Q&A for How to Calibrate a Torque Wrench

Usually with torque wrenches, you need to zero them out. Different manufacturers have different processes for doing this, though. I'd recommend looking at the manufacturer's literature, then follow their instructions on how to zero them out.

The functional length of a torque wrench is the distance from the center of the square drive at the head to the center of the handle or grip, and varies depending on the range of torque the wrench is designed for. The overall physical length will be a little longer and is not necessarily relevant to the function.

Not necessarily. While there is no harm in considering local gravity in your uncertainty equations as required by ISO certification, it is relatively insignificant in terms of torque devices. If your weights are calibrated locally, then you would have no need for a correction.

You will need to take your torque wrench to a professional to be calibrated if you would like a certificate.

No, this is not needed. The resulting torque is found by: TA=(Ft/Lb setting)*(D1/D2) Torque is rotational force and the torque applied is always slightly more than the resulting torque from the work. For instance, you may apply around 17 pounds of force to get 15 ft-lb of torque.

Your torque wrench may require a new spring if it has been left out for too long. This is because when raising the torque spec of the wrench, you are essentially tightening the pull on that spring. Leaving it at a higher level than its lowest torque spec will wear the spring. You can try to take it to someone to have it calibrated; if it can't be done, some stores stock parts for the torque wrenches they sell.

The only way to know if your wrench is accurate is to have it calibrated by a qualified calibration lab or do it yourself as was described above with a vise and weights. Proper calibration, however, requires certified weights. Whether is was calibrated "correctly" depends on whether it was done by a certified Calibration Lab qualified to calibrate torque tools or someone in their garage with whatever weight they have lying around.

If your wrench has an interchangeable head, as is sold by several manufacturers, you can use whatever head they offer for that model wrench. It all depends on the length of the mounting point to the center of the head gear or socket. If you take a head that is 2 inches long and replace it with a head that is 4 inches long, you will significantly alter the output of the wrench and it will not be accurate. If they are the same length from mount to center of socket, regardless of bulk or socket size, you should have no issues.

That is not necessary for several reasons, but primarily, those equations are factored into the wrench by the engineers designing these wrenches as far as spring tension, pawl and such. Also, the weight of the wrench itself is insignificant compared to the tolerances and range of any given wrench. I calibrate several hundred of torque devices for ISO17025 certification each year.

The same method, however, you would read the output displayed after hanging weights rather than listen for a click, since digital torque wrenches do not "break'' like traditional torque wrenches.