Understand unloader valves

[Karl_T]

I bought a nice ten horse quincy air compressor at auction. Worked there, not when I got home

First was electrical. installed new contactor and relaced burnt up wiring. Still no joy.

Went completely through head. Removed ten pounds of burnt carbon several reed valves and springs. Man was it a dirty mess. Still does not run, but I am close.

The unloader valve is not releasing. If I undo the unloader valve air line at the blue arrow in the top of quincy pic, the compressor begins pumping.



Look at the unloader valve assembly - side of quincy pic.

I think this stuff is for both an engine type compressor and an electric motor start unit.





Now look at my tractor PTO unit. This compressor only unloads on high pressure allwoing the PTO to keep turning. Sure looks exactly like the parts in green arrows on the quincy.


Back to the side of the quincy. I want to verify this, but I am pretty sure I've seen compressors with only the bottom device (red arrow in side of Quincy view). So, am I right? all the stuff in the green arrows has nothing to do with starting unloader operation? So, it can be removed? FWIW, the valve ,orange arrow, is turned off.

That still won't get it working. My guess is replace the bottom, red arrow, unit. plumb it direct to the unloader valves on top the compressor.

I may be all wet here, sure would like to find someone that know how it is supposed to work.

 

[matthewsx]

I can’t help with your specific problem but definitely find an expert who understands your unit. Also make sure the tank is good before running it up to pressure, they can be deadly. I have a local compressor service company that always gives good advice, if I hadn’t gone into their shop my compressor would still be wired wrong. I also changed to a lower pressure switch that stresses both the motor and me out less.

Cheers,

John

 

[Latinrascalrg1]

So to understand what your dealing with, you have an electrical air compressor that is not working if there is pressure in the line between then head and the check valve? If yes have you checked the start and run capacitors (assuming that it has them)?

 

[Karl_T]

electrical end is fine, motor starts and stop with pressure switch.

There is always air pressure to the unloaders on top of the air compressor so the reed valves are held open and it does not pump.

Correct operation is for the unloaders to be held open for a few seconds while motor gets up to speed. Then they close and it starts pumping air. Unloaders are present on large compressors to reduce startup load.

 

[Latinrascalrg1]

Ok so regardless if the motor side is working or not the Unloader portion should be holding the "Start Valves" Open so that there is no pressure at startup, Yes/No? So the next questions would be, What tells the valve when its time to close and how is it activated?

 

[Karl_T]

I am a bit in the dark on how it works, that's why the query.

I can tell it is backward from your description. It takes air pressure to the unloaders on top to depress them and disable the reed valves. That's why it pumps when this air line is disconnected.

I *think* air pressure in the crank case is the switch, not real sure. but there is a connection to the crank case. I *think* that's what the bottom device , red arrow, does.

 

[Karl_T]

here's the parts manual for the unloader control parts.

Mine has dual controls with pilot valve.

It shows an option for hydraulic unloader only. I *think* that's what i should go to and then repair / replace the hydraulic unloader.

 

[JimDawson]

Karl, I think you're on the right track. It looks like the starting unloader is activated by oil pressure. You might put a gauge on the oil pump and see if the oil pump is working. Or maybe the shuttle is just stuck.

 

[Latinrascalrg1]

Ok looking over a few different styles of the unloader valves there seems to be 2 default settings either Normally open or Normally closed. Either way it sounds like your valve is stuck but not sure whether that is stuck open or closed! This is probably being caused by a broken/corroded spring and or Ball bearing inside the valve which may be replaceable.

 

[westerner]

One old compressor I am familiar with uses a centrifugal unloader. Flyweights on the crank move to open (or close, I can't remember) a valve that controls the reed valves. With this gizmo stuck, the reed would be held open, and no pumping going on. This type of mechanism is located on the end of the crankcase, centered about the crankshaft. Air line running from the unloader housing to the cylinder head relays the signal. No oil pump at all on this compressor.