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Chapter 14

MECHANICAL ADVANTAGE

PULLEY SYSTEMS

It is believed that in 1500 BC people in Mesopotamia used
rope pulleys for hoisting water. Archimedes of Syracuse,
Greek mathematician, invented the first compound pulley
systems 287 BC - 212 BC. According to Greek historian,
Plutarch, Archimedes moved an entire warship, laden with
78
men, using compound pulleys and his own strength.

Pulley systems are actually simple machines, which serve
as "force multipliers" (Figure 168). A machine is a device
designed to change the direction or the magnitude of a
force required to do useful work, or one to transform and
79
transfer energy. In rescue applications, pulley systems
permit a rescuer to raise a load by applying a force that is
less than load itself.
Figure 168- Mechanical
The mechanical advantage of a pulley system is advantage pulley system with
calculated as the ratio of the load in comparison to the fixed and moveable pulley. From
First Principles of Physics. H.
amount of force required to move the load. If a pulley Carhart and H. Chute. 1912.
system employs a 1 kN force to move a 2
kN mass, then the mechanical advantage
is calculated as 2:1. Mechanical
advantage is gained at the expense of
endurance. Even though less force is
required, it must be employed over a
greater distance.

Rescue pulleys are comprised of a wheel

the rope rolls over, which is known as a
sheave. The sheave is supported by an
axle and held in place by the side plates. Figure 169- Double sheave pulley. Rock Exotica
mini machined double sheave pulley (model P21-D)
with terminology of major parts listed. Original image
© Rock Exotica.
78 http://www.machine-history.com/node/309
79 H. Carhart and H. Chute. p 126.

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