Abstract:
A hydraulic fluid feeding device for a power steering device is proposed which has a plurality of pumps for feeding hydraulic fluid to the power steering device and a flow path change-over valve for switching the paths between the pumps and the power steering device, wherein the feeding rate of the hydraulic fluid to the power steering device is controlled by the action of the flow path change-over valve. The flow path change-over valve is operated according to the magnitude of the load exerted on the power steering device so that hydraulic fluid from all the pumps combined is fed to the power steering device when the load is great, and the hydraulic fluid from at least one of the pumps is fed to the power steering device when the load is small. With this device, the control of the feeding rate becomes efficient. It also becomes possible to bypass the hydraulic fluid from some of the pumps to a tank, independently of the delivery rates of the pumps, so that the power consumption may be reduced.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a hydraulic fluid feeding device for feeding hydraulic fluid to operate a power steering device. 
     A device has been proposed which utilizes two pumps and a flow change-over valve for, on the one hand, combining the flows from the two pumps to supply them to a power steering device when the delivery rate of each pump is small, and, on the other hand, for switching the flow change-over valve to feed the hydraulic fluid from one pump to the power steering device and to bypass the hydraulic fluid from the other pump to a tank when the delivery rate of each pump becomes large. With such a device, the horsepower requirement to drive one pump may be made extremely small when bypassing the hydraulic fluid from this pump to the tank, and a lower consumption of power may be attained. 
     However, the power steering device requires a relatively high feeding rate of hydraulic fluid when a high load is exerted thereon, that is, when a high output is required. The device proposed above does not necessarily satisfy this requirement of the power steering device. For example, the pumps are normally driven by the engine, and their delivery rates are increased in proportion to an increase in the rotational frequency. Therefore, when a vehicle is travelling along a winding and steep slope, the flow change-over valve is so switched that the hydraulic fluid from only one pump is fed to the power steering device. Under such conditions, the feeding rate of hydraulic fluid fed to the power steering device may become too small upon abrupt turning of the steering wheel, in the clockwise or counterclockwise direction, adversely affecting the steering response. 
     BRIEF SUMMARY OF THE INVENTION 
     It is, therefore, the primary object of the present invention to provide a hydraulic fluid feeding device which is capable of attaining a lower consumption of power, and of feeding hydraulic fluid at a high rate when a high load is exerted on the power steering device. In order to achieve this object, the present invention provides a hydraulic fluid feeding device for a power steering device wherein a plurality of pumps and a flow change-over valve for switching the flow paths between these pumps and the power steering device are operated according to the magnitude of the load of the power steering device, so that the hydraulic fluid from all the pumps combined may be fed when the load is high. 
     It is another object of the present invention to provide a hydraulic fluid feeding device for a power steering device wherein the steering wheel and the flow change-over valve are operated in cooperation with each other, so that the hydraulic fluid from all the pumps combined may be fed to the power steering device when the steering angle of the steering handle is large, and the feeding rate of hydraulic fluid fed under a high load may be increased without failure. 
     It is another object of the present invention to provide a hydraulic fluid feeding device for a power steering device wherein the flow change-over valve is operated according to an increase in the pressure of hydraulic fluid fed upon operation of the power steering device, thus simplifying the construction of the overall device. 
     The above and other objects and features of the present invention will become apparent from the following description when taken in conjunction with the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The attached drawings show the preferred embodiments of the present invention in which: 
     FIG. 1 is a sectional view illustrating the main part of an embodiment of the present invention; 
     FIG. 2 is a sectional view of the main part of the device shown in FIG. 1 under a different condition; 
     FIG. 3 shows characteristic curves representing the flow rate characteristics; and 
     FIG. 4 is a sectional view of the main part of another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described by way of its example with reference to the accompanying drawings. Referring to FIG. 1, the hydraulic fluid delivered from a first pump 1 is fed through a conduit 2, a feeding path 4 formed in a casing 3, and a conduit 5 to a power steering device 6. The hydraulic fluid is then returned through a conduit 7 to a tank 8 and through a conduit 9 to the first pump 1. A known flow rate control valve 10 is assembled in the casing 3. This flow rate control valve 10 has an orifice 11 disposed in the feeding path 4, and a spool valve 13 slidably fitted inside a space 12 formed in the casing 3 in a fluid-tight manner. This spool valve 13 divides the space 12 into a high pressure chamber 14 and a low pressure chamber 15. The high pressure chamber 14 communicates with the upstream side of the orifice 11 through a path 16, and the low pressure chamber 15 communicates with the downstream side of the orifice 11 through a path 17 and an orifice 18 for the prevention of vibrations of the spool valve 13. The spool valve 13 is held at the nonoperative position shown in the figure by a spring 19 disposed inside the low pressure chamber 15, to normally seal the high pressure chamber 14 from a bypass 20. This bypass 20 is connected to the tank 8 through a conduit 21, and a relief valve 22 is disposed inside the spool valve 13. 
     According to this embodiment of the present invention, a flow path change-over valve 25, for switching the flow path of a second pump 24 according to the magnitude of the steering angle of a steering wheel 23, is disposed inside the casing 3. In order to operate the flow path change-over valve 25 according to the steering angle of the steering wheel 23, in this embodiment, one end of a rod 28 extends fluid-tightly from the exterior of the casing 3 into a hole 27 in which a spool valve 26 comprising the flow path change-over valve 25 is slidably fitted in a fluid-tight manner. The other end of this rod 28 is in elastic contact with the cam face of a cam 31 mounted on an output shaft 30 of the power steering device 6. The terminal end of the spool valve 26 is in elastic contact with one end of the rod by a spring 32. This cam 31 has a cam face such that the length of the rod 28 extending into the hole 27 may be minimized when the steering wheel 23 is in the neutral position, that is, when the output shaft 30 operating in cooperation therewith is in the neutral position. Further, this cam face is designed such that the length of the rod 28 extending into the hole 27 may be increased with an increase in the rotation of the output shaft, that is, an increase in the steering angle of the steering wheel 23 when the steering wheel 23 is pivoted clockwise or counterclockwise and the output shaft is correspondingly pivoted. Accordingly, the spool valve 26 is displaced to the right with the increase in the steering angle from the neutral position of the steering wheel 23 so as to switch the flow path in a manner to be described later. 
     Two annular grooves 33 and 34 are formed in the outer circumferential surface of the spool valve 26. One annular groove 33 constantly communicates with the outlet port of the second pump 24 through a path 35 formed in the casing 3 and a conduit 36 connected to the path 35. The other annular groove 34 constantly communicates with the annular groove 33 through a path 37 formed in the interior of the spool valve 26, and is capable of communicating with two paths 38 and 39 formed in the casing 3, according to the operating position of the spool valve 26. One path 38 communicates with the annular groove 34 when the spool valve 26 is located at the nonoperative position shown in the figure (the extreme left end), as well as with the tank 8 through a conduit 40. The other path 39 communicates with the annular groove 34 when the spool valve 26 has been slightly displaced to the right from the nonoperative position shown in the figure. The path 39 is capable of maintaining this communication with the annular groove 34 even when the spool valve 26 is further displaced to the right and the communication of the path 38 with the annular groove 34 is blocked. This path 39 also communicates with the feeding path 4 and with the outlet port of the first pump 1. A check valve 41 is disposed inside the path 39, for allowing the hydraulic fluid to flow from the side of the annular groove 34 to the side of the feeding path 4. The delivery pressure of the first pump 1 is introduced to a left end chamber 42 of the spool valve 26 through a path 43, and the delivery pressure of the second pump 24 is introduced to a right end chamber 44 through a path 37. The area receiving the pressure at the left end is set to be larger than that at the right of the spool valve 26. 
     With such a construction, under the condition that the steering wheel 23 is located at the neutral position, the output shaft 30 operating in cooperation therewith is also located at the neutral position, and the spool valve 26 is located at the nonoperative position, the annular groove 34 is blocked from the path 39 so that the hydraulic fluid from the second pump 24 is bypassed through the conduit 36, the path 35, the annular groove 33, the path 37, the annular groove 34, the path 38 and the conduit 40 to the tank 8. On the other hand, the hydraulic fluid from the first pump 1 is fed to the power steering device 6 through the conduit 2, the feeding path 4 and the conduit 5. When the delivery rate of the first pump 1 exceeds a predetermined value, the flow control valve 10 operates in a known manner to bypass the extra fluid from the feeding path 4 through the path 16, the high pressure chamber 14, the bypass 20 and the conduit 21 to the tank 8 so that the feeding flow rate of the fluid to the power steering device may be kept substantially constant. 
     The flow rate characteristic curve (A) in FIG. 3 shows the above condition. Referring to this figure, (I) is a curve showing the relation between the rotational frequency and the delivery rate of the first pump 1, (J) is a curve showing the same relation for the second pump 24, and (K) is a curve showing the total delivery rate of the two pumps acting in combination. In the case described above, the hydraulic fluid is fed to the power steering device 6 only by the first pump 1. All the fluid delivered by the first pump 1 is fed to the power steering device until the flow rate exceeds a predetermined value (a) at which the flow rate control valve 10 starts operating. When it has exceeded the predetermined value (a), the flow rate control valve 10 operates to keep the feeding rate of the hydraulic fluid to the power steering device 6 substantially constant (the characteristic curve (A) shown in FIG. 3). 
     As may be seen from the above description, with a small steering angle, according to which the steering wheel 23 is located at the neutral position or is only slightly rotated clockwise or counterclockwise from the neutral position, the delivery rates of the pumps 1 and 2 do not change and the hydraulic fluid from the second pump 24 is bypassed to the tank. When the steering angle of the steering wheel 23 is zero or small, a high load is not generally exerted on the power steering device 6, so that only the hydraulic fluid from the first pump 1 may be fed to the power steering device 6. However, when a high load is exerted, the delivery pressure of the first pump 1, and accordingly, the pressure inside the chamber 42 communicating therewith, is raised. Then, the spool valve 26 is displaced to the right by the differential pressure between the chambers 42 and 44. In a manner similar to that described with reference to the case wherein the spool valve is displaced to the right, the hydraulic fluid from the second pump 24 is fed to the power steering device 6. 
     When the steering angle of the steering wheel 23 becomes large and the displacement to the right of the spool valve 26 becomes large through the output shaft 30 operating in cooperation therewith, the cam 31 and the rod 28, the annular groove 34 communicates with both the paths 38 and 39. The flow path between the annular groove 34 and the path 38 communicating with the tank 8 is made narrower by the displacement to the right of the spool valve 26, raising the pressure at the side of the annular groove 34. When this pressure exceeds the pressure at the side of the feeding path 4, part of the hydraulic fluid introduced from the second pump 24 to the inside of the annular groove 34 flows in the feeding path 4 through the path 39 and the check valve 41 to meet with the hydraulic fluid from the first pump 1, and is fed to the power steering device 6 (FIG. 2). When the steering angle of the steering wheel 23 becomes still larger, and the displacement to the right of the spool valve 26 becomes larger, the communication between the annular groove 34 and the path 38 is blocked. Under this condition, all of the hydraulic fluid from the second pump 24 is fed to the power steering device 6. 
     In this manner, when the steering angle of the steering wheel 23 exceeds the small angle described above, the feeding rate of the hydraulic fluid to the power steering device increases, even though the delivery rates of the pumps 1 and 24 are constant. In FIG. 3, when the rotational frequency of the pumps is (b), the first pump 1 has a delivery rate of (c), and the second pump 24 has a delivery rate of (d). When the steering angle is small and the hydraulic fluid is fed to the power steering device 6 from only the first pump 1, the feeding rate of the hydraulic fluid to the power steering device 6 is (c). However, when the steering angle increases under this condition, the hydraulic fluid from the second pump 24 is fed to the power steering device 6 to increase the overall feeding rate. All the delivered fluid from the second pump 2 is finally fed to the power steering device 6, and the total combined feeding rate becomes (e). When the pump rotational frequency increases and the total feeding rate becomes the predetermined value (a) described above, the flow rate control valve 10 operates so that the feeding rate is kept substantially constant (the characteristic curve (B) of FIG. 3). When the steering angle is intermediate or above, the vehicle is, in general, travelling at a low speed, that is, the delivery rates of the pumps 1 and 24 are small. Under such condition, a large load acts on the power steering device 6, and the vehicle requires a relatively high feeding rate of the hydraulic fluid. Thus, the increase in the feeding rate of the hydraulic fluid is capable of responding to this condition. 
     FIG. 4 shows another embodiment of the present invention. In this embodiment, the rod 28, a spring 29, and the cam 31 are eliminated and the spool valve 26 is controlled to advance or withdraw by the delivery pressure of the first pump 1. The construction of the other parts is the same as that of the former embodiment. 
     In accordance with the construction of this embodiment, the spool valve 26 is not operated according to the steering angle. However, the load exerted on the power steering device 6 becomes greater as the steering angle increases. Accordingly, the delivery pressure of the first pump 1 increases to increase the displacement to the right of the spool valve 26, providing substantially the same effects as the former embodiment. 
     Although the present invention has been described in connection with preferred embodiments thereof, many variations and modifications will now become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.