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Barrel-Sealing Piston Problem!

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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Barrel-Sealing Piston Problem!

Unread postAuthor: spudkilla224 » Sun Sep 14, 2008 8:42 pm

I am having troubles with my barrel-sealing cannon. for the piston i use a 1 1/4" coupling. with a 2" fender washer. with a 1/8" thick rubber washer that is also 2" in Diameter. the sealing face seals against a 1.5" pvc pipe. i chamfered the inside of the 1.5" pipe.

when i fill the chamber, the piston seals against the sealing face pipe. but air still leaks out around the 1.5" pipe.

any idea's on how to fix this would be apreciated.

thankyou
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Unread postAuthor: man_o_brass » Sun Sep 14, 2008 8:48 pm

Perhaps someone else can help, but I'm going to need a better description or a picture before I know just what you're describing. Can you give us some more details?
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Unread postAuthor: hi » Sun Sep 14, 2008 10:27 pm

its a little late now, but make sure your cut on the end of the barrel is strait. you may also want to try two or three rubber washers. if that doesnt work try a different sealing material. i find that cutting one of those abs rubber end caps makes an ok sealing face, but the problem with it is it only seals at a fairly high pressure, around 100 psi.
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Unread postAuthor: McThunderThighs » Sun Sep 14, 2008 11:26 pm

If I get you right: Your piston seals against a 1.5" barrel (or whatever goes out as exhaust), and you are wondering why air gets around the piston to the space around the 1.5" pipe? So the barrel seals fine (no air is coming out the barrel) but around it?

If that is right...well, that's how a barrel sealing piston works.

Definitely could use a picture though. Paint is your friend!
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Unread postAuthor: MrCrowley » Mon Sep 15, 2008 12:01 am

So the piston seals against the barrel, but air still leaks through the barrel?

If you have used a bolt on your piston, put some epoxy over the bolt on the sealing face of the piston. Air is probably leaking through your piston or one of the washers and out the bolt.

That's quite a common problem, so try that first.

Make sure you use epoxy too, and not hot glue or some crap like that. Also try not to get epoxy all over the sealing face :P
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Unread postAuthor: JDP12 » Mon Sep 15, 2008 7:57 am

the rubber could be too hard of a rubber- thats what happened to me- now i'm trying a mousepad- neoprene material next. If it's too hard of rubber it cant make a good seal
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Unread postAuthor: mark.f » Mon Sep 15, 2008 8:44 am

2" washer > 1.5" barrel.

That seems to me to be the problem, in which case, a seal of metal against hand-cut PVC plastic ain't going to be too great. :wink:

If this is the case, just make sure your washer is small enough to fit inside of the barrel.


If I read you wrong, first of all, sorry, second of all, what size is the washer that is holding the rubber onto the piston? If it is too small, it will allow the piston-face to fold up and allow leaks through the barrel when an edge pops into it. If it is too big, it will cause the same problem described above.
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