Mechanism & Function -- Power Steering System: Overview Subaru Forester SH

Scheme 1

1. The steering wheel vertical position can be adjusted within a 38 mm (1.50 in) range by using the tilt lever to unlock the steering column and lock it again at the desired position.

Scheme 2

1. To absorb the backward movement energy generated in the engine compartment in the event of a frontal collision, a press-fitted pipes type steering column jacket has been adopted. When an impact load exceeding a certain level is applied to the steering column, jacket A slides on jacket B. Since jacket B is press-fitted in jacket A, friction generated between them absorbs the impact. The column bending load is supported by the press-fitted jackets.
2. Another measure to alleviate impact on the driver in the event of a collision is the ripping plate, which is located between the steering column and the tilt bracket attached to the steering support beam. When a large impact load is applied to the steering column, the ripping plate is deformed and torn progressively. The impact energy is absorbed during this process.

Scheme 3

1. 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 4

1. The fluid pump is directly driven by the engine through a belt.
2. 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.
3. 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.
4. The fluid pressure in chamber A or B acts on the rack piston in the same direction as 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.
5. 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 excessive fluid pressure.
6. Since a damper valve is provided in the fluid passage, the steering stability in straight forward direction is improved and kickback is reduced.

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 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 becomes a maximum.

Scheme 5

Mid- to high-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.

Scheme 6