Viscous coupling unit

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Viscous coupling is an application used in all-wheel drive systems and torque converters for automatic trasmissions. The coupling is caused by a special viscous fluid which is inside of a hermetic container. In the container, there are numerous plates which are installed very close to each other, with the viscous fluid stuck between and around the plates. Half of the plates are connected to the input shaft. The other plates are connected to an output shaft.

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The system works when the spinning speed difference of the inner and outer axles occur (this happens when the front wheels are slipping and the rear wheels are not). The special fluid inside the coupling unit gets heated by the friction created when the plates are spinning at different speeds. The disks that are not slipping attempt to keep up with the viscous fluid, which is turning as fast as the slipping disks. This can be considered a basic clutch. In a viscous coupling, this occurs very quickly to reduce wheelspin.

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This is why it is virtually impossible to perform a burnout in an all-wheel drive vehicle. Attempting to do so can cause significant driveline shock, possibly resulting in damage to the vehicle's transmission and/or drivetrain.

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[edit] Example

You can use an egg to see how basic viscous coupling works. Place an egg on a flat surface. In this state, the egg, shell, and yolk are all stationary. Quickly spin the egg several times on its side. Continue to keep it spinning for several seconds. The shell itself is spinning at a faster rate(rpm) than the yolk for a short time, but the yolk will quickly catch up. This represents the coupling action. If you suddenly stop the egg from spinning and then quickly let go, the yolk inside will still be spinning, and have enough torque to cause the egg to continue spinning on its own power for a couple of seconds. This shows viscous coupling between the yolk and shell. In a viscous coupling, the force is applied between the fluid and the sets of plates in the same way as between the yolk and the shell.

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Examples of cars that use a viscous coupling all-wheel drive unit include some DSM vehicles including the Mitsubishi Eclipse, Eagle Talon, Plymouth Laser, Mitsubishi Galant, Mitsubishi 3000GT.

Also, all recent Subaru vehicles are all-wheel drive including Subaru Impreza, Subaru Legacy, and Subaru Outback.