### Calculating Assembly Torque per ISO 68 & ISO 724

**Engineering Torque Design and Equations**

Calculating Assembly Torque per ISO 68 & ISO 724:

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.

M_{A} = F_{M} (0.16 · P + 0.58 · d_{2} · μ_{G} + μ_{K} · D_{Km}/2)

where

M_{A} is the assembly torque in N m

F_{M} is the preload in kN

P is the pitch in mm

d_{2} is the pitch diameter in mm

μ_{G} is the thread friction coefficient

μ_{K} is the bearing friction coefficient

D_{Km} = (d_{w} +d_{h})/2

d_{w} is the bearing surface outer diameter in mm

d_{h} is the hole diameter in mm

The target preload F_{M} usually is chosen as a certain percentage of the screw yield stress.

F_{M} = ν · σ_{M} · A_{0}

where

ν is the percentage of yield stress chosen (90% is standard)

σ_{M} is stress due to combined tension and torsion

A_{0} is the relevant shank cross-sectional area (Full, reduced, or threaded shank)

σ_{M} = R_{p0.2} / [1 + 3(3/2 · d_{2}/d_{0} · {P/(π · d_{2}) + 1.115 μ_{G}})^{2}]^{0.5}

where

R_{p0.2} is the screw yield stress

d_{0} is the diameter corresponding to A_{0}

π = 3.141 592 654

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