Gearboxes are used for increasing the torque of the source of rotary motion having high angular momentum and low torque. This high torque is necessary for performance of work. This phenomenon of increase in torque is called gear reduction and is brought about by coupling of a smaller gear called the pinion with a larger gear. This results in reduction of torque at the expense of angular momentum. Such a gearbox is called a reducer. One more application of gears is to change the axis or plane of rotary motion with or without gear reduction.

When you { gearbox manufacturers } open a gearbox you will see that the inner construction is very simple. Inside you will find two gears coupled with one another. The gears may be of spur, helical, cycloid, worm or bevel type. In case of gear reduction, the diameter of the output is larger than that of the input gear. If only a change in direction is required, the size of the gears is the same. Spur gears are used for heavy load but are noisy. If the load is comparatively lesser, helical gears are preferred as they are silent in operation due to gradual engagement. If change of plane of rotation is required, hypoid gears are used.

The gear may be either of metal or plastic. This entire arrangement is enclosed in metallic or plastic housing. The point of contact of the gear teeth is well lubricated with gear oil. The gear oil must be very clean and free of abrasive materials to avoid wearing of the gears.

A gearbox is used in turbines, windmills, grinders, etc. to change the direction of the rotary motion. In automobiles a gear box is used for transfer of to power of the engine to the wheels through a differential.

A differential gear is an arrangement of beveled gears in such a manner such that the input torsion is taken up by a ring gear and distributed equally through two planetary gears to the side gears which drive the shafts on either side as used by gears manufacturers.

One of the common applications of differential gears is in automobiles. Consider a vehicle following a curved path. As one can easily imagine, the inner wheel covers a shorter distance than the outer wheel. In case the system consists of only a single shaft connecting the inner and outer wheel, both the wheels are forced to turn at the same angular momentum and hence makes turning difficult leading to slippage and binding of tires. In addition to this, the strain on the inner wheels is passed on to the outer wheels through the axle putting undue strain on it.

This problem is solved by using the differential gear system. In this, the power of the engine is transferred through a drive shaft and pinion arrangement to the ring gear and the torque is further split by the differential gears into two components such that the outer wheel spins at greater angular momentum than the inner wheel. This eliminates slippage of the wheels and strain on the axle.

However simple differential gears have one major disadvantage. If the inner and outer wheels are on surfaces of unequal roughness, the wheel on the smoother surface spins with a higher angular momentum. Since the differential splits the torque equally, the wheel on rougher surface will decrease in angular momentum and hence the vehicle will move at a very low speed or may not move at all. In case the difference in traction is too large i.e. when one of the wheels is off road, torque from the wheel on the road will be transferred to the wheel that is off the surface and it will spin at double angular momentum. Hence the vehicle will remain stationary.

In order to solve this problem, we need to transfer more of the torque to the stationary wheels. LSD, viscous coupling and torsen(R) are some of the technique used for the transfer of torque as said by Gearbox manufacturers india.