3º Compound A compound
pulley is a combination fixed and movable pulley system.
The simplest theory of operation for a pulley system assumes
that the pulleys and ropes are weightless, and that there is no
energy loss due to friction. It is also assumed that the ropes
do not stretch.
The product of the weight lifted times the distance it is
moved ( Wxd) is equal to the product of the lifting force times the distance the lifting line is
moved. The weight lifted divided by the lifting force is defined
as the advantage of the pulley
system, that is:
advantage = weight
lifted/lifting force
1º A basic equation for a
pulley: In equilibrium,
the
force F on the pulley axle is
equal and opposite to the sum of the
tensions in each line leaving the
pulley, and these tensions are equal.
Diagram 2 - A simple pulley system -
a single movable pulley lifting a unit
weight. The tension in each line is half
the unit weight and an advantage
of 2.
Diagram 3 - Another simple pulley
system similar to diagram 2, but in
which the lifting force is redirected
downward.
A practical compound pulley
corresponding to diagram 2a.
Calculate the
advantage of this pulley system
Diagram 4 - A simple compound pulley
system - a movable pulley and a fixed
pulley lifting a unit weight. The
tension in each line is one third the
unit weight, yielding an advantage of 3.
Diagram 5 - - a movable pulley and a
fixed pulley lifting a unit weight, with
an additional pulley redirecting the
lifting force downward. The tension in
each line is one third the unit weight,
yielding an advantage of 3.
Diagram 6 - A more complicated
compound pulley system. The tension in
each line is one quarter of the unit
weight. It has an advantage of 4. An
additional pulley redirecting the
lifting force has been added.
