Truly automatic crossbow, design 1

Hi there,

As promised my build for a truly automatic crossbow. No electric motor or gimmick like that.

The basic principle is the same a for a rubber band powered boat: https://www.youtube.com/watch?v=pM5RXCWxVs8&t=199 for cycling through shots. Loading happens this way: Load one rubber, turn once, add another rubber, turn again and so on. When shooting this is what happens: 3 rubbers.
3 rubbers make it turn, 1 is released, 2 make it turn, 1 is released, 1 makes it turn, 1 is released, done.



Here is before the rotation has happened (trigger has been pressed).



Here same thing, just with the shot happened and the next turn blocked by the back part of the trigger.

I will post the (working) miniature I made some time soon. Keep in mind that I did it with rubbers but it would work with springs or a regular crossbow as well.

Please tell me if anything looks unclear. Would love to have your comments.

Yup - I want to see a miniature, and then a table top model with say a 50#, 18" 'replacement' prod.

Ok, so I took it out of "storage". Unfortunately it is broken now since some of the parts were held together by rubber bands... which dried up.

I put quite some sweat and a few drops of blood in this built. Please keep in mind that I had basically no power tool. Just a small handsaw, a hammer and a file.

Here a few picture of the small scale model:





With the first string loaded.




"Main" trigger blocking the rotationnal trigger string one:





After one rotation, string two loaded:



My attempt at a full scale model:



Which failed...

Sorry for the double post but could you give me a hand?

How could I go to a full scale model without the desastrous result of wood breaking, actually implementing the mechanism in a less crude way while keeping in mind I can't work with power tools...

Finally, I used Thera band for the second band like JS uses, but it proved to be less reliable than the rubber string. Any idea on that? Or should I just use a spring and a regular string?

Hey there!

I truly do not get what this design is about!

At this point I assume you try to preload 3 shots of rubber band on a rotating block.

First of all, this has nothing to do with being automatic.
If it were automatic it would reload and cock itself without adding manual work into the direct process.


All you do is to preload multiple shots, which is by the way with your kind of system when using a crossbow very weak to hold the forces.

No offense meant (I think somebody has to speak it out), but this looks like the overenthusiastic idea of fast shooting things as seen on screen.

You do not have power tools?
Why don´t you start building some simple wooden crossbow without using ducktape and develop some working skill and experience with kinetic devices?

If you dont know how to choose the right material or work with it without breaking you should consider beginning at earlier stages.

Get some decent timber, some simple woodworking equipment and invest a couple of hours reading here or elsewhere online.





Rizzar

Hi!

You actually got exactly what this design is about.

To me "automatic" is shooting multiple times without having to manually reload each time. You can see this in a gun for example: the projectile is loaded automatically and as well as the energy (gunpowder).
Just press the trigger and shoot until the magazine is empty (all rather quickly), wouldn't you agree?

So yes you are right, I am trying to preload a number of shots (3,4 or maybe 7). Why do you think this is not an interesting idea?

Could you explicit in what way it is weak to hold forces when loading multiple shots? (besides the rotation overstretching the rubber)


What I have done so far is read through a lot of topics of this forum, but it's taking time!

I do not intend to argue about the definition of automatic/semi auto/double or single action.
So I get directly to your design.


I think your rubbers lay on the hooks around your axle.
Each time you turn the axle a rubber is released and shoots itself or a powered projectile.

As you recognised the rubbers are overstretched even in your model.


To move an object a designated distance a certain amount of work is required.

So, to move a bolt/arrow/bullet you are going to need a lot of force. (consider a 40lbs bow is quite ok for target purpose, while a 40lbs crossbow is a joke inflicted due to lower efficiency factor)

Normally a crossbow string does not stretch (with a detailed look it does, but this can be neglected right now).

A crossbow has always some kind of bow attached to a stock/tiller or something (usually reminding of a cross).

A speargun is technically not a crossbow.


There are so many points/concerns I do not really know where to start.

If you consider to fulfil the above mentioned plan:
You need a locking mechanism that works with your device that can hold the forces even when in full rotation to stop fire.
You need to work out the tension on the string when rotated around that axle in full draw which most likely damages or destroys it.
You need bows that withstands overdraw, or accept that they work under their specifications or optimise draw in order of shooting.
Reloading seems very difficult dangerous once a single string is "loaded".
Consider the more strings are attached the higher the load on the lock will be.



These difficulties and the fact that you do not even have the opportunity to use power tools let me doubt of project practicability.

I do not want to say that it is impossible do design an automatic crossbow, but I think it would be very difficult to see some real results.

A couple of days ago a friend of mine sent me this link kickstarter rubber band gatling

Funny toy.


Rizzar

Rizzar has pretty much hit all the important points here, so I haven't much to add. I will say that an 'automatic' rather than a repeating crossbow will almost certainly require a separate energy-source for each projectile you want to launch, unless you can arrange one very powerful source and tap it for individual shots.  Compressed gas comes to mind... you might have an air-tank of some sort... or you could have an entire bank of separate prods or big elastic bands. 
   As I see it, an 'automatic' crossbow that launches with compressed air isn't a crossbow at all.  I have seen one semi-automatic that spanned with compressed air... though I rather wondered why not eliminate the prod and launch the bolts with air... it would have been more reliable.
    A gang of crossbow prods becomes really heavy and unwieldy... worse with each added element.  A gang of elastic bands might be your best bet... though what you've got then is a spear-thrower, not a crossbow.  For elastic work, I recommand looking at Jorg's Slingshot Channel.  He seems to be the reigning authority in that field, and indeed he has reported some pretty impressive performance out of properly prepared elastic. 
And that's my two-bits worth:  Geezer.

First of all, thanks for all your input!

My bad if I misused the word "cross bow. As I saw the "mech-crossbow" and the "sling-bow" sections, I got the wrong idea.

For me, the most important part was to use materials that could have been found in medieval times. I admit, rubber is not part of it but I had planed to replace it with a regular string attached to springs for added "authenticity" and more serious power.

Let me answer your different points.

Your are absolutely right, each time the axle turns, one rubber is released and propels a bolt that is in the quite ugly storage compartiment above. You start out with several bolt in the compartiment and several loaded strings and go from there. I intended to use springs at a later stage to increase the stored power.

I have already worked out the locking mechanism as it was an important part of the design. It's what I call the "main trigger" blocking the rotational trigger and you see in one of the pictures.



If I use a solid string and a spring, will there be the same problem of overdraw? Then again, all depends on the quality of your rubber. When I first built it, I was able to make 3 full turns with the rubbers without breaking the rubber, but again, I can't predict when it will...

You could consider reloading dangerous but each turn, the "Main trigger" blocks the rotating trigger, so unless you push it by mistake, except the tension built up. I agree with your comment on the lock but don't you think that this system of perpendicular pressure will hold up better?

To compensate for the lack of powertool, I though of 3D printing some parts but considering the load it might be plain dangerous.

Incredible! The project was funded so quickly! Quite similar to Jorg's design though... (https://www.youtube.com/watch?v=qxtHtPK2MH8)

Yes I have seen that design with compressed air too. I though up a similar design but using a big torsion spring to store the energy however would that be able to store enough? But I'm getting lost, that was a whole different idea.

Last comment, I think you are right with one major flaw of any design more complicated than a classic crossbow: the weight. But then again, I was really interested in the mechanical working of my idea.

I have watched quite some videos of Jorg's channel, I saw he posted also a bit around here. Indeed he knows his stuff!

I really appreciated all of your inputs and I know it's rather a big challenge, hope I'll have the occasion to work on it some time soon.