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pneumatic ram design

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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pneumatic ram design

Unread postAuthor: daccel » Sat Nov 10, 2007 2:23 am

I've been thinking about high pressure too much lately :) . I found a few threads discussing pneumatic rams to increase the performance of small diameter ammo, but they involved using a burst disk or resistance of the projectile and didn't seem to go anywhere. Anyways, my idea is that to get around using a sabot for a given performance with a fixed barrel length, you have to increase pressure. Hpa is limited to 3-4.5k psi, and a standard paintball regulator will take it down to ~800psi for consistent shot to shot performance, and with a pneumatic ram you could increase that to whatever pressure your gun could handle, by changing the ratio of expansion to compression chamber. I picked 6000 psi because that's the highest rated steel fittings I could find. If you were comfortable going beyond the rated pressure given safety factors, you could increase the performance considerably.

To walk through the diagram, the desired chamber volume is 1 cu in at ~6000psi. The volume in the valve housing is ignored for simplicity's sake. To get that pressure from 800 psi, the compression chamber will have to be ~7.5 times the length of 1 cu in. In 3/4" ID pipe 1 cu in is 2.3" long, and the ratio of a 2":3/4" circle is 7.1:1, therefor the compression chamber must be 16.3" long, and expansion chamber 14" long so that the piston doesn't slam the last couple inches when the compression chamber dumps.

First, (1) the compression chamber is filled to 800 psi through the standard piston valve, then that fill line is closed off. (2) The expansion chamber is filled to 800 psi, and the piston moves to the right, venting the air in the middle out (3), and compressing the air in the compression chamber to (4) about 1/7 the volume, which would increase it from 800 to 5700psi. Then the piston valve is fired (5), and the expansion chamber vented (6). The piston will return to its original position when step 1 is repeated.

I can see getting the piston to seal being the most challenging part, but they use o rings in hydraulics that operate at high pressures, so it's possible. Also finding a sealing face that won't be cut by the barrel at that pressure could be difficult. So, difficulties aside, would this work?
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Unread postAuthor: jackssmirkingrevenge » Sat Nov 10, 2007 3:47 am

The theory seems sound, though how much it would survive for another shot, I cannot say.

If you have 800 psi on tap, I would say a normal small calibre piston valve with a large enough chamber and long enough barrel would be fearsome enough in its own right.
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Unread postAuthor: POLAND_SPUD » Sat Nov 10, 2007 7:20 am

what's the point in using a regulator to get pressure down to 800psi and then increase it with a ram ?? If you want to use 6000psi u need very strong piston valve :shock:

Using 3-4k psi from a HPA tank should be easier than
making a ram and valve strong enough to work safely at 6k
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Unread postAuthor: daccel » Sat Nov 10, 2007 10:14 am

Yes, 800 psi with a long enough barrel would do the trick, but the idea was to avoid an overly long barrel. For example, throwing some numbers quickly into GGDT, for a .22 barrel a 3 gram projectile is accelerated to just under the speed of sound in 3'@5700psi vs. 12'@800 psi with the given volumes in my example above (1 cu in at 5700 and 57 cu in (same as the amount used to get the former pressure) at 800).

The point in reducing to 800 and then increasing again is that you get consistent shot to shot performance for accuracy. If you just use unregulated HPA, each subsequent shot will be slower and not hit the same point. Yes, I realize this isn't easy and it's not something I'd try to throw together without the right parts, but it may be a way to reach a new level of performance in spudding in a compact package.

The other thing is it doesn't have to be used for such high pressures, you could pressurize it to 120 psi with a compressor and in the same model you would get 850 psi out. Or you could use a bicycle pump and have a completely mobile, compact, limitless air supply of over 1000psi.

Glad to hear the theory passed, Jack :wink:.
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Unread postAuthor: DYI » Sat Nov 10, 2007 10:57 am

This is a great design. By incorporating a check valve and a third chamber, you could fill larger chambers with the ram and make it more practical. I might just make one of these for myself, to bring my bike pump's output pressure up to something useful and avoid wasting precious nitrogen :lol:
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Unread postAuthor: POLAND_SPUD » Sat Nov 10, 2007 11:38 am

DYI I like your idea... it doesn't have to be a part of the gun but it can be connected to a compressor or pump to 'multiply' pressure, like a (sort of) pneumatic gear-box right ?

I am afraid that there can be some temperature problems due to adiabatic heating - I don't know much about that process so any comments are welcomed
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Unread postAuthor: daccel » Tue Nov 13, 2007 12:38 am

Not sure about the overheating, it would probably only be an issue if you used it many times in a row to fill a large chamber.

Thinking about using co2 instead of hpa, but obviously the first chamber couldn't be co2. Anyone know how long it would take to fill 10 in3 with a shock pump to 300 psi?
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 13, 2007 12:44 am

It takes me about a minute of hard pumping to fill 3 cubic inches...
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Unread postAuthor: daccel » Tue Nov 13, 2007 12:13 pm

:shock: Fun. Back to the drawing board. Damn laws of physics. Two rams and a bike pump would work though, and separating them into a backpack with the co2 would make the gun less of a beast to handle.
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Unread postAuthor: frankrede » Tue Nov 13, 2007 1:25 pm

daccel wrote:Yes, 800 psi with a long enough barrel would do the trick, but the idea was to avoid an overly long barrel. For example, throwing some numbers quickly into GGDT, for a .22 barrel a 3 gram projectile is accelerated to just under the speed of sound in 3'@5700psi vs. 12'@800 psi with the given volumes in my example above (1 cu in at 5700 and 57 cu in (same as the amount used to get the former pressure) at 800).

The point in reducing to 800 and then increasing again is that you get consistent shot to shot performance for accuracy. If you just use unregulated HPA, each subsequent shot will be slower and not hit the same point. Yes, I realize this isn't easy and it's not something I'd try to throw together without the right parts, but it may be a way to reach a new level of performance in spudding in a compact package.

The other thing is it doesn't have to be used for such high pressures, you could pressurize it to 120 psi with a compressor and in the same model you would get 850 psi out. Or you could use a bicycle pump and have a completely mobile, compact, limitless air supply of over 1000psi.

Glad to hear the theory passed, Jack :wink:.

Its an interesting idea, but me personally would just buy a real gun if I wanted compact performance.
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Unread postAuthor: DYI » Tue Nov 13, 2007 4:46 pm

You do realise that the length of rams you need will completely ruin any chance of portability unless you have seamless pipe with and ID of over 3 inch, right? I contemplated the idea, but the sheer cost and difficulty of making one of a decent size put me off.
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