Coil Springs as power source?

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!

I use coil springs to power my ballista, the Suburban Scorpion. Basically the design is like a Roman ballista, scorpion-sized, but with steel extension springs in lieu of the usual torsion bundles of rope. I would post pictures of it here, but I haven't figured out how to do that yet . . . two facts emerged form building and shooting scorpion: the springs need to be pre-tensioned before the draw, and I need to use a shock absorbing bowstring. I had a fellow make a strong, standard style string out of Dacron and Fast Flight, and using it broke one of the 68 pound springs and bent several pieces of 5/16 steel hardware. All the shock of firing was transferred to the hardware in the throwing arms. Using 128 pound test nylon cord as the bowstring, none of these things happen.

Paul

Thanks for the reply Paul, wouldn't have thought of the string issue! The leftover energy must go somewhere...Found a vid of a coil spring powered crossbow:



He disassembles it at the end of the vid. What came to my mind is that maybe there could be a friction issue with compression spring inside the tube better to use pull springs. and also you may get more velocity with reducing mass and friction in the rollers.

But unfortunately cannot understand a word of what he says. In principle the idea with coil springs seems to work but the crossbow seems a little slow. If fine tuned maybe a viable concept. Seems like a homemade version of this:

http://www.crossbowpylenok.aiq.ru/

More and more I come to the conclusion that some kind of pneumatic spring could be better because of the lower mass involved. I really wonder if those bike dampers like http://www.sram.com/rockshox/products/bar-r could be adapted for this purpose? Well, you must find one in which the dampening effect could be reduced to almost zero. or does one know of a similar readily available device which could be used in a similar way?RegardsRolynd

I watched the video. Very interesting. I don't speak Russian, but it was pretty clear what he was doing and how the bow worked. He's using a compression spring instead of an extension type (which I used in my ballista). Compression springs widen under load, so the tube might be binding a bit--can't tell how close the tolerances are. Extension springs thin when stretched.

It also looks like he's using a wire string. Judging by the size of the spring and how he cocked it using those short handles, I bet it's around 80 pounds pull maybe? He was piercing the plywood target but not the metal backing board.

Interesting design. I wonder where he got his pulleys . . . I have considered putting pulleys on the scorpion, but I am trying to keep it as Roman as I can.

Is the same guy in both vids...!!!
He got sponsors it seems...!!!!!
I´m wondering.....the prod turns into a useless part on that design.......don´t you think??
I mean....the string must come from somewere....but the essential spirit of a croosbow is lost.....wath is the next passe?? To use powder cartridges to shoot arrows maybe???

Well, in a strictly speaking sense its more of an "arrow projector" than a classical crossbow. I think as long as its firing arrows and works mechanical it classifies ;-) . If you look at hte more modern designs like leopro, Stryker etc they dont remind one of the clasical medieval crossbow either, using the latest technological materials cnc production cad designd cams etc. Where will you draw the line?I wonder what would have become of the crossbow now if we never found out about blackpowder in the first... You cannot beat the laws of physics but mabe squezze a little more performance out of them with modern materials and lateral thinking than before thought possible? I am a tinkerer and always interested in a new way to skin the cat so to speak. Some may be dead ends but others show pomise.

I totally agree with you, but modern versions of crossbows still use
some bow shape, that actually works like a bow works, larger, shortes,
two limbs, one piece, with or without pulleys, and a long etcetera......but the concept of a bow crossed on a stock is allways pressent. In this case, the "prod" is not acting like we used to see, it´s only there to hold the pulleys.....no movement at all.....and the energy is provided by a coil spring that is´nt even crossed to the stock.....
I don´t know, but I´ll sure classified it into Arrow Shooter (or Projector, like you call it) and not into crossbows......just my opinion...
Anyway, i think that a compression coil spring will work just fine, if it has a couple of guides in the inside diameter, and have some free space in the outside diameter, well lubricated (the receptacle where it fits will work as a grease holder)...just like an air rifle works, string instead air....

I have thought about using springs as the power mechanism before. it still in the thought experiment stage for me but would like to see one made.

Never would have thought about the solid T style limbs but that seems to work. But I think that adding some lever arms instead of going directly to the bolt would be more effecient. Yeah assuming the arms are light but strong and the projectiles are suffcently light. Having the solid limbs might be better for heavier projectiles.

One concern I might have with springs would be the noise. all that metal clanking around. Of course pnuematic springs could be quieter.

The question "is it a crossbow?" is pretty much the same as vertical bow enthusiasts ask of crossbows in general "is it a bow?".

If we break it down logically...if it doesn't have a "bow" for energy storage...it's not a bow of any kind. However I'm of two opinions here also, while we can talk all day about it not being a crossbow (just as bow guys do calling crossbows - CrossGuns ) we are just wasting time.

Think of it this way...if we don't welcome these designs...who will?

Crossbow got it's name after centuries and centuries of world wide use, the name evolved so many times I don't think that every version of the name is even on record. So.....I accepted these designs under one name -"Crossbow Perversions", so as to buy us time in developing names for each of these weapon variations without missing out on the fun of discussing and building these Perv's ...

Spoiler:

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Rolynd does have a point... not all of them will make it, but those that will ...will have a chance to become a weapon of it's own class and one day be granted a unique name just as crossbow once had.

I hope we can agree on postponing the weapon classification discussion until a better time >>> perhaps once several are built and we gain better understanding of these unique design "essences"...and the builders surprise us with the unique names they give their weapons...heh, it happens.

Sorry, I'm at work right now and don't have the time to chip in on the details, but I'll check in at a later time...had to stop by to ease the tension though.
We're all friends here and a topic regarding the weapon classification can be created separately where we can take a constructive route in regards to "calling things names" ...there is history to support us and engineering to inspire us, in the end it is an art, so let us be artistic.

Ivo

Ok, after shake hands with Rolynd ( ), I return to the vids.....and realize that they are vacuum "things"!!!! Searching for more details......so interesting!!!! I think that when they pull the string back, that´s the moment when the vacuum is generated......or the vacuum is permanently on??

juanecho, look at this video:



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!

Yeah!!!! Quite simple!!!!!! Looks like a good idea......no coil sprig, no metal clanking, just a little bit of maintenance with the cilinder and the grease....nothing more....
Is rare that the vacuum are produced just by getting out the air in the tube.....I mean, no vacuum machine is used, maybe the diameter of that tube has the key.......
What if the tube gets thinner, and the vacuum is produced by pulling the air outside by a vacuum machine??

Vacuum IS vacuum! You cannot get more force by more vacuum. The force comes from the outside air pressure which "pushes back". The force generated is proportional to the pressure on the surface area of the plug ,so a thinner tube is less powerful. Thats why the tube has to be that thick i think.
If we assume the tube has 10cm diameter, the surface area of the plug is 78,54cm². At sea level air pressure is 1013,25mbar. this equals 1,033kgf/cm². so the 10cm tube can generate a force of approx 81kg=162lbs. I dont think he will get a total vacuum in the tube and some will be lost in friction.So lets say 150-155lbs with a 10cm tube.The good thing is the air pushes all the way, all the same so the arrow is accelerated with almost the same force during the whole power stroke.

But, as I hear from a sound of this video, vacuum crossbow doesn't seem to have very hight arrow speed.. I think mean mass of the piston is eating all the energy. Anyway, idea is very interesting.

..here is one of my vision how to use coil spring to power a.. somekind ballista..

Power it has enough but not speed. This is i think because the action of the spring/vacuum is directly transferred- working with some kind of lever /moving arms brings more speed. My favourite is so far still mr van house`s, its proven to bring high arrow speeds.

Reg.....thinking in your sketch, I imaginne a "Cross Spring Bow" or something like that.....
Here is my Idea......very poor by now, but I think it works fine....
And, overall, it has a name.... jajajjaja!!!!!!

Juan,
I like your design, that really is very clever.

Just a couple of thoughts. I think the small idler wheels might be more useful if they are closer to the springs. This will keep the force on the spring in pure compression or tension rather than at an angle.

Thanks Bas!!!!
You are wright!! It was a speedy sketch, with the idea still fresh in my
mind.....it deserve a lot of study and plans to get well working....but
I think that it can work just fine!!!!
Once finish my "medieval style" crossbow, y want to get on a modern one....maybe this one will be ok...!!!!!
Maybe we all can get on it......like in the Devil´s ToothPick project...

Here is a picture of my ballista, powered by a pair of extension springs. While this layout works well as a ballista, it's probably too bulky for a handheld weapon.


Paul

It's funny to see the children covered their ears!!! Like it was a cannon!!!!!!!Beyond that is bulky, I think that is not thought to have much power, you are right with the size issue. Anyway, I can see that the kids love it! Good work Apol!!!!!!!!

Wow Paul that is one nice looking ballista. Glad to see your sharing your project with the kids.

that is kind of what I was thinking of for a spring powered crossbow but obviously modified to fit in smaller package.

That looks awesome Juancheco! How did it shoot?

The kids enjoyed the ballista a lot. Some of them were holding their ears because the ratchet on the winch was kind of loud.

I used two 38 pound springs that day, which threw a 21 inch crossbow bolt 100 yards. I can attached more springs for more power, but the field at the school was only 100 yards wide, so more power wasn't necessary.

Paul

Jake, that is Pau´s work!!!
Did you know any specs of those springs??? I´m just thinkin that I´can get any of those in a repair shop, here, n Argentina.....Are they automotive spares???

The springs are general purpose types sold by home-building centers. They're similar to the kind used on trampolines and are rated from 15 to 68 pounds.

Paul