ABSTRACT
A torque wrench appears to respond to torsional stress levels in the bolt as well as to preload levels. The torque required to restart a nut can be less than that required to tighten it in the first place, even if there has been no loss in preload in the meantime, if torsional stress has disappeared or been reduced. Repeated "torque recovery" can, as a result, gradually increase the tension in a fastener until it is substantially above initial anticipated levels. In one set of measurements on tank tread end connector bolts, for example, we first tightened the 5/8-18 X 1 3/4 Grade 8 bolts with a torque of 150 lb-ft. This stretched the bolt 0.0015 in. This was a tapered joint, so the bolt now relaxed to a stretch of only 0.001 in. We reapplied 150 lb-ft of torque, and the stretch returned to 0.0014 in. Incidentally, it took only 100 lb-ft to restart the nut.
