Differential (mechanical device)
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- 100 BC–70 BC: 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.
- 30 BC–20 BC: Differential gear systems possibly used in China
- 658, 666 AD: two Chinese Buddhist monks and engineers create south-pointing chariots for Emperor Tenji of Japan.
- 1027, 1107 AD: Documented Chinese reproductions of the south-pointing chariot by Yan Su and then Wu Deren, which described in detail the mechanical functions and gear ratios of the device much more so than earlier Chinese records.
- 1720: Joseph Williamson uses a differential gear in a clock.
- 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 des Arts et Métiers inFrance for use on a steam cart. (Sources: Britannica Online and)
- 1832: Richard Roberts of England patents 'gear of compensation', a differential for road locomotives.
- 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: first use of differential on an Australian steam car by David Shearer.
- 1958: Vernon Gleasman patents the Torsen dual-drive differential, a type of limited slip differential that relies solely on the action of gearing instead of a combination of clutches and gears.
Application to vehicles
Loss of traction
Automobiles without differentials
- Race cars and trucks in certain classes. Drag racing is done in a straight line (and often on a prepared surface), which obviates the need for a differential. A spool is used to make a solid connection between both drive wheels, which is simpler and less likely to break under very heavy acceleration. Racing on dirt or mud tracks also allows the use of spools, because the loose surface gives way while cornering. NASCAR mandates the use of spools in their cars, which does cause axle wind-up, and degrades handling in turns. Other forms of racing without differentials includes tractor pulling, mud bogging and other 4x4 motorsports where differential action is not needed.
- Vehicles with a single driving wheel. Besides motorcycles, which are generally not classified as automobiles, this group includes most three-wheeled cars. These were quite common in Europe in the mid-20th Century, but have now become rare there. They are still common in some areas of the developing world, such as India. Some early four-wheeled cars also had only one driving wheel to avoid the need for a differential. However, this arrangement led to many problems. The system was unbalanced, the driving wheel would easily spin, etc.. Because of these problems, few such vehicles were made.
- Vehicles using two freewheels. A freewheel, as used on a pedal bicycle for example, allows a road wheel to rotate faster than the mechanism that drives it, allowing a cyclist to stop pedalling while going downhill. Some early automobiles had the engine driving two freewheels, one for each driving road wheel. When the vehicle turned, the engine would continue to drive the wheel on the inside of the curve, but the wheel on the outside was permitted to rotate faster by its freewheel. Thus, while turning, the vehicle had only one driving wheel. Driving in reverse is also impossible as is engine braking due to the freewheels.
- Vehicles with continuously variable transmissions, such as the DAF Daffodil. The Daffodil, and other similar vehicles which were made until the 1970s by the Dutch company DAF, had a type of transmission that used an arrangement of belts and pulleys to provide an infinite number of gear ratios. The engine drove two separate transmissions which ran the two driving wheels. When the vehicle turned, the two wheels could rotate at different speeds, making the two transmissions shift to different gear ratios, thus functionally substituting for a differential. The slower moving wheel received more driving torque than the faster one, so the system had limited-slip characteristics. The duplication also provided redundancy. If one belt broke, the vehicle could still be driven.
- Light vehicles with closely spaced rear wheels, such as the Isetta and Opperman Unicar, or very low mass vehicles.
- Vehicles with separate motors for the driving wheels. Electric cars can have a separate motor for each driving wheel, eliminating the need for a differential, but usually with some form of gearing at each motor to get the large wheel torques necessary. A multi-motor electric vehicle such as the Dual Motor Tesla Model S can electronically control the power distribution between the motors on a millisecond scale, in this case acting as a centre differential where open differentials are still employed left-to-right.