A viscous coupling is a mechanical device which transfers

torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of t ...

and

rotation Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...

by the medium of a

viscous The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shea ...

Design

Rotary viscous couplings with interleaved, perforated plates and filled with viscous fluids are used in automotive systems to transmit torque. 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 Shaft may refer to: Rotating machine elements * Shaft (mechanical engineering), a rotating machine element used to transmit power * Line shaft, a power transmission system * Drive shaft, a shaft for transferring torque * Axle, a shaft around whi ...

at one end of the assembly and a driven shaft at the other end. The drum is filled with a dilatant fluid, often 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 The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

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 couplings such as

torque converter A torque converter is a type of fluid coupling that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the torque converter connects the pow ...

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 Four-wheel drive, also called 4×4 ("four by four") or 4WD, refers to a two-axled vehicle drivetrain capable of providing torque to all of its wheels simultaneously. It may be full-time or on-demand, and is typically linked via a transfer case ...

(4WD)

vehicle A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles ( motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles ( trains, trams ...

s. The first mass-produced viscous couplings for a permanent 4WD off-road-capable vehicle was in the AMC Eagle, which was produced from 1980 to 1988 model years. The AMC Eagle's single-speed model 119 New Process central differential used a viscous coupling filled with a liquid silicone-based material. 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. Viscous couplings are used as the center differential in cars such as the Toyota Celica GT-Four, and also as a limited slip differential (LSD) in rear axles. They offer a cheaper way to implement four-wheel-drive than technologies like the mechanical-transfer 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 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 at the time. Ferguson Research and 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.

References

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External links

Interactive animation

Mechanical devices using viscosity Vehicle parts Clutches

Design

Vehicles

Attributions

See also

References

External links