The Experts: How to determine a suitable tightening torque

Q:  I don’t know the tightening accuracy of my tool. How should I proceed with my torque calculation?

A:  All tightening tools have a certain inaccuracy that must be accounted for in order to determine a suitable tightening torque.

The accuracy of a tightening tool can generally be obtained from the manufacturer or retailer. However, there are standards that categorize tightening tools and their accuracy level including the tool operator. If you do not know the tightening accuracy of your tool, the values in the table could be applicable.

It is also important to remember that friction variance influences the final preload. It often has a greater impact on the final preload than the tool inaccuracy. A cost effective way for an accurate tightening process is therefore to properly lubricate the fasteners.

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