Steering System - Mechanism & Function: Other Subaru Forester SF рестайлинг

Steering Support Beam

The steering column is held by a support beam located close to the steering wheel to reduce the overhang. The upper bearing is also located close to the steering wheel to increase supporting rigidity, as well as to reduce the problem of a shaking or shimmying wheel.

Identifying Steering Support Beam. Scheme 113

Hydraulic System

1. Oil pump is belt-driven from the engine to discharge oil under pressure.
2. Oil under pressure is controlled by the oil pump in response to engine speed and is delivered to control valve via hose A.
3. When the steering wheel is turned, control valve 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 or B.
4. Oil in chamber A or B acts on rack pinion to produce the force required to move rack shaft to the left or the right. This helps reduce the effort required to operate the steering wheel.
5. Movement of rack piston in turn causes oil in the other chamber to return to tank via pipe A or B, control valve and hose B. If the hydraulic system becomes inoperative, the steering shaft will then be connected to the pinion shaft mechanically via control valve. 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 is built into the oil pump to release excess oil pressure.

Identifying Power Steering System - Hydraulic System. Scheme 114

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.

Identifying Gearbox With A Built-In Control Valve & Power Cylinder. Scheme 115

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 V 1 through V 4 .

The pinion and rotor are meshed with adequate clearance. They utilize a fail-safe design.

Identifying Control Valve. Scheme 116

Scheme 117

Scheme 118

1. 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 V 1, V 2, V 3 and V 4, which in turn switches oil passages and controls oil pressure.
2. When no steering force is applied: The rotor and sleeve are held at the neutral position. Oil passages V 1, V 2 and V 3, 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. (Scheme 117): Operating Principle (When No Steering Force Is Applied)
3. When steering force is applied: When the steering wheel is turned to the right, for example, oil passages V 1 and V 3 open while oil passages V 2 and V 4 nearly close. At this point, oil under pressure in chamber A increases in response to the throttle position of oil passages V 2 and V 4 so that the rack piston moves to the right. Oil in chamber B, on the other hand, is discharged through oil passage V 3, returning to the oil reservoir. (Scheme 118): Operating Principle (When Steering Force Is Applied)
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.

Scheme 119

Scheme 120

1. 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 variable capacity type vane pump which controls the delivery rate per rotation to match the engine speed. It is integrated with pump control valve and relief-valve. (Scheme 119): Identifying Oil Pump Components
2. The vane pump consists of a rotor, cam rings, and eleven 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. Oil from the chamber is discharged into the oil circuit via the discharge port. (Scheme 120): Identifying Oil Vane Pump, Rotor, Cam Rings & Vanes

Scheme 121

Scheme 122

Scheme 123

Scheme 124

Scheme 125

1. The oil pump controls the delivery rate per rotation by changing the eccentricity of the cam ring. (Scheme 121): Oil Pump Delivery Rate Graph
2. During low speed operation, the pressure difference between before and after the variable orifice is small. So the control valve is pressed toward the left by action of the control valve spring, and the tank pressure led to pressure chamber A. The pressure led to pressure chamber B is the pressure that has passed through the variable orifice. It is a higher pressure than that applied to pressure chamber A. The cam ring is pressed toward the left by the pressure difference between pressure chambers A and B and action of the cam ring spring, so the eccentricity of the cam ring reaches a maximum. When the eccentricity of the cam ring is maximized, the delivery rate per rotation is maximized. Pump speed range A to B (Scheme 122): Low Speed Operation (Pump Speed Range A To B)
3. During medium speed operation, the delivery rate increases. So the pressure before passing through the variable orifice increases because of action of the variable orifice. When the pressure before passing through the variable orifice increases, the control valve is moved to the right against action of the control valve spring. When the control valve is moved to the right, the pressure before passing through the variable orifice is led to pressure chamber A, and the pressure is higher than that applied to pressure chamber B. Because of the pressure difference between pressure chambers A and B, the cam ring is moved to the right against action of the cam ring spring. When the cam ring is moved to the right, the eccentricity of the cam ring is reduced, and the delivery rate per rotation decreases. Therefore, the delivery rate is decreased even if the pump speed increases. Pump speed range B to C (Scheme 123): Medium Speed Operation (Pump Speed Range B To C)
4. During high speed operation, the cam ring moves further to the right to close the variable orifice and reduce the opening area of the variable orifice. When the opening area of the variable orifice is reduced, the pressure before passing through the variable orifice increases, and the pressure difference between pressure chambers A and B increases, and the cam ring moves further to the right. Movement of the cam ring further to the right further reduces the delivery rate per rotation. Therefore, the delivery rate is decreased even if the pump speed increases. Pump speed range C to D (Scheme 124): High Speed Operation (Pump Speed Range C To D)
5. When the delivery rate exceeds a preset value, the relief valve opens to let the hydraulic fluid to escape toward the upstream side of the pump, thereby controlling the maximum pressure. (Scheme 125): Controlling Maximum Pressure (Delivery Rate Exceeds A Preset Value)