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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: Technician1002 » Mon Nov 29, 2010 1:52 am

Nice nutshell. I didn't have time to mark up the drawing.
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Unread postAuthor: Ragnarok » Mon Nov 29, 2010 1:58 am

Oxbreath wrote:I assure you, this drawing is very accurate.

That new drawing, maybe. The old one however had flaws that meant that the valve could not have worked as drawn.

Jack has done a brilliant job of explaining it.

Occasionally, there may be an exception like Tech said, where the valve starts to "leak" when the net sealing force on the piston falls - but that's not a good design and should be avoided.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: Selador » Mon Nov 29, 2010 2:04 am

jackssmirkingrevenge wrote:In a nutshell:


I do believe that is it, in a nutshell.

Now, about this one:

This is an accurate description of my 1/2" valve.

Yellow is 1/2" pipe.
Green is 1/2" tee.
Red is 3/4 fitting. (Had to sand the 1/2" tee down some, to fit the 3/4" fitting onto it.)
Blue is the piston.
Seal and bumper are brown

The tee has been hogged out, to allow a 1/2" pipe to go all the way through. The 1/2" pipe is inserted almost halfway. The piston slips in the other side, and is the same OD as the 1/2" pipe.


If you'll remember, I had some difficulty getting this one to work with only a blowgun as pilot. But I did finally get it to work when I made the piston fit to the bore... a pretty tight fit.

But it DOES now work just fine with only a blowgun as pilot.

Again, the ID of the barrel is smaller than the OD of the piston. That is probably why this one works.
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Unread postAuthor: Technician1002 » Mon Nov 29, 2010 2:11 am

Very nice. The only changes I would recommend is adding some padding to the thin bumper so the piston retracts the same distance the barrel enters the T from the other side, so when open, the gap between the piston and valve seat is centered in the T. This places the air inlet directly in the widest part of the air inlet and reduces the impact force of the piston on the bumper.
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Unread postAuthor: Selador » Mon Nov 29, 2010 2:29 am

Technician1002 wrote:Very nice. The only changes I would recommend is adding some padding to the thin bumper so the piston retracts the same distance the barrel enters the T from the other side, so when open, the gap between the piston and valve seat is centered in the T. This places the air inlet directly in the widest part of the air inlet and reduces the impact force of the piston on the bumper.


I do intend to take that valve apart again later, and try to get better performance. (It works good, but IMO could work better.)

I'll keep your suggestions in mind when I do so.

Thank you.

saefroch wrote:If you'd like somebody to tutor you on how piston valves work, that can be done.


1. I thought that was pretty much what was already going on, here. ;)

2. I have already built two working piston valves, after reading all the tutorials I could find on this site. I am working on a third. And learning more and incorporating that learning, as I go along. :)

I greatly enjoy these mutual-learning discussions.

I don't, however, consider insults, (couched or otherwise), to be valid teaching/learning tools.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 3:50 am

Rag wrote: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.


Actually they do, for A LOWER RATE OF REDUCTION IN POWAH ONCE THE BULLET LEAVES THE MUZZLE!

Image

Nothing to do with surface area in the barrel, but with a sustantial aerodynamic benefit.

Yes, it's one of those days :roll:

Oxbreath wrote:I don't, however, consider insults, (couched or otherwise), to be valid teaching/learning tools


Looking through the thread, seems like you don't need tutoring and there was simply a misunderstanding, as long as we're agreed on this ;)

Again, the ID of the barrel is smaller than the OD of the piston. That is probably why this one works.
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Unread postAuthor: Selador » Mon Nov 29, 2010 4:03 am

jackssmirkingrevenge wrote:
Oxbreath wrote:I don't, however, consider insults, (couched or otherwise), to be valid teaching/learning tools


Looking through the thread, seems like you don't need tutoring and there was simply a misunderstanding, as long as we're agreed on this ;)


Agreed.

In fact, I just came back in here, to remove that last bit, because on afterthought, it was obviously just a misunderstanding.

Too late. LOL

Oh well. Can I say again, how enjoyable it is to discuss this kind of stuff with people who are not only -not- disinterested... But who actually understand the physics, the WHY, and not just the how, of putting these thigs together.

And better, like to experiment and to watch/learn from others doing the same...

For instance, I just read in a thread where Saefroch got out of bed to sketch out a design and put it on spudfiles. Just so others could discuss it with him. And hopefully somthing would be learned, all around.

That's an admirable kind of obsession ! LOL

:D
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 4:17 am

Oxbreath wrote:That's an admirable kind of obsession ! LOL

:D


Not the first time I fall asleep spudding notebook in hand and wake up with a pen awkwardly attached to my cheek :roll: as my Russian dominatrix whore once declared, "it's not a hobby - it's a lifestyle" ;)
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Unread postAuthor: Crna Legija » Mon Nov 29, 2010 4:21 am

you need to leave a bit around the piston for the pressure to push against
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 4:30 am

-_- wrote:you need to leave a bit around the piston for the pressure to push against


Actually there's no need, this is a common misconception.

As the pilot empties, the piston unseals and air gets into the tiny gap between the piston face and the barrel, and that's enough to push it back.
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Unread postAuthor: Technician1002 » Mon Nov 29, 2010 4:42 am

I have found a narrow seat like that promotes damage to the seal. Cutting back the seal is not nessarry. Leaving it full size is not a problem as the pilot pressure reduces and the force on the seal reduces, the outside edge tends to lift on it's own as the force is trying to blow the seal toward the inside of the barrel, thus making the sealing surface near the ID of the barrel.

The forces on the seal may simply blow it into the barrel.

Leave the seal large for a wide suport to prevent a knife edge from cutting it. If you are concerned, you could very lightly cut small grooves on the seat from the outside edge inward about half way to the ID. This would permit chamber air to infiltrate the outer edges of the seal area and prevent a tight seal at the OD of the valve seat.

On my Mouse Musket, I built an o ring into the seat to define the diameter and left material inside and outside the seal for the piston to rest on without mashing the seal. I built the Dragon with the same o ring valve seat.
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Unread postAuthor: mlz3000 » Mon Nov 29, 2010 8:57 am

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


Are you kidding? I've learned so much more reading this discussion than had you guys kept it on topic. The Topic was basically "look at this, I would have put it in the showcase, but the glue is still drying, so I have not pieced it together or done any testing." :roll:

This was way more informative. I held the misconception that sphere would accelerate down the barrel faster than something with a flat face towards the pressure :lol: now I know better :wink:
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Unread postAuthor: saefroch » Mon Nov 29, 2010 11:50 am

Oxbreath wrote:I don't, however, consider insults, (couched or otherwise), to be valid teaching/learning tools.
I did not intend to be at all insulting, I was merely attempting to offer assistance, which I not understand to be redundant.

mlz3000 wrote:sphere would accelerate down the barrel faster than something with a flat face towards the pressure
I've been thinking about this for a while, and can you prove this with calc? Take the angle of the tangent at any point, resolve it into x and y, and from there you get the force in the relevant direction at any point on the sphere... which turns out to be much easier done with cross-sectional area. Am I correct?

Technician1002 wrote:The forces on the seal may simply blow it into the barrel.
Use a firm enough material and a bolt with a sufficiently large head and you won't blow off a seal. Just don't over-tighten the bolt or the seal will begin to extrude around it.

jackssmirkingrevenge wrote:As the pilot empties, the piston unseals and air gets into the tiny gap between the piston face and the barrel, and that's enough to push it back.
In theory, yes. I blew off a seal a few days ago and managed to get a solid UHMWPE piston to seal to my valve seat without realizing the o-ring had blown off. The full diameter of my seat is greater than that of the piston. The piston did not pilot, even though it was still sealed on the back end. I had to wait for it to slowly leak backwards through the check valve.

mlz3000 wrote:the glue is still drying, so I have not pieced it together or done any testing.
Can we expect an update on your progress any time soon?
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Unread postAuthor: Selador » Mon Nov 29, 2010 4:22 pm

saefroch wrote:I did not intend to be at all insulting, I was merely attempting to offer assistance, which I now understand to be redundant.


Thank you for the PM's.

And once again, please accept my apologies for the misunderstanding.

:D
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Unread postAuthor: mlz3000 » Mon Nov 29, 2010 5:14 pm

saefroch wrote:Can we expect an update on your progress any time soon?


Soon. I know it's getting a paint job on Thursday, hopefully I can get some testing in before then, but probably Thursday at the latest. I've got a lot on my plate right now. :roll:
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