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A set of pulleys assembled in a manner so that they rotate
independently on the same axle form a block. This is known
as a double sheave pulley (Figure 169). A block and tackle
is assembled so one block is attached to a becket (fixed
anchor point) eye and the other is attached to the moving load.
The mechanical advantage of the block and tackle is equal to
the number of rope strands that support the moving block.

Pulley efficiency is reduced as a result of friction loss and

other factors, such as bending and unbending of the rope. The
measure of pulley efficiency is calculated by the output force
coming out of a pulley over the input force going into a pulley,
which is expressed as a percent. As an example, if a 95N
force is required on one side of a pulley to hold a 100N load,
then the efficiency of the pulley is calculated to be 95% Figure 170- A pulley with
(95/100). 90-95% is the typical efficiency of a rescue pulley 95% efficiency is depicted in
(Figure 170). this example. Original image
© Rock Exotica.

Factors influencing the efficiency of a pulley, as well as the entire pulley system include;
• Pulley sheave (wheel which rope runs on) size, since a larger diameter sheave
gives a larger moment to overcome bearing friction and the radius of rope
bending as well as unbending is less, therefore more energy is transferred

through.
• Pulleys with self-lubricating bushings are efficient; however sealed ball bearings
are more efficient and require very little maintenance.
• The Angle of the load line and haul line, since lines kept at 180° result in
improved pulley system efficiency.
• Threading pulleys to ensure ropes don't rub on each other, or result in twisting,
which decreases efficiency. Twisting can be lessened with the use of pulleys
with swivel connections.

Efficiency of a pulley system is also evaluated in terms of what will provide the best
output for the operation. If a 5:1 pulley system will comfortably move a load, then using

a 12:1 pulley system would be less efficient, since it requires more time and movement
of more rope through a system to move the load the same distance.

The following terminology is used to describe the mechanical advantage of a pulley
system:

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