Gears can be used to amplify/reduce the torque/speed. Spur gear is the simplest type of gear which is used to connect or transmit motion from one shaft to another having their axes parallel to each other. The contact surfaces of a pair of spur gears are cylindrical in shape i.e. cylinders become pitch surfaces of the gears. Figure below shows a spur gear drive. Generally, the smaller gear is called a pinion and the mating gear is called gear. If D and d are the diameters of gear and pinion respectively and N and n are their speeds in revolution/s, then DN = dn