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Crossbows - Everything about Building, Modding, and Using your Crossbow Gear

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» Low Draw Weight Build
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» Trigger testing Rig/Jig?
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» Han Dynasty Chinese Crossbow
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» Drawing of Crossbow
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» Medieval crossbow finished
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» Finish wooden stock
by stuckinthemud1 Sat Dec 25, 2021 9:06 am


3 posters

    Pulley/Cam design

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    ora8i
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    Pulley/Cam design Empty Pulley/Cam design

    Post by ora8i Sun Feb 21, 2016 2:06 pm

    A quick question.

    What should the dimension be of the groove in the pulley/cam that the string runs in be?  

    The groove around the outer edge of the pulley were the string passes around the pulley.



    Many Thanks                   ora
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    kenh
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    Pulley/Cam design Empty Re: Pulley/Cam design

    Post by kenh Sun Feb 21, 2016 3:09 pm

    Whoa!   Not NEARLY enough information there!!!! 

    Dimensions of prod
    String path
    Desired Draw Weight
    Weight Weigh sans pulleys
    Power stroke

    and lots more I can't hnk of because I don't care for pulley bows.
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    Pulley/Cam design Empty Re: Pulley/Cam design

    Post by ora8i Sun Feb 21, 2016 3:55 pm

    I'm nowhere near that yet,  I'm thinking of a lowish draw weight bow under 100 lb possibly about 80 and possibly reverse acting.

    I've never built a compound bow so I'm going to keep it simple with round pulleys at each end of the prod rather than true cams. 

    For a prod I would either buy a Chinese fibreglass type or make up a loose laminate (stacked slats) from 3mm fibreglass strips. If I use strips I can change the number/length used to get the draw I want.

    So as you see I'm unsure of most or all of the details but I do need some idea of the groove size and how the string or cable is terminated and attached  to the pulley.

    I expect to use a block of two pulleys locked together one larger than the other to provide mechanical advantage.  The other smaller would carry the cable that compresses the lims. This seems to be the most basic compound setup.

    Or just use the pulleys just as they would be in a lifting block with the pulleys not attached to each other but able to rotate independently, effectively using then as a block and tackle to reduce the pull weight and lengthen the stroke. 

    Slightly off subject what is the minimum radius a string should be bent around for example 3 x String Diameter, 4 x String Diameter ?????? In effect the minimum diameter of pulley I could reasonably use.

    Atb.          Ora
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    Pulley/Cam design Empty Re: Pulley/Cam design

    Post by ora8i Fri Feb 26, 2016 2:00 am

    How about just this then? -


    Slightly off subject what is the minimum radius a string should be bent around for example 3 x String Diameter, 4 x String Diameter ?????? In effect the minimum diameter of pulley I could reasonably use.




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    Pulley/Cam design Empty Re: Pulley/Cam design

    Post by kenh Fri Feb 26, 2016 1:42 pm

    I think you're looking t it backwards.  I think the larger the cams (withing reason) the more efficient the system.  Most prod cams that I've seen are in the 2" to 3" diameter range.
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    Pulley/Cam design Empty Re: Pulley/Cam design

    Post by ora8i Sat Feb 27, 2016 8:08 am

    2" it will be then, its simple pulleys rather than cams.


    Thanks     ora
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    Pulley/Cam design Empty Re: Pulley/Cam design

    Post by ora8i Mon Mar 14, 2016 2:28 pm

    I'm still working out the layout for my postulant build.


    I'd like to do reverse draw but that implies some sort of compound or pulley system.
    I no longer have machining facilities so I was thinking of using a simple pulleys but how do I route the “ back string”?
     
    The one coming off the front of the pulley (front of the bow furthest from the trigger) will be drawn back to the lock and propel the bolt, that's no problem.
    The strings coming off the back of the pulleys go to the limbs on the opposite side of the bow.  This is the most basic of the compound systems.


    The pulleys need to be flat, that is in the same plane as the top of the arrow track, so how do I run the back string without massive friction?. It has to cross through the stock to get to the other side.


    The only alternative I can think off is to lock two pulleys one on top of the other, just like most commercial compound bows, that would put the back string lower than the track so the friction will be reduced and the string miss the fletching.  But it's doubled the weight of the moving parts!


    What would happen it the “back strings” were fixed to the stock rather than run through it to the opposite limb?




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    Last edited by ora8i on Tue Mar 15, 2016 1:03 am; edited 1 time in total
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    c sitas
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    Pulley/Cam design Empty Re: Pulley/Cam design

    Post by c sitas Mon Mar 14, 2016 7:05 pm

    What's with this thing timing out just when your ready to post?

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    Pulley/Cam design Empty Re: Pulley/Cam design

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