Flat washers are one of the most common components used in mechanical assemblies.
Because they are inexpensive and simple, they are often installed without much thought.
You may have heard comments such as:
- "Just add a washer."
- "The hole is too large, so let's use one."
- "The orientation doesn't matter."
While these practices are common, they are not always correct.
A washer is not just an accessory—it is an important part of a bolted joint that directly affects clamping force, joint reliability, and long-term performance.
In this article, we'll explain the real purpose of flat washers, common mistakes to avoid, and why washers are essential in high-strength bolted joints.
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What Is the Real Purpose of a Flat Washer?
A flat washer has three primary functions.
1. Distributing Bearing Pressure
When a bolt or nut is tightened, a large clamping force is concentrated on a relatively small contact area.
Without a washer, this can create excessive local stress that may damage the connected parts.
A properly selected washer helps to:
- Increase the bearing area
- Reduce contact pressure
- Prevent indentation or permanent deformation of the base material
In other words, a washer helps transfer the clamping force more evenly.
2. Providing Stable Friction Conditions
The preload generated in a bolt depends greatly on friction.
Surface conditions such as:
- Paint
- Mill scale
- Burrs
- Surface roughness
can significantly affect the friction coefficient.
Using a washer with a consistent surface helps reduce these variations, allowing a more predictable bolt preload.
3. Protecting the Bearing Surface
In joints that are frequently assembled and disassembled, the rotating nut or bolt head can scratch or gall the contact surface.
A washer protects the parent material from wear and makes maintenance easier.
Practical Uses Beyond Their Original Purpose
In real engineering work, washers are often used for additional practical reasons.
Covering Oversized Holes
Washers are commonly used when dealing with:
- Slotted holes
- Flame-cut holes
- Field-drilled holes
In these cases, the washer prevents the bolt head or nut from sinking into the opening.
This is generally an acceptable engineering practice—as long as the washer provides sufficient bearing area and does not deform under load.
Common Mistakes When Using Washers
Stacking Multiple Washers
A common shortcut is to stack two or more washers to compensate for gaps or adjust height.
Although convenient, this is usually poor engineering practice.
Possible problems include:
- Washers sliding against each other
- Uneven load distribution
- Reduced bolt preload
- Increased risk of loosening
Multiple washers may also compress differently over time, causing preload loss.
If dimensional adjustment is required, use properly manufactured shims instead of stacking washers.
Using Flat Washers on Inclined Surfaces
Installing a flat washer on an inclined or uneven surface creates eccentric loading.
This can lead to:
- Bending stress in the bolt
- Uneven bearing pressure
- Reduced fatigue life
For inclined surfaces, tapered washers should be used.
For example, tapered washers are commonly used with structural steel channels to provide a flat bearing surface.
Tightening Over Burrs or Thick Coatings
Even if a washer is installed, it cannot compensate for poor surface preparation.
Do not tighten bolts over:
- Welding spatter
- Flame-cut burrs
- Thick paint coatings
These materials compress after tightening, reducing bolt preload and increasing the risk of loosening.
Why Are These Mistakes Dangerous?
A bolted joint works because the bolt behaves like a spring.
When tightened, the bolt stretches elastically and produces clamping force.
Anything that compresses or settles after tightening—such as stacked washers, burrs, or soft coatings—reduces that preload.
The result may be:
- Bolt loosening
- Fatigue failure
- Bolt fracture
Good bolted-joint design is about maintaining preload throughout the service life of the assembly.
The Role of Washers in High-Strength Bolted Joints
In structural high-strength bolted connections, washers are not optional—they are essential.
Unlike ordinary bolted joints, high-strength bolts are designed to clamp structural members together so that loads are transferred through friction rather than bolt shear.
Because of this, achieving the specified preload is critical.
Washers help by:
- Providing a uniform bearing surface
- Preventing indentation under extremely high clamping forces
- Maintaining consistent friction during tightening
For these reasons, hardened washers are normally installed beneath both the bolt head and the nut.
The washers are considered part of the engineered fastening system, not merely accessories.
Key Takeaways
Flat washers are much more than simple spacers.
They are designed to:
- Distribute bearing pressure
- Stabilize friction conditions
- Protect contact surfaces
- Help maintain bolt preload
At the same time, washers should not be used as adjustment components.
Stacking washers, tightening over burrs, or installing flat washers on inclined surfaces can reduce joint reliability and eventually lead to loosening or failure.
A good engineer should always be able to answer one simple question:
Why is this washer installed?
If there is no clear engineering reason, it may not belong in the assembly at all.
Small as they are, washers play a major role in the safety and reliability of bolted joints.
If you're interested, feel free to check out my other articles on mechanical design.
