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 (called the "axle ratio" or "diff ratio"). For example, many differentials in motor vehicles provide a gearing reduction by having fewer teeth on the pinion than the ring gear.
History
Milestones in the design or use of differentials include: 100 BCE–70 BCE: The Antikythera mechanism has been dated to this period. It was discovered in 1902 on a shipwreck by sponge divers, and modern research suggests that it used a differential gear to determine the angle between the ecliptic positions of the Sun and Moon, and thus the phase of the Moon. c. 250 CE: Chinese engineer Ma Jun creates the first well-documented south-pointing chariot, a precursor to the compass. Its mechanism of action is unclear, though some 20th century engineers put forward the argument that it used a differential gear. 1810: Rudolph Ackermann of Germany invents a four-wheel steering system for carriages, which some later writers mistakenly report as a differential. 1827: Modern automotive differential patented by watchmaker Onésiphore Pecqueur (1792–1852) of the Conservatoire National des Arts et Métiers in France for use on a steam wagon. 1874: Aveling and Porter of Rochester, Kent list a crane locomotive in their catalogue fitted with their patent differential gear on the rear axle. 1876: James Starley of Coventry invents chain-drive differential for use on bicycles; invention later used on automobiles by Karl Benz. 1897: While building his Australian steam car, David Shearer made the first use of a differential in a motor vehicle. 1958: Vernon Gleasman patents the Torsen limited-slip differential.
Use in wheeled vehicles
Purpose
During cornering, the outer wheels of a vehicle must travel further than the inner wheels (since they are on a larger radius). This is easily accommodated when the wheels are not connected, however it becomes more difficult for the drive wheels, since both wheels are connected to the engine (usually via a transmission). Some vehicles (for example go-karts and trams) use axles without a differential, thus relying on wheel slip when cornering. However, for improved cornering abilities, many vehicles use a differential, which allows the two wheels to rotate at different speeds.
The purpose of a differential is to transfer the engine's power to the wheels while still allowing the wheels to rotate at different speeds when required. An illustration of the operating principle for a ring-and-pinion differential is shown below.
Ring-and-pinion design
A relatively simple design of differential is used in rear-wheel drive vehicles, whereby a ring gear is driven by a pinion gear connected to the transmission. The functions of this design are to change the axis of rotation by 90 degrees (from the propshaft to the half-shafts) and provide a reduction in the gear ratio.
The components of the ring-and-pinion differential shown in the schematic diagram on the right are: 1. Output shafts (axles) 2. Drive gear 3. Output gears 4. Planetary gears 5. Carrier 6. Input gear 7. Input shaft (driveshaft)
Epicyclic design
An epicyclic differential uses epicyclic gearing to send certain proportions of torque to the front axle and the rear axle in an all-wheel drive vehicle. An advantage of the epicyclic design is that it is relatively compact width (when viewed along the axis of its input shaft).
Spur-gear design
A spur-gear differential has an equal-sized spur gears at each end, each of which is connected to an output shaft. The input torque (i.e. from the engine or transmission) is applied to the differential via the rotating carrier. Pinion pairs are located within the carrier and rotate freely on pins supported by the carrier. The pinion pairs only mesh for the part of their length between the two spur gears, and rotate in opposite directions. The remaining length of a given pinion meshes with the nearer spur gear on its axle. Each pinion connects the associated spur gear to the other spur gear (via the other pinion). As the carrier is rotated (by the input torque), the relationship between the speeds of the input (i.e. the carrier) and that of the output shafts is the same as other types of open differentials.
Uses of spur-gear differentials include the Oldsmobile Toronado American front-wheel drive car.