GTA
All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Topic started by: FuzzyGrub on January 25, 2019, 11:53:04 AM
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I have had this love/hate relationship with making 0.187" ID TP from poly tubing for mrod/prod. Love how easy they are to make, but they need to be near perfect length and perfectly square to seal without collapsing over time. On my latest project, a 25 cal Fortitude, having issues with getting a good seal. Part of this, is because it is a custom length to figure out. Getting the tubing square, hasn't been that hard.
Anyway, I had some thoughts on improving it, which I'm sure are not new or novel ideas, and would like some feedback before jumping in.
The biggest issue I have, is sealing at the valve interface. The flat surface at the top of the valve, while surrounded by he ring of the airtube, has a big gap to leak by, if the contact seal is not perfect. My thought is to bore into the valve and let it seal within the valve. The TP would go below the surface of the valve and part of the OD would provide sealing in addition to whatever seal on the bottom. Thoughts?
The other idea, that would go with the above, is the construction of the TP. Use brass tubing, 0.014" wall with 0.187" ID. https://www.mcmaster.com/8859k23 (https://www.mcmaster.com/8859k23) and streach this latex tubing over it: https://www.mcmaster.com/5234k972 (https://www.mcmaster.com/5234k972). ie it would side seal. Test fitting to the breech and barrel seems nice and tight. It would need to insert into a valve opening as I described above.
I appreciate any comments/suggestions.
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OK, I started on the process. I used my step drill process to open the valve exhaust to the 1/4" OD step. The first step on the drill is 3/16" (0.187") which will make a good guide. The pilot point did have to be ground off such that it would not drill into the other side of the pocket. I forgot to mention in my 25 cal Fortitude thread, that the guide step should have the cutting edges filed down. Ie you want it just to guide and keep the next step centered. For this case, the 1/4" step.
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I have one TP made up and ready to try. Skeptical if the latex rubber tubing will hold up. If needed, a second try might be shrink tubing.
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John,
Thinking out loud here...
At the risk of stating the obvious; to have a "perfect seal", the static contact pressure between the sealing surfaces needs to be at least equal with the gas pressure being sealed in (or out).
The above is not true; if the gas pressure creates a force that increases the contact pressure between primary sealing surfaces. It is also not true, if the gas pressure creates a force that tries to separate the primary sealing surfaces (then, the contact pressure needs to be much higher than the gas pressure).
If you are dealing with a long term seal, such as the air reservoir, then that contact pressure would need to be quite a bit higher than the gas pressure you are keeping in. For a TP, the seal only needs to work for perhaps 0.003 second at a time. This means that it does not have to be as robust to creep as an air tank end cap seal.
To achieve a gas-tight seal, you need at least a narrow unbroken line of contact all the way around the interface. This is why O-rings work so well: The contact pressure is focused into a stable predictable line.
I like the idea of a thin walled brass tube to help support the compliant seal from buckling, or squeezing into the TP. I gather that you are trying to achieving a seal between the rubber tube OD, and the valve body TP pocket ID?
I think you are stretching the rubber over your brass tube? That should make it more stable when in compression against the Valve body TP pocket ID:
Compliant seals such as O-rings need to be stretched at the ID to provide a stable contact sealing "path" around the circumference. If you squash an O-ring so hard at its OD that you eliminate the stretch at the ID (or your assembly has no ID stretch on the o-ring to begin with), it will leak.
You have to ask, when you preload your rubber seal, where does the rubber material flow to? Is is stable? You can't push a wet noodle and expect it to remain straight; but you can pull it straight Thus, hoop tension in the seal is your friend...
If you are attempting a face seal with the brass / rubber tube arrangement, then I think the rubber may give too much to seal; unless it is also closely constrained by the valve body pocket ID. In any event, the rubber must not separate from the brass tube when all parts are assembled; or under firing pressure.
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Thx, and agree.
With the current latex rubber outer cover, I have to "screw" the TP into the valve to keep it from bunching up. It is a tighter fit than the breech. While I could get some rubber adhesive on it, think the heat shrink tubing would be better. It can curl over the end of the brass and help hold it in place when the TP is inserted. If that fails, can always go back to the poly which should seal a little better than before.
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The first test is positive. :) Ran a string of 16 shots. No leakage by the breech, and the fps went up about 30fps. The truth will be in how it survives on a much longer basis, though.
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Hi John,
I have been curious about this topic as well. One question, what is the best way to determine if the TP is leaking? Chrono results? Or is there another method? I suspect I have a leak on one of my guns.
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Hi John,
I have been curious about this topic as well. One question, what is the best way to determine if the TP is leaking? Chrono results? Or is there another method? I suspect I have a leak on one of my guns.
I held my hand to the side of my breech, when I fired in my basement range. I was definitely getting a puff of air on every shot.
With the new TP, not getting any "puff" and the fps went up 30fps.
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I have the same love hate with these things as you do. Often times reassembling a gun that was not leaking ends up being a leaky nightmare and gets frustrating for sure
I have seen the approach you are using before but it used a different sealing material. I think it was from alchemy airwerks but I would have to look. Where did you get the tubing?
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The brass and latex rubber tubing was from McMasterCarr, with links in the prior posts. The brass tubing is K&S and can be had at multiple places.
I see that Cobra valves have that same style of step in them, but have never handled one, or seen how their TP is made.
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I was correct the tp I was.talking about was from alchemy airgun. He uses viton tubing for his.
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I held my hand to the side of my breech, when I fired in my basement range. I was definitely getting a puff of air on every shot.
With the new TP, not getting any "puff" and the fps went up 30fps.
Ah ha, I was thinking of using baby powder or something..Thanks!
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I made a transfer port for my Mrod on my drill press with hard delrin. It's really doing a good job too.