Products - Nuts Index - Hex Jam Lock Nuts

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

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

Now a half of a nut (the jam nut) will have less overall force than a fully torque 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 torque first, the threads are still deformed upward. But when a full nut is fully torque, 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.

We have seen two full nuts used in an attempt to provide locking. If both nuts are torque 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.

Since hot-dip galvanizing typically adds 2.2 to 5 mils of thickness to the threaded portion of a fastener, galvanized hex jam nuts are tapped oversize to compensate for the corrosion resistant coating on the bolts.