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A gear is a toothed disk (sometimes also called wheel or cog) 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.

Since, work equals force through a distance (force times distance), the small gear goes through a longer distance and exerts a larger force (the twisting force of torque), so performs more work in the process. Thus, this process converts a weak force applied to the driving wheel into a strong force delivered by the driven wheel. The gear ratio is the ratio of the rotational speeds of the two gears.

Gears are important devices when a specific velocity ratio is needed. However, they are relatively expensive to build and maintain when compared to similar devices such as chains and belts.

An example of early gear trains is the Antikythera mechanism. This gear-driven calendar device made in Rhodes about 87 BC contains at least 25 gears cut in bronze. With it, the positions of the sun and the moon could be predicted as well as the rising and setting of certain stars. By the first century AD all the simple kinds of gears were well known.

There are numerous types of gears. 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 in California, are developing molecule-sized gears and other machine parts in the hopes of producing nano-structures 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, which 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.



Litvin, Faydor L. Gear Geometry and Applied Theory. New York: Cambridge University Press, 2004.

Royston, Angela. Pulleys and Gears. Chicago, IL: Heinemann Library, 2001.

Thompson, Gare. Simple Machines. Washington, DC: National Geographic Society, 2002.

Laurie Toupin

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