We at Hardware Everywhere sell a LOT of lock nuts. It occurs to us, not everyone is aware of what types might be best in what environment. And of course that leads to pontification on the subject by your humble reporter.

A lock nut is a kind of fastener used to secure bolted joints and resist loosening under vibration, shock, impact, and/or torque using a unique design. After inserting a fastener through the assembly, a lock nut is turned onto the end of the fastener from the backside of the assembly using a deep socket (or from the side with a hand tool, though that doesn’t work with all types of lock nuts), using a tool. The lock nut prevents loosening either by utilizing friction or a positive locking device, depending on its design.

The most common lock nut types are all-metal lock nuts and nylon insert lock nuts. There are many sub-types in each category, but this is a basic overview of what you might expect to find. We’ll start with the nylon insert lock nuts as they are, by far, the most common locking nut.

Nylon Insert Lock Nuts / Elastic Stop Nut / Nyloc® 

This type of lock nut has an insert ring made of polyamide nylon inside the crown of the nut which is then crimped in place. When you thread the bolt through, the threads starts to dig into the nylon, which then put pressure on the screw in order to prevent the bolt from vibrating loose or the nut from backing off. The downside is the addition of non-metal material makes the nut far more sensitive to temperature and chemicals. These cannot be installed by hand, and you’ll need a wrench or power tool.

This locking ring can be put into hex nuts, jam nuts, flange nuts, cap nuts, and wing nuts among others, though they aren’t typically “off the shelf” items. These nuts should not be reused as the ring is damaged as part of the installation process. In some instances, the nuts can be reused, but they weaken each time they’re put back to work as the nylon is eaten away – sometimes causing the ring to pop out of the fastener. In applications where safety is paramount, never reuse these nuts.

Also please note, if the nut will be exposed to extremely high temperatures, choose another nut instead. Nylon insert lock nuts can operate in temperatures up to approximately 248˚F (120˚C). In extreme cold, the nylon insert can become brittle and break. The color of the nylon ring is simply a way some manufacturers use to identify their product and it is not an indicator of grade, material, or performance. White is the most common color.

What’s the NM/NE mean when used in a description of a standard Nylon Insert Lock Nut? It refers to the pattern type of the nut. The common patterns are listed in the chart.

Nyloc® (ending with a C) is for a specific brand of nylon insert lock nuts, whereas Nylok® (ending with a K) is for a brand of Nylon Patching having nothing to do with nuts. Nylock with a CK is a generic term used throughout the industry.

We sometimes get complaints that “The nylon insert lock nuts are making my bolts break!” or that they don't go all the way on. They're not supposed to! These complaints are invariably user error in picking a fastener that is too long. The nuts are supposed to lock after one or two threads clear the locking ring; that's how they do their job. This is not a defect in the product. You can force them further, but the locking ring will pop out, or cause the fastener to break. If you insist the nuts go all the way down, applying dry wax to the fastener may work. They also make special waxed nylon insert lock nuts which are substantially more money. You may wish to consider a k-lock nut or serrated flange lock nut instead, which are both supposed to go all the way on.

Serrated Flange Lock Nut

This lock nut has an integral flange built into the base and the bottom bearing surface of the base is serrated; it is typically used in applications where you have metal sheeting or a metal component. The flange acts as an integrated washer making them popular for use on assembly lines as they don’t require a separate washer, which speeds up operation.

This integrated washer enables built-up forces under the nut to spread over a larger area, which results in lower surface pressure at a given point. This minimizes the chance for damage to the surface area while securing the assembly in place to withstand loosening. They also make these without the serrations, but those are not technically lock nuts. 

K-Lock Nut / Keps® Nut

K-Lock nuts have an external tooth lock washer captivated on one base of the nut. The washer is free-spinning (movable). When the teeth get tightened by turning the nut, they dig in which the locking mechanism. Do not over tighten them, because once you over tighten the nut, because the free-spinning washer gets crushed, and the teeth are flattened instead of biting in. 

Two-way Lock Nut / Three-way Lock Nut / Center-Lock Nut / Side-Lock Nut

There is a small indentation in the middle of two opposing sides on the outside of the nut which looks like a punch right in the center. The number of punches (two is the most common) shows it’s a two-way nut, as the name suggests. (These also come one and three side punched, though those are much less common.)

Both Stover-style lock nut and two-way lock nuts use distorted threads to keep the nuts from loosening. The difference is that the distorted thread of Stover-style nuts is at the top and the two-way nuts have it located in the center of the nut where the punch is. They are theoretically re-usable. Keep in mind that the bolt threads can be damaged by this nut, and they may not be able to be reused because of that. Almost any type of nut can be side-punched to create this type of nut. The punches may be either circular or rectangular – that’s a simply function of the punch used, and not a performance issue. 

 Prevailing-Torque Locking Nut (Stover®-Type Lock Nut) / Cone Lock Nut

These are one-piece all-metal locking nuts that have a conical top and a flat bottom bearing surface with chamfered corners. It has a locking mechanism that’s built into the cone, and as you put it on, its self-contained feature creates frictional interference between the threads of the mating components.

When fastening a prevailing-torque lock nut, there is a resistance to rotation during both assembly and disassembly requiring them to be wrenched; that resistance is called prevailing-torque (hence the name of the nut). They can be adjusted or removed after installation. If you look at it straight on from the top, you will see the top of the cone is distorted in one location which is what creates the locking feature. They are very popular in applications where shock is the loosening force. Further, they are not permanently locked, which means maintenance or adjustments can be made. In the rightmost nut in the photo, if you look carefully, you can see the distortion of the thread.

There is also a collar type version of this nut, usually only seen in larger diameters (over 1”). The locking mechanism is part of the collar. Do not confuse these collar types with flexlock type nuts.

They’re also available with flanges, and the flange that provides the more secure hold. Some flange nuts have the prevailing-torque, the nylon insert, and the serrations for three locking mechanisms in one fastener. Absolute overkill!

Tri-Loc® Nuts

This is a version of the standard Stover-style locknut but it’s not conical at the top. The locking mechanism is on the top of the flats. This all-metal locknut features three locking elements deflecting the threads in the top of the nut. It is available in flange nut style as well. Don’t confuse these with other top-lock style nuts where the locking mechanism is punched all the way into the flats or the outside of the top flat. Note how the lock is only on the inside edge, going downwards where the locking deformity takes place. 

Side Top Locknut

This prevailing-torque locknut features a positive thread deflection on two sides at the top of the nut. It’s available on many different types of nuts. Compared to the Tri-Loc® nut, notice the punch is on the outside of the top flat and not the inside. This can be done with one or more punches. The deformity is still created in the same spot as the punch.

Serrated Hex Nut (one or two sides)

These are regular Hex Nuts (or Jam Nuts) which are serrated directly on the top flat surface. They can be serrated on one or both sides. They are typically used where a serrated flange nut is just too wide to use. These are very costly and hard to come by and aren’t seen much outside of the electronics industry.

Castle Nut (Castellated Nut)

Castle lock nuts are a type of nut with slots (notches) cut into one end, which gives the appearance of castle battlements and why the nut was so named. They are typically used in low-torque applications, such as holding a car wheel bearing in place, you can find them on axles when they hold onto wheels and bearings. A castle nut is often used with a bolt that has a pre-drilled radial hole. A cotter pin is slotted through the notches in the nut and the hole in the bolt. Then the cotter pin’s prongs (one or both) are bent. This prevents the nut from turning or the pin from being removed. Many people confuse Castle Nuts with slotted hex nuts, but they are completely different animals. Castle nuts come only in Hex, Heavy Hex, Hex Jam, and Heavy Hex Jam patterns.

Slotted Nuts 

A slotted hex nut looks very similar to a castle nut, but the difference can be seen in their respective style of slots. The slots on castle nuts are located on a rounded area at the top. Slotted nuts lack this specific geometry. On a slotted nut, the diameter of the slotted area is the same as the wrenching area. On a castle nut, the castellated area’s diameter is somewhat smaller than the wrenching section. For slotted 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 prongs are then bent. Slotted nuts often use safety wire instead of cotter pins, making them a sort of wire lock nut in those instances. Slotted nuts are slightly shorter in height than a castellated nut. Many different types of nuts can be slotted.

PM Nuts

A PM nut (originally spelled with a hyphen as in P-M® Nut) was a patented and trademarked fastener invented by NKR Precision in Michigan decades ago. They went out of business and assigned the patent (now expired) to a company named Interstate Screw Corporation which continues making these fasteners to exacting standards at one of their factories for distribution throughout the world. 

These purpose-made parts are made out of a very special alloys to allow proper compression. These lock nuts work by installing the nut with the points towards the metal, and as the bolt compresses the nut, it slowly flattens the nut with the points digging into the metal surface. A number of companies sell these with rounded corners, but they do not work properly. A proper PM nut has very sharp corners, and if you look at the photo you can see the difference. (This is what happens when a company tries to copy a part without the original engineering print. Interstate Screw Corporation has – to our knowledge – the last surviving original engineering print.) 

FlexLoc® Self-Locking Nuts 

Generically these are called flex lock nuts or flexlock nuts). They are one-piece, all-metal, prevailing-torque locknuts that lock positively on bolts, screws, studs, rods, and other threaded fasteners. They keep assemblies intact from impact, shock, torque, or vibration. Flexlock nuts require nothing special to install, nor do they come apart. Because they lock without seating, they stay in place anywhere on a fastener. Once the locking threads are fully engaged, they can also be used as stopping nuts. With flexlock nuts, every thread, including those in the locking section, carries its full share of the load, and resists working free even under the most severe vibration. They have six opposing narrow slots (spaced exactly 60° apart) aligned with the nut corners in a cylindrical top which is a little smaller in diameter than the width of the nut. In fact, they look like a bit like castle nuts expect the slots are much narrower. These are primarily used in automotive, machinery, and electronics applications.

Pin Locking Nut

A pin-locking nut is an all-metal, self-locking nut that features a stainless steel ratchet pin. The pin is drilled all the way through the side of nut and is bent over the edge and the tip of the pin sticks into the thread of the fastener causing a locking action. There is also a notch cut into the side near the base of the nut. The ratchet pin slides along the threads as the nut is threaded onto the bolt, which prevents the nut from backing off. The almost unbreakable ratchet pin also limits damage to fastener threads, and on galvanized parts also reduces the removal of galvanizing while ensuring consistent locking torque. Some types of these lock nuts allow the ratchet pin to be bent/turned so that the nut can be removed, if needed. This type of nut is usually seen in structural nuts (A563DH, 2H, and so forth.)

Jam Nuts

Jam nuts are not lock nuts but are included here because some people, incorrectly, seem to think they are. Some people do use them to create a lock. The jam nut in this case is used with another nut. You should tighten them up against each other – you’re jamming them together that’s why they are called jam nuts. Most people believe 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 some measure of clamping force. Some people believe they should be installed in the reverse order of what was just described. Because it’s not a lock nut, we won’t offer an opinion on this. 

And there you have a quick summary of some of the lock nuts you will see in the field. This list is by no means complete, but it should cover everything you’re likely to see in common use. If you want more information or types not listed, just ask your friendly Hardware Everywhere customer service agent.



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