Coil Springs as power source?

First topic message reminder :

I am thinking of using coil spring/s as a power source for a crossbow. More like a kind of Arrow projector than a conventional crossbow. As I have not seen anyone using this kind of type I wonder if there are any disadvantages with using coil springs. As such a spring could be integrated into the stock of the crossbow it would lead to a much slimmer package.
The force would be transferred via a set of pulleys to lengthen the draw. Before I start a build I wanted to research as much as possible to lessen the chance of faliure, maybe someone here can give me some input on this topic? In his "crossbow perversions" Ivo had a drawing of a spring powered crossbow, I am thinking along those lines but not an exact similar type.

Is it mandatory that you have moving arms on the crossbow to get some kind of "snap" or can a more simpler fixed "T" shaped Stock with rollers at the end of the T´s small arms be used? In some modern crossbows (like armcross leopro, RDT) there is almost no forward movement of the prods, the power is more working to the left and right so this seems possible?.If you use a T shaped stock with fixed rollers how could the performance be improved by using some sort of cams instead of regular symmetrical rollers? Can someone enlighten me on the physics/mechanics and design of cams?

Is the distance between the rollers/cams crucial? What are the disadvantages when you move the cams further together?Any disadvantages from a more acute string angle at full draw?

In the book "Die Armbrust, Egon Harmuth, p195" he mentiones a Swiss crossbowmaker Marcel Burri who designed a crossbow with coil springs but I was not able to find a picture of this thing anywhere. Anyone seen this?

I know that because of the mass of the steelspring it will have a high inertia, making it slower than fiberglass/carbon prods. A coil spring is better than a conventional steel prod because it has a lower neutral Zone. can that be counteracted by movable arms giving the bolt more "snap"?

I think that if you are using more than one spring it is necessary to synchronize these somehow, easier way would be to combine the springs into one power source, right?

Robert van House designed a crossbow http://www.freepatentsonline.com/4169456.pdf and before switching to a air spring used steel coil springs. With this crossbow he was able to get arrow velocities in the 300fps range!When looking at this patent I wonder if one could not use a regular air spring which are used nowadays in mountain bikes as power source? In most of these rebound and overall force could be regulated so you could have a crossbow with variable power!

rolynd wrote:juanecho, look at this video:

https://www.youtube.com/v/VcoiFQwV7u4

here he assembles the vacuum "crossbow". Quite a simple affair, I would have thought the vacuum part to be a more difficult build, just a tube and a greased plug!

Has anybody tried this vacuum approach with bellows instead of piston ?
That would eliminate any issues with sealing and friction.
Just hinged lever expanding a bag full of emptiness.

Say What?

Chaz

http://en.wikipedia.org/wiki/Bellows


Like this , but without the valve and nozzle.

Couple of those fixed from one handle to a static prod facing each other, string between the other handles.

Nerd,

So actually it would work off of compressed air ?

Chaz

No , not compressed air.
A vacuum spring works with external air pressure.

Just create a void and let atmospheric pressure fill it.

Bit like a pneumatic trebuchet.

Nerd,

Keep going, sounds interesting ......... need more pictures ......... you know they are worth a thousand words. I'll be watchin' this one.

Chaz

I can add a little here. I have been in the monkey business  here for quite a while. I tried the springs, went quite large too. They work , but firing one is not for the faint of heart.All that comes with the release surely is not  pleasurable. That being, I went to divers rubber 5/8" - dia.. I don't know how to measure speed or anything. All I can say right now is what I'm shooting can not be coked by hand . My draw is 17.5 , I'm using 5 pieces of rubber 6" long, doubled over and made to pull off a single hook up. The noise is nowhere near what the spring was.Things get kinda spooky when you start to get into this kind of energy. I stop and think  of what went through the minds of some of  the very early people that done this sort of thing. Can't say if it was bravery or -----.