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Linear Piston Valve Idea

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: clide » Tue Jun 17, 2008 10:14 pm

Here is a similar design in a chamber sealing configuration. http://www.geocities.com/man_o_brass/in ... valve.html

Although in this case it would be a much better idea to go with a barrel sealing setup.

I don't believe I've ever seen such a valve actually get made. Although there is a commercial air cannon (for unclogging grain silos) that has a very similar setup.
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<a href="http://gbcannon.com" target="_blank"><img src="http://gbcannon.com/pics/misc/pixel.png" border="0"></a>latest update - debut of the cardapult

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Unread postAuthor: MrCrowley » Tue Jun 17, 2008 10:52 pm

I thought of something, thanks to JSR's diagram, that could make this work quite well. Gimme half an hour to put up a diagram.
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Unread postAuthor: MrCrowley » Tue Jun 17, 2008 11:38 pm

The reason I put the pilot in two directions is just to show that the pilot could go to the end of the chamber, or come out the middle.

If I did build this, I would probably set it how it is in the picture, fill from the rear, fire from the middle.

The purple supports would be a 4"x2" bushing cut in half. So a support at the rear and at the front that is glued to half the circumference of the chamber, should be plenty enough to withstand the piston.

You could put the supports on opposite sides, one top one bottom but that would further reduce the flow. The way it is in the picture will make the air to simply go over the top of the piston housing.

Another limitation in this design is the porting, because a 3" piston housing in a 4" chamber wouldn't have the best flow. I wonder what would be better, larger seat but less flow in the chamber, or larger flow in the chamber but a smaller seat.
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Unread postAuthor: jackssmirkingrevenge » Wed Jun 18, 2008 12:42 am

psycix wrote:When the thin layer of air between it pushes the piston back and the piston starts to move back, the thin layer becomes "doublethin" thickness within no-time, reducing pressure inside the layer unless air from the chamber flows into the layer and the still very thin layer does not allow that much flow.

And because of that, there MIGHT* slip trough a bit of air I think. When it seals against a pipe, the sealing face is very forcefully pressed on that area of sealing, allowing no air to escape.
Also, imperfections on the two surfaces create ways for the air to escape trough the seal.


From the practical aspect, most of the piston guns I've made recently are configured this way, seems to work well enough, both in terms for sealing and performance.
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Unread postAuthor: MrCrowley » Wed Jun 18, 2008 12:45 am

Anything to add about the new diagram I put up JSR?

Just wondering if you think there'll be any 'turbulence'.
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Unread postAuthor: jackssmirkingrevenge » Wed Jun 18, 2008 12:47 am

I'd remove the purple supports and have it rest solely on the fill pipe, that of course assumes rugged construction involving copious amounts of epoxy ;)
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Unread postAuthor: MrCrowley » Wed Jun 18, 2008 12:53 am

jackssmirkingrevenge wrote:I'd remove the purple supports and have it rest solely on the fill pipe, that of course assumes rugged construction involving copious amounts of epoxy ;)

That's the best kind of construction :D

I think that this design will have similar flow to that of a 2" barrel sealer with an over-under 4" chamber. I have some 4" pipe laying around so I just might give this a go.
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Unread postAuthor: jackssmirkingrevenge » Wed Jun 18, 2008 4:04 am

... also, the concept of separate filling and pilot valves is alien to me :? but I guess there's only so much you can pilot with a schrader ;p
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Unread postAuthor: MrCrowley » Wed Jun 18, 2008 4:41 am

Haha i'd love to see you try and pilot 300mm of 15mm pipe with a schrader :P

I'm actually getting serious about this design now with the competition coming up and all.

Depending on the chamber length, I would change how the fill valve/pilot valve is set up. Which would be threaded through the coupler for thickness.

Parts list (Just so I have it written down somewhere other then my head):

3 - 4"x2" PVC bushing (piston support, chamber rear, chamber front)
1 - 2"x1" PVC bushing
1 - 1"x1/2" threaded PVC bushing
1 - 2" PVC female threaded socket adapter
1 - 2" PVC male threaded adapter
1 - 2"x1.5" PVC bushing (Golfball barrel)
1 - 2" PVC end cap
1 - 4" coupler
Various brass fittings

Thinks that's about it.
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Unread postAuthor: jackssmirkingrevenge » Wed Jun 18, 2008 5:44 am

I once made a coaxial with the body of a footpump as a chamber, must have been around 50mm diameter. Managed to pilot it with a schrader, but then again as I was using the (modified) pump head as a piston, and it had a floating o-ring, the fact that it was a good seal made it easy.
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