Hi all,
        If the pulleys are fixed to a common shaft, they have to be the same
size.  Otherwise, any will work.

bill


Jinx wrote:
>
> (made this EE rather than OT in case there are spin-offs
> for the Bot Boys)
>
> I'm working backwards from a load to get a ballpark figure
> for the size of motor required to lift it with a 3-pulley block
> and tackle (which I'm making)
>
> Everything seemd OK until I read this paragraph from a
> technical book
>
> "Ignoring friction, for a simple pulley system with equal sized
> pulleys, the mechanical advantage is equal to the number of
> pulleys. The ratio of the distance moved by the effort to that
> moved by the load (the velocity ratio) will also equal the number
> of pulleys, and is not affected by friction or the weight of the
> equipment. In practice, these factors reduce the mechanical
> advantage, and the efficiency of the system is calculated by
> dividing the mechanical advantage by the velocity ratio"
>
> Pretty straight-forward. To raise 10kg weight 1m with 3 pulleys
> requires a 3.33kg force and 3m of rope. I plan to use bearings
> rather than plain shafts so that should cut the friction down. What
> I hadn't expected was the statement "equal sized pulleys". I've
> got 2 large (200mm) and 1 small (110mm) pulleys. Trying to
> think what difference the size would make. Any Ideas ? Looked
> around a little bit but can't find a mention of this. Plenty of very
> good sites about what I already knew though
>
> --
> http://www.piclist.com hint: The PICList is archived three different
> ways.  See http://www.piclist.com/#archives for details.

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.