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Piston Valves Explained Visually

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 Valves Explained Visually

Unread postAuthor: clide » Mon May 14, 2007 11:21 pm

I had this posted on Spudtech, but I am reposting it here to hopefully help some people out and allow for discussions/updates.

Well I made this flash animation in hopes to explain piston valves for the most part. Let me know if this helps anyone figure them out. Those who already know, please point out if you spot something wrong.

You may need to watch it several times because there are many things going on at once. Stepping through the frames one by one on Phase 3 would probably help too. Try to figure out what things are changing while firing and why they are changing.
This assumes you know the relationship between pressure, surface area, and force (pressure * surface area = force)


Phase Explanations
Applies to all Models


Phase 0: Basically just the gun in its pre-use state. No pressure in the gun.
Phase 1: An air source is connected behind the piston. Air enters the pilot chamber and the pressure pushes the piston against the rear of the barrel or chamber port.
Phase 2: Air continues to flow through the input and leaks around the outside of the piston or through an equalization hole into the main chamber. Once desired pressure is reached, the input flow is cut off and the gun is ready to fire.
Phase 3: The exhaust valve is opened and the pressure begins to fall in the pilot chamber. The exhaust must exhaust faster than the equalization hole can leak air back into the pilot. Once the force pressing on the back of the piston falls below the force acting on the front of the piston, it begins to slide back. Suddenly there is more surface area exposed on the front of the piston, and the jump in force slams the piston back leaving an opening for air to flow into the barrel to accelerate the projectile.

The exhaust valve is closed and a new projectile loaded. The gun is then back to it's original condition, ready for the cycle to repeat.

The main thing to notice between the different types are the forces caused by pressure. Of course forces can be changed by changing the piston/sealing port diameter, but in the general scenario like the one shown, with a constant barrel diameter between all types and a tee slightly larger than the port, the barrel sealing valve will have the same forces as coaxial in the closed position. Barrel sealing will have the highest opening force, coaxial will be in the middle, and chamber sealing will have both a low closing force and a low opening force.
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Attachments

chamberseal.swf [ 41.55 KiB | Viewed 76519 times ]

barrelseal.swf [ 41.19 KiB | Viewed 76519 times ]

coax.swf [ 40.96 KiB | Viewed 76519 times ]

Last edited by clide on Wed Oct 09, 2013 5:57 pm, edited 2 times in total.

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Unread postAuthor: Thursto » Tue May 15, 2007 3:14 am

Thank you so much, that helps me out alot, i never understood how they worked, but might be hard for me to make one :P thankyou so much mate, your a champ. Thursto.
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Last edited by Thursto on Tue May 15, 2007 3:48 am, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Tue May 15, 2007 3:27 am

This should have been a sticky a long time ago, cheers clide :)
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Unread postAuthor: Insomniac » Tue May 15, 2007 4:09 am

Those are some really nice, smoothly animated little diagrams. You should definatly add them to the wiki.
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Unread postAuthor: MrCrowley » Tue May 15, 2007 4:21 am

Im sure these are on this site somewhere. It would be nice if it was a sticky.
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Unread postAuthor: Hotwired » Tue May 15, 2007 4:27 am

I'm sure I've seen them before but it's nice to know exactly where they are now. They're good diagram animations.
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Unread postAuthor: flamerz14 » Tue May 15, 2007 8:25 am

Thanks man! great help..now i just need to figure out how to build 1..
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Unread postAuthor: cork9 » Tue May 15, 2007 6:15 pm

Gread diagrams It helped me alot!! the only thing i dont understand is even with the equalization hole is there still a lot of force pressing on the celing surface?
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Unread postAuthor: ArticWolf » Wed May 16, 2007 5:27 am

Thanks mate for helping me understand how a barrel sealing valve works. I was planning on making a pneumatic cannon but I thought it was hard to make right, it looks and sounds as if this type of valve beats sprinkler valves and qev's hands down. Now I just have to figure out what parts I am going to use to make this work over 300 psi just a thought though.

Just a ?. Could I use a one way check valve in the equalization hole I figure if I recess it in a solid piece of that clear plastic tube it should work and not damage the o rings or the wall Of the PVC T sorry if this was off topic but I figured that we were talking about barrel sealing valves that it wasn't off topic.
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Unread postAuthor: Hotwired » Wed May 16, 2007 5:38 am

Just a flap of rubber over the equalisation hole on the chamber side of the piston should work as a check valve. The rubber will be forced against the hole with the pressure difference as the pilot vents.
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Unread postAuthor: frocksie » Wed May 16, 2007 6:05 pm

Thanks Clide, just built a 2in barrel sealing piston, using your diagrams as a "blueprint." Really helpful.
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Unread postAuthor: clide » Wed May 16, 2007 6:49 pm

Good to know they are helping out.

cork9: Yep, the force comes from an the imbalance of surface area exposed to pressure as seen in the upper right hand corner of the animation. The only thing the equalization hole controls is how fast you can change the pressure on one side of the piston without changing the other side. If your equalization hole is too big then the pressure across the piston won't be able to drop enough while venting to open the valve and all your air will go out the exhaust valve, but the force holding the valve closed when the gun is charged will still be the same.

ArticWolf: Sprinkler valves and QEVs work on the same principles. The real advantage is from better efficiency over sprinkler valves (not as many twists and turns) and there is a huge cost advantage over QEVs and larger sprinkler valves. And of course they are highly adaptable and can be incorporated more cleanly into a gun.

Check valves work great in pistons and I put them in just about all my pistons. There are many ways to do them, just don't stress to much about creating a perfect seal or anything because they are only holding back air for a fraction of a second. In fact having a perfect sealing check valve could be a hazard since a small leak in your pilot area could fire the valve.
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Unread postAuthor: unicyclist » Sat May 19, 2007 11:44 pm

what is the most common?

what is each type used for?
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Unread postAuthor: clide » Sun May 20, 2007 12:04 am

Barrel sealing tee valves and coaxial valves are most common because they are easiest to build since they don't require o-rings for the piston.

Typically the only reason why you would chose one type of valve over the other is what configuration of gun you are going for. The tee valves tend to have slightly more flow than the coaxial, but it is not a huge difference. The tee valves are also more easily made for interchangeable barrels and chambers, but coaxials are easily made compact and clean.

There are a few special case scenarios where you might want to chose your valve because the other type might encounter performance issues, but these scenarios aren't common. I'm too tired to right it up right now, but that will be something that goes in the advanced considerations section once I have time to write it up.
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Unread postAuthor: origin unknown » Sun May 20, 2007 12:17 am

Finally somebody has moved this howto to a new location. Now it is easier than ever to find.
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