Understand unloader valves

There is two types of unloader valves. The ones most are familiar with, unloads the pressure from the head, to the tank when the motor shuts off, making it easy to start up the next time. The other style, many big compressors have, unloads the head, when the pressure is reached, (Like what is used on gas driven compressors,) So the compressor stops pumping, but the motor can still run. Some setups will turn off the motor, and some leave it running (Do not know why, but I had one). Those look like the second style.

Yes, you can remove ort bypass the parts you are looking at. The unloading you want is the one that releases the air in the line from the compressor to the tank (assuming the is a one way valve at the tank).
 
Above post is right on about the unloader valve. Looks like you have a lot going on there. Here’s a pic of my Quincy at work same configuration kind of. I don’t know why they have that gate valve in your setup. The unloader is a one way valve that releases air pressure off the line so the compressor isn’t pressurized when motor starts up.
8D6AEA8C-B3BE-46AF-81BD-2093E6DA311B.jpegDB6D09CD-2B92-47BE-BDEA-CBB3352DD4C1.jpeg
Valve on top of head is piped to square valve on side of block. From that box it goes to the tank gauge not in view. Other small line going in side of head is a oil line coming from crankcase. Are your relief valves working. Their the valve with the key loop thing hanging off the end. They open when you max pressure is reached.
 
Ok, if you search long enough, you can find anything on the web. I attached an exploded view of the hydraulic unloader assembly on my unit.

My plan will be to remove every unloader control not needed and go to the hydraulic unloader only option from the .pdf I found in post 7. install new piping and rebuild the hydraulic unloader.

Stay tuned to see if it runs. Right now, this is spare time when i'm bored. My other compressor is making a bad sound. If/when it dies, this will be top of the list.
 

Attachments

  • Hydraulic unloader assembly.pdf
    2 MB · Views: 13
I have that same pump in the older splash oiling style. Started rebuilding it last year before I left for the winter and should finish it up soon.The one thing I can tell you is that Quincy doesn't use a check valve between the pump outlet and the tank. As said above the unloader dumps pressure at the the head when set pressure is reached, a very distinctive sound. I got my rebuild kit from these very helpful folks (see below) Mike


Pacific Air Compressors

3009 NE 172nd Place

Portland, OR 97230 503-674-2744

​
 
Mine looks to be splash oiling also. Not sure but see no pump system in the parts manual. I will pull the hydraulic unloader and turn over the unit to see if it spurts oil to know for sure.

?? Do you sell parts?

I am looking for a hydrauliuc unloader repair kit. Part number 8063 from this very old manual. see my post 13.
 
Google should be your friend just typed in that part number and this is what came up.
465DB9CF-16DE-42BF-A9DA-884B2469178F.png
 
What did we ever do without the inter web net?
 
OK, got the hydraulic unloader rebuild kit and spent the morning cleaning and rebuilding the unit. kind of tricky but pretty sure it was assembled correctly.

But it still did not work. :( it is always letting air pressure through. So, I'm beat on this route.

I just ordered an air solenoid and a delay off timer from McMaster Carr. I will control the unloading at start up electrically. stay tuned to see if this works.
 
Update. Got the unloader working with an air solenoid and timer. This may actually be a better way to do things as you now have total control of when to bring in the compressor.

OK, all this is only with the RPC running. I want the compressor to run on single phase power, so I ordered this VFD:

CNC 7.5kw 7500w 220v 10HP 34a 10hp Variable Frequency Drive Inverter VFD for Spindle Motor Speed Control - - Amazon.com

CNC 7.5kw 7500w 220v 10HP 34a 10hp Variable Frequency Drive Inverter VFD for Spindle Motor Speed Control - - Amazon.com
www.amazon.com

One question, do these VFDs have the ability to fire an output when they are up to speed? it would work GREAT to engage the unloaders only after the VFD has made full speed.
 
Karl_T said:
One question, do these VFDs have the ability to fire an output when they are up to speed? it would work GREAT to engage the unloaders only after the VFD has made full speed.
Click to expand...

Most VFDs have a programmable output that can be programmed for ''At Speed'', not sure about this specific model.
 
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