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JAM NUT ON TOP OR BOTTOM?



We have seen both variations of jam nut installations but only one can be correct.

When a nut is torqued a deformation of the bolt threads take place. You can visualize that one nut, fully torqued will tend to deform the threads upward.

Now a half of a nut (the jam nut) will have less overall force than a fully torqued full nut. So if the full nut is placed on first, the threads are deformed upward. The jam nut, placed on top will bear against the full nut, tending to reverse the deformation. However since the full nut applies more force, the thread deformation under both the full nut and the jam nut is in the same relative direction.

If the jam nut is torqued first, the threads are still deformed upward. But when a full nut is fully torqued, the bearing against the jam nut will tend to reverse the thread deformation. The threads under the full nut are deformed upward, while the threads under the jam nut are deformed downward. This thread deformation reversal provides the locking mechanism.

Whe have seen two full nuts used in an attempt to provide locking. If both nuts are torqued to the same value, this is the worst of all worlds. Theoretically the bottom nut will impose no deformation on the thread leaving it useless.

The jam nut should be installed on the bottom.




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