Center Differential and Transfer
The layout of the center differential, front differential, transfer drive gear and transfer driven gear is almost the same as with the center differential mechanical locking type, but viscous coupling is built into the transfer block to limit the differential action of the center differential. The center differential assembly consists mainly of the center differential right case, center differential intermediate case, center differential left case, center differential pinion gears, and center differential right side gear.
2. Power transmission
(1) During straight-ahead driving The transmission of engine power from the center differential drive gear to the center differential right and left side gears is the same as that in the center differential mechanical locking type. The power from the center differential right side gear is transmitted via the transaxle mode select sleeve to the transfer drive gear, transfer driven gear and rear propeller shaft. It is also transmitted to the viscous coupling outer plate. Since the No.2 intermediate shaft is inserted into the center differential left side gear, power is also transmitted to the viscous coupling inner plate.
(2) During cornering Since the viscous coupling acts to reduce the difference in the number of revolutions between the center differential right and left side gears, the differential action of the center differential is limited. The differential limiting function is not so strong as to obstruct smooth turning. (2/4)
3. Center differential (viscous coupling)
(1) Construction The center differential gear is the bevel gear type and the center differential limit control mechanism is the viscous coupling type. The viscous coupling is built into the transfer ring gear mounting. The inner plates of the viscous coupling are joined to the center differential left side gear and the front differential case, while the outer plates of the viscous coupling are joined to the ring gear mount case and the center differential right side gear. Since the viscous coupling is built into the transfer ring gear mounting case the construction has been changed from the three-axis center differential shaft, based on the mechanical locking transaxle, to the four-axis type. A viscous coupling is a kind of fluid clutch which transmits torque by the viscous resistance of oil. It uses this viscous resistance to control the operation of the center differential, creating an effect in the center differential like that created by an LSD (Limited Slip Differential).
(2) Operation The center differential with viscous coupling operates when rotational speed differences between the inner plates (front) and outer plates (rear) of the viscous coupling occur, transmitting power and limiting the differential action of the center differential. Torque is transmitted by the viscous resistance of the silicone oil.
<1>When plates rotate at identical speeds No viscous resistance is generated.
<2>When plates rotate at different speeds When the plates begin to turn at different speeds, the silicone oil particles are pulled away from each other and a resistance is generated. This resistance reduces the difference in speed between the two plates.
<3>Transmitted torque characteristics
During normal operation The amount of viscous resistance due to differences in the rotational speeds of the outer and inner plates increases or decreases in proportion to the extent of the difference in speed.
During humping In the viscous coupling continues to operate with the outer and inner plates rotating at large different speeds, the temperature and internal pressure inside the viscous coupling increases. The inner plate, which can move in an axial direction, is pulled toward the side where pressure is lower. As a result, the inner plate contacts the outer plate directly to create a large resistance. This condition is called humping. Since there exists no speed difference between the inner and outer plates during humping, the temperature and internal pressure inside the viscous coupling decreases. The compressed bubbles expand again and separate the inner plate from the outer plate.