Classification of Gear Toothed Wheels

According to the Position of Axis of Shaft

(a) Parallel
(b) Intersecting 
(c) Non-intersecting and non-parallel

According to the peripheral velocity of the gears

(a) Low velocity, less than 3 m/s 
(b) Medium velocity, between 3 and 15 m/s
(c) High velocity. more than 15 m

According to the type of gearing

(a)External gearing
(b) Internal gearing
(c) Rack and pinion

According to position of teeth on the gear surface

(a) straight
(b) inclined
(c) curved