# Viscous coupling unit > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Viscous_coupling_unit > Markdown URL: https://mediated.wiki/source/Viscous_coupling_unit.md > Source: https://en.wikipedia.org/wiki/Viscous_coupling_unit > Source revision: 1348467781 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) {{Short description|Clutch using viscous fluid to transfer torque}} {{More footnotes needed|date=June 2010}} {{Use dmy dates|date=April 2026}} thumb|An example of a viscous coupling unit opened up thumb|The fluid inside the VC thumb|Completely disassembled thumb|The internal plates A '''viscous coupling''' is a mechanical device which transfers [torque](/source/torque) and [rotation](/source/rotation) by the medium of a [viscous](/source/viscosity) [fluid](/source/fluid). One use is in automobiles as a [differential](/source/Differential_(mechanical_device)), especially in [four-wheel drive](/source/four-wheel_drive) vehicles. ==Design== Rotary viscous couplings with interleaved, perforated plates and filled with viscous fluids are used in automotive systems to transmit torque.{{cite book|url= https://books.google.com/books?id=vJJwAgAAQBAJ&q=viscous+coupling+function+to+transmit+torque&pg=PA748 |page=748 |title=Tribology and dynamics of engine and powertrain: fundamentals, applications and future trends |editor-first=Homer |editor-last=Rahnejat |publisher=Woodhead Publishing |year=2010 |isbn=9781845699932 |access-date=1 April 2014}} The device consists of a number of circular plates with tabs or perforations, fitted very close to each other in a sealed drum. Alternate plates are connected to a driving [shaft](/source/Axle) at one end of the assembly and a driven shaft at the other end. The drum is filled with a [dilatant fluid](/source/power-law_fluid), often [silicone](/source/silicone)-based, to about 80% by volume. When the two sets of plates are rotating in unison, the fluid stays cool and remains liquid. When the plates start rotating at different speeds, the shear effect of the tabs or perforations on the fluid will cause it to heat and become nearly solid because the [viscosity](/source/viscosity) of dilatant fluids rapidly increases with shear. The fluid in this state will effectively glue the plates together and transmit power from one set of plates to the other. The size of the tabs or perforations, the number of plates, and the fluid used will determine the strength and onset of this mechanical transfer. This type of device essentially differs from [fluid coupling](/source/fluid_coupling)s such as [torque converter](/source/torque_converter)s by using the viscosity of the medium to transfer torque, rather than its momentum. This makes it potentially useful even on very small scales and requires less cooling. The torque transmitted is sensitive to the difference in speeds of the input and output but is almost independent of their common rate. ==Vehicles== Viscous couplings are used as the center differential in some [four-wheel-drive](/source/four-wheel-drive) (4WD) [vehicle](/source/vehicle)s. The first mass-produced viscous couplings for a permanent 4WD off-road-capable vehicle were in the [AMC Eagle](/source/AMC_Eagle), which was produced from 1980 to 1988 model years.{{cite journal|url= https://books.google.com/books?id=AuQDAAAAMBAJ&q=The+first+production+car+to+use+the+complete+FF+system,+including+the+VC.+was+the+AMC+Eagle+in+1979&pg=PA56 |page=58 |journal=Popular Mechanics |date=August 1987 |first=John |last=Rettie |title=Four-Wheeling Into Your Future |volume=164 |issue=8 |quote=The first production car to use the complete FF system, including the VC. was the AMC Eagle in 1979. |access-date=1 April 2014}} The AMC Eagle's single-speed model 119 New Process central differential used a viscous coupling filled with a liquid silicone-based material.{{cite book|url= https://books.google.com/books?id=4pqgGnFY6w0C&q=The+single+speed+model+119+New+Process+case+used+a+viscous+coupling+consisting+of+a+liquid+silicone-based+material+to+link+the+front+and+rear+differentials |page=9 |title=Standard Catalog of 4 x 4's: a comprehensive guide to four-wheel drive vehicles including trucks, vans and sports sedans and sport utility vehicles, 1945-1993 |first=Robert C. |last=Ackerson |publisher=Krause Publications |year=1993 |isbn=9780873412032 |access-date=1 April 2014}} It linked the front and rear differentials for quiet and smooth transfer of power to the axle with the greatest traction, on wet or dry pavement.{{cite book|url= https://books.google.com/books?id=6jpVtX9Epi4C&q=AMC+Eagle+innovative+central+viscous+coupling |page=29 |title=Complete Book of Collectible Cars: 70 Years of Blue Chip Auto Investments 1930-2000 |first=Richard M. |last=Langworth |publisher=Publications International |year=2000 |isbn=9780785343134 |access-date=1 April 2014}} Viscous couplings are used as the center differential in cars such as the [Toyota Celica GT-Four](/source/Toyota_Celica_GT-Four), and also as a [limited slip differential](/source/limited_slip_differential) (LSD) in rear axles. They offer a cheaper way to implement four-wheel-drive than technologies like the mechanical-transfer [Torsen](/source/Torsen) differentials. Volvo, Subaru, Land Rover, Vauxhall/Opel, and many others have also used viscous couplings in their drivelines at various times. They are now mostly superseded by electronically controlled devices. == Attributions == [Tony Rolt](/source/Tony_Rolt) is often credited with the original idea and development as applied to automatic couplings in vehicle transmission systems, particularly four-wheel drive, working with [Freddie Dixon](/source/Freddie_Dixon) at the time. [Ferguson Research](/source/Ferguson_Research) and [FF Developments](/source/FF_Developments) were companies formed in the 1970s to commercialize much early work into 4WD systems and devices. It is thought that GKN Driveline owns any remaining worldwide patents today and offers several variants and combinations of viscous couplings integrated with other driveline components. == See also == * [Hele-Shaw clutch](/source/Hele-Shaw_clutch) * [Limited-slip differential](/source/Limited-slip_differential) ==References== {{reflist}} == External links == * [http://auto.howstuffworks.com/differential5.htm Interactive animation] * [http://syncro.org/VCTest.html The Viscous Coupling: how to test & replace it] * [https://web.archive.org/web/20070403212405/http://www.autozine.org/technical_school/traction/tech_traction_4wd_2.htm Different types of 4WD] * [https://web.archive.org/web/20081012065224/http://www.club80-90syncro.co.uk/Syncro_website/TechnicalPages/VC%20Refurb%20Torque%20Curves.htm Some torque~temp VC calibration curves] Category:Mechanical devices using viscosity Category:Vehicle parts Category:Clutches --- Adapted from the Wikipedia article [Viscous coupling unit](https://en.wikipedia.org/wiki/Viscous_coupling_unit) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Viscous_coupling_unit?action=history)). 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