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How Much Load Does Each Thread Carry?

22 October 2014


When a bolt is tightened, the nut moves along the bolt thread and its face is pressed against the item to be fastened.

This force at the bearing surface leads to the bolt being loaded in TENSION and the nut in COMPRESSION.

Hence the bolt stretches axially, causing an INCREASE in pitch of the male thread and the nut COMPRESSES – resulting in a REDUCTION in pitch of the female nut.

The consequence of these two actions is that the threads near the loaded nut face are forced into much harder contact than the remainder – resulting in a CONCENTRATION of load and higher stress in this region of the union.

Typically the resulting maximum stress is about THREE times the nominal stress on the initial threads – in some cases the nut can also “mushroom”.

As an approximation – the first thread carries 33% and the second carries 25% of the total load.

Studies have suggested that for the same length of thread engagement – the FINER the thread pitch the higher the average load on the first thread.

This is one of the contributing factors why high tensile fine pitch bolts metric fine and UNF  threads have a higher susceptibility to stripping failures.

Sound engineering practice is to have at least two full threads protruding past the end of the nut.


References:       Technical information supplied courtesy of Hobson update 26 - Nov 14

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Author Warrick Cashman