For more than 150 years, engineers have been using two nuts to provide a locking effect on fasteners.
The question is how effective these jam nuts or lock nuts really are.
In this article we'll investigate the effectiveness of using a jam nut, and we'll also discover that the order that a lock nut is applied can make a huge difference to the overall locking effect.
A jam nut or lock nut is a secondary nut that's added onto a fastener to lock a primary nut in place. Often the jam nut is a lower profile than the primary nut and is typically less than half the height. However, you will sometimes see jam nuts and primary nuts that are the same size.
Jam nuts need to be used when a nut needs to be locked into place without clamping or locking it against another object or surface. They are often used on threaded rods that don't have a traditional bolt head or in situations where a bolt needs to rotate in its position. They were also used in situations where a bolt that was clamped against an object needed to be locked in place.
These days there are more efficient ways to lock a bolt that's clamped against an object. But 100 years ago, there were not the options of nyloc, self locking or the slotted nuts we have access to today. For this reason, you'll tend to see lock nuts on older machinery or vehicles.
If we take a cross section of a jam nut, we can see why a two nut system is so effective in resisting self loosening.
When we install the jam nut into our joint, the nut works the same way as any nut or bolt fastener would. As it's tightened, the Clamp Force (Fc) and Preload Force (Fp) increases.
At this point the pressure is being held by the bottom flanks of the thread in the jam nut. If we zoom in on the jam nut thread, we can see where the load is being held.
In this diagram you can see that the lower flanks (A) of the thread are holding all the load and providing the preload and clamp (Fc) forces into the joint. The upper flanks of the thread (B) are not carrying any of the load.
When the thick nut is installed into the joint and tightened against the jam nut, the pressure points change and new clamping forces are created between the two nuts.
As the tightening continues, the bolt thread eventually touches the top flanks of the jam nut. When this first occurs, the forces measured at F2 and Fc are equal.
However, while we continue to tighten the two nuts against each other, the forces measured at F2 will increase to be more than the forces measured at Fc.
While this is happening, another interesting thing happens with the thread – and it is worth zooming in to have a look.
As we tighten the two nuts against each other, the thread within the jam nut moves from the lower flanks (A) to the upper flanks (B) of the thread and starts to provide clamping forces at F2 instead of Fc. We also find that the lower flanks (C) of the larger nut are now holding load and contributing to the preload (Fp) and clamping forces at F2 and Fc.
The reason why the two nut system is so effective is because the nuts are bearing in opposite directions and are jammed. Because the top thread of one nut and the bottom thread of the other nut are engaged, relative thread movement is not possible. This locking effect will remain even if the bolt tension is lost.
A jam nut works by first installing the smaller jam nut onto the fastener. This is followed by the larger nut which is screwed down on top of the jam nut. The small and larger nuts are then tightened against each other to provide a locking effect.
While it may seem counter intuitive, the smaller lock nut should always be installed against the clamping surface. When the larger nut is installed on top, it relieves the smaller nut of the load on its thread.
When a jam nut is tightened directly against the surface of an object, it's typically tightened to between 25 and 50% of the recommended torque. The primary or top nut is then tightened on top of the jam nut and is tightened to the full torque value. It is important that the jam nut is held by a wrench to stop it from rotating while the torque is applied to the primary nut.
Bolt Science carried out tests with the jam nut on top and below the main nut. The tests were carried out using M10 nuts and bolts and followed the Junker fastener vibration test protocol.
The results are quite dramatic and within less than 50 cycles, the difference between the two methods is quite evident.
When the small nut was on top, the nuts started to rotate together – as if they were a single nut. In fact, having the small nut on top was only marginally better than a single nut without using a jam nut. Within 600 cycles, the version with the small nut on top came completely loose.
In comparison, the test with the jam nut next to the joint demonstrated some relaxation of the join – but it didn't show a significant amount of self loosening.
Proper application of a jam nut can offer an effective locking capability. Particular attention needs to paid to holding the jam nut while the larger nut is tightened to torque. Unless this is done correctly, the threads will not engage properly and much of the locking effect will be lost. This is highlighted further in ISO 989-2:
Thin nuts used as jam nuts should be assembled together with a regular nut or a high nut. In assemblies with jam nut, the thin nut is first tightened against the assembled parts and then the regular or high nut is tightened against the thin nut.
Image sources : Bolt Science