Design and function of power steering pump - RA4600FBL0050X(46-0050)

Designation plate 

Fig 1: Power Steering Pump Designation Plate

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Power steering pump model 

Fig 2: Description Of Power Steering Pump Designation Plate Numbers

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Drive 

The power steering pump is driven by the engine crankshaft via a poly V-belt (110). An automatic retensioner ensures an even preload on the poly V-belt.

Fig 3: Identifying Power Steering Pump Drive, Line Routing

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The poly V-belt pulley (48) is secured to the drive shaft flange with three hexagon bolts (48a).

Fig 4: Identifying Poly V-Belt Pulley (48) And Drive Shaft Flange Bolts (48A)

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Power steering pumps delivered previously (up to 1/85) were driven directly by the crankshaft by a V-belt.

The V-belt is tensioned via a retensioner (20) on the power steering pump (1).

The pulley (3) is secured to the drive shaft via a taper and wedge by means of a clamp nut (4).

Fig 5: Identifying Power Steering Pump (1), Pulley (3) And Clamp Nut (4)

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Fig 6: Identifying Power Steering Pump Components

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The drive shaft (11) is held by plain bearings in the pump flange (4).

It has gear teeth, on which the rotor (7), also toothed, is placed. The rotor is secured on the drive shaft via a snap ring (18).

There are 10 individual cells (slots) in the rotor (7) in which the 10 vanes (16) move smoothly. An internal oval-shaped curved ring (6) is located around this unit.

The cam insert (6), in which the rotor moves, is surrounded by two pressure plates. The two pressure plates and the cam insert are retained in the pump housing by two positioning pins.

Sealing 

The complete pump assembly is sealed by O-rings.

The drive shaft (11) is sealed by a double-lipped radial sealing ring (13).

Line routing 

The pump housing has a pressure connection on which the pressure line (29) is connected to the power steering.

The return line (28) from the power steering is secured to the return connection. Vehicles with a high front axle load (6, 8 and 12 cylinder) need high pump pressures and flow rates but this causes pronounced heating up of the steering gear oil. In these cases a servo oil cooler with bypass is also built into the return line.

Fig 7: Identifying Return Line (28) And Pressure Line (29)

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Fig 8: Identifying Steering Gear, Return Line, Pressure Line And Servo Pump

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Bypass 

Fig 9: Bypass Servo Oil Pressure Diagram

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

At temperatures of approx. -40°C the servo oil is so viscous that in the return line - where a pressure of approx. 2 bar occurs under normal conditions - a pressure of approx. 60 bar is built up.

The built-in bypass reduces the pressure at these low temperatures to approx. 10 bar.

Fig 10: Identifying Oil Delivery Of Power Steering Pump

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The rotation of the drive shaft and thus also the rotor causes the vane positioned in the rotor to be pressed in a radial direction on to the track of the fixed cam insert due to the centrifugal force arising. This is assisted by pressure oil which travels from the pressure chamber to the inner end faces of the vane via bores and grooves. Therefore 10 separate pump cells are formed between ten vanes which extract oil as the volume increases in the two sickle shaped pump chambers and eject it into the pressure chamber as the volume is reduced.

Fig 11: Operating Principle Of Vane-Type Pump

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Since in each case 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 its volume per rotation of the drive shaft.

In addition, the hydraulic radial forces acting on the rotor increase due to this dual arrangement of suction and pressure zones.

The combined flow control and pressure limiting valve of the pump results in a practically constant oil flow being delivered to the power steering over the entire engine speed range. The pressure limiting valve protects the servo system against overload.

The opening pressure differs according to the vehicle and is up to 125 bar max.

The pressure oil pump is driven at different speeds by the engine. However, a constant oil flow is required for servo assistance.

Since at higher engine speeds more oil is delivered than is required, the excess oil is led back to the reservoir via the flow control valve.

Idle 

(Flow control valve closed)

Fig 12: Identifying Oil Flow Control (Flow Control Valve Closed)

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The oil is led from the high pressure chamber (10) to the high pressure outlet (13) via the calibrated bore (15). The high pressure outlet is connected to the spring chamber behind the flow control valve (14) via a bore (12). The calibrated bore (15) throttles the oil flow so that in the spring chamber and on this side of the flow control valve a somewhat lower oil pressure is created than on the side of the high-pressure chamber (10).

If the engine is not running, the flow control valve (11) is closed by the preloaded spring.

• Oil delivery starts at engine idle speed, and a differential pressure arises between the high-pressure chamber (10) and the spring chamber due to the calibrated bore (15).
• The flow control valve remains approximately in its initial position due to the flow control valve spring (14b).

Increased engine speed 

(Flow control valve open)

Fig 13: Identifying Oil Flow Control (Flow Control Valve Open)

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

If the oil flow in the high-pressure chamber (10) increases as engine speed increases, a greater pressure differential arises due to the calibrated bore (15).

This ensures that the flow control valve (14) is pressed against the spring (14b). In this way the high-pressure chamber (10) is connected to the suction duct (9).

The excessive oil flow delivered in the high-pressure chamber (10) is delivered back to the reservoir through the flow control valve via the suction duct (9).

The oil flow delivered to the steering gear remains approximately constant over the entire engine speed range. This is a requirement in order to achieve good servo assistance.

Fig 14: Identifying Power Steering Pump Pressure Limitation

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

A pressure relief valve (11) is installed in the flow control valve (14) for pressure limitation. Depending on the pump model it opens between approx. 65-125 bar pressure. If high servo assistance or full steering lock is required, the operating pressure increases to maximum pressure.

In this way the spring-loaded pressure limiting valve (11) in the flow control valve (14) is opened in the pressure oil line (19) and in the spring chamber and the high-pressure oil can flow back to the reservoir via the return duct (9). The system is protected against overload by this function.