Gears are used to transmit motion from one shaft to another. It is a toothed member designed to transmit or receive motion from another member by successively engaging tooth. Gears are used in metal cutting machine tools, automobiles, tractors, hoisting transporting machinery, rolling mills etc..!!
Coming to your answer why do we use different types of gears. It actually depends upon the need and the purpose, it can be used to increase or decrease the speed of rotation, it can also be used to reverse the direction of rotation and for many more other purposes we use gears.
So if we generally classify the gears then they are classified on three basis :-
According to the position of axes of the shaft:-
Parallel shafts :- Spur, Helical, Double helical gears.
Intersecting shafts :- Straight bevel, Spiral bevel, Zerol bevel and Face gears.
Non parallel and non intersecting shafts:- Spiral, Hypoid, Worm gears.
According to the peripheral velocity of the gears:-
Low velocity :- less than 3 m/s
Medium velocity :- in between 3m/s - 15m/s
High velocity :- more than 15m/s
According to the type of gearing:-
External gearing :- the gears of two shaft mesh externally with each other.
Internal gearing :- the gears of two shaft mesh internally with each other.
Rack and pinion(which converts linear motion into rotary motion or vice-versa):- the gear of a shaft meshes externally and internally with the gears in a straight line. The straight line gear is called rack and the circular wheel is called pinion.
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