Thanks for the definition bellydoc!
Ok NEW QUESTION FOLKS!!!:
I found the wrench i want, it's a 1/2" with 700 ft/lbs of torque, and
"5.5 average SCFM at 90 psi".
Now i have to find an air compressor that is up to the job. Do i have to get a compressor with the same or more SCFM? Or is that irrelevant?
Thanks!
Get the biggest baddest compressor that you can possibly afford. Whatever it's capability, you'll eventually max it.
The concept is that the compressor has a certain amount of oomph. You can get it to put out more flow at a lower pressure and less flow at a higher pressure... but the maximum amount of work it can do is pretty much fixed. The compressor will be rated for a certain flow rate in SCFM (standardized cubic feet per minute) at one pressure (like 60 PSI) and a lesser flow rate
at a higher pressure (for example, 90 PSI).
Your impact wrench is rated to get to it's top torque after it hammers away under the sustained force of air arriving at 90 PSI going at least 5.5 CFM. If it doesn't get that, it'll still work, but not as hard. If your compressor can do 90 PSI at a lesser flow, then you'll deplete your volume tank and your wrench will start to lose power. You'll have to wait in order to get it cranking again. The bigger the volume tank, the longer you can exceed your compressors output before you end up losing motive force. In theory, you could fill a huge tank with a bicycle pump and work for a while on the compressed air... but there's obviously no way you'd be able to keep up for sustained use.
I have a 2 HP, 150 PSI rated compressor and a 33 gallon tank. It's a Sears Craftsman oil-free 1 stage compressor. It's rated for 8.6 SCFM at 60 PSI and 6.4 SCFM at 90 PSI. I have the regulator set to keep it at about 125 PSI because that's what my plasma cutter thrives on. It runs air tools just fine, including an impact wrench, an air ratchet and my air-hydraulic tube bender. It also does blow-gun cleaning duty and tire inflations. That's about everything I use it for.
It's loud. Let me rephrase that. It's WAY TOO LOUD. I ended up building a big particle board cabinet to hide it inside, and I lined it with egg crate foam and carpet scraps like a garage band recording studio. I cut air holes in the bottom and top to let the heat and air in, up and through. It's still a dominating hum when it fires up, but I took the sharpness out of the sound.
I sometimes use powertools late into the night and I've tested the insulating properties of my garage walls and door. I can barely hear the bench grinder eating metal from the sidewalk in front of my driveway, when the garage door is down. From across the street it sounds like it might as well be a mile away. I can hear the harsher squeal of the angle grinder or the drill press hogging metal from across the street, but it's not loud. I expect that it's almost too faint to hear from inside anyone else's home.
The thrumming bass note of the compressor makes it down the street, and I'm sure it can be heard from inside many neighboring houses. I've listened to it from out on the street, standing a couple of houses down. If I didn't know where it was coming from, I might not be able to point it out exactly, but it would take about 30 seconds of investigation to figure it out.
If I have a neighbor that gets bent out of shape about my hobby work, it'll more likely be from the compressor sound than from the grinder.
I'm told that the 2 stage compressors which use oil are substantially quieter. I will likely go with one of those designs once I manage to kill this one.