A gear or gearwheel, also called a toothed wheel, is a rotating machine part typically used to transmit rotational motion or torque by means of a series of "teeth" that engage with compatible teeth of another gear or other part. The teeth can be integral saliences or cavities machined on the part, or separate pegs inserted into it. In the latter case, the gear is usually called a cogwheel. A cog may be one of those pegs or the whole gear. Two or more meshing gears are called a gear train. The smaller member of a pair of meshing gears is often called pinion. Most commonly, gears and gear trains can be used to trade torque for rotational speed between two axles or other rotating parts or to change the axis of rotation or to invert the sense of rotation. A gear may also be used to transmit linear force or linear motion to a rack, a straight bar with a row of compatible teeth.
Gears are among the most common mechanical parts. They come in a great variety of shapes and materials, and are used for many different functions and applications. Diameters may range from a few μm in micromachines, to a few mm in watches and toys to over 10 metres in some mining equipment. Other types of parts that are somewhat similar in shape and function to gears include the sprocket, which is meant to engage with a link chain instead of another gear, and the timing pulley, meant to engage a timing belt. Most gears are round and have equal teeth, designed to operate as smoothly as possible; but there are several applications for non-circular gears, and the Geneva drive has an extremely uneven operation, by design. Gears can be seen as instances of the basic lever "machine". When a small gear drives a larger one, the mechanical advantage of this ideal lever causes the torque T to increase but the rotational speed ω to decrease. The opposite effect is obtained when a large gear drives a small one. The changes are proportional to the gear ratio r, the ratio of the tooth counts: namely, T2/T1 = r = N2/N1, and ω2/ω1 = 1/r = N1/N2. Depending on the geometry of the pair, the sense of rotation may also be inverted (from clockwise to anti-clockwise, or vice versa). Most vehicles have a transmission or "gearbox" containing a set of gears that can be meshed in multiple configurations. The gearbox lets the operator vary the torque that is applied to the wheels without changing the engine's speed. Gearboxes are also used in many other machines, such as lathes and conveyor belts. In all those cases, terms like "first gear", "high gear", and "reverse gear" refer to the overall torque ratios of different meshing configurations, rather than to specific physical gears. These terms may be applied even when the vehicle does not actually contain gears, as in a continuously variable transmission.
History
The earliest surviving gears date to 4th century BC China, during the Warring States period, which have been preserved at the Luoyang Museum in Henan Province, China.
In Europe, Aristotle mentions gears around 330 BC, as wheel drives in windlasses. He observed that the direction of rotation is reversed when one gear wheel drives another gear wheel. Philon of Byzantium was one of the first who used gears in water raising devices. 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 Library of Alexandria in 3rd-century BC Ptolemaic Egypt, and were greatly developed by the Greek polymath Archimedes (287–212 BC). The earliest surviving gears in Europe were found in the Antikythera mechanism, an example of a very early and intricate geared device, designed to calculate astronomical positions of the sun, moon, and planets, and predict eclipses. Its time of construction is now estimated between 150 and 100 BC.
The Chinese engineer Ma Jun (c. 200–265) described a south-pointing chariot. A set of differential gears connected to the wheels and to a pointer on top of the chariot kept the direction of the latter unchanged as the chariot turned. Another early surviving example of geared mechanism is a complex calendrical device showing the phase of the Moon, the day of the month, and the places of the Sun and the Moon in the Zodiac was invented in the Byzantine empire in the early 6th century. Geared mechanical water clocks were built in China by 725. Around 1221, a geared astrolabe was built in Isfahan showing the position of the moon in the zodiac and its phase, and the number of days since the new moon. The worm gear was invented in the Indian subcontinent, for use in roller cotton gins, some time during the 13th–14th centuries. A complex astronomical clock, called the Astrarium, was built between 1348 and 1364 by Giovanni Dondi dell'Orologio. It had seven faces and 107 moving parts; it showed the positions of the sun, the moon and the five planets then known, as well as religious feast days. The Salisbury Cathedral clock, built in 1386, is the world's oldest still working geared mechanical clock. Differential gears were used by the British clock maker Joseph Williamson in 1720.
Etymology The word gear is probably from Old Norse gørvi (plural gørvar) 'apparel, gear,' related to gøra, gørva 'to make, construct, build; set in order, prepare,' a common verb in Old Norse, "used in a wide range of situations from writing a book to dressing meat". In this context, the meaning of 'toothed wheel in machinery' is first attested to the 1520s; the specific mechanical sense of 'parts by which a motor communicates motion' is from 1814; specifically of a vehicle (bicycle, automobile, etc.) by 1888. A cog is a tooth on a wheel. From Middle English cogge, from Old Norse (compare Norwegian kugg ('cog'), Swedish kugg, kugge ('cog, tooth')), from Proto-Germanic *kuggō (compare Dutch kogge ('cog boat'), German Kock), from Proto-Indo-European *gugā ('hump, ball') (compare Lithuanian gugà ('pommel, hump, hill'), from PIE *gēw- ('to bend, arch'). First used c. 1300 in the sense of 'a wheel having teeth or cogs; late 14c., 'tooth on a wheel'; cog-wheel, early 15c.
Materials
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