Description

DESCRIPTION
^ The transfer gearbox is a 4 wheel drive, two speed ratio, reducing gearbox, incorporating High and Low range outputs. A differential is installed between the front and rear outputs to allow the propeller shafts to rotate at different speeds when the vehicle is cornering. Drive is transmitted to the front propeller shaft through a viscous coupling which eliminates the need for a conventional differential lock.
^ The High/Low range change is switch activated and electronically controlled. On manual vehicles, the High/Low request switch is mounted on the fascia. Automatic vehicles use a micro switch in the Low side of the selector 'H' gate.
^ An electric transfer motor alters the position of the selector sleeve, which determines the selected range. The position of the selector sleeve is monitored by the transfer gearbox ECU through four position switches in the transfer motor.

OPERATION
^ Drive is transmitted to the transfer gearbox by the m<. in gearbox output shaft. The gearbox output shaft is permanently engaged into the sun gear of the single semi-epicyclic gear set. The sun gear transmits drive directly to the selector sleeve when in High range. In Low range, drive is transmitted to the selector sleeve through the planet carrier. The selector sleeve transmits drive to the Morse chain sprocket which, in turn drives the differential. Drive from the differential to the forward propeller shaft is via the viscous coupling.





^ The viscous coupling acts in conjunction with the differential to control the proportion of drive torque delivered to the front and rear drive shafts. The viscous coupling comprises a shallow cylinder attached to the front drive shaft, into which the rear drive shaft is introduced. Slotted discs are attached alternately to the rear drive shaft and the inside of the cylinder connected to the front drive shaft. The discs interleave very closely within the cylinder.
^ The viscous coupling is sealed and filled a special silicon jelly which surrounds the discs. This substance has the unusual characteristic of increasing viscosity, and hence its resistance to flow when agitated and heated. During normal driving conditions, the slight variations in the relative speed of each drive shaft as the vehicle negotiates a corner, is not sufficient to increase the viscosity of the silicon jelly, therefore the coupling offers no resistance.
^ In off-road conditions, as the wheels loose grip on loose or muddy surfaces, a greater differential in the speed of the front and rear drive shafts exists. In these conditions the viscosity of the silicon jelly increases due to the initial greater degree of slippage. This increases the physical drag between the discs making it more difficult for the front and rear drive shafts to rotate at different speeds. The reaction of the viscous coupling in these conditions eliminates the need to install a manual differential lock.
^ Lubrication of the transfer gearbox is achieved by a low pressure plunger type oil pump. The pump is driven by the epicyclic gear set. Oil is delivered through oil ways in the components. The viscous coupling, though partially immersed in lubricating oil, is a self contained, sealed unit.