Knowing how to use a lock washer correctly can be the difference between a joint that holds and one that rattles loose. In short: a lock washer sits directly under the nut or bolt head, its inner diameter must match the bolt shank, and it should be tightened to the correct torque so it can grip without going flat.

This guide walks through every part of the process — from picking the right type to avoiding the mistakes that make lock washers fail. It also covers something most DIY articles skip: real engineering research showing that the common split lock washer isn’t always as effective as people think.

What Is a Lock Washer?

A lock washer is a small metal ring placed between a nut or bolt head and the surface it’s fastening. Its job is to keep the joint from loosening on its own when it’s exposed to vibration, movement, or repeated stress.

You’ll find lock washers in cars, appliances, lawn mowers, plumbing fixtures, and machinery — basically anywhere a bolt might shake loose over time. Instead of relying only on friction between threads, a lock washer adds spring tension or physical bite to help the fastener stay put.

There are several common types: split (helical spring) washers, internal-tooth washers, external-tooth washers, high-collar washers, and wedge-lock washers. Each one uses a different method to resist loosening, and picking the right one matters more than most people realize.

Types of Lock Washers and Their Uses

The right lock washer depends on your material, load, and how much vibration the joint will face. Here’s a quick breakdown of the main types and where they work best.

Type	How It Works	Best Used For
Split (helical spring)	Spring tension pushes back against loosening	Light-duty, general household use
Internal-tooth	Teeth on the inside edge bite into the fastener	Smaller bolts, tight spaces
External-tooth	Teeth on the outside edge bite into the surface	Softer materials, electrical grounding
High-collar	Thicker, stiffer spring for higher clamp load	Heavier-duty joints
Wedge-lock (e.g., Nord-Lock)	Cam angles greater than thread pitch lock the joint by geometry, not friction	High-vibration, safety-critical joints

Split washers are the most common and cheapest option, which is why they show up in so many hardware kits. But for anything vibration-heavy — like automotive or machinery work — wedge-lock washers are increasingly recommended as the stronger choice.

Lock Washer vs. Flat Washer vs. Spring Washer

A lock washer is not the same thing as a flat washer, even though people often use the terms loosely. A flat washer spreads out the load and protects the surface material. A lock washer (also called a spring washer in some contexts) resists loosening.

Here’s how they compare:

Washer Type	Main Purpose	Placement
Flat washer	Distributes load, protects surface	Against the material, under the lock washer
Lock washer	Resists loosening from vibration	Directly under the nut or bolt head
Spring/wave washer	Adds slight spring tension and takes up slack	Between flat washer and fastener

If your project needs both load distribution and loosening resistance, you can use them together: flat washer against the surface, lock washer directly under the fastener.

Step-by-Step: How to Install a Lock Washer

Installing a lock washer correctly only takes a few extra seconds, but it’s the step most people rush. Follow this order every time.

1. Check the bolt size. Match the washer’s inner diameter to the bolt or screw’s shank diameter.
2. Clean the surfaces. Remove paint, grease, or rust from the bearing surface so the washer can seat and grip properly.
3. Place the flat washer first (if using one). It goes against the material.
4. Add the lock washer. Place it directly under the nut or bolt head, right against the metal it’s tightening onto.
5. Check the orientation. For toothed washers, internal teeth face the bolt shank and external teeth face the workpiece.
6. Tighten to the correct torque. Use a torque wrench when possible, and stop once you reach the manufacturer’s spec — don’t just tighten until it “feels” right.
7. Inspect the joint. Make sure the washer isn’t fully flattened and that teeth (if present) are actually engaged.

For multi-bolt jobs like flanges, tighten the bolts in stages using a star pattern instead of fully tightening one bolt at a time. This spreads the clamping force evenly and avoids warping the joint.

Which Way Does a Lock Washer Go?

A lock washer always goes directly under the nut or bolt head, never against the surface material on its own. If you’re also using a flat washer, the flat washer sits closest to the material, and the lock washer sits between the flat washer and the fastener.

For toothed washers, orientation matters even more. Internal-tooth washers should have their teeth facing toward the bolt shank. External-tooth washers should have their teeth facing outward, so they bite into the surface being fastened. Getting this backward means the teeth never actually engage, and the washer does almost nothing.

Do You Need a Flat Washer Under a Lock Washer?

Not always, but it’s often a good idea. A flat washer protects softer materials from getting dented or scratched by the lock washer, and it also gives the lock washer a smoother, more even surface to press against.

If you’re working with hard metal directly, a flat washer isn’t strictly necessary. But for wood, thin metal, plastic, or any surface that could get damaged, adding a flat washer underneath is the safer choice.

Does a Split Lock Washer Actually Work?

This is where a lot of common advice falls short. Once a split lock washer is fully tightened, engineering research shows it behaves more like a flat, solid washer than a spring — meaning its ability to resist loosening drops sharply.

This isn’t a new or fringe claim. Engineer Gerhard Junker’s research, dating back to 1969, found that split-ring lock washers can actually loosen faster than joints with no washer at all under certain vibration tests. NASA’s own fastener guidance (RP-1228) states plainly that helical spring lock washers become a solid flat washer once torqued down, calling them “useless for locking” in that state.

That doesn’t mean split washers are pointless — they’re fine for light-duty, low-vibration jobs like household appliances or furniture. But for anything exposed to real vibration, such as engines, machinery, or safety-critical joints, wedge-lock washers, thread-locking compounds, or nylon-insert lock nuts are generally more reliable.

How Do You Choose the Right Lock Washer Size?

Choosing the right size comes down to matching the washer’s inner diameter to your bolt or screw. A washer that’s too loose won’t grip properly, and one that’s too tight won’t fit over the fastener at all.

Standard sizing follows established references like ASME B18.21.1 for inch-series washers and DIN 127 or DIN 7980 for metric ones. If you’re unsure, bring your bolt to the hardware store or check a size chart before buying — inner diameter, outer diameter, and thickness all need to line up with your fastener grade and application.

Common Lock Washer Mistakes (and How to Avoid Them)

Most lock washer failures come down to a handful of repeatable mistakes. Watch out for these:

• Wrong orientation — teeth facing the wrong direction so they never bite.
• Wrong size — a washer that doesn’t match the bolt’s shank diameter.
• Overtightening — crushing the spring washer flat, which kills its function.
• Reusing old washers — once a lock washer is compressed, it usually loses its spring tension for good.
• Dirty surfaces — paint, grease, or rust stops teeth from engaging properly.
• Wrong washer for the job — using a light-duty split washer on a high-vibration joint that needs something stronger.

Avoiding these mistakes is usually more important than which brand or type of washer you buy.

When to Look Beyond a Lock Washer

If your joint keeps loosening even with a lock washer installed, it’s worth considering other options. Thread-locking compound (like Loctite) chemically bonds the threads together. Nylon-insert lock nuts use friction from a nylon ring instead of a spring washer. Wedge-lock washer pairs, such as Nord-Lock, use matched cam angles to resist loosening through geometry rather than friction — and they’ve held up far better in vibration testing.

For safety-critical or high-vibration applications — engines, machinery, structural connections — these alternatives are often a better long-term solution than relying on a split lock washer alone.

Expert Tips for a Secure Fastened Joint

• Clean bearing surfaces before every installation so washers can seat and grip fully.
• Use a calibrated torque wrench rather than tightening by feel.
• For multi-bolt flanges, tighten in stages using a star pattern.
• Aim for at least 65% of the fastener’s proof load as preload — this is often enough to resist vibration loosening on its own.
• Replace any lock washer that looks flattened or worn before reusing a joint.
• For wedge-lock pairs, double-check the cams face each other correctly before tightening.

Conclusion

Using a lock washer correctly isn’t complicated, but it does require attention to a few key details: the right type, the right size, correct orientation, clean surfaces, and proper torque. Split washers work fine for light, low-vibration tasks, but real engineering data shows they lose most of their effectiveness once fully tightened — especially in high-vibration settings. For anything more demanding, wedge-lock washers, thread-locking compound, or nylon lock nuts are worth considering. Get these basics right, and your fastened joints will stay tight far longer.

FAQs

What is a lock washer used for? A lock washer helps stop a nut or bolt from loosening under vibration by adding spring tension or physical bite against the surface it’s tightened onto.

Which way does a lock washer go? It goes directly under the nut or bolt head. For toothed washers, internal teeth face the bolt shank, and external teeth face the workpiece.

Does a split lock washer actually prevent loosening? Once fully tightened, a split lock washer flattens and loses most of its spring tension, making it far less effective at preventing loosening — research from Gerhard Junker and NASA both confirm this.

Should I use a flat washer with a lock washer? It’s often a good idea, especially on softer materials. The flat washer sits against the surface, and the lock washer sits between the flat washer and the fastener.

How tight should I make it? Tighten to the bolt manufacturer’s torque specification using a torque wrench. Avoid overtightening to the point where the spring washer flattens completely.

How do I choose the right size? Match the washer’s inner diameter to your bolt or screw’s shank diameter, referencing a size chart or standard like ASME B18.21.1 if you’re unsure.

Can a lock washer be reused? Generally, no. Once a lock washer is compressed and flattened, it loses its spring action and should be replaced with a new one.