Introduction:
A Haldex coupling is a hydraulic clutch that is electronically controlled and transmits mechanical forces. A Haldex coupling is fitted to cars with four-wheel drive (such as Audi Quattro and VW/Seat/Skoda 4-motion) and is located between the prop shaft and the final drive (the differential).
In these four-wheel drive systems, the front wheels are driven continuously, just like on a front-wheel drive car. The rear wheels are only driven during acceleration or when the front wheels slip (for example due to a soft surface or high engine power). Only in these situations is there actual four-wheel drive. Because the rear wheels are not driven at a constant speed, this system with a Haldex coupling is more fuel-efficient than a system with permanent four-wheel drive.
Operation of the Haldex coupling:
The torque is transferred to the Haldex coupling from the input shaft (the blue shaft in the image below). The pinion drives the differential. When driving straight ahead, the speed of the input (blue) shaft and the output shaft (the red pinion) is the same, because the front and rear wheels rotate at the same speed. There is then still no drive to the rear wheels, because the clutch plates of the Haldex are not yet pressed together. When there is a speed difference between the input and output shafts, for example when the front wheels slip and the rear wheels continue to roll on the road surface, the (yellow) rollers in the feed plunger (see image below) ensure that the clutch plates are pressed together mechanically. When rotating, the rollers end up in a narrowing of the cam disc, causing the grey ring to be pressed further inward.
The image also shows that the cam disc has a thickened section. When this thickened section is at the top, it pushes the grey ring against the blue clutch plates.
Hydraulic circuit:
There is also a hydraulic system connected to it that builds up a certain pressure even in the disengaged state. This is the feed pressure. This pressure ensures that there can be no play on the feed and working plungers of the Haldex coupling.
The pump of the hydraulic system is controlled by a control unit. This regulates the feed pressure and the working pressure. At maximum working pressure, the clutch plates are pressed together. By controlling the pressure relief valves, less pressure can also be allowed to the clutch, so that the clutch plates are allowed to slip slightly. In this way, not all power is transmitted to the rear wheels.
Oil:
The Haldex coupling contains special oil that must be changed periodically. If this is not done, extra wear will occur, just as with engine oil in the engine. The total oil quantity in a Haldex coupling is often less than 1 litre, sometimes even less than half a litre.
In a BMW, four-wheel drive cars do not have a Haldex coupling, but a transfer case that is mounted to the gearbox. This is described on a separate page. Click here for more information about the BMW transfer case.