A pinion is a round gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....

 used in several applications:
  • usually the smallest gear in a gear drive train
    Gear train
    A gear train is formed by mounting gears on a frame so that the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, this provides a smooth transmission of rotation from one gear to the next.The transmission of...

    , although in the case of John Blenkinsop's
    John Blenkinsop
    John Blenkinsop was an English mining engineer and an inventor of steam locomotives, who designed the first practical railway locomotive....

     Salamanca, the pinion was rather large. In many cases, such as remote control
    Remote control
    A remote control is a component of an electronics device, most commonly a television set, used for operating the television device wirelessly from a short line-of-sight distance.The remote control is usually contracted to remote...

    led toy
    A toy is any object that can be used for play. Toys are associated commonly with children and pets. Playing with toys is often thought to be an enjoyable means of training the young for life in human society. Different materials are used to make toys enjoyable and cuddly to both young and old...

    s, the pinion is also the drive gear.
  • the smaller gear that drives in a 90-degree
    Degree (angle)
    A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

     angle towards a crown gear in a differential drive
    Differential (mechanics)
    A differential is a device, usually, but not necessarily, employing gears, capable of transmitting torque and rotation through three shafts, almost always used in one of two ways: in one way, it receives one input and provides two outputs—this is found in most automobiles—and in the other way, it...

  • the small front sprocket on a chain driven motorcycle.
  • the round gear that engages and drives a rack in a rack and pinion
    Rack and pinion
    A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move, thereby...

     mechanism and against a rack in a rack railway
    Rack railway
    A rack-and-pinion railway is a railway with a toothed rack rail, usually between the running rails. The trains are fitted with one or more cog wheels or pinions that mesh with this rack rail...

  • in the case of radio-controlled car
    Radio-controlled car
    Radio-controlled cars are self-powered model cars or trucks that can be controlled from a distance using a specialized transmitter...

    s 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.
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