Steering: Description and Operation
A: 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 (Turbo model).
^ 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 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.
B: GEARBOX ASSEMBLY
1. 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.
2. 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, 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).
1 of 2
2 of 2
^ 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, V3 and V4, 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 Vat 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.
^ 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.
C: FLUID PUMP AND RESERVOIR TANK
1. NON-TURBO MODEL
1 of 2
2 of 2
The fluid pump is a vane type driven by the engine via belt.
The reservoir tank is mounted on the vehicle body.
The fluid pump incorporates the flow control valve and relief valve, each performing the following functions:
^ The flow control valve regulates the flow rate of discharged fluid to a constant level irrespective of the engine speed.
^ The relief valve protects the system from an excessively high pressure, which may occur, for example, when the steering wheel is turned all the way.
^ VANE PUMP
The vane pump consists of a rotor, a cam ring, and ten vanes.
When the rotor rotates, the vane movably fitted in each slot of the rotor is radially moved out by centrifugal force and pressed against the inside wall of the cam ring. Since the inside of the cam ring is oval-shaped, the fluid from the suction port is confined and pressurized in the chamber formed between two adjacent vanes as the rotor rotates and is delivered through the discharge port. The pressurized fluid circulates through the hydraulic circuit.
^ FLOW CONTROL VALVE
The flow control valve regulates flow rate of fluid which otherwise would increase as pump speed increases and deliver dangerously high pressure to the gearbox. It consists of orifices 1 and 2, valve spool, return port and flow control spring. When a pressure difference occurs between the front and rear of orifice 2 as a result of an increase in discharge rate, the valve spool moves against the tension of the flow control spring in such a way that the oil flow rate is controlled by opening and closing of the return port and orifice 2.
^ When the pump begins to rotate, pressure P increases, causing the valve spool to move to the right. If the pump is operating in the A-to-B speed range, the fluid delivered by the pump is sent to the gear box in its whole amount through orifice 1.
Pump speed range: A to B
^ As the pump speed increases, pressure P increases further, pushing the valve spool further to the right. As a result, orifice 2 opens allowing part of fluid to return to the pump circuit. Accordingly, a constant flow of oil is maintained.
Pump speed range: B to D
^ When the pump speed further increases, the valve spool is pushed fully to the right. Now, orifice 1 is restricted while orifice 2 opens wide. Oil flow to the gear box is thus maintained at a low rate.
Pump speed range: D to E
2. TURBO MODEL
^ The reservoir tank is mounted on the vehicle body.
^ The fluid pump is belt-driven by the engine. The fluid pump for the turbo models is different from that for the non-turbo models in that it has additionally an energy-saving valve mechanism. This valve mechanism reduces the rate of fluid recirculation during straight-ahead driving, which helps reduce engine load.
^ VANE PUMP
The vane pump consists of a rotor, a cam ring, and ten vanes.
When the rotor rotates, the vane movably fitted in each slot of the rotor is radially moved out by centrifugal force and pressed against the inside wall of the cam ring. Since the inside of the cam ring is oval-shaped, the fluid from the suction port is confined and pressurized in the chamber formed between two adjacent vanes as the rotor rotates and is delivered through the discharge port. The pressurized fluid circulates through the hydraulic circuit.
^ FLOW CONTROL AND ENERGY-SAVING VALVE ASSEMBLY
^ During engine idling
When the engine is idling, the fluid pump speed is low. As the delivery rate of the pump is small, the metering orifice allows whole fluid to flow passing through it. So, there is no difference in the fluid pressure between the right and left of the metering orifice. This means that pressure P. acting on the left end of the flow control valve is equal to pressure P2 (pressure of the fluid flowing through the pressure sensing holes in the valve body) acting on the right end of the valve. As a result, the flow control valve is held pressed leftward by the force of the control valve spring. With the flow control valve in this position, the passage to the fluid pump suction port is closed, so that the whole fluid delivered from the pump flows toward the power steering gear box.
^ During straight-ahead driving
The speed of the fluid pump increases and the fluid pressure becomes high. The metering orifice does not allow whole fluid to pass through it any longer.
Since the amount of the fluid flowing toward the left end of the flow control valve is larger than the amount of the fluid flowing toward the right end of the valve passing through the pressure sensing holes, pressure P1 becomes larger than pressure P2.
As a result, pressure P1 overcomes the force of the flow control valve spring and pushes the valve rightward. This keeps the passage to the fluid pump suction port open, allowing part of the fluid to return to the pump suction port and thus reducing flow of fluid to the steering gear box.
The energy-saving valve mechanism reduces the fluid pump delivery pressure in this way when steering power assistance is unnecessary, thus reducing load on the engine.
^ During a turn
When the steering wheel is turned, the power steering system starts the steering assist function and the fluid pressure increases.
Although the pressure of the fluid entering chamber B through the pressure sensing holes in the valve body increases, the pump delivery pressure is still higher than the chamber B pressure. So, the flow control valve cannot move leftward.
The fluid in chamber B then enters chamber A at the right end of energy-saving valve piston and increase the pressure of chamber A.
Since the pressure in chambers A and B is higher than the pressure (ambient pressure) in chamber C at the left end of the energy-saving valve piston, the energy-saving valve piston moves leftward, overcoming the force of the energy-saving valve spring.
Leftward movement of the energy-saving valve piston compresses the flow control valve spring. The combination of the tension of the spring and the pressure in chamber B now causes the flow control valve to move leftward, overcoming the fluid pump delivery pressure. As a result, the flow control valve closes the passage to the fluid pump suction port and the passage in the metering orifice opens wider. This enables the fluid delivered by the pump to be supplied to the steering gear box at the same rate as in the system without an energy-saving valve mechanism.