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Piston not sucking back

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 not sucking back

Unread postAuthor: jmeyer1022 » Fri Jan 30, 2009 12:16 pm

So I made a new piston and it's not sucking back? Is my pilot to small for this? I can push it back with my finger and air comes out around it but it wont suck back on its own.
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Unread postAuthor: psycix » Fri Jan 30, 2009 12:23 pm

Some dimensions would be welcome.
What is the pipe and piston diameter? What are you using as pilot valve?
Could I see a picture of your piston?
Its probably not tight enough for the pilot valve you're using.
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Unread postAuthor: jmeyer1022 » Fri Jan 30, 2009 12:32 pm

http://www.spudfiles.com/forums/how-i-b ... 17206.html

piston is pill bottle that is about 2 1/2 inches long

pilot is like 1/8th inch, very small

The piston is tight in there but slides nice. It seals great, when i pilot through the sprinkler valve air does come out but there is still air behind the piston that when i push it back with my finger comes out.


FIXED! I was running at to low of PSI (testing indoors and didn't want it to be to loud) I put it up at 40 PSI and it worked perfectly!

Unfixed! The sealing face I was using was working fine at 40 but when I pushed up to 60 it blew apart... I'm trying rubber but it is vacuum sealing to the piece of pipe even with a steel washer that fits perfectly into the barrel
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Unread postAuthor: jmeyer1022 » Sat Jan 31, 2009 4:23 pm

I've been stuck trying to figure this out, really frustrating seeing I had it working for a bit. No idea why it won't suck back with the rubber, I'm guessing maybe not enough surface area? :/ any other suggestions for pistons besides 1 1/4 coupler or pill bottle. I look for a sanding drum like
http://www.spudfiles.com/forums/sanding ... 14066.html
but saw nothing at home depot. The guns all ready besides a new 100% functional piston.
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Unread postAuthor: ALIHISGREAT » Sat Jan 31, 2009 4:37 pm

The title says it all really.... "piston not sucking back"....

well if you don't understand why a piston works, the chances are your piston valve won't work.

Piston valves are not sucked back, they are pushed back as a result of higher pressure on one side....

So when you vent behind the piston, the higher pressure air on the chamber/barrel sealing side pushes it back.
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Unread postAuthor: jmeyer1022 » Sat Jan 31, 2009 4:51 pm

Sorry and thanks for the correction, But I'm confused as to why it was working with a piece of foam like material, which blew apart at higher PSI, but is not working with rubber, its suctioning onto what it is suppose to be sealing against. I'm guessing there is not enough surface area on my sealing face for it to be pushed back.
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Unread postAuthor: ALIHISGREAT » Sat Jan 31, 2009 4:59 pm

A roughly scale diagram would help alot :wink:
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Unread postAuthor: Pilgrimman » Sat Jan 31, 2009 5:23 pm

This is a long shot, but the problem might be that the rubber is pushing between the miniscule area between the washer and the inner barrel walls and causing too much friction with the barrel walls? :? That would be my best guess because if it worked with foam it should work with anything that seals...
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Unread postAuthor: jmeyer1022 » Sat Jan 31, 2009 5:48 pm

Pilgrimman wrote:This is a long shot, but the problem might be that the rubber is pushing between the miniscule area between the washer and the inner barrel walls and causing too much friction with the barrel walls? :? That would be my best guess because if it worked with foam it should work with anything that seals...


I honestly think this may be it, it drags a little bit when I put it in, I'm going to buy a washer that fits the ID of my 2" guide pipe perfect, right now it's off a bit hopefully this will solve it.
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