Craftsman compressor cannot
       drive impact wrench?



Craftsman compressor cannot drive impact wrench is a response to a question asked recently on this site. Though the answer appears to be product specific, the response actually provides information for anyone who has an air compressor that cannot provide enough air to drive any air tool.

The question: Compressor can't drive 1/2" impact gun? Here is the issue. Compressor seems to lack the PSI to drive a craftsman 1/2 Impact Gun. Compressor operates as normal. Compressor shuts down at 140 PSI on tank guage (a few psi off of spec but close enough) and regulator can be adjusted to 120PSI according to regulator gauge.

Regulator adjusted to 90 PSI with tool running but it does not have enough torque to loosen a clean bolt torqued to 110flbs!

This is with the tank full at 90PSI also tested at up to 120PSI I have tested it with a Craftsman, and two CP 1/2 guns with the same results. Also tested with two different hoses and air separator installed and removed. Compressor is a few years old but with very little use and the same goes for the craftsman gun issue has been there since new but never had a need to use the 1/2 drive gun on this compressor much. Any help would be greatly appreciated.

The impact gun specs are:

  • Impact Gun:
  • Craftsman 1/2in
  • 240flbs
  • 90 PSI
  • 4.0 CFM

  • Hose:
  • Goodyear
  • 25ft 3/8"

  • Compressor:
  • Craftsman #919.167320
  • 6/2 HP 33 Gal tank
  • 8.6SCFM @ 40PSI
  • 6.4 SCFM @ 90PSI
  • 150PSI Max

My response: I suspect that the figures for your compressor are optimistic. What is the voltage of your compressor, please? If it's 120 VAC, then I doubt it has the poop to deliver the flow promised.

It seems to me that your compressor has the power with 140 PSI in the tank, but as soon as you start using the impact wrench, the pressure will drop below that, and your wrench won't have enough force.

With the regulator set at 90 PSI, watch the regulator gauge as you hold the trigger on the wrench. What happens to the pressure setting?

Comment by Rich: Thanks Bill... A flow issue makes total sense as the wrench just does not have the force to break the nut lose. Impact action just seems weak even when I tested at higher air pressure.

My question would now be... is this Craftsman compressor so overrated 6.4 SCFM @90PSI (gun requires 4.0 SCFM @ 90) or is there a restriction in the line? Tried two differentnt 25ft 3/8 hoses with same results. Was trying to see if there was an issue with the compressor that could be repaired.

Think moving up to a more efficientnt gun or a larger hose might do the trick? Like I said before I have read reviews of the same model and people have stated that they had no problem taking off lug nuts! (most modern cars are 85 ft-lbs and up) Last ditch effort before I start shopping for a new compressor, rather not as money is tight but already have 220 in the garage so I am ready to go just need to wire up a receptacle. Thanks again for all your help! Cheers, Rich

My response: Rich, the problem may be the flow rate of the regulator. Take your line feed to the gun upstream of the regulator as a test. Yes, you'll be hitting the gun with higher pressure, but also higher flow, and that might do the trick.

If I were using high demand air tools on a smaller compressor, I would probably change out the regulator from a mini-to a full size of the same port. The flow difference is significant and may help. And, If you can get a 1/2" hose for greater air flow - remember, hose is measured on the I.D., that will help too. B.

Comment by Rich: Bill, Yes it's 120 VAC. Optimistic ratings I would bet on it also, but here's what has me stumped! The Craftsman gun is rated at 4.0 SCFM 90 PSI 240 ft-lbs, compressor is rated for 6.4 SCFM at 90 PSI. There is a 10 PSI drop in line pressure when the gun is in use, I adjust up 10 PSI for 90 PSI under load.

Even if the gun and compressor had an optimistic rating I should have a bit of cushion no?

It's a 33 gallon tank so I would think it would have enough flow and pressure (with 140 PSI in the tank and only 10 PSI drop) to break at least one wheel lug nut torqued to 100 ft-lbs. I also tried two similar rated CP guns with the exact same results. I have read several reviews on this model and a few refer to it having no problem taking off lug nuts. Help I am stumped! Cheers, Rich

My response:Rich, pressure without flow equals no force. Your wrench will work with a low flow for easier removals I expect, but I also expect that your compressor cannot deliver the flow fast enough to give the wrench the oomph needed. And, it's not just the tank volume. Air lines tend to reduce flow, and if the line is too small then that is contributing.

You tried other guns, all had the same problem. Either the lug nut is on too tight for the force you have available, or the compressor is too small. I'm guessing it is the latter. B.