Four-wheel Drive Transmission With Haldex V Clutch: Fifth Rear

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A chill runs through my body - what if it doesn't work? “Skoda Yeti” nods sharply, in the windshield the horizon soars up, I only see the ground and … Tr-tr-tr! The brakes started talking in short machine gun bursts - the hill descent assistant worked. And the Yeti slowly slides down.

Car electronics is improving: it reacts faster and works more precisely. The famous Haldex multi-plate clutch, which is an ingenious mixture of mechanics, hydraulics and electronics, did not stand aside. The fifth generation clutch, the most advanced, is installed on many new cars. Including the new "Skoda".

Haldex 4

Haldex couplings of the fourth [a] and fifth [b] generation. Haldex couplings of the fourth [a] and fifth [b] generation.

TASKS AND RESPONSIBILITIES

Haldex is an electronically controlled coupling. It is installed in front of the rear cross-axle differential and throws traction to the rear wheels - naturally, when there is a need for it. For example, on a slippery road. Or when starting off - to more effectively realize the torque.

The Haldex control unit collects data for the entire car - from engine sensors, gearboxes, anti-lock braking systems, steering. When giving the command to the clutch actuators, the computer takes into account not only wheel slippage, but also speed, lateral acceleration, steering position, movement under traction or coast. Such a large amount of information is needed in order for the all-wheel drive transmission to react in advance to situations arising on the road. For example, with a locked clutch, you can transfer maximum torque to the rear axle - to pull out a car that has been hit by a front wheel on a slippery surface. Or, on the contrary, remove the thrust from the stern and thereby help other systems while stopping the front wheels to stop the occurring skid.

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It is not entirely correct to say that the Haldex connects the rear axle. Even with excellent adhesion of the wheels to the road, aft still flows up to 10% of the torque. This is a kind of "preload". Why is it needed? So that the system is always ready and, if necessary, instantly throws traction - after all, controllability and all-terrain qualities depend on the speed of response.

The principle of operation of "Haldex" has not changed for several decades, but with each generation the clutch became more technologically advanced and more compact, it worked faster and more accurately (details - ЗР, 2011, No. 4). Torque from the engine comes to the drive wheels, while the followers are connected to the rear axle drives. Hydraulic actuators, at the command of the electronics, compress the pack of discs - the stronger they engage, the more traction can be thrown back when the front wheels slip. And the torque transmitted to the second pair of wheels changes smoothly.

FATHERS AND SONS

Four-wheel drive "Skoda" originally installed the fourth "Haldeks". The transmission of new models has a more advanced clutch - the fifth generation. Major changes have occurred in the hydraulic system, which, under the control of electronics, compresses and unloads the discs.

In the fourth Haldex, the electric pump created a working fluid pressure (up to 30 bar), and the control solenoid valve limited its supply to the annular piston compressing the disk pack. The more fluid the valve overflowed, the denser the discs pressed against each other and the higher the torque could be transmitted to the rear axle.

In the fifth-generation coupling, the pump was equipped with a centrifugal regulator, which measures the required working pressure in the system. During rotation, the levers of the regulator under the action of centrifugal force diverge and block the channels through which oil is drained into the sump. In this case, the pressure in the system increases, the piston begins to compress the disks. If you want to unlock the clutch, the automation reduces the speed of the electric motor, the levers return to their original position, the valves open and the pressure drops.

In fact, the centrifugal regulator replaced two parts: a control solenoid valve and a hydraulic accumulator, necessary to maintain pressure.

True, a compact safety valve was introduced for insurance - it opens and bleeds excess oil into the tank when the pressure rises above 44 bar.

The struggle for millimeters and kilograms (by the way, the fifth Chaldex is 1.7 kg lighter than its predecessor) is justified only if it does not compromise reliability. Not sure if it made sense to abandon such an important detail as an oil filter. After all, the fourth "Haldex" had a filter - but in the fifth it is not! It is unlikely that the disks and other rotating parts began to be covered with some kind of magical material, which completely eliminates their wear. And where do wear products go? The accumulated “shavings” in the oil can disrupt the operation of delicate hydraulic mechanisms, and repairing the coupling is expensive. In addition, the lubricant is now recommended to be changed not every 60, 000 km, but every three years. Many motorists during this time, and roll over 100 thousand! We hope the developers have not forgotten about this.

HALDEKS AND COMPANY

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