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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.
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 only be 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 below 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
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This page was last updated on October 19, 2018.