Power Steering System
HYDRAULIC SYSTEM
1) Vane pump (1) is belt-driven from the engine to discharge oil under pressure.
2) Oil under pressure is controlled by the flow control valve (2) located inside the oil pump assembly in response to engine speed and is delivered to control valve (4) via hose A (3).
3) When the steering wheel is turned, control valve (4) connected to the pinion shaft activates to form an oil flow circuit corresponding to the rotation direction of the steering wheel. Oil will then be delivered to chamber A or B via pipe A (5) or B (6).
4) Oil in chamber A or B acts on rack pinion (8) to produce the force required to move rack shaft (9) to the left or the right. This helps reduce the effort required to operate the steering wheel.
5) Movement of rack piston (8) in turn causes oil in the other chamber to return to oil reservoir (11) via pipe A (5) or B (6), control valve (4) and hose B (10).
- If the hydraulic system becomes inoperative, the steering shaft will then be connected to the pinion shaft mechanically via control valve (4). Thus, the steering shaft can act as one similar to a manual steering system to move the rack and pinion.
- To control the maximum oil pressure setting, relief valve (12) is built into flow control valve (2) of the oil pump assembly to release excess oil pressure.
GEARBOX ASSEMBLY
1) Power cylinder
The gearbox is integrated with a built-in control valve and power cylinder. The rack shaft is used as a power cylinder piston and a rotary control valve is located in such a manner as to enclose the pinion shaft.
The control valve and power cylinder are connected to each other by two pipes through which hydraulic oil flows.
2) Control valve
The 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).
Oil grooves C and D are located in the rotor and sleeve to form oil flow passages V1 through V4.
The pinion and rotor are meshed with adequate clearance. They utilize a fail-safe design.
3) Operating principle
When the torsion bar twists in relation to the steering force, a relative rotational displacement occurs between the rotor and sleeve. This displacement changes the cross-sectional area of oil passages V1, V2, V3 and V4, which in turn switches oil passages and controls oil pressure.
(1) When no steering force is applied:
The rotor and sleeve are held at the neutral position. Oil passages Vi, V2 and V3, which are formed by valve grooves C and D are open equally. Under this condition, oil delivered from the oil pump returns to the oil reservoir so that neither oil pressure builds up nor does the power cylinder activate.
(2) When steering force is applied:
When the steering wheel is turned to the right, for example, oil passages V1 and V2 open while oil passages V2 and V4 nearly close.
At this point, oil under pressure in chamber A increases in response to the throttle position of oil passages V2 and V4 so that the rack piston moves to the right. Oil in chamber B, on the other hand, is discharged through oil passage V3 returning to the oil reservoir.
4) Fail-safe function
If oil pressure fails to build up due to a broken oil pump drive belt, torque is transmitted from the valve rotor to the pinion by way of the fail-safe function.
OIL PUMP & TANK
The oil pump is belt-driven from the engine. The oil flow is controlled in response to engine speed so that an adequately "heavy" steering effort is maintained during high-speed operation.
The oil pump is a vane type. It is integrated with an oil reservoir and houses the flow control and relief-valves.
1) Vane pump
The vane pump consists of a rotor, cam rings, and ten vanes.
When the rotor rotates, the vane located in each groove of the rotor is radially swung out by centrifugal force and pressed against the cam ring. The tip of the vane slides along the inner oval wall of the cam ring so that oil is delivered to the chamber formed by the rotor, cam ring and vane by way of a pea-shaped groove. Oil from the chamber is discharged into the oil circuit via the discharge port.
2) Flow control valve
The flow control valve adequately regulates the discharge flow of oil which increases in proportion to pump speed and delivers it to the gearbox. It consists of orifices 1 and 2, valve spool, return port and flow control spring. When a pressure differential occurs between the front and rear of orifice 2 in response to increases in discharge flow, the valve spool moves against the tension of the flow control spring so that the oil flow is controlled by the open and close operation of the return port and orifice 2.
(1) When the pump begins to rotate, P increases, and the valve spool is moved to the right by this pressure. In the pump speed range A to B, the total amount of oil delivered by the pump is sent to the gear box through orifice 1
- Pump speed range A to B
(2) As the pump speed increases, P1 increases further, and the valve spool is pushed further to the right. As a result, orifice 2 opens. Accordingly, a constant flow of oil is maintained.
- Pump speed range B to D
(3) When the pump speed increases further, the valve spool is pushed to the tar right position.
At this spool position, orifice 1 is restricted while orifice 2 opens wide.
Accordingly, the oil flow decreases.
- Pump speed range D to E