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Piston problems...

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 problems...

Unread postAuthor: warhead052 » Mon Apr 09, 2012 3:04 pm

I am getting this weird suction problem with my piston. No matter what I do it won't stop getting a suction where it seals, and it vent's all the air out the pilot before it releases. The only way to fire it is to ram something solid down the barrel to force it to open. No pics because my camera is broken, but I will try to find a working camera if possible... I am using 25 psi right now before I glue the final piece on just for safety (so I don't blow the back part out onto my self...) I know I should glue it, don't bother telling me this but I won't be able to glue it till I fix the piston problem...
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Unread postAuthor: sagthegreat » Mon Apr 09, 2012 4:41 pm

you need the piston to be tighter, try warping it with tape, so all the air doest come out when you pilot
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Unread postAuthor: Gun Freak » Mon Apr 09, 2012 4:43 pm

It's a piston fit issue.

Fix that joint, it'll allow you to use higher pressure and the valve might open.

By the way there is no such thing as suction in a piston valve.
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Unread postAuthor: warhead052 » Mon Apr 09, 2012 5:32 pm

Its not the fit of the piston, it moves freely inside, its just when it seals, it won't balance the pressure and open. Its more of a toolies valve, and I am wondering if that's the problem...
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Unread postAuthor: Gun Freak » Mon Apr 09, 2012 5:56 pm

Can you describe the setup?
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Unread postAuthor: warhead052 » Mon Apr 09, 2012 6:32 pm

2"x2' pvc, with a 2"-1/4" reducer at one end, and a cap at the other, I shaved the inside of the reducer down to be able to slide a 1 1/4"x 3" piece of pvc in for the piston to seal against. Uses a quick disconnect male and female to pilot and fill. Piston is about 1' 8" long and fits inside the tube perfectly, has o-rings on it, non check but have a small eq gap, very small... 1 1/4" sealing face, works perfectly. Anything else you need to know?
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Unread postAuthor: Gun Freak » Mon Apr 09, 2012 7:27 pm

Y U No check valve? I bet your o-rings don't seal enough.
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Unread postAuthor: warhead052 » Mon Apr 09, 2012 8:17 pm

Any idea on why it won't slide back though? My o-rings are a fairly tight fit... The piston can slide freely with no oring's but with them it slides but only when there is a force acting on it.
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Unread postAuthor: Gun Freak » Mon Apr 09, 2012 8:51 pm

Gun Freak wrote:Y U No check valve? I bet your o-rings don't seal enough.


Pilot valve is too small/too much leakage around spool, not enough pressure differential.
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Unread postAuthor: warhead052 » Mon Apr 09, 2012 9:08 pm

Dude, its not too small. What I do to fire is just disconnect the fill line and it releases the air straight out. Just like you would disconnect an air line from an air tool. All the air empties out in like 2 seconds, from 25psi to 0.
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Unread postAuthor: Gun Freak » Mon Apr 09, 2012 9:28 pm

I know what you mean. But something isn't adding up here! The only logical explanation is the piston fit.
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Unread postAuthor: warhead052 » Mon Apr 09, 2012 9:53 pm

I dunno... Plus my mom has banned me from going to lowes or home depot now, because I "need to save my money for more important things", so now I have to solve this without spending money....
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Unread postAuthor: velocity3x » Mon Apr 09, 2012 9:57 pm

warhead052 wrote:Any idea on why it won't slide back though? My o-rings are a fairly tight fit... The piston can slide freely with no oring's but with them it slides but only when there is a force acting on it.


Warhead,
I had the same problem with the first cannon I built. The piston was in a Tee type housing. The valve seat was about .375 " forward of the tee intersection. I didn't allow adequate clearance around the piston at the valve seat. When the piston would try to open, the gas passing around the piston to the barrel would drag and hold the piston on the seat. I increased the bore around the piston adjacent to the valve seat and the problem was cured.
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Unread postAuthor: warhead052 » Mon Apr 09, 2012 9:59 pm

That's part of the problem I guess then, velocity... Its not in a tee, its a toolies valve, but I do have one last idea before I scrap this project and just make a tee piston (which I don't want to do, I wanted to make a M1A1 rocket launcher initially...)
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Unread postAuthor: warhead052 » Tue Apr 10, 2012 2:10 pm

*doublepost I know...* FIXED IT, sort of. Still have a issue with it not sliding back fast enough, but it fires at 45psi now! I am going to glue everything and test it at 100psi and see what happens!

Scratch that... Its broken AGAIN. Now it won't fire. Tried it at 50psi and it won't work. Piston didn't move back, dunno why.
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