Patent Publication Number: US-6213730-B1

Title: Flow control apparatus for a hydraulic pump

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a flow control apparatus for a positive-displacement pump such as rotary-vane pump, plunger pump, gear pump, and more particularly to the flow control apparatus for keeping a flow rate of fluid discharged from the pump at the high rotational speed which is lower than a flow rate of fluid discharged from the pump at the low rotational speed. 
     Generally, a positive-displacement pump, for instance, rotary-vane pump, installed in automotive vehicles which are driven by engines, is operated by the engine acting as power source and utilized as fluid pressure source for supplying hydraulic fluid to actuators of various hydraulic equipment, for instance, power steering systems. 
     Among various types of the power steering systems for assisting torque generated in manual steering by using hydraulic fluid, there is one type adapted to provide relatively great steering assistance at low vehicle speed and relatively small steering assistance at high vehicle speed. This is because the steering is stable at the high vehicle speed. A positive-displacement pump mounted to such type of the power steering system is required to discharge a high flow rate of fluid at the low rotational speed, i.e., at the low vehicle speed, and a low flow rate of fluid at the high rotational speed, i.e., at the high vehicle speed. For this reason, there have been recently proposed flow control apparatuses adapted to control a flow rate of fluid discharged from the pump and exhibit the aforementioned characteristic of the flow rate of fluid with respect to the rotational speed of the pump. Description of the Related Art One example of the flow control apparatuses as proposed is disclosed in German Patent Application First Publication No. DE4433598A1. The apparatus includes a variable flow control valve disposed within a discharge passage communicating with the discharge side of a positive-displacement pump, and a flow control circuit cooperating with the discharge passage to permit fluid to return the suction side of the pump. The flow control circuit includes a drain valve adapted to drain the fluid discharged from the pump in response to a difference between pressures upstream and downstream of the variable flow control valve. The variable flow control valve is operative to vary a flow of fluid that is discharged from the pump and delivered to actuators through the discharge passage. The variable flow control valve includes a spool facing the fluid discharged from the pump and moveable to vary an opening area of the discharge passage, and a spring biasing the spool so as to increase the opening area of the discharge passage. The drain valve and the variable flow control valve cooperate to control the flow rate of the discharged fluid passing through the discharge passage. 
     In this conventionally known apparatus, when the rotational speed of the pump increases beyond a set value up to a greater value than the set value, the drain valve and the variable flow control valve cooperate to reduce the flow rate of fluid passing through the discharge passage down to a predetermined value. Subsequently, when the rotational speed of the pump exceeds the greater set value, the drain valve and the variable flow control valve cooperate in order to keep the flow rate of fluid of the predetermined value. Under such condition as the rotational speed of the pump exceeding the greater set value, the flow rate of fluid discharged from the pump becomes much higher than the flow rate of fluid drained from the drain valve. However, the known apparatus tends to cause undesired increase in flow rate of fluid passing through the discharge passage over the predetermined value. When the pump is operated at the high rotational speed beyond the greater set value, the characteristic of the flow rate of fluid passing through the discharge passage becomes unstable due to the flow rate of fluid increasing as the rotational speed of the pump rises. This leads to decrease of operating accuracy of actuators and then hydraulic equipment to which the fluid discharged from the pump is supplied via the discharged passage. 
     It is an object of the present invention to provide a variable flow control apparatus for a positive-displacement pump that is capable of achieving a desirably stable characteristic of the flow rate of fluid discharged from the pump at the high rotational speed. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided an apparatus for variably controlling a flow rate of fluid discharged from a positive-displacement pump, comprising: 
     a discharge passage communicating with the pump; 
     a variable flow control valve operative to vary a flow of fluid passing through the discharge passage, the variable flow control valve being disposed within the discharge passage; and 
     a flow control circuit cooperative with the discharge passage to permit a predetermined flow of the fluid, the flow control circuit including a drain valve actuatable in response to a difference between pressures upstream and downstream of the variable flow control valve; 
     the variable flow control valve including a spool bore communicating with the discharge side of the pump, a spool moveably disposed in the spool bore and having positions where different opening areas of the discharge passage are defined, and a spring biasing the spool in such one direction as to increase the opening area of the discharge passage, the spool being displaceable between the positions by a biasing force of the spring and a force variably acting on the spool in response to the flow rate of fluid discharged from the pump; 
     wherein the spring includes a first spring and a second spring arranged in series. 
     According to further aspect of the present invention, there is provided an apparatus for variably controlling a flow rate of fluid discharged from a positive-displacement pump, comprising: 
     a discharge passage communicating with the pump; 
     a fixed orifice disposed within the discharge passage; 
     a flow control circuit cooperative with the discharge passage to permit a predetermined flow of the fluid, the flow control circuit including a drain valve actuatable in response to a difference between pressures upstream and downstream of the fixed orifice; and 
     a variable flow control valve operative to vary a flow of fluid passing through the discharge passage, said variable flow control valve being disposed within the discharge passage downstream of the fixed orifice, the variable flow control valve including a spool bore communicating with the discharge side of the pump, a spool moveably disposed in the spool bore and having positions where different opening areas of the discharge passage are defined, and a spring biasing the spool in such one direction as to increase the opening area of the discharge passage, the spool being displaceable between the positions by a biasing force of the spring and a force variably acting on the spool in response to the flow rate of fluid discharged from the pump; 
     wherein the spring includes a first spring and a second spring arranged in series. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal section, taken along an axis of a pump shaft, of a first embodiment of a flow control apparatus for a hydraulic pump, according to the present invention; 
     FIG. 2 is a graph showing a relationship between the discharge flow and the rotational speed of the pump; 
     FIG. 3 is a schematic diagram of the first embodiment; and 
     FIG. 4 is a schematic diagram of a second embodiment of the apparatus according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a first preferred embodiment of a flow control apparatus for a rotary-vane pump, according to the present invention is explained. The rotary-vane pump denoted at  11  in FIG. 1, is usable as a fluid pressure source for hydraulic actuators of various vehicle components such as power steering system, which supplies the actuators with two different flow rates of fluid in response to rotational speed of a power source of the vehicle. Namely, the rotary—vane pump  11  is driven by the power source having variable speed, for example, an engine, and adapted to supply a first relatively large-predetermined flow rate of fluid at a low vehicle speed and a second predetermined flow rate of fluid at a high vehicle speed, that is less than the first one. 
     As illustrated in FIG. 1, the flow control apparatus is built in a pump housing together with a pump body  15  to form the rotary-vane pump  11  as one unit. The rotary-vane pump  11  includes a pump shaft  12  drivingly connected with the power source such as engine, a cover  13  and a casing  14  cooperating with the cover  13  to define a cavity in which the pump body  15  disposed within the cavity. A suction passage  16  is formed in the casing  14  and fluidly connected with the suction side of the pump body  15 . The suction passage  16  is also fluidly connected with a reservoir. A discharge bore  17  is formed in the casing  14  and constitutes a part of a discharge passage permitting fluid discharged from the discharge side of the pump body  15  to pass therethrough and be fed to the hydraulic actuator. Reference numerals  18  and  19  denote a metallic bearing and an oil seal that are disposed within the casing  14 , respectively. 
     The pump body  15  includes a cylindrical rotor  20  operatively connected with the pump shaft  12 , a plurality of vanes  21  radially reciprocally moveably mounted to an outer periphery of the rotor  20 , a cam ring  22  having an internal circumferential cam surface opposed to the outer periphery of the rotor  20 , and two end plates  23  disposed on opposite axial ends of each of the rotor  20  and the cam ring  22 . In FIG. 1, one of the two end plates  23  is illustrated. The vanes  21 , the outer periphery of the rotor  20 , the internal circumferential cam surface of the cam ring  22  and the end plates  23  cooperate to define pumping chambers therebetween. The pumping chambers vary in volume as the rotor  20  rotates and the vanes  21  slide on the internal circumferential cam surface of the cam ring  22  that has a generally elliptic shape in section. The pump body  15  conducts the continuous pumping action by the volumetric change of the pumping chambers, supplying the fluid pressure. The structure of the pump body  15  is generally known and, for example, described in German Patent Application First Publication No. DE4433598A1 published on Mar. 28, 1996, which is incorporated by reference. 
     The end plate  23  has outlet ports  24   a  and  24   b  and inlet ports, not shown, which are communicated with the volumetrically decreasing pumping chamber and the volumetrically increasing pumping chamber of the pump body  15 , respectively. The inlet ports are also fluidly connected with the suction passage  16  to communicate the suction passage  16  with the volumetrically increasing pumping chamber of the pump body  15 . The outlet ports  24   a  and  24   b  are fluidly connected with the discharge bore  17  open to an end face of the casing  14  which mates with one end face of the end plate  23 . The discharge bore  17  is communicated with the volumetrically decreasing pumping chamber of the pump body  15  via a pressure chamber  25  of the pump body  15  and a variable flow control valve  26 , as explained in detail later. The pressure chamber  25  is defined by the cover  13  and the pump body  15  and has a generally annular shape. The outlet port  24   a  extends radially outward to be open into the pressure chamber  25 . The outlet port  24   b  axially extends to be open to the one end face of the end plate  23  and then extends substantially perpendicularly to be open into the pressure chamber  25 . The outlet port  24   b  thus is formed into a bending passage shape. 
     The discharge bore  17  is fluidly connected with the pressure chamber  25  via a communication passage  29  that is formed in the casing  14  to be open to the end face of the casing  14 . The communication passage  29  has an axial passage portion extending along the axis of the pump shaft  12  and a radial passage portion substantially perpendicular to the axial passage portion. The discharge bore  17  and the communication passage  29  constitute the discharge passage through that the fluid discharged from the pump body  15  is delivered to the actuators. 
     The variable flow control valve  26  is disposed within the discharge passage. The variable flow control valve  26  is operative to vary a flow of fluid passing through the discharge passage. The variable flow control valve  26  includes a spool bore  27  communicating with the discharge side of the pump body  15 , a spool  28  moveable in the spool bore  27  between positions where different opening areas of the discharge passage are defined, and a spring  30  biasing the spool  28  in such one direction as to increase the opening area of the discharge passage. The spool  28  is displaceable between the positions by a biasing force of the spring  30  and a force variably acting on the spool  28  in response to the flow rate of fluid discharged from the pump body  15 . The spool  28  has one surface facing the force, i.e., dynamic pressure, of fluid discharged from the pump body  15  via the outlet port  24   b , and an opposite surface facing the biasing force of the spring unit  30 . The spring  30  is in the form of a spring unit including a first spring  38  and a second spring  39  and a displacement stop  40  interconnecting the first and second springs  38  and  39 . The first and second springs  38  and  39  are arranged in series through the displacement stop  40 . The first spring  38  has a first rigidity and a second spring  39  has a second rigidity greater than the first rigidity. The displacement stop  40  restricts the compression of the first spring  38  in a direction opposite to the one direction. Namely, this direction is such a direction that the spool  28  is forced to move to reduce the opening area of the discharge passage. 
     Specifically, the spool bore  27  is formed in the casing  14  and extends in the axial direction of the pump shaft  12  to be open to the end face of the casing  14 . The spool bore  27  intersects the radial passage portion of the communication passage  29 . The variable flow control valve  26  has valve-inlet and valve-outlet ports which communicate with the radial passage portion of the communication passage  29  as to allow the fluid to flow into the spool bore  27  and pass therethrough to enter the discharge bore  17 . Thus, the spool bore  27  extends in a transverse direction relative to the flow of fluid passing through the discharge passage. The spool bore  27  is opposed to the outlet port  24   b  of the end plate  23  to communicate with the volumetrically decreasing pumping chamber of the pump body  15 . The spool  28  is formed into a hollow cylindrical shape having a disk-like bottom wall  28 A and a circumferential side wall  28 B which are joined together to define a spring mount bore accommodating the spring unit  30 . The bottom wall  28 A has an outer surface facing the dynamic pressure of fluid in the outlet port  24   b  and an inner surface facing the biasing force of the spring unit  30 . The circumferential side wall  28 B is opposed to the opening area of the discharge passage. The displacement stop  40  is fitted to the spring mount bore of the spool  28 . The displacement stop  40  has a rod portion  42  extending along the axis of the pump shaft  12  toward a bottom of the spool bore  27 , and a flange portion  41  extending radially outward from the rod portion  42 . The rod portion  42  has such a length as to contact the bottom of the spool bore  27  at an axial end thereof when the first spring  38  is displaced to a compressed state by a predetermined distance due to the movement of the spool  28  against the first spring  38 . The flange portion  41  is interposed between the first and second springs  38  and  39 . The spool bore  27 , the circumferential side wall  28 B of the spool  28 , and the displacement stop  40  cooperate to define a first spring chamber within the spring mount bore that accommodates the first spring  38 . In this embodiment, the first spring  38  is a coil spring, through which the rod portion  42  of the displacement stop  40  extends toward the bottom of the spool bore  27 . The first spring  38  has one end retained by the bottom of spool bore  27  and an opposite end retained by the flange portion  41  of the displacement stop  40 . The bottom wall  28 A and circumferential side wall  28 B of the spool  28  and the flange portion  41  of the displacement stop  40  cooperate to define a second spring chamber within the spring mount bore that accommodates the second spring  39 . The second spring  39  has one end retained by the bottom wall  28 A of the spool  28  and an opposite end retained by the flange portion  41  of the displacement stop  40 . A coned disk spring is used as the second spring  39  in this embodiment. 
     The spool  28  has its normal position shown in FIG. 1, in which the spool  28  is urged against the end plate  23  by the first spring  38  to allow a maximum opening area of the discharge passage. The spool  28  is moveable by the fluid pressure within the pressure chamber  25  against the biasing forces of the first and second springs  38  and  39 , from the normal position to positions in which the spool  28  is spaced leftward as viewed in FIG. 1, from the end plate  23  to allow reduced opening areas of the discharge passage that are smaller than the maximum opening area thereof. 
     As shown in FIG. 3, the discharge passage B has a portion disposed within a flow control circuit A cooperative with the discharge passage B to permit a predetermined flow of the fluid discharged from the pump body  15 . The flow control circuit A includes a drain valve  37  actuatable to drain the fluid in response to a difference between pressures upstream and downstream of the variable flow control valve  26 . The drain valve  37  is fluidly connected with the reservoir. 
     Referring back to FIG. 1, the drain valve  37  includes a spool bore  31  formed in the casing  14  in communication with the pressure chamber  25 , a spool  32  slidably disposed in the spool bore  31 , and a return spring  33  biasing the spool  32  toward the pressure chamber  25 . The spool bore  31  extends substantially parallel to the axis of the pump shaft  12 . A drain passage  34  is open at one end thereof to the spool bore  31  near an open end of the spool bore  31  that is opposed to the pressure chamber  25 . The drain passage  34  communicates with the suction passage  16 . An induction passage  35  is open at one end thereof to the spool bore  31  near a bottom of the spool bore  31 . The induction passage  35  communicates with the discharge bore  17 . The spool  32  divides the spool bore  31  into a spool pressure chamber disposed on the open end side of the spool bore  31 , and a spool back pressure chamber  36  disposed on the bottom side of the spool bore  31 . The spool pressure chamber is in communication with the pressure chamber  25  of the pump body  15  and the spool back pressure chamber  36  is in communication with the discharge bore  17  via the induction passage  35 . The spool  32  is reciprocally moveable in the spool bore  31  to open and close the open end of the drain passage  34  in response to a difference between pressures in the pressure chamber  25  and the discharge bore  17 . Namely, the spool  32  reciprocates in the spool bore  31  to control the fluid communication of the drain passage  34  with the pressure chamber  25  in response to the difference between pressures upstream and downstream of the variable flow control valve  26 . A flow of fluid discharged from the pressure chamber  25  is controlled by the reciprocal movement of the spool  32 . The spool  32  has a normal position shown in FIG. 1, in which the spool  32  is urged by the spring  33  to close the open end of the drain passage  34  to restrain the fluid communication between the pressure chamber  25  and the drain passage  34 . The spool  32  is moveable by the fluid pressure in the pressure chamber  25  from the normal position to a position in which the spool  32  is located leftward as viewed in FIG. 1, against the biasing force of the spring  33  to open the open end of the drain passage  34  to allow the fluid communication between the pressure chamber  25  and the drain passage  34  via the spool pressure chamber. 
     A relief valve, not shown, of a known type is disposed within the discharge bore  17 . The relief valve is adapted to prevent a fluid pressure in the discharge bore  17  from extremely rising up, the structure of that is described in, for instance, U.S. Pat. No. 5,098,259. 
     An operation of the variable flow control apparatus of the invention will be explained hereinafter by referring to FIGS. 1 and 2. 
     When the pump shaft  12  is in its non-rotating state and the pumping action of the pump body  15  is stopped, the spool  28  of the variable flow control valve  26  and the spool  32  of the drain valve  37  are placed in the respective normal positions where the spools  28  and  32  are contacted with the end plate  23  as shown in FIG.  1 . The spool  28  allows the maximum opening area of the discharge passage while the spool  32  prevents the drain passage  34  from being communicated with the pressure chamber  25  of the pump body  15 . 
     When the pump shaft  12  is driven to start its rotation, the pump body  15  actuates to discharge fluid from the volumetrically increasing pumping chamber into the discharge bore  17  via the outlet ports  24   a  and  24   b , the pressure chamber  25 , the communication passage  29 , and the variable flow control valve  26 . In this condition, until the rotational speed of the pump body  15  reaches a first set value a shown in FIG. 2, both of the static pressure of fluid within the pressure chamber  25  and the dynamic pressure of fluid within the outlet port  24   b  are low. The spool  28  of the variable flow control valve  26  and the spool  32  of the drain valve  37  are still placed in the respective normal positions, so that a flow rate of fluid discharged from the discharge bore  17  increases as the rotational speed of the pump body  15  rises. 
     When the rotational speed of the pump rises up to the first set value a and the difference between pressures upstream and downstream of the variable flow control valve  26  becomes greater than a certain value, the spool  32  of the drain valve  37  is moved toward the bottom of the spool bore  31  to allow an excessive amount of the fluid in the pressure chamber  25  to flow into the drain passage  34 . The flow rate of fluid discharged from the discharge bore  17  is kept at a first predetermined value q 1 . This flow control continues until the rotational speed of the pump body  15  reaches a second set value b higher than the first set value a. 
     When the rotational speed of the pump body  15  exceeds the second set value b and the dynamic pressure of fluid discharged from the outlet port  24   b  becomes not less than a certain level, the spool  28  of the variable flow control valve  26  is forced by the dynamic pressure to move toward the bottom of the spool bore  27  against the biasing force of the first spring  38 . The first spring  38  is compressed as the spool  28  is retracted into the spool bore  27 . The opening area of the valve-outlet port connected to the discharge bore  17  is reduced from the maximum depending on the movement of the spool  28 . The flow rate of fluid discharged from the discharge bore  17  becomes lower than the first predetermined value q 1 . Until the rotational speed of the pump body  15  rises up to a third set value c higher than the second set value b, the flow rate of fluid discharged from the discharge bore  17  continues to decrease. 
     When the rotational speed of the pump body  15  reaches the third set value c, the tip end of the rod portion  42  of the displacement stop  40  of the variable flow control valve  26  contacts the bottom of the spool bore  27  so that the first spring  38  is prevented from being further compressed. The flow rate of fluid discharged from the discharge bore  17  reaches a second predetermined value q 2  lower than the first predetermined value q 1 . Subsequently, when the rotational speed of the pump body  15  becomes higher than the third set value c, load is caused by the dynamic pressure of fluid discharged from the outlet port  24   b . Under this condition, assuming that the opening area of the valve-outlet is no longer reduced and besides, for instance, the spool  32  of the drain valve  37  is delayed in response to the raise of the pump rotational speed, the flow rate of fluid discharged from the discharge bore  17  begins to gradually increase to be higher than the second predetermined value q 2  as indicated by the broken line P in FIG.  2 . However, with the arrangement of the apparatus of the first embodiment, the second spring  39  between the spool  28  and the displacement stop  40  is brought into being compressed by the load applied thereto via the spool  28 . The spool  28  is further moved toward the bottom of the spool bore  27  against the biasing force of the second spring  39 , so that the opening area of the valve-outlet port connected with the discharge bore  17  is further reduced. Thus, since the opening area of the valve-outlet port is further reduced by the spool  28  further moving along with the compression of the second spring  39 , the increment of the flow rate of fluid discharged from the discharge bore  17  is eliminated. As a result, after the pump rotational speed becomes higher than the third set value c, the flow rate of fluid discharged from the discharge bore  17  is kept constant at substantially the second predetermined value q 2  as indicated by the solid line R in FIG.  2 . 
     As seen from the above description, the rotary-vane pump  11  with the flow control apparatus can provide the first predetermined flow rate q 1  at the low rotational speed a to b and the second predetermined flow rate q 2  at the high rotational speed c as shown in FIG.  2 . Accordingly, the rotary-vane pump  11  can supply actuators with the fluid pressure required for desirably operating hydraulic equipment connected with the actuators at both the low rotational speed and the high rotational speed. This serves for enhancing the operating performance of the actuators and the hydraulic equipment. The positive-displacement pump may be a plunger pump, a gear pump, or the like. 
     Further, it will be appreciated from the above explanation that, since the spring unit  30  of the variable flow control valve  26  has the serial arrangement of the first spring  38  and the second spring  39  greater in rigidity than the first spring  38 , the compression of the first spring  38  is caused prior to the compression of the second spring  39 , upon the rotational speed of the pump body  15  increasing. By the compression of the first spring  38 , the opening area of the discharge passage is reduced to lower the flow rate of fluid passing through the discharge passage to the second predetermined value q 2 . Owing to the compression of the second spring  39  subsequent to the compression of the first spring  38 , the opening area of the discharge passage is further reduced, causing gradual and slow decrease of the flow rate of fluid passing through the discharge passage. The decrease of the flow rate that is caused by the compression of the second spring  39  can eliminate the increment of the flow rate that occurs, for instance, with the delayed response of the drain valve  37 , in the pump operation at the high rotational speed. As a result, the flow rate of fluid discharged from the pump body  15  at the high rotational speed can be kept constant at substantially the second predetermined value q 2  while the rotational speed of the pump body  15  further increases to exceed the set value c. Therefore, the variable flow control apparatus of the present invention can exhibit the desired characteristic of the flow rate of fluid discharged from the pump body  15  in the pump operation at each of the low rotational speed and the high rotational speed. 
     Furthermore, with the arrangement of the displacement stop  40  restraining the compression of the first spring  38 , the second spring  39  having a greater rigidity than the first spring  38  can be compressed after the compression of the first spring  38  is completely restricted by the displacement stop  40 . Accordingly, the compression of the second spring  39  is assured to occur at the high rotational speed, i.e., the rotational speed higher than c as shown in FIG.  2 . This allows the action of the spool  28  to be readily controlled in the pump operation at the high rotational speed, serving for more accurate control of the flow rate of fluid discharged from the pump body  15  at the high rotational speed. To this end, it will be possible to easily obtain the desirable characteristic of the flow rate of fluid discharged from the pump body  15  at the high rotational speed. 
     In addition, in this embodiment, the use of the coned disk spring as the second spring  39  contributes to volumetric reduction of the second spring chamber within the spool  28 . This results in reduction of dimension of the spring unit  30  and the variable flow control valve  26  as a whole. 
     The second spring  39  is not limited to the coned disk spring as described in the first embodiment but it can be in the form of a coil spring. In the case of using the coil spring as the second spring  39 , the characteristic of the compression displacement relative to load is linearly indicated, so that the desirable characteristic of the flow rate of fluid discharged from the pump body  15  at the high rotational speed will be readily obtained. Further, since the coil spring is easily produced, the use of the coil spring serves for saving the manufacturing cost. 
     Further, the above-described simple structure of the spring unit  30  of the variable flow control valve  26  contributes to easy achievement of the desirable characteristic of the flow rate of fluid discharged from the pump body  15  at each of the low rotational speed and the high rotational speed. The simple structure also serves for reducing the manufacturing cost of the flow control apparatus. 
     Furthermore, in the first embodiment, the dynamic pressure in the outlet port  24   b  is utilized as the force variably acting on the spool  28  in response to the flow rate of fluid discharged from the pump body  15 . However, in a case where an orifice adapted to permit the entire flow of fluid discharged from the pump body  15  to pass therethrough is disposed within the discharge passage, a difference between pressures upstream and downstream of the orifice may be utilized for actuating the spool  28 . In this case, since the difference between pressures upstream and downstream of the orifice varies in response to the flow rate of fluid from the pump body  15 , the spool  28  can be actuated when the rotational speed of the pump body  15  reaches the set value. 
     Referring to FIG. 4, a second preferred embodiment of the flow control apparatus will be explained hereinafter. 
     In FIG. 4, a fixed orifice  100  is disposed within a portion of the discharge passage B which cooperates with the flow control circuit A. The variable flow control valve  126  is disposed within discharge passage B downstream of the fixed orifice  100  and the flow control circuit A. The variable flow control valve  126  has the same structure as the variable flow control valve  26  explained in the first embodiment.