Effective
length
of thread engagement 

External thread fastener: 
Thread  steel grade 
 
Ratio of shear strength to tensile
strength τ/σ 
(steel) 
Internal thread tapped hole: 
Tensile strength σ 
MPa 
Ratio of shear strength to tensile
strength τ/σ 
(cast iron) 

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 
maximum tensile load F_{0.2 }= R_{p0.2} x A_{t} 
kN 
ultimate tensile load F = Rm x A_{t} 
kN 
ultimate shear stress Τ 
MPa 
required
shear
area external thread A_{th}=F/Τ 
mm^{2} 
Effective length of thread engagement L_{e}^{1)} 
mm 
^{1)} Minimum value of thread engagement length to make
the tensile area determinative for the load at which the screw joint
fails. 

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