a mechanism in which mechanical energy is transmitted over a distance by a chain that engages sprockets.

Chain drives are classified according to various criteria: the design of the chains used, the number of sprockets (simple drives with two sprockets and complex drives with three or more sprockets, including one or more driven and idler sprockets for tensioning), the direction of rotation of the driven sprockets (direct, or forward, and reverse), the spatial arrangement of the chain (vertical loops, horizontal loops, and sprockets with crossed axes), the disposition of the line joining the sprocket centers (horizontal, vertical, and oblique), the disposition of the leading or working strand (above or below), the conversion mode of the rotational speed of the drive shaft (reduction or step-up), the number of chain contours in parallel, the means used to adjust chain tension, the means used to protect the chains from contamination (open or protected with leather, housings, or casings), the method of lubrication (manual lubrication for chain velocities up to 2 m per second, drip lubrication for velocities up to 6 m per second, oil baths for velocities up to 8 m per second, and lubrication by means of an oil-circulation system for velocities in excess of 8 m per second), and layout (built-in chain drives for machines and chain reduction gears).

The widespread use of chain drives began with the introduction of pintle chains with bushings and precision roller chains that permit powers up to 5,000 kilowatts to be transmitted at high chain velocities (up to 35 m per second) and high forces (up to 70,000 kg-force, or 700 meganewtons, in chain drives having several parallel, multistrand chains), with a substantial number of gear ratios (up to 12 for a single chain drive), and with high efficiency (up to 0.99). Under particularly light-duty operating conditions (low velocities and light loads), open-hook chains may be used.

Drive chains are versatile, simple, and economical. In comparison with gear drives, they are less sensitive to shaft misalignment and impact loads, they permit almost unlimited distances between centers, and they afford a simpler layout. Compared with belt drives, they have several advantages: there is no slippage, and the average gear ratio is constant; there is no preliminary tensioning or the extra shaft and bearing loads associated with it; large amounts of power may be transmitted at both high and low velocities; satisfactory operation is maintained at high and low temperatures; and the drive may accommodate any design changes by the removal or addition of links.

Chain drives have several disadvantages: the nonuniformity of chain motion, which increases with a reduction in the number of sprocket teeth and an increase in the pitch; high noise and chain wear when the wrong design is chosen, the assembly is careless, or the maintenance is poor; and the necessity for lubrication and for taking up slack in the idling leg as the chain wears.

Chain drives are used for agricultural machinery, bicycles, motorcycles, motor vehicles, and construction and reading machines, as well as for equipment used in the petroleum industry. The most common chain drives are open types operating without lubrication or with only periodic manual lubrication, using single-strand roller chains directly built into machines.