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Quick help needed on a pop-off piloted piston setup (to-scal

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Quick help needed on a pop-off piloted piston setup (to-scal

Unread postAuthor: Leonard » Mon May 11, 2009 10:56 am

Hello Guys! Been a long time since I posted, but i'v been lurking around...

After a terrible fail on trying my luck at a blowback hammer valve setup (never posted my failure because it was not interesting : I simply needed tools I do not have), I though i'd try something easy just to actually finnish a damn working launcher, so I went for a simple piston piloted by a pop-off for full-auto action (and a spring loaded magazine to be made later).

Howhever, im having trouble going full-auto with it, and i just cant figure out my problem.
Here's the specs :
Barrel : 3/8" epoxied onto a 1/2" tee (12" inch long)
Piston : Toolie'ich, weight 3 or 4 grams, mostly made out of rubber (and rubber bumper)
Air inlet : 1/8" (c02 regged down to 115 psi) from a blowgun.
Exhaust : 1/4" pop-off valve (115 psi pop-off pressure)
Chamber : 1/2" wide, 8" long

Here's a quick diagram of the valve (scale is ten pixels = 1mm) :

Image

The goal I had while making this valve was to reduce pilot volume to a MINIMUM, as can be seen on the diagram, so a 1/4" pop-off valve (taken from a cambpell hausfield compressor) would easily pilot the 1/2 inch piston. When i fill the chamber to 110 psi (5 psi below pop-off pressure), I can simply pull on the pop-off to make it fire and it pilot the piston perfecly (nothing out of the ordinary, but it performs well, air blast is harmful when shot to skin at end of 12" barrel). So im like : Great!

But the problem appear when I fill the chamber with more than 115 psi. The pop-off valve act like a goddam bleed valve! It pop and open, but only a little and for a very short period of time : the piston does'nt move. If I keep the blowgun open, the pop-off will open/close/open/close but the piston if not piloted. What could be the problem? Is my pop-off no good? Is my incomming airflow to low? Or not high enough?

Any input would be appreciated, thanks!
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Unread postAuthor: jackssmirkingrevenge » Mon May 11, 2009 11:10 am

The goal I had while making this valve was to reduce pilot volume to a MINIMUM, as can be seen on the diagram


That's your problem, same thing happened to me here. You need to make the pilot volume bigger.
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Pop off valve pilot problem

Unread postAuthor: Technician1002 » Mon May 11, 2009 11:16 am

Getting this to work has been a FAIL for most people. There are a couple reasons this is marginal at best.

1, The valve seat to valve diameter on the main valve sets the pressure that the main valve requires to even consider opening. If the valve seat is too large for a close ratio main valve, the pop off will close at a higher pressure than the main valve needs to open. Setting the pop off to a lower pressure may fix this.

2 Pilot area is too small. I know, WTF??? A pop off valve to fully open count on the pressure to remain high in the pilot area so when it unseats, the air flow increases to lift the larger diameter step in the pop off valve. With a very small pilot, the initial opening step causes a rapid loss of pressure so the pilot never fully opens. This in turn can be too fast and with too little of a pressure drop to lift the main valve. A larger pilot area permits the pop off to remain open longer with a longer draw down time of the pilot permitting the main valve to pop open if the other conditions are met.

3 Main valve leaks too much. The pop off may pop and hold open because too much air blows past the main valve.

4 Main valve has too much friction. The initial opening force is too low to unstick the valve.

Any one of the above or a combination can prevent a pop off and piston valve combination from operating. It is a delicate balance to get one to work properly. Hats off to those who pull it off. :o
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Last edited by Technician1002 on Tue May 12, 2009 5:12 am, edited 1 time in total.
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Re: Pop off valve pilot problem

Unread postAuthor: jackssmirkingrevenge » Mon May 11, 2009 11:21 am

Technician1002 wrote:1) The valve seat to valve diameter on the main valve sets the pressure that the main valve requires to even consider opening. If the valve seat is too large for a close ratio main valve, the pop off will close at a higher pressure than the main valve needs to open.


If I understood you correctly then this is also a valid point, piston diameter should be significantly bigger than the seat diameter.
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Re: Pop off valve pilot problem

Unread postAuthor: Technician1002 » Mon May 11, 2009 11:58 am

jackssmirkingrevenge wrote:
Technician1002 wrote:1) The valve seat to valve diameter on the main valve sets the pressure that the main valve requires to even consider opening. If the valve seat is too large for a close ratio main valve, the pop off will close at a higher pressure than the main valve needs to open.


If I understood you correctly then this is also a valid point, piston diameter should be significantly bigger than the seat diameter.


You did understand me correctly. A pop off valve has 2 setpoints. 1 is the pressure it pops open. 2 is the point it snaps back closed. If point 2 is above the pressure needed to open the main valve, it will never open.

This can be "Adjusted" in some BBMG designs by altering the "Chamber" pressure independent of the pilot pressure. This is why many BBMG designs work well at high supply pressure and quit firing well before the pop-off stops functioning. At higher chamber pressure, the lower pop off pressure can make it function, but it doesn't go low enough to keep it operating as the supply pressure dwindles.
This is why designs with a high pressure source for the chamber and lower pressure to pilot pop off seem to work better.
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Unread postAuthor: Leonard » Mon May 11, 2009 5:22 pm

Thanks for the input guys!

I tried to increase the pilot volume, but i still did not work properly. I will probably double the piston to block the incomming air flow from going direcly into the exhaust (like the previous designs i worked on, but toolie style), i'll post if i get any good results.
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Unread postAuthor: Technician1002 » Tue May 12, 2009 5:16 am

Leonard wrote:Thanks for the input guys!

I tried to increase the pilot volume, but i still did not work properly. I will probably double the piston to block the incomming air flow from going direcly into the exhaust (like the previous designs i worked on, but toolie style), i'll post if i get any good results.


and
When i fill the chamber to 110 psi (5 psi below pop-off pressure), I can simply pull on the pop-off to make it fire and it pilot the piston perfecly


Since the main valve pops when you pull the pop off by hand the pressure is close. Setting the pop off valve to a lower pressure (lower closing) may dip the pressure low enough to get the main valve to pop.

If the pop off is not adjustable, a spring or heavy rubber band could be rigged to apply some pressure on the pull ring. May be worth a try.
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Unread postAuthor: jackssmirkingrevenge » Tue May 12, 2009 5:18 am

That's another good point, the pop-off pressure is quite close to the maximum pressure. I would thing that having it set lower (say 85 psi) would be a better idea.
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Unread postAuthor: Leonard » Tue May 12, 2009 3:37 pm

I understand your point, but i would rather have a setup that would work at any given pressure. Epoxy is curing at home, i should have some results in few days, we'll see ^_^
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Unread postAuthor: Technician1002 » Tue May 12, 2009 6:51 pm

Leonard wrote:I understand your point, but i would rather have a setup that would work at any given pressure.


Pop off valves is not the best trigger if that is your goal. A motor driven small ball valve that rotates a full 360 degrees would work as it would not re-close on dropping pressure. The motor driven modulator valve would not need to be electric. I'll play with some other ideas I'm kicking around that may meet that specific goal. I'm playing with some ideas to create a "Gumball Machine ... Gun" that would have a wide pressure operating range. I still have some math and theory to go over to get it to work on paper before spending $$$$ on a build.
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