| The Pulley is a wheel over which a rope or belt is passed for the
purpose of transmitting energy and doing work.
 |
The simplest
pulley is a grooved wheel on a fixed axle (fixed
pulley). A rope passed over this
wheel is tied to the load to be lifted, and a
pull is applied to the other end of the rope.
This kind of pulley gives no mechanical advantage
of lift, but changes the direction of the force
applied to the load -- its mechanical advantage
is 1 because only one segment of rope supports
the weight; effort and weight are equal. When the task of the fixed pulley is to
carry a continuous turning motion, the two ends
of the rope or belt are laced together. A second
pulley, which is connected to the source of
energy, transmits a steady rotation to the first
pulley. If driver and driven pulleys are of the
same size, the only advantage is a choice of
directions from which the energy may come. If the
pulleys are of different sizes, an advantage of
either speed or of force may be obtained. When
the belt between the two pulleys is crossed, the
direction of turn of the driven pulley is
reversed.
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The second
basic type of pulley is the single
movable pulley. The load is
attached to the axle of this pulley. One end of
the rope that passes through the pulley is
attached to a fixed support above the load. A
pull is applied to the free end of the rope in
the same direction that the load is to move. With
two segments supporting the weight, the single
movable pulley has a mechanical advantage of 2,
meaning that the pull applied to the free end of
the rope need be only half the weight of the
load. |
 |
The block
and tackle allows the user to lift
extremely large loads with a relatively small
amount of effort. The pulleys are mounted in
frames called blocks. Each block has a
hook by which it can be fastened to its support
or to the load. A single block has one
pulley, a double block has two pulleys
and so on. The ideal
mechanical advantage of a block and tackle equals
the number of sections of rope that support the
movable block. The block and tackle shown at left
has three rope segments supporting the weight,
giving it an advantage of 3. But this advantage
comes at a cost; a smaller effort is required to
lift a weight, but the distance through which the
effort must move increases in direct proportion
to mechanical advantage obtained. Thus, using the
block and tackle to raise the 60-pound weight
three feet requires 20 pounds of effort to be
exerted through a distance of nine feet.
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Questions or comments about this
page?
Robert O'Brien. Machines. New York:Time
Incorporated, 1964.
World Book Encyclopedia. Chicago:World
Book-Childcraft International, 1979.
Wheel and Axle
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