Torque wrench calibration

My torque wrench is nearly 15 years old and had never been calibrated since manufacturing.

Youtube was a great help in understanding how a simple job it is to do. The maths is easy as well.

The idea is to apply a constant known force at the end of the lever.

That constant force is provided by a heavy weight that will hang from the handle. I have used 3 counterweights from my telescope, weighing 5.1kg each. That's 15.3kg total.

Conversion to Newtons is just a matter of mutiplying the weight by "g", earth's standard gravity of 9.81 m/s².  The counterweights will thus apply a vertical force equal to 15.3 x 9.81 = 150.093 N

As the groove in the handle is 39.5 cm (=0.395m) from the center of rotation, the resulting torque will be 150N x 0.395m = 59.29 Nm.  -This when the lever is horizontal.-
Here the angle was minimal and no correction was really needed (basic trigonometry would take care of that) and we didn't account for force applied by the wrench weight anyway. The error is likely in the same order of magnitude, and compensating for the slight angle. I'm not crying, you're crying.

59 Nm is perfect as it's right in the middle of the wrench rating. Setting it too low wouldn't be as accurate.

You begin by setting the torque of the wrench to the previously calculated value.

As it's difficult to lock the wrench and apply that kind of force, the easiest solution was to do it on the car. This is ok because the lug nuts are torqued to 85Nm and we are working well below that. - I nearly broke my workbench when testing the procedure in my vise with just 5kg.

The adjustment is made with a hex screw that is covered by a small transparent plastic cap on the side of the lever.

I proceeded by feeling how far the setting was by moving the force (the rope) a few centimeters up and down along the handle. Then set the spring tension with the screw in order to have the ratchet click when the force was applied to the groove at 39.5cm from the center of rotation.
I didn't adjust the screw under tension, but did remove the weight from the wrench a few times for adjustment, then put it back for the calibration check.

My wrench was actually quite alright, maybe 1% too low in spite of my lack of care and letting it set to 85Nm for years on. It is advised to release the stress by setting it low before storage.