Lock Washer vs. Lock Nut: Which One Actually Stops Loosening?

If a bolted joint keeps working loose under vibration, a lock nut is usually the better choice. A split lock washer is a spring, and it does its springing while you tighten the nut. By the time the joint is at full tightness, the washer has been pressed flat and acts like an ordinary flat washer. A nylon insert lock nut keeps a grip on the bolt threads the whole time it is installed, so it resists turning even if the joint loses some clamp.

In practice: use a nylon insert lock nut when you need the nut to stay put and the joint doesn't run hot. Use a serrated hex flange nut when you want the nut to grip the clamped surface and don't mind it marking that surface. Keep split lock washers for light-duty assemblies, or for replacing like-for-like where a drawing calls for them. They aren't a fix for a joint that is already coming loose.

Do split lock washers actually work?

A split lock washer (also called a helical spring lock washer) is a single coil with a cut through it. As you tighten the nut, the coil compresses and pushes back. That spring force only exists while the washer is still springing. Once the nut is fully tightened the washer is pressed flat, and from then on it acts as a flat washer. A Machine Design article on joint loosening puts split-ring washers in the group of spring-based fasteners that are "inexpensive but ineffective against vibrations."

That doesn't make them useless. Our washer guide covers where a split washer still fits, alongside flat and fender washers. What it can't do is hold a tight joint that vibration is already working loose. If a nut keeps backing off, swapping in a new split washer usually won't fix it.

How does a nylon insert lock nut stop loosening?

A nylon insert lock nut (often called a "nyloc" nut) is a hex nut with a nylon ring set into the top. The ring's hole is smaller than the bolt's thread, so the bolt has to form its way through the nylon as you turn the nut. That interference creates prevailing torque: resistance to turning that is there all the time, not only when the joint is tight.

That is the difference that matters. A split washer's spring force disappears once it is flat. A nylon insert keeps gripping whether the nut is snug, fully tightened or has lost some preload. You'll feel it on install: the nut spins freely until the bolt reaches the nylon, then needs a wrench the rest of the way.

A few practical points:

• Thread length. The bolt needs to pass all the way through the nylon ring to lock. If the bolt end stops inside the insert, you're relying on partial engagement. Our guide on matching nuts and bolts covers picking a bolt length that clears the nut.
• Heat. Nylon softens at elevated temperature, and the same Machine Design article notes that nylon inserts degrade at high temperatures. Near exhausts, ovens, heaters or anything else hot, use a locking method that doesn't depend on a plastic insert. Check the manufacturer's temperature rating for the specific nut.
• Stainless on stainless. A lock nut makes you work against thread friction for most of the install, and 18-8 or 316 stainless threads can gall when they bind. Drive it steadily, don't force a nut that suddenly tightens, and see our post on preventing stainless steel galling.

If you need a bearing surface too, flanged nylon insert lock nuts build a washer face into the nut.

How do serrated flange nuts resist loosening?

A serrated hex flange nut has a wide flange at its base with serrations (ridges) on the face that touches the part. When the nut is tightened, those serrations grip the surface they clamp against, and that grip resists the nut turning back off. Fastenio stocks them in 18-8 and 316 stainless steel.

Two things follow from how they work:

• They mark the surface. The serrations grip by pressing into the clamped surface. That's fine on structural steel or a bracket nobody sees, but not on a painted, coated or finished face you need to keep clean.
• Don't add a washer under them. A washer puts the serrations against the washer instead of the part, which defeats the point. The flange already spreads the load.

Tooth lock washers work on a similar idea: twisted teeth that grip the nut and the surface. Fastenio carries external tooth lock washers and internal tooth versions, and K-lock (Kep) nuts come with an external tooth washer already attached, which is handy on assembly lines and panel work.

Lock washer vs. lock nut: how do they compare?

The jam nut row is there because it's the common no-special-parts option: a thin hex jam nut tightened against a full nut on the same bolt. The friction between the two nuts holds them in place.

Can you use a lock washer and a lock nut together?

You can, but on a joint with a nylon insert lock nut a split washer adds little. Once it is flat it behaves like a flat washer, and the nut is doing the locking. If the reason for a washer is load spreading or protecting a soft surface, use a flat washer, which gives a flat, consistent bearing face.

Don't stack a split washer under a serrated flange nut either. The serrations would bite into the washer instead of the part.

Can you reuse a nylon insert lock nut?

Each install forces the bolt threads through the nylon ring, so the ring is formed to the thread the first time it's used. Whether a given nut is still good for another install depends on the manufacturer's rating, and Fastenio's product pages don't publish a reuse rating. The safe rule for anything that matters, like a guard, a mount, or anything that moves or vibrates, is to fit a new nut when you take one off.

Split washers are a similar case. The Machine Design article lists split washers among the parts that aren't reusable.

Bottom line

Split lock washers don't keep a tight joint from loosening under vibration. They flatten as you tighten and then act like flat washers. For a joint that has to stay put, use a lock nut:

• Nylon insert lock nuts, or flanged nylon insert lock nuts, as the default, away from heat.
• Serrated hex flange nuts where heat rules out nylon and marking the surface is acceptable.
• Split lock washers for like-for-like replacement and light-duty assemblies.

All of them are available in 18-8 and 316 stainless steel. Size the nut to the bolt's thread (for example, a 1/4-20 bolt takes a 1/4-20 nut), and make sure the bolt runs fully through the nylon.