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Torque wrench calibration

  1. Jan 25, 2013 #1

    I hope this is in the correct section, please feel free to move it or advise me to move if it's in the incorrect section.

    I work as an aerospace engineer and a sideline of what i do is to maintain the torque wrenches we use on the equipment. We have a set schedule for maintenance every 3 months on where we test them with a 10% tolerance using a accratorque rig.

    We get told like im sure most others do, that you should return the torque wrench to its lowest setting to prevent the wrench from becoming inaccurate. However, ive recently left some of the wrenches set at their in use settings, some at 50-75% of their max setting. Ive found that upon testing them at the 3 month intervals they showed very little discrepancies just like those returned to the low settings.

    Im curious what is the theory behind winding them back to zero? And can anyone explain why there was no difference when i left them set?

  2. jcsd
  3. Jan 25, 2013 #2
    It depends on the type of wrench, but in general they use springs, for their relatively consistent spring constant, to set the torque. By leaving the spring compressed to say, 50-60% of it's maximum rated setting, you are leaving that spring in compression for an extended period of time.

    Springs wear out, and are subject to creep just like any other material. Leaving the spring in compression for extended periods of time (or numerous different times) will lead to changes in the springs characteristics. This will eventually cause it to become innacurate, and likely very difficult to even calibrate properly.

    It's not going to happen instantly, as you've seen after leaving yours set to torque, but these are "precision" tools and you want to take care of them.
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