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Usually present in all-wheel -drive vehicles, viscous coupling is a mechanical device which functions to join the front wheels of the vehicle to the back wheels so that torque /power can be alternated between both sets of wheels as and when required. 

The viscous coupling consists of a coupling housing, containing a dense liquid called viscous fluid, in which there are two sets of plates; one set of plates per output shaft. The plates connect to the corresponding output shaft. 

Usually the plates inside the housing and the liquid rotate at the same speed. If the front wheels begin to rotate faster than the back wheels, for example, commonly caused by the wheels slipping, the plates that link up to the front set of wheels begin to spin quicker than the plates linked to the back wheels. This causes the viscous fluid inside the coupling housing, which is sandwiched between the two sets of plates, to spin faster as it attempts to rotate at the same speed as the spinning plates attached to the front wheels. This causes the plates attached to the back wheels to spin faster, which transfers extra torque to the slower moving back wheels. The difference between the speeds of the rotating plates determines how much torque is transferred from the viscous coupling to the wheels that aren't slipping. The bigger the difference in speed between the two sets of spinning plates, the more torque the viscous coupling transfers. The viscous coupling only transfers torque to the other set of wheels when one set starts to slip. Excess in torque produces wheel slip; Wheels usually slip when full acceleration is applied suddenly, causing the wheels to lose traction with the surface of the road.

Problems can occur with the viscous coupling, for example the coupling can get locked solid and the plates can wear away over time caused by general wear and tear on this mechanical part. The viscous coupling provides the perfect replacement part when the viscous coupling is damaged. Manufactured to be compatible with the make and model of your vehicle, a good quality viscous coupling should install easily, perform well and ultimately last longer.