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

Latest topics

» Beware of Alchem
by Archer46176 Today at 3:33 pm

» How to measure and cut leather for prod
by Celtic Archer Yesterday at 5:56 pm

» Crossbow with built in lever?
by John Edgerton Thu Aug 17, 2017 12:38 pm

» Of Bows and Torsion Engines
by JacobL Mon Aug 14, 2017 12:03 pm

» Early Lever and pin Crossbow
by topfmine Mon Aug 14, 2017 8:56 am

» Crusader crossbow
by topfmine Mon Aug 14, 2017 8:53 am

» First Efforts
by rickraedeke Tue Aug 08, 2017 5:25 am

» aluminum prod
by Phil Abrahams Fri Aug 04, 2017 8:38 am

» First whitetail buck taken with medieval crossbow
by Phil Abrahams Fri Aug 04, 2017 3:38 am

» Airsoft Crossbow
by JacobL Wed Aug 02, 2017 9:11 pm

» Questions about steel prod specs
by JacobL Mon Jul 31, 2017 8:25 pm

» Faking a medieval composite crossbow prod.
by JacobL Mon Jul 31, 2017 8:13 pm

» Airsoft Bow
by JacobL Sun Jul 30, 2017 1:56 pm

» Hello All!
by JacobL Sun Jul 30, 2017 12:54 pm

» Padre Island Bow
by Geezer Wed Jul 26, 2017 12:14 pm

» Blocked pictures by Photobucket
by Crossbowmen Mon Jul 24, 2017 1:33 pm

» Geezer website?
by Geezer Fri Jul 21, 2017 3:01 pm

» Could I get advise on order of operations?
by Bs1110101 Wed Jul 19, 2017 4:02 am

» Renaissance Sporting crossbow
by chaz Mon Jul 10, 2017 12:22 am

» Welcome! Welcome! Welcome!
by Agniznag Sat Jul 08, 2017 12:26 pm

» The Arbalist Guild - Around the World
by Agniznag Sat Jul 08, 2017 11:56 am

» opinions please
by Geezer Thu Jul 06, 2017 10:19 pm

» Hello all!
by Tarzan65 Thu Jul 06, 2017 10:54 am

» Yet another one
by Daniel Levesque Sun Jul 02, 2017 9:37 am

» Renaissance peep sight
by kenh Fri Jun 30, 2017 4:48 am


    Pulley/Cam design

    Share

    ora8i
    Fresh Blood

    Doesn't mean
    I'm new to crossbows


    Fresh Blood Doesn't meanI'm new to crossbows

    Posts : 26
    Join date : 2011-09-23

    Pulley/Cam design

    Post by ora8i on 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
    avatar
    kenh
    Crossbow Junkie

    I live here!


    Crossbow JunkieI live here!

    Posts : 727
    Join date : 2012-08-03
    Age : 69
    Location : Living Aboard a Sailboat in Fort Myers, FL

    Re: Pulley/Cam design

    Post by kenh on 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.

    ora8i
    Fresh Blood

    Doesn't mean
    I'm new to crossbows


    Fresh Blood Doesn't meanI'm new to crossbows

    Posts : 26
    Join date : 2011-09-23

    Re: Pulley/Cam design

    Post by ora8i on 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

    ora8i
    Fresh Blood

    Doesn't mean
    I'm new to crossbows


    Fresh Blood Doesn't meanI'm new to crossbows

    Posts : 26
    Join date : 2011-09-23

    Re: Pulley/Cam design

    Post by ora8i on 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.




    ora
    avatar
    kenh
    Crossbow Junkie

    I live here!


    Crossbow JunkieI live here!

    Posts : 727
    Join date : 2012-08-03
    Age : 69
    Location : Living Aboard a Sailboat in Fort Myers, FL

    Re: Pulley/Cam design

    Post by kenh on 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.

    ora8i
    Fresh Blood

    Doesn't mean
    I'm new to crossbows


    Fresh Blood Doesn't meanI'm new to crossbows

    Posts : 26
    Join date : 2011-09-23

    Re: Pulley/Cam design

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

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


    Thanks     ora

    ora8i
    Fresh Blood

    Doesn't mean
    I'm new to crossbows


    Fresh Blood Doesn't meanI'm new to crossbows

    Posts : 26
    Join date : 2011-09-23

    Re: Pulley/Cam design

    Post by ora8i on 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?




    Atb.            Aamcle

    http://illiweb.com/fa/pbucket.gif


    Last edited by ora8i on Tue Mar 15, 2016 1:03 am; edited 1 time in total

    c sitas
    Crossbow Junkie

    I live here!


    Crossbow JunkieI live here!

    Posts : 427
    Join date : 2012-11-24

    Re: Pulley/Cam design

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

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

    Sponsored content

    Re: Pulley/Cam design

    Post by Sponsored content


      Current date/time is Mon Aug 21, 2017 9:28 pm