Function

FUNCTION





The rotation of the drive shaft and thus also of the rotor causes vanes guided in the rotor to be forced into the path of the fixed cam insert due to the centrifugal force occurring in the radial direction. This is assisted by pressure oil which reaches the inner end faces of the vanes from the pressure chamber via holes and grooves.

Consequently ten separate pump cells are formed between the ten vanes. As the volume increases, oil is now drawn out off the reservoir (B) and oil is forced into the pressure chamber (A) as the volume decreases.

As two suction and pressure zones are opposite each other due to the shape of the cam insert, each of the ten pump cells delivers twice their volume for each rotation of the drive shaft.

In addition, the hydraulic radial forces becoming effective at tire rotor cancel each other out due to this double arrangement of suction and pressure zones.
(c) Depressurized
(f) Pressurized
(A) To steering gear
(B) From reservoir


OIL QUANTITY CONTROL
The power steering pump is driven at variable speeds by the engine. However, a constant oil flow is required for power assistance.

As more oil is delivered at higher engine speeds than is required, the excess oil is led back into the reservoir via the open volume control valve (30t). The volume control valve therefore causes a constant oil flow to be supplied to the steering gear throughout the engine speed range.

Figure 1 Volume Control Valve (30t) Open:

ENGINE ON, STEERING WHEEL NOT TURNED
The pressure oil produced by the vane cells is led to the steering gear via the calibrated bore (9a) and also to the spring chamber behind the volume control valve (30t) via the bore (9b). The calibrated bores (9a, 9b) restrict the oil flow so that a lower oil pressure prevails in the spring chamber behind the volume control valve (30t) than on the side of the high-pressure chamber. The volume control valve (30t) opens (Figure 1) and the surplus pressure oil flows back into the reservoir.

Figure 2 Volume Control Valve (t30) Closed:

ENGINE ON, STEERING WHEEL TURNED
The pressure behind the volume control valve also increases due to the increasing pressure level behind the restrictor (a). The volume control valve (30t) closes (Figure 2) and remains in its initial position.

INCREASED ENGINE SPEED
If the oil quantity increases as engine speed increases, a greater pressure difference occurs again due to the calibrated bore (9a). The volume control valve (30t) is pressed against the spring (30s) and the excess oil quantity delivered can flow back into the reservoir (Figure 1).

PRESSURE LIMITATION
A pressure limiting valve (49d) is installed in the volume control valve (30t) to protect the steering system against overload. It opens between a pressure of 65 and 125 bar depending on the tune of newer steering pump.

This occurs at full steer angle for example, which is audible as a shrill whirring whistling sound.