Like HowStuffWorks on Facebook!

What is the best way to perform a torque wrench calibration?

A torque wrench is a tool that measures the tension, or twist, of bolts and nuts to make sure they’re tight enough without over-tightening them. The torque wrench looks like a long-armed socket wrench that bends when you apply turning force to the handle. The handle contains an internal mechanical indicator that is connected to a calibrated scale. When the indicator determines that you reached the desired torque, you know to stop twisting the bolt. It’s important to calibrate the torque wrench properly to make sure the fastener has the exact tension it needs to function safely.

All threaded fasteners, such as bolts and nuts, have appropriate torque values designed by the manufacturer. Under-tightening a fastener doesn’t provide proper thread loading, while overtightening may cause the fastener to weaken and break prematurely. You need to check your torque wrench periodically to make sure that its reading is accurate. You can pay a professional testing lab to do the calibration, or you can assemble a torque wrench calibrator and do the testing yourself.

To perform a torque wrench calibration on your own, you need to determine the calibration ratio and multiply that amount by the required bolt torque. The torque wrench has the option of measuring distance in inch pounds or foot pounds. First, measure the distance from the center point on the wrench head to the point at which you apply the most pressure on the wrench. Then clamp the wrench bit in the vise and hang a 20-pound (9-kilogram) weight from the handle of the wrench with a string. Move the string until it measures 40 foot pounds (or 480 inch pounds.) Next, measure the distance from the center point on the head of the wrench to the string. Divide the second distance by the first to find the calibration ratio. Finally, set the torque wrench to the product of the calibration ratio and the required bolt torque.