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Piston help

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 help

Unread postAuthor: crazyhockeywako » Sun Sep 05, 2010 10:38 pm

Hey everyone i need help with a piston valve i have tried twice and every time the piston leaks. I use 1/16" rubber washers and they don't do the job. Any ideas or materials to use would be great.



Sorry about the blank posts i'm new to the posting. Yes it does have a center bolt. Yes the threads are sealed. I don't think it is ballooning. Face is flat. i didn't cut it so unless it was manufactured at an angle then its smooth. ok thanks i will try that next time.

It looks flat and smooth.

http://s792.photobucket.com/albums/yy209/crazyhockeywako/Piston/?action=view&current=IMG_4355.jpg

http://s792.photobucket.com/albums/yy209/crazyhockeywako/Piston/?action=view&current=IMG_4358.jpg

http://s792.photobucket.com/albums/yy209/crazyhockeywako/Piston/?action=view&current=IMG_4359.jpg

Edited by jrrdw.
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Last edited by crazyhockeywako on Mon Sep 06, 2010 12:09 am, edited 5 times in total.

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Unread postAuthor: Technician1002 » Sun Sep 05, 2010 11:09 pm

A photo of your piston and valve seat would be helpful.

Does the gasket have a center bolt to hold it in place? Are the threads sealed? Can air get under the seal and balloon it into the barrel and make leaks? Is the piston face flat? Is the valve seat flat?

You are asking for a cure without a diagnosis of the problem. Only a sympton is given.. IE, it leaks.

On my pistons I use plumbing o rings with good results. The o ring on the piston or embedded in the valve seat both work well.
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Unread postAuthor: geardog32 » Sun Sep 05, 2010 11:40 pm

did you pay special attention to the sealing face making sure it is flat and smooth?
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Unread postAuthor: jackssmirkingrevenge » Mon Sep 06, 2010 12:53 am

Image

Image

Image

Are you sure the piston is the correct diameter and can't wobble around in the tee? Why is the sealing face washer smaller than the rear washer?

What pressure have you tried it at?

Have you tried putting a small washer between the piston bolt head and the sealing face washer?
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Unread postAuthor: saefroch » Mon Sep 06, 2010 8:01 am

Unless you really gum up a piston valve with vaseline, I've never built one that seals at all below 80 psi.

Can you find with your fingers or ear exactly where the leak is? It's usually pretty easy, just pump it up and run your fingers over it (then if you really want to be stupid, put it up against your ear and use that method to find the leak, though with brass fittings you should be fine)

Now that I'm looking at your piston again, are you sure when you set the piston in there that the front seal isn't folding over against the hose barb you're using as a seat? Suggestions: Stamp a piston sealing face, don't cut it with scissors. Don't tighten it down so much and deform it around the nut on the front of the piston.

EDIT: suggestions for a good seal: O-rings on a highly polished surface, stamped rubber gasket material (so you control the size of the hole in the middle, and the size of the gasket itself).
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Unread postAuthor: jhalek90 » Mon Sep 06, 2010 10:34 am

I'm sorry, but your sealing face will never seal.

the sealing face looks like crap. so rough cut, and so many rips, there is no way that would ever seal.

remake the sealing face, and take some time on it this time.

make sure your rubber gaskets are ROUND and cut very very carefully.
make sure your bolt is not bent.....


rebuild a decent piston, and you should be fun.
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Unread postAuthor: Gun Freak » Mon Sep 06, 2010 10:40 am

Put a razor blade in a vise then chuck the piston in a drill. Spin the piston and cut the rubber until it perfectly fits in the housing. I agree with Jhalek, that piston will never seal. Buy the right size washer or carefully cut a new one using the method described above.
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Unread postAuthor: MRR » Mon Sep 06, 2010 11:14 am

Hey, that design looks familiar to me.
I agree with GunFreak and jhalek90, the sealing face is really a mess but instead of a blade you should use a piece of sandpaper and a drill (safer and better results).

This is a piston that I have build some time ago. I used 3/4" rubber gaskets from the plumbing section of my local hardware store.

Image

Besides, since you fill the chamber from behind the piston you don't need a spring behind it.
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Unread postAuthor: clemsonguy1125 » Mon Sep 06, 2010 12:09 pm

I agree that piston will never seal. My favorite way of making pistons is to use a rubber stopper that fits nicely in the piston housing. Take a pipe pipe like the one your piston is housed in and lube it up with vasaline. Place the stopper in the pipe and cover the bottom with duct tape. Then pour epoxy around it and when its dry simply slide it out. Ive used this and had them seal at 10-20 psi. Ill make a graphic if you need one.
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Unread postAuthor: saefroch » Mon Sep 06, 2010 12:24 pm

My first piston had a sealing face that looked a LOT like the one crazyhockeywako posted... he just has to loosen the nut on the front. It seems very illogical that such an adjustment would actually help, but what's happening is the sealing face, which is quite flimsy, is being forced around the nut, and probably folding over when it pushes against the hose barb.

Image
Look familiar? I operated this piston from about 60-250 psi, no problems.
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Unread postAuthor: crazyhockeywako » Mon Sep 06, 2010 12:41 pm

It can wobble a little bit. I modeled it off of MRR and his had it that way. i have tried it from 0-100 psi, i can't get my fingers down the hose barb but it sounds like thats were it is coming from. Where can i buy the stamps? ok epoxy piston is next. Thanks everyone of the help.
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Unread postAuthor: MRR » Mon Sep 06, 2010 12:58 pm

Wait, you are not sure if it is really the sealing face?

It can also be the hose barb. At the first try with this design I've made the mistake to wrap the teflon tape into the wrong direction causing a leak that looked like a defective sealing face.
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Unread postAuthor: saefroch » Mon Sep 06, 2010 1:12 pm

To stamp a piston out of gasket material, just use a file to put a bit of an edge on the inside of a pipe nipple, then place it against your gasket material and start hitting it with a hammer. I'd use the same size pipe nipple as whatever size tee that is you're using. Then put a little hole in the gasket you stamped and shove that over the threaded steel rod.

Did you try loosening the front nut a bit? Can you see if your sealing face is folding over at all against the hose barb? Recommend using a flashlight and just looking...
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Unread postAuthor: Gun Freak » Mon Sep 06, 2010 1:20 pm

Put a little bit of water in the barrel port then you can tell where the leak is coming from.
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Unread postAuthor: crazyhockeywako » Mon Sep 06, 2010 8:41 pm

yes i dunked it in water and it was coming from the piston. i got some densely thick gasket material so when i get some time i am going to stamp one out. thanks for the help
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