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Piston Valve not so air tight

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 not so air tight

Unread postAuthor: -T0OL- » Mon Oct 30, 2006 8:32 pm

Hey, im making a coaxial, and i need some info...

What do you guys use for the sealing face on a piston? (preferably from a local hardware store (ie. not internet))

and for those valves that have screws in them so that you can have access to the piston for repairs, how do you make the screw holes air tight?
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Unread postAuthor: schmanman » Mon Oct 30, 2006 8:34 pm

depends on how big the piston is. for smaller pistons, use bike or car innertube rubber. for larger piston valves, neoprene rubber is preferred.
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Unread postAuthor: ProfessorAmadeus » Mon Oct 30, 2006 8:54 pm

For the screws you can use teflon tape.
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Unread postAuthor: -T0OL- » Mon Oct 30, 2006 9:45 pm

cannon is coaxial, chamber is 2" pvc and barrel is 3/4" pvc
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Unread postAuthor: POS » Tue Oct 31, 2006 8:11 am

in a coax you shouldn't make a very big piston. Take a piece of rubber that fits the chamber. Make sure it is 5 mm thick. That's all. It seals of the barrel, and lets enough air trough to fill the chamber. Pilot it from the rear of course
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Unread postAuthor: -T0OL- » Sat Nov 04, 2006 10:08 pm

ok right now what i have is a 1.5" endcap filed down to fit inside chamber with the barrel going inside the end cap. but for some reason i cant get it to seal, whether ther isnt enough pressure fast enough in the pilot area (bike pump) or if the barrel isnt centred or if the barrel wasnt cut square...

I'll try your idea POS. thanks.
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Unread postAuthor: ProfessorAmadeus » Sat Nov 04, 2006 10:13 pm

You usally cant use a bike pump to get a piston to seal. Use a compressor.
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Unread postAuthor: -T0OL- » Sun Nov 05, 2006 5:40 pm

dang... i dont have any compressors near by that i could use... anyone know a cheap compressor that will work?
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Unread postAuthor: jrrdw » Sun Nov 05, 2006 6:55 pm

Turn your end cap around so the outside seals against the barrel. The way they are shaped on the inside, i don't ever see it sealing. If you add a spring (lite weight) to push the piston against the barrel, a bike pump should work. You can get little 12v tire inflaters for cheap on ebay.
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Unread postAuthor: Flinchy » Sun Nov 05, 2006 8:29 pm

that or just build with the piston pressed against the inner barrel so that its sealed even when its not pressurized. that way you can use a bike pump. heck i made a tenis ball cannon with a 10' length of 3" pvc that used two 1" sprinkler valves to let out the pressure. i used a bike pump to pressurize it. believe me, its most likely possible to pressurize a coaxial with a bike pump.

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Unread postAuthor: -T0OL- » Sun Nov 05, 2006 9:40 pm

ok, right now there isnt enough room to turn the plug around unless i shorten the barrel, and i wouldnt know where to find a spring that would work....

so right now my options are (without alot of work)
-air compressor
-different piston (POS idea)
-spring behind piston

did i miss one?
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Unread postAuthor: jrrdw » Mon Nov 06, 2006 7:42 am

Hardware stores that carry lawnmower parts normaly have a wide selection of springs, or ebay should have what you need. I measured from the back of my piston (in the closed/sealed postion) to the sprinkler valve, and used a piece of balsa wood inbetween the spring and sprinkler valve, (to protect the valve from the spring, also to make the spring stronger). You can adjust the spring tenson by useing longer or shorter piece of wood. It has been working good for me so far.
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Unread postAuthor: -T0OL- » Mon Nov 06, 2006 8:36 pm

alright thanks for the comments. ill have to try a few of these
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