Endurance strength of screw joints
 Metric thread -
 Joint stiffness factor Cm=kb/(kb+km)
 Minimum value of clamping force Fm kN
 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
 load at yield F0.2 kN
 endurance strength σe MPa
 maximum value of load fluctuation bolt1 Pb=2At σe kN
 load fluctuation clamped material Pm=Pb(1-Cm)/Cm kN
 Minimum  value of preload Fi,min=Fm+Pm kN
 Maximum value of preload Fi,max=F0.2−Pb kN
 Maximum value of load fluctuation P=Pb+Pm kN
1In infinite life or safe stress designs stresses higher then the endurance limit are avoided and a factor of safety is taken into account. Most rotating and oscillating components must sustain over 10^6 load cycles during service life and thus are to be designed for infinite fatigue life.