Gear train

A gear train is an arrangement of meshing gears that transmits motion and torque. Its geometry determines the relationship between input and output speed, torque, and direction.

Gear ratio: The ratio between the rotational speeds or tooth counts of two connected gears. Tooth-count ratios determine how a gear train changes rotational speed and torque.
Gear: A rotating machine element with teeth that mesh with another toothed element to transmit motion or force. A gear train is built from gears that engage in successive pairs.
Transmission (mechanics): A mechanism that transfers power from a source to a machine’s output, often changing speed or torque. Vehicle transmissions use gear trains to select output speed and torque.
Belt drive: A power-transmission system that uses a flexible belt running over pulleys. Unlike meshing gears, belts can connect separated shafts but may slip.
Idler gear: A gear placed between driving and driven gears to transmit motion without changing the overall ratio. An intermediate gear can reverse rotation direction while preserving the train’s overall ratio.
Gear tooth: A projection on a gear’s rim that engages corresponding teeth on another gear. Tooth shape and spacing govern smooth contact and reliable motion transfer.
Differential (mechanical device): A gear mechanism that allows two outputs to rotate at different speeds while transmitting power between them. Automotive differentials use interconnected gears to let driven wheels turn at different speeds.
Chain drive: A power-transmission system in which a chain engages toothed sprockets. A chain drive links separated shafts without the direct tooth-to-tooth mesh of a gear train.
Compound gear train: A gear train with at least one shaft carrying two rigidly connected gears of different sizes. Compound stages achieve larger overall ratios in less space than a single gear pair.
Pitch circle: An imaginary circle on a gear that rolls without slipping against the pitch circle of a mating gear. Pitch circles provide the geometry used to calculate gear ratios and center distances.

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