How do I calculate an assembly torque

How do I calculate an assembly torque? by Cory Padfield

The following assumes a metric thread form according to:

ISO 68-1 ISO general purpose screw threads – Basic profile – Part 1: Metric screw threads, and
ISO 724 ISO general-purpose metric screw threads – Basic dimensions.

Also, it is assumed that all assembly torque is used to overcome friction and develop joint preload, so the joint members must be elastic and there must not be any prevailing torque.

MA = FM (0.16 · P + 0.58 · d2 · μG + μK · DKm/2)


MA is the assembly torque in N m
FM is the preload in kN
P is the pitch in mm
d2 is the pitch diameter in mm
μG is the thread friction coefficient
μK is the bearing friction coefficient
DKm = (dw +dh)/2
dw is the bearing surface outer diameter in mm
dh is the hole diameter in mm

The target preload FM usually is chosen as a certain percentage of the screw yield stress.

FM = ν · σM · A0


ν is the percentage of yield stress chosen (90% is standard)
σM is stress due to combined tension and torsion
A0 is the relevant shank cross-sectional area (Full, reduced, or threaded shank)

σM = Rp0.2 / [1 + 3(3/2 · d2/d0 · {P/(π · d2) + 1.115 μG})2]0.5


Rp0.2 is the screw yield stress
d0 is the diameter corresponding to A0
π = 3.141 592 654

Thanks for the info… it is my understanding that it is difficult to establish a torque due to the variation in components. Quite a scatter in the results.