Gears are major part of any machine working on the principle of rotary motion. The motion can be produced either by a motor or engine and is called the input. The engine usually has a very high angular momentum but the torque is small. This torque is not enough for performing work under heavy load and hence cannot be used directly in industrial machines. To overcome this problem, coupled gears are used.
In coupled gear system, the input is connected through a driving shaft to a small gear called the pinion. The pinion is further coupled with a gear of larger diameter i.e. high gear ratio. Due to this form of coupling, the torque of the engine increases at the expense of angular momentum. A variety of gears can be used for this purpose viz. spur gears, helical gears, worm gears, etc.
However these gears have a limitation i.e. they can only be used in systems where the input and output shafts are in the same plane. To overcome this problem, a special type of gear called the hypoid gear is used.
The hypoid gears are a subtype of bevel gears. On observation the hypoid gear seems to be similar in appearance to the helical bevel gears. The main difference being that the planes of the input and the output gears are different. This allows for more efficient intermeshing of the pinion and driven gear. Since the contact of the teeth is gradual, the hypoid gear is silent in operation as compared to the spur gears.
These gears are usually used in industrial and automotive application and hence the material used is a metal like stainless steel. A major application of hypoid gears is in car differentials where the axes of engine and crown wheel are in different planes.