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Thread torque

Thread torque

Thread torque


I need to calculate torque needed to tighten the bolt.
Problem is that the bolt and the hole are not of the same material, and the thread engagement length is fixed - usually smaller than recommended.

I just want to know is this right way to do it, or at least on the right track.

L - thread engagement length
P - thread pitch
D - outer diameter of the bolt
d2 - pitch diameter
u - thread coefficient of friction
us - bolt head-surface coefficient of friction
rs - effective radius of bolt head-surface connection
T - shear strength of the weaker material - ~0.6 of tensile strength

As = 0.5*pi*L*d2 - shear area

fi = atan(u/cos(30°))
ro = atan(P/d2*pi)

F = T_hole*As - shear force

T = F*(d2/2 * tan (fi+ro)+us*rs)


RE: Thread torque

Quick and dirty: take production representative parts and torque to failure. Set seating torque at 60% of mean failure torque.
If failure was by breaking the bolt you will be at about 75% of yield using this method. It will also give you some realistic margin of safety compared to your weakest component.

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