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Steve Steven
07-28-2011, 05:11 PM
I have a DeVilbiss air compressor, which I mentioned in this thread about 5 years ago. http://bbs.homeshopmachinist.net/showthread.php?t=21703&highlight=air+compressor A drawing of a similiar one is here http://www.devilbissparts.com/OilLube%20PDF%20files/5e30ad-1.pdf

Mine differs from this one in one way, my check valve (part 29) does not have a 1/4" line attached.

Just found this part drawing, it appears to be like mine: http://www.devilbissparts.com/OilLube%20PDF%20files/l520.pdf

My current problem is the compressor will not restart after shutting off, the pressure is not being relieved from the pistons after shutting down. At first I thought it was a bad check valve, now I am not so sure. I took out the check valve, it seems OK, the surfaces of the valve disc and seat are clean and have no cuts, the spring works.

I have read on the net and found out how the unloader works here http://www.about-air-compressors.com/unloadervalve.html and that might be my problem.

Does anyone know what the 1/4" line shown on the drawing as part 15 does? Is it just a tube, or is there more to it than that? I am thinking I can replace my current check valve with a valve with a unloader built in like this one http://www.grainger.com/Grainger/CDI-CONTROL-DEVICES-Compressor-In-Tank-Check-6X213?Pid=search

Anyone here know about this?

Steve

winchman
07-28-2011, 05:21 PM
I think the tube marked 15 should go to the unloader port on the pressure switch. The port should open when the compressor shuts off to release the pressure between the check valve and the compressor.

Does the switch have an unloader valve built into it? Maybe someone replaced the original switch with a regular water pump switch, which wouldn't have an unloader valve.

The check valve from Grainger along with a switch with an unloader valve and some tubing should fix your problem.

Boucher
07-28-2011, 05:25 PM
Likley the unloader is not working. You should hear the psst of it relieving the pressure immediately after the compressor stops running.

Black_Moons
07-28-2011, 05:27 PM
the problem is the unloader port is supposed to go to a valve in your pressure cutout for your compressor (the dealie that turns it on/off), So that its opened up whenever your compressor turns off.

What I did on my compressor when I replaced the valve on my old compressor, was put a 1/8" NPT plug into the unloader port But I did'nt use any pipe tape, So it leaks slightly. Not enough to noticeabley reduce the amount of air the compressor makes, but enough to bleed out the air in that tube within the few minutes it takes any tool to take my compressor from the shutoff pressure back to turn on pressure.

You could do the same just by loosening the connection beween the compressor and check valve untill air starts escapeing slightly.

And yea, if it just started doing this, your compressors built in unloader (if it has one) has stoped functioning. Or it just used to leak slightly and used that to unload itself.

PS: Consider getting that check valve anyway for your compressor, My old compressor used a much older style of valve that made nasty clackity noises. Now that I have the teflon propet valve like pictured http://www.grainger.com/Grainger/CDI-CONTROL-DEVICES-Compressor-In-Tank-Check-6X213?Pid=search my compressor is about half as noisy, Very quiet

Also make sure you don't make my mistake and buy the wrong one :) 1/2" NPT is really about a 1" hole!

Steve Steven
07-31-2011, 10:55 AM
Well,
I have found that the unloader was plugged up with crud and not working. When I first took it off, the pin was stuck and not working, but a bit of solvent and working with my finger got it to move.

I took off the pressure switch and took off the unloader. It works by the lever attached to the switch poppong up and pushing on the pin of the unloader seen in this picture:

http://img.photobucket.com/albums/v200/Stevesteven/Air%20compressor/Viewofswitchandunloader.jpg

Note that I had already cut the unloader up by this time, not shown is the nut that held it in place nor the top, with the threads for the compression nut to hold the 1/4" air tube. Here is a photo of the unit as it would be after we cut it up:

http://img.photobucket.com/albums/v200/Stevesteven/Air%20compressor/Unloaderaftercuttingup.jpg

After cutting it up, Found the pin had a rubber blob attached to the end that acted as the valve disc. Presumably the air would escape around the stem (pin) of the valve after the switch arm pushed it up after shutdown. Photo here:

http://img.photobucket.com/albums/v200/Stevesteven/Air%20compressor/Unloaderinternalsaftercuttingup.jpg

Not shown is a small plug fitted to the internal bore with a small hole in it to limit air flow. You can see where it was fitted, we were blowing out the internals to try and disloge the plugging "stuff" when somthing blew out with some force, never found it. that allowed the internal bore to be big enough to put in the "valve disc on a pin" and kept it in place, and also limited the air flow.

I had a bit of work finding one of these, finally found one under part number 9049A56 as a Square D item (Square D now owned by Schnieder or somthing like that) at both grainger and Amazon, ordered one should be here by next Wednsday. Found the part number by looking at the Schnieder web site.

Steve

I determined it was plugged by blowing into the top (prior to cutting it up)

bborr01
07-31-2011, 11:29 AM
I had a similar problem with a second hand compressor I picked up.

It would only start if the tank was at 60psi or less. The previous owner had reset the pressure switch to shut off at 60.

Come to find out, the metal tab that actuates the unloader valve on the pressure switch was bent and not pushing the valve enough to actuate the valve.

A slight bend with a pliers took care of that and it now shuts off at 135psi, then starts up again with no problems.

Brian

winchman
07-31-2011, 12:29 PM
I'm confused. Your first post said you didn't have the 1/4" tube from the check valve, so why would the switch have an unloader valve built into it?

Maybe the previous owner replaced the check valve with one that didn't have a port for the unloader, and that didn't work very well.

Steve Steven
07-31-2011, 04:16 PM
Sorry, Winchman for the confusion. My first post was before I found the correct diagram. This is the correct parts list, and has no 1/4" tube from the check valve. http://www.devilbissparts.com/OilLub...files/l520.pdf

The 1/4" tube comes from the compressor head, and goes to the unloader.

The compressor has not been modified since it left the factory, other than having the belt guard and wheels removed. (I got it 25 yeaes ago)

Steve

Boostinjdm
07-31-2011, 05:02 PM
I bought the last few pressure switches with unloader valves from Mills Fleet farm for about $15. Just thought I'd mention it.

wierdscience
07-31-2011, 07:47 PM
Some check valves use a #55 hole drilled in the side just above the check to unload the head.The same can be done on your compressor by drilling the same size hole in the 1/4" unloader line.

macona
07-31-2011, 11:04 PM
On the little compressor I built I used a Normally open solenoid valve in parallel with the compressor motor.