Definition of ring gear in English:

ring gear

noun

Engineering
  • A gear in the form of a ring, having teeth on its inner circumference; (in later use) such a gear or (later) any gear on the flywheel of an internal combustion engine which is driven by the pinion of a starter motor, so communicating power from the motor to the engine.

Origin

Mid 19th century; earliest use found in U.S. Patents.