Subaru Impreza 2002 Service Manual page 57

CENTER DIFFERENTIAL
Manual Transmission and Differential
1. 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 rela-
tionship between the torque transmission and the speed difference is shown in the figure below.
As can be seen from the figure, the smaller the speed difference, the smaller the torque transmis-
sion and the differential action.
NF0122
2. HUMP PHENOMENON
Silicone oil is heated and expanded as differential action continues. This causes the pressure of
air inside the viscous coupling to increase and the pressure of oil between plates to decrease. As
a result, the inner and outer plates are pushed together. This direct plate-to-plate contact causes
a non-viscous operation to occur, and 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 op-
eration. (The hump phenomenon does not occur under normal operating conditions.)
6MT-18