How do you determine tightening torque?
Tightening torque follows from M = K × F × d, where F is the target preload, d the bolt diameter and K a torque coefficient that depends mainly on friction (thread + head). The table shows guide values at μ≈0.14 (lightly oiled) for property classes 8.8, 10.9 and 12.9. Dry threads need a higher torque, heavily lubricated or greased threads a lower one — expect a difference of tens of percent. For critical joints, always follow the design specification.
Frequently asked questions
What is the tightening torque for an M12 bolt?
For M12 class 8.8 it is about 86 Nm, class 10.9 about 125 Nm and class 12.9 about 145 Nm (at μ≈0.14).
What do 8.8, 10.9 and 12.9 mean?
They are the bolt property class: the higher the number, the higher the tensile and yield strength, and thus the allowable preload and torque.
Does lubrication matter?
Yes, a lot. Friction determines most of the torque. Oiled threads (μ≈0.14) need clearly less than dry threads; grease or anti-seize lowers it further.
Do these values apply to fine threads?
The table is for metric coarse thread. For fine thread the values differ slightly; follow the specific specification.