Preload and tightening torque

Thread

Property class

Nut factor K

Assumed, not derived. 0.20 is common shop practice for dry, unplated steel, not standardized.

Target preload

Within proof load

As and Sp, ISO 898-1:2013 · K = 0.20, assumed · 65 % of proof load · Not hot-dip galvanized or structural · T = K·Fi·d

Show the formula

Fp = As·Sp; Fi = (target / 100)·Fp; T = K·Fi·d

As: Tensile stress area, as ISO 898-1:2013 defines it, computed from the ISO 68-1:2023 basic profile, mm²
Sp: Nominal stress under proof load for the property class, ISO 898-1:2013, at the thread's diameter, MPa
Fp: Proof load, N
target: Target preload as a percentage of proof load, %
Fi: Preload, N
K: Nut factor. Assumed, not derived; common shop practice, not standardized
d: Nominal diameter, mm
T: Tightening torque. With d in mm, K·Fi·d is in N·mm and is divided by 1000, N·m