Stress 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 reduces the stressed area compared to fine threads.
The stressed area in a bolt 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 a reduced area will reduce the breaking load of a bolt - coarse threads are used for clamping. Fine threads bolts are weaker due to reduced thread height.