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piston valve being annoying and other things

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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piston valve being annoying and other things

Unread postAuthor: ramses » Sun Apr 26, 2009 4:08 pm

So, as the title might have lead you to believe, I have built a piston valve. The piston is Oringed, and slides in a smoothed out 3/4" galvanized nipple. The piston seals against a 1/2" copper pipe.

Now to the problem part. After 3 shots, the rubber seal gets shot down the barrel. so far, I have fired it 6 times. pictures should be helpful.

I am also having some problems with this one valve. I tried taking it apart and cleaning the seat and needle, but it didn't help. Does anyone have any experience with these valves (pictures)

pictures are jumbled, but whatever.

EDIT: Here is video evidence of 2 shots when it did work, in case that is relevant :D

[youtube]http://www.youtube.com/watch?v=CWypkoV8q1Q&feature=channel_page[/youtube]

[youtube]http://www.youtube.com/watch?v=wxDq5nEcN-s&feature=channel_page[/youtube]


In the second video, you can actually see the rubber seal shoot out the barrel :oops: :D
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Attachments
resized_CIMG2227.jpg
the piston.
CIMG0008.JPG
piston end
CIMG0004.JPG
valve apart
CIMG0003.JPG
valve assembled
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Unread postAuthor: jrrdw » Sun Apr 26, 2009 4:27 pm

To solve the sealing face problem, use a washer all most the size of the inside diameter of your barrel. As for the brass valve once you tighten it up to tight, you might have grooved the seat. Unless it's replaceable, it's done for. :D
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Unread postAuthor: ramses » Sun Apr 26, 2009 4:30 pm

The end of the screw is almost that big. I tried a washer, but it interferes with sealing. When I modded this one copper piece, it wasn't quite straight. There was no seat on the valve, just brass on brass. I will consider looking for a small O-ring, but I doubt that would work. hmmmmm...
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Unread postAuthor: jrrdw » Sun Apr 26, 2009 4:38 pm

Yea, after looking at the pic again I can see what looks to be a groove in the valve body, aka seat.
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Unread postAuthor: Technician1002 » Sun Apr 26, 2009 5:17 pm

Use a washer just big enough to fit inside the barrel. Put it on the bolt to hold the rubber flat against the piston. Make sure the hole in the rubber is just big enough for the bolt shaft, not the bolt head and washer. Trim the rubber to just larger than the barrel so the rush of air doesn't grab the exposed edge.
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Unread postAuthor: Gippeto » Sun Apr 26, 2009 5:23 pm

use a washer all most the size of the inside diameter of your barrel.


Use a washer just big enough to fit inside the barrel.


Do you read the previous posts?

Make sure the hole in the rubber is just big enough for the bolt shaft, not the bolt head and washer.


You DO know that we're not stupid children, ... right?
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Unread postAuthor: Technician1002 » Sun Apr 26, 2009 5:24 pm

ramses wrote:The end of the screw is almost that big. I tried a washer, but it interferes with sealing. When I modded this one copper piece, it wasn't quite straight. There was no seat on the valve, just brass on brass. I will consider looking for a small O-ring, but I doubt that would work. hmmmmm...


O rings tend to get sucked off the groove on a piston. I learned this the hard way. If you make your own valve seat, o rings work well.

Do you read the previous posts?


:oops: Slow refresh. Lots of active threads to read before posting.
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PICT229.JPG
O ring in valve seat. 1 inch PVC barrel.
Last edited by Technician1002 on Mon Apr 27, 2009 1:10 am, edited 1 time in total.
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Unread postAuthor: Sparkie » Sun Apr 26, 2009 5:38 pm

With your sealing of the barrel and the rubber being dragged out I would recommend replacing the rubber with a tap washer rubber.

The rubber they use in a tap washer is a lot denser and shouldn't suck out the barrel.
I tried to use an old bike tire for a rubber washer but as soon as I hit 100psi it would pop like a balloon, so I went searching for a decent bit of rubber and that's how I found a tap washer rubber.
Sometimes you might need to try both sides of the rubber for a best fit but when it works it lasts.

Edit:
Love the camera work. That is some nice slow motion.
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P3270122.JPG
This is what I raid the rubber of for the piston seal. Worked a treat for me.
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Unread postAuthor: Carlman » Mon Apr 27, 2009 12:28 am

The brass needle valve is missing its rubber seat mate, ill get a pic of mine up later.
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Unread postAuthor: inonickname » Mon Apr 27, 2009 12:59 am

Sparkie wrote:With your sealing of the barrel and the rubber being dragged out I would recommend replacing the rubber with a tap washer rubber.

The rubber they use in a tap washer is a lot denser and shouldn't suck out the barrel.
I tried to use an old bike tire for a rubber washer but as soon as I hit 100psi it would pop like a balloon, so I went searching for a decent bit of rubber and that's how I found a tap washer rubber.
Sometimes you might need to try both sides of the rubber for a best fit but when it works it lasts.

Edit:
Love the camera work. That is some nice slow motion.


i can agree with this, one was also used on desert storm. Pity I didn't think of one when looking for a sealing face in my new cannon..now I have to pull it apart to put one in. You can also just drill a hole and epoxy the whole thing in the piston. Also make sure you take off the rubber bit, put some epoxy on and put it back on for a nice secure fit.
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