GEARSA gear or cogwheel is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part to transmit torque.

INTRODUCTION

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. 

HISTORYEarly examples of gears date from the 4th century BCE in China.(Zhan Goo times - Late East Zhou dynasty), which have been preserved at the Luoyang Museum of Henan Province, China. The earliest gears in Europe were circa CE 50 by Hero of Alexandria,[4] but they can be traced back to the Greek mechanics of the Alexandrian school in the 3rd century BCE and were greatly developed by the Greek polymath Archimedes (287–212 BCE).

Examples of further development include:Ma Jun (c. 200–265 CE) used gears as part of a south-pointing chariot.The Antikythera mechanism is an example of a very early and intricate geared device, designed to calculate astronomical positions. Its time of construction is now estimated between 150 and 100 BCE.[6]

The water-powered grain-mill, the water-powered saw mill, fulling mill, and other applications of watermill often used gears.The first mechanical clocks were built in CE 725.The 1386 Salisbury cathedral clock may be the world's oldest working mechanical clock.

COMPARISON WITH DRIVE MECHANISMS

The definite velocity 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.

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.

EXTERNAL VS INTERNAL GEARS

SPURSpur gears or straight-cut gears are the simplest type of gear. They consist of a cylinder or disk with teeth projecting radically.

HELICALHelical 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.

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.{\displaystyle E=\beta _{1}+\beta _{2}} for gears of the same handedness{\displaystyle E=\beta _{1}-\beta _{2}} for gears of opposite handedness

DOUBLE HELICAL

Double helical gears and herringbone gears are similar but the difference is that herringbone gears don't have a groove in the middle like double helical gears do.

BEVELA 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. 

SPIRAL BEVELSMain article: Spiral bevel gearSpiral 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 (Epicycloide with constant tooth depth) or Klingelnberg Palloid.

HYPOIDHypoid 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.

CROWNMain article: Crown gearCrown gears or contrite 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.

WORM

Main articles: Worm driveand Slewing driveWorms resemble screws. A worm is meshed with a worm wheel, which looks similar to a spur gear.

NON-CIRCULARMain article: Non-circular gearNon-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.

RACK AND PINIONMain article: Rack and pinionA 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.

EPICYCLICMain article: Epicyclic gearingIn epicyclical gearing one or more of the gear axes moves. Examples are sun and planet gearing (see below), cyclical drive, and mechanical differentials.

HARMONIC GEARMain article: Harmonic DriveA harmonic 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.

MAGNETIC GEAR

Main article: magnetic couplingMain article: magnetic GearAll 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

GENERAL NOMENCLATURE

MODULE OR MODULUS

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{\displaystyle m=p/\pi }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 diametric pitch, DP, is in English units.

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.

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.

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

TOOTH CONTACT NOMENCLATURE

GEAR MATERIALS

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. 

MANUFACTURE

As of 2014, an estimated 80% of all gearing produced worldwMolded gearing is usually either powder metallurgy or plastic.[30] 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. ide is produced by net shape molding. 

INSPECTION

Overall gear geometry can be inspected and verified using various methods such as industrial CT scanning, coordinate-measuring machines, white light scanner or laser scanning. Particularly useful for plastic gears, industrial CT scanning can inspect internal geometry and imperfections such as porosity.

GEAR MECHANISM IN WOLD

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.

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