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O-ring compression seal- leaking

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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O-ring compression seal- leaking

Unread postAuthor: saefroch » Wed Aug 25, 2010 8:15 am

I have begun a new build (which will be posted in the showcase soon if I can fix this issue) in which I am attempting to remove the rubber or whatever sealing material from the piston itself and place it instead on the sealing face. Quite a challenge. The only material I have found so far that can stand up to that kind of abuse is an o-ring which luckily has the same OD and ID as the side of the bushing I am attempting to seal to. The only problem is that it's not really sealing. It will seal at about 40 psi or so, but once I exceed that air starts to leak down the barrel at an incredible rate. I am sure it is not leaking at all right around 40 psi. The front of the piston is pretty smooth, but not like a mirror. Any suggestions?
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Unread postAuthor: Technician1002 » Wed Aug 25, 2010 10:11 am

On my Mouse Musket (see link in my sig) I used an o ring in the seat. To keep it from leaking, I embedded it in caulking. I used it regularly at 100 PSI with no issues except an occasional cracked piston.

My larger Dragon cannon is built with the same valve seat design.

If your piston is flexing under pressure and the face is distorting under pressure, this can cause the leak you are talking about. What is the piston made of and can it hold the force on the area?
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Last edited by Technician1002 on Wed Aug 25, 2010 10:16 am, edited 1 time in total.
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Unread postAuthor: saefroch » Wed Aug 25, 2010 10:14 am

The piston is solid steel. The o-ring is embedded in tar, I found caulk to be far too weak. It is possible that the o-ring is distorting under the pressure, but that occurs to me as unlikely.

EDIT: Problem identified. The piston is forcing the o-ring more to one side and slipping into it, causing the leak. Any suggestions on how to fix that?
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Unread postAuthor: Ragnarok » Wed Aug 25, 2010 11:26 am

Sorry to hijack this thread, but...

Technician1002 wrote:see link in my sig

On the note of your sig, could you change it so that it's using hyperlinks rather than URLs? As it is, it's taking up more lines than it really needs to (and strictly, more lines than it's allowed to), and is just messier.

For example, if you changed it to (which, with the simplified URLs, still fits within the 300 character limit):

It's taking up ONE size 9 line (at least, it would be without the reduced width of the quote box), not six size 12 lines. I hate to seem quite so pedantic, but people don't come to this forum to read the signatures, and many really take up a lot more space than they need to.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: Technician1002 » Wed Aug 25, 2010 11:59 am

saefroch wrote:EDIT: Problem identified. The piston is forcing the o-ring more to one side and slipping into it, causing the leak. Any suggestions on how to fix that?


If the piston is placing too much pressure on the o ring to distort it to where it leaks, the piston will need support besides the o ring. On my launchers the edge of the o ring barely extends above the valve seat surface. The o ring compresses slightly until the piston and the barrel contact.

Try making a deeper socket for the o ring so it compresses and distorts less.

I don't know how much pressure the Mouse Musket will take. I have tested it to 125 PSI.

Is the o ring smaller than the OD of the piston? The edge (Corner) of the piston should not rest on the o ring. Just the flat face of the piston should rest on the ring. Alignment of the piston and barrel is important. Is the piston hitting off center, and thus tilting and pushing the o ring sideways? Be sure the barrel is carefully centered.

For example, if you changed it to (which, with the simplified URLs, still fits within the 300 character limit):

Thanks. I don't play with HTML much. A PM might have been better than a hijack.
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Unread postAuthor: Ragnarok » Wed Aug 25, 2010 12:21 pm

Technician1002 wrote:A PM might have been better than a hijack.

Unfortunately, I realised that after posting, and after I'd edited out the other half of the post - which was rendered pointless by saefroch's edit that he'd found the problem. :roll:
I would blame the poor night's sleep I had, but for humour factor, I think I'll blame it on the government.
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Unread postAuthor: saefroch » Wed Aug 25, 2010 1:54 pm

Thanks for the help, Technician. I got the problems identified. Now it's just another regular engineering challenge to fix them.
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