Viscous coupling is an application used in all-wheel drive systems. The idea of the coupling is based on a special fluid which is inside of a hermetic container. In the container there are numerous plates which are installed very close to each other. Odd plates are attached to a cylinder-like outer rim which is fixed to a cardan shaft. Even plates are attached to a bar-like inner axle which is fixed to the other half of the cardan shaft. 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 friction when the plates are spinning at different speed and gets sticky or solid in fractions of a second. The fluid returns to liquid in no-time when the spinning speed difference has disappeared due the same rotation speed of front and rear wheels. So as a result, when the front wheels slip, the fluid gets hot from friction and it makes the axles stick together and makes power go to the rear wheels as well. Four wheel drive or 4x4, is a type of four wheeled vehicle drivetrain configuration that enables all four wheels to receive power from the engine simultaneously in order to provide maximum traction. ... Generally, coupling means a mechanical connection between two things. ... An axle is a central shaft for a rotating wheel or gear. ... Friction is the force that opposes the relative motion or tendency of such motion of two surfaces in contact. ...

If the car is driven so that the front wheels are always getting power from the motor and the rear wheels are getting the power from the cardan shaft through a viscous coupling, the power ratio between the front and rear wheels could change from 100% front and 0% rear to 50% front and 50% rear. The 50/50 is naturally a situation of complete stuck between the ends of the cardan shaft // Mechanical power In physics, power (symbol: P) is the amount of work W done per unit of time t. ...