If you work with material handling equipment, you know that inspecting bolted joints is a tedious process for maintenance technicians. For example, mining operations use conveyor systems with dozens of bolted joints to secure guards, supports and other adjoining components; technicians must walk the entire length of the conveyor to individually inspect joints and retighten any that have loosened.
Sometimes, the fix is not as simple as tightening a single joint. Moving machinery, stored energy or interconnected components can make it unsafe to work on the system while it’s still running. In these instances, the entire system must be shut down. This process is time-consuming and costly because it interrupts productivity and requires labor costs to stop, repair and restart equipment.
The Solution
Prevailing torque locknuts play a critical role in applications where vibration, heavy impacts and repeated loading cause conventional fasteners to loosen frequently. Locking fasteners reduce the concern of loose nuts because their design maintains the connection without relying solely on clamp force and friction.
There are three broad design categories of prevailing torque locknuts: friction and spring-based, deformation-based and adhesive-based; all perform differently across varying temperatures and reinstallation cycles.
Each design resists loosening and generates a different amount of prevailing torque through its own locking mechanism.
Common Prevailing Torque Locknut Designs
Friction and Spring-Based
Friction and spring-based locknuts generate prevailing torque by pressing a locking element against the bolt threads without permanently deforming the nut or the bolt. These locknuts can be removed and reinstalled through a different number of cycles depending on their specific locking element.
Nylon-insert locknuts, such as Nyloc, create prevailing torque by pressing a nylon collar into the top of the nut. As it is threaded onto a bolt, the bolt threads cut slightly into the collar and compress it against the threads, which increases the surface friction. Nylon-insert locknuts are cost effective, but they are not suitable for components subjected to extreme temperatures because nylon softens in high temperatures and becomes brittle in cold temperatures. The nylon also degrades with each removal, so they can be reused only a limited number of times.
Split nuts are all-metal locknuts with slots in the upper portion of the nut body which divide it into flexible segments. The segments act as springs that press against the bolt threads to generate friction and prevailing torque. Because the design is all metal, split nuts tolerate extreme temperatures and reuse better than nylon-inserts, but the prevailing torque reduces as the spring segments wear with each reinstallation.
Deformation-Based
Deformed-thread nuts create prevailing torque by intentionally distorting a portion of the nut’s threads during manufacturing. There are several variants; some designs compress the entire nut body toward the top while others pierce the nut from the side to displace the threads. The Stover “Top Lock,” for example, pinches the top of the nut inward to create additional friction between the nut and bolt threads. In each case, the result is an interference fit between misaligned threads from the bolt and the deformed nut.
Because the locking function comes from deformed threads, these nuts are effectively single use. Each removal further deforms and wears the nut to fit the bolt’s threads, and the prevailing torque drops sharply with reuse. However, they are inexpensive and withstand extreme temperatures, so they are appropriate for applications like highway guardrails where fasteners are used for one-time installation until the fastener is replaced.
Adhesive-Based
Adhesive-based locknuts use a patch of nylon or adhesive, such as epoxy, on the nut’s internal threads. As the bolt is threaded into the nut, it compresses the patch or adhesive to create an interference fit. Unlike a nylon-insert, where the collar is a separate component at the top of the nut, the material is applied or bonded directly to the threads.
The resistance created with adhesive-based locknuts is a chemical bond rather than friction. Once the nut is removed or comes loose from vibration, the bond is broken and the locking mechanism no longer functions. While it is debatable whether adhesive-based locknuts are prevailing torque locknuts, the design is increasingly common and is used in applications that prioritize automated assembly and low-cost fasteners that will be replaced rather than reused.
The Security Locknut: A Reusable All-Steel Design
The Security Locknut falls into the friction- and spring-based category. Its innovative design separates friction and clamp load between two components to resist loosening. An oval-shaped steel lock ring sits in a counterbore separate from the nut body. The lock ring flexes into a circular shape as it twists on the bolt, creating a spring-like tension as it tries to revert to an oval. This tension grips the bolt tightly and locks it into place.
The nut body does not rely on friction to maintain clamp load; unlike like a standard nut, clamp load only reduces as the mating parts settle in, bed down or ware away. Retightening is still recommended, but it will be less frequent.
The lock ring makes the Security Locknut unique. Unlike plastic inserts or deformed threads, spring steel reliably returns to its original shape with each removal. Its prevailing torque remains consistent through repeated reuse, temperature changes and exposure to materials that would degrade nylon or adhesives.
Choosing the Right Prevailing Torque Locknut
Each prevailing torque locknut design is suitable for applications where secured joints must resist vibration and hold up to heavy or repeated impact loads. For joints that must withstand temperature extremes, relentless vibration or repeated reassembly, the Security Locknut is engineered for success.
Speak with one of our engineers to determine whether Security Locknut is right for your joints or learn more about how the Security Locknut works.