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How to use it
Choose thread and property class, then state the friction coefficients in the thread and under the head. The tool calculates the preload that takes the bolt to the chosen share of the yield strength, taking into account the torsion that arises during tightening, and the torque needed to reach it.
Formulas used
dKm is the mean diameter of the bearing surface of the hexagon head on a medium-series clearance hole.
What remains to be checked
The value concerns assembly only. The joint must be verified under the service loads: residual clamping force, additional bolt load, fatigue, pressure under the head, embedding and length of thread engagement. With stainless bolts, washers, self-locking nuts or special coatings friction changes considerably. The dedicated service is verification of a bolted joint to VDI 2230.
FAQ
Why do tables give slightly different values?
Many tables in circulation use the formula of the older editions, which treats torsion in the elastic range and gives preloads about 8% lower. The bearing diameter of the head and the assumed friction also differ.
How much does friction matter?
About 90% of the torque is used to overcome friction in the thread and under the head: only the rest becomes preload. That is why a lubricated bolt tightened to the torque meant for a dry bolt can yield.
How accurate is torque-controlled tightening?
With a torque wrench the real preload can vary by about 25% above or below the nominal value. VDI 2230 accounts for this with the tightening factor, about 1.6 for this method.