Twice as long - twice as good

Discussion in 'New project ideas' started by RegulusRubber, Oct 19, 2013.

  1. RegulusRubber

    RegulusRubber Junior Member

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    A few days ago I´ve seen the video "Twice as long - twice as good" and I had an idea to improve the design.
    Last time I´m thinking about making a crossbow using Wheels instead of a bow and when I saw the video I thought why not also use wheels to extend the rubber drawing-length for the winch-operated slinghot.

    Here is my freehand drawing. I hope it´s useful.

    Greetings from Gevelsberg
     

    Attached Files:

    Last edited: Oct 19, 2013
  2. JoergS

    JoergS Administrator

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    Such designs have been discussed here many times, and I have built several pulley equipped models in the past. I no longer follow that line. Why?

    - Performance goes up a little, but not much due to the additional friction and mass
    - Rubber breaks very early as stretched rubber is extremeley vulnerable against abrasion
    - A slightly longer shaft delivers the same performance increase, but has none of the disadvantages

    But by all means, just build it! The fact that I found these enhancements not effective does not mean that there is no solution. Maybe I just haven't found it.
     

  3. Antraxx

    Antraxx #7

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    Twice as long - twice as good

    Thats what she s... oh nevermind :p
     
  4. MountainStorm

    MountainStorm Termite

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    Watching slow motion shot videos by Dev and JS and others showing what happens when the rubber retracts is mandatory education :)

    Close inspection of bands or tubes that have been used hard shows a "banding" pattern from the stresses peculiar to stretched rubber contracting against resistance.

    That is why I think pulley against rubber designs are not going to perform as expected.

    Rope through pulley action is ideal. Rubber to rope transition is probably a work around to avoid the stresses rubber on a pulley experiences in its "sine wave" form.
     
  5. Brazilviking

    Brazilviking Thread Hijacker

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    It's not the size of the wand that counts, Antraxx! It is the magic that it does!:D
     
  6. AttilaTheFun

    AttilaTheFun New Member

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    BV I'm sorry, I dont want to make you feel silly but we're talking about a crossbow not some kind of wand... don't thank me I'm happy to help! :D

    Anyways: Interesting design, build it! It'd be nice to see it in action!
     
  7. xXdoomXx

    xXdoomXx Junior threadjacker ag. J

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    You guys are disgusting :/ :p I would have done that to if antraxx wouldn't steal my line..... Hrmpf

    I'm planing to make one of these or a crossbow

    J
     
  8. ruthiexxxx

    ruthiexxxx ruthiexxxx

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    That is such helpful information. I had been thinking that I ought to try some sort of pulley system and am glad to have been spared the wasted effort.
     
  9. coyote-1

    coyote-1 Member

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    Haven't explored anything like this yet. But it occurs to me that a combo of mechanical stretch (springs or small fiberglass limbs) on one side of the pulley might be a useful assist. (Or might not due to the aforementioned frictions and other mechanical energy losses.) Anyway, you would have a heavy spring (or limb, just like an xbow). It would be connected to a cable, and that cable would connect to the rubber. The stretch of the whole system would be set up so that ONLY the cable comes in contact with the pulley during cocking and release.

    Perhaps with a mechanical assist for cocking, such as with the folding xbow(rifle) slingshot idea, it could generate some extra speed.


    EDIT: In thinking about it, the springs would need to have higher draw weight than the rubber in order to make any difference; the lowest draw component of the system would limit the entire system to that draw. So for an additional component (such as springs or limbs) to assist the rubber, that component would have to have its own separate release that would have to be nearly simultaneous** with the release of the rubber. That would also necessitate separate cocking of each tension component.


    **If not perfectly simultaneous, the springs/limbs would have to release first. That would give momentary higher tension to the rubber, which would then release.
     
    Last edited: Nov 6, 2013