Motor vehicles – Steering gear – With fluid power assist
Reexamination Certificate
1999-03-04
2001-04-17
Johnson, Brian L. (Department: 3618)
Motor vehicles
Steering gear
With fluid power assist
C180S442000, C060S468000
Reexamination Certificate
active
06216815
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hydraulic power steering apparatus.
2. Description of the Related Art
Hydraulic power steering apparatuses have been proposed for assisting steering operation of vehicles and the like. In general, the conventional hydraulic power steering apparatus mainly consists of a pump
50
, a reservoir
90
, a rotary control valve
10
and a power cylinder
30
, as shown in FIG.
1
. The pump
50
is for supplying operating fluid for producing assist power in the steering operation of a driver. The reservoir
90
is for storage of the operating fluid. The rotary control valve
10
locates between the pump
50
and the reservoir
90
, and controls the operating fluid in accordance with the steering operation. The power cylinder
30
works in response to the operating fluid controlled by the rotary control valve
10
.
The rotary control valve
10
adopts a center-closed-type for giving the driver steady steering feeling in the vicinity of the neutral position of the steering wheel (refer to FIG.
2
). The center-closed-type of the rotary control valve
10
substantially closes an inlet port
10
a
leading the operating fluid from the pump
50
in the vicinity of the neutral position of the steering wheel to prevent the operating fluid for the power cylinder
30
. Since difference pressure in the power cylinder
30
is maintained to be extremely low or substantially zero in the vicinity of the neutral position, the steering feeling becomes similar to that of a non-assisted steering apparatus. Therefore, the power steering apparatus is able to give the driver steady steering feeling in the vicinity of the neutral position of the steering wheel, i.e., rigidity of the neutral position is enhanced.
However, the inventors found that the power steering apparatus with the center-closed-type rotary control valve
10
is inferior to that with a center-open-type rotary control valve
10
′ shown in
FIG. 3
in self-return of the steering wheel to the neutral position thereof. The self-return of the steering wheel is a restoring rotation of the steering wheel caused by reaction force from wheels, i.e., self-aligning-torque, so as that the steering wheel automatically returns to the neutral position after the driver stops turning the steering wheel.
The center-open-type rotary control valve
10
′ shown in
FIG. 3
opens an inlet port
10
a′
, cylinder ports
10
b′
and return ports
10
c′
in the neutral position thereof. This configuration allows two discharging passages A′ and B′ in each of the cylinder port
10
b′
to release the operating fluid under the self-return of the steering wheel. When the steering wheel returns to the neutral position, i.e., the self-return starts, a displaced power piston
330
′ returns to the neutral position of the power cylinder
30
′. For example, the power piston
330
′ shown by broken lines moves to the neutral position shown by solid lines. The moving power piston
330
′ discharges the operating fluid from a left-side cylinder chamber to the left-side cylinder port
10
b′
(refer to a broken arrow line of FIG.
3
). Under this state, the operating fluid is discharged from the passages A′ and B′ in the cylinder port
10
b′.
On the other hand, the center-closed-type rotary control valve
10
shown in
FIG. 2
closes the inlet port
10
a
in the neutral position thereof. This configuration allows only one discharging passage A in each of a cylinder port
10
b
to release the operating fluid under the self-return of the steering wheel. When the steering wheel returns to the neutral position, a displaced power piston
330
returns to the neutral position of the power cylinder
30
. For example, the power piston
330
shown by broken lines moves to the neutral position shown by solid lines so as to discharge the operating fluid from a left-side cylinder chamber to the left-side cylinder port
10
b
(refer to a broken arrow line of FIG.
2
). Under this state, the operating fluid is discharged from the passage A in the cylinder port
10
b.
Therefore, the center-closed-type rotary control valve
10
substantially has a half of area for discharging the operating fluid compared with the center-open-type rotary control valve
10
′. This structural difference gives the center-closed-type rotary control valve
10
the sluggish self-return of the steering wheel.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to provide an improved hydraulic power steering apparatus capable of giving smooth self-return of a steering wheel.
Briefly, this and other objects of this invention as hereinafter will become more readily apparent as having been attained broadly by a power steering apparatus, including a pump, a power cylinder, a control valve and a switching valve.
The pump is for discharging operating fluid. The power cylinder has two cylinder chambers divided by a power piston. The control valve is for regulating the operating fluid supplied to the power cylinder in accordance with rotation of a steering wheel. The switching valve is for connecting the cylinder chambers in accordance with pressure change of the operating fluid responsive to operation of the control valve in order to return the steering wheel toward a neutral position.
In the power steering apparatus, when the steering wheel is rotated by a driver, the control valve supplies the operating fluid discharged from the pump to the power cylinder. In this state, the switching valve disconnects the cylinder chambers, so as that hydraulic pressure acts on the power piston of the power cylinder to assist the steering operation.
When the rotation of the steering wheel is substantially stopped, the power piston returns toward the neutral position of the power cylinder by reaction force from wheels, i.e., self-aligning-torque. In this state, the switching valve reacts to the pressure change of the operating fluid in order to connect the cylinder chambers, so as that the higher hydraulic pressure of one of the cylinder chamber releases to the other. This hydraulic pressure releasing of the operating fluid smoothly returns the power piston toward the neural position. As a result, the steering wheel also returns toward the neutral position thereof in accordance with the return of the cylinder piston, so as that restoring force of the steering wheel is enhanced. Therefore, the power steering apparatus provides the smooth self-return of steering wheel with no costly electric device to help the self-return.
It is a preferable feature of the invention that the control valve includes a center-closed-type variable orifice for preventing the operating fluid supply to the power cylinder in vicinity of the neutral position of the steering wheel.
Sine the center-closed-type control valve prevents supplying the operating fluid to the power cylinder in vicinity of the neutral position, the power steering apparatus does not assist the steering operation in vicinity of the neutral position. This means that steering feeling of the driver is similar to that of the manual steering apparatus, so as that rigidity, i.e., strength of stability, of the steering operation is enhanced around neutral position. Therefore, directional stability of the vehicle is also enhanced.
Namely, the power steering apparatus copes with both enhancements that restoring force for self-return of the steering wheel and the rigidity of the steering operation around neutral position.
It is another preferable feature of the invention that the switching valve has a spool moving in accordance with differential pressure between discharge pressure of the pump and higher pressure of the cylinder chambers.
When the steering wheel is rotated by a driver, the differential pressure is relatively small, so as that the switching valve maintains the disconnection between the cylinder chambers. When the steering wheel is stopped, the control valve returns to its neutral position.
Ando Kazuya
Inaguma Yoshiharu
Johnson Brian L.
Lerner Avraham H.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Toyoda Koki Kabushiki Kaisha
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