Patent Publication Number: US-6709242-B2

Title: Variable displacement pump

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a variable displacement pump employed for a power steering device or the like of a motor vehicle. 
     2. Description of the Related Art 
     Conventionally, there has been a variable displacement pump, as disclosed in Japanese Patent No. 2932236, arranged to assist steering force by means of a hydraulic power steering device of a motor vehicle. This conventional variable displacement pump is directly rotated and driven by means of an engine of the motor vehicle. This device provides a rotor in a cam ring engaged movably and displaceably with an adapter ring engaged with a pump casing, and forms a pump chamber between the cam ring and the periphery of the rotor. 
     Further, in this conventional art, the structure allows the cam ring to be movable within the adapter ring, and a biasing force, making the capacity of the pump chamber maximum, is applied to the cam ring by a spring. The first and second fluid pressure chambers are separately formed between the cam ring and the adapter ring. There is a switch valve operated by a pressure difference between upstream and downstream sides of a main throttle provided in a pump discharge side passage, which controls a fluid pressure supplied to both the fluid pressure chambers in correspondence to a discharge flow amount of a pressurized fluid from the pump chamber. A cam ring is moved, thereby changing the capacity of the pump chamber so as to control the discharge flow amount from the pump chamber. Accordingly, in this variable displacement pump, the discharge flow amount is controlled to be relatively large so as to produce a large steering assist force when the motor vehicle stops or runs at a low speed, or where the motor vehicle has a low rotational speed. The discharge flow amount is controlled to be equal to or less than a fixed amount so as to make the steering assist force small when the motor vehicle runs at a high speed, or where the motor vehicle has a high rotational speed, whereby it is possible to generate the appropriate steering assist force required for the power steering device. 
     In this case, in the conventional art (Japanese Patent No. 2932236), an opening range around a pump shaft of a discharge port which opens to a discharge area in a downstream side in a rotor rotating direction of the pump chamber, is arranged so as to be shifted to a side of a second fluid pressure chamber. Then, a force based on a pressure fluctuation (an increase of internal pressure of a cam ring) generated within the pump chamber moves the cam ring to a side of the second fluid pressure chamber so as to fluctuate the discharge flow amount of the pump when a load is generated on the basis of operation of equipment to be used, such as a steering operation of a power steering device or the like. In Japanese Patent No. 2932236, it is described that since the fluid pressure in the downstream section of the main throttle is substantially close to the discharge pressure which can resist against the increase of the internal pressure of the cam ring, which is mentioned above, and when this pressure is introduced into the second fluid pressure chamber, the movement mentioned above of the cam ring can be restricted by the introduction of pressure, and the fluctuation of the flow amount mentioned above can be prevented. However, this description is in error. It is impossible to prevent the flow amount from being adjusted in this manner. 
     Because the force (except the spring) applied to the cam ring is constituted by the fluid pressure of the first fluid pressure chamber, the second fluid pressure chamber, and the pump chamber, the fluctuation of the pressure is transmitted to all the area of the discharge system from the pump chamber to the equipment in use when the load is generated. At this time, since the force based on the pressure fluctuation generated in the first fluid pressure chamber, and the force based on the pressure fluctuation generated in the second fluid pressure chamber have substantially the same area in their pressure receiving surfaces and are opposed to each other, they cancel each other. However, the force based on the pressure fluctuation generated in the pump chamber leaves as before. This force moves the cam ring to the side of the second fluid pressure chamber so as to fluctuate the flow amount. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to restrict a fluctuation of a discharge flow amount when a load is generated, in a variable displacement pump. 
     According to the present invention, there is disclosed a variable displacement pump comprising: 
     A rotor rotated and driven in a state of being fixed to a pump shaft inserted to a pump casing and receiving a multiplicity of vanes in a groove so as to be movable in a radial direction. 
     A cam ring is fitted to a fitting hole in the pump casing so as to form a pump chamber between the cam ring and an outer peripheral portion of the rotor, making it movable within the pump casing and forming first and second fluid pressure chambers between the cam ring and the pump casing. 
     An opening range around a pump shaft of a discharge port open to a discharge area in a downstream side in a rotor rotational direction of the pump chamber is shifted to one side of the second fluid pressure chamber. 
     A pressurizing cylinder is provided on an opposite side of the first fluid pressure chamber holding the cam ring between, and a piston which is inserted to a pressurizing cylinder, which collides with the cam ring. 
     An oil chamber of the pressurizing cylinder is interposed in a pump discharge side passage. 
     Pressure in an upstream side of the main throttle provided in the pump discharge side passage is introduced to the first fluid pressure chamber and the oil chamber of the pressurizing cylinder. Pressure in a downstream side of the main throttle is introduced to the second fluid pressure chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully understood from the detailed description given below and from the accompanying drawings which should not be taken to be a limitation on the invention, but are intended for explanation and understanding only. The drawings: 
     FIG. 1 is a sectional view showing a variable displacement pump; 
     FIG. 2 is a sectional view taken along line II—II of FIG. 1; 
     FIG. 3 is a cross sectional view showing a switch valve; and 
     FIG. 4 is a cross sectional view showing a modified embodiment of a variable displacement pump. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A variable displacement pump  10  is a vane pump that is a hydraulic power generation source of a hydraulic power steering device of a motor vehicle. As shown in FIG.  1  and FIG. 2, the pump  10  includes a rotor  13  fixed to a pump shaft  12  inserted into a pump casing  11  by means of a serration to be driven rotatably. The pump casing  11  is arranged so as to integrate a pump housing  11 A with a cover  11 B by means of a bolt  14  to support the pump shaft  12  via bearings  15 A to  15 C. The pump shaft  12  can be directly driven rotatably by means of a motor vehicle engine. 
     The rotor  13  houses vanes  17  in grooves  16  provided at a plurality of peripheral positions, respectively, thereby making it possible to move each vane  17  in a radial direction along the groove  16 . 
     A pressure plate  18  and an adapter ring  19  are engaged with an engagement hole  20  of the pump housing  11 A of the pump casing  11  in a layered state. The plate  18  and ring  19  are fixed laterally by a cover  11 B while they are positioned in the peripheral direction by means of a fulcrum pin  21  described later. 
     A cam ring  22  is engaged with the aforementioned adapter ring  19  fixed to the pump housing  11 A of the pump casing  11 . The cam ring  22  surrounds the rotor  13  with a certain quantity of eccentricity, and forms a pump chamber  23  between the pressure plate  18  and the cover  11 B or the periphery of the rotor  13 . In a suction area in the upstream side in the rotor rotation direction of the pump chamber  23 , a suction port  24  provided at the cover  11 B. A suction opening  26  of the pump  10  communicates with this suction port  24  via suction passages (drain passages)  25 A and  25 B provided at the housing  11 A and the cover  11 B. On a downstream area in the downstream side of the rotor rotation direction of the pump chamber  23 , a discharge port  27  provided at a pressure plate  18  opens. A discharge opening  29  of the pump  10  is communicated with the discharge port  27  via a high pressure chamber  28 A and a discharge passage  28 B provided at the housing  11 A. 
     In this manner, in the variable displacement pump  10 , when the rotor  13  is rotatably driven by means of the pump shaft  12 , and the vane  17  of the rotor  13  rotates while it is pressed to the cam ring  22  with centrifugal force, a capacitance between an interval of the adjacent vanes  17  and the cam ring  22  is expanded together with rotation on the upstream side in the rotor rotation direction of the pump chamber  23 . Then, working fluid is suctioned from the suction port  24 , the capacitance between the interval of the adjacent vanes  17  and the cam ring  22  is reduced together with rotation on the downstream side in the rotor rotation direction of the pump chamber  23 , and the working fluid is ejected from the discharge port  27 . 
     The variable displacement pump  10  is structured, as shown in FIG. 2, such that an opening range α around the pump shaft  12  of the discharge port  27  is arranged so as to be shifted to the side of a second fluid pressure chamber  42  mentioned below at an angle β. 
     Accordingly, the variable displacement pump  10  has a discharge flow amount control apparatus  40 . 
     The discharge flow amount control apparatus  40  is structured such that the fulcrum pin  21  mentioned above is mounted on a vertical lowermost portion of the adapter ring  19  mentioned above, fixed to the pump casing  11 . The vertical lowermost portion of the cam ring  22  is supported to the fulcrum pin  21 , and the cam ring  22  can be swingably displaced within the adapter ring  19 . 
     The discharge flow amount control apparatus  40  is provided in the pump housing  11 A constituting the pump casing, in an opposite side to a first fluid pressure chamber mentioned below in regard to the cam ring  22 . A pressurizing cylinder  50  is provided to engage in a sealed state in the pump housing  11 A holding an O-ring in between. An oil chamber  51  of the pressurizing cylinder  50  is interposed in a middle of the discharge passage  28 B, and a piston  52  inserted to the oil chamber  51  is in slidable contact with an outer surface of the cam ring  22  through a piston hole  53  provided in the adapter ring  19 . A spring  54  corresponding to an energizing means is arranged in the oil chamber  51  of the pressurizing cylinder  50 . The spring  54  energizes the cam ring  22  via the piston  52  with respect to the outer peripheral portion of the rotor  13  in a direction making a capacity (a pump capacity) of the pump chamber  23  maximum. The piston  52  is constituted by a closed-end cylindrical hollow body provided with a cavity receiving the spring  54 . 
     In this case, the adapter ring  19  is structured such that a cam ring movement restricting stopper  19 A is formed in a protruding shape in a part of an inner peripheral portion of the first fluid pressure chamber  41 , whereby it is possible to restrict a moving limit of the cam ring  22  for making the capacity of the pump chamber  23  maximum as mentioned below. The adapter ring  19  is structured such that a cam ring movement restricting stopper  19 B is formed in a protruding shape in a part of an inner peripheral portion forming of a second fluid pressure chamber  42  mentioned below, so as to restrict a moving limit of the cam ring  22  for making the capacity of the pump chamber  23  minimum as mentioned below. 
     The discharge flow amount control apparatus  40  controls the size of the first and second fluid pressure chambers  41  and  42  between the cam ring  22  and the adapter ring  19 . The first fluid pressure chamber  41  and the second fluid pressure chamber  42  are separated between the cam ring  22  and the adapter  19  by the fulcrum pin  21  and a seal member  43  provided at an axially symmetrical position. The first and second fluid pressure chambers  41  and  42  are formed between the cam ring  22 , the adapter ring  19 , the cover  11 B and the pressure plate  18 . The first fluid pressure chamber  41  is provided with a communicating groove communicating a first area of the first fluid pressure chamber  41  formed on one side of stopper  19 A with a second area of the first fluid pressure chamber  41  formed on the other side of stopper  19 A, and the second fluid pressure chamber  42  is provided with a communicating groove communicating a first area of the second fluid pressure chamber  42  formed on one side of stopper  19 B with a second area of the second fluid pressure chamber  42  formed in the other side of stopper  19 B. The cam ring  22  collides and aligns with the cam ring movement restricting stoppers  19 A and  19 B mentioned above in the adapter ring  19 , in the pressure plate  18 . 
     In this case, the oil chamber  51  of the pressurizing cylinder  50  mentioned above is interposed in the middle of the discharge passage  28 B of the pump  10 . Accordingly, in the discharge path of the pump  10 , the pressurized fluid discharged from the pump chamber  23  and reaching the discharge passage  28 B via the discharge port  27  of the pressure plate  18  and the high pressure chamber  28 A of the pump housing  11 A is fed in a downstream side  28 C of the discharge passage  28 B from an annular groove  55 A around the pressurizing cylinder  50  and a passage  55 B open onto a wall surface of the pressurizing cylinder  50  through the oil chamber  51 . A piston  52  inserted to the oil chamber  51  of the pressurizing cylinder  50  has a hole-like communication passage  56  for communicating the oil chamber  51  with the downstream side  28 C of the discharge passage  28 B in such a manner as to be pierced on a wall surface of the hollow body of the piston  52 . This changes an opening area of the communication passage  56  with the downstream side  28 C of the discharge passage  28 B by a front end edge  57  of the pressurizing cylinder  50  when the piston  52  moves in correspondence to the movement of the cam ring  22 , thereby constituting a variable main throttle  58 . 
     (1) The discharge flow amount control apparatus  40  may introduce the pressure in an upstream  15  side of the main throttle  58  to the first fluid pressure chamber  41 , applying the moving displacement in the direction making the capacity of the pump chamber  23  minimum to the cam ring  22 , via a switch valve apparatus  60  mentioned below. (2) The discharge flow amount control apparatus  40  may introduce the pressure in a downstream side of the main throttle  58  to the second fluid pressure chamber  42 , applying the moving displacement in the direction making the capacity of the pump chamber  23  maximum to the cam ring  22 , from the discharge passage  28 B via the piston hole  53  of the adapter ring  19 . (3) The discharge flow amount control apparatus may directly introduce the pressure in the upstream side of the main throttle  58  to the oil chamber  51  of the pressurizing cylinder  50  applying the moving displacement in the direction making the capacity of the pump chamber  23  maximum to the cam ring  22 . Due to a balance of the pressures applied to the first fluid pressure chamber  41 , the second fluid pressure chamber  42  and the oil chamber  51  of the pressurizing cylinder  50 , it is possible to move the cam ring  22  against the biasing force of the spring  54  and change the capacity of the pump chamber  23 , thereby controlling the discharge flow amount of the pump  10 . 
     In this case, in the discharge flow amount control apparatus  40 , there is provided the switch valve apparatus  60  operating on the basis of the pressure difference between the upstream and downstream sides of the main throttle  58 . This controls the fluid pressure supplied to the first fluid pressure chamber  41  in correspondence to the discharge flow amount of the pressurized fluid from the pump chamber  23 . In particular, the switch valve apparatus  60  is interposed between a communication passage  61  connected to the first fluid pressure chamber  41  and a communication passage  67  disposed in an upstream side of the main throttle  58  in the discharge passage  28 B. This closes the first fluid pressure chamber  41  with respect to the communication passage  67  in a low rotational range of the pump  10  in association with a throttle  61 A provided in the communication passage  61  and connects the first fluid pressure chamber  41  to the communication passage  67  in a high rotational range. 
     In this case, the switch valve apparatus  60  is structured such that a spring  63  and a switch valve  64  are received in a valve receiving hole  62  pierced in the pump housing  11 A, and the switch valve  64  energized by the spring  63  is supported by a cap  65  engaged with the pump housing  11 A. The switch valve  64  is provided with a valve body  64 A and a switch valve body  64 B, and is structured such that the communication passage  67  in the upstream side rather than the main throttle  58  of the discharge passage  28 B is communicated with a pressurizing chamber  66 A provided in one end side of the valve body  64 A. A communication passage  68  in the downstream side rather than the main throttle  58  of the discharge passage  28 B is communicated with a back pressure chamber  66 B in which a spring  63 , provided in another end side of the switch valve body  64 B, is stored, via the second fluid pressure chamber  42 . Further, a suction passage (a drain passage)  25 A mentioned above is formed through a drain chamber  66 C between the valve body  64 A and the switch valve body  64 B, and is in communication with a tank. The switch valve body  64 B can open and close the communication passage  61  mentioned above. In a low rotational range having a low discharge pressure of the pump  10 , the switch valve body  64 B sets the switch valve  64  to an original position shown in FIG. 2 due to the biasing force of the spring  63 . This closes the communication between the first fluid pressure chamber  41  and the communication passage  67  by the switch valve body  64 B. In a middle and high rotational range of the pump  10 , the switch valve body  64 B moves the switch valve  64  due to the high pressure fluid of the communication passage  67  applied to the pressurizing chamber  66 A so as to open the communication passage  61 , thereby introducing the high pressure fluid of the communication passage  67  to the first fluid pressure chamber  41 . In this case, a throttle  67 A is provided in the communication passage  67  so as to make it possible to absorb a pulsation from the upstream sides of the main throttle  58 . 
     Accordingly, a discharge flow amount characteristic of the pump  10  provided with the discharge flow amount control apparatus  40  is as follows. 
     (1) In a low speed running range of a motor vehicle in which the rotational speed of the pump  10  is relatively low, the pressure of the fluid discharged from the pump chamber  23  to the pressurizing chamber  66 A of the switch valve apparatus  60  is also low. The switch valve  64  is positioned at the original position and the switch valve  64  closes the communication passage  61  with the first fluid pressure chamber  41 . Accordingly, the pressure in the upstream side of the main throttle  58  is not supplied to the first fluid pressure chamber  41 . The pressure in the downstream side of the main throttle  58  is applied to the second fluid pressure chamber  42 , and the pressure in the upstream side of the main throttle  58  is applied to the oil chamber  51  of the pressurizing cylinder  50 . Accordingly, the cam ring  22  is maintained in the side making the capacity of the pump chamber  23  maximum due to the pressure difference between the first fluid pressure chamber  41  and the second fluid pressure chamber  42 , and due to the pressing force o£ the piston  52  of the pressurizing cylinder  50  and the biasing force of the spring  54 . The discharge flow amount of the pump  10  is increased in proportion to the rotational speed. 
     (2) When the pressure of the fluid discharged from the pump chamber  23  to the pressurizing chamber  66 A of the switch valve apparatus  60  becomes high due to an increase of the rotational speed of the pump  10 , the switch valve apparatus  60  moves the switch valve  64  against the biasing force of the spring  63  so as to open the communication passage  61  with the first fluid pressure chamber  41 . Accordingly, the pressure of the first fluid pressure chamber  41  is increased and the cam ring  22  moves to the side reducing the capacity of the pump chamber  23 . Therefore, the discharge flow amount of the pump  10  cancels the flow amount increase caused by the increase of the rotational speed and the flow amount reduction caused by the reduction of the capacity in the pump chamber  23 , so as to maintain a fixed relatively large flow amount. 
     (3) When the rotational speed of the pump  10  is continuously increased and the cam ring  22  is further moved, whereby the cam ring  22  presses the spring  52  of the pressuring cylinder  50  at an amount over a fixed amount, the main throttle  58  is throttled due to the movement of the piston  52 . Accordingly, the discharge flow amount pressure fed to the downstream side of the discharge passage  28 B of the pump  10  is reduced in proportion to the throttling amount of the main throttle  58 . 
     (4) When reaching a high speed drive range of the motor vehicle in which the rotational speed of the pump  10  is over a fixed value, the cam ring  22  reaches a moving limit where the cam ring  22  is collided and aligned with the stopper  19 B of the adapter ring  19 . The throttling amount of the main throttle  58  becomes maximum, and the discharge flow amount of the pump  10  maintains a fixed small flow amount. 
     In this case, the pump  10  has a relief valve  70  corresponding to the switch valve relieving an excessive fluid pressure in the pump discharge side among the high pressure chamber  28 A, the suction passage (the drain passage)  25 A and the drain chamber  66 C. Further, in the pump  10 , a lubricating oil supply passage  121  from the suction passage  25 B toward the bearing  15 C of the pump shaft  12  is pierced in the cover  11 B, and a lubricating oil return passage  122  returning from a peripheral portion of the bearing  15 B of the pump shaft  12  to the suction passage  25 A is pierced in the pump housing  11 A. 
     The relief valve  70  is structured in a pilot-drive type in which a ball  73  constructing a pilot valve is added to a main valve  71  installed in the switch valve apparatus  60  and is constituted by the switch valve  64  itself as shown in FIG.  3 . Further, the main valve  71  can open and close an upstream side passage of the main throttle  58  provided in the pump discharge side passage, that is, a first valve chamber (the same as the pressurizing chamber  66 A)  81  with respect to the drain passage  25 A (suction passage). A fluid pressure in the downstream side of the main throttle  58  is provided in the pump discharge side passage, and further a fluid pressure of the second valve chamber (the same as the back pressure chamber  66 B)  82  is applied to the ball  73 . 
     In particular, the relief valve  70  is provided with the following structure (a) to (c). 
     (a) The relief valve  70  is provided with the main valve  71  (the switch valve  64 ) slidably within the valve receiving hole  62  and applies the fluid pressure in the upstream side of the main throttle  58  provided in the discharge side passage of the pump  10  to the first valve chamber  81  (the pressurizing chamber  66 A) defined in one end side of the valve receiving hole  62  with respect to the main valve  71 . The relief valve  70  applies the fluid pressure in the downstream side of the main throttle  58  to the second valve chamber  82  (the back pressure chamber  66 B) defined in another end side of the valve receiving hole  62  with respect to the main valve  71 . The relief valve  70  is provided with a relief passage (not shown) communicating the first valve chamber  81  with the drain passage  25 A via the drain chamber  66 C in the valve receiving hole  62 , and is provided with a spring  84  (the same as the spring  63 ) energizing the main valve  71  to a side of the first valve chamber  81  so as to set the main valve  71  to a close position of the relief passage. 
     (b) The relief valve  70  has a main valve  71  in which an axial hole  71 A for relieving the fluid pressure is formed and a relief hole  71 B crossing the axial hole  71 A is formed so as to be slidably provided in the valve receiving hole  62 . A valve seat  72  is provided with a communication hole  72 A inserted and attached to an inflow side opening end of the axial hole  71 A in the main valve  71  so as to communicate the internal and external portions of the axial hole  71 A. This includes a ball receiving surface  72 B formed in an outflow side end of the communication hole  72 A, a ball  73  movably provided in the axial hole  71 A of the main valve  71  which is capable of being brought into contact with the ball receiving surface  72 B in the valve seat  72 , and a spring presser  74  provided with a ball pressing surface  74 A provided in the axial hole  71 A of the main valve  71 , which presses the ball  73  to the ball receiving surface  72 B of the valve seat  72  while being backed up by a spring  75 . In this case, reference symbol  71 C denotes a fluid pressure relief hole (a relief hole) provided in a side wall of the axial hole  71 A receiving the spring  75  of the main valve  71  and opposing to the drain chamber  66 C and the drain passage  25 A for making the movement of the spring presser  74  smooth. 
     (c) The ball receiving surface  72 B of the valve seat  72  in the relief valve  70  is formed as a tapered surface expanding toward a direction in which the fluid flows out in an axial direction of the communication hole  72 A. At the same time, the peripheral end surface  74 B of the ball pressing surface  74 A in the spring presser  74  is formed as a tapered surface expanding toward an opposite direction to the ball pressing direction in the axial direction of the spring presser  74 . 
     The relief valve  70  is structured such that when the fluid pressure in the pump discharge side becomes excessive due to a continuous static turn steering state generated by the power steering device in which the pump  10  is used, or the like, and the fluid pressure of the second valve chamber  82  connected to the discharge passage in the downstream side of the main throttle  58  reaches the relief set pressure, the fluid pressure of the second valve chamber  82  opens the ball  73  against the urging of the spring  75 . Accordingly, it is possible to relieve the fluid pressure of the second valve chamber  82  from the relief hole  71 B to the drain passage  25 A via the drain passage  66 C so as to open the main valve  71  against the spring  84  due to the fluid pressure of the first valve chamber  81 . This occurs when the fluid pressure of the second valve chamber  82  is reduced by this relief, so that it is possible to relieve the fluid pressure of the first valve chamber  81  from the relief passage  83  to the drain passage  25 A via the drain chamber  66 C. Therefore, it is possible to relieve the excessive fluid pressure in the pump discharge side. 
     According to the present embodiment, the following operations can be obtained. 
     (1) The force (except the spring  54 ) applied to the cam ring  22  is constituted by the fluid pressure of the first fluid pressure chamber  41 , the second fluid pressure chamber  42 , the oil chamber  51  of the pressurizing cylinder  50  and the pump chamber  23 . Because of this condition, the fluctuation of the pressure is transmitted to all the entire area of the discharge system from the pump chamber  23  to the equipment to be used, when the load is generated. At this time, since the force based on the pressure fluctuation generated in the first fluid pressure chamber  41  and the force based on the pressure fluctuation generated in the second fluid pressure chamber  42  have substantially the same area in their pressure receiving surfaces and are opposed to each other, they cancel each other. The force based on the pressure fluctuation generated in the pump chamber  23  is opposed by the pressing force of the piston  52  based on the pressure fluctuation generated in the oil chamber  51  of the pressurizing cylinder  50 , so that the force based on the pressure fluctuation generated in the pump chamber  23  moves the cam ring  22  in the side of the second fluid pressure chamber  42  so as to restrict the fluctuation of the discharge flow amount. 
     (2) Since the oil chamber  51  of the pressurizing cylinder  50  is interposed in the discharge passage  28 B, it is not necessary to independently provide the communication passage of the pressurizing cylinder  50  branched from the discharge passage  28 B with the oil chamber  51  and it is possible to make it simply from the oil passage. 
     (3) Since the communication passage  56  of the piston  52  communicated with the oil chamber  51  of the pressurizing cylinder  50  is set to the main throttle  58 , the rotational speed of the pump  10  is increased. When the cam ring  22  is going to move to the side reducing the capacity of the pump chamber  23  due to the balance of the force mentioned in the item (1) mentioned above, it is possible to throttle the main throttle  58  due to the movement of the piston  52  together with the movement of the cam ring  22 . It is also possible to reduce the discharge flow amount pressure fed to the downstream side  28 C of the discharge passage  28 B of the pump  10  in proportion to the throttle amount of the main throttle  58 . 
     (4) Since the spring  54  corresponding to the energizing means for energizing the cam ring  22  in the direction in which the capacity of the pump chamber  23  becomes maximum is provided, the cam ring  22  can always be maintained in the original state in which the capacity of the pump chamber  23  becomes maximum when starting the rotation of the pump  10  so as to stabilize the moving control of the cam ring  22 . Since the spring  54  is arranged in the oil chamber  51  of the pressurizing cylinder  50 , it is possible to make the shape of the pump  10  compact while having both the pressurizing cylinder  50  and the spring  54 . 
     The pump  10  in FIG. 4 is different from the pump  10  in FIGS. 1 to  3  in that in the pressurizing cylinder  50 , an annular band-like groove  56 A connecting to the outer periphery of the piston  52  is provided in the communication passage  56  provided in the piston  52  and an opening area of the band-like groove  56 A with the discharge passage  28 B is changed by the front end edge  57  of the pressurizing cylinder  50 , thereby constituting the main throttle  58 . 
     As mentioned above, according to the present invention, in the variable displacement pump, it is possible to restrict the fluctuation of the discharge flow amount when the load is generated. 
     As heretofore explained, embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configurations of the present invention are not limited to the embodiments but those having a modification of the design within the range of the present invention are also included in the present invention. 
     Although the invention has been illustrated and described with respect to several exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made to the present invention without departing from the spirit and scope thereof. Therefore, the present invention should not be understood as limited to the specific embodiment set out above, but should be understood to include all possible embodiments which can be embodied within a scope encompassed and equivalents thereof with respect to the features set out in the appended claims.