Calibrate a Torque Wrench
Having reliable tools in your box is very important. Certain tools needs special care and adjustment. Proper torque wrench calibration by a professional should be done every year or so, but, in a pinch, there is a way to calibrate one yourself. This article will tell you how to calibrate a torque wrench using this method.
Contents
Steps
Weight calibration
- Mark the back of the wrench at the center of the drive of the wrench.
- Measure from that mark to where you would position your hand while using the tool and make a second mark (or a line). Determine the distance between the two marks.
- Secure the square head in a bench vise, making sure no other part of the wrench touches the vise. Move the handle to a horizontal position.
- Align the torque value to the setting of the distance you got in step 2 x 20 pounds (9.07 kilograms).
- Dangle a 20 pound (9.07 kilogram) weight from marks you drew in steps 1 and 2.
- If a click from the tool is heard, lift the weight and slowly move it toward the wrench head until the clicking stops. Make a tentative mark and repeat the step to make sure the spot is correct.
- If there is no click at first, move the weight away from the head of the wrench until you hear a click. Make a tentative mark and repeat to make sure the spot is correct.
- You can make your mark more definitive after two or more checks land in the same spot.
- Measure the distance between the square head and the click mark you just made. This is the other number you will need for your calibration equation. To find the true amount of torque, multiply the distance by 20 pounds (9.07 kilograms).
- Use the formula Ta=Ts x(D1/D2). Plug your numbers into this equation, remembering that Ta is torque applied, Ts is your torque setting, D1 stands for the distance you found in step 2 and D2 stands for the last distance you found.
- Check your math several times and adjust your wrench accordingly.
- Note that the distance that matters is the distance from the center of the drive head to the place you hang the weight. It has nothing to do with where you usually put your hand. It is foot pounds. The feet refers to the moment arm. In this case, the moment arm is the distance from the center of the drive head to the place you hang the weight.
- So, if you hang the weight {{safesubst:#invoke:convert|convert}} from the drive head center line and hang a 20# weight you are applying 20 pounds x {{safesubst:#invoke:convert|convert}} to the drive head or {{safesubst:#invoke:convert|convert}} pounds.
- If you hang the weight 6" from the center line of the head, you would have {{safesubst:#invoke:convert|convert}} pounds ( 20 pounds x {{safesubst:#invoke:convert|convert}} = {{safesubst:#invoke:convert|convert}} pounds. The wrench handle should be parallel to the floor when you do this but for more exact measurements, account for the extra weight of the pivoting wrench handle itself, measured from the hang point. This would typically be ~1 lb for inch pound size wrenches and ~ 2 lbs for foot lb size wrenches if you don't have a weigh scale.
Fish scale calibration
- Secure the wrench drive in a vise.
- Attach a fish scale one foot from center of the drive.
- Determine the weight of the pull on the scale for a specific wrench setting.
- Calculate the percentage of error.
- Re-do the preceding steps at a different wrench setting to determine the consistency of the error rate.
- Apply the percentage of error to the the scale on the wrench.
Tips
- Remember to lift the weight off the wrench's handle when finding and double checking the click spot to ensure the spot's accuracy.
- If you have serious doubts about your ability to calibrate your tool this way, send it to a professional shop. The shop will have the right equipment and personnel to calibrate it correctly.
- The weight you are using must be exactly 20 pounds (9.07 kilograms).
Things You'll Need
- Bench clamp or vise
- An object that weighs 20 pounds (9.07 kilograms)
- Thin rope or twine
- Measuring tape
- Marker
- Calculator or paper and pencil (optional)
