In this application, when high-range is engaged for driving on firm road surfaces, differences in speed between the front and rear wheels, are catered for by the third differential gears in the transfer case output.
However, the third differential operation means that if a front wheel spins on a slippery surface, no drive is transmitted to the rear.
To reduce this action of the third differential operation, a viscous coupling unit between the differential case and the rear output shaft acts as a limited slip differential.
The outer plates of the viscous coupling are splined to the differential case and the inner plates are splined to the output shaft.
Torque transfer through the coupling is achieved by the high viscosity of the silicone fluid sealed in the unit. As the fluid expands when it is heated, the unit is only filled to about 80% of its capacity.
The outer plates are separated by spacer rings. The inner plates are free to move axially and are not mechanically centered between the outer plates. In normal driving conditions, the third differential operates to provide the small degree of differential action necessary. There is only a minor speed difference between the viscous coupling plates.
However, when a wheel spins, there is a higher level of third differential action, and a greater speed difference between the front and rear propeller shafts.
This speed difference also applies to the viscous coupling plates. Fluid in the unit is sheared by the relative motion between them.
This shearing action causes localized expansion, forcing the inner plates into contact with the outer plates.
The degree of locking achieved depends on the severity of the wheel slippage occurring.
100% locking is only achieved under severe conditions.
When the plates are fully locked the fluid starts to cool, and the cycle of shearing and locking re-commences.