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Need a simpler solution to piston valve problem

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: psycix » Mon Sep 15, 2008 8:17 am

:(
Such an awesome cannon and then there are evil parts on it :(
Whats the use of dual 4" piston valves if you can only take it up to 50PSI?

frankrede wrote:
McThunderThighs wrote:@frank: I don't think you are understanding. I am using a coaxial(x) valve, the air to push the piston forward and to fill the reservoir comes from behind the piston, seals the piston with a gasket against the barrel (or the stuff to the barrel) and then goes around the piston to fill the rest of the reservoir. If the piston sealed to the sides of the valve, then the reservoir would never fill and then what's the point?
No no no, I understand completely, I'm just saying that you need an almost airtight fit around the piston.
Air will leak past.
Your cannon isn't a coaxial, its a barrel sealing piston valve in a T.

McTT, I understand that you think you will not be able to fill the chamber, and with a perfect seal, that is right, but for optimal results, a prefect seal is required around the piston.
Solution to the filling problem:
A small (VERY SMALL) equalisation hole in the piston.
This hole is usually drilled under the flap of the sealing face so it basically has a one-way valve. Air can get into the chamber, but no air can leak back outta pilot.
Frank is right about the airtight fit.
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Unread postAuthor: JDP12 » Mon Sep 15, 2008 8:21 am

my eyes, my eyes are hurt by looking at DWV! seriously though, its a cool cannon, but i wouldn't use it because of the DWV. there are a fair number of parts on there that are, not just one or two.

If you have to use it, 50 PSI max. and even then, be careful.
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Unread postAuthor: McThunderThighs » Mon Sep 15, 2008 5:42 pm

@ MrCrowley: Show me a "safe" cannon... I am aware of that, that is the point of not using the ball valve design. The piston chambers and receivers are made of forming putty (stuff is stronger than steel in volume:strength). The rest of the stuff is pretty much expected to explode, and that is why my nice trigger is a long ways away.

@ frank: It is coaxial, valves are mounted at 90deg to the barrel. Take a second look at the pic. In the case of the cannon, they are coaxial-y, in case of the valve, they are coaxial-x. Remember that I said I was using a coaxial valve, not a coaxial cannon. In fact, there is no such thing as a coaxial cannon, unless you are saying that the cannon is sharing an axis with itself. The exhaust and the reservoir share the x axis in the valves.

@ everyone else: When you have a stipend and a time limit, you spend a lot of money to build something cheap...

Diaphragm valves (sprinklers), piston valves, and anything with concentric circles is coaxial.

Thanks!
-MTT
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Unread postAuthor: MrCrowley » Mon Sep 15, 2008 5:52 pm

Look at the attached picture of your cannon below:

1) Very short socket depths in DWV fittings, hence why the bushing still has ~50% of it's spigot exposed.

2) Small diameter, non pressure rated pipe and fittings are taking the extreme stress when the piston fires back like a projectile into your bumper. After a few decent powered shots I can guarantee that this weak point will fail.

3) Clean out cap, these things are notorious for blowing out in basic combustions, which generate only about 40psi. I'm sure you have or plan on using this cannon at higher pressures then 40psi. You also have a few elsewhere on your cannon.

A safe cannon will have an operating pressure lower then that of the weakest rated component.

This is my latest cannon, lowest rated fitting is 125PSI, which is the sprinkler valve:
http://www.spudfiles.com/forums/v-a-l-p ... 14702.html
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Unread postAuthor: McThunderThighs » Mon Sep 15, 2008 6:21 pm

Haven't taken it above 20psi on one valve, and we were putting holes in a trash can. I can't get the valves to seal under 15psi, so I intend an operation between 20psi and 30psi.

The pistons, large slides, and receivers are made of the stuff they use to fix planes with (forming epoxy). It won't fail, ever.

The connections are temporary to allow for a large number of tests, cleanouts are being replaced with bushings.

The DWV is good enough, it doesn't encounter any of the stress from the piston because the entire piston assembly is made of the plastic equivalent to steel (again volume:strength, I think that was the 3rd time I said that).

Anyway, I have resolved the problem that was stopping us. Thanks for the comments but I am not here to argue about whether or not you are going to see pictures of me with PVC shrapnel (and yes, I know it WILL HAPPEN, that is sorta the point).

Luego!
-MTT
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Unread postAuthor: PotatoNick » Mon Sep 15, 2008 8:41 pm

jackssmirkingrevenge wrote:
Velocity wrote:For a 4" piston, you should aim to have it moving back only 1". If you have lathes and such to meet closer tolerances, you can reduce the pilot volume even more to have faster actuation time (its generally diameter/4 for pilot volume)


Worth clarifying that the piston needs only to move back around 25% (maximum theoretical efficiency) to 50% (allowing a bit of tolerance for piston bounce) of the barrel/seat diameter for maximum efficiency.



Simple explanation of the "maximum theoretical efficiency". Using the equation for circular area (pi)r^2 we would observe that a 4" (i.d.) tube has a cross sectional flow area of 12.5664sq.in. or 4pi. The flow area between the tube and the piston would be found by multiplying lift x circumference, or (.25d)(2(pi)r), so 12.664sq.in. or 1*4pi.
(Just for clarification, d is the inner diameter of your "out port" pipe, not piston.)

Where I disagree with you is the reason for having lift of up to .50d(or any value greater than .25d) . I don't believe that much, if any, piston bounce occurs during the time a projectile is in the barrel because you have so much force constantly being applied from the pressure pushing the piston backwards I would think that it would more than cancel out any opposing force from the very low spring rate of PVC. The reason for using larger amounts of lift has to due with fluid dynamics. You don't get to utilize the full flow area because your ratio of "edge" or perimeter to flow area gets larger. A circle is the most effective way to have the lowest "edge" to flow area ratio. This is important because wherever you have an edge, you have a boundary layer of air that restricts flow. There are also lots of bends in order to utilize the full flow area between the tube and the piston. All of these things really add up when working with such high pressure differentials.

The Fluid Dynamics of a Spudchucker :D
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Unread postAuthor: McThunderThighs » Mon Sep 15, 2008 10:42 pm

Ok, how about some good news?

We had a little team meeting, and since the 2" valves would have to be replaced anyway (they were put together in a half hour, for $7 a piece, they were trial runs) we are going to loose the DWV.

So now here comes the question, if I walk into Lowes/Home Depot what the heck am I looking for that is better?
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Unread postAuthor: MrCrowley » Mon Sep 15, 2008 10:53 pm

This article I wrote should explain it all with pictures to clarify:
http://www.spudfiles.com/spud_wiki/inde ... _Rated_PVC

Ignore the sections that only relate to Australia and New Zealand., which are clearly marked.
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Unread postAuthor: frankrede » Mon Sep 15, 2008 10:59 pm

McThunderThighs wrote:
@ frank: It is coaxial, valves are mounted at 90deg to the barrel. Take a second look at the pic. In the case of the cannon, they are coaxial-y, in case of the valve, they are coaxial-x. Remember that I said I was using a coaxial valve, not a coaxial cannon. In fact, there is no such thing as a coaxial cannon, unless you are saying that the cannon is sharing an axis with itself. The exhaust and the reservoir share the x axis in the valves.



Diaphragm valves (sprinklers), piston valves, and anything with concentric circles is coaxial.

Thanks!
-MTT


Regardless of your unorthidox ways of describing your cannon

lets just clarify one thing.

OBVIOUSLY YOU HAVE A LIMITED KNOWLEDGE ON THIS SUBJECT.

TAKE OUR ADVICE YOU ARROGANT FOOL.
WE ARE TELLING YOU THESE THINGS SO WE DON'T HAVE TO SEE YOU IN THE NEWS REGARDING A CANNON EXPLOSION IN WHICH YOU LOST YOUR EYES AND HAVE PVC EMBEDDED IN YOUR FLESH.
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Unread postAuthor: MrCrowley » Mon Sep 15, 2008 11:03 pm

frankrede wrote:Regardless of your unorthidox ways of describing your cannon

lets just clarify one thing.

OBVIOUSLY YOU HAVE A LIMITED KNOWLEDGE ON THIS SUBJECT.

TAKE OUR ADVICE YOU ARROGANT FOOL.
WE ARE TELLING YOU THESE THINGS SO WE DON'T HAVE TO SEE YOU IN THE NEWS REGARDING A CANNON EXPLOSION IN WHICH YOU LOST YOUR EYES AND HAVE PVC EMBEDDED IN YOUR FLESH.

Easy Frank, he said he's only using at 30psi. Which it may still fail at but probably only the cleanout or the small bore piping leading up to the valves, which is a minor failure that shouldn't do any harm if he's wearing safety goggles.

I know it is dangerous, but he's already said he'll swap it all out for pressure rated fittings, so i'm not too bothered about it. I don't think he'll change his mind on using the DWV cannon at 30psi anyway.

The DWV is good enough, it doesn't encounter any of the stress from the piston because the entire piston assembly is made of the plastic equivalent to steel (again volume:strength, I think that was the 3rd time I said that).

The piston assembly isn't the problem, the force of the piston hitting the back of the valve is enough to snap the small bore piping vertically leading up to the valve. It's often regarded that firing a barrel sealer without the barrel can be enough to fracture the fitting leading up to the valve. And since you are using very small diameter pipe compared to that of the valve housing, and the PVC isn't, there is a high chance it ill fail.

I don't see how the valve assembly has anything to do with the pipe and fittings below the tee. Diagram maybe?
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Unread postAuthor: mark.f » Mon Sep 15, 2008 11:28 pm

Just the recoil itself is enough to snap a weak elbow, when the assembly is not braced well enough. And for this cannon, recoil would definitely be a factor. Image

It's good to hear you are losing the DWV. It really shouldn't be used in any pneumatic launcher, no matter how low the pressure.
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Unread postAuthor: ALIHISGREAT » Tue Sep 16, 2008 1:27 am

just out of interest can i see a diagram of your pistons?

other than that, once you loose all the non pressure rated stuff, i would mount the Sprinler valves directly onto the back of the two tees housing the pistons to minimise the pilot volume.
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Unread postAuthor: jackssmirkingrevenge » Tue Sep 16, 2008 2:46 pm

PotatoNick wrote:Where I disagree with you is the reason for having lift of up to .50d(or any value greater than .25d) . I don't believe that much, if any, piston bounce occurs during the time a projectile is in the barrel because you have so much force constantly being applied from the pressure pushing the piston backwards I would think that it would more than cancel out any opposing force from the very low spring rate of PVC.


I used to agree religiously with that view, however my change of heart is explained in reasonable detail here.
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Unread postAuthor: Sticky_Tape » Tue Sep 16, 2008 4:13 pm

When you make the next cannon I would suggest lowering your pilot volume it's quite huge.
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You can tell how awesome a cannon is by the pressure used.
http://www.spudfiles.com/forums/high-pr ... 12803.html
xnt rnm ne z ahtbg
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Unread postAuthor: McThunderThighs » Tue Sep 16, 2008 6:34 pm

frankrede wrote:OBVIOUSLY YOU HAVE A LIMITED KNOWLEDGE ON THIS SUBJECT.

TAKE OUR ADVICE YOU ARROGANT FOOL.
WE ARE TELLING YOU THESE THINGS SO WE DON'T HAVE TO SEE YOU IN THE NEWS REGARDING A CANNON EXPLOSION IN WHICH YOU LOST YOUR EYES AND HAVE PVC EMBEDDED IN YOUR FLESH.


Thanks buddy. I know that the DWV valves are dangerous. That is why I took your advice and advised the build guy to replace them, which is happening. Chill out, operation is going to be pretty low thanks to being able to deliver a greater quantity of air.

I'm working on a diagram of the valves real quick for whoever wanted one. Gimme a few minutes.

EDIT: I have no idea how to export to an image in our cad software, so here goes a good description:

Two 4" piston valves are driven by two 2" piston valves which are driven by two 3/4" sprinkler valves.

The pistons are constructed like this: The piston sits at one end of the top of the T (if you look at it like it is a t) and the barrel at the other. The reservoir is coming off the 90deg part of the T. The piston has it's own special carriage that is made from forming putty and screws into the NPT fittings on the back of the Ts. This carriage both allows us to use cheaper material for a piston, and to seal the piston when it returns (so air doesn't keep running through the pilot) Just imagine a bushing that is made to go all the way into the valve, and has threads on the very end.

If I can figure out how to export I'll put up a diagram that does the design justice.

Thanks!
-MTT
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