The present invention relates to a vehicle-use hydraulic power steering apparatus useable in a vehicle, the apparatus having a damper valve. In particular, the invention relates to a hydraulic power steering apparatus in which the dampering effect is changeable according to the number of revolutions of an internal combustion engine so that steering follow-up performance in a low speed range of the vehicle is improved and stability of steering in a high speed range is improved.
Conventionally, Japanese Utility Model Application Publication No. 2-49109 discloses a hydraulic power steering apparatus used in a vehicle in which a damper valve is disposed in a hydraulic oil circuit for connecting an oil passage selector valve in a gear box with right and left oil chambers of a power cylinder.
In this apparatus, as shown in FIG. 5, hydraulic oil circuits 08 and 09, which connect an oil passage selector valve 011 housed in a gear box of a hydraulic power steering apparatus 01 with right and left oil chambers of a power cylinder 06, is provided with check valves 010a, 010a which allow oil only to flow from the oil passage selector valve 011 to the right and left oil chambers, and throttle check valves 010b, 010b which have an initial load and allow oil only from the oil chambers to the oil passage selector valve 011. The check valves 010a, 010a and the throttle check valves 010b, 010b comprise a damper valve.
A hydraulic pump 013 which is a hydraulic source for supplying hydraulic oil to the power cylinder 06 includes a flow control valve 023, a relief valve 024 and a fixed orifice 025, and their cooperation allows the hydraulic oil to be discharged by a discharge amount in compliance with a characteristic chart shown in FIG. 4 according to the number of revolutions.
In other words, the hydraulic oil which gradually increases and reaches a substantially constantly high level is discharged in a low revolving range where the number of revolutions of the hydraulic pump 013 reaches a predetermined low revolving speed N, and when it exceeds the number of revolutions N, the hydraulic oil whose amount gradually decrease and reaches a substantially constantly low level is discharged. The hydraulic pump 013 is connected to a crank shaft of an internal combustion engine, not shown, via a transmission mechanism, and it rotates at the number of revolutions which is the same as the number of revolutions of the internal combustion engine.
Since the damper valve in the above-mentioned conventional hydraulic power steering apparatus 01 does not have a portion where friction is generated, returning performance of a steering handle is excellent. Moreover, since each of the throttle check valves 010b, 010b has the initial load, even if a rack shaft 04 which serves also as a piston rod of the power cylinder 06 tries to slightly oscillate due to unbalance of tires or the like, the flow of the hydraulic oil from the right and left oil chambers to the oil passage selector valve 011 is restricted, and thus the rack shaft 04 hardly moves. For this reason, slight oscillation of the steering handle, namely, a shimmy phenomenon, can be prevented. In this regard, reference numeral 012 is a reservoir tank.
However, since the conventional damper valve has a constant damper effect regardless of the number of revolutions of the internal combustion engine (the number of revolution of the hydraulic pump) and the speed of a vehicle, the damper effect is not required so much at the time of low-speed revolution of the internal combustion engine and in a low-speed operation of the vehicle. In fact the damper effect is required in the occasion opposite to the above one. However, in the past the damper effect could not be adjusted according to the degree of necessity. For this reason, there is a need to improve steering follow-up performance of the steering handle particularly, at the low speed of the vehicle.
The present invention is disclosed in order to solve the above problem. It therefore is an object of the present invention to provide a hydraulic power steering apparatus in which the damper effect of a damper valve is changeable according to the number of revolutions of an internal combustion engine so that steering follow-up performance of a steering handle, particularly in a low speed range of a vehicle can be improved, and stability of steering in a high speed range of the vehicle can be improved, and a shimmy phenomenon and the kick-back phenomenon can be prevented simultaneously.
The present invention relates to a hydraulic power steering apparatus which solves the above problem. The invention provides a hydraulic power steering apparatus, which is constituted so that a damper valve is disposed in hydraulic oil circuits for connecting an oil passage selector valve in a gear box with right and left oil chambers of a power cylinder, characterized in that: the damper valve is composed of a spool valve. A control oil chamber of the spool valve is connected with a hydraulic oil feed passage for connecting a hydraulic pump which is driven by an internal combustion engine so as to discharge hydraulic oil with the oil passage selector valve. Its back pressure chamber is connected with a hydraulic oil circulating passage for connecting the oil passage selector valve with a reservoir tank. Orifices are provided respectively in the connecting oil passages, and when a discharged oil pressure of the hydraulic pump rises, the opening of the spool valve increases.
Since the invention is constituted as mentioned above, the spool valve, which composes the damper valve disposed in the hydraulic oil circuits for connecting the oil passage selector valve with the right and left oil chambers of the power cylinder, is constituted so that its control oil chamber is connected with the hydraulic oil feed passage for connecting the hydraulic pump with the oil passage selector valve. Its back pressure chamber is connected with the hydraulic oil circulating passage for connecting the oil passage selector valve with the reservoir tank. When the spool valve is actuated in compliance with a discharge oil pressure of the hydraulic pump, which is driven by the internal combustion engine so as to discharge hydraulic oil and the discharge oil pressure rises, the opening of the spool valve increases.
As a result, when the number of revolutions of the internal combustion engine is low and thus the number of revolutions of the hydraulic pump is low, and a discharge amount of the hydraulic oil increases based on a characteristic chart of the power steering, when a steering handle is steered, a discharge pressure of the hydraulic oil also rises. As a result, opening of the damper valve increases, and thus the hydraulic oil circuits are opened to a greater extent. As a result, the hydraulic oil easily flows through the circuits, the damper effect of the damper valve is weakened, and steering follow-up performance of the steering handle in a low-speed range of a vehicle is improved so that an operator feels lightness of steering.
In addition, in the case where the number of revolutions of the internal combustion engine is high, and thus the number of revolutions of the hydraulic pump is high and a discharge amount of the hydraulic oil becomes small based on the characteristic chart of the power steering, even when the steering handle is steered, the discharge pressure of the hydraulic oil does not rise so much. As a result, the opening of the damper valve becomes small so that the hydraulic oil circuits are opened to a lesser extent. As a result, the hydraulic oil flows with difficultly through the circuits, and the damper effect of the damper valve is manifested. Then, the steering follow-up performance of the steering handle in a high-speed range of the vehicle is lowered, and instability of the steering wheel in a neutral position is reduced, so that the operator feels stability of the steering.
Further, in this case, since the damper valve displays the essential damper effect, slight oscillation (shimmy phenomenon), which is transmitted from the wheels to the steering handle via a piston rod (rack shaft) of the power cylinder while the vehicle is running, is suppressed. Moreover, a so-called kick-back phenomenon such that the steering handle rotates in a reverse manner, which occurs when the wheels run onto a rock or the like, is also suppressed. As a result, comfortableness of the steering is improved.
In addition, since orifices are disposed respectively in a connecting oil passage for connecting the control oil chamber of the damper valve (spool valve) with the hydraulic oil feed passage and in a connecting oil passage for connecting the back pressure chamber with the hydraulic oil circulating passage, the movement of a spool of the damper valve is slowed down. As a result, when the steering handle is returned to its neutral position, particularly in a low-speed range of a vehicle, the steering handle can be returned before the damper effect is generated so that the operator feels the steering being more light.