[Synchro Joe]

Harry D is right to follow the calibration lab advice. I was a PMEL lab chief in another lifetime and our customers were aeronautical, maintenance, medical, scientific and munitions types of organizations. Almost all of these applications required certified traceability back to a National Standard and regular calibrations and inspections to ensure accuracy.

When a torque wrench is regularly used, it does not need to be wound back after each use. However, when storing a torque wrench for an extended periods, it should always be wound down to the minimum scale setting and never to zero. Adjustable wrenches use springs that must meet a linear capability test. After use, externally adjustable click wrenches should be turned back to the minimum scale value. This helps to preserve the linearity of the spring and calibration of the wrench. Calibrated torque wrenches use a spring with a known linear range. Manufacturers believe that leaving a calibrated wrench at one setting can impact spring linearity. It does not happen every time, but over time linearity can be impacted causing errors. This is the reason for the recommendation to turn a wrench back to its minimum setting when not in use. If adjusted and stored below the lower limit, it can allow the clicker mechanism to shift out of place, requiring the wrench be disassembled to get it repositioned and re-calibrated.

Torque wrenches are normally calibrated at three points: 20%, 60% and 100% of their range) Accuracy of +-4% means that if your tightening to a specification of 100 ft-lb., the torque wrench will click to indicate a proper torque somewhere between 96 and 104 ft-lb.

One accidental drop to the shop floor probably isn’t a big deal, but repeated drops can affect the wrench’s calibration, even if it doesn’t show signs of damage. Professional shops normally have several wrenches and send them in for calibration every six months, annually, or every 5,000 cycles depending upon the required accuracy.

The head of click-type torque wrenches houses the gear and pawl mechanism. Repetitive forces applied to its mechanism leads to the need for regular maintenance and re-calibration.

Due to the simpler torque mechanism, dial-type torque wrenches do not require as much maintenance and re-calibration as the click-type torque wrenches.

Digital torque wrenches are the most expensive but also are very accurate instruments. The more delicate construction requires extra care while handling and of course need batteries in order to operate. Most digital wrenches can be considered 4 torque wrenches in one, due to their ability to set torque values in four scales (ft/lbs, in/lbs, Nm and kg./cm).