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Piston valve problem (valve opening prematurely)

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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Piston valve problem (valve opening prematurely)

Unread postAuthor: noob of noobs » Sun Sep 02, 2007 2:58 pm

I'm working on a piston valve, and I'm having a problem. (Yes, I've searched) When I pressurize the gun over about 15 PSI, the valve opens without being triggered. After confirming the problem, I took the valve apart, and looked at the internals. Everything seems to be working fine. The piston seal is tight when I press it against the lip, and the piston is moving fine. The back cap structure is pretty much airtight except for a small leak due to a schrader valve, which I don't think is enough to vent the pilot area . I think that the sealing face on the piston might keep getting sucked into the barrel due to suction because everytime the valve opens prematurely, the rubber is warped on one end.
How would you recomend I fix this? I think that the edges of the ruber need to be secured to the body of piston better, but how?

Thank's for your time
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Unread postAuthor: hi » Sun Sep 02, 2007 3:55 pm

i had a simulare problem with my piston valve. the seal face kept getting sucked out. i fixed that with jb weld. it hasnt moved since... :wink:
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Unread postAuthor: CannonCreator » Sun Sep 02, 2007 7:47 pm

dude try a putting a long, kinda weack spring in the piston vlave behind the piston. It might help a little bit.
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Unread postAuthor: mark.f » Sun Sep 02, 2007 8:46 pm

How is the sealing face attached to the front of the piston?

Usually, if the connection is not strong enough, (i.e. it was GLUED on there, or a washer of sufficient size was not used), the sealing face will be forced through the breech, resulting in something getting shot you did not want shot. :?

Usually, the problem can be fixed by using a washer a little smaller than the inside diameter of the breech, and the right size for the screw you are using to attach the sealing face to the piston.

EDIT - and if the edge is just getting pulled into the breech, try making a bigger sealing face that extends past the ends of the breech better, while you're changing the washer. If the sealing face is as big as it can be and it's edges are getting sucked into the breech, that's a telltale sign to me that you're breech might be too close to the piston carriage I.D., which is usually bad as well.
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Unread postAuthor: frankrede » Sun Sep 02, 2007 9:11 pm

CannonCreator wrote:dude try a putting a long, kinda weack spring in the piston vlave behind the piston. It might help a little bit.
What would that do?
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Unread postAuthor: noob of noobs » Sun Sep 02, 2007 9:15 pm

Thanks guys
The sealing face is a rubber washer with a screw and washer holding it in the center of a 1" PVC female adapter. When the screw/bolt is tightened, the washer (which is a bit small come to think of it) kind of flares, so I don't tighten it that much, but enough to make it airtight.
The sealing face is about the same size as the lip's outer diameter, because the lip is made of half a 1" PVC coupler. I thought this'd provide a more uniform lip.
The inner diameter of the breech is still 1" but the coupler makes the total outer diameter almost 1.5" (the diameter of my piston) Do I need to take the coupler off?

Again, thanks for your time.

EDIT: I think Cannon Creator is trying to put more force on the sealing face so that that force would overide the suction. However, a spring might make the valve a bit slower, and since there'd still probably be more suction than spring power, I'm not sure that would work.

ANOTHER EDIT: I also forgot to mention my schrader squeaks when I'm pressurizing it.
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