Now, anyone working in torque knows that this isn’t an easily agreed upon topic.
Certainly, if a torque wrench is wound back at all it should not be adjusted below the minimum scale marking (usually 20% of maximum) - never to zero as this can adversely affect the calibration of the wrench. Beyond that, it really depends on the application. We have thousands of Production Type wrenches in service that are left at their setting for months on end without a problem. Whilst the occasional user should adjust back to the minimum scale setting after use, if you use the wrench in an environment such as a commercial garage, the choice is yours. Either way is acceptable.
However, to make sure we were absolutely 100% certain of this, we carried out tests over the past month. We wanted to ensure that we could answer this long-debated industry question with the upmost accuracy. The test was completed recently, designed to show the effects of leaving a torque wrench wound up at 100% of full scale against one wound down to 20% of full scale. Firstly, four wrenches were taken and the calibration lab took results. Two wrenches were then left wound up and two wound down. After 24 hours the calibration lab took further results, and again after a week, then again after one month. At the end of the month the results showed (bar one single result) that all the readings taken were within the ISO tolerance of 4% of reading, regardless of whether the tool was wound up or down.
So, there you have it. Although considered by some to be good practice, extensive testing has shown that unwinding a Norbar torque wrench to its minimum setting, hence relaxing the spring between uses, has no effect upon the wrench calibration. However, there is a level of considered good practice that should be adhered to when using a torque wrench even if you don’t need to unwind it after use. For a start, torque wrenches have moving parts and can be affected by corrosion and dirt in the mechanism. They should always be kept clean and in a storage box. They should not be dismantled without calibration and adjustment afterwards as dismantling and reassembly will affect the torque values. Further to this, if a torque wrench has not been used for a day, it should be exercised about five times before use to redistribute any grease that had dried up or been squeezed out. For safety, critical or high-volume applications it is common to check the wrench every day or every week.
This check makes sure that the reading is inside the allowable tolerance. It is not the same as a calibration, where the difference between the true value and the wrench value are compared and documented.
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Head of Marketing & Product Development