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Piston Valve Design

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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Unread postAuthor: DonTheLegend » Sat May 10, 2008 1:47 pm

I can't figure out what you're saying will not work. I have looked at clides animations again and still don't see what won't work. The only thing I can see that you might be talking about is a lack of seals in the diagrams. The O-rings on the piston and and backplate are not shown, nor is the seal on the face of the piston. We left these out of the diagrams for simplicity's sake, but mentioned them in the description.
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Unread postAuthor: clide » Sat May 10, 2008 2:35 pm

The last one looks like it would work to me. It may not be the fastest piston valve out there because it is a chamber sealer with the sealing port close to the size of the piston, but it should work (a large pilot valve may be needed to prevent bouncing).
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Unread postAuthor: DonTheLegend » Sat May 10, 2008 2:54 pm

For a pilot we are planning on using a 1" sprinkler valve.
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Unread postAuthor: Gippeto » Sun May 11, 2008 12:02 am

I already told you that your diagrams and Clides have the same problem.

The lack of seals in your diagram is not a consideration. (I did read your description)

So, thinking it through;

If surface area and o-rings are not the problem, what's left?

You built a barrel sealer right? Think about this;

WHY does it seal? ("Because it's supposed to" is not the correct answer.)

Are you getting close yet?


Big hint;

The problem with Clides animation is at phase 2

As drawn, it'll never see phase 3
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Unread postAuthor: clide » Sun May 11, 2008 12:19 am

Why haven't you brought this up before? I would like my animations to be as accurate as possible so if there is a problem I would really want to fix it. However I still have no clue what you are hinting at, and I doubt Don does either. Can you just tell us?

If not does anyone else have any idea what he is talking about?
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Unread postAuthor: Velocity » Sun May 11, 2008 12:41 am

If this is about your diagram of the Chamber Sealing Valve, I too am puzzled as to why Gippeto believes it will not work as drawn, specificially at phase 2.

Considering that F=PA, the pressure on both sides of the piston is equal, yet the area on the pilot side of the piston is greater, so the force pushing the piston shut is greater than the force pushing the piston open. So that can't be a problem.

The equalization hole diameter is much smaller than the pilot valve diameter, so the air in the chamber will not be able to equalize quickly enough to keep the piston shut. So that's not a problem.

I don't see a problem...
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Unread postAuthor: Gippeto » Sun May 11, 2008 12:45 am

As drawn, there is no spring holding the piston closed.

Nor is there any manner to use air pressure to keep the piston closed.

Having an equalization hole in the piston would seem to be the natural way to fill the chamber.

But, after the chamber is filled, pressure equalized, what keeps the piston forced onto its seat?

In your animation @ phase 2;

High pressure in the chamber is trying to leak into the barrel.

By providing a path for the pressure to continue to equalize (between pilot and chamber), you allow the piston to move back, and the pressure to leak out the barrel.

If a suitable spring can be used to maintain the seal, it'll work, but won't dump the whole chamber.

Alternatively, isolate the pilot volume, and fill the pilot and chamber separately.
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Unread postAuthor: DonTheLegend » Sun May 11, 2008 12:46 am

You start with a wrong assumption. Its a chamber sealer.

And I am still missing what you say is wrong with clide's animation.
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Unread postAuthor: DonTheLegend » Sun May 11, 2008 12:49 am

Air pressure will equalize continuously, but since there is a larger surface area on the pilot side, there is more force at the same pressure, thus the piston stays closed. No spring required.
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Unread postAuthor: Velocity » Sun May 11, 2008 12:57 am

Gippeto wrote:As drawn, there is no spring holding the piston closed.

Nor is there any manner to use air pressure to keep the piston closed.

Having an equalization hole in the piston would seem to be the natural way to fill the chamber.

But, after the chamber is filled, pressure equalized, what keeps the piston forced onto its seat?

In your animation @ phase 2;

High pressure in the chamber is trying to leak into the barrel.

By providing a path for the pressure to continue to equalize (between pilot and chamber), you allow the piston to move back, and the pressure to leak out the barrel.

If a suitable spring can be used to maintain the seal, it'll work, but won't dump the whole chamber.

Alternatively, isolate the pilot volume, and fill the pilot and chamber separately.


If we are talking about the chamber sealing valve diagram, you must consider the following:

Pressure = Force/Area
P = F / A; therefore:
F = PA
F = P(r^2)(pi)

The force on the pilot side is PR^2(pi), where the force on the chamber side is Pr^2(pi), where R > r. Because of this, there will always be a greater force pushing the piston shut than the force trying to push the piston open. As a result, the net force on the piston is P(R^2 - r^2)(pi), pushing the piston shut.
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Unread postAuthor: Gippeto » Sun May 11, 2008 1:19 am

I know perfectly well that we're talking about a chamber sealer.

I am also quite familiar with the relationships between pressure, area and force. Thanks.

By continuing to equalize the pressure between the pilot and the chamber, you are nullifying the force created by the area differential.

The piston will NOT seal.

The pressure in the chamber WILL leak out the barrel.
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Unread postAuthor: DonTheLegend » Sun May 11, 2008 2:08 am

Air movement depends only on pressure differentials, so it will flow into the chamber until there are equal pressures. At this point the system has reached a stable state, and the F=PA equation can be used to figure out the force. Flowing air will not magically nullify the net force equation.
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Unread postAuthor: Velocity » Sun May 11, 2008 9:09 am

I know perfectly well that we're talking about a chamber sealer.


That was more for my reference, because I came into the conversation late without reading the past posts :oops:

I am also quite familiar with the relationships between pressure, area and force. Thanks.

By continuing to equalize the pressure between the pilot and the chamber, you are nullifying the force created by the area differential.

The piston will NOT seal.

The pressure in the chamber WILL leak out the barrel.

I have not taken so much physics in my lifetime, so it is possible that there is some factor that I overlooked. But does the equation not apply here? Is it not true that there would be a resultant force pushing the piston closed? I wasn't trying to insult your intelligence in any way when I posted that formula; I was just using it to show my own thought process.
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Unread postAuthor: Hotwired » Sun May 11, 2008 9:53 am

Uhhh Gippeto,

I'm not digging this non-sealing theory.

Force does not mean a pressure difference.

A greater force will be present on the pilot side because of a larger surface area.

The difference between the two forces on the piston is found at the 3.5" chamber opening that is the point of sealing for the 4" piston.

The extra force on the pilot side is matched by an opposing force from the 3.5" stopper, which, not being able to move, is going to create a seal against the piston.
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Last edited by Hotwired on Sun May 11, 2008 11:21 am, edited 1 time in total.
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Unread postAuthor: Gippeto » Sun May 11, 2008 10:46 am

I've been tossing this around all bloody night.

Putting my "pigheadedness" aside, I concluded that I am the one not "thinking it through".

No excuses, attempted explanations, or passing the blame, I AM wrong.

My apologies to everyone.


In the way of a minor act;

It sounded like you guys had access to a lathe.

If you were to shape your piston more like a thread spool, you could accomplish the weight reduction without the increase in pilot volume.

But evidently, don't take my word for it. :oops:
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