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PVC Semi Auto Valve Idea

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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PVC Semi Auto Valve Idea

Unread postAuthor: shardbearer » Fri May 20, 2011 3:53 am

So I was trying to develop a semi auto, low pressure (50-100 psi), semi auto valve that was self contained (only one piece), cheap, around $10, and all the parts could be bought at any hardware store. This is eventually what I came up with. The three nesting pipes are 1/2" pvc, 1/2" cpvc, and 3/8" pex. In order to get them to fit properly, the cpvc is sanded on the outside and the pex is hammered and solvent welded to the inside.

Black=PVC
Blue=PEX
Green=Locknuts
Purple=Threaded Rod
Red=O rings

Filling:
Image

Firing:
Image

Slide forwards to fill the small tank from the big one or air compressor, and slide back to disconnect and fire.
Do you guys think it would work? Could I make the forward piston sliding like a QDV for faster opening?
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Last edited by shardbearer on Fri May 20, 2011 5:49 am, edited 1 time in total.

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Unread postAuthor: POLAND_SPUD » Fri May 20, 2011 4:14 am

Could I make the forward piston sliding like a QDV for faster opening?
Yeah, why not

Nice design
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Unread postAuthor: shardbearer » Fri May 20, 2011 4:17 am

I was thinking that it would be useless cause there would be pressure behind the piston pushing it forward, as opposed to all of Technician's cannons, where there is no pressure back there. All it would accomplish would be a longer trigger pull.
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Unread postAuthor: shiny455 » Fri May 20, 2011 5:23 am

im not sure if your going to understand me but instead of having a small chamber could you not use the space between the two pistons as the small chamber and put the barrel where you put the small chamber but i do like the idea
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Unread postAuthor: shardbearer » Fri May 20, 2011 5:54 am

I thought about that for while, but there are a couple problems with that.
1. This would be a 2 way valve instead of 3 way. That means that each shot would be a different amount of air, because it is dependent on the time you leave the trigger open. You might as well use a high flow blowgun or spring closing ball valve.

2.That would mean either a very small air tank or a very large trigger mech, and I also want interchangable air tanks.
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Unread postAuthor: Copperboy » Fri May 20, 2011 6:29 am

shardbearer wrote:I was thinking that it would be useless cause there would be pressure behind the piston pushing it forward, as opposed to all of Technician's cannons, where there is no pressure back there. All it would accomplish would be a longer trigger pull.


There would indeed be pressure behind the piston (if by piston you mean the forward sealing part), but keep in mind that the very same pressure also acts on the other sealing part. The resulting force acting on the whole thing equals zero, so the only thing acting against moving it back is friction. (Exactly the same case as Tech's QDV). However, ensure that it can't fly out of the casing and into your face. :wink:

EDIT

And in the diagram, the casing is to short. Lengthen it so that the backside of the piston can't escape.
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Unread postAuthor: jackssmirkingrevenge » Fri May 20, 2011 7:20 am

Why not have the "small tank" coaxial to the valve tube? What you want is for flow from the main tank to be blocked off and the small tank to be opened to the barrel at the same time, a bit like a slide valve.
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Unread postAuthor: Gaderelguitarist » Fri May 20, 2011 10:37 am

And with a coaxial firing chamber, you could increase the porting area by mimicking a QDV housing and having three ports.

I wonder about working the charging handle into a more ergonomic solution. I'm thinking pump action shot gun, where the foregrip charged the firing chamber, and the trigger would release a sear on the piston. You would of course have to throw a spring in there, but it would definitely be interesting.
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Unread postAuthor: POLAND_SPUD » Fri May 20, 2011 10:58 am

1. This would be a 2 way valve instead of 3 way. That means that each shot would be a different amount of air, because it is dependent on the time you leave the trigger open. You might as well use a high flow blowgun or spring closing ball valve.
nahh dude it is ok... it won't be a problem
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Unread postAuthor: Technician1002 » Fri May 20, 2011 12:05 pm

Instead of a "Main Tank" and a "Small tank" The space between the pistons could become the tank by using reducers and making the tank larger. The basic valving would be like the ShureShot fuel meter. Instead of metering a shot of propane into the chamber, it could meter a shot of pressure air into the barrel.

I'll play with it on how a tank arrangement like the Marshmallow cannon can get a second piston to charge the tank and then seal and vent the pipe so the front valve can be a QDV. It is a concept worth considering.
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Unread postAuthor: shardbearer » Sat May 21, 2011 3:10 pm

Anyone have opinions on neoprene washers sandwiched between metal ones vs floating o rings vs non floating? And metal washer piston vs PVC?

And I am not using the space in the middle as a tank because I want to be able to change the tank size.
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