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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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Linear Piston Valve Idea

Unread postAuthor: MrCrowley » Tue Jun 17, 2008 12:23 am

I thought of this when trying to sleep last night, I think I have everything covered but want to make sure and share this idea unlike Mr.Sandman. :roll:


Key:
Red = 4" Coupler and Ball Valve
Black = Various pipe (4", 2", 1.5" and bushings)
Purple = Modified Bushing for Piston Housing Support (allows air to travel through it, like a support in a co-axial, flow may be an issue)
Orange = Check valve
Blue = O-Rings (not required)
Yellow = Fill Valve
Brown = Various metal fittings for the pilot setup
Green = Piston
Dark Green = Sealing Face

I have a few ideas for the piston housing support to allow maximum flow, but any ideas will be appreciated. The only problems would be this would be difficult to do on small bore and finding a suitable support that can withstand the piston actuating and allow enough flow through.

Edit: Oh and it's not to scale, just what I could fit in the MSPaint window. The chamber is represented as 4", barrel 1.5" and piston housing 2".

One more thing, the support would probably be doubled up, one at the front and one at the back of the piston housing. Which should be enough to withstand the piston.
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Unread postAuthor: jackssmirkingrevenge » Tue Jun 17, 2008 1:03 am

It would work but what would the advantage be, I can see lots of potential turbulence...
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Unread postAuthor: MrCrowley » Tue Jun 17, 2008 1:10 am

Well the only advantage would be a linear design, that's it. The flow wouldn't match that of a barrel sealer or co-axial, so linear design is all that I can think of.

Yeah, i'm gonna take a look around this weekend to see what I could use as a good support, bushings probably don't have enough flow. But once glued in place, one at the rear and one at the front of the 2" pipe, i'm sure it will hold fine.

I probably wouldn't build this anytime soon anyway, 4" fittings just cost too much.
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Unread postAuthor: starman » Tue Jun 17, 2008 1:14 am

Functionally, you look covered, although I don't claim to be a pneumatic genius. The pilot and piston/ barrel seals look workable. Will an O-ringed piston allow fast enough response?...and still seal properly?

My impression is that it would be difficult to do with glued up PVC and possibly not very servicable. Unless you use some screw-on sections it looks like your piston will be forever trapped in there. Not sure what all you had planned there...I know it's a concept so you may not have thought much about a practical implementation.
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Unread postAuthor: jackssmirkingrevenge » Tue Jun 17, 2008 1:15 am

I had come up with a similar idea for a variation of the preloaded cartridge:

Image

Note how the barrel doesn't protrude into the chamber, allowing for even flow, and since the piston housing is affixed to to the rear of the chamber theair is free to flow around it smoothly.
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Unread postAuthor: MrCrowley » Tue Jun 17, 2008 2:10 am

Does your idea work on the fact that when the pilot pressure is gone the air pressure from the chamber will seap out the barrel pushing the piston back? Because other then that there is no frontal pressure on the piston.

Good idea nonetheless, gives me some more ideas.

@Starman,

You could always have a female threaded adapter on the front instead of the bushing.
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Unread postAuthor: jackssmirkingrevenge » Tue Jun 17, 2008 2:17 am

MrCrowley wrote:Because other then that there is no frontal pressure on the piston.


That's what you think ;)

The minute the pilot chamber starts to empty, all you need is a tiny layer of air to get between the piston and the sealing face, and it will work just like any piston. Remember that air will impart the same pressure in terms of pounds per square inch, regardless of how thick that layer is. If you're still not convinced, you can always round off the front edge of the piston :)
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Unread postAuthor: MrCrowley » Tue Jun 17, 2008 2:31 am

Oh no I believe it will work alright, just wasn't sure where you were getting that frontal pressure :)
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Unread postAuthor: biged » Tue Jun 17, 2008 6:07 am

This is about as simple a design as I have made.

Image
Image

It's a 3 inch pipe with a 2.5 ID washer welded to one end.
2.75" piston face, but the piston itself is 3" and 15" long.

I've made them in 2" and 3".

Image

This one is a linear coaxial I have in my bazooka.
Image
Before welding:
Image

///ed///
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Unread postAuthor: Mitchza89 » Tue Jun 17, 2008 6:19 am

That's a nice idea Crowley. I came up with something pretty much exactly the same at the start of the year but never got round to coming up with something practical. The guts of the gun are the same as what you came up with, but I don't have it labled as clearly as you do. This bloke who I work with is into spudguns just as much as me. He's a bloody top mig welder, he just finished his custom air tank for his bb piston repeater. I'd love to get some stainless steel, and make a linear valve with the air chamber as a stock, so it looks EXACTLY like a rifle. Something ultra realistic looking.

Best of luck bud,

Mitch.
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Unread postAuthor: MrCrowley » Tue Jun 17, 2008 2:47 pm

I'm actually in the process of making a BB Sniper that looks like a rifle too. I bored out the middle of the stock to hide 30cm of 15mm pipe in it. You would never know. :P
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Unread postAuthor: psycix » Tue Jun 17, 2008 2:53 pm

jackssmirkingrevenge wrote:I had come up with a similar idea for a variation of the preloaded cartridge:
<pic>
Note how the barrel doesn't protrude into the chamber, allowing for even flow, and since the piston housing is affixed to to the rear of the chamber theair is free to flow around it smoothly.


For such cardridges this might be very useful (though it does generate extra pilot volume EFFICIENCY!!!)

MrCrowley wrote:Does your idea work on the fact that when the pilot pressure is gone the air pressure from the chamber will seap out the barrel pushing the piston back? Because other then that there is no frontal pressure on the piston.

I forsee that problem too. Even if the valve still may work (due to the below quoted idea), it may open slower then usual.
Maybe just give it a very short piece of pipe to seal against, instead of sealing against the bushing. This will also give a better seal I think.

jackssmirkingrevenge wrote:The minute the pilot chamber starts to empty, all you need is a tiny layer of air to get between the piston and the sealing face, and it will work just like any piston. Remember that air will impart the same pressure in terms of pounds per square inch, regardless of how thick that layer is.

A minute?! Id rather have my valve open within a second... :D
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Unread postAuthor: jackssmirkingrevenge » Tue Jun 17, 2008 3:06 pm

psycix wrote:For such cartridges this might be very useful (though it does generate extra pilot volume EFFICIENCY!!!)


I know, I know, but sometimes you have to compromise :p

Even if the valve still may work (due to the below quoted idea), it may open slower then usual.
Maybe just give it a very short piece of pipe to seal against, instead of sealing against the bushing. This will also give a better seal I think.


I disagree, there is no reason why it shouldn't be as effective as a "normal" piston valve. You also get the benefit of reduced wear on the piston seal as the force on it is spread out over a larger area.

A minute?! Id rather have my valve open within a second... :D


The instant then, better :p ;)
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Unread postAuthor: psycix » Tue Jun 17, 2008 3:49 pm

jackssmirkingrevenge wrote:I disagree, there is no reason why it shouldn't be as effective as a "normal" piston valve.

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.

You also get the benefit of reduced wear on the piston seal as the force on it is spread out over a larger area.

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.

*I said "might" because it also might not do it, and thus it seals off just fine.

The instant then, better :p ;)

:D
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Unread postAuthor: koolaidman » Tue Jun 17, 2008 10:05 pm

Nice design. You probably will not need a check valve, unless you do in fact use orings. I like the originality and the practicality of a linear piston. Quite simply however though, this is a pretty basic concept. Instead of having the chamber going forward (coaxial) or under and forward (tee setup,) you have it wrap around the piston and go backwards. This is really just a small tweak on a regular piston. It should definately work though and could serve as a practical linear valve setup. good luck.
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