Differential (mechanical device)
A differential is a gear train with three drive shafts that has the property that the rotational speed of one shaft is the average of the speeds of the others. A common use of differentials is in motor vehicles, to allow the wheels at each end of a drive axle to rotate at different speeds while cornering. Other uses include clocks and analog computers.
Differentials can also provide a gear ratio between the input and output shafts. For example, many differentials in motor vehicles provide a gearing reduction by having fewer teeth on the pinion than the ring gear.
Differential unit for a rear-wheel drive car, built by ZF circa 2004
Differential gears (in yellow) in a punched tape reader, built by Tally circa 1962
Illustration of a ring-and-pinion differential for a rear-wheel drive vehicle
Schematic diagram of a ring-and pinion differential
A gear train or gear set is a machine element of a mechanical system formed by mounting two or more gears on a frame such that the teeth of the gears engage.
Transmission of motion and force by gear wheels, compound train.
Illustration by Georgius Agricola (1580) showing a toothed wheel that engages a slotted cylinder to form a gear train that transmits power from a human-powered treadmill to mining pump.
2 gears and an idler gear on a piece of farm equipment, with a ratio of 42/13 = 3.23:1
Bicycle with toothed belt drive to transmit torque from crank to rear sprocket
