Dear Brett Nordgren,

    The 'zero force' hinge analysis given in zerohng2.pdf as referred to the 
rolling cylinders, may be easier to understand if you think of it in a 
different manner. Consider two equal diameter pulleys with fixed parallel 
axes of rotation having a crossed belt running around them, giving 
contra-rotation. When you turn one pulley, the other moves through the same 
angle, but in the opposite sense. 

    If you now fix one pulley and allow the axis of the other to rotate 
around it, the moving pulley rotates twice while going once around the fixed 
pulley. The position of the crossover of the belt moves in a circle half way 
between the pulleys. This analysis can be extended to circles of different 
diameter with different 'belt' lengths.

    The slight 'catch' about this 'zero force' design is that unless the belt 
is 'completely flexible', bending the belt over the pulleys involves a force 
which tends to pull the pulleys together - the natural shape of the 'straight 
bit' forming the hinge is actually 'S' shaped.

    Regards,

    Chris Chapman
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