What is locknut and how it works?

Metal locking nut

So, what are lock nuts used for? Ordinary nuts provide clamping force to a bolt or screw to prevent axial movement. When the application involves vibration, the nuts risk loosening, risking damage to the application. This is where lock nuts prove their worth: they resist vibration and shock. There’s no one answer for how to use a lock nut, as they vary. Let’s take a closer look.

How does a lock nut work?

There are essentially two lock-nut types:

  • Uses friction between mating threads to prevent loosening

These lock nuts are designed with pitted or flanged grooves and need greater torque to tighten and loosen. They don’t spin freely along the fastener’s shaft, as they begin to clamp down as you rotate them. This means longer assembly time for your application. However, they cost a lot less than the other lock-nut style. These types of nuts are called prevailing-torque nuts due to the resistance to rotation when assembling and removing.

  • Designed with a positive locking device

These are essentially nuts with locks. They easily rotate during tightening and loosening for fast assembly. Locked into position by a positive action. Some lock nuts require inserting a pin while others present a small lip or crown that is crimped inward when installed, gripping the shaft. While assembly is fast and more secure, this type of lock nut costs more.

It’s for you to decide where the value is: save money on assembly or the hardware. Compare costs – sometimes nuts with locking devices can cost significantly more than their cousins, but the faster assembly time may balance this out.

Friction: lock nuts

Also known as self-locking nuts, common examples include:

Nyloc nuts

1. Nyloc nuts

Nylon lock nuts or stop nuts, as they’re also called, are economic and extremely effective. They have a nylon insert, or collar, which is smaller than the thread diameter. As the nylon insert lock nut is tightened, the collar deforms over the mating diameter, creating friction. The nylon nut is then locked into position. The collar of the nylon insert nut identifies it as either imperial threads (white) or metric threads (blue), as shown here.

Can nylon lock nuts be reused?

That’s a matter for debate. In some instances they can, but they weaken each time they’re put back to work. In applications where safety is paramount, do NOT reuse. On another note, if the application will be exposed to extremely high temperatures, choose another nut. Nyloc nuts can operate as intended in temperatures up to 120˚C (248˚F).

Applications: Elevators, streetlights, furniture, lifting equipment, pipelines

Self-locking nylon flange hex nut

2. Self-locking nylon flange hex nut

The nylon hex lock nut is designed for use with metal screws. The flange acts as an integrated, stable washer. This enables built-up forces under the nut head to spread over a larger area, which results in lower surface pressure. This minimises the chance for damage to the surface area while securing it in place to withstand loosening.

These are popular choices on assembly lines. They don’t require a separate washer, which speeds up operations.

Applications: Electronics, construction, metalworking

Jam nut

3. Jam nuts

Also called a half lock nut, a jam nut has a lower profile than standard nuts and designed to be tightened – or jammed – against another nut. The jam nut by itself is not a lock nut – it’s the function it performs that earns its place here.

The jam nut should be installed and tightened first and then the primary nut, which is thicker, is fitted over the top. As the two nuts are tightened against each other, the thread within the jam nut moves, providing clamping force. Using two nuts can be effective for a simple reason: the nuts are jammed against each other, bearing in opposite directions.

Jam nuts are not considered best practice in industrial applications, as predicting loads under stress is difficult. If using this method, a skilled installer is critical.

Jam nuts are also ideal as threaded rod lock nuts. Learn more about threaded rods. Another good use for jam nuts is in areas with restricted space, where a full-sized nut isn’t viable.

Jam nut vs. a hex nut

There isn’t a lot of difference between jam nuts and hex nuts. It comes down to size and purpose. Jam nuts are thinner and as noted, have a low profile. They work in concert with a heavy hex nut, which is the main nut.

Applications: Automotive, specialty vehicles, furniture, pipelines, air & rail

Stover nuts

4. Stover nuts

These are metal and have a conical top and a flat bottom bearing surface with chamfered corners. The locking action is created by controlled distortion of their top heads. Their cone shape enables orientation during automated assembly, which is why they’re a popular choice for high-volume applications. They’re available without flanges, but it’s the flange that provides the more secure hold. Even though these are considered lock nuts, they are not permanently locked, which means maintenance or making adjustments can be carried out. Prevailing torque nuts is also used specifically to refer to stover nuts.

Applications: Agricultural equipment, automotive, metalworking

Positive locking nuts

Your two choices are almost interchangeable, but not quite. Both are used as safety nuts and use bolts that come with pre-drilled holes, or you can drill holes in the bolts yourself.

Castle nut

1. Castle nuts

One end has notches, which gives the appearance of castle battlements. A castle nut is used with a bolt that has a pre-drill radial hole. A cotter pin is slotted through the notches and the hole in the bolt. The pin’s twin twines are then bent. This prevents the nut from turning or the pin from being removed. Once applied, it’s not possible to tweak the torque, so castle nuts are best used for low-torque applications, when no specific preload is required.

Safety or lock wire, or an R-Clip, can be used instead of a cotter pin.

Applications: Automotive (popular for use in wheel hubs), aircraft, and rail

Slotted nut

2. Slotted nuts

This is available as round, or as shown here, a hex lock nut that looks very similar to a castle nut. Castle and slotted nuts are similar. The difference can be seen in their style of slots. The slots on castle nuts are located on a round area at the top. Slotted nuts lack this geometry.

On a slotted nut, the diameter of the castellated area is the same as the wrenching area. On a castle nut, the castellated area’s diameter is somewhat smaller than the wrenching section. Like castle nuts, a cotter pin is pushed through the slot, through the fastener’s pre-drilled hole, and out of the other slot. With pliers, the pin’s twines are bent.

Slotted nuts often use safety wire instead of cotter pins, making them a sort of wire lock nut in those instances.

Applications: Trailer wheel axles, axle nuts on motorcycles

Castle nuts vs. slotted nuts

What exactly is the difference other than the slight variation in geometry? Slotted nuts are shorter than castle nuts with the same thread size.

Locking nut in application

At a glance: which lock nut?

Where used Nyloc nuts Self-locking flange hex nuts Jam nuts Stover nuts Castle nuts Slotted nuts


Specialty equipment and vehicles








Air and Rail    



Lock washer vs. lock nut

Should you use a lock nut? Or an ordinary nut with a lock washer? That really depends on the type of lock nut and lock washer and, of course, the application. The ability to handle pressure of torquing down a bolt or screw is best done with a lock washer. Yet for durability and tenacity in joint integrity, a Nyloc nut is preferred. If the space you’re working with is compact, then that Nyloc nut might prove too bulky, in which case you’ll want to go with a lock-washer.

There are so many variables here. Both are excellent choices.

Which way do self-locking nuts go on?

All lock nuts have a round, smooth end. The top might be chamfered or raised, but the bottom is smooth. It’s the smooth end that goes over the bolt first.

How to tighten a lock nut

It’s a straightforward process:

  1. Just as you’d do with a typical nut, place the lock nut against the threaded end of the bolt.
  2. Hand-tighten the lock nut. If the lock nut has a raised centre area, ensure that the nut’s flat surface sits flush against the surface once tightened.
  3. Use a torque wrench to tighten until the lock nut reaches the specified torque.


If your application is industrial or heavy duty, choose one of the types of metal nuts. For lock nuts, this will typically be:

  • Stainless steel
  • Zinc-plated mild steel
  • Brass

Metal lock nuts increase the resistance to rotation in an assembly. They’re also excellent at withstanding high temperatures – something plastic lock nuts can’t do. However, plastic is a great choice for applications where the torque forces aren’t too much. A nylon jam nut or hex nylon lock nut can handle some of the same applications that metal can, and for less money.

How do you measure a nut size?

There are several different ways to do this. But first, know there’s more to matching a nut to a bolt than just sizing. Each are graded, and that plays a part in your choice. We urge you to read What are the different types of nuts and bolts to better understand how to match these fasteners.

Measuring for diameter

Step 1: Lay the nut on a flat surface so that the hole faces up.

Step 2: Stretch the measuring tape across the widest opening from one side of the inner threads to the other side’s inner threads.

Step 3: If the nut is U.S. standard, use the inch side of the tape and count the lines in sixteenths. If the nut is metric, measure in millimetres.

Measuring for coarse or fine thread

Step 1: Count the number of threads along the inside of the nut. If it helps, use need a magnifying glass and pin or pencil with a sharp point to keep count.

Step 2: Write down the number of threads and determine the threads per inch.

  • Coarse: 16 threads per inch
  • Fine: 24 threads per inch

If your nut is 1/4" and you counted 4 threads, then the nut is coarse thread. If that same nut has five threads, then it’s fine thread.

Download free CADs and try before you buy

Free CADs are available for most solutions, which you can download. You can also request free samples to ensure the lock nut you’ve chosen is exactly what you need. You might find The ultimate guide to fasteners helpful. If you’re not quite sure which product will work best, our experts are always happy to advise you.

Whatever your requirements, you can depend on fast despatch. Request your free samples or download free CADs now.


Email us at sales@essentracomponents.co.uk or speak to one of our experts for further information on the ideal solution for your application 0345 528 0474.