A pinion is a round gear—usually the smaller of two meshed gears—used in several applications, including drivetrain and rack and pinion systems.

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

• the typically smaller gear in a gear drive train (although in the first commercially successful steam locomotive—the Salamanca—the pinion was rather large).^[1] In many cases, such as remote controlled toys, the pinion is also the drive 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 a 90-degree angle towards a crown gear in a differential drive.
• the small front sprocket on a chain driven motorcycle.
• the clutch bell gear when paired with a centrifugal clutch, in radio-controlled cars with an engine (e.g., nitro).^[2]

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

• List of gear nomenclature