The simplest effective rope pulley block system, or block and tackle, always involves at least two pulley blocks, which yields a force advantage of two. In order to lift a 100-pound (45.36-kilogram) weight from the floor of a building in this manner, a single pulley is attached to the weight and another to the ceiling above the weight. A rope would be attached on one end to the ceiling, threaded through both pulleys, and be pulled by a force of 50 pounds (22.68 kilograms) on the free end in order to lift the weight.
More complicated rope pulley block systems can yield even higher advantage ratios through the use of compound pulley systems. Many cranes use a pulley system where two movable pulleys are connected together by a single axle shaft, with the lifting hook situated between the two. A third fixed pulley is mounted perpendicular to the two movable pulleys on the crane's frame. The lifting cable is then connected to the frame on the side opposite of the fixed pulley, as is the lifting winch. The cable runs through the first movable pulley, up to the fixed pulley, back down to the second movable pulley, and up to the lifting winch to create a lifting advantage of four.
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