A torque wrench, as the name suggests, is a wrench used to apply a specific torque to a fastener such as a bolt or a nut. This should be the only thing that a torque wrench is used for.
Do I need to Calibrate Torque Wrench?
The torque wrench is a surprisingly delicate tool prone to losing calibration or even breakage. The weight and length of a torque wrench make it tempting for one to use as a breaker bar or even a hammer, especially if it is in arms reach and your hammer is all the way across the room. This is never to be done, even if you tell yourself that it’s just one time.
One of the most common uses of the torque wrench is to tighten the wheel nuts. If you do not apply sufficient torque the lug nuts can get loosened. On the other hand, over-torquing can damage the nuts and break the bolts.
How Often Should You Calibrate?
The torque wrench can even lose calibration when used with the utmost care. The spring or other internal mechanism can wear or loosen over time from normal usage. Because of this, it is important to check the calibration of your torque wrench often and calibrate it as necessary. At least every 12 months or after 5000 cycles is recommended or after a fall or impact.
You can always take your torque wrench to a reputable calibration technician but if you are the adventurous do-it-yourself type, you can calibrate it yourself.
In this article, I will explain the procedures for calibrating your torque wrench at home or in your shop. No one says no to saving a few bucks and learning a new skill, do they?
Things You Will Need
Before you begin your calibration, you will need the following items:
- A torque wrench (obviously)
- Bench vice
- Known weight (about 20 to 30kg will suffice)
- A string (to hang the weight)
- Marker
- Tape measure
- Screwdriver or hex key
Checking Your Torque Wrench Accuracy
How do I know if my torque wrench is accurate? The answer is you should check it with weights. Here are the steps you should follow.
Step 1. Measure and mark the length to your reference point
Your first step is to measure from the center of the torque wrench drive to a point on the handle. Make your measurement in inches and for easy calculations, use a whole number. Use the marker to mark the point where you measured on the handle. Write down this measurement as you will need it for your upcoming calculations.
Step 2. Calculate your expected torque
It’s time to awaken the scientist within for some nuclear level calculations. Just kidding, it’s way easier. One wrongly torqued bolt, in theory, could cause a NASA rocket to fail so what we’re doing is important.
The most popular torque wrench measurements are Newton-meter (Nm) and foot-pound (ft-lb). We’ll stick with foot-pound for easier calculations but you can always convert your answers to Nm if that’s what your torque wrench reads. You can choose the long way of conversion using a table or formulas or you can just Google it; your choice.
As an example, let’s say you’re using a 20lb weight and you measured 18 inches from the drive to your marked point on the wrench.
You expected torque will be: (20lb x 18”)/12 = 30ft-lb
You will now set your torque wrench to 30ft-lb or whatever result you arrived at using your specific weight and distance.
Step 3. Secure your torque wrench in the vice
Now, with your torque wrench set, secure it in the vice.
Lock the drive of the torque wrench in the vice so that the handle of the wrench is in a horizontal position and hanging away from your workbench. There should be enough clearance to the sides and below the wrench handle so that you can freely hang the weight from it.
Be careful not to damage your torque wrench drive when locking it in the vice. If necessary, use a soft metal such as aluminum between the vice jaws and the drive so as not to cause any damage.
Also, ensure that your wrench handle is perfectly horizontal as any other angle will change your readings.
Step 4. Test calibration by hanging the weight
Now, use your string to hang the weight freely on the torque wrench at the exact spot that you marked. What happened? Did it click?
If it didn’t click, you’ll have to move on to the calibration instructions.
If it did click, we have some more testing to do. Lift the weight and place it further up the handle in the direction of the drive. You can start with about a 1 cm interval. If it still clicked, move on to the calibration instructions. If it didn’t click, move the weight slightly toward your marked point until it clicks.
Depending on the precision requirements of your job, you may be able to work with a torque wrench with a couple of millimeters tolerance and can, therefore, consider you torque wrench to be calibrated.
How to Calibrate a Torque Wrench
If you’ve moved on to this step, it means that you did not get a click when you hung your weight from the marked point on your torque wrench or it was clicking even when you moved your weight closer to the drive. Either way, it needs calibration.
Correcting High Reading
If the wrench didn’t click, look for the calibration screw about midway the wrench shaft. Tighten the screw at increments and lift and lower the weight each time until it clicks.
When it does click, loosen the screw ever so slightly until it stops clicking, then tighten at very small intervals to get the exact spot at which the wrench starts to click. This is your calibration point.
For those who are not seeing a screw, your wrench may have a lock nut and an adjustment screw at the top of the handle. You will follow almost the same procedure except that you will have to lock the nut each time you adjust the screw before testing.
Correcting Low Reading Errors
If the wrench clicks and keeps clicking when you move the weight towards the drive, move the weight back to the marked point. You will then loosen the screw at intervals while lifting and lowering the weight until it stops clicking.
When it stops clicking, start to tighten the screw slightly while testing with the weights until it starts to click again. Feel free to repeat the process at smaller and smaller intervals until you are satisfied with its accuracy.
Now that you have successfully completed your calibration, remember to take care of this delicate precision tool. Always unwind it to its lowest reading when you are finished using it. Clean it up and put it back in its case if this is available. Otherwise, find a safe place to put it that is out of harm’s way.
Your torque wrench is a spoiled child in your tool set so remember to treat it as such.
Dan Miller is a professional toolmaker with extensive experience in the metalworking and tooling industry.
An avid DIY enthusiast and passionate hobbyist woodworker, Dan shares his expert knowledge here.