Torque Wrench Accuracy + Calibration
Techniques for Accuracy
Torque wrenches are only as accurate as your ability to use them. Too many people focus just on the factory calibration. There’s no doubt that proper calibration is important, but, in reality, your own technique often has a bigger effect on accuracy. Focusing solely on factory calibration is a bit like worrying only whether your fishing line can hold a 30 pound trout. The equipment isn’t often the limiting factor.
4 Tips for Maximizing Accuracy When Using Your Tekton Torque Wrench
1. Look carefully. You have to set the torque wrench with your own eyes. If you accidentally set the dial at 98 or 102 when you mean to set it at 100, you have introduced extra error. This can happen if you are looking at the scale at an angle.
2. Turn the torque wrench smoothly. You have to turn the the torque wrench slowly and steadily, stopping when you hear or feel the click (when using a click-type torque wrench, that is). Turning the fastener past the click means you have applied extra, unintended clamping load. Swinging the torque wrench rapidly means you might swing right past the click.
3. Pay attention to the conditions under which torque is measured. If the specifications indicate a 120 ft-lb. torque on a dry fastener, don’t expect to achieve highly precise measurements when your fastener is greasy or unusually hot, for example. Depending on the conditions, you might still get quite close. You will have to judge whether close is adequate for your purposes. Over-torquing can result in broken parts and early deterioration. Under-torquing can result in loose parts, which can fail or come apart unexpectedly.
4. Push on the handle, not anywhere on the wrench body. The torque wrench measures force at the pivot point just below its head. It has been calibrated so that this force accurately corresponds to the amount of torque shown on its scale only when you apply force on the handle and nowhere else.
The Science Behind Measuring Torque
Torque is rotational force—the force you exert when swinging a bat, pushing a merry-go-round, or closing a door, for example. When you turn a fastener (say, a screw connecting two pieces of wood or a lug nut attaching a wheel to your car), you use torque, or rotational force. As it turns, the fastener compresses the materials being fastened. They push back on the fastener like a spring and can even stretch it. These forces increase friction and make the fastener harder to turn. Torque wrenches measure fastener tightness—how much force it takes to keep turning the fastener against rising friction. With this measurement, you can clamp down a fastener with the right amount of pressure or load.
But there is more to understanding whether your torque readings are accurate. The torque wrench only measures how much rotational force it takes to turn the fastener, not the clamping load itself. So the torque wrench will only achieve the clamping load you expect if you are turning a fastener under the conditions that the manufacturer assumed when setting the torque specification. In general, the amount of friction depends on multiple factors including surface coatings, temperature, materials, and fastener shape and size. If those factors vary unexpectedly, you may get unintended results that are outside the calibrated accuracy range of the torque wrench.
For example, if you are working with bolts covered in grease, but your torque specification is for dry metal bolts, you can assume you’ll overtighten the fastener. With grease on the threads, the fastener will slip more easily against the material in which it is turning. It won’t feel tight enough even as it is clamping down with greater and greater load as it turns.
Tekton’s torque wrenches are each calibrated at three points: 20%, 60%, and 100% of their range. Accuracy of +-4% means that if your fastener should be tightened to a specification of 100 ft-lb., the torque wrench will click to indicate a proper torque somewhere between 96 and 104 ft-lb. This accuracy is typical of high-quality torque wrenches and is more than sufficient for normal uses.
What If I Think My Torque Wrench Is Out Of Calibration?
If you need to know for sure whether your torque wrench is calibrated correctly, you should take it to a professional or use well-maintained calibration equipment. However, here’s a quick check you can do yourself:
1. Get a large fixed weight (such as a 45 lb. steel plate) and a way of hanging it on the end of your torque wrench (such as a rope). Add up the weight of the objects you’ll be hanging on the torque wrench (for example, total weight = 46 lb. if the steel plate weighs 45 lb. and the rope weighs 1 lb.)
2. Measure the distance in inches from the center of the torque wrench handle to the drive tang. Divide by 12 to get the number of feet—for example, 21 in. = 1.75 ft.
3. Set the torque wrench to the total weight of the objects you will hang on it multiplied by the number of feet. In this example, 46 lb. * 1.75 ft. = 80.5 ft-lb. of torque.
4. Fix the drive end on a fastener or other object that won’t move so that the torque wrench is parallel to level ground. You can use a level to make sure the wrench is really parallel.
5. Hang the weight from the center of the handle. Don’t let the weight touch the ground.
6. Your torque wrench is accurate if it clicks when set to the same torque as you are generating, as long as it doesn't click when less torque is applied. You can check this last part by moving the weight in slightly toward the drive, which will reduce the torque. The torque wrench shouldn’t click when the weight is moved closer. If it does, it is under-measuring the torque, meaning that your fastener won't be tight enough.