> >   If the pulleys are fixed to a common shaft, they have to be the
> >   same
> >   size.  Otherwise, any will work.
>
>     The pulleys *must* be able to rotate independently anyway.  If they
are all
>     the same diameter than the rotation rates have to be different.


In PRACTICE I'm sure it is very wise to allow each pulley to rotate
independently as any slight differences in pulleys or rope will lead to rope
slip or tension changes etc which could cause losses that markedly reduce
the efficiency.

My mental picture (which may be wrong) suggests that in THEORY, with equal
size pulleys, each group of pulleys can be fixed to a common axle.
This is because the rope must maintain a constant tension and velocity
throughout and this would occur in these (theoretical) circumstances.
With care one could perhaps use a single rotating axle with spacer disks
between ropes (Teflon?) although even resisting the side forces which occur
along the axle due to the slight horizontal offset in each rope could be
enough to introduce unacceptable losses. The fact that pulley blocks that I
have seen all seem to use multiple independent pulleys suggests that this is
the result of practical experience.



        Russell

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