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What Is a Lock Nut?

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  • Written By: Dale Marshall
  • Edited By: Kristen Osborne
  • Last Modified Date: 17 August 2014
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A lock nut is a type of nut that resists loosening when properly threaded onto a bolt. Nut-and-bolt fasteners are extremely common in industry and home applications, and finding an efficient, reliable lock nut is a priority for many. Many are universal in application, while others work best under certain circumstances. For instance, some of the most popular lock nuts have nylon inserts that increase the gripping force on the bolt, and are relatively impervious to vibration and corrosion; however, these lock nuts' capacity for re-use is questioned, and re-use is forbidden in some aerospace applications.

Nut-and-bolt fasteners are used to fasten together pre-drilled items, whether wood, plastic or metal. Nut-and-bolt fasteners are key components of much structural construction. Additionally, most machinery, including the engines that power transportation vehicles, relies on nuts and bolts to keep components fastened together. A universal problem with these fasteners is that over time, nuts will lose their grip on the bolt. This problem is exacerbated when machinery is involved, causing vibrations that hasten the loosening process. When nuts loosen, time is spent identifying the problem and correcting it. In addition, if the loosened nut caused other problems their solution will also involve expenditures of time and money. To solve the problem of nuts loosening from their bolts, lock nuts were devised.

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One of the simplest lock nut applications, when there's sufficient space on the bolt protruding beyond the nut, is to tighten the nut and then thread a second nut onto the bolt, tightening it firmly against the first. Nuts that are specifically made for this purpose are thinner than regular nuts, and are called jam nuts. A jam nut will sometimes be used to secure a nut in place on a bolt where it's not secured up against the workpiece and some slack is desired.

Many applications, however, won't accommodate such ad hoc solutions, and many manufacturers produce specialized lock nuts to meet the demands of industry. Among the most popular of these is the lock nut with nylon collar insert. The nylon insert's diameter is slightly smaller than the bolt's diameter, and as the nut is threaded onto the bolt and tightened down, the nylon insert is deformed onto the threads of the bolt, increasing the friction between the nylon insert and the bolt threads, as well as between the threaded section of the nut and the bolt's threads.

Many lock nuts' efficacy is usually achieved by increasing the friction between the threads of the nut and the bolt. Interfering thread lock nuts and distorted thread lock nuts utilize this concept by deliberately making the nut and bolt inconsistent; the nut's diameter may be more oval than circular, or the opening may be tapered. In either case, as the nut is threaded onto the bolt, more friction exists between the two sets of threads, thus increasing resistance to loosening.

Another approach to resisting loosening is to provide resistance at the point of fastening, as in the case of a serrated face nut. The serrations are angled against the rotation of the nut when being tightened, and bite into the surface of the item being fastened. The work surface itself, then, provides resistance against loosening. These nuts should be used when the installation is intended to be permanent.

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