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!

I try for simplicity since I basically have no real tools to work with, and love designing inswinger torsion based crossbows.
I use the extension spring to power both levers and use no cams only simple pulley wheels. On a Scorpyd frame with 20" draw, my very simple design returns 175fps with a 400grain arrow. This would translate into 200fps for a 350grain and even more for a 300 grain arrow. This is with 45-50lbs of pull.
The current levers are made from 5/8 diameter metal rods so will downsize to 3/8 for a good amount of weight savings that should effect the fps postively as well.
My goal is approach 275-300fps with 50-65lbs of pull with a 300 - 325 grain arrow. That way when I bump up the power two or three times I will have a spring powered bow to rival the high speed production bows.
The new Scorpyd build will be a 20 inches min. axle to axle. I go against the grain of most ballista builders in that I go for short levers instead of the longest possible. If I can get even 260 - 275 with 50lbs draw and 300 grain arrow I will be happy.
Hopefully be done over the next month.

Here is a little information on the inswinging design itself. My work is a little rad and often goes against the sensibilities of an educated mind. A lot of research has been put into naturally occurring instances of acceleration where no net force is applied.
My kind of thinking is referred to as "tin-foil cap thinking" by most folks, but thats where I am at.
http://warhammer1.wordpress.com/nu-inswinger-data-to-digest/
An instance is where I get the math guy to recalculate string to lever speed for shorter arms on a inswinger. Despite the educated mind of the math Dude, my tin foil cap thinking wins out.
I did not manage to get him to calculate my preferred lever length to axle to axle ratio which is even faster.
One day perhaps Nick will see the light, as the proof is in the pudding (reproducable results upon demand). While math calculations are fine, its hard to argue with results.
BTW, if I remember correctly House's design achieved over 400fps with a lightweight arrow under 300 grains, powered by a compression spring. I will continue to use the most ineffecient spring (slow memory) which I believe is the extension spring. When engineering is all done, I will upgrade to a faster spring like matched left and right double torsion springs, or perhaps a constant force spring or even like House, a compression spring.
Hoping to see 250-300fps from my next 50lbs pull build with a 300 grain arrow.

All very fascinating stuff on here and i think i may be able to help out with using coil spring's.Yes coil spring's are a very good way for alternative power system's.As you already know i have been working with these on powering crossbow type weaponry for a while now and i have found that to get any real speed you need a lot of spring energy and this is where it can get rather tricky as space is rather limited for hand held crossbow's.I employ 16 spring's in total to power the Beast crossbow,these spring's are from a high powered air rifle that have been modified with smaller spring's fitted inside them so i got 8 outer spring's and 8 inner spring's all compression spring's,now the total energy storage is 1.5 ton of spring power and this is geared to the mechanical limb's by pushrod's.To date i have hit 461.5 fps using a chroni chronograph and i hit this speed with a projectile that was 260 grn's so not much K.E but that is the speed i managed to hit using spring's.I could get more with a lighter projectile say 160 grn's.I have more work to do in order to get the full potential as there is a lot more potential with this concept but patience is required in enormouse amount's if you are going for the absolute highest speed's.So this system is well worth the time and the patience and energy as it can pay off but this is all new stuff when compared to what is out there in the shop's and so forth so we are all on our own doing this stuff with not a lot of history to go on.We are in unchartered territory here as we are going against the grain with this kind thing but oh what fun it all is i say let's keep on doing this and seeing what we can achieve.Tomorrow is another day and i got some more idea's to put to work to get more power and speed.As i cannot say too much on here as i am getting a brand new concept in crossbow design and innovation into production but hope i have give you some inspiring thought's if you are planning to have a go with the spring thing then you do not need any special tool's as all my work to date has been made using very basic tooling i have not even got a lathe,but i can get part's turned if i need them.All i am saying is please do not think you are limited if you have not got a designer work station,lol.Anyway have fun as it's all interesting stuff and you never know you may find a way that could be totally awesome and deliver to your expectation's.Of course not everyone is after very high speed's so you would not need as much energy as what i use as a very good performance say around 200/220 fps is still going some as even speed's like these are not easy to get they may not sound a lot but i think myself they are pretty ample for deer and largish sized game,i just like very high speed's for the rush i get when shooting it.

Hi phill
I am realy glade to have the occasion to tell you here how i admire your beautifull work in Talisman crossbow it seem like a dream of intelligence and art together . Is there any recoil when shooting and what is the sound of the arrow flying at 400 fps?
I am very interested by new horizon for crossbow and the use of spring ,rubber or compressed air for propulsion. Like Warhammer i am a fan of "in-swinging" it is a so simple and elegant way to reach hight speed whit relatively low power but long draw length . It haven't been used only in antiquity but officially like a grenade launcher by the French army during the trench war ww1. The power was provided by "torsion springs" . I had the occasion to work on two of those artefacts and i write a complete studied about it and all the other mechanic tool used as grenade launcher by the French. It is to heavy to put this work here . You'll find it in the new issues of the "archers antiquary journal" One of the two "arbalète d'Imphy " had the springs bow missing I make a complete reconstruction and shot video of the test. I plan to use smallers torsion springs for a smaller hand crossbow . This is the original



Sorry video to heavy!!!!! Close view of the new pair of
torsion spring i tell you man,what a djob to do!!! But the result work
pretty well it is very powerful, the grenade is 3#.
I am actualy working in a in-swinging hand crossbow using rubber for propulsion.

Thank's Jeep,i love making these thing's and shooting them well i managed to hit 501 fps today using the same lightweight arrow and it is phenomenal shooting a crossbow at this very high speed and there is virtually no recoil and very quiet as well as most crossbow's that shoot at high power and speed are sometime's quite loud but there is a few out there that are said to be quiet.I managed the extra 100 fps by fine tuning of the limb's i am incredibly happy at the result.The beauty of the design is that it is possible to top 600 fps with this bow system shooting a very light bullet bolt of around 100 grn's i will try this out with a 100 grn projectile tomorrow and see if 600 fps can be hit.Also i am going to uprate the spring power to hopefully attain higher speed's than this i am hoping to hit around 800 fps with a crossbow using a 100 grn crossbow bullet and that will be madness i know but what fun you will be able to have with a thing that can do this,it will be mindblowing to hit such velocitie's with anything other than gun powder especially a bow even a mechanical compound bow system will be crazy,lol.Anyway that grenade launcher is a wild looking piece of apparatus and also good luck with your project and if i can be of any help then please feel free

You may see this very crude and weird artefact!!!! But it is right in or discussion...It can be seen in the Canadian war museum , this is a spring crossbow build by Germans soldiers in WW2 prisoner camp in Canada. Fortunately the war and before they could use it!!!!!! They used materiel coming from a bed (spring) and pick-up around in there barack. Draw length and power unknown (ha!ha!)

heres my idea











Do we need all the sticky out arm bits? Looking at all the modern compound bows, they all run with pulleys and various limb placements to give the tension to the string, through the pulleys allowing a fairly long pull back with minimum arm movement...

Like the thread starter I thought of powering the string with a spring.. a tension spring replaces the tension applied by the limbs, and a system of pulleys gives me a mechanical advantage and lets me pull back the string.. One very strong spring can repace the two bow arms..usng a single spring will be the simplest method

So I made this proof of concept.. It is only a rough model, just to check my theories and to work out the pulley system before I start spending money and going full size, to emulate the pulleys I used key rings and let the string slide through them.. to emulate a tension spring Ive used bungee tube..

For the full size I was thinking vertical window sash pulleys to replace the big key rings at the front, then working back down the parts a fixed vertical pair .. linked by the string to a double pulley loose block which attaches to a large tension spring on the springs ring.

please point me in any better directions, mechanical wise, and tell me where I am going wrong

dave

Couldnt open the demo video, but seems there is potential for a lot of excessive weight to accelerate. That isnt so bad in itself, but any mass you put into motion must be brought to a stop.

Every pulley robs power through friction sometimes less is more. With catapults its a machine and will also seek to balance itself. Every change you make effects something somewhere else. You have to think a few steps ahead and get the result you want.

Dont try and skip steps and cheat yourself of the inventing/innovation process. Sounds silly but I've spent days staring at a failed design to figure out exactly why it did not produce the desired effects. Way more effective than merely saying "this should work".

Innovation is a results driven process. Dont avoid the failures - they are crucial to success. If you have the passion, failures dont mean a thing except you are one step closer to your goal.

I think the basic concept has potential. I like the setup as a quick test rig.

Like Warhammer mentioned you will loose some power to the pulleys so you want pulleys that are as light as possible and have the least friction.

I assume your going with a fixed limb design? All the mechanical advantage of turning power in to speed will be done with the pulley setup?

I presume you were talking about my simple trial..

I appreciate the problems mentioned, and I am looking for a lightweight 2 pulley rear block and other bits at the moment.. I am looking at nylon pulleys for less friction and weight.

Full size its only the rear pulley and coil spring that will move, and other pulleys just have to spin..

Is it much different to the arms on a bow moving ? those compound bows have pulleys attatched, and enough mass to accelerate and stop..and the strain on the pulleys must also be similar

As for limbs, I want to get rid of them and keep the spread at the front and the width of the body to the minimum that will enclose the works..



edit..I am having problems finding tension springs with a strong enough spec.. I am now looking at compression springs from motor cars.. to fit these I would have to have the floating pulley inside the spring with the action compressing the spring as it pulls

would these be any good as the base for a trial..

I'll drop in with a quick reply. Hopefully come back with something a little deeper later on.
I like the idea with the compression coil, I had a similar idea myself.

It looks good overall but one issue is how are you keeping the line on the pulleys? The few spring powered crossbows that I have seen have had a small T at the end so that the string could stop there like a more traditional crossbow. Not saying this couldn't work just tossing that out there.

My drawing is only the basic idea.. lots more needs thinking about.. on a small model to test the mechanics of the pulleys I fitted two pegs into the top rail before the pulleys as a stop for the string.. when fired the string hits the pegs and there is some tension remaining which keeps the string nicely in place.. The pegs can be replaced with a front sight loop over and this can be padded to take any impact from the string. side plates looping over the pulleys would likely be a good idea to prevent any bounce out..

The main worrys at the moment are building a decent trigger/nut mechanism,....How strong will the pulleys have to be...How much force will be lost to friction...The stronger the bow the higher the forces and the more friction is generated at the pulleys where the string changes direction

Another worry... with a 3:1 mechanical advantage to get 100 lbs at the full pull I have to look for 300 at the spring.. thats a lot of strain on the mechanics and the rifle itself..

I am more of a small scale modeler..rc planes..not an engineer or mechanic, so making something like this is something well out of my comfort zone

hey guys,

i did some math on it, to find out how fast a arrow would fly by useing a coil-spring. In my opinion is the biggest problem with a coilspring the big mass that must be accelerated too. So I think it is necessary to use a pulley or something like that to accelerate the arrow more than the spring.

In my calculation I used a pulley with a ratio of 3. As spring could be used a spring of door of a garage. I do not know the exact mass of it but I calculated with 1kg per spring and a spring constant of 3.5 N/mm. This kind of spring can be extend to something like 400mm. So if you use 2 springs and 400mm you store 560J of potential energy. The problem is, that you can't get this all in to kinetic energy of the arrow because of the mass of the spring.

For the math i only used the half of the mass of each spring because one side of it is fixed at the shaft. In this case we have a accelarated mass of two spring of 1kg and an arrow weight+ cable of probably 70g. The distance of the accelaration of the arrow will be 3*400mm=1,2m (I think a crossbow should not be longer).

In this set-up, without using friction in the model the arrow will have a speed 78.55m/s which is 257.7 feet per second. It stored a energy of 215J. The difference to the potential energy will hit you.


If I have more time i will make some parameter variations to find a good setup. And I think I will include damping in to the model.


I'm sorry about my englisch,
grüße H

Hi guys,
found a photo of the mentioned Burri spring crossbow on the french forum:





does anyone have some specs on this crossbow? I wonder what kind of fps this produces?

I don't know how to do it. but I bet some of you engineer types could produce a weapon that would use a bolt with a highly positive charged nock at the back of the bolt that would be used in a stock with a high powered battery pack with a type of electro-magnet that when a little red button (in place of the trigger) is pushed, would produce a very powerful positive charge that would spit that bolt out at a gazillion fps. Just a thought ................ just think probably no recoil, nosprings, no rubber bands, no prods,no spanners, no trigger assemblies to jack with ...........where is all this goin' ?

Just a thought..............that is when I get in the most trouble

Chaz

Haha! That woudl be a rail-gun mate. That's a whole other forum...I mean a completely different site to be exact.

As much as we push it with trying to accept as many "perversion of tradition", we still try to stick with crossbow being a crossbow and only needing a man to be operated and not a charging station etc.

Sorry to kill your high.

Ivo

Didn't kill my high. You only helped make my piont ! Yea CROSSBOW !

Chaz

just think probably no recoil, nosprings, no rubber bands, no prods,no spanners, no trigger assemblies to jack with...

No mechanical energy storage material (bow, spring, rubber) would make this wep no longer classified as a crossbow, that's what we're all about here actually - taking a material that stores energy when stressed, stressing it and retaining the stored energy via trigger mechanism, and transferring that energy upon release to the arrow...preferably with a bowstring tied into the energy transfer part (kinda essential to the *bow* part of the deal ). It's an art of sort, and the game we play ~ knowing the limitation and trying to get around them by designing a better "crossbow".

Ivo

Tip o' the hat to ya !

Love the CROSSBOW !

Thanks

Chaz

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.