defeats the load spreading it was fitted for. The right answer is the specified hardened washer, not the soft one you have.

Checking you got this right

• Say the washer's job out loud before fitting it. If the sentence is "because that is how it comes apart," take it out or find out what it was doing.
• Compare the bearing area against the softest member in the stack, not against the bolt. The limit lives in the weakest bearing surface.
• Check washer thickness on any soft or thin member. A washer that dishes is not distributing, and you can often see the dish on the removed part.
• Look at the removed washer as evidence. A bright polished ring means the nut turned on it, which is normal; a washer indented by the head means the material was too soft for the load; a flattened split washer tells you it contributed nothing and had not for a long time.
• After changing anything in the stack, treat the published torque value as no longer verified for that assembly, and go back to the source for a value that matches what is now in the joint.

References

• 29 CFR 1910.147, control of hazardous energy, and 29 CFR 1910.333(b)(2) for the electrical case that 1910.147 excludes at (a)(1)(ii)(C)
• Equipment and fastener manufacturer documentation for the specified washer type, hardness and thickness, and the torque value developed for that stack
• Published wood and metal design references for bearing and compression capacity perpendicular to the grain or across a thin section, routed through the engineer of record where the connection is structural
• See related: Why a Bolted Joint Loosens; Why Preload and Not Friction Holds a Joint Together; The Fastener That Was the Wrong Material