What Is Locknut?- Definition, Types & How it works

What is Locknut?

When discussing fasteners that can withstand vibration and repeated impacts, the term “lock nut” often comes up. Unlike ordinary hex nuts, lock nuts are specifically designed to resist loosening, even in conditions where movement or shaking is common. That’s why they’re a staple in assemblies exposed to persistent vibration or shock—they simply hold things together more reliably over time.

Structurally, a lock nut is a special kind of internally threaded fastener that features built-in locking capabilities. Sometimes, this function is achieved by the nut alone; other times, it works together with an additional part. The key point is that, by their very design, lock nuts remain firmly in place, even when subjected to forces that might otherwise twist or rattle them loose.

You might hear lock nuts referred to by a handful of names: self-locking nuts, stiffnuts, prevailing torque nuts, elastic stop nuts, or even just locking nuts. Regardless of the label, the idea is the same—these nuts grip onto a bolt or threaded rod just like standard nuts, but they go a step further by resisting loosening when torque or vibration is in play.

Certain lock nuts, such as elastic stop nuts and prevailing torque nuts, get their holding power from a clever design trick: a part of the nut itself deforms slightly (but elastically) as it’s tightened, creating extra friction and locking action.

A bit of historical context helps here. The development of lock nuts dates back to the 1930s, a time when manufacturers were searching for ways to cut costs in machine assembly.

Prior to their invention, the go-to method for keeping nuts tight under vibration was to use two standard nuts on a single bolt—an effective fix, but one that quickly doubled the number of nuts and washers required. For small-scale projects, this wasn’t such a big deal, but as production ramped up, so did the expense.

Lock nuts offered a smart solution. Although each lock nut is a bit pricier than a regular nut, it’s still far more economical than using two nuts every time. This cost-effectiveness, combined with their reliability, made lock nuts an industry standard.

Nowadays, you’ll find lock nuts practically everywhere on threaded rods and studs in all sorts of equipment and across a broad range of industries.

They’re just one of several tools used to combat the problem of vibration loosening. Other common solutions include jam nuts, lock washers, and thread-locking fluids, all of which aim to keep bolted joints secure when the going gets rough.

Lock Nuts with Nylon Inserts

Locknuts featuring nylon inserts often referred to as self-locking or locking nuts are designed to maintain a secure fit even under conditions of vibration and routine use. Unlike standard nuts that spin freely, these locknuts employ an elastic or metallic element that deforms slightly, helping them stay firmly in place despite torque and impact.

While nylon-insert locknuts are widely used across various applications due to their reliability, it is important to note their limitations regarding temperature. Specifically, they are best suited for environments where temperatures do not exceed 250°F, as exposure beyond this threshold can compromise the integrity of the nylon insert.

Related Posts: What are Nuts (Hardware)?- Definition, and Types

How Do Lock Nuts with Nylon Inserts Work?

Hex lock nuts are designed with a nylon insert at the top, which adds a bit of tension between the nut itself and the threads of whatever part it’s being fastened to. When you start threading the nut onto a bolt or rod, it will turn easily at first. But once those threads reach the nylon ring, that’s when things change the nylon pushes back, creating noticeable resistance.

This little bit of friction is exactly what keeps the lock nut from coming loose on its own, even if there’s vibration or movement in the assembly. However, it’s worth noting that the nylon insert isn’t invincible. With repeated tightening and loosening, it can start to lose its grip, becoming less effective over time.

If you ever need to take apart something that uses these nuts, it’s best practice to replace them rather than reusing the old ones. Swapping in a fresh lock nut ensures you get the most secure hold and don’t run into problems down the line.

Types of Lock nut

There are various kinds of specialized lock nuts, including: 

1. Jam nut.
2. Castellated nut or Castle nut.
3. Sawn nut.
4. Penn, ring, or grooved nut.
5. Locking with pin.
6. Locking with plate.
7. Spring lock washer.

#1. Jam Nut.

A jam nut is a low-profile fastener, typically about half the thickness of a standard nut. Its main role is to serve as a type of lock nut essentially, it’s tightened against another nut to keep both securely in place. This makes jam nuts particularly useful in situations where a full-sized nut would be too bulky or simply wouldn’t fit due to space constraints.

Interestingly, the term “jam nut” isn’t limited strictly to these thin nuts. In practice, any nut used to “jam” another meaning, to lock it in place can be called a jam nut, even if it’s just a regular nut doing double duty. Alongside jam nuts, people often rely on other locking methods like lock washers and thread-locking fluids to combat vibration and keep bolted joints from working loose over time.

Jam nuts are especially handy when you need to secure a nut in place but can’t clamp it directly against another component. By tightening two nuts against each other, the jam nut effectively stands in for a separate object, locking everything up neatly.

This technique also comes in handy when you need to attach something to a fastener without actually putting any pressure on the object itself a delicate task in some assemblies.

Here’s how it typically works: you start by tightening the first nut onto the threaded item, then add a second nut on top. Next, you back off the inner nut so it presses firmly against the outer nut, locking both in place.

There’s another clever use for jam nuts when dealing with threaded rods. Since these rods lack a traditional screw head, gripping and turning them can be a challenge. Using a pair of jam nuts, you can create a secure spot for a wrench, making it much easier to apply torque and rotate the rod as needed.

However, it’s worth noting that jam nuts do have their limitations particularly under heavy loads. If you overtighten the inner nut relative to the outer, there’s a risk that the outer nut will yield. On the flip side, if the outer nut is tightened too much, the inner nut might eventually work itself loose.

So, while jam nuts are handy for many applications, they’re best used with an awareness of these quirks.

#2. Castellated nut or Castle Nut.

A castellated nut, often referred to as a castle nut, is distinguished by a series of slots or notches cut into one end, resembling the battlements you’d see on a medieval castle hence the name. Sometimes people mistakenly call them “castigated nuts,” but that’s actually a mix-up.

Although castellated nuts are commonly grouped together with slotted nuts, there are some subtle differences worth noting. The main one is that castellated nuts have a rounded section at the top where the slots are cut, while slotted nuts have straight sides running all the way from top to bottom, without that curved top edge.

Both types are designed to work with a pin usually a cotter pin that’s inserted through the slots and a hole in the bolt or screw. This setup keeps the nut securely in place and prevents it from coming loose, which is pretty important in any mechanical assembly.

An interesting point is that castellated nuts let you position the cotter pin a little closer to the edge of the nut, which can make the whole arrangement even more secure compared to standard slotted nuts.

When installing, the bolt typically has one or two holes drilled through the threaded end. If the slots in the nut don’t quite line up with the hole in the bolt, you just turn the nut forward to the next slot never backward to get the right alignment.

Then, the pin (or sometimes an R-clip or safety wire) is pushed through to lock everything in place. This method is considered a “positive lock” because it physically prevents the nut from rotating and backing off.

In practice, castellated nuts show up most often in applications where you don’t need a ton of torque, like holding a wheel bearing in position. They’re a simple solution for making sure critical fasteners stay put, especially when safety is on the line.

#3. Sawn locknut.

A slotted nut is designed with a slot cut halfway through its body. Once the nut is securely fastened onto the bolt, a small screw is then tightened into place. This added step increases the friction between the nut and the bolt threads, effectively reducing the likelihood of the nut coming loose during operation.

#4. Ring Nut, Penn Nut, or Grooved Nut.

This component features a hexagonal shape at the top and a cylindrical form at the bottom. It’s commonly found in situations where bolts need to pass through parts that are situated fairly close to their edges—marine-type connecting rod ends are a typical example.

The cylindrical lower section is specially recessed to accommodate the tip of a locking set screw. For proper fitting, the bolt hole itself must be counter-bored so it can accept the cylindrical part of the nut. Additionally, to avoid any potential damage caused by the hardened tip of the set screw pressing against the nut, a recessed area is provided for extra protection.

#5. Locking with Pin.

Nuts can be secured by inserting a taper pin or a cotter pin through the center of the nut itself. However, it is more common to use a split pin, which is typically inserted through a hole in the bolt, just above the nut, to prevent it from loosening.

#6. Locking with plate.

These devices are essentially thin plates made of metal. After tightening the nut, you can hold its position by using a set screw, which keeps everything from shifting. Typically, the plate itself has a hole—often a twelve-sided one which makes it a lot easier to adjust things as needed.

To make sure the locking plate stays secure, a spring washer is paired with the set screw. There are also double-sided versions of these locking plates available; these are designed to lock two nuts at the same time, which can be handy in certain setups.

#7. Spring Locknut Washer.

When the nut is tightened, it presses the washer firmly against the surface beneath it. This action causes one edge of the washer to bite into the underlying material, which creates extra resistance and makes it much less likely for the nut to come loose unintentionally.

There’s actually a wide variety of spring lock washers out there, and while their effectiveness can vary, some types do a pretty solid job at keeping things secure.

Interestingly, you don’t always need a special locknut to achieve this effect. A standard nut can serve the same purpose if you use a thread locker compound or even take the time to drill and pin it in place.

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