**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)**

where,

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**

where,

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**

where,

Rp0.2 is the screw yield stress

d0 is the diameter corresponding to A0

π = 3.141 592 654