However, when the front and rear axles start to rotate at different speeds, the swash-plate rotates relative to the rollers
which generates the hydraulic pressure. This pressure is used to force the opposing clutch plates together, increasing the
transmission of torque to the rear axle. As the difference in axle speed increases the hydraulic pressure pushes the clutch
plates further together to increase the torque to the rear axle.
A control valve/axial solenoid controls the amount of pressure applied to the clutch plates, and hence the amount of
torque transmitted to the rear wheels. Close manufacturing tolerances and exceptionally low component wear ensure
torque control remains accurate throughout the vehicle's life.
Mechanical Hydraulic Pump - Generation 4 Couplings - Vehicles from 2009MY
The Generation 4 coupling does not use the swash plate to mechanically raise hydraulic pressure; instead, a new hydraulic
pump is used to generate hydraulic pressure and force the clutch plates together. The removal of the swash plate allows a
larger overall clutch plate surface area, which in turn reduces the hydraulic pressure requirement. The pressure required to
achieve 1500Nm (1106 lb ft) has been reduced from 100 Bar for Generation 3 couplings to 40 Bar for Generation 4
couplings.
The positioning of the Generation 4 coupling's input and output remains the same as the Generation 3 coupling, as
described above.
By-pass Valve
On very low friction surfaces, driveline drag torque can occur, for example:
reverse torque from engine braking, or
forced movement of the driveshaft by the front wheels.
This can influence rear wheel speed, making it impossible to determine the true friction capability of the rear wheels, by
distorting the wheel speed signal. To prevent this, the active on-demand coupling is designed to open immediately in
response to a stability control event. This is achieved by a by-pass valve instantly reducing system pressure to nominal.
To balance the 4 bar base pressure (see below for details), the Generation 3 coupling uses a large Belleville spring to force
the clutch plates clear of each other to prevent torque transmission through the coupling. Even at 0°C, torque transmission
is reduced from 300 Nm to Zero within 10 ms. The Generation 4 coupling however, does not require a Bellville spring to
separate the plates as the coupling does not have a significant base pressure pushing the plates together.
Accumulator
The further the clutch plates have to move in order to contact each other, the longer it takes to displace the hydraulic fluid
necessary to build pressure and transmit torque. To counter this, the Generation 3 coupling incorporates an accumulator.
This retains a nominal 4 bar pressure within the hydraulic circuit. Although this is not enough pressure to cause significant
torque transmission through the coupling, it forces the plates very close together so that very little fluid displacement is
required to achieve full engagement and maximum torque transfer. Full torque transmission can be achieved in 150 ms.
On Generation 4 couplings, the 150ms activation time is achieved through the use of a Bellville spring that acts to push
the plates together (without causing significant torque transmission through the coupling). As the pump is not
continuously used to create this base pressure (as it was with Generation 3 coupling), improvements in fuel economy have
been achieved.
Wet Clutch Pack
The clutch pack is made up of 7 pairs of plates; the inner discs are produced from hardened steel with the outer discs
manufactured from steel with a sintered face. The clutch plates operate in transmission fluid.
Torque transmission across the clutch pack is limited to 1500 Nm (1106 lb ft). This ensures the lower gears retain an
element of front-wheel-drive for traction stability. Within the higher gears the coupling is theoretically capable of
transmitting all the drive to the rear axle; although conditions would have to be extreme for this to occur.
PRINCIPLES OF OPERATION
Generation 3 Couplings - up to 2009MY
An internal electronically-controlled pump provides hydraulic pre-charge pressure within the coupling. The pre-charge
pressure supplies the required operating pressure to the clutch plates to eliminate initial wheel-spin as the vehicle
accelerates from standstill.
In conjunction with the pre-charge pressure a mechanical hydraulic pump operates within the clutch plates to supply the
coupling's main hydraulic operating pressure. The mechanical pump is functioned by the 'input' and 'output' of the coupling:
input - driveshaft connection from the front axle,
output - differential connection to the rear axle.
Any speed difference between the front and rear axles will start the operation of the mechanical hydraulic pump. The
amount of hydraulic pressure applied to the clutch pack by the pump determines the gap between the clutch plates. For
example, the greater the hydraulic pressure, the smaller the gap between the plates and subsequently the greater the
torque transmitted through the coupling from the front axle to the rear axle.
This main hydraulic pressure is designed to transmit the torque for traction demands of off-road driving, and to provide
lock-up as required.
Generation 4 Couplings - from 2009MY
The Generation 4 coupling still uses an electrically-controlled pump to provide hydraulic pre-charge to eliminate wheel-spin
from standstill; however, the same pump is also used to provide the coupling's main hydraulic operating pressure.