Viscous Coupling: Description and Operation

System overview
Active On-demand Coupling





The main role of Active On-demand Coupling and the control module for all wheel drive ([1][2]Differential Electronic Module) is to regulate the all wheel drive function, i.e. the distribution of drive between the front and rear axles. The system consists of a coupling assembly with a mechanical, hydraulic and electronic component. The coupling is located on the rear axle, between the axle gear and the propeller shaft. The mechanical and hydraulic component is driven by the rotation of the propeller shaft.
The Active On-demand Coupling provides the following characteristics:
- Permanent all wheel drive with electronic control of torque transfer
- Driving characteristics similar to all wheel drive
- Fast reaction
- No wind-up forces during low speed maneuvering
- The system is not sensitive to differences between tires (when driving with a spare wheel for example)
- The system is not sensitive to being towed with one axle raised
- The system is not sensitive to brake tests on a chassis dynamometer.

Coupling unit and control module ([1][2]differential electronic module)





The hydraulic component consists of 1-5, the mechanical component consists of 6-10 and the electronic component consists of 11-14.
1. Pressure valves
2. Accumulator
3. Oil filter
4. Annular pistons
5. Control valves
6. Input shaft
7. Inner and outer wet plates
8. Sinusoidal cam disc
9. Rollers with annular pistons
10. Output shaft
11. Electric oil pump
12. Stepper motor for the control valve
13. Temperature sensor
14. Control module for all wheel drive ([1][2]differential electronic module).

The [1][2]differential electronic module is integrated with the coupling unit. The differential electronic module communicates with other control modules via CAN. The control module has no internal diagnostics.

Signals








The table summarizes the input and output signals to and from the [1][2]differential electronic module. The illustration displays the same information with the Volvo part numbers.

Design
Control module
The [1][2]differential electronic module is mounted directly onto the housing for the coupling unit. It forms part of a unit with the stepper motor and the control valve.
The control module receives signals from the Engine Control Module (ECU), central electronic module and brake control module via the Control Area Network (CAN). The control module uses these signals to determine the oil pressure for the clutch pack in the coupling unit. The oil pressure on the multi-plate clutch determines the torque that can be transferred to the rear wheels.
All wheel drive is disengaged if there is a fault in the [1][2]differential electronic module.

Temperature sensor





The temperature sensor in the coupling unit is mounted close to the control valve in the housing for the control module. The sensor is immersed in the hydraulic fluid. The sensor measures the temperature of the hydraulic fluid and transmits a signal to the [1][2]differential electronic module. The differential electronic module uses the temperature signal to adapt the system when the temperature, and therefore the viscosity, of the fluid changes.
If the temperature of the fluid exceeds 100°C there is no pressure in the coupling and all wheel drive is disengaged. When the temperature falls below 100°C the coupling is pressurized again.
All wheel drive is disengaged if there is a fault in the temperature sensor.

Stepper motor





The stepper motor (4) is built into the housing for the [1][2]differential electronic module. The differential electronic module supplies the stepper motor with power. A control signal to the stepper motor from the differential electronic module changes the position of a control spindle (3) in the control valve (2) via a small gear wheel (5). The position of the control spindle alters the cross section of a return duct (1) in the valve. This controls the pressure to the operating piston for the multi-plate clutch.

Electric feed pump
The electric feed pump is built into the coupling unit. The pump is supplied with power by the [1][2]differential electronic module. When the engine is started and exceeds 400 rpm the differential electronic module supplies the feed pump with power.
The feed pump pumps fluid to the annular pistons. This pushes the rollers for the pistons into contact with the cam disc. At the same time hydraulic fluid flows to the operating piston. This eliminates play in the multi-plate clutch pack. The coupling is now able to react and regulate torque distribution rapidly.
All wheel drive is disengaged if there is a fault in the feed pump.

Function
Electronic function





1. Control module for all wheel drive ([1][2]differential electronic module)
2. Stepper motor
3. Control valve
4. Pressure flow
5. Control valve cover.

The [1][2]differential electronic module is mounted directly onto the housing for the coupling unit. It forms part of a unit with the stepper motor and the control valve. The differential electronic module communicates with the Engine Control Module (ECU) and brake control module via the Control Area Network (CAN). The differential electronic module uses the sensor signals to control the oil pressure on the multi-plate clutch by adjusting the control valve. The oil pressure on the multi-plate clutch determines the torque that can be transferred to the rear wheels.

Note: If the car has dynamic stability and traction control (DSTC), the stability control function has priority over all wheel drive.

The Active On-demand Coupling has a temperature sensor. This measures the temperature of the hydraulic fluid and transmits it to the [1][2]differential electronic module. All wheel drive is disengaged if the temperature is too high. This prevents damage to the coupling unit.

Mechanical function, overview





1. Pressure plate
2. Rollers for annular pistons
3. Rollers for the operating piston
4. Annular piston
5. Operating piston
6. Input shaft
7. Cam disc
8. Housing for the outer clutch driven plates
9. Outer clutch driven plates
10. Inner clutch driven plates
11. Springs
12. Output shaft.

The engine torque is transferred to the propeller shaft via the gearbox. The input shaft is separate to the output shaft. The multi-plate clutch pack in the coupling must be loaded for torque to be transferred to the rear axle. The inner and outer wet plates are the points at which drive is transferred between the input and output shafts.
The sinusoidal cam disc rotates at the same speed as the propeller shaft. The rollers for the annular piston act on the peaks and troughs of the cam disc. This presses the annular piston in and out, generating an oil pressure which acts on the operating piston, causing it to engage the multi plate clutch pack.

Hydraulic function, overview





1. Cam disc
2. Rollers
3. Pressure valves
4. Overload protection valve
5. Control valve cover
6. Control valve
7. Stepper motor
8. Accumulator
9. Oil screen
10. Electric oil pump
11. Oil filter
12. Suction valves
13. Piston pump
14. Operating piston
15. Wet multi-plate clutch pack
16. Bearing
17. Springs.

Active On-demand Coupling can be described as a hydraulic pump.
The basic pressure in the system is generated by the electric oil pump. This puts pressure on the annular pistons pressing them against the cam disc via the rollers. The annular piston generates an operating pressure which is supplied to the operating piston via the pressure valve. This pressure connects the input and output shaft. Maximum pressure is limited by the overload protection valve. The difference in speed between the input and output shafts is in direct proportion to the oil pressure on the annular pistons. The greater the difference in speed between the input and output shafts, the higher the oil pressure on the annular pistons. If the speed of the input and output shafts is the same, the oil pressure on the annular pistons is less. The oil pressure on the multi-plate clutch is controlled by the control valve. The stepper motor operates the control valve. The pressure on the multi-plate clutch is greatest when the control valve is closed. This provides maximum torque to the rear wheels. The pressure on multi-plate clutch is lowest when the control valve is open. This limits the torque transferred to the rear wheels.
The accumulator maintains the basic pressure in the system. The oil filter keeps the oil clean and free of small particles which could damage the system.