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A gear or cogwheel is a rotating machine part having cut ''teeth'', or cogs, which mesh with another toothed part to transmit torque, in most cases with teeth on the one gear being of identical shape, and often also with that shape on the other gear.〔http://www.merriam-webster.com/dictionary/gear〕 Two or more gears working in a sequence (train) are called a gear train or, in many cases, a ''transmission''; such gear arrangements can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared devices can change the speed, torque, and direction of a power source. The most common situation is for a gear to mesh with another gear; however, a gear can also mesh with a non-rotating toothed part, called a rack, thereby producing translation instead of rotation. 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. When two gears mesh, and one gear is bigger than the other (even though the size of the teeth must match), a mechanical advantage is produced, with the rotational speeds and the torques of the two gears differing in an inverse relationship. 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.〔(【引用サイトリンク】title=Transmission Basics )〕 ==History== Early examples of gears date from the 4th century BCE in China〔Derek J. de Solla Price, (On the Origin of Clockwork, Perpetual Motion Devices, and the Compass ), p.84〕 (Zhan Guo times - Late East Zhou dynasty), which have been preserved at the Luoyang Museum of Henan Province, China. The earliest gears in Europe were circa CE 50 by Hero of Alexandria, but they can be traced back to the Greek mechanics of the Alexandrian school in the 3rd century BCE and were greatly developed by the Greek polymath Archimedes (287–212 BCE). Examples of further development include: *Ma Jun (c. 200–265 CE) used gears as part of a south-pointing chariot. *The Antikythera mechanism is 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 BCE.〔(【引用サイトリンク】title=The Antikythera Mechanism Research Project: Why is it so important? )〕 *The water-powered grain-mill, the water-powered saw mill, fulling mill, and other applications of watermill often used gears. *The first mechanical clocks were built in CE 725. * *The 1386 Salisbury cathedral clock may be the world's oldest working mechanical clock. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「gear」の詳細全文を読む スポンサード リンク
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