Apinion is a roundgear—usually the smaller of two meshed gears—used in several applications, includingdrivetrain andrack and pinion systems.

Drivetrains usually feature a gear known as the pinion, which may vary in different systems, including

• the typically smaller gear in agear drive train (although in the first commercially successful steam locomotive—theSalamanca—thepinion was rather large).^[1] In many cases, such asremote controlledtoys, the pinion is also thedrive gear for a reduction in speed, since electric motors operate at higher speed and lower torque than desirable at the wheels. However the reverse is true in watches, where gear trains commence with a high-torque, low-speed spring and terminate in the fast-and-weak escapement.
• the smaller gear that drives in a90-degree angle towards acrown gear in adifferential drive.
• the small frontsprocket on achain driven motorcycle.
• theclutch bell gear when paired with acentrifugal clutch, inradio-controlled cars with anengine (e.g.,nitro).^[2]

Inrack and pinion systems, the pinion is the round gear that engages and moves along the linear rack.

• List of gear nomenclature

Look uppinion in Wiktionary, the free dictionary.

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