Maximum tightening torque to preload a bolt
 Metric thread : -
 Maximum equivalent stress : σe = xRp0.2
 Thread coeff. of friction : thread =
 Head face coeff. of friction : head =
 pitch P mm
 pitch diameter d2 mm
 root diameter d3 mm
 tensile stress area At = π/4 d02 ; d0=(d2+d3)/2 mm2
 ultimate tensile strength Rm  MPa
 yield strength Rp0.2 MPa
 equivalent stress σe = (σt2 + 3 τ 2) MPa
 torsion stress τ = MG / ( π/16 d3) MPa
 tension stress σt = Fi/At MPa
 thread friction MG Nm
 thread friction MG- Nm
 head face friction MWD Nm
 Total tightening torque MA = MG + MWD Nm
 Stress relaxation (σe - σt)/ σe %
 Initial preload Fi = σt At kN
 Load reserve Pb = F0.2 - Fi kN
 1) the equivalent stress is a result of the stress by tension and torsion.
 2) stress relaxation is the reduction of the (equivalent) stress by setting, where torsional stresses fade away.
 3) load at yield F0.2=Rp0.2.At
 4) torque to yield stress TTY is obtained with σe=Rp0.2.