pulley simple machine consisting of a wheel over which a rope, belt, chain, or cable runs. 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...
pulley simple machine consisting of a wheel over which a rope, belt, chain, or cable runs. 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...
