Air tools are a new thing to me. At this point all I own is a brad nailer which I have not used in anger.
I have been looking at the general principles guiding air tool use and there are questions that I have not found satisfactory answer to:
1) Air tools are usually given specs including operating pressure (often 90 psi) and air flow (in CFM). If the available compressor is not rated for the given flow "the tool will not perform properly". How is this manifested? E.g. a grinder: Will it turn slower? Less torque? Both? Will it not turn at all? Will it keep turning till the operating pressure drops to some level and it stops, i.e is it an on-off thing?
2) Lowering the compressor operating pressure should increase the available flow. What does lowering the operating pressure do to the air-tool? I am looking at pressure x flow as similar to current x voltage but I am not sure this analogy holds in this application. If the electrical analogy holds I cannot see the air tool "suck" more air because the pressure dropped, in fact I would intuitively assume *less* flow.
3) In practical terms, if a tool requiring "average" 3 CFM is operated from a 2.7 CFM @90 psi will it just underperform all the time or will it require a shorter duty cycle (and when "on" it will run at full power)?
At this point I do not see much use for air tools in my shop but one never knows.
Generally, you need both the rated pressure and rated flow to get reasonable performance out of an air tool. The pressure feeding an air motor primarily influences torque, since an increase in pressure increases the force on the vanes inside the turbine. For an air drill, for example, not enough torque and the tool will stall. Flow rate in an air motor primarily influences speed since it is the amount of air flowing through the tool. In a tool, this directly correlates to speed, which is also important to get any sort of useful work out of an air tool. Many air tools have a fair amount of internal resistance and the valving is dependent on the components spinning, so if you try to start them under load or the pressure is too low, air will simply flow around the vanes and the tool will not spin. One way to boost your compressor's capabilities a bit to use tools that are just a bit too large is to connect one or more air storage tanks downstream from your compressor. You will still have to take breaks to allow the compressor to catch up, but the buffer will help you, for example, to run a 3cfm tool on a 2.5cfm compressor. Don't forget that your compressor has a duty cycle (usually under 50%) so even if you can make it work by running the compressor continously, you and the compressor won't be very happy afterwards.
You can run any tool off a tanked compressor at full power. The tank represents a near-infinite CFM source. The duty cycle is the limitation. If you put an air-hungry tool like a grinder on a small compressor, the tank drains in a matter of seconds.
Beware that tools are typically way underrated for air consumption and compressors overrated for air production.
It all depends on the tool. Yes, they do go up and down in performance as pressure in the tank does. The trick is to buy a compressor with a tank so big that the tool can never get ahead of it. Air tools are very useful. Whenever I use mine, they save a lot of time and work.
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Seat of the pants answer for you is that my compressor kicks on when tank pressure drops below about 90. When it kicks on I stop working until it kicks off. Rated CFM duration is based on tank volume and compressor volume and speed. Also possibly by pressure switch settings, but a small compressor is not going to be able to fill the tank faster than you can empty it.
You can run a 1/2 impact on a one gallon compressor if it exceeds 90 PSI, but only at full power for a few seconds. I am sure one of these very knowledgeable guys will give you a more complete answer.
I have no problems with impacts and air wrenches. With air grinders and air sanders I can work about 30-40% duty cycle. With the impacts you rarely work at close to that duty cycle so you never notice. Oddly enough I never seem to have an issue with paint sprayers. They tend to be used continuous duty, but they operate at much lower pressure for most of the paints I have sprayed. (Just remember a water separator)
One of the big reasons we have some of the air tools on the FD rigs is because you can use them in and under water. Hard to use a battery or electric sawzall that way. You can also use them in environments where gas/oil are around with MUCH less danger than other tools.
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-- Happiness is not a station you arrive at, but a manner of traveling. -- Margaret Lee Runbeck
The sizing of the hose to the air tool being used is very important. Try running a one inch impact gun with a three eighths hose and you immediately get the idea. Just like running a two hp motor on a pair of #24 wires.
I have used air tools for many years and like them better than electric tools. The are lighter, no chance of getting shocked ( unless you drill into a live wire) and you will not burn them up with hard use or is you stall them. Just keep them oiled and they will last a long time. The biggest drawback is having an air hose handy but that is determined on how you set up your shop.
The air compressor has a regulator to maintain a constant output pressure as the pressure in the tank drops -- until the pressure in the tank gets close to the set pressure on the regulator, at which time the output pressure drops. If this set pressure is lower than the pressure at which the compressor motor turns back on, then you will maintain the output pressure full time -- unless you are using a tool which draws more air than the compressor can provide, in which case the output pressure starts to drop, and the speed and torque of your tool start to drop. At that point, you should pause while waiting for the compressor to re-fill the tank.
If you are using short bursts of work from the tool, and the CFM draw from the tool is not too much greater than the pump's capability, you will not even see the problem -- simply the pump will turn on and off from time to time as you work.
However -- beware of two things:
1) Some compressors are optimistically rated. (Though this is more common in the horsepower ratings.) So you may not see the CFM@pressure which the rating claims.
2) Some tools are also optimistically rated. In reality, they use more air than they claim to use.
Less power available from the tool. It is designed to work properly at a specific pressure. If you can live with it running slower, and having less torque, you can use it at a lower pressure. But normally it is a little more efficient to pause with the tool and let the compressor catch up.
It would be less flow. This is because you don't have an equivalent to "Back EMF" which is what allows an electric motor to draw more current at lower voltages.
A shorter duty cycle. It has no way of knowing that your compressor can't supply the full 3 CFM -- until the pressure starts to drop. So if you use it in bursts, you are fine.
Hah! I picked up an old Sears compressor at an estate sale (from before they started lying about horsepower) and picked up a tool or two. More got added (some from hamfests), and each time I see a tool which I don't have, I pick it up. Two sizes of impact wrench, ratchet wrench spun by the compressed air, 3/8" drill, screwdriver, special tool for Screw-Sticks (a stack of screws with hex heads but no slots, all turned from a single length of brass. The tool screws in the screw, pulls out a little so the torque is applied to the next head back, and the screw wrings off at the narrow neck between the head and the end of the next screw. Now if I could just find more screws for it -- but it was free anyway. Air hammer (I was thinking for driving rivets, but it has been a lifesaver in auto work when I had to remove four 7/16" rivets to repair part of the suspension.) The other tool which turned out to be a winner for that was the drill. And I just used it for drilling holes in studs when running new wiring for my lathe and air compressor, instead of plugging them into outlets the length of the shop away. It was a lot faster than the electric Milwaukee right-angle drill motor, and small enough to fit nicely between the studs, even when there was one closer than normal because of a door.) Another new tool which was used in the car work was a high speed disc grinder with about a 2-1/4" disc for cutting off hardened steel.
Oh yes -- the air drill will stall before it twists your wrist off, unlike a 1/2" electric drill in the confined space under a car, which brings the back of your hand in violent contact with something sharp or otherwise uncomfortable.
For a lot of these I had to pause to let the compressor catch up. That didn't keep them from being very useful. Oh yes, I also have a die grinder, which I haven't had much use for yet -- but it came at a good price from a hamfest.
Oh yes -- avoid the "direct drive" compressors. They are *very* annoyingly noisy. Go for the belt driven ones which are oil wetted. (Not as good for spray painting, but much nicer for your ears. But when using the air hammer to chisel off the heads of the rivets, I had to have hearing protection under that car. :-) The belt drive ones start up and run from time to time -- but they don't make you jump out of your skin. :-)