Please help with an old air compressor

1) Should I re-shive and stay at 1750 on the motor, or go to 3550 RPM on the motor?
I would'nt change the compressor rpm. Maybe the 3550 with a jackshaft would work.

Dave, Hold on a minute....
Compressor RPM's are determined (in part) by the amount of heat that can be dissapated. The higher the operating pressure, the lower the RPM. That being said, for general shop use, 120 PSI, the RPM for these cast iron, piston, fan cooled compressors generally fall in the 900 RPM range. I am basing this solely off of the many compressors that I have worked on. If you have he ability to dissapate more heat, ie aftercoolers, water cooled heads, blowers, water cooled inter and after coolers, etc, then you can run them faster.
A double speed motor will have 1/2 of the torque for the same HP size. High torque is required to get compressors rolling and overcome drag and pumping forces. Up the motor HP, stay at 1750 RPM.
Another way to gauge your motor selection is to look in a Granger or IR catalog. Copy what they do for the same size compressor.
Bob

The reason for using a two pole motor ( about 3450 rpm ) is that they are cheaper for the same horsepower than a four pole motor ( about 1750 ). It won't be easy to find a 7.5 to 10 hp motor that is not three phase. If you are going to use a three phase motor, then I would see what I could find in a used motor before deciding on how to procede.
I don't know of a rule of thumb on compressor rpm. It probably depends on piston speed. I would look at what rpms other compressors run at to get some idea of what is reasonable.
Dan

Looking at the other side of the equation.... I'm not sure you have the horsepower to change sheaves.... rather than overheat the compressor, you'll likely burn up the motor. I'd look for a 5 to 7.5 hp motor in the 1750 RPM range and rig for a compressor speed of 800-900 RPM.
A rule of thumb is: A quality compressor in good shape will deliver a true 4 CFM at 100 psig, per true horsepower. That means no Chinese motors with 5hp ratings on a 2hp motor.
Machinery's Handbook has some good info on this.

- to double the CFM (7 or 8 to 15) requires doubling the compressor speed - doubling the compressor speed, at the same PSI, requires twice the driving HP - single phase 220v 5 hp motors are common - it doesn't matter what the motor speed is, given the proper sheave ratio to get the compressor RPM you need
Bob

A buddy of mine has a single phase 5hp motor for sale in the Seattle area. He wants like $125 for it, firm, plus shipping. Seems real high until you go price them. I think this one is new. Guy is honest and reputable.
Any air pump has a speed it's supposed to be run at. It's OK to run it slower, but if you run it faster you're asking for big trouble.
Grant Erwin
Gene Kearns wrote:

My 2x2.5" Gardner Denver (very old, three dollar garage sale item) is plated for 400-600 RPM, max 125 PSI. Someone (sorry to not quote,but thanks for the info) comes up with 4-6CFM @ 100PSI per HP Ah, there he is, Richard J. Kinch, excellent source of information!
Gerry :-)} London, Canada

Do I recall 3 scfm/HP at the 175 psi max of a 2 stage, that seems consistant w/ about 4 scfm/HP at 100psi on a single stage, no?
As for 1725 vs 3450 RPM the faster two pole motor will require half the sheeve diameter, and as sheeve dial becomes small it gets tough on the belt(s), another argument for 1725's.

Dunno bout the higher pressures, perhaps Richard could help out there. Before I found the 1/2HP motor for the little G-D I used two smaller motors belted together, worked fine. Gerry :-)} London, Canada

I have a PAR Model 20. I put a 5 HP/1750 RPM Baldor on it. Looks good, and seems to operate okay. I have not used it very much, so I really do not have a feeling for it's efficiency (CFM).
Nice details on these old compressors. I like the motor's cradle mount for the belt adjustment. The drain on the side of the tank is something that I have not seen elsewhere. (The water is pushed up through a internal tube.)
I also have an old PAR single cylinder/single stage compressor without a flywheel. The crankshaft has what apparently is a unique taper, and I have not found a flywheel to fit.

Sounds like it's under-powered to me, too.
You'll have to scare up some specs on maximum RPM and suggested motor rating of the compressor. The reason it's working now may well be because they slowed it way down (smaller motor sheave, 1750 motor) to control the motor's maximum loading - to make sure, get a clamp ammeter and watch the running full load amps from restart up to pressure switch cutoff, and make sure its not overloaded past the motor nameplate amps.
That's why they make many sizes of sheaves, including the "adjustable" ones used primarily on HVAC blower motors. If you can't control the back-pressure on the fan, you have to control the max motor load by adjusting the motor sheave diameter.
And if it's a 120V/240V motor and you have a 2-pole pressure switch and 240V outlet available, rewire it. It will be much happier running on 240V with a lower inherent voltage drop...
If you change the motor you will probably need a new adjustable pulley, unless fortuosity smiles on you. ;-) They're cheap enough, Browning pulleys through Johnstone range from $8.50 for a Zinc 3.25" OD by 1/2 or 5/8 bore, to $77 for 6.5" x 1 1/8" bore in cast iron. Two groove cast iron range from $58 to $195
Same as any other reciprocating device (like a car engine) - spin it too fast, and things have a nasty habit of coming apart. Knowing how fast "too fast" is ahead of time is a good thing... ;-)
And there might also be a minimum speed, below which the splash lubrication dippers don't splash enough for effective lubrication - better find that, too.
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I appreciate all the responses very much, thank you all!
Based on what you've all written, I've measured some things and have this additional information:
Rotational Speed: The present shive rotates the air head at 480 RPM, while the motor is rotating at 1768 RPM. (These numbers are from a digital tach that I recently checked the calibration on; should be good.) One gentleman who wrote to me privately mentioned that, in his experience with this style of air head, 900 RPM at the air head was quite normal. Interestingly, the air head speed did not drop as the receiver filled. At 30 psi in the receiver, the air head was rotating at 480 RPM, at 180 psi it was rotating at 479 RPM. I would have expected quite a bit more slip on the motor as the work load went up.
Temperature: Right now, the air head and exchanger tubing are really only warm at their hottest, which makes me think there is headroom for raising the speed. If anyone knows of a good temperature test, I'd appreciate it if they would pass it on.
Shaft Diameter: The motor shaft diameter is 7/8" with a key. NEMA seems to stop 7/8" 3600 RPM motor shafts at 3 HP, 5 HP gets you up to a 1.125" shaft.
Unless someone has a good differential temperature test I can do on the air head, or some additional thoughts, I'm going to buy a 5 HP motor and shive, sized to turn the air head at 900 RPM or so.
Thank you all again for the help!
Dave

But old electric motors can be quite large to their horsepower as compared to today's motors - I traded away an old 1 hp motor which was larger and (ugh) heavier than many modern 3 hp motors not long ago.
Many 1-2 hp vacuum pumps use 2 belts, IME.
Hopefully someone can actually come up with information about a Lynch PAR model 15 for you (from an old catalog or the like) - that would be the best bet to getting it right. Sounds like a nice unit.

I looked, and that single stage PAR is a Model 21, and has 2 cylinders. (Dispelling the theory that model # = CFM.)
RE: 5HP 3450RPM 7/8" Shaft Reversible Electric Motor Northern has a Century brand (USA) for $165 with "free" shipping.