My Compressor Loads Down?

by Curt Garrett
(Canyon Lake Texas)

I have an older (new in May 1985) Craftsman/DeVilbiss 2-HP 12 gallon recipreciating type portable air compressor.

I had it stored for about 3 years with good oil in the sump, after building my new garage shop I brough her out of moth balls and changed the oil, checked the taknk for water agin and cleaned the think felt air intake filter in thinner ( it was pretty neww and not reaaly dirty at all). This compressor has ALWYS run like a top all thiese years and still lokks like new also.

So, here's my problem now: this 919.174350 Model compressor runs to about 40 psi and then the motor starts to load down to where the drive belt starts to squeel (bett i in good shape and under proper tension)...I had this problem in te past ONLY where it was very cold outside here in Texas and the oil in the sump was very thick, so what I did was too open the drain vale and place a Milton nipple in the hose quick connect at the tank regulator the run the nit under no pressure to circulate the oil and warm it up to like 10 minutes...

Now, my load/bog down problem is occuring when the oil in the case is warm...I can run her under noo pressue fine and dandy,when I cut off the air outlets I opened up she will then run to about 40 psi and start to bog down the motor where I just have to flip the cut off stop lever switch?

I absolutly know beyond a shadow of a doubt I have the compressor on a proper 25 AMP 110 volt outlet....in te past I could run her on a 15 or 20 Amp outlet with out any problem (boat work on a dock, etc...) This unit can be wired for 120 Volt or 240 Volt operation, my unit came from DeVilbiss factory wired for 110 use. I havr that motor rebuilt/rewound in around 1991/92 at an industrial motor shop in Houston where I was living at that time.


Any thoughts or suggestions on where to look to at this issue would be greatlty appreciated...

Capt Curt
Canyon Lake Texas

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Bill says...

Capt. Curt, let's look at what happens when the compressor runs... and this is as much for the benefit of other folks as you, as it's pretty clear that you know what goes on.

As the motor turns the compressor pump sucks air in and drives it down into the tank. As the pressure in the tank increases, the compressor motor and pump have to work harder to keep running as the air is now being pumped into a tank with pressure that's trying to flow back up the check valve.

If the air in the tank is below the compressor cut out pressure level, then the compressor will keep running, working harder and harder, until the pressure in the tank hits the cut out setting.

The compressor can handle that.

What it can't handle is if the check valve is not opening properly when air is flowing down into the tank. This causes back pressure.

What it can't handle is if the the pump itself has a mechanical issue that doesn't present itself until it has to pump against load.

And what it can't handle is if the motor is wearing out, and as the load increases with the pressure in the tank, the motor can no longer provide enough power to turn the shaft against the load of the pump.

If it were me, and although it's not likely the check valve, I'd pull it an check it to be sure.

I'd then try to turn the pump sheave by hand to see if I felt any issues (even though with no air in the tank there wouldn't be back pressure).

Then, I'd take my motor to a rebuild shop, and ask them to do a load test to see what's what.

Cheers,

Bill