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Old December 19th, 2012, 09:28 PM
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Charles Galpin
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Air compressor question

I went to look at a used air compressor for sale this evening (a Campbell Hausfeld HS781003AJ). It's a monster 80 gallon, 4 HP, 2 stage 220 (single phase) compressor that puts out 12 CFM at 175 PSI. When it's working...

So it was hard wired before so I couldn't just plug it in to test it, and during the visit the guy revealed it had been lying on it's side (not sure how long) but had been upright again for about 6-8 hours. This came out because after "hot wiring" it to run the motor it was suspiciously quiet and no air was coming out of the open hole on the side where it used to be plumbed. I would expect to immediately hear air rushing out if it puts out that kind of CFM. I confirmed the pump was turning in the direction indicated on the flywheel (if that even matters) and could turn the wheel with a single finger.

So can anyone tell me if this is a sign of a major issue, or the pump just needing cleaned out? The shutoff valve wasn't hooked up, but as far as I can tell all that does is shut off the electric motor and the pump is all mechanical (not some electrical clutch say).

If anyone has any insight on this please let me know. I'm willing to go back and buy it if I have a reasonable level of confidence it can be repaired cheaply or is going to be some simple fix, but I somehow doubt it. I'm not looking for a project, but it would be a hell of a deal if it turned out to be an easy fix.

tia,
charles
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Old December 19th, 2012, 09:34 PM
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A compressor of that size should make a very loud noise while running.

I'm not totally sure I understand what you saw based on your description, but if the electric motor was spinning, and the belt was turning, and the flywheel on the pump was rotating, then the pump itself should be making a heck of a racket and air should be shooting out of the pump like crazy.

There should be no electrical clutch. If the flywheel is spinning but the pump is making no noise, then two things are possible:

1. The woodruff key on the crankshaft has sheared and the flywheel is just spinning on its own. Easy fix.
2. The crankshaft has cracked. This means you have to replace the compressor pump in its entirety.

Even if the compressor was totally fubar, you can still buy another pump from Harbor Freight for pretty cheap, bolt it on, put on a belt, and call it a day.

Either way, what you describe is not normal
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Old December 19th, 2012, 09:36 PM
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Yes I expected exactly what you described. The electric motor was spinning, the belt was turning, the flywheel on the pump was rotating, but it was not pumping

I would have expected to hear bad noises or grinding or something if it had a broken crank shaft or something.
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Old December 19th, 2012, 09:41 PM
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Yes I expected exactly what you described. The electric motor was spinning, the belt was turning, the flywheel on the pump was rotating, but it was not pumping

I would have expected to hear bad noises or grinding or something if it had a broken crank shaft or something.
Cast iron crankshafts can shear before the first eccentric. Pistons hold the rest of the crankshaft in place while the main bearing closest to the flywheel lets it spin freely.

But a sheared key is the most common and is a couple of bucks to fix.

The other thing to do is look into the oil window on the compressor. If there's clean oil in it, then the compressor is usually ok. If there's no oil or black sludge, then there's something probably badly wrong with the compressor.
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Old December 19th, 2012, 10:01 PM
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Where would the key be? - the flywheel seemed secure to the shaft and I couldn't feel any end play.

Keeping in mind it was dark, the oil was either very dirty, sludge, over full, or empty. It was just really dark and I couldn't see the level.
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Old December 19th, 2012, 10:03 PM
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If that resulted in the key breaking or the crankshaft, and it spins freely after that, then yes I would think so. But not enough history to know.

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Its possible that it was hydrolocked with oil above the rings.

-Jeff
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Old December 19th, 2012, 10:11 PM
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I did. It was smooth as butter with one finger - including the motor resistance. So clearly nothing gong on inside the pump.
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Old December 19th, 2012, 10:20 PM
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Originally Posted by cgalpin View Post
Where would the key be? - the flywheel seemed secure to the shaft and I couldn't feel any end play.

Keeping in mind it was dark, the oil was either very dirty, sludge, over full, or empty. It was just really dark and I couldn't see the level.
The flywheel is bolted against the crankshaft, which has a taper. In the taper of the crankshaft, there is a keyway (a groove or a slot) machined into it. Therefore if the keyway is broken, the flywheel will spin but the crankshaft itself will not.
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Old December 19th, 2012, 10:45 PM
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Could be the reed valves (if it has them). They are just spring steel shim looking things where one bends down as the piston goes down to BDC and it seals as the discharge one bends up to allow the air to be forced out as the piston goes up to TDC.

AFAIK, you can't "hydro" a compressor, as they compress, they discharge to the tank.
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