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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: chartreusesnot » Thu Sep 20, 2007 10:43 pm

I have recently made another potato cannon. THis one is a lot smaller, like almost pocket sized. It is completely stainless steel, and all the parts are rated for 300 psi and up. I made my own piston valve, which is the problem. The is is more like a "pistaphragm" because it can move freely, but is only a sheet of relatively thick neoprene on a bolt. This gun is coaxial. The piston is housed in a threaded metal reducer, so it is going over some rough uneven surfaces. The problem is, that air is escaping through the barrel, which should be easily fixed, by just airtightening the pistaphragm. the main problem, is that I think that more air is getting out of the pilot valve than the barrel, or at least such a significant amount that it is severely impairing the performance. I have only done very little testing, but due to the barrel leak, i can only take it up to 90 psi before the compressor and barrel leak reach equilibrium.

the piston looks like this:
||
||
<||==========
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||

EDIT:
The tail of the piston goes into the smaller end of the reducer, so the piston cant flip.
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Unread postAuthor: CannonCreator » Thu Sep 20, 2007 11:40 pm

Im having the exact same problem with mysprinkler valve, more air coming out of the pilot then the barrel. You can fix this by if the pilot is a blowgun put a smaller output fitting on it like a needle for filling basket balls and cut the needle at the start of it so there is a smaller hole. This will make more air escape the barrel im pretty sure.
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Unread postAuthor: Pete Zaria » Thu Sep 20, 2007 11:56 pm

CannonCreator wrote:Im having the exact same problem with mysprinkler valve, more air coming out of the pilot then the barrel. You can fix this by if the pilot is a blowgun put a smaller output fitting on it like a needle for filling basket balls and cut the needle at the start of it so there is a smaller hole. This will make more air escape the barrel im pretty sure.


This sounds like putting a band-aid on the problem to me... Rather than forcing less air to escape from the pilot, figure out WHY the valve isn't actuating properly.

I'm not very familiar with coaxial piston/diaphragm setups, so I hope one of the senior pneumatic pro's will help you out. My first ideas would be lubricating things and checking the seals, but this isn't my department.

Peace,
Pete Zaria.
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Unread postAuthor: CannonCreator » Fri Sep 21, 2007 12:05 am

You might also want to try putting a diffrent neoprane seal on the sealing face that might help out a bit.
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Re: piston problems

Unread postAuthor: clide » Fri Sep 21, 2007 3:51 pm

chartreusesnot wrote:The piston is housed in a threaded metal reducer, so it is going over some rough uneven surfaces.


That sounds like your problem to me. You want where the piston is to be smooth and round so you can get the best fit and won't have much air flowing back into the pilot as you are trying to fire.
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