How bolt torque relates to preload
Torque is only a proxy for what matters, which is the clamp force (preload) in the bolt. The short-form relation uses a nut factor K that lumps together thread and underhead friction, so lubrication changes the preload you get from a given torque more than anything else.
- Torque:
T = K·F·d - Tensile stress area (metric):
A_s = π/4·(d − 0.9382·P)² - Proof load:
F_p = S_p·A_s; target preload is usually 65–85% of it
Worked example: M10 class 8.8, dry
An M10×1.5 class 8.8 bolt has a stress area of 58 mm² and a proof load of about 33.6 kN. Preloading it to 75% (25.2 kN) with dry steel threads (K = 0.20) takes about 50.5 N·m.
Good to know
- Dry threads can vary K from about 0.18 to 0.25, so the same torque can produce preloads that differ by a third.
- Pro shows the preload range from friction and tool accuracy, and checks the joint against slip under shear and separation under tension.