Underpowering air ratchets, or impact wrenches.

[hornedfrog1985]

Ok, I was looking at buying a Kobalt 30 gallon Compressor but the air ratchet I was looking at says it needs 6cfm to run. The compressor says it only powers 5.3cfm if I remeber correctly. So what would happen if I don't give it the full 6cfm? Will it underperform greatly, or would it be hardly noticeable? Thanks.

 

[garfunkle24]

It would only have trouble keeping up if you ran the gun continuously. For light - medium work the compressor and tank will keep up fine with an impact or ratchet. As for the performance, there is usually a fairly linear relationship between psi and torque output with an air motor.

 

[hornedfrog1985]

Hahaha, man this gets more, and more confusing the more I read about it. So I'm more concerned with the PSI as for the torque of the tool, and not the CFM?

 

[Kirbot]

The lower the cfm rating of the compressor, the more often you will have to stop and wait for the tank to charge.

It shouldn't effect the power at all.

 

[hornedfrog1985]

Oh, so the CFM just refers to how fast the compressor pumps the air into the tank, but the PSI refers to how hard it comes out of the hose? If a tool requires 100PSI to operate, will it use even more PSI if its available?

 

[Kev442]

Correct. I used to take a tire off and put it back on and my 11 gallon would only kick on at the last couple of lugnuts with an eleven gallon tank, so 30 gallons is pretty good sized.
An air ratchet will work it a lot harder, but you will only slowly take 1 CFM more out of the tank than the compressor can replace once it kicks on, so there will not be much waiting for it to catch up.

 

[Roman G]

Oh, so the CFM just refers to how fast the compressor pumps the air into the tank, but the PSI refers to how hard it comes out of the hose? If a tool requires 100PSI to operate, will it use even more PSI if its available?

So if this is the case is there a calculation that we can run that will tell us what the actually cfm is versus what the label says?

Thanks
Roman

 

[darkk]

Oh, so the CFM just refers to how fast the compressor pumps the air into the tank, but the PSI refers to how hard it comes out of the hose? If a tool requires 100PSI to operate, will it use even more PSI if its available?

CFM=cubic feet per minute (volume of air produced every minute)
PSI=pounds of air per square inch (pressure of compressed air per square inch)
For....example. your compressor will produce 5.3 cubic foot of air when compressed to 90 pounds per square inch every minute.

 

[garfunkle24]

See the thing is, it's the pressure that's going to change the tools torque. However, if you don't feed enough CFMs, you will create a pressure drop. So until you've ran your tank down some, you should have good power and torque at the tool. Clear as mud?

 

[hornedfrog1985]

Yeah, sounds like I'm going to have to save up a little more money to get what I want I think. I don't want to buy something I regret.

 

[garfunkle24]

Also keep in mind that those tools are not spectacularly air-hungry like some. Die grinders, air grinders, media blasting and even decent use of a blowgun beyond cleaning a bench require a lot more air.

 

[GSteg]

How often will you run your tool? If you need to run it continuously all day long, then 30 gallon might not be enough. Before I went cordless, my 19 gallon compressor worked okay for small jobs here and there. Once I started to do more work, the compressor ran all day long.

Also, the tank size is just half the equation. Pressure is the other half. If you have a 20 gallon tank rated for 155psi, it's going to allow you to do more work than a 25 gallon tank rated at 115psi.

Don't worry too much about the CFM. Your tool will need a lot more CFM than what the pump can generate. It's not uncommon for tools to be rated at 6 CFM but in actuality, they need 20+ CFM at full load. Just get a large tank with a reasonable CFM output and you'll be fine.

 

[pipsters]

First don't let anyone tell you that compressor (I assume this one right?) won't power what you are going to be using it for - it will. It's one of the best 120v compressors you can buy these days, and for the 20% off they are offing it now ($329? I believe) I would be jumping on that like white on rice if I needed a compressor right now.

The compressor output (CFM @ 90psi) is only what the pump will replenish the tank with. That is a great size tank (30 gallon) with higher PSI than normally found (155) that you will have NO PROBLEM running an impact and air ratchet for everyday DIY auto stuff.

I have a 25 gallon 150 craftsman compressor (oiled like the Lowes - separate pump and motor) and I can hold down the trigger of my 1/2" Earthquake impact (excellent gun for the $$ BTW) for 30+ seconds before it drops below 90 psi, even though my compressor output is 5.7 CFM @ 90 psi and the impact uses around 20 CFM running. It uses stored air.

I can take off crank pulley bolts with that setup with ease. OK maybe not a Honda (haven't tried, and doubt it) but it pulled the 30mm nut off my 15 year old salt encrusted Volvo 850 like butter, and same for the 34mm axle nuts.

I also use a die grinder every now and then and for my purposes (sharpening lawn mower blades) it works awesome.

About the only thing you probably won't be able to do much of is sand blast. You need a 220v unit for that.

The only reason I bought my Craftsman over the Kobalt is the output (5.7 vs. 5.5) as I thought as you originally did, and everyone told me that as well. Unfortunately, there are many out there who don't understand the concept of stored air.

Oh and in my experience, you'll find a 3/8" air ratchet really isn't that handy. Personally I prefer a quiet 1/4" air ratchet. The 3/8" is too unwieldy and loud for my tastes.

 

[garfunkle24]

Pipsters

Not sure if that was pointed my way at all, but I made no comment on the suitability of said compressor....

 

[hornedfrog1985]

Do you think the HF 21 gallon compressor would work with those Earthquake tools? Or would you stay away from it. I'm only going to be using this probably once a month (if even).

I see the Kobalt has gone up in price..... figures.

 

[Zrexxer]

You're missing the most important point about a compressed air system. Once that air's stored in the tank, the only thing that's limiting how much volume (cfm) it can flow is the size of the outlet hole. That tank can supply 10 cfm, 20 cfm, probably 30 or 40 cfm - for as long as it takes to reach the lower cut-in pressure. Once the pump starts operating, it can only put 5.3 cubic feet per minute back into the tank. If you're using more than that, you will have to stop and wait for the tank to refill.

The tank provides a reservoir, a buffer of stored air, so that you're not running the pump 100% of the time for every tool. By its very nature, of an impact wrench for automotive work is an intermittent load. If you were running an air grinder in a production setting at 100% duty cycle, that's a different story. But a 5.3 cfm compressor will run a 6 cfm tool intermittently just fine.

 

[Cryptic1911]

^ that was my point in his other thread. I have that exact setup with the hf21 gallon and the earthquake 1/2" and it works great. I did however swap all the fittings to high flow fittings when I had to buy a new hose and couldn't get the old fittings off our old hose

 

[hornedfrog1985]

Cryptic. Can you point me to the exact fittings/hose you use? I tried to PM you, but it doesn't seem to work.

 

[Cryptic1911]

it worked, I just didn't scroll up enough to see the new pm's alert flashing

 

[hornedfrog1985]

Thanks again for the help everyone.

 

[larry_g]

How often will you run your tool? If you need to run it continuously all day long, then 30 gallon might not be enough. Before I went cordless, my 19 gallon compressor worked okay for small jobs here and there. Once I started to do more work, the compressor ran all day long.

Also, the tank size is just half the equation. Pressure is the other half. If you have a 20 gallon tank rated for 155psi, it's going to allow you to do more work than a 25 gallon tank rated at 115psi.

Don't worry too much about the CFM. Your tool will need a lot more CFM than what the pump can generate. It's not uncommon for tools to be rated at 6 CFM but in actuality, they need 20+ CFM at full load. Just get a large tank with a reasonable CFM output and you'll be fine.

There is so much wrong information in the above post that I'm not even going to try to address it.

lg
no neat sig line

 

[GSteg]

Care to point out the wrong information?

 

[pipsters]

Care to point out the wrong information?

I don't see much if anything wrong about your post.

 

[Zrexxer]

I'm not even going to try to address it.

That would be wise, because there was pretty much nothing patently wrong in gstegs' post.

 

[diggerrick]

I've had a 30 gallon Craftsman for about 10 years running every air tool we have short of a paint gun (we have used a touch-up gun, but without a filter & dryer we won't do it again). The only time I've ever had to wait on the compressor was when I used an air drill (running slow) with a buffing wheel & cone to polish my injector hat, motor plate, & wheels before a race. Even with the regulator set to like 40-50 psi it ran down several times, but I was polishing for at least 30 minutes.

My little 15 gallon C-H even works well for zipping off lugnuts & cutting exhaust pipe, but the little inpact hammer really taxes it.

Last winter I bought a new compressor to put under the stairs in my back shop (2-story w/ finished workshop/office area upstairs). I've couldn't see the need for a 60 gallon compressor for occasional use, so I bought a 20 gallon Tool Shop compressor on sale at Menard's that runs a right angle die grinder, impact wrench, impact hammer, and powder coater - not more than one at a time.

From my experience, 30 gallons is a nice hobby/amateur-wrench size if you're not doing bodywork or painting.

BTW - race car & most work is done in front garage. I mostly work on power equipment, transmissions, and powder coat in the back shop these days.

 

[Charles (in GA)]

look at it like a car's electrical system. The alternator (the pump) only puts out 60 amps, the starter draws 300 or more amps, momentarily. The battery (the storage tank) has the ability to flow this current to the starter for a reasonable amount of time, before it runs out, just as you use air from a tank and then stop using the air. The alternator, then can "pump up" the battery, it just takes a while to do so, just as your air compressor pump will slowly pump up its tank (technically called a receiver).

Charles

 

[lonestarky]

So SCFM, or standard cubic feet per minute, is the volume of air at standard temperature and pressure your air tools require, or the compressor can compress.

At the outlet of your tank, the air will be regulated down to 90 PSI. Your tank itself will be at either 115 or 155 or whatever PSI its designed to stay at. As long as your tank is above 90 psi, you'll always see 90 psi air in your hose, and at your tools (minus pressure loss due to hose length, fittings, etc, but thats minimal most of the time). Period. End of story. Your compressor will not under-power any tool, if you have 90 PSI at your hose.

Now, for the more complicated explanation-

When you use an air ratchet, or a die grinder that pulls 7-9, or more SCFM, you're going to deplete the 155 or 115 psi air in the tank faster than your pump can replenish it. Your compressor will kick on when the pressure in the tank reaches 120 psi (or whatever the set point is) and will run until the tank charges back up to its pressure. Its at this point that your risk dropping below 90psi (if you're using more air than you can compress) and decreasing the output of your tool.

None of this matters occasionally. Your compressor and motor will have a duty cycle. 10 minutes on every 30 or something like that. You'll use your tools, and everything will cycle on and off, charge and discharge, and then you'll drain the condensation out of your tank when your done, and that's it. A 26-30 gal compressor has enough storage capacity at 155 psi to run anything moderately. Just being mindful of your duty cycle is the most important thing. Most 110 V compressors are sealed, and oil-less. If they're used too much they overheat and fail. So when you're running your air ratchet, give your compressor a break to stay within its duty cycle, and it will be just fine.

Like everyone else said, painting, sand-blasting, etc, these things all basically vent 90psi directly to atmospheric pressure, so you drain your tank very quickly. Doing these things, you risk exceeding the duty cycle of your compressor and overheating it if you don't watch the time your compressor runs.

 

[a390st]

I use that compressor (the previous Coleman version, which is rated very similar) all the time. I've used impact wrenches, grinders, and ratchets. If you're going crazy, you'll have to wait for it to catch up some, but it isn't as bad as I expected. It's also somewhat portable which is very convenient. It's great for nail guns if you have electric onsite.

 

[hornedfrog1985]

I won't be doing things like tire rotations all the time, or things like that. What I need it to be able to do is get something like an axle nut off, or a stubborn bolt that I can't get my club hands in, and around, or whatever. Just so I don't have to bust up my knuckles tinkering. Usually I'm working on family members cars just to save them money, and they throw me a few bucks for my time.

For example. My parents had me do their brake pads on there car last weekend, and I figure sure why not, probably take me an hour. Wrong. I couldn't get a couple little bolts off the caliper because I didn't have the leverage. Not to mention some ******* stole all my sockets out of my garage the day before. I didn't realize that till I started on my parents car though....

 

[GSteg]

If that's the case, then the Kobalt compressor should be good enough. It'll kick in once in a while, but it's quiet enough to tolerate. I got by with a 19 gallon doing suspension and brakes and it turned on maybe 2 or 3 times for every car I worked on. Use a good impact gun and you're good to go. A weak gun will end up wasting more air if it has to hammer a lot.

 

[jvitez]

Use a good impact gun and you're good to go. A weak gun will end up wasting more air if it has to hammer a lot.

Very true. I started out by buying the cheapest 1/2" air impact gun I could find. Ya it works, but sometimes it has to whack away for quite a while before the nut loosens. Then I found a clearance deal on a Porter Cable branded air impact. Ziiiiiiiiiip! The nut is loose. Way more fun to use, and uses far less air.

To buy cheap is to buy twice......