How Does the Security Locknut Work?
Last time, we discussed why you may not need a locknut if the joint is properly designed and assembled. But, in the real world, on critical applications, this is often difficult to consistently achieve. In these common situations, a locknut can improve joint integrity, reduce maintenance costs, and help prevent catastrophic failure.
Here’s how the Security Locknut works and what makes it different.
It holds tight.
Unlike other locknuts, the load and locking function on the Security Locknut are isolated from each other. The illustrations below will show why this makes a big difference in performance.
If you consider the thread contact surface between the nut and bolt as a simple sloped plane, it will look as shown in the figure below. The resulting forces at the contact surface are illustrated in the free body diagram in the figure below. Many locknuts try to prevent slippage between the mating surfaces by increasing the friction. The difficulty with this method is the slip surface is still subject to the same load and vibration forces as the rest of the nut. The vibration only needs to overcome the small resulting friction force (F3) for the nut to slide along the mating surface (direction A and B) and loosen.
The Security Locknut solves this problem by creating a separate locking feature (the spring steel lock ring) that is independent and isolated from the nut body. This is illustrated with a red block in the figure below. This lock ring is not connected to the main nut body, so it is not subject to the upward clamp load force. With this configuration, the nut body takes the load and vibration and the lock ring only needs to resist the small friction force (F3) to prevent movement.
The resulting Security Locknut product looks like this:
The lock ring lifts and separates from the main nut body on the base and side walls (colored yellow below) when the bolt is installed. This separation isolates the clamping load and vibration from the locking function.
This subtle difference in the way the Security Locknut functions makes a big difference in how it performs.
The locking mechanism on the Security Locknut is an oval shaped part made of spring steel, as seen below. Just like a spring, it bounces back to its original position after each use. The thread on the lock ring matches the shape of the thread on the bolt, also seen below. The spring on the lock ring creates a force (F4) normal to the axis of the bolt and squeezes hard in the direction shown.
When it’s removed, it springs back to its original shape, ready for use again and again. This also allows the Security Locknut to be retightened as the bolted joint relaxes or settles after initial machine run in so full preload tension can be maintained.
Having problems with bolted joints? Give us a call. We can help.