Gears

Gears are toothed machine elements that transmit rotation and torque between shafts. Their tooth geometry constrains motion, often changing speed, torque, or direction.

Gear ratio: The ratio of rotational speeds or tooth counts between meshing gears. It quantifies how a gear pair changes speed and torque.
Pitch circle: An imaginary circle on a gear that rolls without slipping against the pitch circle of a mating gear. It provides the reference geometry for speed ratios and center spacing.
Automotive transmission: A vehicle drivetrain assembly that varies the ratio between engine or motor speed and wheel speed. Its gearsets provide selectable ratios for acceleration and cruising.
Belt drive: A power-transmission system that uses a flexible belt running over pulleys. Unlike gears, it can connect separated shafts and may slip under overload.
Antikythera mechanism: An ancient Greek geared device, built in the second century BCE, used to model astronomical cycles. Its surviving bronze gears show sophisticated ancient gearing in a specific machine.
Involute gear: A gear whose tooth flanks follow an involute curve, maintaining a constant pressure angle during meshing. Its tooth profile keeps transmission smooth despite small center-distance errors.
Module (gear): A metric measure of gear tooth size equal to pitch diameter divided by tooth count. Matching modules is necessary for standard metric gears to mesh.
Differential (mechanical device): A gear mechanism that allows connected wheels to rotate at different speeds while receiving torque. Differentials use gears to accommodate unequal wheel speeds in turns.
Chain drive: A transmission system that transfers motion through a chain engaging toothed sprockets. It links separated shafts without the continuous tooth-to-tooth mesh of gears.
Chinese south-pointing chariot: A traditional Chinese vehicle mechanism that used differential gearing to keep a figure pointing south. It illustrates an early application of gear relationships to directional indication.

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