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Help! Piston Sealing 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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Help! Piston Sealing Problem

Unread postAuthor: Yonderknight » Wed Jul 11, 2007 5:28 pm

Hi everyone!

I finally got my barrel sealing tee valve working, but there's a problem! It's a 2" valve with 1" ports, and a 1" ball valve for an exhaust. I'm using some converters attached to the 1" ball valve to connect a quick-release hose to fill it up. Then to fire it, I just remove the quick-release. The problem is that I'm not getting the full 1" exhaust (because it is reduced to 1/4" for the quick-release).

So to try to fix this, I thought I could fill it up, close the ball valve, take the quck-release off then fire it. The problem is when I close the ball valve and stop applying air to it, air starts leaking out of the barrel. I think it's because it's not sealing properly. I also have a spring pushing on the piston pretty hard, but it doesn't help it.

Also, I have a schrader valve on the tee, and when I try to fill from that, all the air just comes out the barrel.

I think the reason why this is all happening is because my sealing material is bad and it only seals when there is a great amount of pressure pushing on it. When filling from the schrader, there isn't enough flow to push the piston enough.

For the piston, I had a sheet of this orangy rubber stuff used for garden hose gaskets. I cut out a circle of that and hot-glued it onto my pison. Since that didn't work very well, I tried hot-gluing a rubber O-ring onto the piston where it hits the 1" pipe. This also didn't maker any difference. I tried lubeing everything with vasoline, but still nothing.

Does anybody know how I can get the barrel to seal better? I think when I cut the 1" pipe that the piston seals against it might be kind of rough, but it's too hard to reach it now to smooth it.

Thanks!
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Unread postAuthor: origin unknown » Wed Jul 11, 2007 5:33 pm

I have a similar problem on my gun exept it leaks around the back cap of my piston valve. I do the same thing with the quick disconnect because I forgot to get the Tee fitting I needed from the store. Just get some 1/8" rubber from lowes to use as a sealing face. It is found in the plumbing section and it is only like $1.50.
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Unread postAuthor: mopherman » Wed Jul 11, 2007 5:35 pm

my guess is that your brrel port isnt cut very squarly. try puting a coupler on the sealing end of your barrel.
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Unread postAuthor: noname » Wed Jul 11, 2007 5:37 pm

Put a 1/8" thick layer of silicone sealant on the sealing face. Silicone is very stretchy and will (hopefully) get itself into all the little imperfections on the piston seat.
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Unread postAuthor: Yonderknight » Wed Jul 11, 2007 5:43 pm

mopherman, I tried putting a coupler on it, but it comes out too far in front of he chamber port. Like, before the coupler is on, it's halfway above the chamber port. Afterwards, it's in front of the hole, so there's less room for the air to get in. I was thinking this would limit flow, would it? Thanks anyways!

noname, do you mean the silicone stuff that's used for pool o-rings? I will definitely try that, thanks!
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Unread postAuthor: noname » Wed Jul 11, 2007 6:08 pm

I don't know, I just mean silicone sealant. I usually use it to fix leaks and to seal up steel pipe threads.
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Unread postAuthor: mopherman » Wed Jul 11, 2007 6:15 pm

well you could try glueing a circle of rubber on the barrel port.
it may get ripped off though
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Unread postAuthor: MrCrowley » Wed Jul 11, 2007 6:44 pm

Ok the glue will rip off once the piston actuates, so it needs to be bolted on, then you need to make sure the bolt isn't leaking air so epxy the bolt up and make sure the seat(barrel) is perfectly flat, I used a bushing for mine so it already comes perfect for sealing.

I've used the orange gasket stuff and it seals fine so no problems there.
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Unread postAuthor: Yonderknight » Wed Jul 11, 2007 7:11 pm

The glue ripped off a couple of times so I just nailed it down (the nail is outside of the barrel port though, so there is no need to seal it).

Once I put vasoline all over it, it doesnt really stick to the barrel anymore though.

noname, are you talking about a silicone glue, or a silicone lubricant?
I have plenty of this stuff that's used to lubricate pool O-rings and other parts, will that work?

I think I could try glueing a coupler on the barrel port, I think that's the main problem. It's just I don't want the flow to be too limited.

Thanks everyone.
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Unread postAuthor: noname » Wed Jul 11, 2007 7:17 pm

I'm talking about sealant. It's made to seal stuff. You can get a tube of it for a buck or two at a hardware store. It's not glue or lubricant.
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Unread postAuthor: Yonderknight » Wed Jul 11, 2007 8:04 pm

Oooh okay, like the stuff you use to seal windows or something? Thanks a lot, I'll go by and pick some up sometime! In the meantime I'll just use the quick-release =\.
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Unread postAuthor: leachja » Thu Jul 12, 2007 1:05 pm

He doesn't mean the stuff you use to seal windows. That is silicone caulk. He is referring to silicone lubricant, it comes in a similar tube, but the tube is cardboard and it usually slightly smaller in diameter. You should be able to find it at a hardware store or Home Depot
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Unread postAuthor: spud yeti » Thu Jul 12, 2007 1:06 pm

Not putty, like the stuff you put on the edge of baths/sinks. The white/grey stuff that is almost rubbery when hard, but paste when not dry.
This stuff: http://www.bostik-us.com/images/subcate ... RTVgrp.jpg
btw, tell me if Im wrong! :oops:
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Unread postAuthor: Yonderknight » Thu Jul 12, 2007 3:10 pm

Yeah okay, I just picked up a tube of 100% silicone sealant used for baths/sinks for like 50 cents =D (it was on clearance). I'm gonna try it today, I'll post how it goes.
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Unread postAuthor: noname » Thu Jul 12, 2007 4:20 pm

Leachja, not to be a hardass, but shut up if you have no idea what you're talking about, especially when I (the person who you're talking about) know's exactly what I'm talking about. You're saying something 100% opposite of what I said.
leachja wrote:He doesn't mean the stuff you use to seal windows. That is silicone caulk. He is referring to silicone lubricant, it comes in a similar tube, but the tube is cardboard and it usually slightly smaller in diameter. You should be able to find it at a hardware store or Home Depot


noname wrote:I'm talking about sealant. It's made to seal stuff. You can get a tube of it for a buck or two at a hardware store. It's not glue or lubricant.



Yonderknight, sounds like you got the right stuff, hope it works. Also, most silicone sealants say the cure time is 24 hours. Do not mess with it before 24 (or more) hours are up or you will have to get it off and do it all over again.
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