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    Can we use mechanical pulleys to increase load tension?

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    whiteraven
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    Can we use mechanical pulleys to increase load tension?

    Post by whiteraven on Thu May 31, 2012 4:43 pm




    I've been doing a little research on pulley systems and how to use one to increase the power of a mechanical crossbow. I found this little clip below and it got me wondering, but my mind just can't fully fathom the pulley systems without having a handleful of pulleys and some rope to try them all out.
    Could this work to improve the power of a crossbow?


    Fig. 5. Spanish burton.
    So now try your skills on this pulley system, Fig. 5, called the
    "Spanish burton". It is shown in Leonardo da Vinci's notebooks. What's
    the number of rope segments 'supporting' the load? That's hard to count,
    for only two forces from one rope act directly on the load. That "count
    the ropes" rule is useless for anything other than block and tackle
    systems. But the analysis of this system (or any pulley system) is easy.
    First assume the system is in equilibrium (either at rest, or with
    everything moving at constant speeds). Start at the right, calling the
    tension in the right rope F. This rope passes over the pulley second
    from the right, exerting an upward force 2F on that pulley. So, to
    maintain equilibrium, the rope acting downward on that pulley must have
    tension 2F. At the next pulley the downward force is 4F, and at the next
    it is 8F, and by the time we get to the last pulley, the downward force
    must be 32F = L. So the mechanical advantage is 32, and there are
    nowhere near that many ropes or even rope segments in the system. (There
    are 5 ropes and 10 segments.)
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    DARIVS ARCHITECTVS
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    Re: Can we use mechanical pulleys to increase load tension?

    Post by DARIVS ARCHITECTVS on Thu May 31, 2012 5:27 pm

    Pulleys trade distance of motion for tension. They increase distance while decreasing the force of tension. Not usually a thing you want to do in a crossbow. You want to increase tension AND increase distance (draw length). Modern crossbows use cams (NOT PULLEYS) to alter the bow string tension at different points along the throwing distance as the bolt is thrown in order to make the transfer of pressure driving the bolt more efficient than a simple leaf spring prod is capable of. The pressure curve behind the bolt is thus modified for maximum transfer of energy to the bolt.
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    Basilisk120
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    Re: Can we use mechanical pulleys to increase load tension?

    Post by Basilisk120 on Thu May 31, 2012 5:42 pm

    Might work but would be complicated and have lots of pulleys hanging out in mid air.

    Some randomw thoughts on this pulley system in a crossbow



    This would make for a long draw lenght. The distance F travels is 32 x the distance L is lifted.



    At least with the way it is setup in the picture the prod would be reversed from the a normale crossbow. This may be easily fixed with one more pulley or a slight layout change.



    What is likely to be a big issue is that each pully adds friction and inertia to the system. For a fast moving dynamic system like a crossbow during release this could be an issue.



    I would say that this system would be most useful if you have really strong prod that has a really short throw.



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    whiteraven
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    Re: Can we use mechanical pulleys to increase load tension?

    Post by whiteraven on Thu May 31, 2012 5:43 pm

    But could employing the use of pulleys in the loading device allow one to draw a heavier tensioned string than your standard cranking loader?
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    Re: Can we use mechanical pulleys to increase load tension?

    Post by DARIVS ARCHITECTVS on Thu May 31, 2012 8:29 pm

    whiteraven wrote:But could employing the use of pulleys in the loading device allow one to draw a heavier tensioned string than your standard cranking loader?

    Yes, and such a system has been used extensively for large siege crossbows. However, the time it takes to draw the cord in the pulleys in order to span the bow makes the reloading time rather long. As such, it was used primarily for crossbows used in defense. A single pulley for loading was often used for lighter crossbows in the field.

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