a wheel over
which a rope or belt is passed for the purpose of
transmitting energy and doing work
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.
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.
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.
Robert O'Brien Machines
New York: Time Incorporated, 1964
The World Book Encyclopedia
Chicago: World Book-Childcraft International,
Wheel and Axle
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