Gears

A gear is a rotating machine part having cut teeth or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared devices can change the speed, torque, and direction of a power source. An advantage of gears is that the teeth of a gear prevent slipping.

gears

Types of gears:

• External gear: An external gear is one with the teeth formed on the outer surface of a cylinder.

external gears

• Internal gear: An internal gear is one with the teeth formed on the inner surface of a cylinder or cone.

internal gears

• Spur gear: Spur gears are the simplest type of gear. They consist of a cylinder with the teeth projecting radially, the edge of each tooth is straight and aligned parallel to the axis of rotation. These gears can be meshed together correctly only if they are fitted to parallel shafts.

spur gears

• Helical gear: 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 causes the tooth shape to be a segment of a helix. Helical gears can be meshed in parallel or crossed orientations. The angled teeth engage more gradually than do spur gear teeth, causing them to run more smoothly and quietly.

helical gears

• 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

• 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 hyperboloids of revolution. Hypoid gears are almost always designed to operate with shafts at 90 degrees.

hypoid gears

• Crown:  Crown 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 of a crown. A crown gear can only mesh accurately with another bevel gear, although crown gears are seen meshing with spur gears. A crown gear is also sometimes meshed with an escapement such as found in mechanical clocks.

crown gears

• Worm gear: Worm gears resemble screws. A worm gear is usually meshed with a spur gear or a helical gear, which is called the gear, or worm wheel. Worm-and-gear sets are a simple and compact way to achieve a high torque, low speed gear ratio.

worm gears

• 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. Torque can be converted into 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.

rack and pinion gears