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Sealing face issues

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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Sealing face issues

Unread postAuthor: Mr.Tallahassee » Sat Jul 16, 2011 8:57 pm

I finished a 1.5'' piston valve and I cannot get a seal and I have polished the face. I even adhered a rubber face to the barrel. :?
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Unread postAuthor: Gun Freak » Sat Jul 16, 2011 9:06 pm

Pictures? It could be that the seat is not perpendicular to the piston.
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Unread postAuthor: daniel0663 » Sat Jul 16, 2011 9:09 pm

I also had this problem with my 1" valve, the problem was not the piston or the sealing face. But the problem was the piston travel, if your piston travels to far your piston housing wont be able to guide the piston straight to seal the sealing face.

If you are confidant that your pistons construction is well built and you have a 90 degree sealing face.

Try tweaking the piston housing or the sealing face closer to the piston so it will be able to seal, your piston travel shouldn't be to far or to close. But in the middle somewhere. So you get a good seal and good performing valve.

I may be wrong, but it worked for me.
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Unread postAuthor: Mr.Tallahassee » Sat Jul 16, 2011 9:10 pm

It wasn't at first, I had to re-cut the end of the pipe. I'll have pictures in about ten minutes. It's not visible to the naked eye whether or not its perfectly perpendicular.
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Unread postAuthor: Mr.Tallahassee » Sat Jul 16, 2011 9:42 pm

Pictures. I've greased the seat as well and the travel is show in the pictures.
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Unread postAuthor: MrCrowley » Sat Jul 16, 2011 10:06 pm

Epoxy the bolt/nut and washer on the sealing face. Make sure to get epoxy in the nut threads, between sealing face and washer and between washer and nut. You may also want to put a washer behind the sealing face for support, epoxy that to the sealing face too. Pretty much epoxy everything except the sealing face area that needs to seal :D

I'm sure that will solve your problems.
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Unread postAuthor: daniel0663 » Sat Jul 16, 2011 10:10 pm

Judging from the photos it all looks fine,

Have you tried moving the sealing face forward closer to the piston?
i recognized that the sealing face is near the half way of the T.
(if you can) try moving it closer and see if it seals.

If that doesn't work,
make sure you identify where exactly the leak is coming from.
You could be mistaken that the 'leak' may be coming from is the piston seal, but in fact the leak may be coming from the connection between the sealing face pipe and the PVC fitting in the last picture on the top right corner.

besides from that everything looks good.....
if that still doesn't work,
i would suggest you PM users that are more experienced in piston valve.
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Unread postAuthor: Mr.Tallahassee » Sat Jul 16, 2011 10:26 pm

I actually prevented the leak in the threaded portions using teflon pipe dope. its designed to seal high pressure fittings for both pvc and steel. I even used it on the rod. The sealing face is thoroughly glued to the washer. The only issue i have with epoxy on this valve is that i have to be able to move the face in order to remove the valve components. I've had to re-seat the o-rings six times. I'll try it though, might actually work.
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Unread postAuthor: MrCrowley » Sat Jul 16, 2011 11:19 pm

If you can get it to leak at low pressure (>30PSI), remove the barrel and pour water on the piston face and you'll find the leak.

There could be a leak between washer and nut, even if the rubber is glued to the washer, it could leak past the inside diameter of the washer and out between the washer and nut.
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Unread postAuthor: Mr.Tallahassee » Sat Jul 16, 2011 11:37 pm

It leaks at any pressure which makes it difficult. It's a fast leak. I'll try that though.
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Unread postAuthor: Technician1002 » Sat Jul 16, 2011 11:48 pm

Any time I cut a barrel for a piston valve, I stand the barrel on end on a sheet of plywood that has a piece of sandpaper glued on it. I rotate the barrel while it is standing on end and look for any wobble as it rotates. If I can see any leaning, I use the sandpaper to sand off the high side until the barrel has no wobble as it is rotated standing on end. Once the barrel is true, the face of the piston can be tested against the end of the barrel for a proper fit. With a small amount of valve grinding compound, the piston face and the barrel can be lapped together.
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Unread postAuthor: Mr.Tallahassee » Sat Jul 16, 2011 11:51 pm

That's exactly what I did originally. Took a whole day but it still leaked badly. I finally broke down and adhered rubber to the pipe surface. It worked numerous times last night but wouldn't get past 20psi. Now it won't hold ANY air hardly.
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Unread postAuthor: Gippeto » Sat Jul 16, 2011 11:51 pm

Suggestion:

Replace the rubber sealing face of the piston with peel and stick sandpaper.

Color the sealing end of the barrel with a black sharpie marker.

Insert the piston until it contacts the barrel.

Give the piston a couple rotations.

Remove piston.

Have a look at the end of the barrel. Any low spots will still have ink on them. If you find low spots...sand using the piston...and do the test again. Repeat until you're done.

Failing that....it's leaking at the all thread/washer...use silicone for low pressure applications...and let it cure over night.
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Unread postAuthor: Mr.Tallahassee » Sat Jul 16, 2011 11:54 pm

Gippeto- The sealing face and barrel end are BOTH rubber surfaces.
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Unread postAuthor: Gippeto » Sat Jul 16, 2011 11:57 pm

Doesn't matter if you've got rubber on both surfaces....if the surfaces aren't square with one another...it's not going to seal.

The method I suggested will ENSURE that the barrel face and sealing face of the piston are square to one another when installed.
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