What are Locking Fasteners?

Threaded fasteners such as bolts and screws are common to countless assemblies, allowing for two or more pieces to be secured together with ease. When exposed to various forces such as vibration and shock, such fasteners can often become loose over time. To prevent such incidents from occurring, specialized components known as locking fasteners may be used. Generally, these are components that are specifically designed to prevent loosening through various locking schemes. In this blog, we will discuss the most common forms of non-permanent locking schemes, allowing you to find the most optimal locking fasteners for your needs.

Free-running locking fasteners are those that rely on an extra component for remaining secure, and lock washers and jam nuts serve as the most common types. Lock washers are commonly paired with small fasteners, and spring action and tooth lock washers are the two primary sub-types. The spring action washer is placed under a nut or the head of a screw during installation, and their compression and spring-back tension mitigates the chance of the fastener becoming loose. With tooth lock washers, on the other hand, a ratchet action is formed as the component bites into the nut or screw head and the contacting surface.

Jam nuts are considered to be one of the first types of locking fasteners, coming in the form of two components that work together to enhance security. The first jam nut is a thin component that is tightened onto the fastener, followed by a thicker nut that is tightened against the thinner one. While presenting an effective method of locking fasteners in place, the bolt that the nuts are installed on has to be long enough to accommodate both. Additionally, the installation process can be longer than is seen with other types due to having to individually tighten each jam nut.

Alongside lock washers and jam nuts, the slotted or castle nut is another free-running option. Generally, such components are paired with bolts or studs that feature a hole drilled in their threaded shank. After tightening the nut over the fastener, a cotter pin or safety wire may be passed through the bolt hole and the nut slots to secure the assembly in place. The major benefit of such installations is the fact that the wire or pin must break or be removed before the nut can loosen.

Prevailing torque locking fasteners are the other major type, such components utilizing a form of interference to maintain security through the friction of mating threads. This can be beneficial for holding an assembly together, but also makes the installation process more difficult. Generally, such locking fasteners come in the form of nuts and screws, the former being more widely used.

Prevailing torque locking fasteners may be all-metal or feature a non-metallic element. All-metal nuts often have less resilience than other types, though are more capable of withstanding temperatures and chemicals. Non-metallic types may be limited by temperatures, though have the advantage of being capable of being threaded onto longer bolts without causing thread damage. Flex top and nylon insert locknuts are two common types that can be very reliable, though they are considered one-way fasteners due to their locking feature.


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