Over time, a wrench’s performance degrades due to the wear and tear of industrial use. Many pieces of equipment are subject to vibrations, repetitious movements and pollutants that can have a negative impact on the accuracy of the torque wrench; calibration readjusts it so that the performance it delivers matches the amount of torque indicated on the gauge or screen.
Calibration is a comparison between a measurement of a known magnitude and a measurement taken from the wrench that is being calibrated. The difference is observed and then the technician makes the necessary changes and adjustments before measuring again to ensure that the values are identical. The calibration process is important to many industries and companies because the accurate functioning of their torque wrenches is fundamental to their daily operations.
Since torque wrenches are widely used for the correct tightening of nuts and bolts, a torque wrench or other torque tool that incorrectly displays the force may lead to under-tightening which does not provide the proper thread loading or over-tightening which stretches the fastener beyond its limits, causing it to fail.
Torque wrench calibration may take place in a torque lab or onsite. In a torque laboratory, professional calibrators test, repair and adjust torque wrenches and other tools. They service and calibrate torque wrenches to levels of 0.5% to 1% margin of error. A trained technician performs the calibration by attaching a device that is able to produce a correct reading in contrast to the measurement of the torque wrench.
The technician who performs the calibration usually completes a summary report and certificate to show that the torque wrench has been calibrated to industrial and national standards. Another option is to adjust the tools onsite using a system of weights and distances to “teach” the torque wrench what certain measurements are. That is, when a known weight is applied, the wrench can be adjusted to display that measure which it then will use as a reference.
Many mechanics rig up their own systems using a vise and hanging weights. Because torque is the product of force and distance, a torque wrench can be calibrated using a weight that is accurately measured and a known distance. Calibrations are usually performed for the middle of the torque wrench’s range and they are usually done within 4% of the exact amount.