The cruel hoax of air tool ratings

On Sat, 13 Dec 2003 06:28:20 GMT, Anthony wrote something ......and in reply I say!:

What you say above is true, but is not going to help much in the overall picture, which is that the tools are so way out, there is nothing to be done about it.

The main problem, as I see it, is not the comps but the tools, whose ratings are just rubbish at the cheap end. They regularly do seem to use 4-5 times as much as the claimed requirement, if used heavily.

At least that was true until recently, when the comp ratings are starting to fly into cloud cuckoo land as well.

I was frightened by the idea of a conspiracy that was causing it all. But then I was terrified that maybe there was no plan, really. Is this unpleasant mess all a mistake?

A good compressor, per true HP, will deliver a true 4 to 5 CFM at 100 psig.

Probably peak HP. Used on everything these days, totally meaningless though it is.

Tim

-- "That's for the courts to decide." - Homer Simpson Website @

On Sun, 14 Dec 2003 03:55:38 GMT, "Tom Young" wrote something ......and in reply I say!:

Which makes it even more amazing. The tool is still way over rated requirements, even at pressures way under its stated pressure need.

I was frightened by the idea of a conspiracy that was causing it all. But then I was terrified that maybe there was no plan, really. Is this unpleasant mess all a mistake?

On Sun, 14 Dec 2003 01:49:43 GMT, Loren Coe wrote something ......and in reply I say!:

Because of the wildy varying efficiencies of motors and pumps, there is no such figure.

But for a good quality, belt drive unit in good condition 3, or _maybe_ 4CFM/HP is a good figure.

My units are reasonable quality, but direct drive. rated at 2.5Hp, they give about 7CFM @ 90 PSI. So there is a figure for non-belt drive. Watch out though! These were "12CFM" compressors!

Some of the more recent ones are simply BS. 5HP (and even on 240 volts that's a solid 15 amps) etc getting maybe 5-8 CFM @ 90 PSI.

I was frightened by the idea of a conspiracy that was causing it all. But then I was terrified that maybe there was no plan, really. Is this unpleasant mess all a mistake?

"Tom Young" wrote in news: snipped-for-privacy@attbi.com:

Something else you can do is check the condition of the air blades and bore. This will require disassembly. The blades should slip-fit in the slots, but move freely. I've taken quite a few apart to find one or two blades stuck in the slots, meaning they never seal and air just blows by. The edges should be straight and no air gaps against the bore. The blades should be flat when laid on a surface plate or straight edge. Any bow in the blades will let air leak by in the slot, and will slightly bind the blade causing intermittent sealing against the bore.

The blowby and cylinder life is one factor in the industrial side switch to screw compressors from piston compresssors.

At least the consumption did come down a bit, but......

I'd be interesting in learning what the drill consumes when it *is* stalled.

If it's still ridiculously high, then I guess the motor sure isn't a positive displacement type, or if it is, the seals must be terrible.

Maybe it's just some kind of turbine, huh? (Can you disassemble it easily and let us know?)

Jeff

gee, i now think i know why laptop batteries always go flat. the manufacture/s use the most optimistic spec, the laptop is spec'd at some incredibly low load that is theoretically the minimun possible.

of course the battery is spec'd at its maximum theoretical capacity. it just the old 10x or .1x factor, used most of the 20th century by the marketing dept. --Loren

"Tim Williams" wrote in message news:...

Compressors are often rated according to actual displacement of the cylinder(s). By this rule, a high-speed compressor will suffer more volumetric efficiency losses, which is another way of saying that the piston is moving too fast and the intake/discharge piping is too draggy to allow full atmospheric pressure at the bottom of the piston stroke. So a compressor rated at 20 CFM, based on cylinder volumes and RPM, might produce 10 CFM if you're lucky, unless it's running really slowly. Further, ANY compressed air left in the cylinder at top dead centre will also expand on the intake stroke, keeping intake valving shut until cylinder pressure drops below ambient pressure, and the stiffness if any intake valve springs and the weight of the intake valves will also work against efficiency. The best compressor runs at a low RPM, has a large intake duct and big filter, large but light and tight intake and discharge valving, and a piston that very nearly hammers the head. The valves must be designed to minimize any volume between them and the piston at TDC. I used to remanufacture compressors used on large trucks, earthmoving equipment and so on. 17,000 of them in 12 years. We tested every one, and measured their output. A tiny nick in a valve seat or rings that weren't just right affected output big time. Doubling the RPM, even on a really tight unit, did not nearly double the output. And that was with a wide-open intake port; no filter or piping. Drag in such systems increases by the square of the airspeed in the system. The same RPM/volumetric losses affect internal combustion engines. On the aircraft engines I look after now, the volumetric eficiency at 2700 RPM is about 50%. This is why turbochargers make such a difference: they make sure the cylinder pressures during intake are near sea-level atmospheric in a turbonormalizing system, or well above it in a boosted system. As far as air tool consumption, the endplay of rotors and vanes is usually the biggest leak. Taking the tool apart and miking the housing, rotor, bore, and vanes will often show where some judicious lapping will tighten things up.

Dan

Okay, thanks for the advise. I'm not at the point where I want to take the pump apart yet, but I'll keep this in mind if/when I do.

-Tom

I didn't check the rate, but it definitely still blows air out the exhaust even when fully stalled.

I guess I'm not quite curious enough about it to start taking my tools apart just yet. Plus, I wouldn't recognize a turbine motor if I tripped over it. But if a turbine is inexpensive and inefficient, it sounds like that's probably what they used.

-Tom

I can say my el-cheapo die grinder (when it was new) didn't use a whole lot of air when stalled. Sometimes the vanes would fall inward when stalled, and then the air would just shoot through. But, otherwise, the air coming out was just a little hiss when stalled. I took it apart to see if the bearings were replacable (Boy, were they NOT!) and it now leaks a bit. I just can't believe Jeff's unit is so VASTLY worse than mine. Nobody with a cheap 2 Hp compressor could possibly use his drill. It would kill one of those pancake tanks in about 5 seconds.

I still think he must somehow be miscalculating this, somehow.

Jon

did you actually use a current probe? that's the only way i know of to verify figures like we are discussing. from my previous scribblings, i extrapolate your electrical "input power" at 2hp, and that does comport w/your observation (at least within a 2x factor) of 5-8 CFM @90psi. that's allowing for some amount of loss in the motor and compressor.

i may have disremembered the Craftsman rating of 6cu.ft.@90psi. it may well have been 4. of course, that was just the "claimed" output from a 1hp motor@110v. i ran it at 220 most of it's life and it started much better. at the time we speculated that the motor actually output more hp on 220v, maybe 20-30%?

there is a dearth of general interest here, but i find this of great interest and plan to do some googling tonight. Regards, --Loren

I have to say that I flat don't believe you can get any more horsepower out of a motor by connecting it to a different voltage. Yes, a single-phase capacitor-start motor will START faster on 220 vis-a-vis 110, but basically there are 4 windings and you are connecting them in serial or parallel and either way they all have 110V across them so its a dead push overall run-wise.

Sometimes a struggling motor will slow down enough so that the start cap will kick back in for a bit. I'm *not* talking about times like that.

Grant Erw> >

"Tom Young" wrote in news: snipped-for-privacy@attbi.com:

I was referring to the air motor of the tools. The compressor is most likely a piston type compressor, although you have never said. To get it up to snuff would most likely require a rebore and rings / pistons.

Jeff Wisnia wrote in news: snipped-for-privacy@conversent.net:

Typical construction for air tool motors is a rotor with 6 or 8 slots lengthwise, in these slots set blades, which centrifugally are slung out against the outer bore, creating a seal. These blades must also seal against the slot.

On Mon, 15 Dec 2003 00:03:48 -0600, Jon Elson wrote something ......and in reply I say!: I don't think there's any miscalculation. The results seem to be based on tests, which are showing not good results.

The results mirror mine and when I asked in the industry, others agreed. These cheap tools are turbine, I'd bet. They chew air!

I was frightened by the idea of a conspiracy that was causing it all. But then I was terrified that maybe there was no plan, really. Is this unpleasant mess all a mistake?

Armed with your info I checked out a little compressor my neighbor bought himself from HF, and just received from them:

The motor draw was claimed to be 2.5 amps, and I don't think that little energy could deliver anywhere near the claimed performance, even on a good day with the wind at its back.

As the neighbor only needs it for filling bike tires, SWMBO's hobby airbrushing, and maybe winterizing his lawn sprinklers, the oilless feature is nice for that sort of stuff, so he's gonna keep it.

At least it's not very noisy. It must have a separate constantly running cooling fan under the shroud, since there's a little whir and some airflow whenever it's switched on, whether or not the compressor's pumping.

Happy Holidays,

Jeff

-- Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can keep smiling when things go wrong, you've thought of someone to place the blame on."

That all seems consistent.