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.