You should NOT use a pulley block in this manner with a common shaft.
 If you intend to use a block to amplify the force, then the pulley
closest to the object to be lifted will move very little relative to the
pulley closest to the pulling force.

-Adam

Russell McMahon wrote:

>>>  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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>
>
>
>

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