What’s a Locknut?

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Locknuts prevent nuts from loosening on bolts, which is a common problem in machinery due to vibrations. There are various types of locknuts, including nylon collar insert locknuts and serrated face nuts, that increase friction between the nut and bolt threads or provide resistance at the fastening point. Some locknuts are designed for specific applications, while others have universal use.

A locknut is a type of nut that resists loosening when properly threaded onto a bolt. Nut and bolt fasteners are extremely common in industry and domestic applications and finding an efficient and reliable lock nut is a priority for many. Many have universal application, while others work better under certain circumstances. For example, some of the more popular locknuts have nylon inserts that increase the gripping force on the bolt and are relatively impervious to vibration and corrosion; however, the ability to reuse these locknuts is questioned and reuse is prohibited in some aerospace applications.

Nut and bolt fasteners are used to secure pre-drilled items together, whether they are wood, plastic or metal. Nut and bolt fasteners are key components of many structural constructions. Additionally, most machinery, including the engines that power transportation vehicles, rely on nuts and bolts to keep components fastened together. A universal problem with these fasteners is that over time the nuts will lose their grip on the bolt. This problem is compounded when machinery is involved, causing vibrations that speed up the loosening process. When the nuts loosen, time is spent identifying the problem and correcting it. Also, if the loose nut caused other problems, fixing them will also cost time and money. To solve the problem of nuts coming loose from their bolts, locknuts were devised.

One of the simpler applications of the locknut, when there is enough space on the bolt to protrude beyond the nut, is to tighten the nut and then screw a second nut onto the bolt, tightening it tightly against the first. Nuts that are made specifically for this purpose are thinner than regular nuts and are called jam nuts. A locknut is sometimes used to secure a nut in place on a bolt where it is not secured against the workpiece and some slack is desired.

However, many applications are not suited to such bespoke solutions and many manufacturers produce specialized lock nuts to meet the needs of industry. Among the most popular is the nylon collar insert locknut. The diameter of the nylon insert is slightly smaller than the diameter of the bolt and as the nut is screwed onto the bolt and tightened, the nylon insert is deformed on the bolt threads, increasing the friction between the nylon insert and the threads of the bolt, as well as between the threaded section of the nut and the threads of the bolt.

The effectiveness of many locknuts is usually achieved by increasing the friction between the nut and bolt threads. Interfering thread locknuts and distorted thread locknuts use this concept by deliberately making the nut and bolt inconsistent; the diameter of the nut may be more oval than circular, or the opening may be tapered. In both cases, as the nut is threaded onto the bolt, there is more friction between the two sets of threads, thus increasing the resistance to loosening.

Another approach to resist loosening is to provide resistance at the fastening point, such as with a serrated face nut. The serrations are angled against the rotation of the nut during tightening and bite into the surface of the item to be fastened. The worktop itself, therefore, provides resistance against loosening. These nuts are to be used when the installation is intended to be permanent.

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