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My First Pressure gun ever. Over/Under Design, need help.

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 » Wed Nov 24, 2010 10:05 am

mlz3000 wrote:Alright, that's what I thought, thanks.

While we're kind of on the topic though, what's the biggest piston valve one could make where a blowgun would be sufficient as a pilot valve? Or would it just almost always better to use a ball valve?

Would a sprinkler valve modded with a blowgun perform appreciably better than one with a blowgun?


To answer that one simply has to understand how a piston valve works. It remains closed until the pressure in the pilot area drops enough to unbalance it so the force holding it closed is less than the force trying to open it.

In order to get the pilot area pressure low enough, the pilot valve vents that area. The larger the volume and the lower the flow of the pilot valve, the longer this takes.

If the piston has some leakage either intentionally or accidentally due to an EQ port or piston blow by, air is added to the pilot area at the same time you are trying to let it out. If the leakage is large, the blowgun will be unable to drop the pressure enough to unbalance the piston and the chamber will bleed out to the pilot without firing.

If the leakage is sealed or very small, a small blowgun will be able to pilot very large valves, but it may take a while to vent the pilot area low enough to fire.

Because most piston valves leak some between the chamber and pilot area, taking a long time to vent the pilot area provides longer time for air to leak out of the chamber into the pilot area resulting in the pressure dropping in the chamber before the cannon fires. For this reason it is a good idea to use a faster flowing pilot valve. Less pressure is lost from the chamber before it fires.

I hope this answers the question.
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Unread postAuthor: Yukondano2 » Wed Nov 24, 2010 1:47 pm

-Snip due to weird errors-
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Last edited by Yukondano2 on Wed Nov 24, 2010 1:48 pm, edited 1 time in total.
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Glitches

Unread postAuthor: Yukondano2 » Wed Nov 24, 2010 1:47 pm

I really like that Trident. So just wondering, Is there any advantage whatsoever to having multiple chambers? I always thought i could make it more compact that way. I wanted to make something that looked kinda like a grenade launcher. http://www.exchange3d.com/images/upload ... 19_aaa.jpg

I could also put in a breech load. I would LIKE to have 6 shots in one gun but i'm not SURE how. That would require 6 valves... which sounds REALLY expensive. So it would only be for the looks, and have one shot. What do you think?

Also i'm having some very strange positing problems. Posts came up as completely blank. Weird.

Trying to understand the concept you put out. The needle slowly moving up? What do you mean? Each one would be directly connected to each other, my idea is just a compact gun, For Instance the Grenade launcher idea.

Not trying to get overly complicated, but a standard over/under design that everyone repeats is somewhat unoriginal.

Actually, I could be wrong. When your building one of these things, is the goal really performance? 'Cause i can't figure out how to use ggdt or whatever.
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Last edited by Yukondano2 on Wed Nov 24, 2010 2:07 pm, edited 3 times in total.
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Unread postAuthor: Yukondano2 » Wed Nov 24, 2010 1:52 pm

If the leakage is sealed or very small, a small blowgun will be able to pilot very large valves, but it may take a while to vent the pilot area low enough to fire.


And the slower the less power because the piston won't open very quickly, less air will flow out the barrel and then you get more of a continuous stream as apposed to a large, Quick blast. Correct? :?
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Unread postAuthor: saefroch » Wed Nov 24, 2010 4:22 pm

Yukondano2 wrote:And the slower the less power because the piston won't open very quickly, less air will flow out the barrel and then you get more of a continuous stream as apposed to a large, Quick blast. Correct?
Not really. As soon as the front of the piston lifts off the sealing face it experiences a force pushing it back equal to the area across the "sealed" diameter of the piston times the pressure. Once the piston "cracks," it moves back with such force that the flow out the pilot valve becomes negligible. Most of the loss from a poor pilot valve comes from the delay and the loss in chamber pressure before the piston unseals. Keep in mind that the piston doesn't begin to move as soon as you open the pilot valve, which is just there to get the piston moving. Once the piston is moving, other forces take over.

No advantage to multiple chambers, just an abnormally high C:B ratio, and since the chamber volume is so tiny anyways, it's not much more effort to pump.

A multiple shot design doesn't require multiple valves necessarily. You could set it up like a grenade launcher, where the grenades are a cylinder of compressed air, and you breech-load from a separate magazine.

Yukondano2 wrote:When your building one of these things, is the goal really performance?
It's whatever you want it to be.

To use GGDT: Fill in all the fields above the graph and click "Calculate performance." If you have any troubles understanding what numbers you're supposed to put in, just ask.
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Re: Glitches

Unread postAuthor: mlz3000 » Wed Nov 24, 2010 5:07 pm

Yukondano2 wrote:When your building one of these things, is the goal really performance? 'Cause i can't figure out how to use ggdt or whatever.


saefroch wrote:It's whatever you want it to be.


I agree. For example, I'm building a piston valve to increase performance over my sprinkler valve, but as a first piston valve, I'm more worried about it functioning at all more than worrying about performance.
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Multiple Shot Design Confusion

Unread postAuthor: Yukondano2 » Wed Nov 24, 2010 11:09 pm

So is there any DISADVANTAGE to multiple chambers? Because again, Its for looks and compactness. However if it reduces functionality then i'm worried.
saefroch wrote:A multiple shot design doesn't require multiple valves necessarily. You could set it up like a grenade launcher, where the grenades are a cylinder of compressed air, and you breech-load from a separate magazine.


I don't follow. What do you mean? How do I separate the airflow from each cylinder? My idea wasn't to have multiple shots but now it occurs to me that MIGHT be a good idea. In order to be true to the original I would be using Six Canisters. I'm assuming this would be manual, so I would do something like reconnect the canister to the gun by like a hose? Of course If i do that then its going to work terribly going through such a small space. I wish I could just have it rotate and fire instead. A breech obviously wouldn't work unless I somehow cut the flow, closed the breech, and opened the flow. Of course at that point I would ALREADY be using a valve on each chamber.

-EDIT-
Anyone know where to find some good Two dimensional Design software?
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Multiple Shot Design Confusion

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Unread postAuthor: jackssmirkingrevenge » Thu Nov 25, 2010 1:32 am

Here's a conceptual drawing of how one could achieve multiple shots from a single chamber with a spring loaded piston valve:

Image

Note that the piston is not completely airtight, meaning air is free to flow back into the pilot chamber after each shot.

You could combine this system with a revolving magazine for reloading, maybe coaxial with the chamber.

Another option would be to have "valveless" cartridges conceptually along these lines (or using burst disks if your projectiles aren't suitable for a friction breech) but with the "cartridges" fixed in a revolver type mechanism, something like this:
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Unread postAuthor: saefroch » Thu Nov 25, 2010 10:55 am

There is NO disadvantage to having "multiple" chambers, like on my trident. It's actually modeled as one 28" long .495" inner diameter chamber.

If you were to use multiple cylinders like on the grenade launcher, you'd have each pre-pressurized and sealed. As you fire one, the next one slides into position and you use some method to unseal it, maybe a check valve where you simply push in on the sealing face of the check valve to force it open. Rather like JSR's second diagram, except with the barrel in the middle and the chambers on the outside. I wouldn't advise trying to build pneumatic cartridge system (here JSR and I are talking about the same thing) before you've built a piston valve though...

2-D Design software? I actually just use autoshapes in PowerPoint, I know some people use paint. There's really no need for high-tech 2-D design, just use what you have available.
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Unread postAuthor: Yukondano2 » Thu Nov 25, 2010 12:07 pm

jackssmirkingrevenge wrote:Look at me still talking when there's science to do


When I look up there it makes me glad I'm not you.
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Unread postAuthor: Yukondano2 » Thu Nov 25, 2010 12:31 pm

The Friction based ammo thing is a bit odd...
What about a check valve that blocks air from the chamber? Let me see if I can get something here...
Image
There we go :D
-EDIT-
FAIL!
forgot to draw the piston in the first slide
XD
-EDIT-
What can i make a check valve from anyways? I'd prefer not to use a piece of rubber, considering that could NOT hold to 300 Psi. I think....
But that doesn't matter. Any good ideas for check valve material? Also I won't be using a hose to connect the chambers, wouldn't work. I'll just have a small slot in the piston.
-EDIT2-
It just occurred to me that the air flowing from the chamber will slam against the piston with such force that it would open it instantly because the pilot isn't pressurized. Perhaps I could have a hose connect from the chamber to the Piston like I had in the diagram and put a small check valve in the pilot area to make it more efficient. This way I could wait for the piston to match pressure with the chamber, measuring via a pressure gauge on the piston. If its moving up, I wait. If it stops, I disconnect it and release the check valve. Would that work?
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Last edited by Yukondano2 on Thu Nov 25, 2010 9:57 pm, edited 2 times in total.
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Unread postAuthor: Yukondano2 » Thu Nov 25, 2010 12:39 pm

saefroch wrote:I wouldn't advise trying to build pneumatic cartridge system (here JSR and I are talking about the same thing) before you've built a piston valve though...


I understand. I'll construct a PVC Piston valve and connect it to my STILL not constructed gun after sawing off part of the barrel and connecting it.

-Edit-
You guys all gone for thanksgiving err something? :?
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