The ISO 6789 standard calls for manual torque screwdrivers to be within +/- 6% accuracy. Although the tool sales rep and company materials may have touted +/- 6% accuracy, is that the real output when the bit hits the slot? The answer in most cases is no. How could that possibly be? The answer is simple. The crux of the problem is that down thrust must be applied to the screwdriver in order to keep the bit in the screw slot and turn the fastener. The down thrust that is used to keep the bit in the screw slot is what changes the output and repeatability on most torque. HOW MUCH DOES OUTPUT CHANGE? The design and materials have a significant impact on how much output variance is in each screwdriver brand. As you do the testing yourself you will be able to see the impact for your self. During testing, some preset +/- 6% torque screwdrivers click over at +/- 18% when down thrust is applied. Some preset screwdrivers click over at +/- 26%. The results are all over the board based on brand and the amount of down thrust that is applied to the screwdriver. WHAT ABOUT REPEATABILITY? You bought your torque screwdrivers for both accuracy And repeatability. As you put your torque screwdrivers on the tester and add down thrust you will see the variance in output. You will also see the dramatic swings in repeatability. Not all torque screwdrivers lose accuracy and repeatability. Some have output that remains relatively stable. Some actually improve both accuracy and repeatability with the application of down thrust. TEST PROCEDURES TO DETERMINE ACTUAL OUTPUT You will find the process of determining actual output of torque screwdrivers as easy as it is eye opening. 1. Exercise the screwdriver at least three times at the full capacity to ensure the screwdriver is fully functional. Not exercising any torque tool prior to testing is the most common torque-testing mistake. For more information go to the Newton Metre Channel on YouTube to view the video “The Most Common Torque Testing Mistake.” https://www.youtube.com/ watch?v=ttSYpwxFrDo 2. Test the screwdriver 15 times with no load. Record the results. 3. With that same screwdriver, add ten (10) pounds of weight to the screwdriver. A simple calibration weight works fine. 4. Test the screwdriver 15 times with the ten-pound weight, recording the results. 5. Change the additional weight from ten pounds to twenty (20) pounds. 6. Test that same screwdriver 15 times with the twentypound weight, recording the results.
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