Stress in Bolts

Calculating the stressed area in UN and UNR bolts

The weakest points in a bolt is the stressed area in the thread root. The stressed area depends on the type of threads - coarse threads will reduce the stressed area compared with fine threads.

The stressed area can be calculated as

A = π / 4  (dn - 0.9743/ n)2         (1)


A = stressed area (square inch)

dn = nominal bolt diameter (inch)

n = 1 / p = number of threads per inch

p = pitch (length per thread)

Despite that reduced area reduces the braking load, coarse threads are used for clamping. Fine threads are weaker due to the reduced thread height.

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