A gear is a transmission device that transmits rotational force to another gear or device. A gear is different from a pulley in that a gear is a round wheel which has linkages (teeth or cogs) that mesh with other gear teeth, allowing force to be fully transferred without slippage. Depending on their construction and arrangement, geared devices can transmit forces at different speeds ,torques, or in a different direction, from the power source. Gears are a very useful simple machine.

Gears transmit rotary motion from one shaft to another through meshing perimetral teeth on wheels mounted on the shafts. It is with a high degree of mechanical efficiency. The simplest gear mechanism has two meshing gears (toothed wheels) mounted in a fixed cage or frame (Fig.1).

Since the two gears intermesh, they must rotate with the same circumferential velocity. This means that the smaller gear has to rotate at a higher angular velocity, i.e., has to perform more revolutions per minute, than the larger. The rotational speeds of the gears are inversely proportional to their respective diameters or their respective numbers of teeth. Since the force exerted by two teeth in contact must be of equal magnitude for both the torque acting on the larger gear (=F x R) must be of greater magnitude than that acting on the smaller gear (=F x r). The ratio of the speed of the driving shaft to that of the driven shaft is called the gear ratio or transmission ratio.

If, instead of the cage, one of the gears is held stationary and other gear is driven, the cage will perform a rotary motion; a planetary gear system is thus obtained (Fig.2), comprising the sun wheel, the planet wheel (as a rule there are two or more planet wheels), the cage, and the internally toothed annulus which meshes with the panel wheel, while the latter meshes with the sun wheel. In Fig.2 the sun wheel is conceived as being stationary, while the annulus drives the planet wheel. Alternatively, any one of the three elements - sun wheel, planet wheel, and cage – may be assigned the role of the stationary element and any of the two others of this trio may be driven. Thus six different possibilities of transmission are available, as well as the possibility of locking the sun wheel and the planet wheels, so that direct drive is obtained.

Generally speaking, gear systems are named after the arrangement of their teeth. Shafts whose center lines cross but do not intersect are connected by spiral gears (Fig.3). In the worm gear (Fig.4) the basic shape of the worm is a cylinder or a globoid, while that of the worm wheel is a globoid. A globoid is a body of revolution that is generated by the rotation of a circular arc about any axis.

Change-speed gears allow the selection of various transmission ratios. The simplest method of doing this is by the removal and replacement of gear wheels of different sizes. Greater ease and convenience are provided by a change-speed gearbox, such as is used on motor vehicles and certain machine tools. Such gearboxes permit the selection of various transmission ratios between driving shaft and driven shaft by appropriate operation of a lever.

The speed changes are achieved by the action of sliding pinions that are moved into or out of mesh with gear wheels. A type of transmission that provides as many ratios as there are gear pairs and that takes up little space is the draw-key transmission (Fig.5). Whenever the draw key is shifted, one of the gears that are mounted loose on shaft II is locked to the shaft, so that the gear pair concerned then transmits power. This type of gear is used as feed gearing on machine tools.

A reversing gear serves to reverse the direction of rotation. In the bevel-gear device of this kind (Fig.6) shaft I is the driving shaft, on which an engaging element with claws is slidably mounted. When this element is shifted to the left, the bevel gear C and therefore the driven shaft II is driven through the agency of the bevel gear A; when the engaging element is slid to the right, the drive is effected through the agency of the bevel gear B instead of A, so that now the shaft II rotates in the opposite direction.