Endurance
strength of screw joints 

Metric thread 
 
Joint stiffness factor C_{m}=k_{b}/(k_{b}+k_{m}) 

Minimum value of clamping force F_{m} 
kN 

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 
load at yield F_{0.2} 
kN 
endurance strength σ_{e} 
MPa 
maximum value of load fluctuation bolt^{1} P_{b}=2A_{t }
σ_{e} 
kN 
load fluctuation clamped material P_{m}=P_{b}(1C_{m})/C_{m} 
kN 
Minimum value of preload F_{i,min}=F_{m}+P_{m} 
kN 
Maximum value of preload F_{i,max}=F_{0.2}−P_{b} 
kN 
Maximum value of load fluctuation P=P_{b}+P_{m} 
kN 
^{1}In 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. 

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