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Cartridge Loader: Overlap vs. No Overlap

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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Cartridge Loader: Overlap vs. No Overlap

Unread postAuthor: PVC Arsenal 17 » Sat Apr 17, 2010 12:13 am

I discussed this briefly not too long ago, but I want some additional input before I go ahead and build anything.

Here's the situation:

I am working on a semi-automatic Nerf gun that uses a rather simple shell-fed loading mechanism.

When the gun is not pressurized (immediately after firing, trigger still depressed), the bolt/barrel is pushed forward by a reverse acting (spring extend) cylinder and the breech is opened.

One shell then enters the gun from the bottom.

When the trigger is released, the gun pressurizes.

The bolt/barrel then gets pulled backwards, taking the newly loaded shell along with it to a location behind* where it entered the gun.

The bolt/barrel keeps constant pressure against the shell to seal itself against the shell and to seal the shell against the firing port.

The gun is now pressurized, loaded, and ready to fire.

When the trigger is depressed, air flows through the exhaust port and pushes the dart out through the shell and the barrel/bolt.

The barrel/bolt then starts moving forward again under spring pressure from the cylinder.

With the spent shell no longer locked against the firing port [and secured by the detent rod*], it is free to be ejected out of the right side by the ejection mechanism*.

The barrel/bolt continues to move forward until the breech is again exposed and a new shell enters.



The question is: How far back should the barrel/bolt push the newly loaded shell? 100% of its length from the place where it entered the gun? 50% (overlap)? More than 50%?

Possibly important to note: In my design, there is a detent rod (black line in diagrams) connected to the barrel/bolt and it prevents the shell from being able to fall out of the ejection port due to the ejection mechanism (simple spring loaded ramp that will get pushed out of the way by the shell as long as the shell is secured by the detent rod, but will push the shell out once the detent rod separates from the shell). In the diagrams, it's not shown exactly where it would be, so I've attached another diagram for clarification.



Advantages of Overlap:
-Shorter overall length of gun
-Shorter cylinder stroke necessary
-Easier to increase the distance the barrel/bolt travels forward from the breech (allows more time for shells to enter gun and could reduce jams)
-Possibly allows for loading through the ejection port when reloading a whole magazine is inconvenient. (Being that the barrel/bolt does move forward from the breech, a removable detent pin would be needed when loading so that shells are not pushed too far forward, preventing them from reentering the magazine.

Disadvantages of Overlap:
-Harder to build (cutting breech and ejection ports will be a bit challenging. Attaching a magazine the way I planned to would have to be adjusted.)
-Possibility that fresh shells could fall out of the ejection port after loading because they are at least 50% exposed and not yet secured by detent rod.
-Slight complications to my detent rod setup.



Advantages of No Overlap:
-Easier to build.
-Detent rod easily incorporated.
-Guaranteed shell security (no exposure to ejection port except when secured by detent rod)

Disadvantages of No Overlap:
-Overall length increased.
-Necessary cylinder stroke increased.
-Potentially less distance that bolt/barrel can travel forward from the breech which could lead to incomplete loading and jams)
-Loading through ejection port almost impossible.
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Attachments
RSP1zero.gif
No Overlap
RSP1zero.gif (18.5 KiB) Viewed 620 times
RSP2half.gif
50% overlap
RSP2half.gif (15.08 KiB) Viewed 620 times
detent.PNG
The square is the body of the gun. Internal dimensions are .625". Diameter of shells and barrel/bolt are JUST slightly under .625" for a sliding fit. The detent rod slides in the corner between the square (body) and circle (shell).

For those who enjoy math, calculating the maximum possible diameter of the detent rod given those dimensions is a pretty fun algebra problem.

If anyone cares I can explain the solution, but I doubt anyone does...
detent.PNG (2.2 KiB) Viewed 620 times

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Unread postAuthor: Ragnarok » Sat Apr 17, 2010 9:01 am

I don't know exactly how useful this will be to you, but here's something I was working on a few years back for a project called Timberwolf.

Animated with transparency
Animated without transparency

As you can see, I designed it to overlap, as you say.

The design for the launcher in question, already had quite a lot of dead space, and overlapping the magazine well and the breech saved on both the length of the bolt and the space that the bolt would need to move into.

However, in the end, I scrapped the design completely - while it was cool, it wasn't exactly entirely practical.
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Unread postAuthor: PVC Arsenal 17 » Sun Apr 18, 2010 9:37 am

What wasn't practical about the design? If you adjusted the loading mechanism to push new shells backwards against the firing port, you would eliminate dead space and construction shouldn't be terribly difficult.
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Unread postAuthor: POLAND_SPUD » Sun Apr 18, 2010 10:43 am

yeah you need overlap... well at least you would have ejecting cartridges... how it could work without overlap ?

I don't know why you need that detend rod ? just get two tubes: one (barrel) that fits in the other and cut ports in the outer one
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Unread postAuthor: PVC Arsenal 17 » Sun Apr 18, 2010 6:09 pm

The detent rod is there because my ejection mechanism is not controlled by the cylinder. The detent mechanism is a simple spring-loaded ramp that always applies a force to the shell it's touching. When the shell needs to stay in position, the detent rod holds it there. When it needs to eject, the detent rod retracts and since there is no longer anything holding the shell in place, the ejection ramp pushes it out.

With that said, overlap is not necessary at all for the gun to function normally, but it could add certain advantages in terms of reloading.
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Unread postAuthor: POLAND_SPUD » Sun Apr 18, 2010 6:30 pm

well I don't think you need a ramp...

I presume that the barrel will be able to push the cartridge against the surface of the main valve fast enough... so once the gun fires and the barrel slides forward the cartridge will be ejected by the next round
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Unread postAuthor: PVC Arsenal 17 » Sun Apr 18, 2010 7:18 pm

It could if I had the gun eject out the top, but I'm looking to have it eject out the right side instead of the top. Maybe I need to reconsider some things... that could be a helpful feature.
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Unread postAuthor: iknowmy3tables » Sun Apr 18, 2010 7:33 pm

at first I thought this design was like this one
Image
however if I understand correctly this is still a bottom loading side ejecting design as opposed to a bottom loading top ejecting design.

so this design intend to remove the possibility of cartridges sliping that was in the original TFS design. why not make a modified ejector like an arc that encapsulates the top half of the cartridge, such an ejector could retain cartridges from potentially ejecting. I was considering building such an ejector so I have a diagram of a pivoting arc ejector below, you could also do a horizontally sliding arc.
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TFS type ejector system.PNG
what do ya know a diagram made by ikm3t that wasn't made on MS paint or powerpoint
TFS type ejector system.PNG (24.75 KiB) Viewed 442 times

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Unread postAuthor: PVC Arsenal 17 » Sun Apr 18, 2010 9:04 pm

That was another idea I considered... and while it has great potential, I can't apply it effectively to my Nerf guns without still incorporating some sort of barrel/bolt because the issue is not just to keep the shells in place, but to press them firmly up against the firing port to provide an effective seal. A pivoting arc/horizontal slide solves the problem of retaining the shells, but not exactly sealing them...in this case...

If I had the equipment to machine a gun with tight enough tolerances, making an adequate seal wouldn't be much of an issue.
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Unread postAuthor: POLAND_SPUD » Sun Apr 18, 2010 9:20 pm

@PVC
but I'm looking to have it eject out the right side instead of the top
well who said you can't cut the port at 30 deg not at 0 (zero) deg ?

in this way it will eject to the right/left
if you use telescoping tubes you can test and adjust it easily
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Unread postAuthor: iknowmy3tables » Sun Apr 18, 2010 9:48 pm

well then I think the best choice in this design involves some overlap, because the larger your stroke is the more volume and pressure your cylinder will need to maintain force on the cartridges to seal. so there is less chance of a compromised seal while firing
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Unread postAuthor: PVC Arsenal 17 » Wed Apr 21, 2010 9:02 pm

Well I went with the 50% overlap, out-the-top ejection configuration. No more ejection mechanism, I plan to make use of the spring pressure from the magazine. I have to make some mags and find a good way to attach them. After that, just a few parts left to order and hopefully the gun will be operational.

Picture: http://www.spudfiles.com/forums/viewtop ... tml#284622
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