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

What is Locknut?

Locknuts are internally threaded fastener that possesses locking capabilities either by themselves or in concert with another component. By design, lock nuts are engineered to resist loosening when subjected to vibration or torque.

A locknut, also known as a lock nut, locking nut, self-locking nut, prevailing torque nut, stiff nut, or elastic stop nut, is a nut that resists loosening under vibrations and torque.

Elastic stop nuts and prevailing torque nuts are of the particular type where some portion of the nut deforms elastically to provide a locking action. The first type used fiber instead of nylon and was invented in 1931.

Locknuts, jam nuts, lock washers, and thread-locking fluid are ways to prevent vibration from loosening a bolted joint.

Lock Nuts with Nylon Inserts

Locknuts with nylon inserts (self‑locking nut, locking nut) are a prevailing torque, locking nut that resists loosening caused by vibration and normal use. Unlike free-spinning nuts, lock nuts make use of a deforming elastic or metal material to stay in position against torque and shock.

Locknuts with nylon inserts are a popular solution for many applications, however, they are limited to applications exposed to temperatures under 250°.

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

How Do Lock Nuts with Nylon Inserts Work?

Hex lock nuts use a nylon insert at the top of the nut creating tension between the nut and the threads of the mating component. As the nut is threaded onto a shaft or rod, it will spin freely until the threads from the host part engage the nylon insert.

The threads of the mating shaft will displace the nylon and create resistance. This resistance prevents unintentional moving of the lock nut. The nylon in traditional hex lock nuts can wear and become less effective after repeated removal and installation.

For optimum holding power, replace the nut if it needs to be removed during the disassembly of a component.

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 nut that is usually half the size of a standard nut. It is commonly used as a type of lock nut where it is “jammed” up against a standard nut to secure the two in place. It is also used in situations where a standard nut would not fit.

The term “jam nut” can also refer to any nut that is used for the same function (even a standard nut that is used for jamming purposes). Jam nuts, other types of lock nuts, lock washers, and thread-locking fluid prevent vibrations from loosening a bolted joint.

A jam nut is often used when a nut needs to be locked into place without clamping against another object. The jam nut essentially acts as “another object” as the two nuts are tightened against each other. They can also be used to attach an object to a fastener without applying force to that object.

This is accomplished by first tightening one of the nuts on the item. Then the other nut is screwed on top of the first nut. The inner nut is then loosened and tightened against the outer nut.

Jam nuts can also be used in situations where a threaded rod needs to be turned. Since threaded rods do not have screw heads, it is difficult or impossible to apply torque to a threaded rod. A pair of jam nuts are used to create a point where a wrench can be used.

Jam nuts can be unreliable under significant loads. When the inner nut is tightened more than the outer nut, the outer nut can give way. When the outer nut is tightened more than the inner nut, the inner nut can loosen.


2. Castellated nut or Castle Nut

A castellated nut, sometimes called a castle nut, is a nut with slots (notches) cut into one end. The name comes from the similarity of the nut the crenelated parapet of a medieval castle. Castellated nuts are sometimes mistakenly called castigated nuts.

While castellated nuts are often referred to as slotted nuts, slotted nuts differ slightly from castellated nuts from a technical point of view. Castellated nuts are formed with a round section at the top where the slots are located. Slotted nuts do not have this rounded modification. The flat sides of the slotted nuts extend completely from the top to the bottom of the nut.

Both castellated and slotted nuts are designed using a pin (usually a cotter pin) that fits through the slots and through a hole in the screw that the nut attaches to. This pin prevents the nut from turning and loosening. Compared to slotted nuts, castellated nuts allow the cotter pin to be attached closer to the edges of the nut itself, which provides additional security.

The bolt has one or two holes drilled through its threaded end. The mother is properly dressed. If the slot is not aligned with the hole in the fastener, the nut is rotated forward to the next slot. The nut is then secured with a cotter pin, an R-clip, or a safety wire. It’s a positive lock.

Castellated nuts are used in low-torque applications, such as holding a wheel bearing in place.

3. Sawn locknut

The sawn nut has a slot sawn halfway through. After the nut is screwed down, the small screw is tightened which produces more friction between the nut and the bolt. This prevents the loosening of the nut.

4. Ring Nut, Penn Nut, or Grooved Nut

It has an upper portion hexagonal and a lower part cylindrical. It is largely used where bolts pass through connected pieces reasonably near their edges such as in marine-type connecting rod ends.

The bottom portion is cylindrical and is recessed to receive the tip of the locking set screw. The bolt hole requires counter-boring to receive the cylindrical portion of the nut. To prevent bruising of the latter by the case-hardened tip of the set screw, it is recessed.

5. Locking with Pin

The nuts may be locked using a taper pin or cotter pin passing through the middle of the nut. But a split pin is often driven through the bolt above the nut.

6. Locking with plate

These come in the form of thin metal plates. With the help of a set screw, its rotation is retained around a nut after the nut has been tightened. The plate consists of a hole which may be usually a twelve-sided one for enabling easy adjustments.

The set screw used for securing the locking plate is kept intact with a spring washer. Double-sided locking plates are also available and are used for locking two nuts simultaneously.

7. Spring Locknut Washer

As the nut tightens the washer against the piece below, one edge of the washer is caused to dig itself into that piece, thus increasing the resistance so that the nut will not loosen so easily. There are many kinds of spring lock washers manufactured, some of which are fairly effective.

A regular nut may also be a locknut by applying a thread locker or by drilling and pinning.