29 May 2001

Retorquing—A Screwy Practice

Although "retorquing" as a way of checking tightness is common in many PM procedures, it violates basic mechanical principles and does more harm than good. A screw has maximum clamping power at a torque value specific to its size, shape, and composition. Exceeding that torque value permanently reduces the screw's clamping power by reducing its elasticity and deforming it. Loosening and then retorquing still reduces elasticity, which still means a loss of clamping power. Doing this to a lock washer results in a permanent 50% loss. What should you do? Use an infrared (IR) thermometer to note hot connections. Check their torque. If they've merely worked loose, try retightening them. Note which screws were loose, and be sure to give them an IR check at the next PM cycle. If they're loose again, replace them. Finally, don't forget the "tug test." This checks crimps, as well as screw connections. Don't do this while the drive's online running the process, though, or you may cause some very expensive disturbances.