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Sliding o-ring seal question

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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Sliding o-ring seal question

Unread postAuthor: Demon » Sun Feb 21, 2016 12:46 am

Hi, it's been quite a while that I've come here. Most likely nobody remembers me,

I'm in the process of patenting a compact paintball pistol design (If you have a TiPX or Tiberius, you know how bulky they are)

Right now, I have a standard inline piston valve with a three way valve combo for the air engine.
But, I am exploring simpler designs and I've come across a technical concern I hope someone can shed some light on.

1-In the linked picture, can I expect the red o-rings to perform a perfect seal ? They are go back and forth each time the action is cycled (and pop into the chamber). I am worried that could, over time, dry the o-rings of their lube/grease and thus make them leak.

2-How tight does the o-ring need to be at high pressures (thus have more friction, something I could do without) to seal properly ?

Basically, I am worried about the seal/friction ratio needed at high pressures (250-350 psi). If it needs to be so tight that you'd need a hammer to make it budge, it would be problematic.
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Re: Sliding o-ring seal question

Unread postAuthor: matti » Mon Feb 22, 2016 3:42 pm

https://www.parker.com/literature/ORD%2 ... ndbook.pdf :wink:

And nice to see u here, I remember you and sure many others do as well ! You can find all information for o-ring seal designs in that link.. You probably could use floating o ring seal to reduce friction. You should also pay attention to o-ring "blow-out" possibility which is not good :D
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Re: Sliding o-ring seal question

Unread postAuthor: wyz2285 » Mon Feb 22, 2016 6:01 pm

I have patented a similar design but for airsoft.
Gonna save you some time and give you the lines you need to follow:
1 The o-ring OD matches the outlet, both free floating (otherwise your valve will jam)
2 Fillet the edge of the outlet, or the o-ring is gonna get chopped.
3 Use thick o-ring, dunno the dimension of your valve but thick enough not to get blown out.
4 Dunno about your design but as drawn you need something to guide/stable the valve stem.
If 1 and 2 are followed the valve will work even if no lub involved.
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Re: Sliding o-ring seal question

Unread postAuthor: Demon » Tue Feb 23, 2016 2:08 am

Thank you both matti and wyz. I'll have some reading to do.

The main issues I had were chopping of the o-rings, but as you suggested wyz filleting should fix the problem.
Another issue was that drill bits just don't seem to make a seamless surface enough even when using a metal lathe at max speed. I am thinking about inserting precision brass tubing to serve as the sealing surface, unless I am missing something obvious.

Also, I suppose that floating o-ring seals are dependent of pressure, and that to have such a seal rated to higher pressures, the friction proportionately increases ?

Edit:
Also matti, that picture is just a reference :D The actual patents don't apply to the air engine. Its the last kink I need to get worked out before the prototype is fully operational.
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Re: Sliding o-ring seal question

Unread postAuthor: wyz2285 » Tue Feb 23, 2016 12:10 pm

Use a reamer for the finish. However I always use brass wherever I can, it's relatively easy to machine, damn good to thread/tap and leaves a nice surface finish.
If the o-ring OD matches the inner surface, it will seal at nearly any pressure point (supposing the parts are decently made).
The friction is minimal at 200 PSI, you can easily open it by hand.
Again, dunno where you plan to apply the above valve but from my experience, a theoretical perfectly balanced valve is troublesome in real life. It's ok to use for us because we know how to fix it and don't mind doing so but I would not apply it into a product, specially sport goods since the working condition is often poor. Learnt this the hard way.
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Re: Sliding o-ring seal question

Unread postAuthor: Demon » Tue Feb 23, 2016 5:55 pm

I definitely have to get myself a reamer bit.

Do you mind telling me why/how a balanced valve is troublesome in real life ? Mind that in the design, it is under tension from a spring and rests against a rim that keeps it on one side rather than the other. I assumed it would operate like a spool valve.
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Re: Sliding o-ring seal question

Unread postAuthor: wyz2285 » Wed Feb 24, 2016 8:18 am

I originally used a 6mm-6.5mm design counter balanced hammer valve hoping the 0.5mm difference would be enough to seal, but once the valve is opened the air flow over comes the spring and the area difference, the valve simply would not close reliably (we are talking about jamming every hundreds of shots). After changing into 6mm-7mm ratio the problem got solved. However this is related to my design which maybe very different from yours, even tho we are both playing with a balanced valve.
If your valve is exactly the same as a DCV or spool valve, forget about what I said. If the valve exit (where the air flows through) is indeed one of the side of the diagram you drawn, I would have my concerns.
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Re: Sliding o-ring seal question

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Re: Sliding o-ring seal question

Unread postAuthor: Demon » Thu Feb 25, 2016 10:32 am

Indeed, the diagram isn't very clear as I only wanted to illustrate sliding o-ring seals.

The O-rings only ever come out of their seal inside the chamber, not outside, so the air flow shouldn't be a problem.

I got myself a 10mm reamer bit. How come didn't I know about those before, they are awesome.
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