Operation

OPERATION

INTRODUCTION
^ The Borg Warner transfer box splits the drive from the main gearbox to the front and rear axles through the propeller shafts. Two speed ratios and a neutral position are provided by means of a single, epicyclic gear set. The two speed ratios, High and Low range, and neutral position are selected electronically by the ratio control motor.
^ A Morse chain transmits the drive through a gear from the intermediate shaft to the differential unit. The differential unit allows the front and rear output shafts to rotate at different speeds. A Viscous Coupling Unit (VCU) limits the amount of slippage allowed between the front and rear output shafts and renders a conventional differential lock unnecessary.





HIGH AND LOW RANGE
^ High and Low range are selected by the driver from a dashboard mounted switch on manual vehicles and via the selector lever on automatic vehicles. Transfer neutral is selected by inserting a 5 amp fuse in position 11 of the driver's seat fuse box. Refer to Owner's Handbook.
^ Drive from the main gearbox is permanently engaged to the sun gear of the epicyclic gear set. When in High range (position B) the sun gear transmits drive directly to the reduction hub. The reduction hub and intermediate shaft rotate at the same speed as the main gearbox output shaft. When in Low range (position A) the reduction hub is driven through the planet carrier. The reduction hub and intermediate shaft rotate at a lower speed than the gearbox output shaft. When in transfer neutral, the reduction hub is positioned between the sun gear and planet carrier.
^ Changing between ratios is achieved by the reduction hub sliding along a splined shaft of the intermediate shaft to engage with the required gear. The ratio change mechanism comprises the ratio control motor, reduction hub, interlock spool and selector fork.
^ The interlock spool consists of a shaft which locates in the front casing and protrudes through the rear casing to engage with the ratio control motor. The shaft carries an aluminium casting with a helical cam track and two springs, which are anchored at one end to the shaft and at the other to the cam track casting.
^ The selector fork is mounted on a second shaft located within the front and rear casings but is able to slide in the casing mountings. A cam follower on the selector fork engages with the cam track of the interlock spool. When the ratio control motor rotates the interlock spool, the cam follower of the selector fork follows the cam track of the interlock spool. This converts rotational movement of the interlock spool into linear movement of the selector fork.
^ The selector fork is engaged to the reduction hub. Linear movement of the selector fork is transmitted to the reduction hub, moving it between High, Low and neutral positions. In the event of the reduction hub gear failing to mesh with the epicyclic gear set, wind-up of the interlock is prevented by the interlock spool springs. The springs apply a constant torque to the cam track casting until the reduction hub engages with the epicyclic gear.





RATIO CONTROL MOTOR
^ The ratio control motor drives the selector mechanism and is controlled by the transfer box electronic control unit (ECU). The ECU monitors a number of variables including the position of the ratio control motor, the speed of the vehicle, and the drive ratio selected by the driver.
^ When a ratio change is requested, the transfer box ECU checks that conditions are favorable for the change, for example that the vehicle speed is sufficiently low to permit engagement. The ECU then drives the ratio control motor to the required position.
^ When the conditions for a ratio change are unfavorable, the transfer box ECU communicates instructions to the driver through the Message Center and will not attempt to change ratios until the correct conditions are met.





DIFFERENTIAL UNIT
^ The differential unit is driven from the intermediate shaft through a Morse chain. The outer casing of the differential unit is the differential input, while the sun gear provides the front output and the planet carrier the rear output.
^ The planet carrier contains three sets of gears, which mesh in pairs to maintain the correct directional relationship between front and rear differential outputs. The rear output shaft passes through the differential unit, engaging with the planet carrier and protruding through the sun gear shaft to locate to the VCU inner spline. The sun gear shaft locates to the VCU outer spline.





VISCOUS COUPLING (VCU)
^ The Viscous Coupling (VCU) comprises a short cylinder which contains an inner shaft with slotted discs attached to its outer surface, and a similar set of discs attached to the inner surface of the cylinder. Both sets of discs are arranged so that they interleave alternately and in close proximity to each other. The VCU is sealed, and filled with a type of silicone jelly which has the property of increasing its viscosity with rises in temperature and shear forces.
^ Variations in speed between the front and rear output shafts are transmitted through the VCU, with the speed differential occurring between the inner shaft and the cylinder. In normal road conditions where the speed variation between front and rear shafts is low, the difference in rotational speed between the VCU discs is also low. As a result, the shear forces acting on the silicon jelly are marginal and offer little resistance to the different rotational speeds of the output shafts.
^ In cases where large rotational speed differences occur between the front and rear output shafts, such as in rough terrain conditions, the speed variation between the discs is high with a subsequent increase in the shear forces acting on the viscous jelly. The resulting increase in viscosity generates sufficient shear resistance to force both sets of discs to rotate at similar speeds, reducing axle slippage and loss of traction.

LUBRICATION
Internal lubrication of the transfer gearbox is provided by a low pressure, plunger-type oil pump mounted on the rear of the intermediate shaft. Oil pick-up is through a strainer in the transfer box oil pan. From the pump, oil is supplied through the intermediate shaft to the epicyclic gear set. The differential and Morse chain are partially immersed-in oil and lubricated as the components rotate. The VCU is a fully sealed unit and does not require separate lubrication.