Power Steering Fluid
| Type | 93 160 548 Power steering fluid | |
|---|---|---|
| Volume (completely drained system) | 1.1 liter | 1.16 qts |
POWER STEERING FLUID SPECIFICATIONS
Scheme 76
| Maximum pressure | Bar | 120 |
|---|---|---|
| Maximum flow | L/min | 7.2 |
POWER STEERING PUMP SPECIFICATIONS
Scheme 77
| Power steering pump | Nm (lbf ft) | 25 (20) |
|---|---|---|
| Delivery pipe, power steering pump | Nm (lbf ft) | 30 (25) |
| Delivery pipe, valve body | Nm (lbf ft) | 30 (25) |
| Return pipe, valve body | Nm (lbf ft) | 30 (25) |
TIGHTENING TORQUES SPECIFICATIONS
Scheme 78
- Power steering fluid reservoir
- Power steering pump
- Control valve
- Servo cylinder
The power-assisted steering gear is of the rack-and-pinion type consisting of control valve and servo cylinder in a common housing. Together with the power steering pump and power steering fluid reservoir, these are the main components of the system.
Power steering fluid is pumped from the power steering pump to the control valve where, depending on which way the steering wheel is turned, it is directed to either the right or the left side of the servo cylinder. The steering servo fluid inside the cylinder acts on the rack's piston, thus providing power assistance to the rack-and- pinion steering gear.
The mechanical components of the steering gear are lubricated by high-viscosity grease and sealed from the hydraulic circuit and other parts of the system by seals and rubber gaiters.
Scheme 79
The control valve consists of a valve spool (1), a sleeve (2), a torsion bar (3) and a pinion (4). The steering column's intermediate shaft is connected to the valve by means of a universal joint. The torsion bar is connected to the upper end of the valve by means of a pin (5). The other end of the torsion bar is a press fit in the pinion. The sleeve is secured to the pinion by a pin (6) and follows the rotation of the pinion exactly. There is also a fail-safe connection between the valve spool and the pinion.
The sleeve has three radial grooves (7), the power steering fluid being pumped to the middle one. When the steering wheel is in the straight-ahead position, the control valve is open and the fluid flows up through the valve and back to the power steering pump via the chamber above the sleeve.
The upper end of the pinion is supported in a needle bearing and the lower end in a ball bearing. A plastic bush is fitted between the valve sleeve and pinion.
A spring loaded plunger presses the rack against the pinion.
The valve housing, which is part of the rack housing, has four hydraulic connections for the flow of power steering fluid: Servo delivery from the power steering pump (A), return to the power steering fluid reservoir (B), to the servo cylinder when turning right or from the servo cylinder when turning left (C) and to the servo cylinder when turning left or from the servo cylinder when turning right (D).
Scheme 80
When the steering wheel is turned, the movement is transferred via the torsion bar to the pinion. Since the torsion bar twists slightly, there will be a difference between the degree of rotation of the valve spool (which follows the rotation of the intermediate shaft) and the sleeve which is fixed to the pinion. As a result, the fluid can no longer flow through the control valve and back to the power steering fluid reservoir directly. Instead, delivery and return passages open for the servo cylinder.
Scheme 81
During a left turn, servo fluid is pumped to the left side of the servo cylinder via the radial groove in the upper sleeve. The servo fluid passes through the valve to the area above the spool and back to the power steering fluid reservoir. During a right turn, the procedure is the opposite.
As long as the torsion bar is twisted, power steering fluid will act on the rack to provide power assistance. The difference between the valve spool and the sleeve will be reduced when the power steering fluid acts on the rack in the same direction as the pinion. When this difference no longer exists, the return passage through the valve is opened again and the servo fluid can flow directly back to the power steering fluid reservoir.
A certain flow of power steering fluid through the valve is always present, except when the steering wheel is turned to full left or right lock. This is necessary for the control valve in the power steering pump to operate and it also helps to cool the fluid.
Scheme 82
The cylinder is part of the rack housing. The rack (1) is equipped with a piston (2) complete with seals. For the flow of power steering fluid to and from the control valve there are two connections on the servo cylinder, one on each side of the piston. For turning right, power steering fluid is pumped to the right-hand section of the SERVO CYLINDER .
Piston and rack are forced to the left and power steering fluid is discharged from the left-hand section of the SERVO CYLINDER . The rubber gaiter on the left-hand side is distended at the same time as the one on the right-hand side is compressed.
Movement of the rack is transferred via the inner ball joints (3), track rod and outer track-rod ends to the steering arms of the steering swivel member. Both the inner ball joints and the outer track-rod ends are lubricated for life and self-adjusting, with no further lubrication or adjustment being necessary or possible.
Scheme 83
The power steering pump is driven by a multigroove V-belt from the crankshaft pulley.
In addition to pump elements, the pump incorporates a control valve for regulation of the pressure and flow.
| WARNING | To avoid damage to the pump, the following points should be observed: ? Never forcibly hold the steering wheel at full lock for any length of time with the engine running as the pump may then overheat and be damaged. ? Take care to prevent any particles of dirt from entering the hydraulic system when checking the fluid level or topping up with fluid. ? Never allow the pump to run dry, with no fluid in the system. |
Scheme 84
The pump element consists of a rotor containing a number of slots, a vane for each slot, a pump casing and two end plates with inlet and outlet ports for the power steering fluid.
Due to the oval shape of the pump casing, the volume between the vanes increases and decreases twice during each revolution of the rotor. Inlet ports lead to the areas in which the volume increases and outlet ports lead from those in which the volume decreases, thereby producing a pumping effect. Apart from being forced outwards by centrifugal force, the vanes are also pressed outwards against the pump casing by the pressure of the fluid. The fluid is directed into the slots inside the vanes.
Scheme 85
The purpose of the control valve is to regulate the flow from the pump so that it remains constant, regardless of engine pump RPM.
One side of the control valve is in direct contact with the delivery side of the pump (A). The outlet (B) from the pump incorporates a restriction from which a connecting line (1) leads to the other side of the valve, which is fitted with a spring (2). When not actuated, the valve is pressed against the outlet side.
Inside the control valve is an overflow valve (3) which, when the pressure exceeds a certain limit, is actuated by the pressure of the power steering fluid on the spring-loaded side of the control valve.
For the control valve to operate, a certain amount of power steering fluid must circulate through it continuously at (A) and (C), although not when the steering wheel is at full lock.
The pump delivers a maximum pressure of about 120 bar and a maximum flow of about 7.2 litres per minute.
Scheme 86
The pressure delivered by the pump (shaded portion) is reduced slightly by the restriction in the pump outlet.
This reduced pressure is also communicated to the spring-loaded side of the control valve, creating a small pressure difference across the valve. Owing to the low speed of the pump, however, the difference in pressure is insufficient to overcome the force of the spring and actuate the valve.
Scheme 87
The flow of power steering fluid inside the pump increases with increasing engine RPM and owing to the restriction in the pump outlet the flow velocity also increases. This reduces the pressure in the connecting passage, with the result that the pressure on the spring-loaded side of the control valve will be lower than that acting on the outlet side of the valve. The valve therefor overcomes the force of the spring, opening a port to the suction side of the pump and allowing a certain amount of internal recirculation of the fluid to take place so that the flow from the pump is maintained at a constant rate, regardless of engine pump RPM.
Scheme 88
Pump speed in this case is often low. When the steering wheel is turned to full lock the control valve of the steering gear closes. The flow of fluid from the pump will then be zero.
The resulting high-pressure is directed via the connecting line to the spring-loaded side of the control valve. The pressure opens the overflow valve and allows the fluid to pass to the inlet side of the pump. The pressure difference across the control valve forces it to move against the spring and so open the port for recirculation of the full delivery flow from the pump.
The predetermined maximum pressure is maintained as long as the control valve remains closed.