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Semi-Auto Pneumatic

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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Semi-Auto Pneumatic

Unread postAuthor: JohnnyBOOM » Fri Jan 23, 2009 3:48 pm

I was looking for thoughts, criticisms and improvements for the following design before I build it. Thanks for your input! :D
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Unread postAuthor: iknowmy3tables » Fri Jan 23, 2009 5:23 pm

could you please indicate for me the path of the air to the barrel, it appears the air will escape way before the bolt closes, air dumps really fast on barrel sealing valves the which is why the typically the recommended stroke length for piston valves in less than an inch, speaking of piston stroke length the you have a HUGE amount of dead space due to your long piston stroke
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Unread postAuthor: skyjive » Fri Jan 23, 2009 5:42 pm

I agree, as soon as the valve opens the gun will fire with the hopper only like 5% sealed off, if I understand it correctly
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Unread postAuthor: JohnnyBOOM » Fri Jan 23, 2009 5:58 pm

The valve follows the barrel forward with the piston. The valve bottoms out just before the piston closes the forward air space. After the seal is broken, spring force pulls the valve back to it's seated position. This should be a very fast opening valve. When the air switch is released, the forward and rear chamber of the gun are pressurized at equal rates which will make the barrel/piston assembly move back to re-seat against the valve, since there is much less volume in the forward section.
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Unread postAuthor: psycix » Fri Jan 23, 2009 6:52 pm

Its possible, but you'd have one heck of a big dead volume there.
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Unread postAuthor: iknowmy3tables » Fri Jan 23, 2009 7:03 pm

I see it's a moving sealing face, the spring could makes the seal pop off at inconsistent positions with different pressures, you could eliminate it completely since the piston will push is back in place and it wont speed up the opening be much, I also see no way to make sure the piston resets when filled such as a return spring, or a flow restriction on the inlet to the chamber , with triple o-ring seals will cause huge amount of friction and they are not necessary for your design and will make the piston/bolt more resistant to move and reset, you also should not make your gun dependent on an exotic valve there is absolutely no reason for such a valve, use a 3-way valve and a check valve (see diagram) this way you can buy a cheaper bigger aster opening more common valve

my sujestion for changes below, how every I think you should rework it even more because the ridiculous pilot volume and dead space
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Revised Design

Unread postAuthor: JohnnyBOOM » Sun Jan 25, 2009 5:01 am

I made a new drawing to clarify the operation and incorporated some of your advice. I still don't think a spring is necessary in front of the piston since it will be very close to bottomed out just before the valve pops. I did put in a cushion however.
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Revised Design

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Unread postAuthor: jackssmirkingrevenge » Sun Jan 25, 2009 6:26 am

Looks awfully complex, what's the advantage over the blow-forward valve?
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Unread postAuthor: JohnnyBOOM » Sun Jan 25, 2009 11:26 am

Well, I suppose the thinking behind this design was that it might also be used as a Piston Hybrid. I wanted to make it work as a pneumatic first though.
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Unread postAuthor: iknowmy3tables » Sun Jan 25, 2009 5:34 pm

for you massive dead space I who'd insist on making the piston smaller, and putting in a tee so the is space for much more chamber volume in relation to that dead space
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