STEERING SUPPORT BEAM
The steering column is held in position by a support beam which is installed crosswise in the vehicle body at a level close to the steering wheel to reduce the overhang distance of the steering wheel from the supporting point of the column. The steering shaft upper bearing is also located close to the steering wheel to increase supporting efficiency as well as to minimize vibration of the steering wheel.
Scheme 9
Scheme 10
- The fluid pump is directly driven by the engine through a belt.
- The fluid flow is maintained almost constant regardless of change in the engine speed by the function of the flow control valve. The flow-regulated fluid is delivered to the control valve through 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 operate 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.
POWER CYLINDER
The gear box 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.
Scheme 11
ROTARY CONTROL VALVE
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 V 1 through V 4 .
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).
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- 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 V 1 , V 2 , V 3 and V 4 . 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 V 1 , V 2 , V 3 and V 4 , 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 V 1 and V 3 are opened while fluid passages V 2 and V 4 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 V 2 and V 4 so that the rack piston moves to the right. The fluid in chamber B, on the other hand, is drained through fluid passage V 3 into the reservoir tank. Fail-safe feature If fluid pressure fails to build up due to, for example, a broken fluid pump drive belt, the steering wheel rotating torque is transmitted from the valve rotor to the pinion through mechanical engagement between them.
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).
Scheme 15
Note. In the following description, pump speed ranges will be indicated using the speed points A through D shown in the drawing above.
Maximum pressure control
When the overall delivery rate of the pump exceeds a preset value, the relief valve opens to allow part of the discharged fluid to flow toward the suction side of the pump, thereby controlling the maximum pressure.
Scheme 16
VARIABLE GEAR RATIO (VGR) POWER STEERING
All models are equipped with a VGR power steering system.
The steering gear ratio of this system is 1:19 at around the straight-ahead position for higher stability during high-speed driving.
Near the right and left maximum steering angle positions, the gear ratio is made smaller than that of the straight-ahead position so that the system can respond quickly to steering wheel inputs.
