Definition
A mechanical arrangement in which a small toothed wheel, called the pinion, meshes with a larger toothed wheel, called the gear, to transmit rotary motion and change either the speed or the torque between the two shafts. When the small pinion drives the larger gear, speed is reduced and torque is increased; when the larger gear drives the pinion, speed is increased and torque is reduced.
Plain English
Two toothed wheels of different sizes that turn together. The small one is the pinion, the big one is the gear. By meshing their teeth, they pass motion from one shaft to another and trade speed for turning force, or the other way around.
Context Anchor
Seen in aircraft maintenance and system descriptions for mechanical drives, actuators, engine accessories, trim systems, and some landing gear or control mechanisms.
Derivation
The word pinion comes from the Latin pinna, meaning feather or wing tip, and was later applied to the small toothed wheel because its short teeth resembled feathers around a hub. Knowing this helps fix in mind that the pinion is always the smaller of the two meshing wheels.
Why Pilots Care
Many critical aircraft systems, from the starter that cranks the engine to the motor that lowers the landing gear, rely on a gear-and-pinion arrangement. Understanding that a small pinion driving a large gear multiplies torque while reducing speed explains why a small electric motor can move a heavy load like landing gear or flaps.
Analogy
Think of two bicycle sprockets connected by a chain, except in a gear and pinion mechanism the toothed wheels touch each other directly. When one turns, its teeth push the other one around.
Intuition Check
Do not read “gear” here as only “landing gear.” In this term, gear means a toothed wheel in a mechanical system, and pinion means the smaller toothed wheel that works with it.
Example Sentence 1
The starter motor uses a gear and pinion mechanism so that a small, fast-spinning electric motor can turn the heavy engine crankshaft slowly enough to start.
Example Sentence 2
The propeller reduction unit uses a gear and pinion mechanism to lower high engine RPM to a speed suitable for the blades.