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{{Short description|Type of simple planetary gear train}}
{{Use
{{Use dmy dates|date=
{{More citations needed|date=July 2015}}
{{Multiple image
}}
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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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 diving|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.<ref name="Wright-Reconsidered">{{cite journal |last=Wright |first=M. T. |date=2007 |title=The Antikythera Mechanism Reconsidered |journal=Interdisciplinary Science Reviews |volume=32 |number=1 |pages=27–43 |doi=10.1179/030801807X163670 |bibcode=2007ISRv...32...27W |s2cid=54663891 |url=http://fsoso.free.fr/antikythera/DOCS/TheAntikytheraMechanismReconsidered.pdf |access-date=8 June 2023 }}</ref>{{clarify|Please provide a quote to support that this mechanism is a predecessor to the modern differential devices|date=February 2023}}
*
* 1810: [[Rudolph Ackermann]] of Germany invents a four-wheel steering system for carriages, which some later writers mistakenly report as a differential.
* 1823: [[Aza Arnold]] develops a differential drive train for use in [[Cotton-spinning machinery|cotton-spinning]]. The design quickly spreads across the United States and into the United Kingdom.<ref>{{Cite book |last=Mitman |first=Carl W. |date=1947 |editor-last=Johnson |editor-first=Allen |url=https://archive.org/details/dictionaryofamer01ilamer |title=Dictionary of American Biography |chapter=Arnold, Aza |publisher=[[Charles Scribner's Sons]], [[American Council of Learned Societies]] |volume=1 |pages=361–362 |location=New York }}</ref>
* 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]].<ref>{{cite web|url=http://www.gmcanada.com/inm/gmcanada/english/about/OverviewHist/hist_auto.html |title=History of the Automobile |publisher=General Motors Canada |access-date=9 January 2011 }}</ref>
* 1874: [[Aveling and Porter]] of [[Rochester, Kent]] list a crane locomotive in their catalogue fitted with their patent differential gear on the rear axle.<ref>{{cite book |last=Preston |first=J.M. |title=Aveling & Porter, Ltd. Rochester |publisher=North Kent Books |date=1987 |isbn=0-948305-03-7 |pages=13–14 }}</ref>
* 1876: [[James Starley]] of [[Coventry]] invents chain-drive differential for use on [[bicycle]]s; invention later used on automobiles by [[Karl Benz]].
* 1897: While building his Australian [[steam car]], [[David Shearer (engineer)|David Shearer]] made the first use of a differential in a motor vehicle.<ref>{{cite web |title=David Shearer's Steam Car at Mannum in 1897 – Australia's First – with World-First Differential |url=https://adelaideaz.com/articles/david-shearer-s-steam--car--at-mannum-1897--australia-s-first---with--world-first-differential- |website=AdelaideAZ.com |access-date=27 February 2023 }}</ref>
* 1958: Vernon Gleasman patents the [[Torsen]] [[limited-slip differential]].<ref>{{cite web |title=Inventor Of Automotive Technologies – Vernon Gleasman's Legacy |url=https://www.theautochannel.com/news/2004/11/19/285488.html |website=TheAutoChannel.com |access-date=27 August 2023 }}</ref>
== Use in wheeled vehicles ==
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=== Ring-and-pinion design ===
[[File:Transmission diagram.JPG|thumb|left|Illustration of a ring-and-pinion differential for a rear-wheel drive vehicle]]
[[File:Differential (Manual of Driving and Maintenance).jpg|thumb
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]].
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=== Epicyclic design ===
<!--
[[File:Epicyclic gear ratios.png|thumb
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.{{citation needed|date=March 2023}} An advantage of the epicyclic design is that it is relatively compact width (when viewed along the axis of its input shaft).{{citation needed|date=March 2023}}
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{{Main|Limited-slip differential}}
An undesirable side-effect of a regular ("open") differential is that it can send most of the power to the wheel with the lesser traction (grip).<ref>{{cite book |last=Bonnick |first=Allan |date=2001 |url=https://books.google.com/books?id=odUZ0O_OWZwC&pg=PA22 |title=Automotive Computer Controlled Systems |isbn=9780750650892 |page=22 |publisher=Elsevier Science & Technology Books }}</ref><ref>{{cite book |last=Bonnick |first=Allan |date=2008 |url=https://books.google.com/books?id=78HoZb-ohbIC&pg=PA123 |title=Automotive Science and Mathematics |isbn=9780750685221 |page=123 |publisher=Butterworth-Heinemann }}</ref> In situation when one wheel has reduced grip (e.g. due to cornering forces or a low-grip surface under one wheel), an open differential can cause [[wheelspin]] in the tyre with less grip, while the tyre with more grip receives very little power to propel the vehicle forward.<ref>{{cite web |last=Chocholek |first=S. E. |date=1988 |url=http://zhome.com/ZCMnL/tech/Torsen/Torsen.htm |title=The Development of a Differential for the Improvement of Traction Control }}</ref>
In order to avoid this situation, various designs of ''limited-slip differentials'' are used to limit the difference in power sent to each of the wheels.
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=== Torque vectoring ===
{{main|Torque vectoring}}
''Torque vectoring'' is a technology employed in automobile differentials that has the ability to vary the torque to each half-shaft with an electronic system; or in rail vehicles which achieve the same using individually motored wheels. In the case of automobiles, it is used to augment the stability or cornering ability of the vehicle.
== Other uses ==
{{More citations needed section|date=March 2023}}
[[File:ChartDriveDetail.JPG|thumb|upright|Planetary differential used to drive a [[chart recorder]]
Non-automotive uses of differentials include performing [[Analog signal|
===
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.
===
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.
===
[[Differential analyzer|Differential analysers]]
▲=== Vehicle suspension ===
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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