A gear is a toothed disk attached to a rotating rod or shaft that transmits and modifies rotary motion by working in conjunction with another gear. Usually circular in shape, the protrusions of one gear mesh into the profile of its mate to obtain a predetermined mechanical advantage. For example, if one gear wheel has ten times as many teeth as the wheel that drives it, it will make one tenth of a turn for every full turn of the latter, while simultaneously exerting ten times the torque or turning force applied to it by the driving wheel. This process converts a weak force applied to the driving wheel into a strong force delivered by the driven wheel.
An example of early gear trains is the Antikythera mechanism. This gear-driven calendar device made in Rhodes about 87 B.C. contains at least 25 gears cut in bronze. With it, the positions of the sun and moon could be predicted as well as the rising and setting of certain stars. By the first century A.D. all the simple kinds of gears were well known.
A pinion is a gear with a small number of teeth engaging with a rack or larger gear. A bevel gear is one of a pair of toothed wheels whose working surfaces are inclined to nonparallel axes. A worm gear transmits power from one shaft to another, usually at right angles. Automobiles employ a differential gear, which permits power from the engine to be transferred to a pair of driving wheels, dividing the force equally between them but permitting them to follow paths of different lengths, as when turning a corner or traversing an uneven road.
Researchers at the NASA Ames Research Center are developing molecule-sized gears and other machine parts in the hopes of producing nanostructures capable of self-repair or that could adapt to a given environment. The Ames team "built" hypothetical gears by forming tubes from fullerenes, a class of molecules consisting of 60 carbon atoms arranged in a ball-like lattice. They attached benzene molecules onto these fullerenes for "teeth." Researchers propose to turn the gears with a laser that will create an electronic field around the nanotube that will drag the tube around similar to a shaft turning. Although these gears presently exist only in computer simulations, the simulations predict that the gears would rotate best at about 100 billion turns per second, or six trillion rotations per minute and are virtually unbreakable.
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Hall, Alan. "A Turn of the Gear." Scientific American April 28, 1997.