pinion

A pinion is a gear with a relatively small number of teeth, typically between 12 and 60, that meshes with a larger gear called a spur gear or ring gear. It is not simply the smallest gear in any system, but rather the smaller member of a gear pair. When two gears mesh, the one with fewer teeth is called the pinion regardless of its absolute size. The pinion's primary job is to transmit motion and torque between shafts, either stepping up speed or stepping down speed depending on which way power flows through the pair.

Pinions are classified by the shape of their teeth and their orientation relative to the mating gear. A spur pinion has straight teeth parallel to its axis and meshes with a spur gear on a parallel shaft. A bevel pinion has angled teeth and meshes with a bevel gear at an angle, often 90 degrees, to change the direction of power transmission. A worm pinion, called a worm gear, has a thread-like profile and drives a worm wheel in a right-angle arrangement with high reduction ratios. Helical pinions have angled teeth that produce smoother, quieter meshing than spur teeth and can carry higher loads.

The size and tooth count of a pinion determine the speed ratio and torque multiplication between driver and driven shafts. A pinion with 20 teeth meshing with a 60-tooth gear produces a 3:1 ratio. If the pinion is the driver, output torque increases threefold while output speed drops to one-third of input speed. The material, typically steel or ductile iron, must be hard enough to resist wear and pitting where teeth contact under load. Tooth surfaces are often case-hardened to depths of 0.5 to 1.5 millimeters to extend service life.

Common failure modes and design considerations

Pinion teeth wear prematurely under inadequate lubrication, misalignment, or overload. Spalling, the flaking away of tooth surfaces, happens when contact stresses exceed the material's fatigue limit. Bending fatigue at the tooth root can cause teeth to break cleanly. The module or diametral pitch of the pinion must exactly match that of its mating gear; a 2-module pinion will not mesh correctly with a 3-module gear. Center distance between shafts is set by the sum of the pitch radii and must be held tight to maintain consistent tooth engagement.

In automotive transmissions, the input pinion on the mainshaft is one of the first gears to receive power from the engine. In machine tool spindles and industrial reducers, pinions are chosen to provide the exact speed and torque needed for the application. The term pinion derives from Old French pignon, meaning small cog. In differential gearing on vehicle axles, small conical pinions rotate on cross shafts to allow wheels to turn at different speeds during cornering. Pinion selection directly affects machine efficiency, noise, longevity, and initial cost.