A pinion is a round gear used in several applications:
usually the smaller gear in a gear drive train, although in the first commercially successful steam locomotive (the Salamanca), the pinion was rather large. In many cases, such as remote controlled toys, the pinion is also the drive gear.
the smaller gear that drives in a 90-degree angle towards a crown gear in a differential drive.
the small front sprocket on a chain driven motorcycle.
the round gear that engages and drives a rack in a rack and pinion mechanism and against a rack in a rack railway.
in the case of radio-controlled cars with an engine (i.e. nitro) this pinion gear can be referred to as a clutch bell when it is paired with a centrifugal clutch.
