Pulley

A chain hoist is a pulley system joined together by a closed loop of chain that is pulled by hand. Chain hoists are sometimes used to lift automobile engines out of cars. The pulleys on a chain hoist have teeth that hold the chain, much like the sprockets that hold a bicycle chain in place. A chain hoist is made up of two sections. The top has a large pulley and a small pulley joined side by side on the same axle. The large and small pulleys turn together as a unit. The bottom section of a chain hoist is a movable pulley attached to the load.

The chain hangs down from the large pulley on one side, and then threads back up around the small pulley, down through the movable pulley, and back up to the large pulley. When a user pulls on the chain hanging down from the large pulley, that pulley pulls in chain from the movable pulley. The chain threading through the movable pulley is fed from the small pulley on top. When the chain is pulled, the large pulley brings in more chain than the small pulley lets out, and so the load is raised. Since the effort travels a greater distance than the load, the chain hoist multiplies force. The MA of a chain hoist depends on the diameters of the large and small pulleys.

As is the case with all the simple machines, the origin of the pulley is unknown. When early peoples lifted heavy objects by throwing vines or other crude ropes over tree limbs, they used the idea of a single fixed pulley to change the direction of a force. But since there was no wheel to turn, this use resulted in considerable friction. It is believed that by 1500 bc people in Mesopotamia used rope pulleys for hoisting water. Legend has it that the Greek inventor Archimedes (287-212 bc) used a block and tackle system to pull ships onto dry land.

Modern pulley systems are often combined with motors to create hoists for lifting heavy loads. By using a motor, the user only has to push a button to lift or lower the load. Construction cranes and cranes used at shipyards move heavy loads using block and tackle systems connected to powerful motors. An elevator in a building uses a pulley system to raise and lower the elevator cars.