The present invention relates to a variable displacement vane pump for use in various types of equipment using pressure fluid such as a power steering apparatus for alleviating the force of operating the steering wheel of an automobile.
Conventionally, volume-type vane pumps which are directly driven by an automobile engine to rotate have generally been employed as pumps for power steering apparatus. Since a discharge flow rate increases or decreases in proportion to the number of revolutions of the engine, however, such volume-type pumps have characteristics mutually contradictory to those of the power steering 0apparatus in that a large steering-assisting force is required during a standstill or low-speed running, whereas a small steering-assisting force is required during high-speed running. Therefore, the volume of such a pump has to be large enough to secure a discharge flow rate which makes it possible to obtain a required steering-assisting force even during low-speed running when the number of revolutions is small. Moreover, a flow control valve becomes indispensable to controlling the discharge flow rate so as to keep it at a fixed level or lower during high-speed running when the number of revolutions is large. For this reason, the number of component parts for use in constituting the pump tends to increase and not only the structure of each pump but also that of passages therein become complex, thus inevitably making the overall apparatus large in size and costly.
In order to obviate drawbacks characteristic of volume-type pumps, there have been proposed various variable displacement vane pumps capable of reducing a discharge flow rate per turn (cc/rev) in proportion to an increase in the number of revolutions as in Japanese Patent Laid-Open Publications Nos. SHO-53-130505/1978, SHO-56-143383/1981, SHO-58-93978/1983, and Japanese Utility Model Publication No. SHO-63-14078/1988. These variable displacement pumps each require no flow control valves as those used in the volume type, prevent a wasteful increase in driving power and excel in the energy efficiency. Moreover, such variable displacement pumps are capable of preventing the oil temperature from rising as there is no return flow to the tank side, and solving problems arising from leakage in the pump interior and a decline in the volume efficiency.
The variable displacement pumps disclosed in Japanese Patent Laid-Open No. SHO-56-143383 are arranged as follows: A cam ring is provided movably in a pump casing; a pair of fluid-pressure chambers serving as control chambers are formed in a gap between the cam ring and the pump casing; and the pressure on the upstream and downstream sides of an orifice provided midway in a discharge passage is made to act directly on the cam ring so as to move the cam ring against the urging force of a spring to change the volume of the pump chamber, whereby discharge flow-rate control is properly effected.
Referring now to FIG. 6, there is shown an example of the variable displacement vane pump described above. In FIG. 6, reference numeral 1 designates a pump body; 1a, an adaptor ring; and 2, a cam ring which is provided within an elliptical space 1b formed in the adaptor ring 1a in a swinging, displaceable manner via a pivotally supporting portion 2a, and to which an urging force is imparted by a press means in the direction indicated by a dropout arrow in the drawing. Further, reference numeral 3 designates a rotor which is accommodated in the cam ring 2 while being situated to one side with reference to the center in such a manner as to form a pump chamber 4 on the other side, and which allows vanes 3a to move back and forth, the vanes 3a being held in a manner capable of radially advancing or retracting when the rotor 3 is driven by an external driving source to rotate.
Incidentally, reference numeral 3b in FIG. 6 designates the drive shaft of the rotor 3, which is driven to rotate in the direction of the arrow.
Further, reference numerals 5, 6 designate a pair of fluid- pressure chambers which become high- and low-pressure sides each formed on both sides of the outer periphery of the cam ring 2 in the elliptical space 1b of the adaptor ring 1a of the body 1, and there are passages 5a, 6a opened to the chambers 5, 6 and used for introducing control pressure for swinging and displacing the cam ring 2, for example, fluid pressure on the upstream and downstream sides of a variable orifice provided in a pump discharge-side passage. When the fluid pressure on the upstream and downstream sides of the variable orifice in the pump discharge-side passage is thus introduced through the passages 5a, 6a, the cam ring 2 is swung and displaced in a desired direction to render variable the volume of the pump chamber 4, so that a discharge-side flow rate is variably controlled in proportion to the flow rate on the discharge side of the pump. In other words, the discharge-side flow rate is so controlled as to decrease the discharge-side flow rate as the number of revolutions of the pump increases.
Reference numeral 7 designates a pump suction-side opening which is open in face-to-face relation to a pump suction-side region 4A in the pump chamber 5; and 8, a pump discharge-side opening which is open in face-to-face relation to a pump discharge-side region 4B. These openings 7, 8 are formed in either pressure or side plate (neither is shown) and both of them are fixed wall portions for holding the rotor 4 and the cam ring 2 constituting a pump component element by clamping the same from both sides thereof.
In this case, an urging force is imparted to the cam ring 2 from the side of the fluid-pressure chamber 6 as shown by F of FIG. 6, so that the volume of the pump chamber 5 is normally maintained at a maximum level. In addition, reference numeral 2b in FIG. 6 designates a seal member provided on the outer periphery of the cam ring 2 so as to define the fluid-pressure chambers 5, 6 on both left- and right-hand sides in association with the pivotally supporting portion 2a provided on the outer periphery thereof.
Further, reference numeral 8a designates a goatee-shaped notch formed in such a manner as to continue from a terminating portion, in the rotational direction of the pump, of the pump suction-side opening 8. When leading ends of the vanes 3a are brought into sliding contact with the inner periphery of the cam ring 2 as the rotor 3 rotates to make them perform pumping action, the notch 8a functions as what allows the fluid pressure to escape gradually from the-high-pressure side to the low-pressure side between the space held by vanes approaching the end portion of each of the openings 7, 8 and the space between vanes adjacent thereto. The notch 8a is effective in preventing the occurrence of surge pressure and the problem of pulsation arising therefrom.
With the variable displacement pump thus constructed as described above, a relief valve for relieving excessive fluid pressure is additionally installed on a part of the pump discharge side.
In the conventional variable displacement vane pump described above, the pump chamber (the chamber partitioned by the vanes 3a, 3a) 4 has the pump discharge pressure and the pump suction pressure alternately in a pump cartridge (pump acting portion) with the pump component elements including the rotor 3, the cam ring 2 and the like when it is positioned in the region ranging from the terminating point of the suction-side opening 4A up to the starting point of the discharge-side opening 4B in the pump chamber and when it is positioned in the intermediate region (the portion indicated with symbols 9A, 9B of FIG. 6) ranging from the terminating point of the discharge-side opening 4B up to the starting point of the suction-side opening 4A.
This is due to the fact that when the preceding vane 3a in the direction in which the rotor 3 rotates reaches the opening 4B or 4A on the leading end side in the rotational direction, the vane 3a has pressure equal to the port pressure on the pump discharge or suction side in the opening 4A or 4B and when the following vane 3a stays at the opening 4A or 4B on the rear end side in the rotational direction, it has pressure equal to the port pressure because of the opening that follows.
An odd number of vanes 3a are employed in a variable displacement vane pump of this type in particular, the vanes 3a are unevenly arranged in the direction in which the rotor 3 rotates and consequently, the space formed between the vanes 3a, 3a passing through the intermediate region 9A and what is formed between those which pass through the intermediate region 9B facing the former with the rotary shaft 3b of the rotor 3 are set asymmetrical, so that the pressure balance tends to become disturbed.
As thrust due to the mutual difference between the pump chambers in the opposing intermediate regions 9A, 9B originating from such unbalanced pressure as well as pressure fluctuation acts on the inner face of the cam ring 2, the cam ring 2 is caused to swing thereby, which results in producing phenomena of flow-rate fluctuation and oil-pressure pulsation on the discharge side of the pump, thus making a noise problem. The pulsating phenomenon appears as shown in a characteristic graph of FIG. 5(b).
For the reason stated above, it has been proposed to provide a metering orifice midway in the pump discharge-side passage of the aforesaid variable displacement vane pump. The fluid pressure on the upstream and downstream sides of the orifice is then used for switching the operation of spool-type control valve so to supply the fluid pressure on the upstream and downstream sides of the orifice and to apply the suction side of the pump selectively to the chambers 5, 6 on both sides of the outer periphery of the cam ring 2, whereby the swinging phenomenon of the cam ring 2 is suppressed. Nevertheless, the arrangement thus proposed still remains unsatisfactory and some countermeasures have been desired to be taken.
Particularly when utilizing equipment to be supplied with fluid pressure from the variable displacement pump operates, the fluid pressure in the main supply passages rises and thereby the pressure difference between the upstream and downstream sides of the metering orifice installed in the passage or the pump discharge-side passage increases. It is therefore necessitated to solve a problem arising from the fluctuation of the pump discharge-side pressure which becomes increased and conspicuous.
When the utilizing equipment is a power steering wheel, for example, the power steering wheel may become difficult or easy to manipulate as a high and a low flow rate are applied to the power steering wheel side. Instability like this needs obviating.
In the conventional variable displacement pump-above, there is also a problem arising from the swinging phenomenon cause to a spool in the control valve for controlling the fluid pressure supplied to the high- and low-pressure sides of the fluid-pressure chamber so as to move and displace the cam ring.
In other words, the pump discharge-side fluid on the upstream side of the metering orifice is introduced into the one chamber of the spool in the control valve, whereas the pump discharge-side fluid on the downstream side of the metering orifice is introduced into the other chamber having the spring. Moreover, the pressure difference between the front and rear of the orifice increases as the flow rate of the discharge-side fluid rises, and the desired fluid pressure is introduced to the high-pressure side of the fluid-pressure chamber when the spool of the valve moves to the other chamber side to cause the cam ring to be moved and displaced, so that the flow rate of the discharge-side fluid is reduced.
When, however, the load on the utilizing equipment side causes the fluid pressure on the discharge side of the pump in such a control valve, the spool within the valve is also caused to swing and the so-called swinging phenomenon occurs. This point is also desired to be taken into consideration.
In the conventional variable displacement pump, a dampening orifice is formed in the fluid passage for use in introducing the fluid pressure on the downstream side of the metering orifice into the other chamber having the spring of the control valve to stabilize the movement of the spool in the valve. However, only the provision of the damping orifice has little throttling effect and allows the spool in the valve to readily swing because the passage flow rate of the fluid is low, which results in not only rendering unstable the fluid pressure in each fluid-pressure chamber under the control of the valve but also causing the cam ring to swing. Consequently, it is desired to clear away those problems mentioned above as they are impossible to suppress.