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# Welding, Bonding & Fastener engineering FAQ

## Fastening calculations

 How do I calculate an assembly torque? by CoryPad faq725-536 Posted: 27 Feb 03 (Edited 6 Aug 03) 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)whereMA is the assembly torque in N mFM is the preload in kNP is the pitch in mmd2 is the pitch diameter in mmμG is the thread friction coefficientμK is the bearing friction coefficientDKm = (dw +dh)/2dw is the bearing surface outer diameter in mmdh is the hole diameter in mmThe target preload FM usually is chosen as a certain percentage of the screw yield stress.FM = ν · σM · A0whereν is the percentage of yield stress chosen (90% is standard)σM is stress due to combined tension and torsionA0 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]0.5whereRp0.2 is the screw yield stressd0 is the diameter corresponding to A0π = 3.141 592 654 Back to Welding, Bonding & Fastener engineering FAQ Index Back to Welding, Bonding & Fastener engineering Forum

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