gear

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OR:

A gear is a rotating circular
machine A machine is a physical system using Power (physics), power to apply Force, forces and control Motion, movement to perform an action. The term is commonly applied to artificial devices, such as those employing engines or motors, but also to na ...
part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called ''cogs''), which mesh with another (compatible) toothed part to transmit (convert)
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of the ...

and speed. The basic principle behind the operation of gears is analogous to the basic principle of levers. A gear may also be known informally as a cog. Geared devices can change the speed, torque, and direction of a power source. Gears of different sizes produce a change in torque, creating a
mechanical advantage Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for t ...
, through their ''gear ratio'', and thus may be considered a
simple machine A simple machine is a machine, mechanical device that changes the direction or magnitude of a force. In general, they can be defined as the simplest mechanisms that use mechanical advantage (also called leverage) to multiply force. Usually the t ...
. The
rotational speed Rotational frequency (also known as rotational speed or rate of rotation) of an object Rotation around a fixed axis, rotating around an axis is the frequency of rotation of the object. Its unit is revolution per minute (rpm), cycle per second ...
s, and the torques, of two meshing gears differ in proportion to their diameters. The teeth on the two meshing gears all have the same shape. Two or more meshing gears, working in a sequence, are called a
gear train A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage. Gear teeth are designed to ensure the Pitch circle diameter (gears), pitch circles of engaging gears roll on each other without slipping, pr ...

or a ''
transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission *** ...
''. The gears in a transmission are analogous to the wheels in a crossed, belt
pulley A pulley is a wheel on an axle or shaft that is designed to support movement and change of direction of a taut cable or belt, or transfer of power between the shaft and cable or belt. In the case of a pulley supported by a frame or shell tha ...

system. An advantage of gears is that the teeth of a gear prevent slippage. In transmissions with multiple gear ratios—such as bicycles, motorcycles, and cars—the term "gear" (e.g., "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 A continuously variable transmission (CVT) is an automatic transmission that can change seamlessly through a continuous range of gear ratios. This contrasts with other transmissions that provide a limited number of gear ratios in fixed steps. T ...
(CVT). Sometimes a CVT is referred to as an "infinitely variable transmission". Furthermore, a gear can mesh with a linear toothed part, called a ''rack'', producing movement in a straight line instead of
rotation Rotation, or spin, is the circular movement of an object around a ''axis of rotation, central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A t ...

(movement in a circle). See
Rack and Pinion A rack and pinion is a type of linear actuator that comprises a circular gear (the ''pinion'') engaging a linear gear (the ''rack''). Together, they convert rotational motion into linear motion. Rotating the pinion causes the rack to be driven i ...
for an example.

# History

Early examples of gears date from the 4th century BC 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 The Zhou dynasty ( ; Old Chinese (Reconstructions of Old Chinese#Baxter–Sagart (2014), B&S): *''tiw'') was a Dynasties in Chinese history, royal dynasty of China that followed the Shang dynasty. Having lasted 789 years, the Zhou dynasty was t ...
), which have been preserved at the Luoyang Museum of Henan Province,
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...

. The earliest preserved gears in Europe were found in the
Antikythera mechanism The Antikythera mechanism ( ) is an Ancient Greece, Ancient Greek hand-powered orrery, described as the oldest example of an analogue computer used to predict astronomy, astronomical positions and eclipses decades in advance. It could also be ...

an example of a very early and intricate geared device, designed to calculate
astronomical Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
positions. Its time of construction is now estimated between 150 and 100 BC. Gears appear in works connected to
Hero of Alexandria Hero of Alexandria (; grc-gre, Ἥρων ὁ Ἀλεξανδρεύς, ''Heron ho Alexandreus'', also known as Heron of Alexandria ; 60 AD) was a Greece, Greek mathematician and engineer who was active in his native city of Alexandria, Roman Egy ...

, in
Roman Egypt , conventional_long_name = Roman Egypt , common_name = Egypt , subdivision = Roman province, Province , nation = the Roman Empire , era = Late antiquity , capital = Alexandria , title_leader = Praefectus Augustalis , image_ ...

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 A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans a substantial number of subjects, known to draw on complex bodies of knowledge to solve specific pro ...

Archimedes Archimedes of Syracuse (;; ) was a Greek mathematician A mathematician is someone who uses an extensive knowledge of mathematics in their work, typically to solve mathematical problems. Mathematicians are concerned with numbers, data, ...

(287–212 BC). A complex geared 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 CE. The
worm gear A worm drive is a gear train, gear arrangement in which a worm (which is a gear in the form of a Screw thread, screw) meshes with a worm wheel (which is similar in appearance to a gear#Spur, spur gear). The two machine element, elements are als ...

was invented in the
Indian subcontinent The Indian subcontinent is a list of the physiographic regions of the world, physiographical region in United Nations geoscheme for Asia#Southern Asia, Southern Asia. It is situated on the Indian Plate, projecting southwards into the Indian O ...

, for use in roller
cotton gin A cotton gin—meaning "cotton engine"—is a machine that quickly and easily separates cotton fibers from their seeds, enabling much greater productivity than manual cotton separation.. Reprinted by McGraw-Hill, New York and London, 1926 (); ...
s, some time during the 13th–14th centuries.
Irfan Habib Irfan Habib (born August 10, 1931) is an Indian historian of ancient and medieval India, following the methodology of Marxist historiography Marxist historiography, or historical materialist historiography, is an influential school of his ...

''Economic History of Medieval India, 1200-1500'', page 53
Pearson Education Pearson Education is a British-owned education publishing and assessment service to schools and corporations, as well for students directly. Pearson owns educational media brands including Addison–Wesley, Peachpit, Prentice Hall, eCollege, ...
Differential gears may have been used in some of the Chinese
south-pointing chariot The south-pointing chariot (or carriage) was an ancient Chinese two-wheeled vehicle that carried a movable pointer to indicate the south, no matter how the chariot turned. Usually, the pointer took the form of a doll or figure with an outstretch ...
s, but the first verifiable use of differential gears was by the British clock maker Joseph Williamson in 1720. Examples of early gear applications include: *1386 CE: The Salisbury Cathedral clock: it is the world's oldest still working geared mechanical clock. *The Astrarium of Giovanni Dondi dell'Orologio was a complex astronomical clock built between 1348 and 1364 by
Giovanni Dondi dell'Orologio Giovanni Dondi dell'Orologio (c. 1330 – 1388), also known as Giovanni de' Dondi, was an Italian physician, astronomer and mechanical engineer in Padua Padua ( ; it, Padova ; vec, Pàdova) is a city and ''comune'' in Veneto, northern ...
. The Astrarium 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. *c. 13th–14th centuries: The
worm gear A worm drive is a gear train, gear arrangement in which a worm (which is a gear in the form of a Screw thread, screw) meshes with a worm wheel (which is similar in appearance to a gear#Spur, spur gear). The two machine element, elements are als ...

was invented as part of a roller
cotton gin A cotton gin—meaning "cotton engine"—is a machine that quickly and easily separates cotton fibers from their seeds, enabling much greater productivity than manual cotton separation.. Reprinted by McGraw-Hill, New York and London, 1926 (); ...
in the
Indian subcontinent The Indian subcontinent is a list of the physiographic regions of the world, physiographical region in United Nations geoscheme for Asia#Southern Asia, Southern Asia. It is situated on the Indian Plate, projecting southwards into the Indian O ...

. *c. 1221 CE The geared
astrolabe An astrolabe ( grc, ἀστρολάβος ; ar, ٱلأَسْطُرلاب ; persian, ستاره‌یاب ) is an ancient astronomical instrument that was a handheld model of the universe. Its various functions also make it an elaborate inclin ...

was built in
Isfahan Isfahan ( fa, اصفهان, Esfahân ), from its Achaemenid empire, ancient designation ''Aspadana'' and, later, ''Spahan'' in Sassanian Empire, middle Persian, rendered in English as ''Ispahan'', is a major city in the Greater Isfahan Regio ...

showing the position of the
moon The Moon is Earth's only natural satellite. It is the List of natural satellites, fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth ( ...

in the
zodiac The zodiac is a belt-shaped region of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic, the Sun path, apparent path of the Sun across the celestial sphere over the course of the year. ...

and its phase, and the number of days since new moon. *c. 6th century CE: A geared calendrical device showing the phase of the Moon, the day of the month and the
Zodiac The zodiac is a belt-shaped region of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic, the Sun path, apparent path of the Sun across the celestial sphere over the course of the year. ...

was invented in the Byzantine empire. *725 CE: The first geared mechanical clocks were built in
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...

. *2nd century BC: The
Antikythera mechanism The Antikythera mechanism ( ) is an Ancient Greece, Ancient Greek hand-powered orrery, described as the oldest example of an analogue computer used to predict astronomy, astronomical positions and eclipses decades in advance. It could also be ...

, the world's oldest
analog computer An analog computer or analogue computer is a type of Computation, computer that uses the continuous variation aspect of physical phenomena such as Electrical network, electrical, Mechanics, mechanical, or Hydraulics, hydraulic quantities (''a ...
is built. It could predict the movement and position of the sun, moon and
planets A planet is a large, rounded Astronomical object, astronomical body that is neither a star nor its Stellar remnant, remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud colla ...

decades in advance and could solve different astronomical problems. *c. 200–265 CE: Ma Jun used gears as part of a
south-pointing chariot The south-pointing chariot (or carriage) was an ancient Chinese two-wheeled vehicle that carried a movable pointer to indicate the south, no matter how the chariot turned. Usually, the pointer took the form of a doll or figure with an outstretch ...
. *In nature: in the hind legs of the nymphs of the planthopper insect '' Issus coleoptratus''.

# Etymology

The word ''gear'' is probably from
Old Norse Old Norse, Old Nordic, or Old Scandinavian, is a stage of development of North Germanic languages, North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants of Scandinavia and t ...
''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' first attested 1520s; 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 Middle English (abbreviated to ME) is a form of the English language that was spoken after the Norman conquest of England, Norman conquest of 1066, until the late 15th century. The English language underwent distinct variations and developments ...
cogge, from
Old Norse Old Norse, Old Nordic, or Old Scandinavian, is a stage of development of North Germanic languages, North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants of Scandinavia and t ...
(compare Norwegian ''kugg'' ('cog'), Swedish ''kugg'', ''kugge'' ('cog, tooth')), from
Proto-Germanic Proto-Germanic (abbreviated PGmc; also called Common Germanic) is the linguistic reconstruction, reconstructed proto-language of the Germanic languages, Germanic branch of the Indo-European languages. Proto-Germanic eventually developed from ...
*''kuggō'' (compare Dutch ''kogge'' (' cogboat'), German ''Kock''), from
Proto-Indo-European Proto-Indo-European (PIE) is the reconstructed common ancestor of the Indo-European language family. Its proposed features have been derived by linguistic reconstruction from documented Indo-European languages. No direct record of Proto-Indo-E ...
*''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. Historically, cogs were teeth made of wood rather than metal, and a cogwheel technically consisted of a series of wooden gear teeth located around a mortise wheel, each tooth forming a type of specialised joint. The wheel can be made of wood,
cast iron Cast iron is a class of iron–carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuriti ...
, or other material. Wooden cogs were formerly used when large metal gears could not be cut, when the cast tooth was not even approximately of the proper shape, or the size of the wheel made manufacture impractical. The cogs were often made of
maple ''Acer'' () is a genus of trees and shrubs commonly known as maples. The genus is placed in the family Sapindaceae.Stevens, P. F. (2001 onwards). Angiosperm Phylogeny Website. Version 9, June 2008 nd more or less continuously updated since h ...

wood. In 1967 the Thompson Manufacturing Company of
Lancaster, New Hampshire Lancaster is a New England town, town located along the Connecticut River in Coös County, New Hampshire, Coös County, New Hampshire, United States. The town is named after the city of Lancaster, Lancashire, Lancaster in England. As of the 2020 ce ...
still had a very active business in supplying tens of thousands of maple gear teeth per year, mostly for use in
paper mill A paper mill is a factory devoted to making paper from vegetable fibres such as wood pulp, old rags, and other ingredients. Prior to the invention and adoption of the Fourdrinier machine and other types of paper machine that use an endless belt, ...
s and grist mills, some dating back over 100 years. Since a wooden cog performs exactly the same function as a cast or machined metal tooth, the word was applied by extension to both, and the distinction has been generally lost.

# Comparison with drive mechanisms

The definite ratio that teeth give gears provides an advantage over other drives (such as traction drives and V-belts) in precision machines such as watches that depend upon an exact velocity ratio. In cases where driver and follower are proximal, gears also have an advantage over other drives in the reduced number of parts required. The downside is that gears are more expensive to manufacture and their lubrication requirements may impose a higher operating cost per hour.

# Types

## External versus internal gears

An ''external gear'' is one with the teeth formed on the outer surface of a cylinder or cone. Conversely, an ''internal gear'' is one with the teeth formed on the inner surface of a cylinder or cone. For
bevel gear Bevel gears are gears where the axes of the two Shaft (mechanical engineering), shafts intersect and the gear tooth, tooth-bearing faces of the gears themselves are conically shaped. Bevel gears are most often mounted on shafts that are 90 degree ...

s, an internal gear is one with the pitch angle exceeding 90 degrees. Internal gears do not cause output shaft direction reversal.

## Spur

''Spur gears'' or ''straight-cut gears'' are the simplest type of gear. They consist of a cylinder or disk with teeth projecting radially. Though the teeth are not straight-sided (but usually of special form to achieve a constant drive ratio, mainly
involute In mathematics, an involute (also known as an evolvent) is a particular type of curve that is dependent on another shape or curve. An involute of a curve is the Locus (mathematics), locus of a point on a piece of taut string as the string is eith ...
but less commonly cycloidal), the edge of each tooth is straight and aligned parallel to the axis of rotation. These gears mesh together correctly only if fitted to parallel shafts. No axial thrust is created by the tooth loads. Spur gears are excellent at moderate speeds but tend to be noisy at high speeds.

## Helical

''Helical'' or "dry fixed" gears offer a refinement over spur gears. The leading edges of the teeth are not parallel to the axis of rotation, but are set at an angle. Since the gear is curved, this angling makes the tooth shape a segment of a
helix A helix () is a shape like a corkscrew or spiral staircase. It is a type of smoothness (mathematics), smooth space curve with tangent lines at a constant angle to a fixed axis. Helices are important in biology, as the DNA molecule is formed as ...

. Helical gears can be meshed in ''parallel'' or ''crossed'' orientations. The former refers to when the shafts are parallel to each other; this is the most common orientation. In the latter, the shafts are non-parallel, and in this configuration the gears are sometimes known as "skew gears". The angled teeth engage more gradually than do spur gear teeth, causing them to run more smoothly and quietly. With parallel helical gears, each pair of teeth first make contact at a single point at one side of the gear wheel; a moving curve of contact then grows gradually across the tooth face to a maximum, then recedes until the teeth break contact at a single point on the opposite side. In spur gears, teeth suddenly meet at a line contact across their entire width, causing stress and noise. Spur gears make a characteristic whine at high speeds. For this reason spur gears are used in low-speed applications and in situations where noise control is not a problem, and helical gears are used in high-speed applications, large power transmission, or where noise abatement is important. The speed is considered high when the pitch line velocity exceeds 25 m/s. A disadvantage of helical gears is a resultant
thrust Thrust is a reaction (physics), reaction force (physics), force described quantitatively by Newton's third law. When a system expels or acceleration, accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude ...

along the axis of the gear, which must be accommodated by appropriate
thrust bearing A thrust bearing is a particular type of rotary Bearing (mechanical), bearing. Like other bearings they permanently rotate between parts, but they are designed to support a predominantly Axis of rotation, axial load. Thrust bearings come in seve ...
s. However, this issue can be turned into an advantage when using a herringbone gear or ''double helical gear'', which has no axial thrust - and also provides self-aligning of the gears. This results in less axial thrust than a comparable spur gear. A second disadvantage of helical gears is also a greater degree of sliding friction between the meshing teeth, often addressed with additives in the lubricant.

### Skew gears

For a "crossed" or "skew" configuration, the gears must have the same pressure angle and normal pitch; however, the helix angle and handedness can be different. The relationship between the two shafts is actually defined by the helix angle(s) of the two shafts and the handedness, as defined: :$E = \beta_1 + \beta_2$ for gears of the same handedness, :$E = \beta_1 - \beta_2$ for gears of opposite handedness, where $\beta$ is the helix angle for the gear. The crossed configuration is less mechanically sound because there is only a point contact between the gears, whereas in the parallel configuration there is a line contact. Quite commonly, helical gears are used with the helix angle of one having the negative of the helix angle of the other; such a pair might also be referred to as having a right-handed helix and a left-handed helix of equal angles. The two equal but opposite angles add to zero: the angle between shafts is zero—that is, the shafts are ''parallel''. Where the sum or the difference (as described in the equations above) is not zero, the shafts are ''crossed''. For shafts ''crossed'' at right angles, the helix angles are of the same hand because they must add to 90 degrees. (This is the case with the gears in the illustration above: they mesh correctly in the crossed configuration: for the parallel configuration, one of the helix angles should be reversed. The gears illustrated cannot mesh with the shafts parallel.)
3D animation of helical gears (parallel axis)

3D animation of helical gears (crossed axis)

## Double helical

Double helical gears overcome the problem of axial thrust presented by single helical gears by using a double set of teeth, slanted in opposite directions. A double helical gear can be thought of as two mirrored helical gears mounted closely together on a common axle. This arrangement cancels out the net axial thrust, since each half of the gear thrusts in the opposite direction, resulting in a net axial force of zero. This arrangement can also remove the need for thrust bearings. However, double helical gears are more difficult to manufacture due to their more complicated shape. Herringbone gears are a special type of helical gears. They do not have a groove in the middle like some other double helical gears do; the two mirrored helical gears are joined so that their teeth form a V shape. This can also be applied to
bevel gear Bevel gears are gears where the axes of the two Shaft (mechanical engineering), shafts intersect and the gear tooth, tooth-bearing faces of the gears themselves are conically shaped. Bevel gears are most often mounted on shafts that are 90 degree ...

s, as in the final drive of the Citroën Type A. For both possible rotational directions, there exist two possible arrangements for the oppositely-oriented helical gears or gear faces. One arrangement is called stable, and the other unstable. In a stable arrangement, the helical gear faces are oriented so that each axial force is directed toward the center of the gear. In an unstable arrangement, both axial forces are directed away from the center of the gear. In either arrangement, the total (or ''net'') axial force on each gear is zero when the gears are aligned correctly. If the gears become misaligned in the axial direction, the unstable arrangement generates a net force that may lead to disassembly of the gear train, while the stable arrangement generates a net corrective force. If the direction of rotation is reversed, the direction of the axial thrusts is also reversed, so a stable configuration becomes unstable, and vice versa. Stable double helical gears can be directly interchanged with spur gears without any need for different bearings.

## Bevel

A bevel gear is shaped like . When two bevel gears mesh, their imaginary vertices must occupy the same point. Their shaft axes also intersect at this point, forming an arbitrary non-straight angle between the shafts. The angle between the shafts can be anything except zero or 180 degrees. Bevel gears with equal numbers of teeth and shaft axes at 90 degrees are called miter (US) or mitre (UK) gears.

## Spiral bevels

Spiral bevel gears can be manufactured as Gleason types (circular arc with non-constant tooth depth), Oerlikon and Curvex types (circular arc with constant tooth depth), Klingelnberg Cyclo-Palloid (Epicycloid with constant tooth depth) or Klingelnberg Palloid. Spiral bevel gears have the same advantages and disadvantages relative to their straight-cut cousins as helical gears do to spur gears. Straight bevel gears are generally used only at speeds below 5 m/s (1000 ft/min), or, for small gears, 1000 r.p.m.. The cylindrical gear tooth profile corresponds to an involute, but the bevel gear tooth profile to an octoid. All traditional bevel gear generators (like Gleason, Klingelnberg, Heidenreich & Harbeck, WMW Modul) manufacture bevel gears with an octoidal tooth profile. For 5-axis milled bevel gear sets it is important to choose the same calculation / layout like the conventional manufacturing method. Simplified calculated bevel gears on the basis of an equivalent cylindrical gear in normal section with an involute tooth form show a deviant tooth form with reduced tooth strength by 10-28% without offset and 45% with offset iss. Hünecke, TU Dresden Furthermore, the "involute bevel gear sets" cause more noise.

## Hypoid

Hypoid gears resemble spiral bevel gears except the shaft axes do not intersect. The pitch surfaces appear conical but, to compensate for the offset shaft, are in fact
hyperboloid In geometry, a hyperboloid of revolution, sometimes called a circular hyperboloid, is the surface (mathematics), surface generated by rotating a hyperbola around one of its Hyperbola#Nomenclature and features, principal axes. A hyperboloid is th ...
s of revolution. Hypoid gears are almost always designed to operate with shafts at 90 degrees. Depending on which side the shaft is offset to, relative to the angling of the teeth, contact between hypoid gear teeth may be even smoother and more gradual than with spiral bevel gear teeth, but also have a sliding action along the meshing teeth as it rotates and therefore usually require some of the most viscous types of gear oil to avoid it being extruded from the mating tooth faces, the oil is normally designated HP (for hypoid) followed by a number denoting the viscosity. Also, the pinion can be designed with fewer teeth than a spiral bevel pinion, with the result that gear ratios of 60:1 and higher are feasible using a single set of hypoid gears.. This style of gear is most common in motor vehicle drive trains, in concert with a differential. Whereas a regular (nonhypoid) ring-and-pinion gear set is suitable for many applications, it is not ideal for vehicle drive trains because it generates more noise and vibration than a hypoid does. Bringing hypoid gears to market for mass-production applications was an engineering improvement of the 1920s.

## Crown gear

''Crown gears'' or ''contrate gears'' are a particular form of bevel gear whose teeth project at right angles to the plane of the wheel; in their orientation the teeth resemble the points on a crown. A crown gear can only mesh accurately with another bevel gear, although crown gears are sometimes seen meshing with spur gears. A crown gear is also sometimes meshed with an
escapement An escapement is a mechanical linkage in mechanical watches and clocks that gives impulses to the timekeeping element and periodically releases the gear train to move forward, advancing the clock's hands. The impulse action transfers energy to ...

such as found in mechanical clocks.

## Worm

''Worms'' resemble
screw A screw and a Bolt (fastener), bolt (see ''#Differentiation between bolt and screw, Differentiation between bolt and screw'' below) are similar types of fastener typically made of metal and characterized by a helix, helical ridge, called a ''m ...

s. A worm is meshed with a ''worm wheel'', which looks similar to a spur gear. Worm-and-gear sets are a simple and compact way to achieve a high torque, low speed gear ratio. For example, helical gears are normally limited to gear ratios of less than 10:1 while worm-and-gear sets vary from 10:1 to 500:1.. A disadvantage is the potential for considerable sliding action, leading to low efficiency. A worm gear is a species of helical gear, but its helix angle is usually somewhat large (close to 90 degrees) and its body is usually fairly long in the axial direction. These attributes give it screw like qualities. The distinction between a worm and a helical gear is that at least one tooth persists for a full rotation around the helix. If this occurs, it is a 'worm'; if not, it is a 'helical gear'. A worm may have as few as one tooth. If that tooth persists for several turns around the helix, the worm appears, superficially, to have more than one tooth, but what one in fact sees is the same tooth reappearing at intervals along the length of the worm. The usual screw nomenclature applies: a one-toothed worm is called ''single thread'' or ''single start''; a worm with more than one tooth is called ''multiple thread'' or ''multiple start''. The helix angle of a worm is not usually specified. Instead, the lead angle, which is equal to 90 degrees minus the helix angle, is given. In a worm-and-gear set, the worm can always drive the gear. However, if the gear attempts to drive the worm, it may or may not succeed. Particularly if the lead angle is small, the gear's teeth may simply lock against the worm's teeth, because the force component circumferential to the worm is not sufficient to overcome friction. In traditional
music box A music box (American English) or musical box (British English) is an automatic musical instrument in a box that produces Musical note, musical notes by using a set of pins placed on a revolving cylinder (geometry), cylinder or disc to pluck ...

es, however, the gear drives the worm, which has a large helix angle. This mesh drives the speed-limiter vanes which are mounted on the worm shaft. Worm-and-gear sets that do lock are called self locking, which can be used to advantage, as when it is desired to set the position of a mechanism by turning the worm and then have the mechanism hold that position. An example is the
machine head A machine head (also referred to as a tuning machine, tuner, or gear head) is a geared apparatus for tuning stringed musical instruments by adjusting string tension. Machine heads are used on mandolins, guitars, double basses and others, and ar ...

found on some types of
stringed instrument String instruments, stringed instruments, or chordophones are musical instruments that produce sound from vibrating strings when a performer plays or sounds the strings in some manner. Musicians play some string instruments by plucking the Str ...
s. If the gear in a worm-and-gear set is an ordinary helical gear only a single point of contact is achieved.. If medium to high power transmission is desired, the tooth shape of the gear is modified to achieve more intimate contact by making both gears partially envelop each other. This is done by making both concave and joining them at a
saddle point In mathematics, a saddle point or minimax point is a Point (geometry), point on the surface (mathematics), surface of the graph of a function where the slopes (derivatives) in orthogonal directions are all zero (a Critical point (mathematics), ...

; this is called a cone-drive or "Double enveloping". Worm gears can be right or left-handed, following the long-established practice for screw threads.

## Non-circular

Non-circular gears are designed for special purposes. While a regular gear is optimized to transmit torque to another engaged member with minimum noise and wear and maximum
efficiency Efficiency is the often measurable ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without ...
, a non-circular gear's main objective might be
ratio In mathematics, a ratio shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...

variations, axle displacement
oscillation Oscillation is the repetitive or Periodic function, periodic variation, typically in time, of some measure about a central value (often a point of Mechanical equilibrium, equilibrium) or between two or more different states. Familiar examples o ...

s and more. Common applications include textile machines,
potentiometer A potentiometer is a three-terminal (electronics), terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat. ...

s and
continuously variable transmission A continuously variable transmission (CVT) is an automatic transmission that can change seamlessly through a continuous range of gear ratios. This contrasts with other transmissions that provide a limited number of gear ratios in fixed steps. T ...
s.

## Rack and pinion

A ''rack'' is a toothed bar or rod that can be thought of as a sector gear with an infinitely large
radius of curvature In differential geometry, the radius of curvature, , is the reciprocal of the curvature. For a curve, it equals the radius of the circular arc which best approximates the curve at that point. For surface (mathematics), surfaces, the radius of cu ...

.
Torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of the ...

can be converted to linear force by meshing a rack with a round gear called a ''pinion'': the pinion turns, while the rack moves in a straight line. Such a mechanism is used in automobiles to convert the rotation of the
steering wheel A steering wheel (also called a driving wheel (UK), a hand wheel, or simply wheel) is a type of steering control in vehicles. Steering wheels are used in most modern land vehicles, including all mass-production automobiles, buses, light and ...

into the left-to-right motion of the
tie rod A tie rod or tie bar (also known as a hanger rod if vertical) is a slender structural unit used as a tie and (in most applications) capable of carrying tension (physics), tensile loads only. It is any rod or bar-shaped structural member designed to ...
(s). Racks also feature in the theory of gear geometry, where, for instance, the tooth shape of an interchangeable set of gears may be specified for the rack (infinite radius), and the tooth shapes for gears of particular actual radii are then derived from that. The rack and pinion gear type is also used in a rack railway.

## Epicyclic gear train

In epicyclic gearing, one or more of the gear Axis of rotation, axes moves. Examples are sun and planet gearing (see below), cycloidal drive, automatic transmissions, and differential (mechanical device), mechanical differentials.

### Sun and planet

Sun and planet gearing is a method of converting reciprocating motion into rotary motion that was used in steam engines. James Watt used it on his early steam engines to get around the patent on the Crank (mechanism), crank, but it also provided the advantage of increasing the flywheel speed so Watt could use a lighter flywheel. In the illustration, the sun is yellow, the planet red, the reciprocating arm is blue, the flywheel is green and the driveshaft is gray.

## Harmonic gear

A ''harmonic gear'' or ''strain wave gearing, strain wave gear'' is a specialized gearing mechanism often used in industrial motion control, robotics and aerospace for its advantages over traditional gearing systems, including lack of backlash, compactness and high gear ratios. Though the diagram does not demonstrate the correct configuration, it is a "timing gear," conventionally with far more teeth than a traditional gear to ensure a higher degree of precision.

## Cage gear

A ''cage gear'', also called a ''lantern gear'' or ''lantern pinion'', has cylindrical rods for teeth, parallel to the axle and arranged in a circle around it, much as the bars on a round bird cage or lantern. The assembly is held together by disks at each end, into which the tooth rods and axle are set. Cage gears are more efficient than solid pinions, and dirt can fall through the rods rather than becoming trapped and increasing wear. They can be constructed with very simple tools as the teeth are not formed by cutting or milling, but rather by drilling holes and inserting rods. Sometimes used in clocks, the ''cage gear'' should always be driven by a gearwheel, not used as the driver. The ''cage gear'' was not initially favoured by conservative clock makers. It became popular in turret clocks where dirty working conditions were most commonplace. Domestic American clock movements often used them.

## Magnetic gear

All cogs of each gear component of magnetic gears act as a constant magnet with periodic alternation of opposite magnetic poles on mating surfaces. Gear components are mounted with a Backlash (engineering), backlash capability similar to other mechanical gearings. Although they cannot exert as much force as a traditional gear due to limits on magnetic field strength, such gears work without touching and so are immune to wear, have very low noise, minimal power losses from friction and can slip without damage making them very reliable. They can be used in configurations that are not possible for gears that must be physically touching and can operate with a non-metallic barrier completely separating the driving force from the load. The magnetic coupling can transmit force into a Hermetic seal, hermetically sealed enclosure without using a radial shaft seal, which may leak.

# Nomenclature

## General

; Rotational frequency, ''n'' : Measured in rotation over time, such as revolutions per minute (RPM or rpm). ; Angular frequency, ω : Measured in Radians per second, radians/second. 1RPM = 2πrad/minute = π/30rad/second. ; Number of teeth, ''N'' : How many teeth a gear has, an integer. In the case of worms, it is the number of thread starts that the worm has. ; Gear, wheel : The larger of two interacting gears or a gear on its own. ; Pinion : The smaller of two interacting gears. ; Path of contact : Path followed by the point of contact between two meshing gear teeth. ; Line of action, pressure line : Line along which the force between two meshing gear teeth is directed. It has the same direction as the force vector. In general, the line of action changes from moment to moment during the period of engagement of a pair of teeth. For involute gears, however, the tooth-to-tooth force is always directed along the same line—that is, the line of action is constant. This implies that for involute gears the path of contact is also a straight line, coincident with the line of action—as is indeed the case. ; Axis : Axis of revolution of the gear; center line of the shaft. ; Pitch point : Point where the line of action crosses a line joining the two gear axes. ; Pitch circle, pitch line : Circle centered on and perpendicular to the axis, and passing through the pitch point. A predefined diametral position on the gear where the circular tooth thickness, pressure angle and helix angles are defined. ; Pitch diameter, ''d'': A predefined diametral position on the gear where the circular tooth thickness, pressure angle and helix angles are defined. The standard pitch diameter is a design dimension and cannot be measured, but is a location where other measurements are made. Its value is based on the number of teeth (''N''), the normal module (''mn''; or normal diametral pitch, ''Pd''), and the helix angle ($\psi$): :: $d = \frac$ in metric units or $d = \frac$ in imperial units.ISO/DIS 21771:2007 : "Gears – Cylindrical Involute Gears and Gear Pairs – Concepts and Geometry", ''International Organization for Standardization'', (2007) ; Module or modulus, ''m'': Since it is impractical to calculate circular pitch with irrational numbers, mechanical engineers usually use a scaling factor that replaces it with a regular value instead. This is known as the ''module'' or ''modulus'' of the wheel and is simply defined as: ::$m = \frac$ : where m is the module and p the circular pitch. The Unit (measurement), units of module are customarily millimeters; an ''English Module'' is sometimes used with the units of inches. When the diametral pitch, DP, is in English units, :: $m = \frac$ in conventional metric units. : The distance between the two axis becomes: :: $a = \frac\left(z_1 + z_2\right)$ : where a is the axis distance, z1 and z2 are the number of cogs (teeth) for each of the two wheels (gears). These numbers (or at least one of them) is often chosen among primes to create an even contact between every cog of both wheels, and thereby avoid unnecessary wear and damage. An even uniform gear wear is achieved by ensuring the tooth counts of the two gears meshing together are relatively prime to each other; this occurs when the Greatest common divisor, greatest common divisor (GCD) of each gear tooth count equals 1, e.g. GCD(16,25)=1; if a 1:1 gear ratio is desired a relatively prime gear may be inserted in between the two gears; this maintains the 1:1 ratio but reverses the gear direction; a second relatively prime gear could also be inserted to restore the original rotational direction while maintaining uniform wear with all 4 gears in this case. Mechanical engineers, at least in continental Europe, usually use the module instead of circular pitch. The module, just like the circular pitch, can be used for all types of cogs, not just evolvent based straight cogs. ; Operating pitch diameters : Diameters determined from the number of teeth and the center distance at which gears operate. Example for pinion: :: $d_w = \frac = \frac.$ ; Pitch surface : In cylindrical gears, cylinder formed by projecting a pitch circle in the axial direction. More generally, the surface formed by the sum of all the pitch circles as one moves along the axis. For bevel gears it is a cone. ; Angle of action : Angle with vertex at the gear center, one leg on the point where mating teeth first make contact, the other leg on the point where they disengage. ; Arc of action : Segment of a pitch circle subtended by the angle of action. ; Pressure angle, $\theta$ : The complement of the angle between the direction that the teeth exert force on each other, and the line joining the centers of the two gears. For involute gears, the teeth always exert force along the line of action, which, for involute gears, is a straight line; and thus, for involute gears, the pressure angle is constant. ; Outside diameter, $D_o$ : Diameter of the gear, measured from the tops of the teeth. ; Root diameter : Diameter of the gear, measured at the base of the tooth. ; Addendum, ''a'' : Radial distance from the pitch surface to the outermost point of the tooth. $a = \frac\left(D_o - D\right)$ ; Dedendum, ''b'' : Radial distance from the depth of the tooth trough to the pitch surface. $b = \frac\left(D - \text\right)$ ; Whole depth, $h_t$ : The distance from the top of the tooth to the root; it is equal to addendum plus dedendum or to working depth plus clearance. ; Clearance : Distance between the root circle of a gear and the addendum circle of its mate. ; Working depth : Depth of engagement of two gears, that is, the sum of their operating addendums. ; Circular pitch, ''p'' : Distance from one face of a tooth to the corresponding face of an adjacent tooth on the same gear, measured along the pitch circle. ; Diametral pitch, ''DP'' : :: $DP = \frac = \frac$ : Ratio of the number of teeth to the pitch diameter. Could be measured in teeth per inch or teeth per centimeter, but conventionally has units of per inch of diameter. Where the module, m, is in metric units :: $DP = \frac$ in English units ; Base circle : In involute gears, the tooth profile is generated by the involute of the base circle. The radius of the base circle is somewhat smaller than that of the pitch circle ; Base pitch, normal pitch, $p_b$ : In involute gears, distance from one face of a tooth to the corresponding face of an adjacent tooth on the same gear, measured along the base circle ; Interference : Contact between teeth other than at the intended parts of their surfaces ; Interchangeable set : A set of gears, any of which mates properly with any other

## Helical gear

; Helix angle, $\psi$ : the Angle between a tangent to the helix and the gear axis. It is zero in the limiting case of a spur gear, albeit it can considered as the hypotenuse angle as well. ; Normal circular pitch, $p_n$ : Circular pitch in the plane normal to the teeth. ; Transverse circular pitch, ''p'' : Circular pitch in the plane of rotation of the gear. Sometimes just called "circular pitch". $p_n = p\cos\left(\psi\right)$ Several other helix parameters can be viewed either in the normal or transverse planes. The subscript n usually indicates the normal.

## Worm gear

; Lead : Distance from any point on a thread to the corresponding point on the next turn of the same thread, measured parallel to the axis. ; Linear pitch, p : Distance from any point on a thread to the corresponding point on the adjacent thread, measured parallel to the axis. For a single-thread worm, lead and linear pitch are the same. ; Lead angle, $\lambda$ : Angle between a tangent to the helix and a plane perpendicular to the axis. Note that the complement of the helix angle is usually given for helical gears. ; Pitch diameter, $d_w$ : Same as described earlier in this list. Note that for a worm it is still measured in a plane perpendicular to the gear axis, not a tilted plane. Subscript w denotes the worm, subscript g denotes the gear.

## Tooth contact

File:Contact line.jpg, Line of contact File:Action path.jpg, Path of action File:Action line.jpg, Line of action File:Action plane.jpg, Plane of action File:Contact lines.jpg, Lines of contact (helical gear) File:Action arc.jpg, Arc of action File:Action length.jpg, Length of action File:Limit diameter.jpg, Limit diameter File:Face advance.svg, Face advance File:Action zone.jpg, Zone of action ; Point of contact : Any point at which two tooth profiles touch each other. ; Line of contact : A line or curve along which two tooth surfaces are tangent to each other. ; Path of action : The locus of successive contact points between a pair of gear teeth, during the phase of engagement. For conjugate gear teeth, the path of action passes through the pitch point. It is the trace of the surface of action in the plane of rotation. ; Line of action : The path of action for involute gears. It is the straight line passing through the pitch point and tangent to both base circles. ; Surface of action : The imaginary surface in which contact occurs between two engaging tooth surfaces. It is the summation of the paths of action in all sections of the engaging teeth. ; Plane of action: The surface of action for involute, parallel axis gears with either spur or helical teeth. It is tangent to the base cylinders. ; Zone of action (contact zone) : For involute, parallel-axis gears with either spur or helical teeth, is the rectangular area in the plane of action bounded by the length of action and the effective face width. ; Path of contact: The curve on either tooth surface along which theoretical single point contact occurs during the engagement of gears with crowned tooth surfaces or gears that normally engage with only single point contact. ; Length of action: The distance on the line of action through which the point of contact moves during the action of the tooth profile. ; Arc of action, Qt : The arc of the pitch circle through which a tooth profile moves from the beginning to the end of contact with a mating profile. ; Arc of approach, Qa : The arc of the pitch circle through which a tooth profile moves from its beginning of contact until the point of contact arrives at the pitch point. ; Arc of recess, Qr : The arc of the pitch circle through which a tooth profile moves from contact at the pitch point until contact ends. ; Contact ratio, mc, ε : The number of angular pitches through which a tooth surface rotates from the beginning to the end of contact. In a simple way, it can be defined as a measure of the average number of teeth in contact during the period during which a tooth comes and goes out of contact with the mating gear. ; Transverse contact ratio, mp, εα : The contact ratio in a transverse plane. It is the ratio of the angle of action to the angular pitch. For involute gears it is most directly obtained as the ratio of the length of action to the base pitch. ; Face contact ratio, mF, εβ : The contact ratio in an axial plane, or the ratio of the face width to the axial pitch. For bevel and hypoid gears it is the ratio of face advance to circular pitch. ; Total contact ratio, mt, εγ : The sum of the transverse contact ratio and the face contact ratio. :: $\epsilon_\gamma = \epsilon_\alpha + \epsilon_\beta$ :: $m_ = m_ + m_$ ; Modified contact ratio, mo : For bevel gears, the square root of the sum of the squares of the transverse and face contact ratios. :: $m_ = \sqrt$ ; Limit diameter : Diameter on a gear at which the line of action intersects the maximum (or minimum for internal pinion) addendum circle of the mating gear. This is also referred to as the start of active profile, the start of contact, the end of contact, or the end of active profile. ; Start of active profile (SAP) : Intersection of the limit diameter and the involute profile. ; Face advance : Distance on a pitch circle through which a helical or spiral tooth moves from the position at which contact begins at one end of the tooth trace on the pitch surface to the position where contact ceases at the other end.

## Pitch

Pitch is the distance between a point on one tooth and the corresponding point on an adjacent tooth. It is a dimension measured along a line or curve in the transverse, normal, or axial directions. The use of the single word ''pitch'' without qualification may be ambiguous, and for this reason it is preferable to use specific designations such as transverse circular pitch, normal base pitch, axial pitch. File:Pitches.svg, Pitch File:Tooth pitches.jpg, Tooth pitch File:Base pitch.jpg, Base pitch relationships File:Principal pitches.jpg, Principal pitches ; Circular pitch, ''p'' : Arc distance along a specified pitch circle or pitch line between corresponding profiles of adjacent teeth. ; Transverse circular pitch, ''p''t : Circular pitch in the transverse plane. ; Normal circular pitch, ''p''n, ''p''e : Circular pitch in the normal plane, and also the length of the arc along the normal pitch helix between helical teeth or threads. ; Axial pitch, ''p''x : Linear pitch in an axial plane and in a pitch surface. In helical gears and worms, axial pitch has the same value at all diameters. In gearing of other types, axial pitch may be confined to the pitch surface and may be a circular measurement. The term axial pitch is preferred to the term linear pitch. The axial pitch of a helical worm and the circular pitch of its worm gear are the same. ; Normal base pitch, ''p''N, ''p''bn :An involute helical gear is the base pitch in the normal plane. It is the normal distance between parallel helical involute surfaces on the plane of action in the normal plane, or is the length of arc on the normal base helix. It is a constant distance in any helical involute gear. ; Transverse base pitch, ''p''b, ''p''bt : In an involute gear, the pitch is on the base circle or along the line of action. Corresponding sides of involute gear teeth are parallel curves, and the base pitch is the constant and fundamental distance between them along a common normal in a transverse plane. ; Diametral pitch (transverse), ''P''d : Ratio of the number of teeth to the standard pitch diameter in inches. :: $P_ = \frac = \frac = \frac$ ; Normal diametrical pitch, ''P''nd : Value of diametrical pitch in a normal plane of a helical gear or worm. :: $P_ = \frac$ ; Angular pitch, θN, τ : Angle subtended by the circular pitch, usually expressed in radians. :: $\tau = \frac$ degrees or $\frac$ radians

# Backlash

Backlash (gear), Backlash is the error in motion that occurs when gears change direction. It exists because there is always some gap between the trailing face of the driving tooth and the leading face of the tooth behind it on the driven gear, and that gap must be closed before force can be transferred in the new direction. The term "backlash" can also be used to refer to the size of the gap, not just the phenomenon it causes; thus, one could speak of a pair of gears as having, for example, "0.1 mm of backlash." A pair of gears could be designed to have zero backlash, but this would presuppose perfection in manufacturing, uniform thermal expansion characteristics throughout the system, and no lubricant. Therefore, gear pairs are designed to have some backlash. It is usually provided by reducing the tooth thickness of each gear by half the desired gap distance. In the case of a large gear and a small pinion, however, the backlash is usually taken entirely off the gear and the pinion is given full sized teeth. Backlash can also be provided by moving the gears further apart. The backlash of a
gear train A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage. Gear teeth are designed to ensure the Pitch circle diameter (gears), pitch circles of engaging gears roll on each other without slipping, pr ...

equals the sum of the backlash of each pair of gears, so in long trains backlash can become a problem. For situations that require precision, such as instrumentation and control, backlash can be minimized through one of several techniques. For instance, the gear can be split along a plane perpendicular to the axis, one half fixed to the shaft in the usual manner, the other half placed alongside it, free to rotate about the shaft, but with springs between the two-halves providing relative torque between them, so that one achieves, in effect, a single gear with expanding teeth. Another method involves tapering the teeth in the axial direction and letting the gear slide in the axial direction to take up slack.

# Shifting of gears

In some machines (e.g., automobiles) it is necessary to alter the gear ratio to suit the task, a process known as gear shifting or changing gear. There are several ways of shifting gears, for example: *Manual transmission *Automatic transmission *Derailleur gears, which are actually sprockets in combination with a roller chain *Hub gears (also called epicyclic gearing or sun-and-planet gears) There are several outcomes of gear shifting in motor vehicles. In the case of roadway noise, vehicle noise emissions, there are higher Sound intensity level, sound levels emitted when the vehicle is engaged in lower gears. The design life of the lower ratio gears is shorter, so cheaper gears may be used, which tend to generate more noise due to smaller overlap ratio and a lower mesh stiffness etc. than the helical gears used for the high ratios. This fact has been used to analyze vehicle-generated sound since the late 1960s, and has been incorporated into the simulation of urban roadway noise and corresponding design of urban noise barriers along roadways.

# Tooth profile

File:Tooth surface.jpg, Profile of a spur gear File:Undercuts.svg, Undercut A profile is one side of a tooth in a cross section between the outside circle and the root circle. Usually a profile is the curve of intersection of a tooth surface and a plane or surface normal to the pitch surface, such as the transverse, normal, or axial plane. The fillet curve (root fillet) is the concave portion of the tooth profile where it joins the bottom of the tooth space.#Notes, 2 As mentioned near the beginning of the article, the attainment of a nonfluctuating velocity ratio is dependent on the profile of the teeth. Friction and wear between two gears is also dependent on the tooth profile. There are a great many tooth profiles that provide constant velocity ratios. In many cases, given an arbitrary tooth shape, it is possible to develop a tooth profile for the mating gear that provides a constant velocity ratio. However, two constant velocity tooth profiles are the most commonly used in modern times: the ''cycloid gear, cycloid'' and the ''involute gear, involute''. The cycloid was more common until the late 1800s. Since then, the involute has largely superseded it, particularly in drive train applications. The cycloid is in some ways the more interesting and flexible shape; however the involute has two advantages: it is easier to manufacture, and it permits the center-to-center spacing of the gears to vary over some range without ruining the constancy of the velocity ratio. Cycloidal gears only work properly if the center spacing is exactly right. Cycloidal gears are still used in mechanical clocks. An Undercut (manufacturing), undercut is a condition in generated gear teeth when any part of the fillet curve lies inside of a line drawn tangent to the working profile at its point of juncture with the fillet. Undercut may be deliberately introduced to facilitate finishing operations. With undercut the fillet curve intersects the working profile. Without undercut the fillet curve and the working profile have a common tangent.

# Gear materials

Numerous nonferrous alloys, cast irons, powder-metallurgy and plastics can be used in the manufacture of gears. However, steels are most commonly used because of their high strength-to-weight ratio and low cost. Plastic is commonly used where cost or weight is a concern. A properly designed plastic gear can replace steel in many cases because it has many desirable properties, including dirt tolerance, low speed meshing, the ability to "skip" quite well and the ability to be made with materials that don't need additional lubrication. Manufacturers have used plastic gears to reduce costs in consumer items including copy machines, optical storage devices, cheap dynamos, consumer audio equipment, servo motors, and printers. Another advantage of the use of plastics, formerly (such as in the 1980s), was the reduction of repair costs for certain expensive machines. In cases of severe jamming (as of the paper in a printer), the plastic gear teeth would be torn free of their substrate, allowing the drive mechanism to then spin freely (instead of damaging itself by straining against the jam). This use of "sacrificial" gear teeth avoided destroying the much more expensive motor and related parts. This method has been superseded, in more recent designs, by the use of clutches and torque- or current-limited motors.

# Standard pitches and the module system

Although gears can be made with any pitch, for convenience and interchangeability standard pitches are frequently used. Pitch is a property associated with linear dimensions and so differs whether the standard values are in the Imperial units, imperial (inch) or metric system, metric systems. Using ''inch'' measurements, standard diametral pitch values with units of "per inch" are chosen; the ''diametrical pitch'' is the number of teeth on a gear of one inch pitch diameter. Common standard values for spur gears are 3, 4, 5, 6, 8, 10, 12, 16, 20, 24, 32, 48, 64, 72, 80, 96, 100, 120, and 200. Certain standard pitches such as ''1/10'' and ''1/20'' in inch measurements, which mesh with linear rack, are actually (linear) ''circular pitch'' values with units of "inches" When gear dimensions are in the metric system the pitch specification is generally in terms of module or ''modulus'', which is effectively a length measurement across the ''pitch diameter''. The term module is understood to mean the pitch diameter in millimetres divided by the number of teeth. When the module is based upon inch measurements, it is known as the ''English module'' to avoid confusion with the metric module. Module is a direct dimension, unlike diametrical pitch, which is an inverse dimension ("threads per inch"). Thus, if the pitch diameter of a gear is 40 mm and the number of teeth 20, the module is 2, which means that there are 2 mm of pitch diameter for each tooth. The preferred standard module values are 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1.0, 1.25, 1.5, 2.0, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 20, 25, 32, 40 and 50.

# Manufacture

As of 2014, an estimated 80% of all gearing produced worldwide is produced by net shape molding. Molded gearing is usually either powder metallurgy or plastic. Many gears are done when they leave the mold (including injection molded plastic and die cast metal gears), but powdered metal gears require sintering and sand castings or investment castings require gear cutting or other machining to finish them. The most common form of gear cutting is hobbing, but gear shaping, milling (machining), milling, and Broaching (metalworking), broaching also exist. 3D printing as a production method is expanding rapidly. For metal gears in the transmission (mechanics), transmissions of cars and trucks, the teeth are heat treating, heat treated to make them hard and more wear resistance, wear resistant while leaving the core soft and Toughness, tough. For large gears that are prone to warp, a quench press is used.

# Gear model in modern physics

Modern physics adopted the gear model in different ways. In the nineteenth century, James Clerk Maxwell developed a model of electromagnetism in which magnetic field lines were rotating tubes of incompressible fluid. Maxwell used a gear wheel and called it an "idle wheel" to explain the electric current as a rotation of particles in opposite directions to that of the rotating field lines. More recently, quantum physics uses "quantum gears" in their model. A group of gears can serve as a model for several different systems, such as an artificially constructed nanomechanical device or a group of ring molecules. The three wave hypothesis compares the wave–particle duality to a bevel gear.

# Gear mechanism in natural world

The gear mechanism was previously considered exclusively artificial, but as early as 1957, gears had been recognized in the hind legs of various species of planthoppers and scientists from the University of Cambridge characterized their functional significance in 2013 by doing high-speed photography of the nymphs of '' Issus coleoptratus'' at Cambridge University. These gears are found only in the nymph forms of all planthoppers, and are lost during the final molt to the adult stage. In ''I. coleoptratus'', each leg has a 400-micrometer strip of teeth, pitch radius 200 micrometers, with 10 to 12 fully interlocking spur-type gear teeth, including fillet (mechanics), filleted curves at the base of each tooth to reduce the risk of shearing. The joint rotates like mechanical gears, and synchronizes ''Issus's'' hind legs when it jumps to within 30 microseconds, preventing yaw rotation. The gears are not connected all the time. One is located on each of the juvenile insect's hind legs, and when it prepares to jump, the two sets of teeth lock together. As a result, the legs move in almost perfect unison, giving the insect more power as the gears rotate to their stopping point and then unlock.

* Gear box * Sprocket * Differential * Superposition principle * Kinematic chain

# References

Bibliography * * * *Industrial Press (2012), Machinery's Handbook (29th ed.), *

* * * * Kravchenko A.I., Bovda A.M. Gear with magnetic couple. Pat. of Ukraine N. 56700 – Bul. N. 2, 2011 – F16H 49/00. * Sclater, Neil. (2011). "Gears: devices, drives and mechanisms." ''Mechanisms and Mechanical Devices Sourcebook.'' 5th ed. New York: McGraw Hill. pp. 131–174. . Drawings and designs of various gearings.
"Wheels That Can't Slip."
''Popular Science'', February 1945, pp. 120–125.