Mechanism of Viscous Coupling

MECHANISM OF VISCOUS COUPLING




The viscous coupling consists of a number of alternately arranged inner and outer plates and air- and-silicone oil mixture filled into a sealed space that is formed by the center differential case and the rear side gear of the differential gear set. The inner plates have their inner perimeters splined to the side gear and the outer plates have their outer perimeters splined to the center differential case. The outer plates are held apart by spacer rings. There are no spacer rings between the inner rings, so the inner rings are movable slightly in axial directions. X-section rings are used to prevent leakage of silicone oil, which would otherwise occur if the oil is pressurized due to large difference in front and rear axle speeds.

TORQUE CHARACTERISTICS




When a speed difference occurs between the center differential case and the rear side gear, a shear force is generated in the silicone oil placed between the outer and inner plates. The torque is then transmitted by the silicone oil between the center differential case and the rear side gear.

The greater the speed difference, the greater the shear force generated in the silicone oil. The relationship between the torque transmission and the speed difference is shown in the figure. As can be seen from the figure, the smaller the speed difference, the smaller the torque transmission and the differential action.

HUMP PHENOMENON
Silicone oil is heated and expanded as differential action continues. The oil in the viscous coupling expands and compresses the inside air to cause direct plate-to-plate contact or a non-viscous operation to occur; this phenomenon is called hump.

The hump eliminates the rotating speed difference between the center differential case and the rear side gear (or locks the differential), so soon after it has occurred, the internal pressure and temperature drop. The viscous coupling then returns to the normal shear torque transmitting operation. (The hump phenomenon does not occur under normal operating conditions.)