A shop might do that for a head bolt, but not for a lug nut
Many shops use a “torque stick” to prevent over tightening. In my experience, it does no good to use it if you let the air gun keep hammering at it.
@“oldtimer 11” 11
Are you saying torque sticks don’t work?
I believe the replies saying cordless is more powerful than a corded, but it puzzles me. Is it because the cordless battery can deliver a larger surge of current than the corded?
Yes, a battery is capable of very large surge current due to it’s relatively low internal resistance. electronics not designed for battery power can often fry the protective fusing due to larger than expected in-rush current capability if a battery is used to power them.
But that’s only part of the story. These battery operated tools often use a switch mode power supply to feed a DC motor and they are very efficient at converting the energy with very little waste. High (relatively) frequency switching and pulse width modulation combine to provide superior capability and efficiency over the conventional brushed motor driven by AC power. Hear that high pitched squeal at partial speed? Imagine a bucket brigade- one moving at 60 large buckets a second and another with smaller buckets but at a rate of 18,000 per second…a simplistic analogy but gives you the basic idea…
I bought a corded impact wrench when I was much younger and looking to get started. Used it twice and has sat in a drawer since. Way too bulky to be used for anything but lug nuts and it lacked any real power…
Is there a reason mechanics use air impact drivers instead of electric ones?
I use both.
The cordless impact gun is for lighter duty removal/installation of fasteners.
The pneumatic impact gun is for higher torque requirements.
For example, you’re not going to remove the crankshaft bolt from a Honda with a cordless impact gun.
Tester
I’m finding this thread interesting because I’ve never used an electric impact wrench and often wondered if they’re any good. Sounds like they’re okay for limited applications.
Re: the torque sticks, I don’t trust them, and I agree that if you let the gun keep hammering on them you’ll overtorque the fastener. They work by using a spring steel alloy made and treated to let loose like a spring at a specific torque. But they don’t prevent the operator from continuing to twist, even with the metal having let go. I just don’t trust them.
For many applications, especially where torque is critical, I prefer a “beam type” torque wrench. I like them 'cause I can see exactly what’s happening. For some applications I use a “click-stop”. I realize that they’re as susceptible to misuse as the torque sticks, but for changing a wheel you can feel confident even on a cold, dark, rainy night with no streetlights around. Can’t do that with a beam-type torque wrench… too easy to misread in low-light situations. I keep a click-stop with a deep socket in my trunk preset to the proper lug nut torque. I’m the only driver of the car, so I know it’ll always be there and properly set when I get a flat tire.
A lot of this is actually just preference. If you’re using any type of torque wrenches, you’re already using far better practices than a whole lot of people. 
@“the same mountainbike”
You do realize you never store a torque wrench without zeroing it out?
Doing that knocks the calibration off.
By the way?
When was the last time your torque wrench was cal/cert’d?
I’m sure it needs it if you’ve been storing it preset.
Tester
Yup, and it’s still far, far, far better than just guessing.
I’m not using it to build an assembly for the space shuttle. It doesn’t comply with NASA5300.4. Or even MIL-STD-55110.
I’m using it to put lug nuts on. When you get a flat, what do YOU use to put your spare on?
I use the lug wrench that came with the vehicle.
Then pull the vehicle into the shop and then check torque with my torque wrench.
Tester
You use the lug wrench that came with the vehicle to install the lug nuts and you’re criticizing ME for the torque wrench in my trunk not being stored and calibrated IAW MIL-STD-44662?
Gimme a break.
http://www.mountztorque.com/learning-center/guide/preventive-maintenance-torque-wrench
Just sayin’
Tester
Is there a reason mechanics use air impact drivers instead of electric ones?
One practical reason, everything else being equal, I’d guess the air version weighs less, so isn’t as tiring for the mechanics-staff to use.
No George.
Mechanics prefer cordless impacts because they weigh the same as a pneumatic impact, but you don’t have to fight with an air hose to get into tight places.
Besides, a cordless impact will remove a majority of the fasteners that are found on a vehicle.
Tester
Tester , you seem to be answering a different question than the OP posed. But I can see your point that a cordless version would be easier due to improved access.
There’s a pretty good comparison summary on the web, link below. OP should look at the second to last paragraph.
I bought a Kawasaki cordless impact wrench a few years ago. When the battery is charged…it has no problem removing any lug-nut. Very powerful. I almost bought a pneumatic and compressor…but I really had no other need for the compressor except to blow up basketballs.
Tester, I don’t need your lecture. End of debate.
I’m just educating people on what NOT to do with a torque wrench.
Tester
You haven’t taught anybody anything. You just think you have.
Now let’s move on.