Gears are seldom used as a single entity. Almost all machines have gears working in pairs. Gear coupling has the following advantages:
Gear reduction is the major application of Gear coupling. The motor, which supplies the power to the system usually, has high power but lacks the essential torque required for work. In order to increase its torque the motor is connected directly to a small gear that is coupled with a larger gear. This results in an increase in torque at the expense of speed. Gear reduction is used in cars to convert the power of the motor into higher torque for motion of the wheels along the surface of the road.
Change in Direction of Movement
Most often the direction of application of torque is different than the direction of movement of the supply. The change in direction can be brought about by using gear coupling consisting of a system of two or more bevel gears. An example is the gear differential used in vehicles.
Change in Axis of Motion
When the rotary motion is to be changed to a different axis, gear coupling is used. Two or more gears are coupled in the same plane to change the axis of rotation. Either spur or helical gears can be used for this purpose. Helical gears are preferred as they work more silently in comparison to spur gears. An example is a windmill where the blades and motor are on different axis of rotation.
Change of Rotary to Linear Motion
Industrial machines like conveyor belts require conversion of the rotary motion of the motor to vertical motion for the movement of material. This is brought about by using gear coupling of a rack and pinion gear. The motor is connected to a small pinion gear which moves the rack gear for linear motion. This type of gear coupling is also using in crushers and water pumps.