Maximum tightening torque to
preload a bolt 

Metric thread : 
 
Maximum equivalent stress : σ_{e
}= 
xR_{p0.2} 
Thread coeff. of friction : µ_{thread
}= 

Head face coeff. of friction : µ_{head }= 



pitch P 
mm 
pitch diameter d_{2} 
mm 
root diameter d_{3} 
mm 
tensile stress area A_{t }= π/4
d_{0}^{2 }; d_{0}=(d_{2}+d_{3})/2 
mm^{2} 
ultimate tensile strength R_{m} 
MPa 
yield strength R_{p0.2} 
MPa 
equivalent stress σ_{e} = (σ_{t}^{2
}+ 3 τ ^{2})^{½} 
MPa 
torsion stress τ = M_{G }/ ( π/16
d_{3}^{3 }) 
MPa 
tension stress σ_{t} = F_{i}/A_{t} 
MPa 
thread friction M_{G} 
Nm

thread friction M_{G} 
Nm

head face friction M_{WD} 
Nm 
Total tightening torque M_{A }= M_{G} + M_{WD} 
Nm 
Stress relaxation (σ_{e}  σ_{t})/ σ_{e} 
% 
Initial preload F_{i }=
σ_{t }·A_{t} 
kN 
Load reserve P_{b} = F_{0.2}  F_{i } 
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 F_{0.2}=R_{p0.2}.A_{t}
^{4)} torque to yield stress TTY is obtained with σ_{e}=R_{p0.2}. 

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