Static and Dynamic friction. As the bolt is tensioned, two surfaces are
sliding against each other. The dynamic friction coefficient determines the
force on the torque wrench while sliding occurs. But, as the operator gets near
the target tension, he may stop and restart by repositioning the wrench. This
allows the much higher static friction to cause the operator to think the bolt
has already reached the target torque.
Surface irregularities. At the microscopic level, surface irregularities
cause changes in the friction coefficient. The changes may be uniform or more
pronounced at certain torque positions.
Lubricants. Lubricants are generally used to get more consistent
readings. Again, the surface irregularities at the microscopic level cause
lubricants to help some irregularities better than others. A more consistent reading
(more precision) may also be more consistently wrong. Repeatability does not
4. Operator mechanics. The operator may not keep the torque wrench at a
perfect 90 degrees to the surface during torqueing.
Bolt threads. At the microscopic level the threads do not load up evenly.
The same problem with surface irregularities applies to each thread that is in
contact with its mating surface.
Calibration. Each bolt/nut pair is different. Unless each bolt/nut
combination is individually calibrated using the same lubricant, the
repeatability of the final tension varies.
Side load tension. If the bolt has any side load tension, the tension
profile will not be uniform. One side of the bolt will be in more tension than the other
many more. Some engineers claim over 70 factors that affect torque. See:
notes for additional confirmation and further errors.
Additionally the reader should visit the Twining Labs and Bolt Science