Count the ropes on a pulley block and you know how much it multiplies your strength
Plutarch tells how Archimedes, using a set of compound pulleys, drew a fully loaded ship toward himself as smoothly as if it floated on calm water. The trick behind that feat is almost embarrassingly simple: every extra strand of rope holding up the load divides the effort needed to lift it.
A pulley is just a wheel on an axle, usually grooved so a rope, cable, belt or chain stays put as it runs over. On its own, a fixed pulley bolted to a beam gains no strength; it only changes direction, letting you pull down to raise something up. Strength appears once a pulley is allowed to move. A single movable pulley hangs in a loop of rope, so two strands share the weight and you need only half the force.
Stack several wheels into a block, hang one block from a fixed point and another from the load, and thread a single rope through both, and you have a block and tackle. In an ideal system with weightless wheels, no friction and rope that never stretches, the tension is identical in every strand. If p strands hold the moving block, each carries a share of W divided by p, so the pull you apply is cut by that same factor. Sailors can even rig a tackle to advantage by tying the rope to the moving block, adding one more supporting strand for free. The price is distance: you haul in far more rope than the load rises.
The idea is old. Pulleys appear in Egypt's Twelfth Dynasty, between 1991 and 1802 BC, and in early 2nd-millennium BC Mesopotamia. Archytas of Tarentum described them in Greece, and Hero of Alexandria later named the pulley among six simple machines for lifting.
Pulleys also pass power from one shaft to another with belts, and wheels of different sizes trade speed for force like gears do. Drill presses shift belts between stepped pulleys to change spindle speed. Slightly domed crowned pulleys, once common on factory line shafts, still spin the brushes in upright vacuum cleaners, and farm tractors carried flat-belt pulleys until the early 1950s.
Source: Pulley
