Maple teeth, bronze wheels and plastic gears built to break on purpose
As late as 1967, a New Hampshire firm, Thompson Manufacturing of Lancaster, was selling tens of thousands of maple gear teeth a year. Its customers were paper mills and grist mills, some running machinery more than a century old, and wooden cogs were simply pegged into the rims of big wheels.
Toothed wheels are very old. The earliest surviving examples, made of iron, come from China's Warring States period in the 4th century BC and are kept at the Luoyang Museum in Henan. Aristotle, around 330 BC, noticed that one gear driving another reverses the direction of spin. Europe's oldest surviving gears are the bronze wheels of the Antikythera mechanism, now dated to between 150 and 100 BC, which computed the positions of sun, moon and planets and forecast eclipses. In China, Ma Jun described a south-pointing chariot in the 3rd century whose differential gearing kept a pointer aimed the same way however the cart turned.
Clockmakers pushed the craft furthest. Geared water clocks existed in China by 725, and a geared astrolabe built in Isfahan around 1221 tracked the moon's phase. Giovanni Dondi dell'Orologio's Astrarium, finished between 1348 and 1364, had seven faces and 107 moving parts showing the sun, moon, five known planets and church feast days. The clock at Salisbury Cathedral, built in 1386, is the oldest geared mechanical clock still working. The worm gear, meanwhile, came from the Indian subcontinent in the 13th or 14th century, used in roller cotton gins.
The word gear likely comes from Old Norse gørvi, apparel or equipment, tied to a verb meaning to make or prepare that Norse speakers used for everything from writing books to dressing meat. The toothed-wheel sense appears in the 1520s, and the vehicle sense by 1888. Cog descends through Old Norse from a Proto-Indo-European root for a hump or ball.
Materials moved from wood to cast iron, then steel and aluminium, and increasingly to engineering plastics that need no oil and can be injection moulded cheaply. Designers sometimes make a plastic gear the deliberately weakest link, so that a jam strips it before costlier parts suffer. By 2014 about 80 percent of the world's gears were produced by net shape moulding.
Source: Gear
