One needs to understand how the impact wrench works & what the torque stick does before the calibration process makes sense.
Basically, a torque stick is a torsion spring. The more you twist it, the more toque it transmits.
An impact wrench has hammers that strike anvils to produce twist on the output shaft. This twist is only for a few degrees of rotation.
After the desired torque is reached, the energy of the hammers striking the anvils is absorbed in the twisting of the torque stick & not the fastener.
Torque stick sets are sized to apply a given torque based on a common fixed input torque.
If you have a set of torque sticks, AND you 'calibrate' your impact wrench, all you have to do to apply the correct torque is use the right stick.
They are most beneficial to shops that have several different torque requirements for the fasteners they are applying when time is limited.
The impact wrench is usually calibrated by:
- Tighten a nut to the torque that matches the torque stick.
- Use a quality regulator (that has repeatable settings) on the air supply to your impact wrench.
- Using the torque stick, increase air pressure to the impact wrench until the nut just moves.
- Back off the air pressure a little & tighten another nut.
- Check this nut with a torque wrench to verify actual torque.
- Fine tune air pressure as required to get the accuracy desired.
- Record air pressure & impact wrench setting.
- If you use these settings with the same impact wrench & torque stick set, you will enjoy the =/-4% accuracy.
Different impact wrenches may require different air pressure settings.
Different torque stick manufacturers may require different impact wrench settings.
If you apply too much input torque to a torque stick, you will over torque the fastener & may damage the torque stick.
(The tire shop did this to my 1988 420SEL & some of the wheel bolts had to be drilled out after the heads wrung off! I also enjoyed replacing the warped rims & hubs.
(Needless to say, I don't use them anymore.)
I hope all this makes sense.
It is for the above reasons, I chose to NOT spend my $$ on torque sticks. No real benefit in my garage.
For my needs, I can 'calibrate' my impact wrench to know the torque vs air pressure & not have to worry about over torqued fasteners. You still need to be carefull about letting it hammer after the nut stops turning.
AND, I still use a torque wrench when required.