> >     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
>
>
> Chris,
> Thanks for the nice explanation.  It is a weird configuration to
visualize.
>
> I think because of symetry, there should actually be zero residual
> moment.  If you didn't mark the ends of the cylinders, you wouldn't be
able
> to tell how much it had rotated.  The flexures would all continue to have
> the same shape, whatever it was.  In effect, as one flexure winds up,
> another is unwinding in exactly the opposite way.  Think everything
cancels
> out.

The above comment is correct.  If the flexures are not bent prior to
wrapping them around a cylinder, there is no energy required to move the
hinge.  As
one flexure is bent, the other is unbent in exactly an equal amount.

The secret to making the stiffness high is to make the flexures as thin as
possible.  As stated in the first message, there is a slight "s" shape
between
the cylinders.  As the force on the hinge increases, these "s" shapes
straighten
out and get stiffer.

I have been using this type of hinge on both verticals and horizontals for a
couple of years, and they perform very well.  My preferred configuration is
1/2 inch cylinders with .002 high strength material for the bands.

George Harris

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