Patent Publication Number: US-6217296-B1

Title: Variable displacement pump

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a variable displacement pump used in, e.g., a pressure fluid utilizing device such as a power steering device for decreasing the force required to operate the steering wheel of a vehicle. 
     As a pump for a power steering device of this type, a displacement vane pump directly driven to rotate by a vehicle engine is used. In this displacement pump, the discharge flow rate increases or decreases in accordance with the rotational speed of the engine. A power steering device requires an auxiliary steering force which increases while the vehicle is stopped or is traveling at a low speed and decreases while the vehicle is traveling at a high speed. The characteristics of the displacement pump must be contradictory to this auxiliary steering force. Accordingly, a displacement pump having a large volume must be used so that it can maintain a discharge flow rate necessary to produce a required auxiliary steering force even during low-speed driving with a low rotational speed. For high-speed driving with a high rotational speed, a flow control valve that controls the discharge flow rate to a redetermined value or less is indispensable. For these reasons, the number of constituent components relatively increases, and the structure and path arrangement are complicated, inevitably leading to an increase in entire size and cost. 
     In order to solve these inconveniences, variable displacement vane pumps each capable of decreasing the discharge flow rate per revolution (cc/rev) in proportion to an increase in rotational speed are proposed in, e.g., Japanese Patent Laid-Open Nos. 56-143383 and 58-93978, U.S. Pat. Nos. 5,538,400, 5,518,380, and 5,562,432, and the like. According to these variable displacement pumps, a flow control valve provided to the displacement pump is unnecessary. The driving power can be decreased to provide an excellent energy efficiency. 
     An example of such a variable displacement vane pump will be described briefly with reference to FIG. 16 showing the pump structure in, e.g., U.S. Pat. No. 5,562,432 or the like. Referring to FIG. 16, reference numeral  1  denotes a pump body;  1   a,  an adapter ring; and  2 , a cam ring. The cam ring  2  is free to swing in an elliptic space  1   b,  formed in the adapter ring  1   a  of the pump body  1 , through a swing fulcrum pin  2   a  serving as a support shaft. A spring means (compression coil spring  2   b ) biases the cam ring  2  to the left in FIG.  16 . 
     A rotor  3  is accommodated in the cam ring  2  to be eccentric on one side to form a pump chamber  4  on the other side. When the rotor  3  is rotatably driven by an external drive source, vanes  3   a  held to be movable forward/backward in the radial direction are projected and retracted. Reference numeral  3   b  denotes a driving shaft of the rotor  3 . The rotor  3  is driven by the rotating shaft  3   b  to rotate in a direction indicated by an arrow in FIG.  16 . In the following description, the pump chamber  4  is a space formed in the cam ring  2  on one side of the rotor  3  to have an almost crescent-like shape, and extends from a suction opening  7  (to be described later) to a discharge opening  8 . 
     First and second fluid pressure chambers  5  and  6  are formed on two sides around the cam ring  2  in the elliptic space  1   b  of the adapter ring  1   a  set in the pump body  1 , and serve as high- and low-pressure chambers, respectively. Paths  5   a  and  6   a  are open to the chambers  5  and  6 , respectively, through a spool type control valve  10  (to be described later), to guide as the control pressure for swinging the cam ring  2  the fluid pressures obtained upstream and downstream of a metering restrictor formed in a pump discharge path  11 . 
     In this example, a variable metering restrictor  12  is formed of a hole  12   a  formed in the side wall surface of the pump body  1  that forms the second fluid pressure chamber  6 , and a side edge  12   b  of the cam ring  2  that moves to change the opening area by selectively covering the hole  12   a.  For this reason, the second fluid pressure chamber  6  is under the fluid pressure obtained downstream of the variable metering restrictor  12 . This fluid pressure is guided to the low-pressure chamber of the control valve  10  through the path  6   a.    
     Reference numeral  13  denotes a pump discharge path formed downstream of the variable metering restrictor  12 . 
     In FIG. 16, a pump suction opening (suction port)  7  is formed to oppose a pump suction region  4 A of the pump chamber  4 . A pump discharge opening (discharge port)  8  is formed to oppose a pump discharge region  4 B of the pump chamber  4 . These openings  7  and  8  are formed in at least corresponding ones of a pressure plate and a side plate (not shown) serving as stationary wall portions for holding pump constituent elements composed of the rotor  3  and cam ring  2  by sandwiching them from two sides. 
     The cam ring  2  is biased by the compression coil spring  2   b  from the fluid pressure chamber  6  and is urged in a direction to keep the volume (pump volume) in the pump chamber  4  maximum. A seal member  2   c  is placed in the outer surface portion of the cam ring  2  to define the fluid pressure chambers  5  and  6 , together with the swing fulcrum pin  2   a,  on the right and left sides. 
     The spool type control valve  10  is actuated by differential pressures P 1  and P 2  obtained upstream and downstream of the variable metering restrictor  12  serving as a metering orifice and formed between the pump discharge paths  11  and  13 . The control valve  10  introduces a fluid pressure P 3  corresponding to the magnitude of the pump discharge flow rate to the high-pressure fluid pressure chamber  5  outside the cam ring  2 , to maintain a sufficiently large flow rate even immediately after the pump is started. 
     More specifically, as described above, when the fluid pressures obtained upstream and downstream of the variable metering restrictor  12  between the pump discharge paths  11  and  13  are controlled by the control valve  10  and guided into the fluid pressure chambers  5  and  6  on two sides of the cam ring  2 , the cam ring  2  is swung in a required direction, as indicated by a solid arrow or a white arrow in FIG. 16, to change the volume of the pump chamber  4 , so that the discharge flow rate can be controlled to match the pump discharge flow rate, as shown by the flow rate curve of FIG.  17 . Also, flow rate control can be performed as follows. As the rotational speed of the pump increases, the discharge flow rate can be raised to a predetermined value, and this state is maintained. When the rotational speed of the pump is in a high speed range, the flow rate is decreased. 
     FIG. 16 described above shows a state that takes place from region A to B in FIG.  17 . When the rotational speed of the pump reaches a predetermined value or more, the difference between the fluid pressures obtained upstream and downstream of the variable metering restrictor  12  increases. As a result, the cam ring  2  swings to the right (a direction indicated by a solid arrow) in FIG. 16 to restrict the variable metering restrictor  12 . The discharge flow rate of the pump decreases in accordance with the restriction amount. When the variable metering restrictor  12  is restricted to the minimum position, the pump discharge flow rate is maintained at the predetermined value, as indicated in a region C. 
     While the pressure fluid utilizing device (for example, the power cylinder of the power steering device and indicated by PS in FIG. 16) is actuated to apply a load, when the differential pressures obtained upstream and downstream of the variable metering restrictor  12  become equal to or higher than a predetermined value, the control valve  10  introduces the fluid pressure P 1  obtained upstream of the variable metering restrictor  12  as a control pressure to the high-pressure fluid pressure chamber  5  outside the cam ring  2 , to prevent swing of the cam ring  2 . 
     The pump body  1  is formed with a pump suction path  14  extending from a tank T to the pump suction region  4 A of the pump chamber  4  through the low-pressure chamber of the spool type control valve  10 . The pump discharge path  13  is formed with a direct driven type relief valve  15  serving as a pressure control valve. The relief valve  15  is formed at such a position that, when the pump discharge fluid pressure becomes equal to or higher than a predetermined value, it relieves the pressure fluid to the pump suction side (or tank T side) through the pump suction path  14 . 
     In the variable displacement pump having the structure described above, the fluid pressure obtained downstream of the variable metering restrictor  12  is directly introduced to, of the pair of fluid pressure chambers  5  and  6  that swing the cam ring  2 , the fluid pressure chamber  6 . More specifically, the hole  12   a  formed in the side wall of the pump body  1  constituting the second fluid pressure chamber  6  and the outer surface of the cam ring  2  which swings form the variable metering restrictor  12 . The fluid pressure is supplied to the pump discharge path  13  through the second fluid pressure chamber  6 . 
     In the conventional variable displacement pump having the structure described above, the cam ring  2  is swung by the pressures of the first and second fluid pressure chambers  5  and  6  and the biasing force of the compression coil spring  2   b  formed in the second fluid pressure chamber  6  in accordance with an increase/decrease of the supply flow rate of the fluid accompanying a change in rotational speed of the pump, thereby controlling the pump volume to a required value. A problem exists, however, in appropriately controlling the swing motion of the cam ring  2 . 
     Assume that the rotational speed of the pump reaches a high range. The first fluid pressure chamber  5  which introduces the fluid pressure obtained upstream of the variable metering restrictor  12  by means of the control valve  10  has a structure of introducing the fluid pressure through the path  5   a  partly having a restrictor. When the cam ring  2  swings toward the first fluid pressure chamber  5 , a required braking force can be exerted on the cam ring  2  by the damper function of the restrictor portion of the path  5   a.    
     In contrast to this, merely the compression coil spring  2   b  is provided to the second fluid pressure chamber  6 . A means having the damper function of braking the cam ring  2  is not provided to the second fluid pressure chamber  6 , unlike in the first fluid pressure chamber  5  described above. 
     When the cam ring  2  swings toward the second fluid pressure chamber  6 , although a spring force generated by flexure of the compression coil spring  2   b  may somewhat act, a braking force produced by the damper function cannot be effected. Accordingly, the swing motion of the cam ring  2  toward the first and second fluid pressure chambers  5  and  6  (particularly the swing motion from the first fluid pressure chamber  5  toward the second fluid pressure chamber  6 ) tends to become unstable. Then, the cam ring  2  may vibrate or pulsation occurs in the pump discharge fluid pressure inevitably. This pulsation state is indicated by a broken line in FIG.  17 . 
     This will be described in detail. When the pump discharge fluid pressure flows in the form of a jet into the second fluid pressure chamber  6  through the hole  12   a  formed in the fluid pressure chamber  6  and when the hole  12   a  is to be closed or opened by the outer edge of the cam ring  2 , the cam ring  2  tends to vibrate. When the jet from the hole  12   a  is blocked by the outer edge of the cam ring  2  or is passed through the hole  12   a,  pulsation increases in the pump discharge side. When such vibration or pulsation occurs, in a power steering device, the steering force may fluctuate, or the noise such as the sound produced by the fluid may increase. 
     In the variable displacement pump described above, it is sought for to simplify the path structure for the pressure fluid in the pump body and the structure of the control valve that swings the cam ring, and to make compact the structure of the entire pump. In a variable displacement pump, it is sought for to take countermeasures that can simplify the structure of the entire pump as much as possible and the structure of the path in the pump body through which the pressure fluid flows, and to improve the machinability and assembly easiness, thereby decreasing the manufacturing cost. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a principal object of the present invention to provide a variable displacement vane pump which can discharge a fluid pressure with predetermined supply flow rate characteristics. 
     It is, therefore, another object of the present invention to provide a variable displacement vane pump in which vibration occurring in a cam ring that swings in a pump body and pulsation in the pump discharge fluid pressure caused by the vibration are attenuated so that the problem of noise resulting from the vibration and the pulsation can be solved. 
     It is, therefore, still another object of the present invention to provide a variable displacement vane pump in which the motion of a cam ring that swings in a pump body is controlled to a normal state so that the cam ring can be swung more smoothly and reliably than in a conventional variable displacement vane pump. 
     In order to achieve these objects, according to the present invention, there is provided a variable displacement pump comprising pump bodies having an inner space and formed with a pump suction path and pump discharge paths communicating with the inner space, a cam ring swingably supported in the inner space of the pump bodies through a swing fulcrum formed on part of an outer surface of the cam ring to extend in an axial direction, first and second fluid pressure chambers divisionally formed in the inner space of the pump bodies with respect to the outer surface of the cam ring through seal means, a rotor having a plurality of vanes and arranged inside the cam ring, a rotating shaft axially supported by the pump bodies and mounted with the rotor, a pump chamber having an opening for the suction path and an opening for the discharge paths and formed between an inner surface of the cam ring and an outer surface of the rotor, biasing means for biasing the cam ring from the second fluid pressure chamber toward the first fluid pressure chamber, a metering restrictor provided midway along the discharge paths of a pressure fluid discharged from the pump chamber, and a control valve connected to the discharge paths formed upstream and downstream, respectively, of the metering restrictor and to the first and second fluid pressure chambers and driven by fluid pressures obtained upstream and downstream of the metering restrictor, wherein a plunger damper is formed to incorporate the biasing means such that a distal end thereof abuts against a side portion of the cam ring in the second fluid pressure chamber, and a variable metering restrictor constituting the metering restrictor is formed at a position, where the variable metering restrictor is opened/closed by a slidable motion of the plunger damper during a swing motion of the cam ring and is partitioned from the second fluid pressure chamber, so that an opening area of the variable metering restrictor changes in an interlocking manner to the swing motion of the cam ring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of the main part of a variable displacement pump having drooping type flow rate characteristics according to the first embodiment of the present invention, in a state wherein the pump rotates at a low rotational speed (from â to immediately before {circle around (b)} in FIG.  5 ); 
     FIG. 2 is a sectional view of one side obtained taken along the line II—II of FIG. 1; 
     FIG. 3A is a diagram for explaining the relationship between three small holes formed in a pressure plate and the outer surface of a cam ring in accordance with the swing motion; 
     FIG. 3B is a diagram for explaining the shape of the pressure plate in the pump structure when a variable displacement pump having the dropping type flow rate characteristics shown in FIGS. 1 and 2 is used as a constant flow rate type pump; 
     FIG. 4 is a side view of the pressure plate disposed on one side of the cam ring in the variable displacement pump shown in FIGS. 1 and 2; 
     FIG. 5 is a graph for explaining the supply flow rate of the variable displacement pump shown in FIGS. 1 and 2 as a function of the rotational speed of the pump; 
     FIG. 6A is a sectional view of a control valve portion to explain a control pressure applied by a control valve to a first fluid pressure chamber when the pump rotates at a low rotational speed (from â to immediately before {circle around (b)} in FIG.  5 ); 
     FIG. 6B is an enlarged view of the main part of the same; 
     FIG. 7A is a sectional view of the control valve portion to explain the control pressure applied by the control valve to the first fluid pressure chamber when the pump rotates at a low rotational speed (from {circle around (b)} to ê in FIG.  5 ); 
     FIG. 7B is an enlarged view of the main part of the same; 
     FIGS. 8A and 8B show a variable displacement pump having drooping type flow rate characteristics according to the second embodiment of the present invention, in which FIG. 8A is a sectional view of the main part of the pump rotating at a low rotational speed (from â to immediately before {circle around (b)} in FIG. 5 described above), and FIG. 8B is an enlarged view of the main part of the same; 
     FIG. 9 is a sectional view of one side taken along the line IX—IX of FIG. 8A; 
     FIG. 10 is a view for explaining the outline of the entire flow of the fluid in the variable displacement pump shown in FIGS. 8A and 8B and FIG. 9; 
     FIG. 11A is a side sectional view for explaining the relationship between a plug, which forms a metering restrictor portion comprised of a stationary metering restrictor and variable metering restrictors and which characterizes the second embodiment of the present invention, and components related to the plug; 
     FIG. 11B is a sectional view of the main part taken where small holes serving as the variable metering restrictors of the plug are formed; 
     FIG. 12 is a view showing a variable displacement pump having drooping type flow rate characteristics according to the third embodiment of the present invention to explain the outline of the entire flow of the fluid in the variable displacement pump; 
     FIG. 13 is a sectional view of the main part of a variable displacement pump having drooping type flow rate characteristics according to the fourth embodiment of the present invention, in a state wherein the pump rotates at a low rotational speed; 
     FIG. 14 is a sectional view of one side taken along the line XIII—XIII of FIG. 13; 
     FIG. 15 is a sectional view showing a modification of FIG. 14; 
     FIG. 16 is a view for explaining the operation of a conventional variable displacement pump in a state wherein the pump rotates at a low speed; and 
     FIG. 17 is a graph for explaining the supply flow rate of the pump of FIG. 16 with respect to the rotational speed of the pump. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 to  7 B show a variable displacement pump according to the first embodiment of the present invention. The first embodiment exemplifies a case wherein a vane pump according to the present invention is a vane type oil pump serving as the oil pressure generating source of a power steering device, and has so-called drooping characteristics. According to the drooping characteristics, as the rotational speed of the pump increases, the discharge flow rate from the pump decreases to a predetermined value lower than the maximum discharge flow rate, and is maintained at this predetermined value. In this embodiment, the pump has a direct driven type relief valve, as shown in FIG.  2 . 
     Referring to FIGS. 1 and 2, a vane type variable displacement pump denoted by reference numeral  20  has a front body  21  and a rear body  22  constituting a pump body. The entire portion of the front body  21  forms a substantially cup-like shape, as shown in FIGS. 1 and 2. A housing space  24  for housing pump constituent elements  23  as a pump cartridge is formed in the front body  21 . The rear body  22  is integrally combined with the front body  21  to close the opening end of the housing space  24 . A driving shaft  26  for externally, rotatably driving a rotor  25  of the pump constituent elements  23  extends through the front body  21 , and is rotatably supported by the front body  21  through bearings  26   a  and  26   b  (the bearing  26   a  is disposed on the front body  21  while the bearing  26   b  is disposed on the rear body  22 ). Reference numeral  26   c  denotes an oil seal. 
     A cam ring  27  has an inner cam surface  27   a  fitted on the outer surface of the rotor  25  having vanes  25   a,  to form a pump chamber  28  between the inner cam surface  27   a  and rotor  25 . The cam ring  27  is movably arranged in an adapter ring  29  that fits the inner wall portion of the housing space  24 , to be able to change the volume (pump volume) of the pump chamber  28 , as will be described later. 
     The adapter ring  29  serves to hold the cam ring  27  in the housing space  24  of the front body  21  to be movable. 
     Referring to FIG.  2  and FIGS. 3A and 3B, a pressure plate  30  is stacked on the front body  21  of the pump cartridge (pump constituent elements  23 ), constituted by the rotor  25 , cam ring  27 , and adapter ring  29  described above, to press against it. The end face of the rear body  22  is pressed against the opposite side surface of the pump cartridge as a side plate. When the front body  21  and rear body  22  are integrally assembled, the pump cartridge is assembled in a required state. These members construct the pump constituent elements  23 . 
     The pressure plate  30  and the rear body  22  stacked on it through the cam ring  27  to serve as the side plate are integrally assembled and fixed to each other while they are positioned in the rotational direction by a swing fulcrum pin  31  (to be described later) and appropriate rotation preventive means (not shown). The swing fulcrum pin  31  also serves as a positioning pin and axial support portion for enabling the cam ring  27  to swing, and has a seal function to define a fluid pressure chamber where the cam ring  27  swings. 
     A pump discharge pressure chamber  33  is formed in the housing space  24  of the front body  21  on the bottom portion side. The pump discharge pressure chamber  33  exerts the pump discharge pressure on the pressure plate  30 . A pump discharge opening  34  is formed in the pressure plate  30  to guide the hydraulic oil from the pump chamber  28  to the pump discharge pressure chamber  33 . 
     Although not shown, a pump suction opening  35  (an opening position with respect to the pump chamber  28  as shown in FIG. 1) is formed in part of the rear body  22 . A suction fluid entering from a tank T through the suction opening  35  flows from a suction port (not shown) formed in part of the rear body  22  into a pump suction path (not shown) formed in the rear body  22 , and is supplied into the pump chamber  28  through the pump suction opening  35  formed in the end face of the rear body  22 . In FIGS. 3A and 3B, a groove  35   a  is formed in the pressure plate  30  to oppose the pump suction opening  35 . 
     A control valve  40  is composed of a spool  42  and a valve hole  41  formed in the upper portion of the front body  21  in a direction perpendicular to the driving shaft  26 . The control valve  40  controls the fluid pressures to be introduced into first and second fluid pressure chambers  43  and  44 , divisionally formed on two sides of the cam ring  27  in the adapter ring  29  by the swing fulcrum pin  31  and a seal member  45  axially symmetric to it. 
     Although not shown, a path  51  (indicated by broken lines in FIG. 1) extending from the pump discharge pressure chamber  33  is connected to one end of the valve hole  41 . 
     A path  52  is formed in the spool  42  in the axial direction. A stationary metering restrictor  53  is formed in part of the path  52 , in this case, on a side of a spring chamber  46  having a spring  46   a  formed on the other end of the spool  42 . A pump discharge port  55  is formed on the outer end of the spring chamber  46  through a pass hole  54 , to supply a hydraulic oil to a power steering device (not shown) serving as a pressure fluid utilizing device (hydraulic pressure utilizing device). 
     As described above, the spool  42  introduces the fluid pressures obtained upstream and downstream of the stationary metering restrictor  53  to the first and second fluid pressure chambers  43  and  44  in accordance with the rotational speed of the pump. The fluid pressure obtained upstream of the stationary metering restrictor  53  is introduced into the valve hole  41  of the control valve  40  through a path hole  47  formed close to one end of the valve hole  41 . The path hole  47  is blocked by a land  42   a  in the initial state when the spool  42  is located left in FIG.  1 . At this time, the fluid pressure from the tank T is introduced via a pump suction path  56  open in this portion through a central annular groove of the spool  42 , through a gap path  42   b  around the small-diameter portion of the land  42   a.    
     As the spool  42  is moved to the left in FIG. 1 by the differential pressure between the fluid pressures obtained upstream and downstream of the stationary metering restrictor  53  and that of a variable metering restrictor (described later), the spool  42  is disconnected from the pump suction side described above, and the fluid pressure obtained upstream of the upstream is introduced to the first fluid pressure chamber  43  through the path hole  47 . The fluid pressure supplied to the path hole  47  is controlled by the control valve  40  as shown in FIGS. 6A and 6B, and FIGS. 7A and 7B corresponding to FIG.  5 . 
     A portion of the path hole  47  forms a damper restrictor  47   a.    
     The fluid pressure obtained downstream of the stationary metering restrictor  53  acts on the second fluid pressure chamber  44  through a path hole  49  open to part of the pump discharge port  55  to serve as a damper restrictor. 
     Part of the pump discharge path, i.e., in this embodiment, paths formed by three small holes  58  formed in the pressure plate  30 , branch from the pump discharge pressure chamber  33  independently of the discharge path  51 , and open to the second fluid pressure chamber  44 . The opening ends of the small holes  58  and the edge portion of the outer surface of the cam ring  27  form a variable metering restrictor  59 . The fluid pressure passing through the variable metering restrictor  59  flows through the second fluid pressure chamber  44  and the notched portion of the adapter ring  29  to be supplied to the pump discharge port  55  through the path hole  49 . 
     Referring to FIGS. 1 and 2, a compression coil spring  61  biases the cam ring  27 . The compression coil spring  61  is arranged in a circular space opposing part of the second fluid pressure chamber  44 . This circular space is formed in the cylindrical portion of a plug  63  screwed to close a hole  62  formed from the outside of the front body  21 . In this cylindrical portion, a plunger damper  64  having one open end abuts against the outer surface of the cam ring  27  due to the spring force of the compression coil spring  61 . The plunger damper  64  always exerts the biasing force of the compression coil spring  61  on the cam ring  27  regardless of the swing motion of the cam ring  27 . In FIGS. 1 and 2, an O-ring  64   a  serves as a seal member for sealing the gap between the outer surface of the plunger damper  64  and the cylindrical portion of the plug  63 . 
     In part of the plunger damper  64 , a damper restrictor  64   b  is formed of a small hole through which the interior of the plunger damper  64  where the compression coil spring  61  is disposed, and the second fluid pressure chamber  44  communicate with each other. In place of the damper restrictor  64   b,  a bleed hole  63   a  may be formed in part of the plug  63  to open to the atmosphere. The bleed hole  63   a  serves to achieve a damper function together with the compression coil spring  61  and plunger damper  64 . 
     The damper restrictor  64   b  may be formed to have a hole diameter of, e.g., about 0.6 mm. The O-ring  64   a  is fitted on the plunger damper  64  to seal its outer surface. The O-ring  64   a  also suppresses vibration of the cam ring  27 . 
     In FIG. 2, a relief valve  38  is formed in part of the rear body  22 . The relief valve  38  opens to the second fluid pressure chamber  44 . Thus, the relief valve  38  is connected to part of the pump discharge path so that the pump discharge fluid pressure can be relieved to the pump suction side through a path  38   a  formed in the rear body  22 . 
     In the above vane type variable displacement pump  20 , the arrangement other than that described above is identical to that conventionally known widely, and a detailed description thereof will accordingly be omitted. 
     According to the variable displacement pump  20  having the above structure, the discharge paths  51 ,  52 ,  54 ,  58 , and  49  extending from the pump discharge pressure chamber  33  are divided into two systems consisting of one provided with the stationary metering restrictor  53  and one provided with the variable metering restrictor  59 . With the fluid pressures obtained upstream of the metering restrictors  53  and  59  and the pump suction fluid pressure (tank pressure), the control pressure controlled by the control valve  40  is introduced into the first fluid pressure chamber  43  located on one side in the swing direction of the cam ring  27 . The fluid pressures obtained downstream of the metering restrictors  53  and  59  are introduced into the second fluid pressure chamber  44  located on the other side in the swing direction of the cam ring  27 . 
     With this structure, the cam ring  27  can be swung in a required state in accordance with the magnitude of the pump discharge flow rate, and the supply flow rate to the pump discharge side can be maintained at a constant value, or an arbitrary value equal to or less than the predetermined value, as the rotational speed of the pump increases, as shown in FIG.  5 . 
     Referring to FIG. 5, when the rotational speed of the pump increases from a low speed range to a medium speed range, the supply flow rate changes as indicated by â-{circle around (b)} and ĉ. As shown in FIGS. 6A and 6B, when the pump rotates at a low rotational speed, the control valve  40  guides the pump suction fluid pressure (tank pressure) to the first fluid pressure chamber  43  through the path hole  47  and damper restrictor  47   a,  to maintain a constant supply flow rate determined by the differential pressure obtained from the restriction amounts of the metering restrictors  53  and  59 . 
     When the rotational speed of the pump reaches a high speed range, the spool  42  of the control valve  40  moves to the left, as shown in FIGS. 7A and 7B, to switch the pressure in the path hole  47  extending to the first fluid pressure chamber  43  to the fluid pressure obtained upstream of the metering restrictor  53  or  59 . Accordingly, the cam ring  27  swings toward the second fluid pressure chamber  44  where the compression coil spring  61  is provided, to gradually close the variable metering restrictor  59 . 
     When the small holes  58  constituting the variable metering restrictor  59  are completely closed by the outer surface of the cam ring  27 , the control valve  40  is controlled by the differential pressure between the fluid pressures obtained upstream and downstream of the stationary metering restrictor  53 , so that the determined flow rate can be maintained (indicated by {circle around (d)}-ê in FIG.  5 ). These flow rate characteristics are so-called drooping characteristics. 
     When the relationship between the small holes  58  constituting the variable metering restrictor  59  and the opening amount determined by the displacement of the outer edge of the cam ring  27  is changed, the flow rate characteristics can be changed as indicated by an alternate long and short dashed line in FIG.  5 . 
     In this embodiment, three small holes  58  described above are used. The variable metering restrictor  59  formed by the small holes  58  has a smaller opening amount than that of a conventional widely-known variable restrictor. The variable metering restrictor  59  is not limited to be formed of the three small holes  58  opened/closed by the outer edge of the cam ring  2  to change their opening area, as shown in FIGS. 1 to  4 , but can be formed of one or more small holes  58 . 
     The swing amount of the cam ring  27  is, e.g., about 1.9 mm with the existing product. If a plurality of small holes  58  (the total opening amount of which is identical to that obtained when the variable metering restrictor  59  is formed of one small hole  58 ) are formed, the opening area obtained by restriction can be changed by small displacement of the cam ring  27 , which is convenient in setting the pump performance. In this embodiment, as the three small holes  58 , for example, one 1-mm diameter small hole  58  (the leading end side in the displacing direction of the cam ring  27 ) and two 1.1-mm diameter small holes  58  (the trailing end side in the displacing direction) may be used. However, the present invention is not limited to this. To change the characteristics as described above, these hole diameters may be appropriately changed, the opening positions may be shifted so that the small holes are aligned in the moving direction of the cam ring  27 , or the opening amounts may be varied along the moving direction. 
     The small holes  58  need not be circular, but can be square or can have any other shape. 
     The first and second fluid pressure chambers  43  and  44  for swinging the cam ring  27  are connected to the control valve  40  and the pump discharge path (pump discharge port  55 ) through the damper restrictors  47   a  and  49 . When the cam ring  27  swings in accordance with the differential pressure between the fluid pressures obtained upstream and downstream of the stationary metering restrictor  53  and the differential pressure between the fluid pressures obtained upstream and downstream of the variable metering restrictor  59  due to an increase/decrease in rotational speed of the pump, a required braking force can be applied to the cam ring  27  in two swing directions. 
     The damper restrictor  47   a  described above may have a hole diameter of, e.g., about 1.2 mm. The path hole  49  serving as a damper restrictor located downstream of the variable metering restrictor  59  may be formed to have a diameter of, e.g., about 2 mm. 
     According to this structure, an appropriate braking force can be applied to the cam ring  27  when it swings toward the first or second fluid pressure chamber  43  or  44 . The cam ring  27  can thus be swung smoothly in a required state so the cam ring  27  does not vibrate or pulsation is not produced on the pump discharge side. It suffices if the path hole  49  described above to serve as the damper restrictor is formed downstream of the stationary metering restrictor  53 . Hence, the path hole  49  can communicate with, e.g., the spring chamber  46  of the control valve  40 . 
     In this embodiment, the biasing force of the compression coil spring  61  serving as the biasing means is exerted on the cam ring  27  through the plunger damper  64 . Therefore, a biasing force and a braking force can be appropriately exerted on the cam ring  27 , and a smooth swing motion can be obtained more effectively. 
     To appropriately control the motion of the plunger damper  64 , the bleed hole  63   a  is formed, and the space where the compression coil spring  61  is provided is open to the atmosphere through a predetermined restrictor, so that the effect is further improved. 
     In the variable displacement pump  20  according to the first embodiment, the pump has so-called drooping type supply flow rate characteristics. However, the present invention is not limited to this, and the supply flow rate characteristics can be easily changed to constant flow rate type characteristics. 
     More specifically, in a constant flow rate type pump, a variable metering restrictor is not required, unlike in the drooping type pump described above. Thus, the small holes  58  in the pressure plate  30  that open to the second fluid pressure chamber  44  may be omitted, as shown in FIG.  3 B. The stationary metering restrictor  53  in the spool  42  of the control valve  40  may be formed to have an appropriate restricting diameter in accordance with the required pump characteristics. 
     The path hole  49  which guides the fluid pressure obtained downstream of the stationary metering restrictor  53  formed in the spool  42  of the control valve  40  may be formed to have such a diameter that it serves as a restricting portion. Alternatively, a restricting portion may be formed in part of the path hole  49 . 
     Even with this structure, a damper effect to the cam ring  27  can obviously be exerted on the second fluid pressure chamber  44  as well with the plunger damper  64  and the path hole  49  serving as the damper restrictor. Therefore, even in the pump having this structure, vibration produced when the cam ring  27  swings can be attenuated, and pulsation on the pump discharge side can be decreased, so that noise is suppressed, in the same manner as in the embodiment described above. 
     As described above, in the pump structure according to the first embodiment, of the variable displacement pump  20 , components except for those constituting the variable metering restrictor can be shared between the drooping type pump and the constant flow rate type pump, and any change in specifications can be coped with simply. 
     FIGS. 8A to  11  show a variable displacement pump according to the second embodiment of the present invention. The second embodiment exemplifies a case wherein a vane pump according to the present invention is a vane type oil pump serving as the oil pressure generating source of a power steering device, and has so-called drooping characteristics. According to the drooping characteristics, as the rotational speed of the pump increases, the discharge flow rate from the pump decreases to a predetermined value lower than the maximum discharge flow rate, and is maintained at this predetermined value. In this embodiment, the pump has a direct driven type relief valve, as shown in FIGS. 8A and 8B, and FIG.  10 . 
     Referring to FIGS. 8A and 8B, and FIG. 9, a vane type variable displacement pump denoted by reference numeral  20  has a front body  21  and a rear body  22  constituting a pump body, in the same manner as in the first embodiment described above. The entire portion of the front body  21  forms a substantially cup-like shape, as shown in FIGS. 8A and 8B, and FIG. 9. A housing space  24  for housing pump constituent elements  23  as a pump cartridge is formed in the front body  21 . The rear body  22  is integrally combined with the front body  21  to close the opening end of the housing space  24 . A driving shaft  26  for externally, rotatably driving a rotor  25  constituting the pump constituent elements  23  extends through the front body  21 , and is rotatably supported by the front body  21  through bearings  26   a  and  26   b.  Reference numeral  26   c  denotes an oil seal. 
     A cam ring  27  has an inner cam surface  27   a  fitted on the outer surface of the rotor  25  having vanes  25   a,  to form a pump chamber  28  between the inner cam surface  27   a  and rotor  25 . The cam ring  27  is movably arranged in an adapter ring  29  that fits the inner wall portion of the housing space  24 , to be able to change the volume (pump volume) of the pump chamber  28 , as will be described later. 
     The adapter ring  29  serves to hold the cam ring  27  in the housing space  24  of the front body  21  to be movable. 
     Referring to FIG. 9, a pressure plate  30  is stacked on the front body  21  of the pump cartridge (pump constituent elements  23 ), constituted by the rotor  25 , cam ring  27 , and adapter ring  29  described above, to press against it. The end face of the rear body  22  is pressed against the opposite side surface of the pump cartridge as a side plate. When the front body  21  and rear body  22  are integrally assembled, the pump cartridge is assembled in a required state. These members construct the pump constituent elements  23 . 
     The pressure plate  30  and the rear body  22  stacked on it through the cam ring  27  to serve as the side plate are integrally assembled and fixed to each other while they are positioned in the rotational direction by a swing fulcrum pin  31  (to be described later) and appropriate rotation preventive means (not shown). The swing fulcrum pin  31  also serves as a positioning pin and axial support portion for enabling the cam ring  27  to swing, and has a seal function to define a fluid pressure chamber where the cam ring  27  swings. 
     A pump discharge pressure chamber  33  is formed in the housing space  24  of the front body  21  on the bottom portion side. The pump discharge pressure chamber  33  exerts the pump discharge pressure on the pressure plate  30 . A pump discharge opening  34  is formed in the pressure plate  30  to guide the hydraulic oil from the pump chamber  28  to the pump discharge pressure chamber  33 . 
     Although not shown, a pump suction opening  35  (an opening position with respect to the pump chamber  28  is shown in FIG. 1) is formed in part of the rear body  22 . A suction fluid entering from a tank T (pump suction side) through the suction opening  35  flows from a suction port (not shown) formed in part of the rear body  22  into a pump suction path (not shown) formed in the rear body  22 , and is supplied into the pump chamber  28  through the pump suction opening  35  open to the end face of the rear body  22 . In FIG. 8A, a groove  35   a  is formed in the pressure plate  30  to oppose the pump suction opening  35  on the rear body  22  side. 
     A control valve  40  is composed of a spool  42  and a valve hole  41  formed in the upper portion of the front body  21  in a direction perpendicular to the driving shaft  26 . The control valve  40  controls the fluid pressures to be introduced into first and second fluid pressure chambers  43  and  44 , divisionally formed on two sides of the cam ring  27  in the adapter ring  29  by the swing fulcrum pin  31  and a seal member  45  axially symmetric to it, in accordance with the rotational speed of the pump. 
     Although not shown, a path  51  (indicated by a broken line in FIG. 8A) extending from the pump discharge pressure chamber  33  is connected to one end of the valve hole  41 . 
     A spring chamber  46  having a spring  46   a  for biasing the spool  42  to one end side is formed on the other end side of the valve hole  41 . The spring  46   a  biases the spool  42  to the right in FIG.  8 A. In this embodiment, the spring chamber  46  is connected to a pilot pressure path  54  formed to extend from a pump discharge port  55  serving as the terminal end portion of the pump discharge path in the front body  21 . 
     The spring chamber  46  is connected to the second fluid pressure chamber  44  through a connection path  50 . A damper restrictor  50   a  is formed in part of the connection path  50 . A high-pressure chamber  48  formed on one end side of the spool  42  is selectively connected to the first fluid pressure chamber  43  through a connection path  47  which is gradually disconnected from the pump discharge side when the spool  42  moves toward the spring chamber  46  (to the left in FIG.  8 A). 
     In FIG. 8A, the connection path  47  is connected to an annular space, formed of an annular groove  42   c  at the central portion in the axial direction of the spool  42 , through a gap path formed of a small-diameter portion  42   b  formed in a land  42   a  on one end side of the spool  42 . As shown in FIGS. 8A and 10, this annular space is connected to the tank T through a pump suction path  56 . A fluid pressure P 1  on the pilot pressure path  51  side is selectively connected to the first fluid pressure chamber  43  through the connection path  47  in accordance with the amount of displacement of the spool  42 . 
     The fluid pressure P 1  and a fluid pressure P 2  obtained upstream and downstream, respectively, of a metering restrictor portion  60  (to be described above) formed midway along the pump discharge path are introduced to the chambers  48  and  46  on two end sides of the control valve  40  through the pilot pressure path  51  and a pilot pressure path  52 , respectively. 
     At the start of operation of the pump and while the pressure fluid utilizing device (PS) operates, the differential pressure between the fluid pressures obtained upstream and downstream of the metering restrictor portion  60  is small. The spool  42  is thus located at the position shown in FIGS. 8A and 10, and a pump suction fluid pressure P 0  is introduced to the first fluid pressure chamber  43 . At this time, the pump discharge fluid pressure P 2  obtained downstream of the metering restrictor portion  60  is introduced to the second fluid pressure chamber  44 , and the cam ring  27  maintains the volume of the pump chamber  28  maximum. 
     When the rotational speed of the pump reaches the medium or high speed range and the pressure fluid utilizing device (PS) is inoperative, the spool  42  moves in a direction to flex the spring  46   a,  and accordingly the chamber  48  connected to the pilot pressure path  51  is connected to the connection path  47 . Then, the fluid pressure obtained upstream of the metering restrictor portion  60  is introduced into the first fluid pressure chamber  43  in accordance with the moving amount of the spool  42 . As a result, the cam ring  27  swings counterclockwise in FIG. 8A to reduce the volume of the pump chamber  28 . 
     This state is indicated by â-{circle around (b)} and {circle around (b)}-ĉ-{circle around (d)}-ê in FIG. 5 described above in the first embodiment. In the control valve  40 , the gap path formed by the small-diameter portion  42   b  is in the state shown in FIG. 6A or  6 B described above. As the spool  42  moves, the pump suction or discharge fluid pressure is supplied to the first fluid pressure chamber  43 , and required supply fluid pressure control is performed. 
     In this embodiment, as shown in FIGS. 8A and 10, a damper restrictor  51   a  is formed in part of the pilot pressure path  51  to suppress unnecessary movement of the spool  42  accompanying variations in fluid pressure in the pump discharge path. At this time, a fluid pressure P 4  acts on the chamber  48 . 
     In this embodiment, the path is formed to have an ordinary diameter. Alternatively, a damper restrictor (e.g., see a portion indicated by reference numeral  54   a  in FIG. 12 to be described later) may be formed in part of the path  54  that connects the pump discharge path located downstream of the metering restrictor portion  60  to the spring chamber  46 . 
     The damper restrictors  47   a  and  50   a  serve to prevent variations in fluid pressure in the first and second fluid pressure chambers  43  and  44  described above, thereby suppressing unnecessary movement of the cam ring  27 . 
     A path  57  partly constituting the pump discharge path is formed to branch from the pump discharge pressure chamber  33  independently of the pilot pressure path  51 , and opens to the inner wall on the outer end side of a plug hole  62  provided with a compression coil spring  61 . The compression coil spring  61  biases the cam ring  27  in a direction to maximize the volume of the pump chamber  28  on the second fluid pressure chamber  44  side. 
     A plug  63  is set in the plug hole  62  to close its opening end, as shown in FIG.  8 A and FIGS. 9 to  11 B. A plunger damper  64  for exerting the biasing force of the compression coil spring  61  on the cam ring  27  is slidably held in a cylindrical portion  63   b  of the plug  63 . 
     In this embodiment, the cylindrical portion  63   b  of the plug  63  and the plunger damper  64  constitute the metering restrictor portion  60 . 
     This will be described in detail. The outer end side of the plunger damper  64  forms a small-diameter portion  64   c,  and an annular path space  65  is formed between the small-diameter portion  64   c  and the inner wall of the cylindrical portion  63   b  of the plug  63 . 
     The path  57  from the pump discharge pressure chamber  33  communicates with the annular path space  65  through a path hole  66  formed radially in the plug  63  from a space between the plug hole  62  of the front body  21  and the plug  63 . 
     The fluid introduced axially in the annular path space  65  is guided to a second path space  70  comprised of small holes  68  and  69  and formed in the cylindrical portion  63   b  of the plug  63  along the axial direction to be defined from the path space  65 . The small hole  68  serves as a stationary metering restrictor. The small hole  69  serves as a movable metering restrictor. The second path space  70  communicates with the pump discharge port  55  through a path  71 . 
     The small hole  69  serving as the variable metering restrictor described above can be opened and closed such that its opening area is changed by a step close to the small-diameter portion  64   c  of the plunger damper  64  which moves in the axial direction along with the swing motion of the cam ring  27 . 
     The small hole  69  serving as the variable metering restrictor whose opening area can be changed by the plunger damper  64  may be formed equidistantly at a plurality of locations (four in this embodiment) on the cylindrical portion  63   b  of the plug  63  in the circumferential direction. Obviously, the present invention is not limited to this structure. 
     Other than the small hole  69 , the small holes  68  and path holes  66  may be formed at balanced positions around the plug  63  in the circumferential direction. In this embodiment, the small holes  68  and path holes  66  are formed at two locations. 
     Referring to FIGS. 8A and 9, the compression coil spring  61  for biasing the cam ring  27  is placed in the plug hole  62  that forms a circular space opposing part of the second fluid pressure chamber  44 , and is formed in the cylindrical portion  63   b  of the plug  63  which is screwed into the hole  62  to close its opening end. In the cylindrical portion  63   b,  the plunger damper  64  having one opening end abuts against the outer surface of the cam ring  27  with the spring force of the compression coil spring  61 . Accordingly, the biasing force generated by the compression coil spring  61  always acts on the cam ring  27  regardless of the swing motion of the cam ring  27 . 
     In part of the plunger damper  64 , a damper restrictor  64   d  is formed, between the plunger damper  64  and the cam ring  27 , of a small hole through which the interior where the compression coil spring  61  is disposed communicates with the second fluid pressure chamber  44 . In place of the damper restrictor  64   d,  a bleed hole that opens to the atmosphere may be formed in part of the plug  63 , and a damper function may be obtained with the compression coil spring  61  and plunger damper  64  by the function of the bleed hole. 
     Referring to FIGS. 8A and 10, a relief valve  74  is formed in the spool  42  of the control valve  40 . The relief valve  74  is connected to the pump discharge port  55 , partly forming the pump discharge path, through the spring chamber  46  and pilot pressure path  52 . Hence, the pump discharge fluid pressure can be relieved to the pump suction side through the hole  75  formed in the small-diameter portion  42   c  of the spool  42 . 
     In the above vane type variable displacement pump  20 , the arrangement other than that described above is identical to that conventionally known widely, and a detailed description thereof will accordingly be omitted. 
     According to the variable displacement pump  20  having the above structure, the pressure fluid flowing through the discharge paths  57 ,  62 ,  66 , and  65  from the pump discharge pressure chamber  33  is guided to the metering restrictor portion  60  comprised of the stationary metering restrictor (small hole  68 ) and a variable metering restrictor (small hole  69 ) which are formed of the plunger damper  64  and plug  63  constituting the damper functional portion. The pressure fluid that has passed through the metering restrictor portion  60  reaches the pump discharge port  55  through the discharge paths  70  and  71 , and is supplied to a power cylinder PS of a power steering device serving as a pressure fluid utilizing device (not shown). 
     The fluid pressures obtained upstream of the metering restrictors  68  and  69  are introduced to one chamber  48  of the control valve  40  through the pilot pressure path  51 . Hence, with the fluid pressure and the pump suction fluid pressure (tank pressure), the control pressure controlled by the control valve  40  is introduced into the first fluid pressure chamber  43  located on one side in the swing direction of the cam ring  27 . The fluid pressures obtained downstream of the metering restrictors  68  and  69  are introduced into the second fluid pressure chamber  44 , located on the other side in the swing direction of the cam ring  27 , through the pilot pressure path  54 , spring chamber  46 , and path  50 . 
     With this structure, the cam ring  27  can be swung in a required state in accordance with the magnitude of the pump discharge flow rate, and the supply flow rate to the pump discharge side can be controlled to be maintained at a predetermined value, or an arbitrary value equal to or less than the predetermined value, as the rotational speed of the pump increases, as shown in FIG. 5 described in the first embodiment. 
     Referring to FIG. 5, when the rotational speed of the pump increases from a low speed range to a medium speed range, the supply flow rate changes as indicated by â-{circle around (b)} and ĉ. As shown in FIG. 6A described in the first embodiment, when the pump rotates at a low rotational speed, the control valve  40  guides the pump suction fluid pressure (tank pressure) to the first fluid pressure chamber  43  through the path hole  47  and a damper restrictor  47   a,  to maintain a constant supply flow rate determined by the differential pressure obtained from the restriction amounts of the metering restrictors  68  and  69 . 
     When the rotational speed of the pump reaches a high speed range, the spool  42  of the control valve  40  moves to the left, as shown in FIG. 6A described in the first embodiment, to switch the pressure in the path hole  47  extending to the first fluid pressure chamber  43  to the fluid pressure obtained upstream of the metering restrictor  68  or  69 . Accordingly, the cam ring  27  swings toward the second fluid pressure chamber  44  where the compression coil spring  61  is provided, to gradually close the small holes  69  serving as variable metering restrictor with the large-diameter portion of the plunger damper  64  in accordance with the movement of the plunger damper  64  moved by the swing motion of the cam ring  27 . 
     When the small holes  69  constituting the variable metering restrictor are completely closed by the large-diameter portion of the plunger damper  64  in accordance with the movement of the cam ring  27 , the control valve  40  is controlled by the differential pressure between the fluid pressures obtained upstream and downstream of the small hole  68  serving as the stationary metering restrictor, so that the determined flow rate can be maintained (indicated by {circle around (d)}-ê in FIG. 5 described in the first embodiment). These flow rate characteristics are so-called drooping characteristics. 
     When the relationship between the small holes  69  constituting the variable metering restrictor and the opening amount determined by the displacement of the large-diameter portion of the plunger damper  64  is changed, the flow rate characteristics shown in FIG. 5 described in the first embodiment can be changed. 
     In the second embodiment, the small holes  69  described above are formed as circular holes formed equidistantly in the outer surface of the cylindrical portion  63   b  of the plug  63 , that holds the plunger damper  64 , in the circumferential direction. However, the present invention is not limited to this, and one small hole or a plurality of small holes may be formed. When a plurality of small holes  69  are to be formed, the shape of the holes need not be circular but can be elliptic or rectangular. Depending on the required characteristics, a plurality of holes may be formed to be shifted from each other in the axial direction. 
     The first and second fluid pressure chambers  43  and  44  for swinging the cam ring  27  are connected to the control valve  40  and the pump discharge path (pump discharge port  55 ) through the damper restrictors  47   a  and  50   a.  When the cam ring  27  swings in accordance with the differential pressure between the fluid pressures obtained upstream and downstream of the stationary metering restrictor  68  and the differential pressure between the fluid pressures obtained upstream and downstream of the variable metering restrictor  69  due to an increase/decrease in rotational speed of the pump, a required braking force can be applied to the cam ring  27  in two swing directions. 
     According to this structure, an appropriate braking force can be applied to the cam ring  27  when it swings toward the first or second fluid pressure chamber  43  or  44 . The cam ring  27  can thus be swung smoothly in a required state so the cam ring  27  does not vibrate or pulsation is not produced on the pump discharge side. 
     In the second embodiment, it suffices if the path hole  50  having the damper restrictor  50   a  described above is formed downstream of the variable metering restrictor  60 . Hence, in place of the spring chamber  46  of the control valve  40 , the second fluid pressure  44  can be made to directly communicate with the discharge port  55  (downstream of the variable metering restrictor  60 ) through, e.g., a restrictor hole  82  indicated by broken lines in FIG.  8 A. 
     In this case, the pilot pressure path  54  serves to guide the fluid pressure obtained downstream of the metering restrictor portion  60  to the spring chamber  46  of the control valve  40 . Also, the pressure fluid from the pump discharge path is guided to the relief valve  74 , formed in the spool  42 , through the pilot pressure path  54 . 
     In this embodiment, the biasing force of the compression coil spring  61  serving as the biasing means is exerted on the cam ring  27  through the plunger damper  64 . Therefore, a biasing force and a braking force can be appropriately exerted on the cam ring  27 , and a smooth swing motion can be obtained more effectively. Also, since the opening area of the variable metering restrictor  69  is changed by the movement of the plunger damper  64 , the function as the variable metering restrictor can be exhibited. 
     In the above embodiment, the plunger damper  64  and plug  63  form the variable metering restrictor. Therefore, the variable displacement pump  20  can be converted from a drooping type pump to a constant flow rate type pump by only omitting the variable metering restrictor. 
     When this structure is employed, of the variable displacement pump  20 , components except for those constituting the variable metering restrictor can be shared between the drooping type pump and the constant flow rate type pump, and any change in specification can be coped with simply. 
     In this embodiment, since the relief valve  74  can be incorporated in the spool  42  constituting the control valve  40 , the front body  21  can be more prevented from projecting outwardly than in a case wherein the relief valve  74  is provided in any other portion of the front bodies  21  and  22 , so that the entire pump can be made compact. 
     In this embodiment, the hole for housing the relief valve  74  can be machined easily, and the valve spool  42  is used as the holder. Therefore, the number of components and the cost can be reduced. 
     According to the present invention, the control valve  40  is a valve operated by the pilot pressure, and the pump discharge fluid pressure is not positively flowed into the control valve  40 . Therefore, the valve hole of the control valve  40  can be machined easily. 
     FIG. 12 shows a variable displacement pump  20  according to the third embodiment of the present invention. Referring to FIG. 12, components identical or corresponding to those in FIGS. 8A to  11 B described above are denoted by the same reference numerals as in FIGS. 8A to  11 B, and a detailed description thereof will be omitted. 
     The pump according to the third embodiment is a variable displacement pump having so-called drooping characteristics, in the same manner as in the second embodiment, with which as the rotational speed of the pump increases, the supply flow rate on the pump discharge side is decreased to be smaller than the maximum flow rate. 
     In this embodiment, different from the second embodiment described above, a pilot restrictor  54   a  is formed in a pilot pressure path  54  which connects a spring chamber  46  of a control valve  40  to the downstream side of a metering restrictor portion  60  on the pump discharge side. 
     In the presence of the pilot restrictor  54   a,  when a relief valve  74  performs relief operation, the pressure in the spring chamber  46  of the control valve  40  drops. Thus, a supply fluid pressure P 5  to be supplied to a second fluid pressure chamber  44 , on a side where the volume a pump chamber  28  of a cam ring  27  becomes the maximum volume, can be decreased. 
     In the pump having this pilot restrictor  54   a,  when the relief valve  74  performs relief operation, the cam ring  27  can be swung in a direction to decrease the volume of the pump chamber  28 . Since the discharge amount from the pump can accordingly be further decreased, energy saving of the pump can be achieved. 
     According to this structure, of the flow rate obtained through the metering restrictor portion  60  formed in the pump discharge path and comprised of the stationary metering restrictor and the variable metering restrictor, only a flow rate decreased by a value corresponding to a value inversely proportional to the restriction amount of the pilot restrictor  54   a  is relieved to the pump suction side through the relief valve  74 . Therefore, the relief valve  74  of this embodiment is a so-called quasi-direct driven type pump the relief amount of which is somewhat smaller than in a direct driven type pump in which the pressure fluid in the pump discharge path is entirely relieved as in the embodiments described above. 
     With the pilot restrictor  54   a  described above, an adverse influence accompanying variations in fluid pressure to be supplied to a spool  42  of the control valve  40  can be prevented. 
     FIGS. 13 and 14 show a variable displacement pump  20  according to the fourth embodiment of the present invention. Referring to FIGS. 13 and 14, components identical or corresponding to those in FIGS. 1 to  7 B, FIGS. 8A to  11 B, and FIG. 12 described above are denoted by the same reference numerals as in FIGS. 1 to  7 B, FIGS. 8A to  11 B, and FIG. 12, and a detailed description thereof will be omitted. 
     The pump according to the fourth embodiment is a variable displacement pump having so-called drooping characteristics, in the same manner as in the first, second, and third embodiments. 
     In this embodiment, an annular groove  64   e  is formed in the outer surface of a plunger damper  64 . A compression coil spring  61  for biasing a cam ring  27  in a direction to maximize the volume of a pump chamber  28  is disposed in the plunger damper  64 . An annular path space  65  is formed, in the annular groove  64   e,  with respect to the inner wall of a cylindrical portion  63   b  of a plug  63 . 
     A path  57  extending from a pump discharge pressure chamber  33  communicates with a first path space  91  formed annularly between the outer surface at substantially the central portion in the axial direction of the cylindrical portion  63   b,  and a plug hole  62  of a body  21 . A plurality of small holes  68  serving as stationary metering restrictors and a plurality of small holes  69  serving as movable metering restrictors are axially formed in a portion of the cylindrical portion  63   b  corresponding to the first path space  91  so as to form a metering restrictor portion  60 . The opening areas of the small holes  69  serving as the variable metering restrictors are changed by the groove end edge portion of the annular groove  64   e  of the plunger damper  64  which moves in the axial direction along with the swing motion of the cam ring  27 . 
     Accordingly, the pump discharge fluid flowing from the pump discharge path  57  into the first path space  91  flows into the annular path space  65 , comprised of the annular groove  64   e  of the plunger damper  64 , through the small holes  68  and  69  constituting the metering restrictor portion  60 . Hence, the interior of the annular path space  65  is set at a fluid pressure obtained downstream of the metering restrictor portion  60 . 
     The fluid obtained downstream of the restrictor portion and flowing into the annular path space  65  flows in the path space  65  in a direction to separate from the cam ring  27 , and is guided to a second path space  92 , formed annularly in a portion on the outer surface of the cylindrical portion  63   b  close to the opening end of the plug hole  62 , through a path hole  66  formed in the cylindrical portion  63   b  of the plug in the radial direction. The second path space  92  communicates with the pump discharge port  55  through the path hole  93  constituting the pump discharge path. 
     The fourth embodiment is different from the second and third embodiments described above in that the fluid pressure obtained downstream of the metering restrictor portion  60  is introduced into the annular path space  65  comprised of the annular groove  64   e  of the plunger damper  64 . 
     According to this arrangement, the pressure in a second fluid pressure chamber  44  can be set almost equal to the pressure in a space in the plunger damper  64  where the compression coil spring  61  is provided. 
     In the second and third embodiments described above, internal leakage of the pump discharge fluid may occur because the fluid pressure (pressure identical to that of the second fluid pressure chamber  44  communicating with the fluid pressure obtained downstream of the metering restrictor portion  60 ) in the plunger damper  64  where the compression coil spring  61  is provided is lower than the fluid pressure in the annular path space  65  which is the pressure obtained upstream of the metering restrictor portion  60 . However, according to the structure of the fourth embodiment, such a problem does not arise. 
     More specifically, in order to prevent internal leakage described above, leakage preventive countermeasures are required, e.g., the inner surface of the cylindrical portion  63   b  and the outer surface of the plunger damper  64  may be machined at high precision, or seal member may be interposed between them, leading to an increase in cost. In order to ensure the high machining precision described above, these components must be machined at high precision by lathe machining or the like. When internal leakage occurs, depending on the leakage amount, the fluid flow rate decreases on the pump discharge side, and so-called N (rotational speed of pump)−Q (discharge flow rate of fluid) characteristics vary. 
     As described above, when the fluid pressure obtained upstream of the metering restrictor portion  60  is introduced to the path space  65  around the plunger damper  64 , a thrust in a direction to interfere with the swing displacement of the cam ring  27  may undesirably act on the plunger damper  64 . 
     More specifically, since the distal end portion of the plunger damper  64  opposes the second fluid pressure chamber  44  and abuts against the outer surface of the cam ring  27 , the end portion of the plunger damper  64  on the cam ring side is under a pressure obtained by controlling the fluid pressure obtained downstream of the metering restrictor portion  60 . Since a fluid pressure obtained upstream of the metering restrictor portion  60  acts on the opposite side of the plunger damper  64 , a thrust in a direction to press the cam ring  27  acts on the plunger damper  64 . Accordingly, the smooth swing motion of the cam ring  27  is interfered with, and variable adjustment of the pump discharge flow rate cannot be performed appropriately. 
     In contrast to this, according to the fourth embodiment, the fluid pressure in the path space  65  comprised of the O-ring  64   a  formed in the outer surface of the plunger damper  64  is set at the pressure obtained downstream of the metering restrictor portion  60 . The pressure in the path space  65  thus becomes almost equal to the fluid pressure in the plunger damper  64 , and the internal leakage as described above does not arise. Countermeasures for ensuring strict machining precision and sealing performance at these portions become unnecessary, thus decreasing the cost. 
     According to this structure, since the fluid pressures on the two end sides of the plunger damper  64  become almost equal to each other, the plunger damper  64  presses the cam ring  27  with the biasing force of the compression coil spring  61 , so that the cam ring  27  can perform a required motion. 
     The flow of the hydraulic oil in this embodiment will be described. As shown in FIGS. 13 and 14, the hydraulic oil discharged from the pump chamber  28  flows through a pump discharge opening  34  and is guided to the first path space  91  between the plug hole  62  of the front body  21  and the cylindrical portion  63   b  through the pump discharge pressure chamber  33  and pump discharge path  57 . The hydraulic oil then flows from the first path space  91  to the path space  65  around the plunger damper  64  through the small holes  68  and  69 , formed in the cylindrical portion  63   b  of the plug to constitute the metering restrictor portion  60 , and is then guided to the second path space  92  defined by the plug hole  62  through the path hole  66  formed in the cylindrical portion  63   b.  The hydraulic oil flows from the second path space  92  to the path hole  93  and is discharged to outside the pump through a pump discharge port  55 . 
     In this embodiment, the path space  65  formed of the annular groove  64   e  around the plunger damper  64  is set at a fluid pressure obtained downstream of the metering restrictor portion  60 . Therefore, all of the housing space in the cylindrical portion  63   b  for housing the plunger damper  64 , two end sides of the plunger damper  64 , and the path space  65  formed of the annular groove  64   e  in the plunger damper  64  are set at the fluid pressure obtained downstream of the metering restrictor portion  60 , leading to a balanced state in terms of the fluid pressure. 
     According to this structure, even when the plunger damper  64  is provided, a thrust that suppress the swing motion of the cam ring  27  is not produced in the plunger damper  64 . The cam ring  27  can be swung smoothly and appropriately, and can be suppressed from unwanted vibration. 
     Since internal leakage of the fluid pressure does not occur near the plunger damper  64 , the N-Q characteristics (rotational speed—supply flow rate characteristics) of the pump can be stabilized. Since the problem of internal leakage does not arise, high machining precision is not needed for the plunger damper  64  and the cylindrical portion  63   b  that holds the plunger damper  64 . The plunger damper  64  and cylindrical portion  63   b  can be formed of molded components such as sintered components, thus reducing the manufacturing cost. 
     FIG. 15 shows a modification of the fourth embodiment described above. In the fifth embodiment, when forming an annular path space  65  around a plunger damper  64 , a small-diameter portion  64   c  is formed, as in the second embodiment described above, and an inner-diameter portion  63   f  for holding the distal end portion of the small-diameter portion  64   c  is formed in a plug  63 . 
     With this structure as well, a function and an effect identical to those of the fourth embodiment described above can be obviously obtained, and a detailed description thereof will be omitted. 
     The present invention is not limited to the structures of the embodiments described above. The shapes, structures, and the like of the respective components of the variable displacement pump  20  can be arbitrarily modified or changed freely when necessary, and various modifications can be possible. 
     In the embodiments described above, the metering restrictors are explained merely as “restrictors”, as in the stationary metering restrictor  53  and the variable metering restrictor  59 , or the stationary metering restrictor and the variable metering restrictor which constitute the metering restrictor portion and which are comprised of the small holes  68  and  69  formed in the plug  63  and of the plunger damper  64  which changes the opening area of the small hole  69 , among the small holes  68  and  69 . This is because these restrictor portions can be either orifices or chokes. 
     As has been described above, in the variable displacement pump according to the present invention, the first and second fluid chambers formed on two sides of the cam ring are formed to be partitioned from the pump discharge path, and a damper function is added to each fluid chamber, so that the damper function can appropriately be effected in both of the swing directions of the cam ring. As a result, a required braking force can be applied to the motion of the cam ring to the two swing directions. Vibration that occurs when the cam ring swings can be attenuated appropriately, and pulsation on the pump discharge side can be improved. 
     Hence, the conventional problem of noise can be decreased. 
     According to the present invention, the pump discharge fluid pressure is supplied not through the control valve but through the damper functional portion which applies a biasing force to the cam ring, and the plunger damper constituting the damper functional portion, and the plug constitute the metering restrictor. The supply flow rate characteristics with respect to the rotational speed of the pump can be adjusted or changed easily by only altering this damper functional portion. 
     According to the present invention, since the metering restrictor portion comprised of the stationary metering restrictor and the variable metering restrictor is provided to the plunger damper portion, vibration produced when the cam ring swings is not directly transmitted to the metering restrictor portion. Therefore, pulsation in the pressure fluid passing through the metering restrictor portion can be decreased. Moreover, such a plunger damper can be easily added when necessary, so that the conventional pump can be converted comparatively easily. 
     According to the present invention, the pump discharge fluid pressure flowing through the annular path space formed between the plunger damper and the cylindrical member that holds the plunger damper can be set at the fluid pressure obtained downstream of the metering restrictor portion, and can be set almost equal to the fluid pressures on two end sides of the plunger damper. Therefore, internal leakage from the pump discharge path does not occur, and the supply flow rate characteristics (N-Q characteristics) as the pump can be maintained at a required state. 
     According to the present invention, even when the plunger damper is provided, a thrust that suppresses the swing motion of the cam ring is not produced in the plunger damper by the fluid pressure. The cam ring can be swung smoothly and appropriately, and unwanted vibration of the cam ring can be suppressed. 
     Since the present invention is free from the problem of internal leakage, high machining precision is not required for the plunger damper and the cylindrical member that holds the plunger damper. The plunger damper and the cylindrical member can be comprised of molded components such as sintered components, thus decreasing the manufacturing cost. 
     According to the present invention, when the variable metering restrictor is formed in the metering restrictor portion, a pump having drooping type flow rate characteristics can be obtained. When the variable metering restrictor is omitted, a pump having constant volume type flow rate characteristics can be obtained. 
     According to the present invention, since the stationary metering restrictor and the variable metering restrictor can be provided to the two branch discharge path systems, the damper function can be appropriately effected in both of the swing directions of the cam ring. As a result, vibration that occurs when the cam ring swings can be attenuated appropriately, and pulsation on the pump discharge side can be improved to reduce the noise problem. 
     According to the present invention, when constituting a pump having drooping type flow rate characteristics, a pump discharge path structure having two systems respectively extending through the stationary metering restrictor and the variable metering restrictor is employed. Therefore, the supply flow rate characteristics with respect to the rotational speed of the pump can be adjusted and altered easily. 
     According to the present invention, since one system of the pump discharge path is formed to extend through the control valve, pulsation can be reduced. Also, a variable displacement pump having the advantages described above can be formed easily to have the same size as that of the conventional pump. 
     According to the present invention, since the variable metering restrictor is provided to the plunger damper portion, vibration produced when the cam ring swings is not directly transmitted to the variable metering restrictor. Therefore, pulsation in the pressure fluid passing through the variable metering restrictor can be decreased. Moreover, such a plunger damper can be easily added when necessary, so that the conventional pump can be altered easily.