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Technology: Engineering Our World
Pulleys are used to lift heavy objects. See Figure 8-19. A bale of hay
can be lifted into a hayloft using a single pulley suspended from a beam.
A car engine hoist enables one person to lift a car engine having a mass of
over 450 lb. (200 kg). Cranes use pulleys to lift enormous loads.
Two types of pulleys are used to lift heavy objects: fixed and movable.
In a single, fixed pulley system, Figure 8-20, the effort is equal to the
resistance. There is no mechanical advantage. It is easier, however, for the
operator to pull down instead of up. There has been a change in direction
of force. The distance moved by the effort (effort distance) is equal to the
distance moved by the resistance (resistance distance).
A single, movable pulley system has a mechanical advantage of two.
Both ropes support the resistance equally. See Figure 8-21. The amount
of effort required is half of the resistance. The disadvantage is that the
operator must pull upward. Also, the effort distance is two times the
resistance distance. In all pulley systems, as the effort decreases, the effort
distance increases.
To have the advantages of change of direction and decreased effort,
movable and fixed pulleys can be combined as shown in Figure 8-22.
In both examples, the mechanical advantage is two. However, the effort
must be exerted over twice the distance.
E
R
F
gure 8-20. With a single, fixed pulley,
the effort is equal to the resistance. There is
no mechanical advantage.
gure 8-19. Pulleys can be used alone or
together in different ways to lift and move large
objects easily.