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Pneumatic Crossbow

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 Crossbow

Unread postAuthor: goose_man » Fri Nov 09, 2007 7:25 am

I’ve had a specific project idea for a while, and I wanted to get some input. The idea is for a pneumatic “crossbow.” There would be no bow, but inside the stock would be a pneumatic cylinder. When the bow-string is pulled back, this would (via pulleys and such) compress the air in the cylinder using an O-ringed piston. When the trigger releases the bow-string, the compressed air in the cylinder would be able to force the piston back to its rest position with significant speed, pulling the string with it. The air in the cylinder would never exit the cylinder. The point of this is to have a cross-bow like device that shoots the same bolts but has no visible bow. At first I wanted to use a steel spring instead of a pneumatic cylinder, but that would vibrate and such (think pellet guns: gas-ram vs. spring-piston).

Does any one have any ideas or criticism?

-goose_man
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Unread postAuthor: spanerman » Fri Nov 09, 2007 7:52 am

the heat from the air compressing may be a factor, and maybe you could fill it with a gas which compresses more than air? what kind of draw force where you planning on achiving?
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Unread postAuthor: Ragnarok » Fri Nov 09, 2007 8:50 am

Crossbows are effective because the string is so light, and the vast majority of the stored energy goes into the projectile (over 99% I believe)

With a pneumatic cylinder, the piston is a significant weight, and may have a lot of friction - large amounts of energy will be lost to that, and the efficiency will drop a lot. The heat changes in decompression and recompression will also hit the efficiency hard.

Same sort of thing with a spring.

My idea of a pneumatic crossbow would to build a crossbow that used a pneumatic cylinder to pull the string back to get automatic fire, like in the "Van Helsing" film. The rest of the film was questionable, but that crossbow was pretty good.
I saw something on TV a while back that showed an auto loader for a ballista, but although I think I found the show on Youtube, it seems to be a shorter cut that no longer shows the replica sadly.
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Unread postAuthor: jackssmirkingrevenge » Fri Nov 09, 2007 9:07 am

Couldn't you mimic the design of a speargun and replace the rubber with a pneumatic piston?

I suggest a long, narrow piston to give you power over a long stroke that won't be too hard to cock.

I saw something on TV a while back that showed an auto loader for a ballista, but although I think I found the show on Youtube, it seems to be a shorter cut that no longer shows the replica sadly.


How about the chinese repeating crossbow
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Unread postAuthor: Hawkeye » Fri Nov 09, 2007 9:21 am

If there is no bow, why is there a bow string? Or how? There are examples of crossbows that are basically just a speargun principle. Heavy rubber bands power the bolts. That is much simpler.
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Unread postAuthor: Ragnarok » Fri Nov 09, 2007 9:38 am

@JSR: That's an interesting design, not the same one as I remember, but still interesting.

What I remember of the design I saw was a cylinder with a notch in it that a bolt would drop into from a gravity magazine, and through a track on the side of the cylinder, it was turned round through 180 degrees by the motion of the draw mechanism, dropping the bolt into position.

On the return, the cylinder was twisted back to pick up another bolt.

If the draw mechanism was moved by a pneumatic cylinder, possibly on a CO2 set-up, that could be pretty neat.
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Unread postAuthor: goose_man » Fri Nov 09, 2007 11:25 am

The point is that I don't want a bow or anything to extend outside the body of the crossbow. Also, I don't have a problem with drawing the string back myself, I just want the entire unit to fit in the body of the crossbow.

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Unread postAuthor: williamfeldmann » Fri Nov 09, 2007 1:27 pm

Ok, so where does the bow string go if you don't have an actual bow anymore?

Are you going to use negative pressure inside the cylinder so it snaps closed, or high pressure so it snaps back open?

Maybe I am just confused :?
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Unread postAuthor: clide » Fri Nov 09, 2007 4:28 pm

Ragnarok wrote:My idea of a pneumatic crossbow would to build a crossbow that used a pneumatic cylinder to pull the string back to get automatic fire, like in the "Van Helsing" film. The rest of the film was questionable, but that crossbow was pretty good.


Indeed, I've been wanting to build something like that for quite some time. I think it would be possible to build nearly an exact replica that would actually function if you had the time, money, skill, and machining facilities. Usually gadgets like that in movies aren't possible in real life.

Maybe the topic starter could combine that idea and the idea of a bowless crossbow and use a large spring inside the stock as power storage and use a pneumatic ram to draw back the string.
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Unread postAuthor: PVC Arsenal 17 » Fri Nov 09, 2007 4:34 pm

That idea reminds me of this.
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Unread postAuthor: boilingleadbath » Fri Nov 09, 2007 4:35 pm

Well, you could make a system similar to the <a href="http://www.swivelmachine.com/html/rimfire.htm">airrow</a>. . . but you'd either have to

a) Use high pressure air/CO2

or

b) put up with fairly anemic performance.

Numbers indicate that 300 psi air would probably be acceptable if you where firing 20 gram arrows; 190 fps.

That's with a 4ci chamber and an oversized valve that opens in 2.5ms. (Yeah, I used "generic". I'm lazy.)

Barrel dwell time is long enough that there's not really any benefit to using a faster valve.

The above probably isn't a terribly accurate... but, if the porting of the valve is .1"x60%, you still get 160 fps with that arrow.
******************

You'll note that I'm using arrows, not bolts... because bolts, while lighter, are also a good deal shorter.
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Unread postAuthor: goose_man » Sat Nov 17, 2007 10:45 pm

I have decided to, for now, go with a "real" bow. I was thinking of using large diameter pvc pipe and cutting a section off down the length to make the bow. Doews anyone have any suggestions such as cellular core pipe vs. pressure pipe, what diameter pipe to start with, etc?

-goose_man
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Unread postAuthor: Hubb » Sun Nov 18, 2007 1:06 am

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Unread postAuthor: Skywalker » Sun Nov 18, 2007 8:33 pm

I had a similar idea, gooseman. My thought was to use some pulleys to approximate the action of a real bow, but only using a single pneumatic cylinder. I was inspired by a marble-shooting gun that used a large screen-door spring and a block-and-tackle. I wish I could remember where I saw that thing...

I doubt you will get very good preformance from a PVC bow. PLastic in general is not a good material for a bow, because it deforms eventually if left under tension for a long time (that's how it get's its name!)
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Unread postAuthor: goose_man » Sun Nov 18, 2007 8:55 pm

Thanks for the links. Do you think whole 1/2" PVC pipe is better than a slice of larger diameter pvc pipe?

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