Power Steering System

3.0 L Model:

Hydraulic System
- The fluid pump is directly driven by the engine through a belt.
- When the steering wheel is not being turned, the pressure-sensitive valve in the pump opens to drain the fluid into the fluid reservoir tank.
- The fluid pressure is maintained almost constant regardless of change in the engine speed by the function of the flow control valve. The pressure-regulated fluid is delivered to the control valve via hose A.
- When the steering wheel is turned, the rotary control valve connected to the pinion shaft opens the hydraulic circuit corresponding to the direction in which the steering wheel is turned. The fluid then flows into chamber A or B via pipe A or B.
- The fluid pressure in chamber A or B acts on the rack piston in the same direction as that in which the rack shaft is moved by rotation of the steering wheel. This helps reduce the effort required of the driver to operate the steering wheel.
- Movement of the rack piston causes the fluid in the other chamber to return to the reservoir tank via pipe A or B, control valve, and hose B.
- As the steering shaft is connected to the pinion shaft mechanically via the rotary control valve, the steering system can be operated as a manual system even if the hydraulic system becomes inoperative.
- To control the maximum fluid pressure, a relief valve is built into the fluid pump to prevent buildup of an excessive fluid pressure.

Gearbox Assembly
Power Cylinder





The gearbox integrates the control valve and power cylinder into a single unit. The rack shaft serves as a power cylinder piston. The rotary control valve is located around the pinion shaft.
The rotary control valve and power cylinder are connected to each other by two pipes through which hydraulic fluid flows.

Rotary Control Valve

Part 1:

Part 2:

The rotary control valve consists of a rotor (which rotates together with the steering shaft), a pinion (which is connected to the rotor and torsion bar), and a sleeve (which rotates together with the pinion). The rotor and sleeve have grooves C and D, respectively, which form fluid passages V1 through V4.

The pinion is in mesh with the rotor with adequate clearance, which enable the rack to be moved manually by rotating the steering shaft (fail-safe feature).

- Principle of operation
When the torsion bar is twisted by a rotational force applied to the steering wheel, the relative position between the rotor and sleeve changes. This changes the cross-sectional area of fluid passages V1, V2, V3 and V4. The fluid passages are thus switched and the fluid pressure is controlled in accordance with the operation of the steering wheel.




- When no steering force is applied:
The rotor and sleeve are held at the neutral position. Fluid passages V1, V2 and V3, which are formed by grooves C and D are open equally. Under this condition, the fluid from the pump returns to the reservoir tank so that neither fluid pressure builds up nor the rack piston moves in the power cylinder.





- When steering force is applied:
When the steering wheel is turned to the right, for example, fluid passages V1 and V3 are opened while fluid passages V2 and V4 are nearly closed. At this point, the fluid pressure in chamber A of the power cylinder increases depending on the degree of closure of fluid passages V2 and V4 so that the rack piston moves to the right. The fluid in chamber B, on the other hand, is drained through fluid passage V3 into the reservoir tank.

Fluid Pump And Reservoir Tank

- The reservoir tank is mounted on the vehicle body.





- The fluid pump is belt-driven by the engine. The fluid flow is controlled according to the engine speed so that an adequate steering resistance is given during high-speed operation.
The fluid pump is a variable capacity type vane pump whose delivery rate per rotation decreases as the engine speed increases. The pump is integral with a pump control valve and relief valve.





- The vane pump consists of a rotor, a cam ring, and eleven vanes.
When the rotor rotates, the vane in each slot of the rotor is radially moved out by centrifugal force and pressed against the cam ring. The fluid from the suction port is confined in chambers formed between two adjacent vanes and carried to the discharge port. Since the cam ring is movable in relation to the rotor, the volume of each chamber is variable. This enables the delivery rate per rotation of the pump to be changed.





Flow Control
The variable capacity pump changes its delivery rate per rotation by changing the degree of eccentricity of the cam ring according to its rotating speed (engine speed).

NOTE: In the following description, pump speed ranges will be indicated using the speed points A through D shown in the drawing above.





Low-range-speed operation (A - B range)
In this speed range, as well as in all the other speed ranges, two different pump discharge pressures are always applied to the control valve; one is directly led from the discharge port to the left end of the valve and the other is led through an orifice (variable orifice) to the right end of the valve. Since the orifice has a pressure reducing effect, the latter pressure is lower than the former.
When the pump is operating at a low speed, its discharge pressure is also low, resulting in only small difference between the two pressures. In this condition, the valve stays pushed leftward by the spring, allowing the non-pressurized reservoir tank fluid to enter chamber A. To chamber B, on the other hand, the orifice-reduced discharge pressure is applied, so the cam ring is pushed leftward by the cam ring spring. This makes the eccentricity of the cam ring a maximum and, therefore, the delivery rate per rotation of the pump become a maximum.





Mid-range-speed operation (B - D range)
During mid-range speed operation, the pump increases its delivery rate. Since the pressure before passing through the variable orifice increases, the control valve moves rightward, overcoming the tension of the control valve spring. This movement of the control valve allows the pressure upstream of the variable orifice to be directed to chamber A after being adjusted to a necessary pressure* by the port opening area created by the control valve. On the other hand, chamber B receives the reservoir pressure (suction pressure). This means that the pressure in chamber A is higher than that in chamber B. As a result, the cam ring moves rightward against the tension of the cam spring. This causes the delivery rate per rotation of the pump to be reduced, so that the flow rate of the fluid to the steering gear box decreases accordingly.
The above control is performed when the pump is operating at a speed in the B - D range.

* The "necessary pressure" for chamber A is a pressure required to move the cam ring to the position corresponding to each predetermined flow rate (pump delivery rate). The pressure is obtained by changing the port opening area appropriately through displacement of the control valve. The displacement of the control valve is determined by how much the pressure before the variable orifice is different from that after the orifice.





Maximum pressure control
When the hydraulic circuit in the steering gear box is closed as a result of a steering action, the pressure in the circuit increases to a very high level. The relief valve prevents the pressure from exceeding a preset safe level in the following way:

If the fluid in the circuit is pressurized to the preset pressure, the fluid pushes the ball of the valve overcoming the tension of the relief spring. Through the opened relief valve, the fluid makes its way to the pump's suction side passage, thus maintaining the circuit pressure at a level lower than the preset pressure.