A gear or cogwheel is a rotating machine part having cut teeth, or in the case of a cogwheel, inserted teeth (called cogs), which mesh with another toothed part to transmit torque. Geared devices can change the speed, torque, and direction of a power source. Gears almost always produce a change in torque, creating a mechanical advantage, through their gear ratio, and thus may be considered a simple machine. The teeth on the two meshing gears all have the same shape.[1] Two or more meshing gears, working in a sequence, are called a gear train or a transmission. A gear can mesh with a linear toothed part, called a rack, 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, 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.

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.[2]

Gears animation
Two meshing gears transmitting rotational motion. Note that the smaller gear is rotating faster. Since the larger gear is rotating less quickly, its torque is proportionally greater. One subtlety of this particular arrangement is that the linear speed at the pitch diameter is the same on both gears.
Multiple reducer gears
Multiple reducer gears in microwave oven (ruler for scale)


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.[3] Gears appear in works connected to Hero of Alexandria, in Roman Egypt circa AD 50,[4] 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).[5]

The segmental gear, which receives/communicates reciprocating motion from/to a cogwheel, consisting of a sector of a circular gear/ring having cogs on the periphery,[6] was invented by Arab engineer Al-Jazari in 1206.[7] The worm gear was invented in the Indian subcontinent, for use in roller cotton gins, some time during the 13th–14th centuries.[8] Differential gears may have been used in some of the Chinese south-pointing chariots,[9] but the first verifiable use of differential gears was by the British clock maker Joseph Williamson in 1720.

Gear reducer
Single-stage gear reducer

Examples of early gear applications include:

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.


External vs internal gears

Inside gear
Internal gear

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 gears, an internal gear is one with the pitch angle exceeding 90 degrees. Internal gears do not cause output shaft direction reversal.[10]


Spur Gear 12mm, 18t
Spur gear

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 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.[11] No axial thrust is created by the tooth loads. Spur gears are excellent at moderate speeds but tend to be noisy at high speeds.[12]


Anim engrenages helicoidaux
An external contact helical gear in action
Helical Gears
Helical gears
Top: parallel configuration
Bottom: crossed configuration

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. 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.[13] 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.[14] The speed is considered high when the pitch line velocity exceeds 25 m/s.[15]

A disadvantage of helical gears is a resultant thrust along the axis of the gear, which must be accommodated by appropriate thrust bearings, and 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:[16]

for gears of the same handedness,
for gears of opposite handedness,

where 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.[16]

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.)

Double helical

Herringbone gears (Bentley, Sketches of Engine and Machine Details)
Herringbone gears

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 together so that their teeths form a V shape. This can also be applied to bevel gears, 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 conversely.

Stable double helical gears can be directly interchanged with spur gears without any need for different bearings.


Engranaje cónico, Nymphenburg, Múnich, Alemania4
Bevel gear

A bevel gear is shaped like a right circular cone with most of its tip cut off. 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 gears.

Spiral bevels

Spiral bevel gears

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.[17]

Note: 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. IMPORTANT: 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 [Diss. Hünecke, TU Dresden]. Furthermore, the "involute bevel gear sets" cause more noise.


Hypoid gear

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 hyperboloids of revolution.[18][19] 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.[20] 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 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 such as found in mechanical clocks.


Worm Gear and Pinion
Worm gear
Worm Gear
4-start worm and wheel

Worms resemble screws. 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.[21] A disadvantage is the potential for considerable sliding action, leading to low efficiency.[22]

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 boxes, 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 found on some types of stringed instruments.

If the gear in a worm-and-gear set is an ordinary helical gear only a single point of contact is achieved.[20][23] 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; this is called a cone-drive[24] or "Double enveloping".

Worm gears can be right or left-handed, following the long-established practice for screw threads.[10]


Non-circular gear
Non-circular gears

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, a non-circular gear's main objective might be ratio variations, axle displacement oscillations and more. Common applications include textile machines, potentiometers and continuously variable transmissions.

Rack and pinion

Rack and pinion animation
Rack and pinion gearing

A rack is a toothed bar or rod that can be thought of as a sector gear with an infinitely large radius of curvature. Torque can be converted to linear force by meshing a rack with a pinion: the pinion turns; the rack moves in a straight line. Such a mechanism is used in automobiles to convert the rotation of the steering wheel into the left-to-right motion of the tie rod(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 employed in a rack railway.


Epicyclic gear ratios
Epicyclic gearing

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

Sun and planet

Sun and planet gears
Sun (yellow) and planet (red) gearing

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, 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

Harmonic drive animation
Harmonic gearing

A harmonic gear or 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.

Cage gear

Cage Gear
Cage gear in Pantigo Windmill, Long Island (with the driving gearwheel disengaged)

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 capability similar to other mechanical gearings. Although they cannot exert as much force as a traditional gear, such gears work without touching and so are immune to wear, have very low noise and can slip without damage making them very reliable.[25] 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 hermetically sealed enclosure without using a radial shaft seal, which may leak.


General nomenclature

Gear words

Gear words
Rotational frequency, n 
Measured in rotation over time, such as revolutions per minute (RPM or rpm).
Angular frequency, ω 
Measured in radians/second. 1 RPM = 2π rad/minute = π/30 rad/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.
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 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 basic dimension and cannot be measured, but is a location where other measurements are made. Its value is based on the number of teeth, the normal module (or normal diametral pitch), and the helix angle. It is calculated as:
in metric units or in imperial units.[26]
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
where m is the module and p the circular pitch. The 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,
in conventional metric units.
The distance between the two axis becomes
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 (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.[27]
Operating pitch diameters 
Diameters determined from the number of teeth and the center distance at which gears operate.[10] Example for pinion:
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,  
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,  
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.
Dedendum, b 
Radial distance from the depth of the tooth trough to the pitch surface.
Whole depth,  
The distance from the top of the tooth to the root; it is equal to addendum plus dedendum or to working depth plus 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 
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
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,  
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
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 nomenclature

Helix angle,  
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,  
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".

Several other helix parameters can be viewed either in the normal or transverse planes. The subscript n usually indicates the normal.

Worm gear nomenclature

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,  
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,  
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 nomenclature

Contact line

Line of contact

Action path

Path of action

Action line

Line of action

Action plane

Plane of action

Contact lines

Lines of contact (helical gear)

Action arc

Arc of action

Action length

Length of action

Limit diameter

Limit diameter

Face advance

Face advance

Action zone

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.
Modified contact ratio, mo 
For bevel gears, the square root of the sum of the squares of the transverse and face contact ratios.
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.

Tooth thickness nomenclature

Tooth thickness

Tooth thickness

Thickness relationships

Thickness relationships

Chordial thickness

Chordal thickness

Pin measurement

Tooth thickness measurement over pins

Span measurement

Span measurement

Addendum teeth

Long and short addendum teeth

Circular thickness 
Length of arc between the two sides of a gear tooth, on the specified datum circle.
Transverse circular thickness 
Circular thickness in the transverse plane.
Normal circular thickness 
Circular thickness in the normal plane. In a helical gear it may be considered as the length of arc along a normal helix.
Axial thickness
In helical gears and worms, tooth thickness in an axial cross section at the standard pitch diameter.
Base circular thickness
In involute teeth, length of arc on the base circle between the two involute curves forming the profile of a tooth.
Normal chordal thickness
Length of the chord that subtends a circular thickness arc in the plane normal to the pitch helix. Any convenient measuring diameter may be selected, not necessarily the standard pitch diameter.
Chordal addendum (chordal height) 
Height from the top of the tooth to the chord subtending the circular thickness arc. Any convenient measuring diameter may be selected, not necessarily the standard pitch diameter.
Profile shift 
Displacement of the basic rack datum line from the reference cylinder, made non-dimensional by dividing by the normal module. It is used to specify the tooth thickness, often for zero backlash.
Rack shift 
Displacement of the tool datum line from the reference cylinder, made non-dimensional by dividing by the normal module. It is used to specify the tooth thickness.
Measurement over pins 
Measurement of the distance taken over a pin positioned in a tooth space and a reference surface. The reference surface may be the reference axis of the gear, a datum surface or either one or two pins positioned in the tooth space or spaces opposite the first. This measurement is used to determine tooth thickness.
Span measurement 
Measurement of the distance across several teeth in a normal plane. As long as the measuring device has parallel measuring surfaces that contact on an unmodified portion of the involute, the measurement wis along a line tangent to the base cylinder. It is used to determine tooth thickness.
Modified addendum teeth 
Teeth of engaging gears, one or both of which have non-standard addendum.
Full-depth teeth 
Teeth in which the working depth equals 2.000 divided by the normal diametral pitch.
Stub teeth 
Teeth in which the working depth is less than 2.000 divided by the normal diametral pitch.
Equal addendum teeth 
Teeth in which two engaging gears have equal addendums.
Long and short-addendum teeth 
Teeth in which the addendums of two engaging gears are unequal.

Pitch nomenclature

Pitch is the distance between a point on one tooth and the corresponding point on an adjacent tooth.[10] 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.



Tooth pitches

Tooth pitch

Base pitch

Base pitch relationships

Principal pitches

Principal pitches

Circular pitch, p 
Arc distance along a specified pitch circle or pitch line between corresponding profiles of adjacent teeth.
Transverse circular pitch, pt 
Circular pitch in the transverse plane.
Normal circular pitch, pn, pe 
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, px 
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, pN, pbn
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, pb, pbt 
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), Pd 
Ratio of the number of teeth to the standard pitch diameter in inches.
Normal diametral pitch, Pnd 
Value of diametral pitch in a normal plane of a helical gear or worm.
Angular pitch, θN, τ 
Angle subtended by the circular pitch, usually expressed in radians.
degrees or radians


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 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 minimised 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:

There are several outcomes of gear shifting in motor vehicles. In the case of vehicle noise emissions, there are higher 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.[28]

Tooth profile

Tooth surface

Profile of a spur gear



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.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 and the 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 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

Cogwheel in Malbork
Wooden gears of a historic windmill

Numerous nonferrous alloys, cast irons, powder-metallurgy and plastics are 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[29] 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 (inch) or metric systems. Using inch measurements, standard diametral pitch values with units of "per inch" are chosen; the diametral 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.[30] 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"[30]

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 millimeters 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 diametral 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.[31] 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.[32]


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.[33] 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, and broaching also exist. 3D printing as a production method is expanding rapidly. For metal gears in the transmissions of cars and trucks, the teeth are heat treated to make them hard and more wear resistant while leaving the core soft and 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.[34]

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.[35]

The three wave hypothesis compares the wave–particle duality to a bevel gear.[36]

Gear mechanism in natural world

Issus coleoptratus
Interactive gears in the hind legs of Issus coleoptratus from Cambridge gears-3
A functioning gear mechanism discovered in Issus coleoptratus, a planthopper species common in Europe

The gear mechanism was previously considered exclusively artificial, but in 2013, scientists from the University of Cambridge announced their discovery that the juvenile form of a common insect Issus (species Issus coleoptratus), found in many European gardens, has a gear-like mechanism in its hind legs. Each leg has a 400 micrometer strip, pitch radius 200 micrometers, with 12 fully interlocking spur-type gear teeth, including filleted curves at the base of each tooth to reduce the risk of shearing. The joint rotates like mechanical gears and synchronizes Issus's legs when it jumps to within 30 microseconds, preventing yaw rotation.[37][38][39][40]

See also


  1. ^ "Definition of GEAR". Retrieved 20 September 2018.
  2. ^ "Transmission Basics". HowStuffWorks.
  3. ^ "The Antikythera Mechanism Research Project: Why is it so important?". Retrieved 2011-01-10. The Mechanism is thought to date from between 150 and 100 BC
  4. ^ Norton 2004, p. 462
  5. ^ Lewis, M. J. T. (1993). "Gearing in the Ancient World". Endeavour. 17 (3): 110–115. doi:10.1016/0160-9327(93)90099-O.
  6. ^ "Segment gear". Retrieved 20 September 2018.
  7. ^ a b Donald Hill (2012), The Book of Knowledge of Ingenious Mechanical Devices, page 273, Springer Science + Business Media
  8. ^ a b Irfan Habib, Economic History of Medieval India, 1200-1500, page 53, Pearson Education
  9. ^ Joseph Needham (1986). Science and Civilization in China: Volume 4, Part 2, page 298. Taipei: Caves Books, Ltd.
  10. ^ a b c d American Gear Manufacturers Association; American National Standards Institute, Gear Nomenclature, Definitions of Terms with Symbols (ANSI/AGMA 1012-G05 ed.), American Gear Manufacturers Association
  11. ^ "How Gears Work". 16 November 2000. Retrieved 20 September 2018.
  12. ^ Machinery's Handbook. New York: Industrial Press. 2012. p. 2125. ISBN 978-0-8311-2900-2.
  13. ^ Khurmi, R. S., Theory of Machines, S.CHAND
  14. ^ Schunck, Richard, "Minimizing gearbox noise inside and outside the box", Motion System Design.
  15. ^ Vallance & Doughtie 1964, p. 281
  16. ^ a b Helical gears, archived from the original on 26 June 2009, retrieved 15 June 2009.
  17. ^ McGraw-Hill 2007, p. 742.
  18. ^ Canfield, Stephen (1997), "Gear Types", Dynamics of Machinery, Tennessee Tech University, Department of Mechanical Engineering, ME 362 lecture notes, archived from the original on 29 August 2008.
  19. ^ Hilbert, David; Cohn-Vossen, Stephan (1952), Geometry and the Imagination (2nd ed.), New York: Chelsea, p. 287, ISBN 978-0-8284-1087-8.
  20. ^ a b McGraw-Hill 2007, p. 743.
  21. ^ Vallance & Doughtie 1964, p. 287.
  22. ^ Vallance & Doughtie 1964, pp. 280, 296.
  23. ^ Vallance & Doughtie 1964, p. 290.
  24. ^ McGraw-Hill 2007, p. 744
  25. ^ Kravchenko A.I., Bovda A.M. Gear with magnetic couple. Pat. of Ukraine N. 56700 – Bul. N. 2, 2011 – F16H 49/00.
  26. ^ ISO/DIS 21771:2007 : "Gears – Cylindrical Involute Gears and Gear Pairs – Concepts and Geometry", International Organization for Standardization, (2007)
  27. ^ Gunnar Dahlvig, "Construction elements and machine construction", Konstruktionselement och maskinbyggnad (in Swedish), 7, ISBN 978-9140115546
  28. ^ Hogan, C. Michael; Latshaw, Gary L. (21–23 May 1973). The Relationship Between Highway Planning and Urban Noise. Proceedings of the ASCE, Urban Transportation Division Specialty Conference. Chicago, Illinois: American Society of Civil Engineers, Urban Transportation Division.
  29. ^ Smith, Zan (2000), "Plastic gears are more reliable when engineers account for material properties and manufacturing processes during design.", Motion System Design.
  30. ^ a b "W. M. Berg Gear Reference Guide" (PDF). Archived from the original (PDF) on 21 April 2015.
  31. ^ Oberg, E.; Jones, F. D.; Horton, H. L.; Ryffell, H. H. (2000), Machinery's Handbook (26th ed.), Industrial Press, p. 2649, ISBN 978-0-8311-2666-7.
  32. ^ "Elements of metric gear technology" (PDF).
  33. ^ Fred Eberle (August 2014). "Materials Matter". Gear Solutions: 22.
  34. ^ Siegel, Daniel M. (1991). Innovation in Maxwell's Electromagnetic Theory: Molecular Vortices, Displacement Current, and Light. University of Chicago Press. ISBN 978-0521353656.
  35. ^ MacKinnon, Angus (2002). "Quantum Gears: A Simple Mechanical System in the Quantum Regime". Nanotechnology. 13 (5): 678–681. arXiv:cond-mat/0205647. Bibcode:2002Nanot..13..678M. doi:10.1088/0957-4484/13/5/328.
  36. ^ Sanduk, M. I. (2007). "Does the Three Wave Hypothesis Imply Hidden Structure?" (PDF). Apeiron. 14 (2): 113–125. Bibcode:2007Apei...14..113S.
  37. ^ Robertson, Adi (September 12, 2013). "The first-ever naturally occurring gears are found on an insect's legs". The Verge. Retrieved September 14, 2013.
  38. ^ Functioning 'mechanical gears' seen in nature for the first time, Cambridge University, 2013.
  39. ^ Functioning 'mechanical gears' seen in nature for the first time
  40. ^ Burrows, Malcolm; Sutton, Gregory (13 September 2013). "Interacting Gears Synchronize Propulsive Leg Movements in a Jumping Insect". Science. 341 (6151): 1254–1256. doi:10.1126/science.1240284. hdl:1983/69cf1502-217a-4dca-a0d3-f8b247794e92. PMID 24031019.


Further reading

  • American Gear Manufacturers Association; American National Standards Institute (2005), Gear Nomenclature: Definitions of Terms with Symbols (ANSI/AGMA 1012-F90 ed.), American Gear Manufacturers Association, ISBN 978-1-55589-846-5.
  • Buckingham, Earle (1949), Analytical Mechanics of Gears, McGraw-Hill Book Co..
  • Coy, John J.; Townsend, Dennis P.; Zaretsky, Erwin V. (1985), Gearing (PDF), NASA Scientific and Technical Information Branch, NASA-RP-1152; AVSCOM Technical Report 84-C-15.
  • 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. ISBN 9780071704427. Drawings and designs of various gearings.

External links

Automatic transmission

An automatic transmission, also called auto, self-shifting transmission, n-speed automatic (where n is its number of forward gear ratios), or AT, is a type of motor vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the driver from having to shift gears manually. Like other transmission systems on vehicles, it allows an internal combustion engine, best suited to run at a relatively high rotational speed, to provide a range of speed and torque outputs necessary for vehicular travel. The number of forward gear ratios is often expressed for manual transmissions as well (e.g., 6-speed manual).

The most popular form found in automobiles is the hydraulic automatic transmission. Similar but larger devices are also used for heavy-duty commercial and industrial vehicles and equipment. This system uses a fluid coupling in place of a friction clutch, and accomplishes gear changes by hydraulically locking and unlocking a system of planetary gears. These systems have a defined set of gear ranges, often with a parking pawl that locks the output shaft of the transmission to keep the vehicle from rolling either forward or backward. Some machines with limited speed ranges or fixed engine speeds, such as some forklifts and lawn mowers, only use a torque converter to provide a variable gearing of the engine to the wheels.

Besides the traditional hydraulic automatic transmissions, there are also other types of automated transmissions, such as a continuously variable transmission (CVT) and semi-automatic transmissions, that free the driver from having to shift gears manually, by using the transmission's computer to change gear, if for example the driver were redlining the engine. Despite superficial similarity to other transmissions, traditional automatic transmissions differ significantly in internal operation and driver's feel from semi-automatics and CVTs. In contrast to conventional automatic transmissions, a CVT uses a belt or other torque transmission scheme to allow an "infinite" number of gear ratios instead of a fixed number of gear ratios. A semi-automatic retains a clutch like a manual transmission, but controls the clutch through electrohydraulic means. The ability to shift gears manually, often via paddle shifters, can also be found on certain automated transmissions (manumatics such as Tiptronic), semi-automatics (BMW SMG, VW Group DSG), and CVTs (such as Lineartronic).

The obvious advantage of an automatic transmission to the driver is the lack of a clutch pedal and manual shift pattern in normal driving. This allows the driver to operate the car with as few as two limbs (possibly using assist devices to position controls within reach of usable limbs), allowing individuals with disabilities to drive. The lack of manual shifting also reduces the attention and workload required inside the cabin, such as monitoring the tachometer and taking a hand off the wheel to move the shifter, allowing the driver to ideally keep both hands on the wheel at all times and to focus more on the road. Control of the car at low speeds is often easier with an automatic than a manual, due to a side effect of the clutchless fluid-coupling design called "creep" that causes the car to want to move while in a driving gear, even at idle. The primary disadvantage of the most popular hydraulic designs is reduced mechanical efficiency of the power transfer between engine and drivetrain, due to the fluid coupling connecting the engine to the gearbox. This can result in lower power/torque ratings for automatics compared to manuals with the same engine specs, as well as reduced fuel efficiency in city driving as the engine must maintain idle against the resistance of the fluid coupling. Advances in transmission and coupler design have narrowed this gap considerably, but clutch-based transmissions (manual or semi-automatic) are still preferred in sport-tuned trim levels of various production cars, as well as in many auto racing leagues.

The automatic transmission was invented in 1921 by Alfred Horner Munro of Regina, Saskatchewan, Canada, and patented under Canadian patent CA 235757 in 1923. (Munro obtained UK patent GB215669 215,669 for his invention in 1924 and US patent 1,613,525 on 4 January 1927). Being a steam engineer, Munro designed his device to use compressed air rather than hydraulic fluid, and so it lacked power and never found commercial application. The first automatic transmission using hydraulic fluid may have been developed in 1932 by two Brazilian engineers, José Braz Araripe and Fernando Lehly Lemos; subsequently the prototype and plans were sold to General Motors who introduced it in the 1940 Oldsmobile as the "Hydra-Matic" transmission. They were incorporated into GM-built tanks during World War II and, after the war, GM marketed them as being "battle-tested." However, a Wall Street Journal article credits ZF Friedrichshafen with the invention, occurring shortly after World War I. ZF's origins were in manufacturing gears for airship engines beginning in 1915; the company was founded by Ferdinand von Zeppelin.

Differential (mechanical device)

A differential is a gear train with three shafts that has the property that the rotational speed of one shaft is the average of the speeds of the others, or a fixed multiple of that average.

Game Gear

The Game Gear is an 8-bit fourth generation handheld game console released by Sega on October 6, 1990 in Japan, in April 1991 throughout North America and Europe, and during 1992 in Australia. The Game Gear primarily competed with Nintendo's Game Boy, the Atari Lynx, and NEC's TurboExpress. It shares much of its hardware with the Master System, and can play Master System games by the use of an adapter. Sega positioned the Game Gear, which had a full-color backlit screen with a landscape format, as a technologically superior handheld to the Game Boy.

Though the Game Gear was rushed to market, its unique game library and price point gave it an edge over the Atari Lynx and TurboExpress. However, due its short battery life, lack of original games, and weak support from Sega, the Game Gear was unable to surpass the Game Boy, selling 10.62 million units by March 1996. The Game Gear was succeeded by the Genesis Nomad in 1995 and discontinued in 1997. It was re-released as a budget system by Majesco Entertainment in 2000, under license from Sega.

Reception of the Game Gear was mixed, with praise for its full-color backlit screen and processing power for its time, criticisms over its large size and short battery life, and questions over the quality of its game library.

Hideo Kojima

Hideo Kojima (小島 秀夫, Kojima Hideo, born August 24, 1963) is a Japanese video game designer, screenwriter, director and game producer.

Regarded as an auteur of video games, during his childhood and adolescence he developed a strong passion for action/adventure cinema and literature. He was hired by Konami in 1986 for which he designed and wrote, in 1987, Metal Gear for MSX platform, a title that laid the foundations for stealth games and his best known and most appreciated series. The title that consecrated him as one of the most acclaimed video game designers is Metal Gear Solid, released in 1998 for PlayStation. Other notable video games he directed are visual novels Snatcher, released in 1988, and Policenauts, released in 1994.

In 2005, Kojima founded Kojima Productions, a software house controlled by Konami, and by 2011 he rose as vice president of Konami Digital Entertainment.In 2015, Kojima Productions split from Konami, becoming an independent software company. Kojima announced a collaboration with Sony Interactive Entertainment for a new action game, Death Stranding, which is currently in development for PlayStation 4. From 2017 to 2018, he also edited a column for Rolling Stone dedicated to cinema, video games and analysis of the differences and similarities between the two mediums.

James May

James Daniel May (born 16 January 1963) is an English television presenter and journalist. He is best known as a co-presenter of the motoring programme Top Gear alongside Jeremy Clarkson and Richard Hammond from 2003 until 2015. As of 2016 he is a director of the production company W. Chump & Sons (founded July 2015) and is also a co-presenter in the television series The Grand Tour for Amazon Video, alongside his former Top Gear colleagues, Clarkson and Hammond, as well as Top Gear's former producer Andy Wilman.

May has presented other programmes on themes including science and technology, toys, wine culture, and the plight of manliness in modern times. He wrote a weekly column for The Daily Telegraph's motoring section from 2003 to 2011.

Jeremy Clarkson

Jeremy Charles Robert Clarkson (born 11 April 1960) is an English broadcaster, journalist and writer who specialises in motoring. He is best known for co-presenting the BBC TV show Top Gear with Richard Hammond and James May from October 2002 to March 2015. He also currently writes weekly columns for The Sunday Times and The Sun.

From a career as a local journalist in Northern England, Clarkson rose to public prominence as a presenter of the original format of Top Gear in 1988. Since the mid-1990s, he has become a recognised public personality, regularly appearing on British television presenting his own shows for BBC and appearing as a guest on other shows. As well as motoring, Clarkson has produced programmes and books on subjects such as history and engineering. In 1998, he hosted the first series of Robot Wars, and from 1998 to 2000 he also hosted his own talk show, Clarkson.

In 2015, the BBC decided not to renew Clarkson's contract with the company after a dispute with a Top Gear producer while filming on location. That year, Clarkson and his Top Gear co-presenters and producer Andy Wilman formed the production company W. Chump & Sons to produce The Grand Tour for Amazon Video. In 2018, he became the new host of Who Wants to Be a Millionaire? for ITV.

His opinionated but humorous tongue-in-cheek writing and presenting style has often provoked a public reaction. His actions, both privately and as a Top Gear presenter have also sometimes resulted in criticism from the media, politicians, pressure groups and the public. He also has a significant public following, being credited as a major factor in the resurgence of Top Gear as one of the most popular shows on the BBC.

Landing gear

Landing gear is the undercarriage of an aircraft or spacecraft and may be used for either takeoff or landing. For aircraft it is generally both. It was also formerly called alighting gear by some manufacturers, such as the Glenn L. Martin Company.

For aircraft, the landing gear supports the craft when it is not flying, allowing it to take off, land, and taxi without damage. Wheels are typically used but skids, skis, floats or a combination of these and other elements can be deployed depending both on the surface and on whether the craft only operates vertically (VTOL) or is able to taxi along the surface. Faster aircraft usually have retractable undercarriages, which fold away during flight to reduce air resistance or drag.

For launch vehicles and spacecraft landers, the landing gear is typically designed to support the vehicle only post-flight, and are typically not used for takeoff or surface movement.

List of Metal Gear characters

The Metal Gear franchise features a large number of characters created by Hideo Kojima and designed by Yoji Shinkawa. Its setting features several soldiers with supernatural powers provided by the new advancements of science.

The series follows mercenary Solid Snake given government missions of finding the Metal Gear weapon, resulting in encounters with Gray Fox and Big Boss in Outer Heaven (Metal Gear) and Zanzibar Land (Metal Gear 2: Solid Snake). Later, Solid Snake meets Otacon and opposes Liquid Snake's FOXHOUND in Metal Gear Solid then assists Raiden in fighting both Solidus Snake and the Patriots in Metal Gear Solid 2: Sons of Liberty. Additionally, there are several prequel games that follow Big Boss's past and legend development as well as the origins of FOXHOUND, Outer Heaven and the Patriots.

While the original Metal Gear games had their characters designs modeled after Hollywood actors, the Metal Gear Solid games established a series of consistent designs based on Shinkawa's ideas of what would appeal to gamers. Additionally, several of the characters he designs follow Kojima and the other staff's ideas. Critical reception of the game's cast has been positive as publications praised their personalities and roles within the series.

List of Top Gear episodes

Top Gear is a British television series that focuses on various motor vehicles, primarily cars, in which its hosts conduct reviews on new models, vintage classics, as well as tackling various motoring related challenges, and inviting celebrities to set a time on their specially designed race-course. The programme is a relaunched version of the original 1977 show of the same name.

For its first series, the show was presented by Jeremy Clarkson, Richard Hammond, and Jason Dawe, with support from anonymous race driver, The Stig. After the first series, Dawe was replaced by James May. After the twenty-second series, the line-up was changed after the departure of Clarkson, Hammond and May, in which Chris Evans and Matt LeBlanc took over as the main hosts, with a team of co-presenters consisting of Chris Harris, Rory Reid, Eddie Jordan and Sabine Schmitz. After the twenty-third series, Evans departed from the show, leading to LeBlanc being joined by Harris and Reid as the main hosts, with occasional appearances from Jordan and Schmitz. LeBlanc is set to depart the show following the twenty-sixth series in 2019.The following is a list of episodes, listed in order of their original UK air date along with featured cars, challenges, and guests. For more information on features and challenges included in each series, visit each series' respective page. Comprehensive lists of challenges and races can be found at Top Gear challenges and Top Gear Races. The list does not include the following episodes: two shorter episodes produced for charity - Top Gear of the Pops, produced for Red Nose Day, and Top Ground Gear Force, produced for Sport Relief; and an Ashes to Ashes parody for BBC Children in Need.

As of 1 April 2018, 197 episodes of Top Gear have aired.

Manual transmission

A manual transmission, also known as a manual gearbox, a standard transmission or colloquially in some countries (e.g. the United States) as a stick shift, is a type of transmission used in motor vehicle applications. It uses a driver-operated clutch, usually engaged and disengaged by a foot pedal or hand lever, for regulating torque transfer from the engine to the transmission; and a gear selector that can be operated by hand or foot.

A conventional 5-speed manual transmission is often the standard equipment in a base-model vehicle, while more expensive manual vehicles are usually equipped with a 6-speed transmission instead; other options include automatic transmissions such as a traditional automatic (hydraulic planetary) transmission (often a manumatic), a semi-automatic transmission, or a continuously variable transmission (CVT). The number of forward gear ratios is often expressed for automatic transmissions as well (e.g., 9-speed automatic).

Matt LeBlanc

Matthew Steven LeBlanc (; born July 25, 1967) is an American actor, comedian and television host. He received international recognition for his portrayal of dim-witted, yet well-intentioned womaniser Joey Tribbiani on the NBC sitcom Friends, which ran from 1994 to 2004. For his work on Friends, LeBlanc received three Emmy Award nominations. LeBlanc has also starred as a fictionalized version of himself in the BBC/Showtime comedy series Episodes (2011–2017), for which he won a Golden Globe Award and received four additional Emmy Award nominations. Since 2016, LeBlanc has hosted the BBC series Top Gear. He has played Adam Burns in the CBS sitcom Man with a Plan since 2016.

Metal Gear

Metal Gear (Japanese: メタルギア, Hepburn: Metaru Gia) is a series of action-adventure stealth video games, created by Hideo Kojima and developed and published by Konami. The first game, Metal Gear, was released in 1987 for MSX home computers. The player often takes control of a special forces operative (usually either Solid Snake or Big Boss), who is assigned to find the titular superweapon "Metal Gear", a bipedal walking tank with the ability to launch nuclear weapons. Several sequels have been released for multiple consoles, which have expanded the original game's plot adding characters opposing and supporting Snake, while there have also been a few prequels exploring the origins of the Metal Gear and recurring characters.

The series is credited for pioneering and popularizing stealth video games and cinematic video games. Notable traits of the series include stealth mechanics, cinematic cutscenes, intricate storylines, offbeat and fourth wall humour, and exploration of cyberpunk, dystopian, political and philosophical themes, with references to Hollywood films to add flavor. As of March 2018, over 53.8 million copies of the game franchise have been sold worldwide, with individual installments having been critically and commercially acclaimed and having received several awards. The franchise has also been adapted into other media such as comics, novels, and drama CDs. Solid Snake also appeared as a guest character in Super Smash Bros. Brawl and Super Smash Bros. Ultimate.

Metal Gear Solid

Metal Gear Solid is an action-adventure stealth video game developed by Konami Computer Entertainment Japan and released for the PlayStation in 1998. The game was directed, produced, and written by Hideo Kojima, and serves as a sequel to the MSX2 video games Metal Gear and Metal Gear 2: Solid Snake, which Kojima also worked on. The game started development in 1996 and was officially unveiled in the Electronic Entertainment Expo in 1997, before eventually releasing in late 1998.The game follows Solid Snake, a soldier who infiltrates a nuclear weapons facility to neutralize the terrorist threat from FOXHOUND, a renegade special forces unit. Snake must liberate two hostages, the head of DARPA and the president of a major arms manufacturer, confront the terrorists, and stop them from launching a nuclear strike. Cinematic cutscenes were rendered using the in-game engine and graphics, and voice acting was used throughout the entire game.Metal Gear Solid was well received, shipping more than six million copies, along with 12 million demos, and scoring an average of 94/100 on the aggregate website Metacritic. It is regarded as one of the greatest and most important video games and helped popularize the stealth genre. Its success prompted the release of an expanded version for the PlayStation and PC, Metal Gear Solid: Integral, and a GameCube remake, Metal Gear Solid: The Twin Snakes. The game has also spawned numerous sequels, prequels, and spin-offs, including several games, a radio drama, comics, and novels.

Richard Hammond

Richard Mark Hammond (born 19 December 1969) is an English presenter, writer, and journalist, best known for co-hosting the BBC Two car programme Top Gear from 2002 until 2015 with Jeremy Clarkson and James May.

He has also presented Brainiac: Science Abuse (2003–2008), Total Wipeout (2009–2012) and Planet Earth Live (2012).

In 2016, Hammond began presenting The Grand Tour television series, produced by W. Chump & Sons. The show is co-presented with his former Top Gear co-hosts, Clarkson and May, as an exclusive distributed via Amazon Video to Amazon Prime customers.

In November 2016, Hammond, alongside the co-presenters of The Grand Tour, Jeremy Clarkson and James May, launched the automotive social media website DriveTribe, where he regularly provides content on his tribe "Hammond's Fob Jockeys".

The Grand Tour

The Grand Tour is a British motoring television series, conceived by Jeremy Clarkson, Richard Hammond, James May, and Andy Wilman, produced by Amazon Studios, launched on 18 November 2016, and made exclusively for streaming from Amazon Prime Video. The programme's format is similar to that of the BBC series Top Gear: each episode is hosted by Clarkson, Hammond and May, features a mixture of live-audience segments and pre-recorded films, and focuses on reviews of cars, discussions on motoring topics, celebrity timed laps (only for the second series), races and special motoring challenges.

The programme was conceived when Clarkson was dismissed from Top Gear, as a result of a disciplinary investigation by the BBC of his behaviour during and behind-the-scenes of the later series of the programme, with Hammond, May and Wilman subsequently leaving the programme in the wake of his dismissal. All four were later approached by Amazon Prime to create a brand new programme; their initial agreement was to produce 36 episodes over three years. Episodes are released weekly to those Amazon Prime Video accounts, and repeats of the first series were made available on traditional broadcasters in late 2017. Until the beginning of the second series, studio segments were filmed using a travelling tent in various countries, before it was decided to set it in a permanent location in the Cotswolds.

As of December 2016 the show was made available to 195 more countries and various territories, and has attracted favourable viewing figures after "The Holy Trinity" became Amazon Video's most watched premiere episode. Overall, the show has received positive reviews from critics. At present, the show is currently airing its third series since 18 January 2019.

Top Gear (2002 TV series)

Top Gear is a British motoring magazine, factual television series, conceived by Jeremy Clarkson and Andy Wilman, launched on 20 October 2002, and broadcast in the United Kingdom on BBC Two. The programme is a relaunched version of the original 1977 show of the same name, which looks at various motor vehicles, primarily cars. While the original format focused mainly on review of cars, the 2002 version expanded on this with motoring-based challenges, special races, timed laps of notable cars, and celebrity timed laps on a course specially-designed for the relaunched programme, with its format developing over time to focus on a more quirky, humorous and sometimes controversial style of presentation. The programme has received acclaim for its visual style and presentation, as well as criticism for its content.Since 2002, the programme has been presented by several hosts. In its first series, the show's line-up was Clarkson, Richard Hammond and Jason Dawe, with Wilman as the show's executive producer, and introducing anonymous test driver "The Stig", an individual played by numerous racing drivers over the course of the show's history. Following the first series, Dawe was replaced by James May, with the line-up unchanged until the end of the twenty-second series, when the BBC chose to not renew Clarkson's contract on 25 March 2015, following an incident during filming. His dismissal from Top Gear prompted the departure of Hammond, May and Wilman from the programme, and led to them joining Clarkson in forming a new motoring series. For the twenty-third series, the programme was presented by Chris Evans and American Matt LeBlanc, with them joined by four co-presenters who would make occasional appearances during its run: Rory Reid, Sabine Schmitz, Chris Harris and Eddie Jordan. After negative feedback on this series led to Evans resigning from the programme, Harris and Reid became the main hosts alongside LeBlanc, with Schmitz and Jordan making occasional appearances as co-presenters, from the twenty-fourth series onwards.

Since its relaunch, Top Gear is one of the BBC's most commercially successful programmes. It has become a significant show in British popular culture, with episodes also broadcast internationally in many countries in Europe, North America, South-East Asia and more, making it the most widely broadcast factual television programme in the world. Its success has led to various forms of merchandising, including live tours, special DVD editions, and books, as well as spawning a variety of international versions in various countries, including the United States, Australia, South Korea, China and France.

Transmission (mechanics)

A transmission is a machine in a power transmission system, which provides controlled application of the power. Often the term transmission refers simply to the gearbox that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device.In British English, the term transmission refers to the whole drivetrain, including clutch, gearbox, prop shaft (for rear-wheel drive), differential, and final drive shafts. In American English, however, the term refers more specifically to the gearbox alone, and detailed usage differs.The most common use is in motor vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels. Such engines need to operate at a relatively high rotational speed, which is inappropriate for starting, stopping, and slower travel. The transmission reduces the higher engine speed to the slower wheel speed, increasing torque in the process. Transmissions are also used on pedal bicycles, fixed machines, and where different rotational speeds and torques are adapted.

Often, a transmission has multiple gear ratios (or simply "gears") with the ability to switch between them as speed varies. This switching may be done manually (by the operator) or automatically. Directional (forward and reverse) control may also be provided. Single-ratio transmissions also exist, which simply change the speed and torque (and sometimes direction) of motor output.

In motor vehicles, the transmission generally is connected to the engine crankshaft via a flywheel or clutch or fluid coupling, partly because internal combustion engines cannot run below a particular speed. The output of the transmission is transmitted via the driveshaft to one or more differentials, which drives the wheels. While a differential may also provide gear reduction, its primary purpose is to permit the wheels at either end of an axle to rotate at different speeds (essential to avoid wheel slippage on turns) as it changes the direction of rotation.

Conventional gear/belt transmissions are not the only mechanism for speed/torque adaptation. Alternative mechanisms include torque converters and power transformation (e.g. diesel-electric transmission and hydraulic drive system). Hybrid configurations also exist. Automatic transmissions use a valve body to shift gears using fluid pressures in response to speed and throttle input.

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