Vane pump with annular groove in rotor which connects undervane chambers
BACKGROUND OF THE INVENTION 
     The invention relates to a vane pump which may find its application in power steering apparatus for vehicle or the like. 
     Generally, a vane pump includes a cam ring having a substantially elliptical cam surface formed around its inner periphery, a rotor disposed for rotation within the cam ring, a plurality of vanes which are slidably disposed in slits formed around the outer periphery of the rotor at an equal circumferential interval, and plates which hold the combination of the cam ring and the rotor sandwiched therebetween. As the rotor rotates, each vane moves in sliding contact with the cam ring to increase or decrease the volume of the pump chamber which is defined between a pair of mutually adjacent vanes, thus performing an oil suction or discharge operation. 
     In a vane pump as described, the oil which is discharged from the pump is introduced into the inside of each slit in order to drive each vane through the slit of the rotor to achieve a positive contact of the vane with the internal surface of the cam ring. In the conventional practice, in their surfaces which are adapted to abut against the rotor, both of the plates are formed with four arcuate grooves which are positioned on a common circle, with the discharged oil being directly introduced into two of these grooves while the discharged oil is introduced into the remaining two grooves through a narrow path which communicates with the grooves. 
     Upon starting the vane pump, the vane which is slightly driven outward by the centrifugal effect, will abut against the cam to be pushed back in the opposite direction upon entering discharge stroke, whereby the oil is expelled from the slit into which the vane is driven. The expelled oil passes through the narrow path to be delivered to the discharge side of the pump. A flow resistance which the oil experiences during its passage through the narrow path causes an increase in the oil pressure inside the slit, thereby urging adjacent vanes outward which have not yet been significantly driven outward. By repeating such process, all of the vanes will eventually be completely driven outward for abutment against the cam ring to initiate the individual discharge operation. 
     The conventional vane pump as described above suffers from a number of difficulties, including poor starting response which is caused by a failure of a sufficient rise in the oil pressure inside the vane during its low speed rotation if the path which interconnects the arcuate grooves formed in both of the plates is not narrow enough, and an increase in the power loss and a rise in the oil temperature which may be caused by an increased flow resistance occurring during a high speed operation if the path is made excessively narrow. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the invention to provide a vane pump which exhibits an improved starting response and which reduces a power loss even during its high speed rotation. 
     The above object of the invention is accomplished by forming an annular groove in the lateral side of the rotor so that the annular groove may be interrupted by a vane as such vane retracts inward of the rotor while allowing the annular groove to provide communication between a plurality of arcuate grooves, which are formed on the surfaces of the plates which are used to hold the rotor and the cam ring sandwiched therebetween whenever the vane projects outward of the rotor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal section of vane pump according to one embodiment of the invention; 
     FIG. 2 is a cross section taken along the line II--II shown in FIG. 1; 
     FIG. 3 is a fragmentary section illustrating the starting phase; 
     FIG. 4 is a cross section taken along the line IV--IV shown in FIG. 3; and 
     FIG. 5 and 6 are views corresponding to FIGS. 3 and 4, respectively, for a second embodiment of the invention. 
     FIG. 7 is a view corresponding to FIG. 4 for a third embodiment of the invention. 
    
    
     DESCRIPTION OF EMBODIMENTS 
     Referring to the drawings, several embodiments of the invention will now be described. FIG. 1 is a longitudinal section, and FIG. 2 is a cross section taken along the line II--II, shown in FIG. 1, of a vane pump according to one embodiment of the invention. The pump comprises a substantially cylindrical front body 2 and a dish-shaped rear body 4 which are disposed in abutment against each other to define a pump body 6, in which a pump cartridge 8 is received. 
     The front body 2 includes a cylindrical portion 2a of a reduced diameter, through which an input shaft 10 is inserted into the pump body 6 and is rotatably supported by a pair of bearings 12 and 14. An oil seal is indicated at 16. The inner end 10a of the input shaft 10 which is disposed within the pump body 6 has a rotor 18 splined thereto for driving it for rotation. The rotor 18 is formed with radially extending slits 15 at an equal circumferential interval, with a vane 20 slidably fitted in each of the slits 15. A cam ring 22 having a substantially elliptical cam surface formed around its internal surface is disposed in surrounding relationship with the rotor 18. 
     The rotor 18 and the cam ring 22 are held sandwiched between a pressure plate 24 which is disposed inside the front body 2 and the rear body 4, with the rear body 4, the cam ring 22 and the pressure plate 24 being angularly positioned by a pin 26. The front body 2 and the rear body 4 are formed with a suction port 2b and suction passages 2c, 4a. Oil which is drawn into each pump chamber, defined by a pair of adjacent vanes 20, through the passages 2c and 4a, lows through a discharge passage 24a formed in the pressure plate 24 to be discharged into a discharge chamber 2d defined in the bottom of the front body 2. 
     At a location opposite to the inner end of each of the slits 15 formed in the rotor 18, the surface of the pressure plate 24 which faces the rotor 18 is formed with an arcuate groove 24b, which is divided into four segments, while arcuate grooves 4b are formed in the surface of the rear body 4 which faces the rotor 18 at a location corresponding to the arcuate groove 24b. The arcuate groove 24b formed in the pressure plate 24 communicates with the discharge chamber 2d through a through-opening 24c. In its both lateral sides, the rotor 18 is formed with annular grooves 18a which are located to be opposite to those portions of the arcuate grooves 4b and 24b which are situated toward their outer periphery. As will be noted from FIG. 2, such annular groove 18a extends across a portion of each slit 15 in which the vane 20 is received that is located toward their inner end, whereby when the vane 20 has been retracted to its inner limit, such annular groove 18a will be interrupted by the vane 20. On the other hand, when the vane 20 is driven outwardly to assume its outer limit, the annular groove 18a is effective to provide a communication between the separate arcuate grooves 4b and 24b. It is to be noted that the cross-sectional area of the annular groove 18a formed in the rotor 18 is chosen to be greater than that of a narrow path which is formed in a conventional plate (corresponding to the pressure plate and the rear body) to provide a communication between arcuate grooves. 
     Describing the operation of the vane pump described above, the vane 20 assumes its inner or retracted position within the slit 15 to interrupt the annular groove 18a before the pump operation is started (see solid line position in FIG. 4). However, as the rotor 18 begins to rotate, the vane 20 will be driven outward to a degree by the centrifugal effect (see a central vane shown in FIG. 3), and when it reaches a discharge stroke, it abuts against the internal surface of the cam ring 22 to be pushed back in the opposite direction, as indicated by a right-hand vane shown in FIG. 3. As the vane 20 is pushed back in this manner, the oil which is present inside the slit 15 in which such vane 20 is disposed will be expelled therefrom, and flows through the arcuate grooves 4b and 24b to push the vane 20 located in the adjacent slit 15 up into the abutment against the cam ring 22. 
     The fact that the oil inside the slit 15 which is expelled by the vane 20 which is pushed back is utilized in its entirety to push up the vane fitted in another slit 15 which is located within the common arcuate grooves 4b and 24b is effective to improve the starting response of the vane pump. 
     In the event the vane pump operates at high speed, the oil will flow from the arcuate grooves 4b and 24b disposed in the discharge region to other arcuate grooves 4b and 24b through the annular groove 18a which exhibits a greater cross-sectional area than that of a conventional path, whereby the resistance which such an oil flow experiences will be reduced, with a corresponding reduction in the power loss. In addition, a temperature rise of the oil is suppressed, thus preventing a degradation of the oil quality, 
     FIGS. 5 and 6 show a second embodiment. Similar parts as those used to describe the first embodiment will be designated by like reference numerals and characters without repeating their description. In this embodiment, narrow paths 4c and 24d are formed in the rear body 4 and the pressure plate 24, respectively, for providing a communication between four arcuate grooves 4b and 24b in the similar manner as in a conventional arrangement. In addition, an annular groove 18a which is similar in nature to the described embodiment is formed in the opposite lateral surfaces of the rotor 18. 
     Upon starting, the operation of this embodiment is similar to that of conventional vane pump, in that the annular groove 18a formed in the rotor 18 is interrupted by the vane 20 and a communication between the arcuate grooves 4b and 24b is provided by narrow paths 4c and 24d. However, during normal operation, the vane 20 will be driven outward to enable a communication through the annular groove 18a, whereby the cross-sectional area of the flow channel which provides a communication between the arcuate grooves 4b and 24b increases to reduce a resistance presented to an oil flow and to reduce the resulting power loss. In addition, the cross-sectional area of the narrow paths 4c and 24d may be reduced than that of the prior art while enabling a reduction in the power loss, due to the presence of the annular groove 18a having an increased cross-sectional area, with consequent improvement in the starting response. 
     Further, as shown in FIG. 7, it may be arranged that the annular groove 18a is formed on the only one surface of the rotor 18, thus either of the narrow paths for communicating the annular grooves 4b and 24b can be provided. 
     It is to be understood that the annular groove 18a need not be constructed so that the communication therethrough is completely interrupted by the vane 20 as it retracts, but may be constructed to provide an essential interruption while leaving partial communication. 
     While the invention has been illustrated and described above in connection with several embodiments thereof, it should be understood that the invention is not limited to the precise embodiments disclosed herein, but that a number of changes, modifications and substitutions therein will readily occur to one skilled in the art from the above disclosure without departing from the scope and spirit of the invention defined by the appended claims.