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CAKE III Preview

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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Re: CAKE III Preview

Unread postAuthor: Ragnarok » Sun Nov 28, 2010 9:29 pm

Oxbreath wrote:Look again at the .gif I posted above. The second one IS my valve. And it works very well.

It will work. It won't work better.

Well, technically, none of those would work as drawn there - there's not the required area differential, but we'll leave that for now.

Only cross-sectional area matters - look again at diagrams #2 and 3. All that area on the right side of the sealing face - there's pressure on that too, creating an opposite force that completely cancels out the extra area you think you've added.

And curved surfaces, while they have a greater area, don't add anything either.
Pressure is always perpendicular to the surface, so the actual axial component of the forces... mathematically, all that extra surface area times the axial components - it works out as identical to the cross sectional area.

Like I said, there's two disproofs of this. One, bullets are not made with curved or grooved bases to add surface area for MOAR POWAH! If it did, they'd do it. But they don't, because it doesn't work.

And an example I've used before. Put your piston on a surface. Any flat surface, as long as it's in a pressurized atmosphere. Our 101,325 Pascal average will do.

Is it spontaneously starting to accelerate? Well then, the net forces on it must be completely equal, so clearly, by example that extra surface area cannot be adding "extra push" in one direction.

In short:
A) You need an area differential for a piston valve to work. If you think a valve is working without one, you're either mistaken, or have broken the laws of physics.
B) Only cross sectional area counts. Not total surface area.
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Unread postAuthor: mlz3000 » Sun Nov 28, 2010 9:58 pm

Alright, I don't know if nobody read it, (I can't blame you, my first post was pretty long) or if no one knows, but how do I figure out what value to put into ggdt as the flow coefficient?

And is 1" plenty of piston travel?
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Re: CAKE III Preview

Unread postAuthor: Selador » Sun Nov 28, 2010 10:17 pm

Ragnarok wrote:It will work. It won't work better.

Well, technically, none of those would work as drawn there - there's not the required area differential, but we'll leave that for now.


Ragnarok wrote:Only cross-sectional area matters - look again at diagrams #2 and 3. All that area on the right side of the sealing face - there's pressure on that too, creating an opposite force that completely cancels out the extra area you think you've added.


Ragnarok wrote:In short:
A) You need an area differential for a piston valve to work. If you think a valve is working without one, you're either mistaken, or have broken the laws of physics.
B) Only cross sectional area counts. Not total surface area.


You know, taken all together, this seems like a logical argument.

Reading it, it just plain makes sense. Especially the part about equal pressure going the other way, in #2 and #3.

But... My piston is exactly as drawn in #2, and it works.

I don't think I am breaking the laws of physics. So why does mine actually work ? Serious question. I am not trolling or being facetious. I want to learn something.

And why do so many other pistons here, built like #3, actually work ?

Also... What do you mean by an area differential ? And cross sectional area ? Can you draw me a picture ? Pretend I am a kindergarten student.
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Unread postAuthor: Technician1002 » Sun Nov 28, 2010 10:24 pm

But... My piston is exactly as drawn in #2, and it works.


Hmm. Is the piston OD exactly the same diameter as the ID of the barrel?

If it is, then the only reason it works is the seal leaks when the force holding it closed goes to zero. When it leaks the pressure rises on the face unbalancing it.

In reality, I expect the piston OD is larger than the ID of the barrel. Is the barrel the same size as the pilot the piston slides into?
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Unread postAuthor: mlz3000 » Sun Nov 28, 2010 10:37 pm

Doesn't #2 work because of the part of the sealing face that sticks out around the seat?
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Unread postAuthor: Selador » Sun Nov 28, 2010 10:39 pm

Technician1002 wrote:Hmm. Is the piston OD exactly the same diameter as the ID of the barrel?


No. Holding an identical end cap, (piston), up to another piece of the same pipe, (barrel), there is no difference in OD, between the two.

Technician1002 wrote:In reality, I expect the piston OD is larger than the ID of the barrel. Is the barrel the same size as the pilot the piston slides into?


Yes. The OD of the barrel is the same size as the ID of the piston bore. (Like I said, the OD of the piston is the same as the OD of the barrel.)

I'm sorry that the drawings don't make that clear.

mlz3000 wrote:Doesn't #2 work because of the part of the sealing face that sticks out around the seat?


That is what I have been thinking.

All other pressures being equal, or cancelling each other out, the tiny bit of seal face that is clear of the diameter of the barrel, is just enough to get the process kick-started.

Edit to add: Yes, you have plenty of piston travel.

Some here would say that you have too much piston travel.

I have almost as much, and am having no problem.
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Re: CAKE III Preview

Unread postAuthor: Ragnarok » Sun Nov 28, 2010 11:06 pm

Oxbreath wrote:But... My piston is exactly as drawn in #2, and it works.

One half or the other of this is in someway false.

I'll trust you on it working, and as you have said that the ID of the piston is greater than the ID of the barrel, then it seems logical that your drawing is at fault.

And why do so many other pistons here, built like #3, actually work?

They're not built like #3. They may just appear to be at first glance.

Also... What do you mean by an area differential? And cross sectional area? Can you draw me a picture?

A good place to start, if you haven't seen it already is here.

We'll start with "Cross sectional area" - imagine cutting an object in two along a flat plane. The flat surface of the cut is the cross section, and the area of it is the cross sectional area. As far as pneumatic systems are concerned, it can get a little more complex... one thing you need to do is assume pistons are solid, even if they are actually hollow. It's hard to explain why, but you'll get a grip on it eventually.

Area differential. What this means is that to fire, when the pilot is vented the cross sectional area exposed to zero pressure (by "zero pressure", I mean atmospheric in this case) in the pilot must be greater than the cross sectional area exposed to zero pressure at the barrel port.

Anything different means no rearwards net force, and therefore no piston movement.
Bear in mind, pistons are NOT "sucked back". Pressure can only push.
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Re: CAKE III Preview

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Unread postAuthor: jackssmirkingrevenge » Sun Nov 28, 2010 11:43 pm

Some discussionon the subject.
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Unread postAuthor: Technician1002 » Mon Nov 29, 2010 12:12 am

Having the barrel ID smaller than the Piston OD makes it work. Otherwise it would need a mechanical trigger like my QDV which does have a barrel ID the same as the piston OD.

The close ratio of the barrel ID to Piston OD would make a very nice fast snap valve. Nice job.
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Unread postAuthor: mlz3000 » Mon Nov 29, 2010 12:25 am

If you look at Oxbreath's post in this thread it's pretty clear that the piston OD is greater than the barrel ID.


And for clarity's sake, in this discussion we're using barrel ID and seat ID synonymously, right? For example, the housing which is the topic of this thread has a 1.5" seat, but I can put on a 2.5" barrel.
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Unread postAuthor: Ragnarok » Mon Nov 29, 2010 12:44 am

I would suggest a better wording than Tech used.

Piston OD is misleading. A less ambiguous, if less succinct, wording is the ID of the piston pipe/tube - the pilot part of it, at least.
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Unread postAuthor: Selador » Mon Nov 29, 2010 1:16 am

First off, mlz3000, I am sorry that we have gotten your thread so sidetracked.

Ragnarok wrote:
Oxbreath wrote:But... My piston is exactly as drawn in #2, and it works.

One half or the other of this is in someway false.


Ragnarok, I hope I haven't gotten you completely frustrated.

In the interest of getting rid of some of the frustration causing innaccuracies, I just spent the last almost three hours drawing my 1-1/4" valve as accurately as possible.

The yellow is all 1-1/4" fittings.
The green is 1-1/4" pipe.
The orange is 1-1/4" grey plastic pipe fitting, meant for electrical conduit.
The purple is the piston. (3/4" cap, pipe and connector.)
The grey is 1" pipe.
The seals and bumpers are probably self-explanatory.

The reason for the conduit fitting (orange), is because the OD of the 1" pipe is slightly smaller than 1-14" pipe. That fitting is smaller in ID, than the regular 1-14" fittings, and the 1" pipe was a snug fit in it.

The piston actually required a wrap of aluminum auto-body tape, to make it a snug fit in the 1-14" pipe.

I assure you, this drawing is very accurate. And the valve -does- work.

~~~

Now I think I'll spend some time drawing my recent 1/2" piston valve. Because according to the current "knowledge", there is just no way that valve could work. But again... it does.
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Unread postAuthor: Lockednloaded » Mon Nov 29, 2010 1:26 am

sorry to side track even further, but if what everyone has been saying is true; how does gort make his 1-1/4" housed 1" porting valves? it seems impossible by the standards addressed in this thread
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Unread postAuthor: saefroch » Mon Nov 29, 2010 1:30 am

Nobody ever said that valve doesn't work. What Rag originally said is this:
Ragnarok wrote:When pressure is concerned, only the cross sectional area is relevant, not the total surface area.
The simplest proof of this is that if the inverse were true, there would be a big business of increasing the surface area of the back of bullets (be it with grooves, curved surfaces, etc) in order to get MOAR POWAH.
What the discussion was on is about how #1 is no better than #2, since the cross-sectional area of the piston face exposed to an area of atmospheric pressure is the same.

Personally, I see all the piston valve designs you posted as exactly the same, except for some silliness with shapes. Rag's already said it:
Ragnarok wrote:Area differential. What this means is that to fire, when the pilot is vented the cross sectional area exposed to zero pressure (by "zero pressure", I mean atmospheric in this case) in the pilot must be greater than the cross sectional area exposed to zero pressure at the barrel port.
All that matters is that there is some area of the sealing face on the piston not against a solid object, and that the area of the port is less than that of the back of what one could consider the sealed area of the piston.

Oxbreath wrote:Now I think I'll spend some time drawing my recent 1/2" piston valve. Because according to the current "knowledge", there is just no way that valve could work. But again... it does.
If you'd like somebody to tutor you on how piston valves work, that can be done.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 1:35 am

In a nutshell:
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