Mechanical Advantage of Pulley Tackle and the Point of Rupture
How each added pulley divides the load, and reading whether a broken cord was moving or still
The folio pairs a study of block-and-tackle with diagrams: four small sketches at the top left show tackles of one to four pulleys with their ratios (4 to 4, 4 to 2, 4 to 1 1/3, 4 to 1), and two large arched frames hold a pulley over hanging weights labelled 10000 and 15000. Leonardo reasons that the fewer the pulleys, the smaller the proportion between the weight's power and the mover's, and that the more cords the tackle has, the more useful load each cord carries. He then distinguishes two kinds of rupture: a still cord breaks directly over the pulley's axle when its hanging weights are equal, while a cord that has been in motion breaks off to one side, toward the mover, the more so as the mover outmatches its load. A caption at the foot reads 'breaking of a cord.'
On this page
Each added pulley divides the four-pound load
With a single pulley, 4 pounds balance 4 in power; with two pulleys 4 balances 2; with three, 4 balances 1 1/3; with four, 4 balances one. The more cords a tackle has, the more useful weight each cord bears: in a four-cord tackle one pound falls to each cord, whereas a single cord must carry all four. A cord good for only one pound would be broken by the surplus three.
Four tackle sketches, one to four pulleys
Four small diagrams set the ratios in order: the first tackle (4 to 4), the second (4 to 2), the third (4 to 1 1/3) and the fourth (4 to 1), showing how added sheaves increase mechanical advantage.
A cord at rest breaks over the pulley axle
When a motionless cord hanging by its ends from a pulley breaks of its own accord, it is a sure sign that the hanging weights are equal to one another, so the break falls directly over the pulley's axle where the cord rested.
A moving cord breaks toward the mover, nearer f
When the cord breaks away from the axle toward the mover's side, it shows the cord had been in motion; and it breaks the nearer to f than to a as the mover's power exceeds that of its load. The point is marked on the arched frame holding weights 10000 and 15000.