A grooved pulley wheel like that used for ropes is called a sheave. A single sheave mounted in a block and fixed in place simply changes the direction of force exerted on the rope passing over it. If the end of the rope that ordinarily would attach to the load is passed around a second, unfixed pulley and back to the fixed pulley, a load attached to the free pulley can be raised with half the effort, or with a mechanical advantage of 2. Thus arranged, the device is called a block and tackle. The number of pulley wheels mounted in the fixed and free blocks can be increased indefinitely to get a higher and higher mechanical advantage, the mechanical advantage equaling the number of strands running to the free pulley. Therefore if the rope is run over the first fixed pulley wheel, around the free pulley, over a second pulley wheel in the fixed block, and back to the free block, the mechanical advantage is 3. A 300-lb load can be raised by a pull of 100 lb on the free end of the rope. To raise the load 10 ft, however, the free end of the rope must be pulled 30 ft.
Disregarding friction, work output will always equal work input. If the action is reversed by attaching the load to the free end of the rope and pulling on the free block, the mechanical advantage becomes a mechanical disadvantage, but a speed advantage. A rope block and tackle is usually for hand operation. To lift larger loads by hand, a chain is substituted for the rope and the pulleys have grooves for gripping the links. A differential pulley consists of two pulleys of different radii connected and rotating as one on a common axle. The pulleys have their circumferences grooved and spiked so that a chain will run in them without slipping. Over the pulleys an endless chain is run, forming two hanging loops. In one loop is placed a movable block, whose pulley is shaped to take the chain. The load is attached to the movable block and is raised by pulling on the other loop of the chain.
Power-operated machinery usually has cables, as in vertical-lift drawbridges, power shovels, and cranes. Before the extensive use of electric motors, steam engines or water turbines often supplied the power for factory machinery. One engine or turbine might run a whole factory through a complicated system of shafts, pulleys, and belts. Pulleys for flat belts are crowned to keep the belt centered. Raised flanges will serve the same purpose, but they wear the edges of the belt. Drive pulleys for conveyor belts often have a covering, called lagging, to provide better grip. Individual electric motors usually provide drive by means of V belts, the pulleys having raised flanges to form slots that match the trapezoidal cross sections of the belts. Cone pulleys consist of a number of pulleys of varying diameters massed in the shape of a cone. They are used with belt drives for machines (e.g., lathes) requiring a variety of speeds.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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