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3" Tee Piston Valve Questions

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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3" Tee Piston Valve Questions

Unread postAuthor: tony48 » Sun Mar 06, 2011 2:23 pm

I'm planning on making a barrel sealing piston valve inside a 3" tee with a 2" barrel. It can be used at 60 to 80 fps. I need some help with the construction and technical stuff

1) What should the overall length of the piston be?

2) How big should the equilization hole be?

3) What volume should the pilot area have?

4) What exhaust valve is needed to vent the pilot chamber?

and lastly
5) I'm thinking that I will make the piston out of pvc or another kind of plastic. I will use a rubber gasket for the sealing face. Not sure about the bumper yet. Should I mount a rod that runs from the back of the tee up through the piston so that the piston can slide on it? The rod will be smooth but threaded on one end to allow for bolting onto the back of the PVC tee so that it runs from the back towards the barrel. It seems like this would be a good idea but I'm new to piston valves so let me know how this all sounds.

Thanks for all of the help because I'm planning a pretty big project and need as much help as I can get to make sure everything works well.
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Unread postAuthor: motorfixer1 » Sun Mar 06, 2011 2:50 pm

You might try reading through the vast supply of free info on this site before asking all these questions. There are not many people willing to spoon feed info here.
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Unread postAuthor: tony48 » Sun Mar 06, 2011 2:53 pm

I've read alot and I have basic ideas but i kinda need them confirmed. And this is hard to do when I can't find examples that have the same aspects of the valve I'm trying to make. I don't need this specific information but it would be nice if someone would give me formulas or general rules of thumb to calculate all of volumes and sizes I need.
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Unread postAuthor: Lockednloaded » Sun Mar 06, 2011 2:55 pm

1.a little less than 3" long

2.you shouldn't need one unless you have an air tight piston

3.a little as possible going by the d/4 rule

4. Depends on your pilot volume and piston tolerances

5. If you're making it out of PVC you'll need a good bumper. You will not need this "guide rod" you're talking about

I'm against spoon feeding as well, but Im in a good mood, and it seems like this rookie has done "some" research
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Unread postAuthor: tony48 » Sun Mar 06, 2011 3:12 pm

When I say equilization hole I just mean the gap between the piston and the tee that air goes through to reach the chamber. Shouldn't this gap be small so that when the pilot area is vented the air from the chamber doesn't go back into the pilot area? I need to know just how small this gap needs to be
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Unread postAuthor: Lockednloaded » Sun Mar 06, 2011 3:25 pm

Just have a piston that fits well with low friction
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Unread postAuthor: tony48 » Sun Mar 06, 2011 4:24 pm

Ok thanks. I have another question. How far into the 3" tee should the 2" barrel come? I am going to use a 2" to 3" bell coupler and the 2" barrel will come through that into the tee. Is there a specific distance I should put it and does it matter how much volume is ahead of the piston?
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Unread postAuthor: Fnord » Sun Mar 06, 2011 4:43 pm

Normally I would extend the port about halfway into the tee. This will ensure a decent air-flow geometry and let you cut down on the piston's length. Make sure you do a good job glueing the port through the bell coupler; if it's your first time solvent welding it could get messy.(Also, check to if the bell coupler says nsf-pw or PN(number) on it. Most bell-style reducers are not pressure rated and have a shallow socket depth.
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Unread postAuthor: tony48 » Sun Mar 06, 2011 5:00 pm

Fnord wrote:Normally I would extend the port about halfway into the tee. This will ensure a decent air-flow geometry and let you cut down on the piston's length.


Are you saying that the barrel should extend about halfway into the piston tee housing?

And I've solvent welded plenty and worked with PVC before so I'll be sure to make sure all components are pressure rated. It's a pain to find 3" parts that are pressure rated though. Anyone have any good sources?
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Unread postAuthor: Technician1002 » Sun Mar 06, 2011 8:20 pm

Fnord wrote:Normally I would extend the port about halfway into the tee.


While this works, better flow is obtained by having the barrel extend slightly less than 1/2 way through the T. Instead of having the barrel / piston seal in the center of the T, for best flow the OPEN valve should have the opening centered. For example if the piston moves back using the 1/4 D distance, When open there would be a 3/4 inch gap between the piston and barrel. The piston would flow best if the barrel was offset 3/8 inch from the center of the T so when open the piston is back 3/8th inch from the center. This would place the open valve gap dead center in the T for maximum flow.


A very high concern with large valves is the kinetic energy they can place on the components. They are known for hammering the pilot area until the valve breaks in half. Keep your body parts away from the breech when firing.
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Unread postAuthor: tony48 » Mon Mar 07, 2011 7:49 pm

I still need to come up with a good bumper system. Other than that I am ready to get started on it. I will start checking around for parts at lowes and local hardware stores.

I've used a series of YouTube videos heavily for information and design theory. The first of the three videos can be seen here:

http://m.youtube.com/index?desktop_uri= ... 5O71AHTIr4

I think I will make my piston the same way his is made since I can't think of a better way to make one except turning one on a lathe (which I have). Anyone know if a piston turned on the lathe or made by sandwiching plywood as seen in this video is best? I like the lathe idea since it will make an overall lighter piston but I like how the sandwich method is adjustable. Opinions?
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Unread postAuthor: Lockednloaded » Mon Mar 07, 2011 7:56 pm

Since you have a lathe, you should buy some HDPE and turn a piston out of it. You could even add O-rings, and your bumper wouldn't have to be near as bulky because it is so impact resistent
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Unread postAuthor: Technician1002 » Mon Mar 07, 2011 8:41 pm

I've used a series of YouTube videos heavily for information and design theory. The first of the three videos can be seen here:


Please check your link. It has nothing to do with a valve or spudding.
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Unread postAuthor: tony48 » Mon Mar 07, 2011 10:09 pm

Technician1002 wrote:
I've used a series of YouTube videos heavily for information and design theory. The first of the three videos can be seen here:


Please check your link. It has nothing to do with a valve or spudding.


It links to the video on my computer....but the video is titled "Barrel Sealing Piston Valve (Part 1)" if you want to look it up. It's a pretty good video and is linked to elsewhere on this site

Also, can you turn HDPE on a wood lathe?

And I'm not really familiar with air tight pistons so I don't think I'll add o rings because then a would need a check valve and equalizer hole and I
don't think I'm ready for all of that just yet. I can always add that on later though so I'm still up for suggestions
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Unread postAuthor: Technician1002 » Tue Mar 08, 2011 12:52 am

If I can turn it on a drill press, I don't see why you can't turn it on a wood lathe. Keep the tool rest close and monitor your progress to keep it even.

In the how to there is a thread on making pistons with o rings.
http://www.spudfiles.com/forums/how-to-database-not-for-help-me-posts-f11.html
It isn't difficult. Making the EQ hole is as simple as drilling a very small hole between the chamber area and the pilot area. Here is a small 1.5 inch piston getting drilled. The hole is drilled from near the piston face back to the rear near the center.

Whether the EQ is from a leaky piston or a drilled EQ, a check valve is not required.
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I could not find the video with your supplied link. I did a search on the title in Youbube. I presume this is the vidoe you are talking about.
[youtube]http://www.youtube.com/watch?v=75O71AHTIr4[/youtube]
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