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A chain drive is a mechanism for transferring mechanical power between two places, and is a common means of locomotion in bicycles and motorcycles. It is also a motive source for many different types of machinery. Chain drives have existed as a technology since the third century BC and have remained much the same in their basic design since that time.
Typically, a chain drive works by having a power source, usually a motor or pedal system, rotate a toothed wheel known as a sprocket, around which a specially designed chain is looped. As the sprocket spins, its teeth catch slots in the chain drive, causing it to rotate around the sprocket. At the other end is a second gear that transforms the mechanical energy delivered by the drive chain into the desired force.
In a bicycle or motorcycle, for instance, the second hub is attached to a shaft which houses a series of gears that power the rear wheel. Depending on the gear, the power is applied in varying capacities. Gears with different numbers of teeth are used to generate propulsion along a wide range of speeds, and the relationship between these gears is known as gear ratio. Typical gear ratio progressions feature multiple sprocket revolutions per single output revolution at the low end. These often then progress to a single revolution of the drive sprocket, causing multiple revolutions of the secondary hub at higher gears.
While simple chain drives are looped designs containing two gears, more complicated shapes can be created by adding additional gears into the design. These additional intermediary gears are known as idler gears, and do not affect the overall ratio of the drive. Only the first and last gears, and specifically the difference in number of their teeth, impacts the gear ratio in a chain drive.
Chain drives are typically constructed of metal, and aside from bikes and motorcycles can been seen in machines ranging from commercial toasters to tanks. They are used on a large scale in mining operations, functioning as a conveyor hauling material from one place to another by a series of buckets connected to the chain. The chain drive was a popular drivetrain in automobiles during the first half of the 20th century, but by the late 1950s had been abandoned altogether in favor of the much more durable, though heavier, driveshaft mechanism. The chain drive is not altogether absent from modern automobiles, however, and is still a popular option to power the timing chain associated with an engine's camshaft.