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Having trouble with my coaxial

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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Having trouble with my coaxial

Unread postAuthor: maverik94 » Tue Oct 20, 2009 4:54 pm

Ok, I am working on a small coaxial. It had a barrel of 1/4" brakeline (really more like 1/8"), and a chamber of 1/4" brass (really more like 3/8") Anyway, I use a schrader valve to fill and shoot the gun. The gun seals just fine and there are no leaks. However, when I press the schrader valve, all the air just comes out of the schrader. I know the problem is that the piston is just not getting sucked back, because when I push down the piston from the muzzle end, all the air comes out the muzzle. I have to use the schrader valve for the actuation of the gun because of my design. Does anyone have any ideas on how to remedy this problem? Thanks!
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Unread postAuthor: cannon monkey » Tue Oct 20, 2009 5:06 pm

u using ur finger to depress the pin or u using a tac or key or somthing?
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I'll give a stab at it.

Unread postAuthor: Procyon » Tue Oct 20, 2009 5:14 pm

My guess would be you need a new piston design. One that seals airtight back to the valve when in the closed position. See if the piston shown on this link would help.

http://www.spudfiles.com/forums/viewtopic.php?t=160
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Unread postAuthor: jor2daje » Tue Oct 20, 2009 5:32 pm

Did you epoxy your shrader in?, the problem is either your piston doesnt fit tight enough or its getting stuck, you could try putting some liquid lube through you shcrader to hopefully fix it if its getting stuck. But I think you probably made your piston too loose, so if its not servicable you may have to start over. :(
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A 2nd thought.

Unread postAuthor: Procyon » Tue Oct 20, 2009 5:46 pm

Since your gun is so small, would it be possible to modify the guts of a Schrader valve to place inside your piston to act as a one-way valve. Sealing off the piston might give you enough of a vacuum that it opens with the set up you have.
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Unread postAuthor: maverik94 » Tue Oct 20, 2009 6:59 pm

its a small length of brass thinwalled pipe filled with epoxy and a neoprene disc on the end
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Unread postAuthor: jmadden91 » Tue Oct 20, 2009 7:19 pm

ok yeh its very similar to mine (a piece of copper with a solid rubber plug inside). I simply wrapped the piston in one thin layer of teflon tape and it was enough for me

Edit: or you can always just use a bigger pilot valve
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Re: Having trouble with my coaxial

Unread postAuthor: boyntonstu » Tue Oct 20, 2009 7:34 pm

maverik94 wrote:Ok, I am working on a small coaxial. It had a barrel of 1/4" brakeline (really more like 1/8"), and a chamber of 1/4" brass (really more like 3/8") Anyway, I use a schrader valve to fill and shoot the gun. The gun seals just fine and there are no leaks. However, when I press the schrader valve, all the air just comes out of the schrader. I know the problem is that the piston is just not getting sucked back, because when I push down the piston from the muzzle end, all the air comes out the muzzle. I have to use the schrader valve for the actuation of the gun because of my design. Does anyone have any ideas on how to remedy this problem? Thanks!


Here is a Schrader pilot firing at 25 psi:
[youtube]http://www.youtube.com/watch?v=VG3jISenqas[/youtube]
It is quite amazing!

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Unread postAuthor: jmadden91 » Tue Oct 20, 2009 7:47 pm

Think thats impressive stu,

Jack pilots this monster with only the schrader valve
Image

Check it out
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Unread postAuthor: maverik94 » Tue Oct 20, 2009 9:32 pm

Ok, Thanks for the input, I'm gonna try some different things tomorrow, I'll get back with y'all, but now I must hit the sack!
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Unread postAuthor: MrCrowley » Wed Oct 21, 2009 1:06 am

Jack could pilot that cannon coz he designed the pilot volume to be tiny and the valve to be air tight if I remember correctly. A schrader valve will have trouble piloting even a few cubic centimeters of air when the piston isn't air-tight, even a blowgun will give you bad performance. Go with a 1/4" ball valve or a homemade push-button.
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 21, 2009 1:52 am

MrCrowley wrote:Jack could pilot that cannon coz he designed the pilot volume to be tiny and the valve to be air tight if I remember correctly.


The piston in that case isn't air tight, but it's an epoxy cast in the tube it travels in so it's an extremely tight fit. Also, as you pointed out I limited piston travel to 5mm (19mm bore) so the pilot volume is as tiny as it can be. Heck, I even cut down the schrader to its minimum possibe length to avoid that little bit of extra pilot volume in its body :D

EFFICIENCY!!!!

:)
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