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{{Short description|Type of simple planetary gear train}}
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{{More citations needed|date=July 2015}}
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A '''differential''' is a [[gear train]] with three [[drive shaft]]s 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 vehicle]]s, to allow the wheels at each end of a [[drive axle]] to rotate at different speeds while cornering. Other uses include clocks and [[
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]].
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=== Epicyclic design ===
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[[File:Epicyclic gear ratios.png|thumb|Diagram of an [[epicyclic gearing]] system]]
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== Other uses ==
{{More citations needed section|date=March 2023}}
[[File:ChartDriveDetail.JPG|thumb|upright|Planetary differential used to drive a [[chart recorder]] c. 1961
Non-automotive uses of differentials include performing [[Analog signal|analogue]] [[arithmetic]]. Two of the differential's three shafts are made to rotate through angles that represent (are proportional to) two numbers, and the angle of the third shaft's rotation represents the sum or difference of the two input numbers. The earliest known use of a differential gear is in the Antikythera mechanism, c. 80 BCE, which used a differential gear to control a small sphere representing the Moon from the difference between the Sun and Moon position pointers. The ball was painted black and white in hemispheres, and graphically showed the phase of the Moon at a particular point in time.<ref name="Wright-Reconsidered" /> An [[equation clock]] that used a differential for addition was made in 1720. In the 20th century, large assemblies of many differentials were used as [[Analog computer|analogue computers]], calculating, for example, the direction in which a gun should be aimed.<ref>{{cite AV media |title=Basic Mechanisms in Fire Control Computers, Part 1, Shafts Gears Cams and Differentials, posted as 'U.S. Navy Vintage Fire Control Computers' |medium=Training Film |url=https://www.youtube.com/watch?v=mQhmmTX5f9Y&list=PLF1A5EC36E8EDF711 |archive-url=https://ghostarchive.org/varchive/youtube/20211118/mQhmmTX5f9Y| archive-date=18 November 2021 |url-status=live |date=1953
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Chinese south-pointing chariots may also have been very early applications of differentials. The chariot had a pointer which constantly pointed to the south, no matter how the chariot turned as it travelled. It could therefore be used as a type of [[compass]]. It is widely thought that a differential mechanism responded to any difference between the speeds of rotation of the two wheels of the chariot, and turned the pointer appropriately. However, the mechanism was not precise enough, and, after a few miles of travel, the dial could be pointing in the wrong direction.
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The earliest verified use of a differential was in a clock made by Joseph Williamson in 1720. It employed a differential to add the [[equation of time]] to [[local mean time]], as determined by the clock mechanism, to produce [[solar time]], which would have been the same as the reading of a [[sundial]]. During the 18th century, sundials were considered to show the "correct" time, so an ordinary clock would frequently have to be readjusted, even if it worked perfectly, because of seasonal variations in the equation of time. Williamson's and other equation clocks showed sundial time without needing readjustment. Nowadays, we consider clocks to be "correct" and sundials usually incorrect, so many sundials carry instructions about how to use their readings to obtain clock time.
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[[Differential analyzer|Differential analysers]], a type of mechanical analogue computer, were used from approximately 1900 to 1950. These devices used differential gear trains to perform addition and subtraction.
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The Mars rovers ''[[Spirit (rover)|Spirit]]'' and ''[[Opportunity (rover)|Opportunity]]'' (both launched in 2004) used differential gears in their [[rocker-bogie]] suspensions to keep the rover body balanced as the wheels on the left and right move up and down over uneven terrain.<ref>{{cite web |title=Rover Wheels |url=https://mars.nasa.gov/mer/mission/rover/wheels-and-legs/ |website=Mars.NASA.gov |access-date=18 January 2023 }}</ref> The ''[[Curiosity (rover)|Curiosity]]'' and ''[[Perseverance (rover)|Perseverance]]'' rovers used a differential bar instead of gears to perform the same function.<ref>{{cite web |title=Curiosity Mobility System, Labeled |url=https://www.planetary.org/space-images/curiosity-mobility-system |website=Planetary.org |access-date=18 January 2023 }}</ref>
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==References==
{{Reflist}}
==Further reading==
* [https://books.google.com/books?id=kiEDAAAAMBAJ&pg=PA76 ''Popular Science'', May 1946, ''How Your Car Turns Corners''], a large article with numerous illustrations on how differentials work▼
==External links==
{{Commons category|Automobile differentials}}
* [https://www.youtube.com/watch?v=vBm-SzO3ggE A video of a 3D model of an open differential]
▲* [https://books.google.com/books?id=kiEDAAAAMBAJ&pg=PA76 ''Popular Science'', May 1946, ''How Your Car Turns Corners''], a large article with numerous illustrations on how differentials work
{{Gears}}
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