Washers Explained: Flat, Lock and Spring Types
Short answer
- A washer spreads a bolt's clamping load over a wider area, protects the surface and can help a joint stay tight, and flat, lock and spring types each do a different job.
- Flat washers spread load, while split lock, tooth and spring-type washers such as conical discs and wave washers try to resist loosening or maintain tension as a joint settles.
- Choose a washer by matching bolt size, strength, surface, material and vibration level, because a washer does not replace correct tightening of a good fastener.
A washer is a thin, usually round plate with a hole, placed under a bolt head or nut to spread the clamping load, protect the surface or help the joint stay tight. Flat washers spread load, lock washers and spring washers try to resist loosening, and each type does a different job.
What is a washer used for?
A bolt or nut bearing directly on a part concentrates all of its clamp force on a small ring of material. On soft metal, thin sheet, plastic or a rough casting, that ring can dent, crush or creep over time, and the joint loses tension. A washer puts a larger, flatter, harder surface between the fastener and the part.
- Spread the load over a wider area so the surface is not damaged.
- Bridge a hole that is larger than the bolt, or a slot.
- Protect the finish so a turning nut does not scar paint, plating or soft material.
- Provide a consistent bearing face so tightening torque produces a predictable clamp force.
- Take up or maintain tension in joints that settle or relax (spring types).
- Insulate or seal when made of fibre, nylon or a bonded rubber insert.
How do washers keep a joint tight?
A bolted joint holds because the bolt is stretched slightly and pulls the parts together. That stretch, called preload, is the real source of clamping force. Anything that lets the parts relax, such as surface embedding, gasket creep or vibration, reduces preload. A flat washer reduces embedding by spreading the load. Spring-type washers add a little elastic movement that can follow small amounts of settling. None of them replaces correct tightening of a good fastener.
What are the main types of washer?
Flat washers
The plain washer is the most common. Narrow, regular and wide (often called fender) patterns differ in outside diameter for the same hole. Inch hardware is generally split between two broad patterns: a narrower one often called SAE and a wider one often called USS, so do not assume two inch washers of the same bore are interchangeable. Metric flat washers are covered by standards such as ISO 7089 and the long-standing DIN 125, but always confirm the dimensions you need rather than relying on the name alone.
Split lock washers
A helical spring washer is a single coil with a cut in it. When flattened it stores a little spring force, and its sharp edges bite slightly into the nut and the surface. They are cheap and familiar, but on hard or hardened surfaces the bite is limited, and many engineers consider their ability to resist vibration loosening modest. Treat them as a light-duty aid rather than a guarantee.
Tooth (star) lock washers
These have serrated teeth, internal, external or both, that dig into the mating surfaces. They suit light electrical and sheet-metal work, and external teeth can also help make an electrical bond through paint. The teeth damage the surface, so they are a poor choice where finish or corrosion protection matters.
Conical (Belleville) and disc springs
A shallow cone-shaped disc that flexes under load. Used singly or stacked, in the same direction or in opposing pairs, discs provide a high spring force in a small space and can keep tension on a bolt that tends to relax. They are also used to apply preload to bearings. The load, deflection and stacking arrangement all matter, so they should be selected from the manufacturer's data rather than guessed.
Wave washers
A ring with a gentle wave in it. Wave washers give a lighter, longer spring travel than a disc and are often used to take up end play or apply light preload in assemblies such as bearings.
Wedge-locking washer pairs
Two washers with matching cam faces on the inside and serrations on the outside. If the bolt tries to turn loose, the cam faces ride up on each other and increase tension instead. These are used where vibration is a known problem. Follow the supplier's instructions on lubrication and tightening.
Specialty washers
- Bonded seal washers with a rubber ring that seals the bolt hole against leaks.
- Nylon or fibre washers for electrical insulation or light sealing.
- Shim and spacer washers for adjusting the height or clearance of an assembly.
- Hardened washers for high-strength bolts, where a soft washer would flatten.
How do you choose the right washer?
- Match the bolt size and the standard. The inside diameter should suit the bolt with a sensible clearance, and the outside diameter and thickness should suit the load.
- Match the strength. A high-strength bolt needs a washer hard enough not to dish or crush under that load.
- Consider the surface. Soft material, plastic or thin sheet generally benefits from a wide washer.
- Consider the material and finish. Plain steel, zinc plated, stainless and brass all behave differently in wet or chemical environments. Check for galvanic issues when mixing metals.
- Think about vibration. If the joint can shake loose, look at a properly designed locking method rather than relying on a basic washer.
- Check the stack. Where a washer is meant to be flat on both sides, make sure a spring or tooth washer sits in the right position relative to it.
What mistakes are common with washers?
- Using a split lock washer under a hardened washer or hard surface where it cannot bite or compress as intended.
- Stacking several washers to take up a gap. Each extra surface can embed and lose preload; use a proper spacer or shorter part instead.
- Reusing flattened spring washers. Once a spring washer has lost its shape it has lost its function.
- Using too narrow a washer on a slotted or oversize hole, where it can be pulled through or deformed.
- Using a soft washer with a high-strength bolt, so the washer yields and the joint loosens.
- Leaving out the washer on soft or painted surfaces, so the nut or bolt head digs in and the joint loses tension as the surface settles.
- Ignoring torque. The washer does not fix an under-tightened or over-tightened fastener.
Do I need a washer under every nut?
Not always. Flanged bolts and nuts have an integral bearing face. But on soft material, oversize holes or slotted holes, a washer is usually worth adding. Follow the joint design or machine manual where one exists.
Does a lock washer stop a bolt coming loose?
Often only partly. Loosening is usually caused by lost preload, so the best defence is a correctly sized, correctly tightened fastener. Where vibration is severe, consider a locking method designed for it, such as a wedge-locking pair, a locking nut or a thread-locking compound.
Can I mix stainless washers with steel bolts?
Often yes in dry indoor conditions, but wet or salty environments can cause galvanic corrosion between dissimilar metals. Check the pairing for your environment.
What is the difference between a flat washer and a fender washer?
A fender washer has a much larger outside diameter for the same hole size, which spreads the load across a wider area. It is a type of flat washer, often chosen for thin sheet or soft material.
Further reading on this site: fasteners and hardware; bolt torque and preload; bolt property classes and grades; thread locking and anti-seize; galvanic corrosion between dissimilar metals; bolt torque calculator.
If you need to match or replace washers or spring discs and have the dimensions, material or a sample description, send us the specification and we will see what we can source for a quote.
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Request a QuoteFrequently asked questions
What is a washer used for on a bolted joint?
A washer spreads a bolt's clamp force over a wider, flatter surface so soft metal, thin sheet, plastic or rough castings do not dent, crush or creep. It can also bridge an oversize hole or slot, protect paint or plating from a turning nut, and give a consistent bearing face so tightening torque produces predictable clamp force.
Does a lock washer stop a bolt from coming loose?
Often only partly. Loosening is usually caused by lost preload, so the main defence is a correctly sized, correctly tightened fastener. Split lock washers are a light-duty aid, and where vibration is severe a locking method designed for it, such as a wedge-locking pair, a locking nut or a thread-locking compound, is the better choice.
What is the difference between a flat washer and a fender washer?
A fender washer has a much larger outside diameter for the same hole size, which spreads the load across a wider area. It is a type of flat washer, often chosen for thin sheet or soft material. Narrow, regular and wide patterns of flat washer differ in outside diameter for the same hole.
Do I need a washer under every nut?
No, a washer is not always needed under every nut. Flanged bolts and nuts have an integral bearing face. But on soft material, oversize holes or slotted holes, a washer is usually worth adding. Follow the joint design or machine manual where one exists, and remember a washer does not fix an under-tightened or over-tightened fastener.