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Floating O ring Not Sealing

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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Floating O ring Not Sealing

Unread postAuthor: shardbearer » Thu Jun 14, 2012 11:58 pm

I just finished my 1 1/4" Tee QDV and found that it leaks. A lot. From what I can tell, the O rings are in the middle of the gap in the piston, not touching either side. They are supposed to be pushed outwards by the pressure, creating a seal with the edge, but aren't.

The housing is 1 1/4" pipe, with the piston made of 3/4" pipe and rings cut from endcaps, solvent welded on. The solvent weld has a thin, kinda rubbery, clear layer over it, and I dont think it supposed to be like that, but don't see how it could have leaked under the pvc rings. The O rings are 1 3/8" x 1 1/8" x 1/8", and fit perfectly in the housing, but are a little loose over the 3/4" piston. The grooves are about 1/4" wide, I can fit two O rings side by side in the grooves with a lot of squishing.

I have only tried blowing into it and filling it with water from the chamber side, not pressurized air yet. With water, theres about a 1/4" stream coming out of both sides. :shock:

Image

I think it may be that I just haven't put any real pressure it yet. Human lungs can only put out about 1-2 psi. I will get access to an air compressor tomorrow though at the shop. I think it could also be that the grooves are too deep, letting too much air through, causing a low pressure difference and not much force on the o rings. Or the grooves are too wide, too much room that they must move, and also the same problem as them being too deep. I guess we'll find out tomorrow.

Edit: I did some math, the cross sectional area of my o rings should be about half a square inch each.

(pi*(1.375/2)^2)-(pi*(1.125/2)^2)

1 psi acting on 1/2" of area is half a pound of force, which is how much pressure my lungs should be pushing the o rings out with, much more than the force needed to push them. So the pressure difference between the two sides of the o rings must be much lower than 1 psi, because the o rings aren't sealing, causing it to not seal. It's a catch 22.
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Unread postAuthor: Technician1002 » Fri Jun 15, 2012 1:08 am

I made my grooves just deep enough so when the o ring was flush with the side of the piston, it touched bottom. This left a very small gap when the piston was in place. A small amount of grease provided enough obstruction to air to ensure the o rings seated reliably. I also did not make the groove any wider than needed so it does not need to move far to seal.

Note in the photo the depth is just enough to contain the ring and the width is barely wider than the ring. Extra space can cause sealing problems.
Image

It drops in and is free to wiggle, but not much. It is important to polish the groove and clean any dirt that may prevent a seal.
Image

The piston to cylinder space should be very small so the ring can't squeeze into the gap if the piston is off center.

Image
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Unread postAuthor: MrCrowley » Fri Jun 15, 2012 6:58 am

It probably is due to not using any 'real' pressure as o-rings really do need some pressure to help them seal properly. You don't want to put more than a 5% 'squeeze' on the o-ring, try to keep it around 2-3%. You can be fairly lenient with the width; my 3.3mm thick o-rings are in a 4.00mm wide groove which is in the middle of the recommended range for that o-ring width. You definitely want a little bit of width so the o-ring can compress and fill out the groove a bit. If you don't have adequate space, the o-ring can pinch between the piston housing walls and the piston. Same thing happens if the piston is too small in diameter as Tech explained.

The above is taken from what I've read about dynamic o-ring seals, it could be different for floating o-rings. There's plenty of literature out there on the subject, just do a google search and some major o-ring manufacturer will give you the formulas you need.
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Unread postAuthor: shardbearer » Sat Jun 16, 2012 1:18 am

Thanks guys. I'll make the grooves thinner, but I can't really do anything to make it shallower. I tried teflon tape under the O rings, it was very lumpy and didn't do anything until it was pushing against the inside of the o ring, making it no longer floating, increasing friction, and only slowing the leak slightly. :? How else would I make the gap shallower?

Also, my cement is pretty much crap, one of my pvc rings fell off—twice. :evil: At least now I can make the gap thinner.

I lost track of the days on summer vacation. I won't get to go to the shop till saturday afternoon or sunday. And my camera went caput, so unless you want grainy photos from a webcam, you'll have to do without.
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Unread postAuthor: MrCrowley » Sat Jun 16, 2012 1:25 am

You could fill the grooves with an epoxy or other type of 'filler' and then sand the grooves back to the necessary depth. To get the epoxy to dry inside the grooves evenly, wrap the piston tightly in plastic wrap; that should keep all the epoxy in the right places.
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Unread postAuthor: shardbearer » Sat Jun 16, 2012 2:36 am

Thanks, thats a good idea.
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Unread postAuthor: mattyzip77 » Sat Jun 16, 2012 2:23 pm

Try using some nail polish. Sounds stupid I know but I havve done it and it worked awesome.
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