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My Pneumatic Crossbow Prototype

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: THUNDERLORD » Sun Nov 23, 2008 11:04 pm

Good job jor2daje!
jor2daje wrote:I know the trigger really sucks haha but as I said I threw the whole cross bow together in basically a day. I'm pretty much just happy that it works.


I've converted a few "releases" into triggers using This one . I just unscrewed the bolt running into it, Drilled a hole through the back, Then cut out (2 plates or bend 1) of sheet metal. (EDIT):Then bolt it inside the plates and bolt the plates through the wood stock with two 1/4" (or +) bolts I made the trigger from a 1/4" rod running back to it.

It works well but I plan on cutting a short slot where it bolts to the sheet metal rather than drilling a hole, that way when the string is drawn back it will close without needing to press the trigger while cocking it.

VH_man wrote:My friend has a childrens bow..... I wonder if you could modify one of theese to be airsoft style......


Sure, but if the rod broke it could shoot out the barrel so it probably shouldn't be fired like [normal] airsoft style.

I was thinking about the energy transfer (Like Starman mentioned) And a professional version linked to by JSR before (can't find it now) I expected the professional model on a 50 or 80lb. bow to be more powerful, but one thing is a normal spring piston air rifle incorporates a lever and the pull on those is something like 30-40 lb.s with the lever so... Perhaps a hydraulic or mechanical jack, or a boat come along pulley(connected to a gear and motor if auto action desired) connected to maybe a car/ truck leaf spring or coiled spring shock Hmmm... :roll: 8)
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Last edited by THUNDERLORD on Mon Nov 24, 2008 1:33 am, edited 6 times in total.
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Unread postAuthor: THUNDERLORD » Sun Nov 23, 2008 11:05 pm

Dang! Accidently double posted :oops:
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-----SPEED,STRENGTH, AND ACCURACY.-----
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 24, 2008 12:40 am

THUNDERLORD wrote:a professional version linked to by JSR before (can't find it now)


This one?

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Unread postAuthor: Ragnarok » Mon Nov 24, 2008 12:54 am

jackssmirkingrevenge wrote:This one?

You do kinda have to ask "Why?"

Sure, neat gimmick and all, but hardly practical for either hunting or paintball, simply by fact of size, rate of fire (at least for PB) and presumable difficulties with accuracy.
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Last edited by Ragnarok on Mon Nov 24, 2008 1:07 am, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 24, 2008 12:59 am

Not to mention that massive and convoluted transfer port :roll:
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Unread postAuthor: THUNDERLORD » Mon Nov 24, 2008 1:24 am

jackssmirkingrevenge wrote:
THUNDERLORD wrote:a professional version linked to by JSR before (can't find it now)


This one?


Hello JSR! Yes, that's the one.
It looks powerful in terms of airguns, and it's interesting. Maybe for someone who's a real archery nut though.

I wonder if it would be more powerful with a burst disc(?).
It would probably become louder too(?)... 8)
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-----SPEED,STRENGTH, AND ACCURACY.-----
"Procrastination" is five syllables for "Sloth".
Theopia 8)
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Unread postAuthor: THUNDERLORD » Fri Nov 28, 2008 1:06 pm

I've been thinking more on the energy transfer,
and basically the pneumatic piston adds power and does not take from it. Think about it, the spring of a spring piston alone could not propel a pellet or spud as quickly as with it.
Maybe that's why I was let down a little by the performance of the professional bow model. :roll: 8)

BTW not much time right now to reply but I'll check back later. :cry: 8)
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-----SPEED,STRENGTH, AND ACCURACY.-----
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Unread postAuthor: Ragnarok » Fri Nov 28, 2008 1:58 pm

THUNDERLORD wrote:I've been thinking more on the energy transfer,
and basically the pneumatic piston adds power and does not take from it.

It adds velocity at the cost of inefficiency - it can't physically add any energy. For these reasons, a springer airgun is usually about 30% efficent, but a bow is usually about 95%+ efficent.
But bows usually struggle to break 100 m/s velocity, whereas a springer can beat 300 m/s (although usually not very accurately)

Consider that a projectile needs to be ideally be heavier than what's propelling it to be able to work with any degree of efficency - an arrow is heavier than a bowstring, and an airgun pellet is heavier than the air.

The spring piston is an inefficient, but workable method of dropping the mass of the propellant to make the use of lighter projectiles feasible.
The use of a spring piston is only really advised for projectiles too light to be propelled by a bowstring.
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