Easy to construct and produces enormous mechanical advantage.
In the summer of 1860, the Second Opium War was reaching its climax. Three years earlier, Britain and France had invaded Canton, China, in order to expand their trade in such unsavory commodities as opium and indentured servants. Chinese Emperor Xianfeng had resisted the outsiders, and a low-intensity war continued in fits and starts.
The European nations, tiring of the expense and anxiety of fighting a war so distant, launched an invasion fleet carrying nearly 18,000 men. After landing at the port of Beitang, the soldiers fought their way westward into Peking (now called Beijing) in October 1860. Not long after, a treaty was signed and an uneasy peace was restored.
Some British soldiers, encamped on the outskirts of Beijing in the aftermath, took note of an ingenious device being used to raise and lower drawbridges throughout the city.
The device consisted of a lifting hook suspended from a broad windlass made from cylinders of unequal diameters. The lifting hook was connected by an enormous length of rope, which came off one side of the windlass, went around the hook, and was coiled onto the other side when the mechanism was operated.
The soldiers observed that this windlass was capable of lifting huge loads with little effort. It soon became well known among Western engineers as the Chinese windlass.
How it works
The Chinese windlass, also known by its more descriptive name, the differential windlass, is easy to construct and produces enormous mechanical advantage.
The lifting power comes from the way the rope is wound around 2 drums of slightly different diameters, on the same axis. When the handle is turned to lift the load, the rope is paid off the smaller drum and onto the larger.
The larger drum winds up a bit more rope than is unwound from the smaller. It’s by this small difference in length, divided by 2, that the load is raised with each turn of the handle. So, while raising the load is very slow and requires many turns of the handle, what’s lost in speed is gained in power. With drums of slightly differing diameter, even the smallest person can lift a very heavy load.
For the engineer or physicist, there’s a simple formula that computes the mechanical advantage or “purchase” that the windlass provides:
R/r ∗ C/D ∗ 2 = P
R = Radius of the crank handle
r = Radius of the large barrel
C = Circumference of the large barrel
D = Difference in circumference between the large and small barrels
P = Purchase (mechanical advantage gained)
Suppose we built a Chinese windlass with a 6" crank handle and 2 steel-pipe barrels, one with a 2¾" diameter and the other 3" in diameter. Could we lift a 600lb engine block?
6/1.5 ∗ 3π/(3π-2.75π) ∗ 2 = 96
Yes, we could lift a 600lb block with 12½lbs of force, or just 1/48 the effort of getting underneath and using our backs. Quite a bit easier indeed!