<P> The gears in a transmission are analogous to the wheels in a crossed, belt pulley system . An advantage of gears is that the teeth of a gear prevent slippage . </P> <P> When two gears mesh, if one gear is bigger than the other, a mechanical advantage is produced, with the rotational speeds, and the torques, of the two gears differing in proportion to their diameters . </P> <P> In transmissions with multiple gear ratios--such as bicycles, motorcycles, and cars--the term "gear" as in "first gear" refers to a gear ratio rather than an actual physical gear . The term describes similar devices, even when the gear ratio is continuous rather than discrete, or when the device does not actually contain gears, as in a continuously variable transmission . </P> <P> Early examples of gears date from the 4th century BC in China (Zhan Guo times--Late East Zhou dynasty), which have been preserved at the Luoyang Museum of Henan Province, China . The earliest preserved gears in Europe were found in the Antikythera mechanism, an example of a very early and intricate geared device, designed to calculate astronomical positions . Its time of construction is now estimated between 150 and 100 BC . Gears appear in works connected to Hero of Alexandria, in Roman Egypt circa AD 50, but can be traced back to the mechanics of the Alexandrian school in 3rd - century BC Ptolemaic Egypt, and were greatly developed by the Greek polymath Archimedes (287--212 BC). </P>

The working depth of an involute gear is equal to addendum