Basic function of rear axle differential

P35.30-2171-76

P35.30-2172-76

Overview

This document contains information on:

 General

 Function requirements

 Function

General

The rear axle differential distributes the torque of the drive train fully variably between the front and rear axles. In addition, the rear axle differential compensates for different wheel speeds at the rear axle. The rear axle differential is part of the all-wheel drive.

Function requirements

 Engine in operation

 Drive train operational

Function

Rear axle gear

The rear axle differential fulfills the following tasks:

 Torque transmission

 Rpm compensation

 Variable torque distribution

The drive torque is distributed between the front and rear axles according to the driving situation. Typically, 100 % of the drive torque is transferred to the front axle and 0 % to the rear axle, e.g. when driving in a straight line without excessive slip at the front wheels.

If a sudden change in the friction coefficient occurs in a borderline case, the drive torque can be up to 50 % at the front axle and up to 50 % at the rear axle.

The actuator motor is initialized at system start. A temperature sensor and a Hall sensor are installed in the rear axle differential actuator motor. The temperature sensor provides information to calculate the temperatures of the clutch and the transmission oil. The Hall sensor detects movements of the electric motor.

Torque transmission

The rear axle differential transfers the torque from the propeller shaft to left and right rear axle shafts.

In order to distribute the torque, a multiplate clutch is integrated in the rear axle differential. The multiplate clutch is actuated electromechanically by the rear axle differential actuator motor. To open or close the plates, the electric motor transmits an axial force onto the plate pack via a spur gear stage and a ball ramp. The rear axle differential actuator motor is actuated irrespective of the engine speed. This allows pilot control of the rear axle differential when stationary.

Depending on the driver's commands and the driving situation, a variable drive torque is generated by the rear axle differential. The possible drive torque at the multiplate clutch varies between 0 and 1000 Nm. For this, the all-wheel drive actuator motor regulates the actuation of the multiplate clutch and the torque transfer starts.

Rpm compensation

For rpm compensation, the rear axle differential is designed as a single-speed differential. It compensates for the different wheel speeds of the rear wheels.

To compensate for the wheel speeds, information about the rpm difference between the front and rear axles is required. The all-wheel drive control unit calculates this rpm difference from the wheel speeds of the individual wheels. The all-wheel drive control unit receives the wheel speeds of the individual wheels from the ESP® control unit and the combustion engine control unit.