BMW 7 Series: Oscillating Motor

Fig. 117: Identifying Oscillating Motor Housing And Oscillating Motor Shaft

INDEX REFERENCE CHART

1. Oscillating motor housing
2. Oscillating motor shaft

The hydraulic motor and the hydraulic motor body are each attached to one half of the anti-roll bar. The active anti-roll bar consists of the oscillating motor and the anti-roll bar halves fitted to the oscillating motor, with press-fitted roller bearings for their connection to the axle carriers. The use of roller bearings ensures optimum comfort thanks to better response and reduced control forces. A thin coating of grease on the roller bearing does not impair the function of the active anti-roll bar.

Operating principle of oscillating motors

The oscillating motor has three functions to perform:

• The oscillating motor transfers the torque into the anti-roll bars.
• The hydraulic motor isolates the two halves of the anti-roll bar
• In the event of system failure (failsafe mode), the front axle anti-roll bar creates sufficient damping via the oscillating motor hydraulic fluid (hydraulic locking). The front suspension anti-roll bar then works like a conventional anti-roll bar.

Exception: if the hydraulic motor chambers no longer contain any fluid as a result of a leak, the front suspension anti-roll bar cannot provide a damping force. The opposing chambers in the oscillating motor are connected to one another. The same pressure exists in both chambers. Two chambers are supplied with high-pressure fluid using one connection. The two other chambers are connected to the tank via the return line.

The forces F-H (High) or F-L (Low) are created as a result of the differences in pressure. Since F-H is greater than F-L , a torque M-S is produced, which causes the shaft to turn in relation to the housing.

Fig. 118: Oscillating Motor Section Diagram

Since one half of the stabilizer bar is connected to the shaft, and the other with the housing, the two halves turn in opposite directions. This torque MS generates the active moment MA around the vehicle longitudinal axis via the anti-roll bar connections which counteracts the rolling moment M during cornering.

The shell is forced upwards on the outside of a curve, and dragged down on the inside of a curve. The maximum level of body torque on the front and rear suspension occurs when there is a high rate of lateral acceleration. The system pressure is then 180 bar on the front suspension and also 180 bar on the rear suspension. If the oscillating motor twists as a consequence of external forces (road excitation, e. g. bumps or potholes), the oscillating motor then acts as a torsional-vibration damper.

As a result of the twisting action, fluid is forced out of the two chambers. The displaced fluid flows through the pipes and the hydraulic valve manifold, the hydraulic resistance of which produces a damping effect. In the event of failsafe locking (hydraulic locking), the oscillating motor can only twist with a very high damping effect as a consequence of the hydraulic jamming in the oscillating motor.

Function of pressure relief valves

When the vehicle is driven on poor road surfaces, the movements of the anti-roll bar produce transient low pressures (cavitation) and pressure peaks in the hydraulic motors which can cause rattling noises. To prevent those noises, pressure relief valves and internal pulsation dampers have been fitted on the front hydraulic motor.

The pressure relief valves allow filtered air to flow into the hydraulic motor to prevent cavitation. That small quantity of air is absorbed by the hydraulic fluid (Pentosin) to form an emulsion which is discharged in the course of subsequent operation of the hydraulic motor. The surplus air is separated out in the expansion tank.

Front Suspension Active Anti-roll Bar

Fig. 119: Identifying Front Suspension Active Anti-Roll Bar Components

INDEX REFERENCE CHART

1. Front-suspension hydraulic motor
2. Tandem pump
3. ARS hydraulic valve manifold
4. VDM
5. Hydraulic cooler
6. Air filter element

The anti-roll bar is mounted on the front suspension subframe. The anti-roll bar links are attached to the pivot bearing. There are two pressure relief valves on the hydraulic motor of the front suspension anti-roll bar.

On the pressure relief valves there are air filter elements (black plastic caps) attached. Those black air filter caps with Goretex inserts must not be removed.

Rear Suspension Active Anti-roll Bar

Fig. 120: Identifying Rear Suspension Active Anti-Roll Bar Components

INDEX REFERENCE CHART

1. Rear suspension hydraulic motor

The anti-roll bar is mounted behind the rear suspension subframe. The anti-roll bar links are attached to the rear suspension swing arms.

On the hydraulic motor for the rear suspension anti-roll bar, blanking plugs are fitted in place of the pressure relief valves.

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