DIY Torque Wrench Calibration How To

If you use your torque wrench once a week then you should have a torque wrench calibration perfomed once a year. Maybe you can go a little longer if you are really careful about keeping it super clean and adjusting the torque back to the lowest setting or increment on the barrel of the wrench after last use.

A torque wrench is a precision instrument and the slightest amount of dirt may throw off the calibration enough to break a stud, strip a thread or leave a nut dangeroulsy loose. Experts recommend "prepping" your torque wrench each time before you use. Here's how. Set the torque to the middle of the range. Put the socket on a nut and twist the wrench with constant pressure until it clicks 5 or 6 times. That's it. Now you can adjust it to the correct setting and go to work.

No question about it. You want your torque wrench to be as accurate as possible. Manufacturers say that a 4% error in actual torque applications is the maximum acceptable. Perhaps no greater day-to-day accuracy is obtainable because of inconsistent or uncalibrated friction between the fastener and its thread. Nevertheless, you need a fairly sophisticated measuring instrument to make sure you stay within that tolerance.

If you are wrenching on an expensive car or motorcycle engine you want to be spot on with the torque specifications for every nut and bolt you turn. You can cost yourself a ton of heartache and a boat load of money by turning your wrench too tight and then adding a few more twists for insurance. Overtightened nuts are more likely to fail than propely torqued fasteners. Under-torqued nuts or bolts can vibrate loose or loosen prematurely from the constant heat and cool cycle.

We'll show you a way to get a backyard calibration below but we strongly recommend that you go with professional torque wrench calibration when safety might become an issue for your application.

This is the simplest method of DIY torque wrench calibration in that it requires the least amount of extra parts and the smallest amount of math. There's no way to get around doing the math.

Let's cover the math first. It's actually pretty simple. This formula is for the torque of a lever; a pry bar or a pipe. Something straight and not spinning.

The formula for torque (T) is Force (Weight) x Distance or T = FxD

The math gets a bit tricky when we have to factor in the Center of Mass of the lever but not too much.

Let's say your click-type torque wrench is adjustable from 10-80 Ft. Pounds. It is 12 inches long and weighs 2 lbs. You want to calibrate the wrench in the middle of the range, or at 35 pounds. Prep the wrench like we recommended above and set the torque to exactly 35 pounds.

Clamp the square drive of the torque wrench in a vise. Make sure the handle is perfectly horizontal when you're done with this part. The next step is to find a pipe that fits snugly over the handle. We are using the pipe as a lever. We can measure the weight and length of the pipe to find out how much torque a given length pipe will apply to the square drive.

Let's assume that we can come up with a 5.5 foot pipe that weighs 10 pounds. After you slip the pipe over the handle measure the distance from the square drive to the end of the pipe. Say it measures 6 feet even.

How much torque is our set up making? Here is where the Center of Mass comes in. We will assume that the center of mass for our set-up is right in the middle or 1/2 the length or 3 feet.

Multiply Weight x Length to find Torque. Be sure to add in the weight of the torque wrench itself. Here's the formula: (2 + 10) x (6/2) = 36 Ft. Pounds.

You can vary the length of the pipe to suit you needs. You can also hang a weight at the end of the pipe. Do the basic calulation and then add the torque of the added weight. For example, if you added a 2 pound weight to the end of the pipe the formula for the added weight would 2 lbs. x 6 Feet = 12 Ft. pounds of torque.

Use a bubble level if you have one. You want the pipe to be perfectly horizontal. Even the slightest angle will throw off the torque measurement and complicate the math beyond practical use.