A viscous coupling is a mechanical device which transfers torque

Torque

Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....

 and rotation

Rotation

A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

 by the medium of a viscous

Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 fluid

Fluid

In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

. It is made 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

Axle

An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...

 at one end of the assembly and a driven shaft at the other end. The drum is filled with a dilatant fluid, often silicone

Silicone

Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....

-based. 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

Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher 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

Fluid coupling

A fluid coupling is a hydrodynamic device used to transmit rotating mechanical power. It has been used in automobile transmissions as an alternative to a mechanical clutch...

s such as torque converter

Torque converter

In modern usage, a torque converter is generally a type of hydrodynamic fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load...

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. It tends to have 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.

Viscous couplings are used as the center differential in some four-wheel-drive (4WD) vehicle

Vehicle

A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....

s such as the Toyota Celica GT-Four

Toyota Celica GT-Four

The Toyota Celica GT-Four is a high performance model of the Celica liftback, with a turbocharged 3S-GTE engine, and full-time AWD. It was created to compete in the World Rally Championship, whose regulations dictate that a manufacturer must build road-going versions of the vehicle in sufficient...

, and also as a limited slip differential

Limited slip differential

A limited slip differential is a type of differential gear arrangement that allows for some difference in angular velocity of the output shafts, but imposes a mechanical bound on the disparity...

 (LSD) in rear axles. They offer a cheaper way to implement four-wheel-drive than technologies like the mechanical-transfer Torsen

Torsen

Torsen is a type of differential used in automobiles. It was invented by American Vernon Gleasman and manufactured by the Gleason Corporation. Torsen is a contraction of Torque-Sensing...

 differentials used by Audi

Audi

Audi AG is a German automobile manufacturer, from supermini to crossover SUVs in various body styles and price ranges that are marketed under the Audi brand , positioned as the premium brand within the Volkswagen Group....

s.

One of the first mass-produced viscous couplings for a permanent 4WD off-road-capable vehicle was used in the AMC Eagle

AMC Eagle

The AMC Eagle is a compact-sized four-wheel drive passenger vehicle that was produced by American Motors Corporation . The AMC Eagle line of vehicles inaugurated a new product category of "sport-utility" or crossover SUV....

, produced from 1980 to 1987. The AMC Eagle's central differential was single-speed (no low range option) and used a thick viscous fluid coupling for quiet and smooth transfer of power to the axle with the greatest traction, on wet or dry pavement

Volvo, Subaru, Range Rover, Vauxhall/Opel and many others have used viscous couplings in their drivelines at various times, now mostly superseded by more complicated electronically controlled devices.

Attributions

Tony Rolt

Tony Rolt

Major Anthony Peter Roylance "Tony" Rolt, MC & Bar, was a British racing driver, soldier and engineer. He won the 1953 24 Hours of Le Mans and participated in three Formula One World Championship Grands Prix without scoring a championship point...

 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 Ltd.

Ferguson Research Ltd.

Harry Ferguson Research Limited was a British company founded by Harry Ferguson who was mostly known as "the father of the modern farm tractor"...

and FF Developments were the companies formed (1970's) to commercialise much early work into 4WD systems and devices. It is thought that GKN Driveline own any remaining worldwide patents today and offer several variants and combinations of Viscous Couplings integrated with other driveline components.

External links

• Interactive animation
• http://syncro.org/VCTest.html
• http://www.autozine.org/technical_school/traction/tech_traction_4wd_2.htm
• Some torque~temp VC calibration curves