A lock washer works by using spring tension, friction, surface bite, or a wedge action to resist the forces that make bolts and nuts spin loose over time. It sits between a fastener and the surface it’s tightened against, and it pushes back against rotation instead of letting the joint relax.

That sounds simple, but the details matter a lot. Not every lock washer works the same way, and some popular types work far less well than people assume. Let’s break down exactly how these small parts do their job.

What Is a Lock Washer?

A lock washer is a fastener accessory built to stop bolts, nuts, and screws from loosening on their own.

Unlike a plain flat washer, which only spreads out load and protects a surface, a lock washer is engineered to actively fight rotation. It’s usually made from spring steel or stainless steel, and it’s shaped to either flex, grip, or wedge against the parts around it.

You’ll find lock washers in cars, machinery, construction, plumbing, and electrical work. They’re cheap, small, and easy to overlook — but choosing the wrong type can leave a joint vulnerable to vibration or thermal cycling.

Why Fasteners Loosen in the First Place

Bolts loosen mainly because of vibration, especially transverse vibration, which is side-to-side movement rather than up-and-down movement.

Other causes include thermal expansion and contraction, settling or creep in gaskets and soft materials, repeated shock loading, and tiny amounts of thread slip that build up over time. A temperature swing of around 200°C between a fastener and the part it’s holding can create thermal stress equal to roughly 15% of the original preload.

Once a bolt starts to lose clamping force, the problem tends to snowball. As the joint loosens slightly, vibration has even more room to work, and the fastener backs out faster. This is exactly the failure mode lock washers are meant to interrupt.

Four Locking Mechanisms: Tension, Friction, Bite, Wedge

Lock washers use one or more of four basic mechanisms to resist loosening.

• Spring tension – The washer compresses when tightened and pushes back continuously against the nut or bolt head.
• Friction – The washer increases resistance between the fastener and the mating surface.
• Surface bite – Teeth or sharp edges dig into the metal surfaces on either side, gripping instead of sliding.
• Wedge action – Interlocking cam faces are angled so that any rotation trying to loosen the joint actually increases clamping force instead of reducing it.

Split and helical spring washers rely mostly on spring tension and friction. Toothed washers add mechanical bite. Wedge-lock washers use geometry rather than pure spring force, which is why engineers now consider them the strongest option for tough vibration environments.

Types of Lock Washers

There isn’t just one kind of lock washer — different shapes are built for different jobs.

Split (helical spring) lock washers are the classic coiled, cut-open ring shape. They compress slightly and are meant to keep pushing against the nut. Internal tooth washers have teeth pointing inward toward the bolt, hidden once installed, which keeps the outer edge smooth. External tooth (star) washers have teeth around the outer rim, biting a wider surface area for stronger grip.

Combined tooth and countersunk washers pair a tapered seat with tooth locking for use with countersunk screws. Belleville (disc spring) washers are cone-shaped and store energy like a spring, often used where consistent preload matters more than anti-rotation. Wedge-lock washers, sold under brands like Nord-Lock and Heico-Lock, use two washers with matching cam faces instead of a single flexible ring.

Type	Mechanism	Best For	Reusable?
Split/helical spring	Spring tension, friction	Light-duty, low-vibration joints	Limited
Internal tooth	Surface bite	Smooth-finish applications	Limited
External tooth (star)	Surface bite (wider grip)	Higher vibration than split type	Limited
Belleville/disc spring	Spring tension	Maintaining consistent preload	Depends on load history
Wedge-lock (Nord-Lock/Heico)	Cam/wedge geometry	High-vibration, safety-critical joints	Yes, if inspected

How Does a Split Lock Washer Really Work?

A split lock washer is a coiled ring of spring steel with the ends cut apart, shaped like a partial helix.

When you tighten the nut down onto it, the coil flattens slightly and stores spring energy. In theory, that stored energy keeps pushing against the nut face, maintaining friction even if the joint starts to relax. The sharp edges at the cut ends are also meant to dig in slightly for extra bite.

In practice, this design has real limits. Once the nut is torqued to a normal working preload, the washer often flattens out almost completely — at which point it behaves much like a flat washer instead of a spring. That’s the core reason engineers have started questioning how much protection it actually offers.

Do Split Lock Washers Actually Work Under Vibration?

Not reliably. Testing shows split lock washers perform only marginally better than having no washer at all under strong transverse vibration.

This conclusion comes from Junker vibration testing, a method developed to measure how bolted joints hold up under side-to-side shaking. In these tests, a fastener with no locking device typically loses nearly all of its clamp force within 300 ± 100 cycles. A properly performing anti-loosening solution is expected to retain at least 80% of clamp force after 2,000 cycles — a standard now formalized in test frameworks like DIN 25201-4 and ISO 16130.

Split washers frequently fail to meet that bar because they flatten under normal tightening torque, losing their spring effect right when it’s needed most. This is why many current engineering guides now steer people toward toothed washers, wedge-lock washers, threadlockers, or nylon-insert (nylock) nuts for anything exposed to real vibration.

How Does a Wedge Lock Washer (Nord-Lock) Work?

A wedge lock washer uses two matching washers with angled cam faces on the inside and radial teeth on the outside.

The teeth grip both the fastener and the surface so the washer pair can’t spin independently. The cam faces between the two washers are cut at an angle greater than the thread pitch of the bolt. That angle difference is the key: if the bolt tries to rotate loose, the cam faces ride up on each other and actually increase clamping tension instead of releasing it.

This is a fundamentally different approach from spring washers — it relies on geometry and tension, not on flexing metal. That’s why wedge-lock systems are commonly recommended for safety-critical assemblies, heavy machinery, and anywhere vibration is a serious concern.

Lock Washer vs. Flat Washer: What’s the Difference?

A flat washer spreads out load and protects the surface; a lock washer’s job is to resist loosening.

Feature	Flat Washer	Lock Washer
Main purpose	Distribute load, protect surface	Resist rotational loosening
Shape	Flat, smooth ring	Split, toothed, or wedge design
Vibration resistance	None on its own	Varies by type
Common pairing	Used under nut/bolt head	Often paired with a flat washer

They’re not competitors — many assemblies use both together, with the flat washer protecting a soft or oversized-hole surface and the lock washer handling anti-rotation.

Does a Lock Washer Go on the Nut or the Bolt Side?

The lock washer should go on the side that actually turns during tightening — almost always the nut side.

If a flat washer is also being used, the usual order is: bolt head, then the part being joined, then a flat washer (if needed for surface protection), then the lock washer, then the nut. Putting the lock washer against the rotating fastener lets it do its job of resisting that rotation directly.

Which Way Should the Split Face When Installing?

Orientation matters more than most people realize — installing a split washer backward can cut its effectiveness by more than half.

For a split lock washer, the cut ends should be positioned so they can engage and bite against the clamping surface as tension is applied. For toothed washers, the sharp tooth tips should face toward the nut or bolt head, not toward the base material, so they don’t gouge soft or painted surfaces unnecessarily.

Locking Methods Compared: Nuts, Threadlockers, Safety Wire

Lock washers aren’t the only anti-loosening option, and sometimes they’re not even the best one.

• Nylon-insert (nylock) nuts use a plastic ring that grips the bolt threads with prevailing torque.
• Threadlockers like Loctite chemically bond the threads together.
• Safety wire and castle nuts with cotter pins provide a mechanical, physical stop — common in aircraft.
• Wedge-lock washers offer geometry-based resistance for high-vibration industrial use.

Aircraft guidance specifically recommends lock washers be used only alongside self-locking nuts, not as a standalone solution in primary or secondary structural joints.

Standards and Sizes (DIN 127, ASME B18.21.1)

Lock washer dimensions and specs are governed by recognized standards, which helps ensure consistent fit and performance.

Common standards include DIN 127 (the long-standing European spec for spring/split lock washers, with square- and tang-end variations), ASME B18.21.1 (the U.S. standard), ISO 7089 (related plain washer sizing), and JIS B 1251 (the Japanese standard). Checking the correct standard matters when ordering replacement parts, since sizes and tolerances can differ slightly between systems.

Can Lock Washers Be Reused?

Spring and toothed lock washers generally shouldn’t be reused once they’ve flattened, cracked, or lost their spring shape.

Before reusing any lock washer, inspect it closely: check for flattening, visible cracks, deformed teeth, or corrosion. Wedge-lock washers, like Nord-Lock and Heico-Lock designs, are built to be reusable as long as they pass inspection, since their locking action comes from geometry rather than a one-time spring flex.

Applications and Real-World Uses

Lock washers show up almost anywhere fasteners are exposed to movement or stress.

Common applications include automotive assembly (wheel hubs, engine components), industrial machinery, construction framing, electrical conduit and grounding hardware, and plumbing fixtures. In aviation, their use is more restricted and typically paired with other locking methods rather than used alone.

Common Mistakes to Avoid

A few installation errors show up again and again in real-world use.

• Installing a split washer backward, which can cut effectiveness by more than half.
• Pointing tooth tips at the workpiece instead of the fastener, causing surface damage.
• Using a lock washer alone on soft, painted, or oversized-hole surfaces instead of pairing it with a flat washer.
• Assuming a split washer will stop vibration loosening when testing shows it often won’t.
• Reusing a flattened or cracked spring washer instead of replacing it.

Conclusion

A lock washer works by fighting rotation through spring tension, friction, surface bite, or wedge geometry — but not every type does this equally well. Split lock washers, despite being the most common, tend to flatten under normal preload and offer limited real protection against vibration, according to Junker-style testing. For light-duty or low-vibration jobs, a properly oriented split or toothed washer may be fine. For anything safety-critical or exposed to heavy vibration, wedge-lock washers, prevailing-torque nuts, or threadlockers are the more reliable choice. Understanding the mechanism behind each type — not just its shape — is what actually helps you pick the right one.

FAQs

How does a lock washer work? It resists loosening through spring tension, friction, surface bite, or wedge action, depending on the type. Spring types push back against the nut, toothed types dig into surfaces, and wedge types increase clamp force if the bolt tries to rotate loose.

Does a lock washer go on the nut or bolt side? It goes on the side that turns during tightening, which is usually the nut side. A flat washer can be added underneath it if the surface needs protection.

Which way does a split lock washer face? The cut ends should be positioned to engage and bite as tension is applied. Installing it backward can reduce effectiveness by more than 50%.

Do split lock washers actually work under vibration? Not reliably. Junker vibration testing shows split lock washers often perform only slightly better than no washer at all, since they tend to flatten under normal preload.

What’s the difference between a lock washer and a flat washer? A flat washer spreads load and protects the surface; a lock washer is designed specifically to resist rotational loosening. They’re often used together.

What is a wedge lock washer? It’s a two-piece washer set with angled cam faces and outer teeth. Because the cam angle is steeper than the bolt’s thread pitch, any loosening rotation increases clamp force instead of reducing it.

Can lock washers be reused? Spring and toothed washers should be inspected for flattening, cracks, or wear before reuse. Wedge-lock washers are generally reusable if they pass inspection.

When should you not use a lock washer? Avoid relying on a lock washer alone in safety-critical or high-vibration joints, on soft or delicate surfaces, or where a dedicated locking mechanism like a self-locking nut is specified.