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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: Gippeto » Fri May 02, 2008 4:14 pm

Releasing the pressure is not the problem.

Keeping it sealed without using 1200+psi in the pilot area is.

If you keep the diameter of your pilot at 1", that is the effective area. Hollowing the face of the piston does not accomplish anything.

Perhaps instead of thinking "pressure differential", you should start thinking "FORCE differential".
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Unread postAuthor: iknowmy3tables » Fri May 02, 2008 4:56 pm

yeah that complication you added doesn't solve anything, you clearly don't understand why pressure produces more force with surface area, because if you did and where applying it to to your design you wouldn't make that mistake.

allow me to explain, compressed air wants to spread out and have more space/volume (decompress), so the reason why larger pistons (with more surface) have more force at the same pressure is because for every inch the larger piston moves, it revels more volume than with a smaller piston, and to calculate volume the first part to find is the volume
refer to the diagrams below

hey do you guys think I should make a thread for this so rather than having to explain to members who misuse area and pressure concepts why their designs are wrong again and again we can just link to the thread
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Attachments
presure area 3.PNG
diagram 3
presure area 3.PNG (15.88 KiB) Viewed 509 times
presure area2.PNG
diagram 2
presure area2.PNG (23.66 KiB) Viewed 509 times
presure area 1.PNG
diagram 1, I spent a good amount of time making these and the explanation so I might be a little late
presure area 1.PNG (13.03 KiB) Viewed 509 times

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Valve Design v2

Unread postAuthor: LynyrdSkynyrd » Thu May 08, 2008 6:58 pm

This is another valve design that i drew up in NX5. Basically modified the old valve. The grey tee is your standard sched. 40 PVC tee, but the inside is machined out so the yellow "sleeve" can be placed inside. The sleeve is made out of sched 40 as well with the inside also machined out for a good sealing surface for o-rings. The blue piston and the green back plate are both machined out of type 2 pvc. On the back plate there is a fill hole (small), and the dump hole which goes to a sprinkler valve. There's a 1/8 inch equalization hole through the center of the piston. The grooves in the green and blue are there for o-rings. The white bushing will be the opening for the tanks. We're not sure on if we're going to have the piston seal against an o-ring or put a rubber face on the front.
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Closeup.JPG
Internal Parts v2.JPG
Internals
All Parts v2.JPG
An exploaded view of the valve
Open v2.JPG
The vavle open.

I removed the tee so the internals can be seen easier.
Closed v2.JPG
Valve closed
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Unread postAuthor: DonTheLegend » Thu May 08, 2008 7:33 pm

@ iknowmy3tables

After reading the responses on the second page we realized that the design obviously wouldn't work. We were trying to throw the design together in a couple days due to availability of shop time. Obviously this didn't work well in the final result. The reason neither of us have been back on since is that we have been working on the above design Sorry for wasting peoples time, but I appreciate the responses we got.

Might not be a bad idea to make it a thread, I've been reading around on valves here and the application of the physics behind them is not extremely clear. Or maybe add those drawings to the sticky on Piston Valves explained visually.

Nice drawings btw, makes it extremely clear.
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Unread postAuthor: LynyrdSkynyrd » Thu May 08, 2008 10:14 pm

Modified the piston and added a guide rod.
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piston v3.JPG
hollowed out the piston to reduce mass and added a guide rod. The hole in the piston is a little bigger than the guide rod to allow equilization.
v3.JPG
Extended the Back plate a quarter inch so we can stagger screws to hold it in.
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Unread postAuthor: Gippeto » Thu May 08, 2008 11:02 pm

Do you guys actually know what you're doing?

Or are you just trying to get someone else to design your piston for you?

It seems to me that you have a very basic understanding, but are too damn lazy to do your own research.

I'm not in a charitable mood right now, so you can either wait for someone who is, or do some reading and figure it out for your selves.
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Unread postAuthor: no-limit » Fri May 09, 2008 12:45 am

Figuring it out yourself is not an easy task. Most people hope they can learn from others who have done it right and not have to read the many pages of crap that is around. If I can tell you exactly how to do it right I will, step by step, then they can make a great sticky out of it. All you have to do then is have fun.

We'll see in the next few days if I have learned enough.
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Unread postAuthor: DonTheLegend » Fri May 09, 2008 10:43 am

If we wanted some one to design it for us, we just would have copied one of the many designs available on this forum. That wasn't the objective of this project though, it was to try and do something different, or at the very least improve on an existing design.

If we were lazy we wouldn't have taken the time to draw everything up in CAD.

The point of this thread was to show a complete design with GOOD drawings and designs to go with it so that the next person to come along would have something to look at instead of reading through pages of people explaining in words how things do or don't work.
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Unread postAuthor: no-limit » Fri May 09, 2008 12:28 pm

Copy and improve if you can is a good thing.
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Unread postAuthor: Gippeto » Fri May 09, 2008 11:35 pm

Is this where I point out that you have YET to put up something which would actually work?
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Unread postAuthor: LynyrdSkynyrd » Sat May 10, 2008 1:08 am

I'm not quite sure why this new design wouldn't work. It wouldn't be the same issue as before with the lack of surface area to hold the piston closed. And this new design is almost exactly like our old one, that was basically two 3" caps sleeved inside a 4" pipe; which worked.
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Unread postAuthor: Gippeto » Sat May 10, 2008 1:51 am

Despite your beautiful cad drawings, your still not thinking it through.

Go back to the basics of WHY a piston valve works (seals and pilots).

Then look at your diagrams again, and (sorry to say it) Clides.

Neither will work as drawn.

Can you figure out why?
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Unread postAuthor: Hotwired » Sat May 10, 2008 5:33 am

Someones still not diggin' the basic principle of having a pilot surface area > than the chamber side surface area on the piston.
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Unread postAuthor: DonTheLegend » Sat May 10, 2008 11:39 am

The latest design DOES have a pilot surface area larger than the chamber side. The chamber is exposed to a 3.5" diamter opening. That's an area of 9.62 sq in. The pilot side has a surface area of the outer ring plus the surface inside the piston, which is just the area of a circle that is the diameter of the piston. The piston is 4" in diameter. That's an area of 12.56 sq in. I'm pretty sure that the pilot surface area is greater that the chamber side surface area.
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Unread postAuthor: Gippeto » Sat May 10, 2008 11:48 am

Look closer.

And from another angle.
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