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Piston Valve Design

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 Design

Unread postAuthor: DonTheLegend » Wed Apr 30, 2008 8:37 pm

This is our design for a new chamber sealing piston valve. Just looking for thoughts and feedback for improvement/ what will or won't work. The internals all have to be machined. Basically, the piston (red) will slide on a tube that is mounted to the back plate (gray) and press against an o-ring in the front of the valve. This assembly is built inside of a 4" SCH 40 tee. The green part is a 4" to 3" reducing bushing. Not pictured is an o-ring on the inner tube to seal it with the piston and an o-ring on the back plate to seal it with the tee. The back plate will be fixed to the tee with machine screws similar to a Supah valve. Right now we are planning on building the piston and backplate out of aluminum.

Note: This isn't the first time we've done this. Built the last piston valve inside a non pressure rated tee and had it blow up (duh we deserved it). Not making that mistake again. It was fun while it lasted though, we shot a tennis ball over a quarter of a mile before it turned into shrapnel.
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Attachments
valve closed.JPG
Valve closed and seated
valve open.JPG
Valve completely opened
all parts.JPG
Exploded view of all parts
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Unread postAuthor: FordGtMan » Wed Apr 30, 2008 8:40 pm

Looks good, but if your machining aluminum, why dont you go ahead and make a whole metal gun instead of pvc? Then there will be 300+ instead of 200... :)
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Unread postAuthor: JDP12 » Wed Apr 30, 2008 8:43 pm

yea, i'd say go with an all metal gun
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Unread postAuthor: DonTheLegend » Wed Apr 30, 2008 8:45 pm

Limited funds. All we are remaking from the original gun is the valves, we are reusing the tanks, barrels, and frame.

Side note: Credit to LynyrdSkynyrd for his part in designing this valve as well as the original cannon.
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Unread postAuthor: FordGtMan » Wed Apr 30, 2008 8:48 pm

Ahh, oh well, but thats going to be one fast opening valve too :shock:
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Unread postAuthor: LynyrdSkynyrd » Wed Apr 30, 2008 8:54 pm

lol hellz ya. The original was damn fast too. Course, as stated, it kinda blew up in my face. :tard:
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Unread postAuthor: THUNDERLORD » Wed Apr 30, 2008 9:21 pm

Looks great.
Were tou planning to vent it through the hole in the back plate (gray)?
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Unread postAuthor: DonTheLegend » Wed Apr 30, 2008 9:32 pm

Yes, the details of the fill port are kind of up in the air, but that part is pretty simple. Possibly a tee with a quick disconnect on one side and the dump valve on the other. In the past we used 3/4" sprinklers for the dump valves. Right now the hole in the backplate is 1" diameter.
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Unread postAuthor: Hotwired » Wed Apr 30, 2008 9:34 pm

Won't work unless you're planning on a unusual two pressure system.

Surface area of the pilot side has to exceed chamber side for the valve to close at all.

From what I can see the red piston has a large surface area exposed to the chamber and a very small pilot area in the "stem" part.
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Unread postAuthor: JDP12 » Wed Apr 30, 2008 9:38 pm

ah yes, hotwired thats a good point that i overlooked thanks
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Unread postAuthor: DonTheLegend » Wed Apr 30, 2008 9:53 pm

Overlooked that point. In theory this would still work if the pressure on the back was much greater to overcome the difference in areas correct? This would follow the formula Force = pressure x area so assuming a 1" pilot ID and a 3.5" sealing face on the piston

F(sealing face) = 38.48 sqin x 100 psi = 384.8 lb

384.8 lb / 3.14 sqin = 122.293 psi required to over come the force on the sealing face and hold the piston closed.


In theory would this allow the valve to open faster due to the reduced pilot volume? (the previous cannon used a split pressure system so the parts are already in place to continue to use it on the new version).
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Unread postAuthor: THUNDERLORD » Wed Apr 30, 2008 9:57 pm

Hotwired made a good point I overlooked.
But it's similar to a design I was thinking of(I'm sure someone's used already), to put holes in the tube part of the gray back plate.

That way when it opens the red part (piston shaft) will actually block the holes to prevent excessive pilot volumes.
Make sense?
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Unread postAuthor: FordGtMan » Wed Apr 30, 2008 9:59 pm

Isn't the equalization going to be inside the piston? If its a chamber sealer, the piston doesnt need a pressure in front of it to knock it open like a barrel sealer. Im confused... :?
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Unread postAuthor: LynyrdSkynyrd » Wed Apr 30, 2008 10:03 pm

On ours the inside of the red piston is all self contained pressure. that pressure seals against the o ring on the green part to seal the chambers. so when the counter pressure in the piston is released it dumps the volume of air.

And the holes wouldn't work because the pressure that escapes through them will just empty out through the barrel.
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Unread postAuthor: DonTheLegend » Wed Apr 30, 2008 10:03 pm

The pilot pressure is inside the piston. But pressure from the chamber is still what opens the valve. More specifically its the force exerted on the piston by the pressure in the chamber. Since Force = Pressure x Area, the opening / closing of the valve still depends on a difference in surface areas or air pressures.
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