Mechanical engineers sometimes don't use gears and rely on the advent of electronic control and the availability of toothed belts because gears for high power machinery are difficult to design. Even though for high power machinery like automotive transmission, gears are the optimal medium for high accuracy and low energy loss.

Gears are of several categories and can be combined in a multitude of ways, some of which are meshing circular spur gears, rack and pinion spur gears, and worm gears. Helical and herringbone gears utilize curved teeth for efficient, high-capacity power transmission. Worm gears, driven by worms transmit motion between non-intersecting right-angle axes. Gears mate or mesh via teeth with a very specific geometry.

Application

Being important parts of a machine, gears have immense usage within various industries, including automotive industries, coal plants industry, steel plants industry, paper industry, mining industry etc. In these industries, they behold a wide area of application. They are used as conveyors, elevators, kilns, separators, cranes and lubrication systems.   

Gears are used for two basic purposes, increase or decrease of rotation speed and increase or decrease of power or torque. Torque is a measure of a force to produce torsion and rotation about an axis. To increase speed and reduce torque, a large drive gear is coupled to a smaller driven gear. To reduce speed and increase torque, a small gear turning a larger gear is used. They are also used for enhancement for positioning systems.

Gear geometry   

The essential features of a gear mesh are:

Center distance: The distance between the centers of two pitch circles.

Pitch diameters: The tangent to two basic circles is the line of contact in gear vernacular. Where this line crosses the line of center establishes the pitch. The ratio of pitch diameters gives the velocity ratio.    

Pitch: It is a measure of tooth spacing along the pitch circle.     

Number of teeth

Pressure angle of the contacting involutes: The angle between the line of force between meshing teeth and the tangent to the pitch circle at the point of mesh.