Examples of compound pulleys include construction cranes, bodybuilding machines, block-and-tackle systems and some elevators. Each of these devices uses more than one pulley to change the direction and multiply the applied force.
Construction cranes make use of multiple pulleys to allow an otherwise underpowered engine to lift building materials far beyond its regular capacity. Bodybuilders who use machines featuring cables attached to weights use multiple pulleys to redirect their muscles' force in ways that are not possible with dumbbell exercises. Elevator designers often add a counterweight to a compound pulley system to let gravity deliver some of the force necessary to move the elevator in a way very similar to the way a block and tackle works.
A compound pulley uses more than one pulley to redirect and exert force on an object. Each additional pulley added to a compound pulley system increases the length of the pull used to achieve the same result, while reducing the amount of force required for each pull. Compound pulleys that make use of large cable distances, such as construction cranes, enjoy a greater mechanical advantage than smaller machines that run over short distances. The mechanical advantage of a particular compound pulley system can be calculated and compared with others, according to Teach Engineering.