How a torque stick works

Torque sticks work by twisting during the hammer phase and then springing back between hammer blows

Their torque value is calculated based on the diameter and length of the shaft. So torque sticks that are designed to tighten to a higher torque usually have larger diameter shafts or shorter shafts.

An impact wrench doesn’t deliver torque like a drill

Squeeze the trigger on a drill and you get continual torque. But an air or battery impact delivers pulses of torque. The sticks work because they apply torque to the lug nut during the torque pulse. If the applied torque exceeds the stick’s rated torque, the shaft twists to absorb the additional and unwanted torque. In-between torque pulses, the stick springs back (untwists)   to its original shape.

Here’s an example: A Chicago Pneumatic CP726 1/2″ Pneumatic Impact Wrench lists its minimum and maximum working torque* at 67.9 ft-lb and 190 ft-lbs with 1,680 blows per minute (28 blows per second). If the impact wrench is set at its lowest setting and the user installs a torque stick rated at 85 ft-lbs, the impact wrench will not properly torque the lug nut. On the flip side, if the impact wrench is set to maximum power, it’s possible to over-torque the lug nut or damage the stick. That’s why every impact wrench must be calibrated to each torque stick.

Let’s assume that the technician has calibrated their impact wrench for all their sticks. They set their wrench to deliver approximately 90 ft-lbs of torque to a stick rated at 85 ft-lbs. The wrench applies torque 28 times per second and transfer the torque to the lug nut until the lug nut reaches 85 ft-lbs. At that point the stick shaft twists slightly. But there’s lag time in-between each blow and that’s when the shaft untwists and relaxes.

Use the same setup but with the wrench set at maximum power and the shaft simply can’t twist enough to absorb the 105 ft-lbs difference (190 ft-lbs maximum torque of the wrench minus 85 ft-lbs of the lug nut). What happens in this case? The lug nut is over-tightened and the stick can be permanently damaged.

How torque sticks are calibrated

To calibrate, a technician must adjust the torque setting and air pressure to the impact wrench and tighten the lug nut with the impact, wait at least 15 minutes for the lug nut to cool, and then loosen it with a torque wrench to measure the force needed to loosen the nut.

* working torque is the maximum torque which can be achieved by tightening nut/bolt combination over a period of 15 seconds.

©, 2023 Rick Muscoplat