Patent Publication Number: US-2009226298-A1

Title: Tandem pump

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a tandem pump capable of increasing a first fluid pressure in a first hydraulic circuit and a second fluid pressure in a second hydraulic circuit by using one driving source (such as a motor). 
     A published Japanese patent Application Publication No. 2007-177687 shows a tandem pump including first and second driving gears of first and second pumps, a drive gear driving the driving gears and a center plate separating the first and second driving gears axially. 
     SUMMARY OF THE INVENTION 
     In the tandem pump of the above-mentioned patent document, the center plate is not positioned in the axial direction. Accordingly, a pressure difference, if produced between the first and second pumps, tends to shift the center plate from the higher pressure side toward the lower pressure side. This shift of the center plate causes interference between the center plate and the driving gear on the lower pressure side and thereby increases the friction therebetween. On the higher pressure side, the center plate moves away from the driving gear, and increases the leakage. 
     Therefore, it is an object of the present invention to provide a tandem pump adapted to restrain the friction and the leak. 
     According to one aspect of the invention, an apparatus includes at least a tandem pump. The tandem pump comprises: a rotation shaft extending in an axial direction; first and second pump sections (or gear sections) driven by the rotation shaft; a housing defining a pump receiving portion including a first pump chamber receiving the first pump section and a second pump chamber receiving the second pump section; and a partition separating the first and second pump chambers from each other. The partition is so arranged that the position of the partition is determined, at least in the axial direction, by the housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a hydraulic circuit diagram showing a hydraulic system including a tandem pump P according to a first embodiment of the present invention. 
         FIG. 2  is a front view showing a z positive side of the tandem pump P. 
         FIG. 3  is a sectional view taken across a line I-I shown in  FIG. 4 . 
         FIG. 4  is a z axis direction sectional view of  FIG. 2  showing a longitudinal or axial section of the tandem pump P. 
         FIG. 5  is a perspective view of a center plate ( 400 ) shown in  FIG. 4 . 
         FIG. 6  is a perspective view of a (first) side plate ( 150 ) shown in  FIG. 4 . 
         FIG. 7  is a perspective view of a leaf spring ( 300 ) shown in  FIG. 2 . 
         FIG. 8  is a z axis direction sectional view of a tandem pump P according to a second embodiment of the present invention. 
         FIG. 9  is a z axis direction sectional view of a tandem pump P according to a third embodiment. 
         FIG. 10  is a perspective view showing a first center plate ( 400 P) shown in  FIG. 9 . 
         FIG. 11  is a perspective view showing a second center plate ( 400 S) shown in  FIG. 9 . 
         FIG. 12  is a z axis direction sectional view of a tandem pump P according to a fourth embodiment. 
         FIG. 13  is a z axis direction sectional view of a tandem pump P according to a fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment 1  
     [Hydraulic Circuit] 
     A tandem pump P is applied to a hydraulic unit H/U as shown in a circuit diagram of  FIG. 1 , according to a first embodiment of the present invention (and subsequent embodiments). Hydraulic unit H/U is connected between a master cylinder M/C and wheel cylinders W/C. Tandem pump P is a combination of a first pump P 1  and a second pump P 2 , which are driven by a single common drive shaft ( 110 ), and so arranged that first and second pumps P 1  and P 2  are equal in the discharge quantity per unit time and the discharge pressure. 
     First pump P 1  is connected with a P subsystem, and second pump P 2  is connected with an S subsystem. First and second pumps P 1  and P 2  are arranged to supply the respective discharge pressures to the P and S subsystems independently. Tandem pump P and other components (such as valves GV-IN, GV-OUT, IN-V, and OUT-V) in the hydraulic circuit are controlled by a control unit CU. 
     This hydraulic brake system includes two independent brake subsystems, the P subsystem including a P route hydraulic circuit  10 P and the S subsystem including an S route hydraulic circuit  20 S. In this example, these two circuits  10 P and  20 S are arranged in a so-called X piping arrangement. The P route circuit  10 P is connected to wheel cylinder W/C(FL) for a front left wheel of the vehicle, and wheel cylinder W/C(RR) for a rear right wheel. The S route circuit  20 S is connected to wheel cylinder W/C(FR) for a front right wheel, and wheel cylinder W/C(RL) for a rear left wheel. It is optional to employ, for the brake circuit, arrangements other than the X piping arrangement. In this example, a wheel cylinder set including the four wheel cylinders is divided into a first subset including two of the fourth wheel cylinders and a second subset including the other two of the fourth wheel cylinders. 
     A brake pedal BP is arranged to transmit a driver&#39;s brake pedal operation through a brake booster BS and an input rod IR, to a master cylinder M/C. This master cylinder M/C is a tandem master cylinder including two pistons arranged in tandem to define two fluid pressure chambers in the cylinder. These two pressure chambers are arranged to receive the supply of a brake fluid from a reservoir tank RES. The first pressure chamber is connected with the first brake circuit  10 P, and the second pressure chamber is connected with the second brake circuit  20 S. 
     When brake pedal BP is depressed, the master cylinder M/C produces fluid pressures (master cylinder pressures Pmc) in the two pressure chambers in accordance with the brake pedal depression quantity, and supplies the produced master cylinder pressures Pmc, respectively, to first and second brake circuits  10 P and  20 S. A cup-shaped seal member (of known type) is provided on the outer circumference of each master cylinder piston, and arranged to shut off the connection between reservoir tank RES and the corresponding pressure chamber to enable a pressure increase in the corresponding pressure chamber at the time of piston stroke. In this case, the brake fluid is not supplied from reservoir tank RES to brake circuits  10 P and  20 S. The brake fluid is supplied only from the pressure chambers of master cylinder M/C to brake circuits  10 P and  20 S. 
     When brake pedal BP is returned, the master cylinder pistons are returned by respective return springs (provided in the pressure chambers). In this case, the cup-shaped seal members make the fluid connection between reservoir tank RES with the pressure chambers again, and thereby enable the supply of the brake fluid from reservoir tank RES to the pressure chambers, again. The following explanation is directed mainly to first brake circuit  10 P. 
     Brake circuit  10 P includes a gate-out valve GV-OUT(P) connected between an upstream second passage  10   n  leading to master cylinder M/C, and a downstream second passage  10   k  extending toward wheel cylinders W/C. Gate-out valve GV-OUT(P) is a normally-open proportional solenoid valve. A check valve  10   p  is disposed in a passage  10   j  connected in parallel to gate-out valve GV-OUT(P), and arranged to prevent the fluid flow in the direction from the downstream side (near the wheel cylinders W/C) to the upstream side (toward master cylinder M/C). 
     Downstream passage  10   k  bifurcates into a first branch circuit  10   a  extending to a first outlet point connected with one of the wheel cylinders W/C(FL, RR) and a second branch circuit  10   b  extending to a second outlet point connected to the other of the wheel cylinders W/C(FL, RR) of the P subsystem. First and second flow-in valves IN/V(FL, RR) are provided, respectively, in the first and second branch circuits  10   a  and  10   b . The flow-in valves IN/V are normally-open proportional solenoid valves. 
     A check valve  10   q  is disposed in a passage  10   l  connected in parallel to the first flow-in valve IN/V(FL), and arranged to prevent the fluid flow from the upstream side to the downstream side. Similarly, a check valve  10   r  is disposed in a passage  10   m  connected in parallel to the in the second flow-in valve IN/V(RR), and arranged to prevent the fluid flow from the upstream side to the downstream side. 
     A first flow-out valve OUT/V(FL) is disposed in a first return passage  10   c  extending from the first outlet point (connected with wheel cylinder W/C(FL)) to a confluent return passage  10   e . A second flow-out valve OUT/V(RR) is disposed in a second return passage  10   d  extending from the second outlet point (connected with wheel cylinder W/C(RR)) to the confluent return passage  10   e . First and second flow-out valves OUT/V are normally-closed on-off solenoid valves. The confluent return passage  10   e  extends to a reservoir  16  provided in hydraulic unit H/U. 
     A gate-in valve GV-IN(P) is provided in a first passage log which is connected with the upstream second passage  10   n  on the upstream side of gate-out valve GV-OUT(P). Gate-in valve GV-IN(P) is a normally-closed on-off solenoid valve arranged to open or close the first passage  10   g . First passage  10   g  joins with a return passage  10   f  extending from reservoir  16 , and forms a suction (or inlet) passage  10   h.    
     The pump P is provided as a secondary pressure source in addition to master cylinder M/C. Pump P of this example is a tandem type gear pump including the first pump P 1  (for the P side) and second pump P 2  (for the S side) both driven by an electric motor M. The suction (inlet) side of first pump P 1  is connected with the suction circuit  10   h . The discharge (outlet) side of first pump P 1  is connected with a discharge (outlet) circuit  10   i , and further connected, through the discharge circuit  10   i , with the downstream side second circuit  10   k.    
     A check valve  10   s  is provided in return circuit  10   f , and arranged to prevent the fluid flow from first circuit  10   g  (gate-in valve GV-IN(P)) to reservoir  16 . A check valve  10   u  is provided in discharge circuit  10   i  and arranged to prevent the fluid flow from downstream second passage  10   k  (gate-out valve GV-OUT(P)) or from branch circuits  10   a  and  10   b  (wheel cylinders W/C), to first pump P 1  (discharge side). The brake circuit  20 S is constructed in the same manner as the brake circuit  10 P, as shown in  FIG. 1  (in which the brake circuits  10 P and  20 S are arranged in a manner of bilateral symmetry). 
     [Brake Control] 
     In a normal brake operation, hydraulic unit H/U enables a boosting control (or pressure increasing control)(as mentioned below), an automatic brake control such as ACC (adaptive cruise control: control for controlling a distance between vehicles) and VDC (vehicle dynamics control or vehicle behavior control), and anti-skid brake control. In the automatic brake control such as the vehicle behavior control, the control unit CU closes the gate-out valve GV-OUT(P), and opens the gate-in valve GV-IN(P)(in the case of the brake circuit  10 P, as an example). At the same time, by driving the pump P, the hydraulic unit HU supplies the brake fluid from master cylinder M/C, through passages  10   g  and  10   h  and discharge circuit  10   i , toward the branch circuits  10   a  and  10   b.    
     Furthermore, the control unit CU controls the gate-out valve GV-OUT(P) or the flow-in valves IN/V(FL, RR) to produce a desired wheel cylinder fluid pressure Pwc* corresponding to a braking force required for stabilizing the vehicle behavior. The brake circuit  20 S is controlled in the same manner. 
     At the time of anti-skid brake control, the control unit CU opens the flow-out valve OUT/V(FL), and closes the flow-in valve IN/V(FL) in the case of wheel FL, as an example. By so doing, the control unit CU decreases the wheel cylinder pressure by discharging the brake fluid from wheel cylinder W/C(FL) to reservoir  16 . When wheel FL recovers from a locking tendency, the control unit CU holds the wheel cylinder pressure by closing the flow-out valve OUT/V(FL). Moreover, control unit CU increases the wheel cylinder pressure appropriately by driving the pump P and opening the flow-in valve IN/V(FL). Pump P functions to return the brake fluid drained to reservoir  16  at the time of pressure decreasing operation, to the second passage  10   k.    
     Thus, as shown in  FIG. 1 , the hydraulic brake system includes at least: a wheel cylinder set including a first subset including at least a first wheel cylinder (W/C) provided for braking a first wheel of a vehicle and a second subset including at least a second wheel cylinder (W/C) provided for braking a second wheel of the vehicle; and a hydraulic (valve) system including a first subsystem including at least a first control valve (IN-V, OUT-V) and connecting the first pump section with the first wheel cylinder to increase the fluid pressure of the first wheel cylinder, and a second subsystem including at least a second control valve (IN-V, OUT-V) and connecting the second pump section with the second wheel cylinder (W/C) to increase the fluid pressure of the second wheel cylinder. 
     [Tandem Pump] 
       FIG. 2  shows a z positive side of the tandem pump P.  FIG. 3  shows tandem pump P in a section taken across a line I-I shown in  FIG. 4 .  FIG. 4  is a z axis (axial) sectional view of tandem pump P. In  FIG. 3 , a leaf spring  300  is omitted.  FIG. 5  shows a center plate (partition)  400  in perspective.  FIG. 6  shows a first side plate  150  in perspective.  FIG. 7  shows the leaf spring  300  in perspective. First and second side plates  150  and  160  are substantially identical in shape, so that only the first side plate  150  is shown in  FIG. 6 . 
     In the following explanation, an x positive direction in which an x axis extends is a direction from a driven shaft  120  to a driving shaft  110  in a pump assembly  100 , a y positive direction in which a y axis extends is a direction which is perpendicular to the x positive direction and which extends toward the position of seal blocks  200 , and a z positive direction of a z axis is an axial direction which is parallel to the axis (Op) of driving shaft  110 , and which extends toward a first end of driving shaft  110  adapted to be connected with motor M. 
     Tandem pump P is a pump of a type driving first and second pumps P 1  and P 2  simultaneously with the single common driving shaft  110 . First and second pumps P 1  and P 2  are substantially identical in construction. First and second pumps P 1  and P 2  produce discharge pressures for the P and S circuits, respectively or independently. Center plate  400  (serving as a partition) is interposed (axially) between first and second pumps P 1  and P 2 , and arranged to seal first and second pumps P 1  and P 2  from each other. 
     [Housing] 
     Housing  1  of tandem pump P is composed of a main housing member  10  (first housing member) and a cover member  20  (second housing member), which are made of metallic material which is aluminum alloy in this example. Main housing member  10  includes an end wall (first end wall) formed with a driving shaft support hole  11  and a surrounding (or circumferential) wall defining a pump receiving portion  12  which, in this example is an inside cavity in the form of a stepped cylinder. Pump assembly  100  is inserted into pump receiving portion  12  from a z negative side (from the left side as viewed in  FIG. 4 ). Driving shaft  110  is supported rotatably through bushing by the drive shaft support hole  11 . 
     The surrounding wall of main housing member  10  includes a step  13  formed in pump receiving portion  12 . Pump receiving portion  12  is composed of a first (smaller cylindrical) portion  14  (serving as a first side receiving portion) and a second (larger cylindrical) portion  15  which is greater in cross sectional size or in diameter than first portion  14  (and which can serve as a middle receiving portion located axially between the first side receiving portion  14  and a second side receiving portion on the z negative side). Step  13  is formed between first and second portions  14  and  15 . Step  13  includes an annular shoulder surface facing in the z negative direction (second axial direction) to abut against the center plate  400  of pump assembly  100  and thereby to limit the movement of pump assembly  100  in the z positive direction (first axial direction). The surrounding wall of main housing member  10  includes a first wall portion surrounding and defining the first (smaller) portion  14 , and a second wall portion surrounding and defining the second (larger) portion  15  located on the z negative side of the first (smaller) portion  14 . 
     As shown in  FIGS. 2 and 3 , the cylindrical pump receiving portion  12  is defined by an inside circumferential (or cylindrical) surface including a first region  12   a  used as a positioning abutment surface and a second region  12   b . The first region  12   a  is adapted to abut on the seal block  200  shown in  FIGS. 2 and 3 , and thereby to position the pump assembly  100 . Therefore, first region  12   a  is formed more accurately than the second region  12   b . Main housing member  10  is formed with discharge circuits  10   h  and  20   h  connecting the pump receiving portion  12  fluidly with the outside. Discharge circuits  10   i  and  20   i  are located on the x positive side. 
     Cover member  20  is fixed to main housing member  10  by bolts B. Cover member  20  is located on the z negative side of main housing member  10 . Pump assembly  100  is enclosed liquid-tightly in the pump receiving portion  12  by cover member  20  and main housing member  10 . Cover member  20  is a cup-shaped member having a bottom. Cover member  20  includes a base portion (forming a second end wall axially confronting the first end wall formed with the drive shaft support hole  11 ) and a (cylindrical) projecting portion  21  projecting in the z positive direction from the base portion and receiving second pump P 2 . The (cylindrical) projecting portion  21  is provided with a seal ring  33  fit in an annular groove formed in the outside circumferential surface of projecting portion  21 , and inserted liquid-tightly in the pump receiving portion  12  of main housing member  10 . The projecting portion  21  of cover member  20  includes a surrounding wall defining a second side receiving portion which is similar to the first portion  14  (defining the first side receiving portion) and which receives the second pump P 2  and second side plate  160  like the first portion  14  of pump receiving portion  12 . 
     [Details of Pump Assembly] 
     Pump assembly  100  includes first and second seal blocks  200 , drive shaft  110 , driven shaft  120 , first and second driving gears  130  and first and second driven gears  140 , first and second side plates  150  and  160  (a pair of side plates) and center plate  400 . 
     Pump assembly  100  is temporarily united by C-shaped leaf springs  300 . Seal blocks  400  are shorter in the width in the x direction than the first and second side plates  150  and  160 . Center plate  400  liquid-tightly defines first and second discharge regions (pump chambers) Dout 1  and Dout 2  formed, respectively, on the radial outer side of first and second side plates  150  and  160 . 
     First driving gear  130 P and first driven gear  140 P for the P route circuit are provided on the z positive side of center plate  400 . Second driving gear  130 S and second driven gear  140 S for the S route circuit are provided on the z negative side of center plate  400 . As shown in  FIG. 4 , the first driving and driven gears  130 P and  140 P are located axially (along the z axis) between the center plate  400  on the z negative side and the first side plate  150  on the z positive side. The second driving and driven gears  130 S and  140 S are located axially between the center plate  400  on the z positive side and the second side plate  160  on the z negative side. Center plate  400  is located axially between the first driving and driven gears  130 P and  140 P on the z positive side, and the second driving and driven gears  130 S and  140 S on the z negative side. 
     A P route discharge circuit  10   i  is formed in the region which is located on the z positive side of center plate  400  and on the x positive side of first side plate  150 . An S route discharge circuit  20   h  is formed in the region which is located on the z negative side of center plate  400  and on the x positive side of second side plate  160 . S route discharge circuit  20   i  is formed by drilling center plate  400 . 
     Two of the seal blocks  200  are disposed, respectively, on the y positive side of first and second side plates  150  and  160 . As shown in  FIG. 4 , along the z axis, the center plate  400  is located between the first pump P 1  for the P route on the z positive side (the right side as viewed in  FIG. 4 ) and the second pump P 2  for the S route on the z negative side (the left side in  FIG. 4 ). However, it is optional to reverse the positions of the first and second pumps of the P route and S route so that first pump P 1  for the P route is located on the z negative side and second pump P 2  for the S route is located on the z positive side. 
     The center plate  400 , first and second side plates  150  and  160  and first and second seal blocks  200  are bilaterally symmetrical with respect to an imaginary straight line II-II (representing an imaginary flat median plane) in a radial plane or cross sectional plane (x-y plane) as shown in  FIG. 2 . This II-II line is located at the middle between driving shaft  110  and driven shaft  120 . Moreover, the leaf spring  300  shown in  FIG. 2  is bilaterally symmetrical with respect to the II-II line, and leaf spring  300  is arranged to produce resilient forces symmetrical with respect to the II-II line. The II-II line extends along the y axis between the driving and driven shafts  110  and  120  both extending along the z axis. 
     In the x-y plane, as shown in  FIG. 2 , pump assembly  100  and leaf spring  300  are set in point contact with each other at three contact points A, B and C. The first contact point A is located on the y positive side of the seal block  200  shown in  FIG. 2 . The second and third contact points B and C are located at two x end portions  152  ( 162  in the case of second side plate  160 ) in a y negative side  151  ( 161 ) of the side plates  150  ( 160 ), respectively, as shown in  FIG. 2 . 
     First contact point A is located on the II-II line (the median plane) which extends through the middle point M between the axes Op and Os of driving shaft  110  and driven shaft  120 , in parallel to the y axis. Second and third contact points B and C are located on the y negative side of an imaginary III-III straight line passing through the axes Op and Os (and representing an imaginary transverse plane), one on the x positive side of the II-II line and the other on the x negative side. 
     The point contact in the x-y plane means a line contact in the x-y-z space on a straight line extending along the z axis. Leaf spring  300  shown in  FIG. 2  contacts with the seal block  200  along a straight line passing through contact point A and extending along the z axis, and further contacts with the (first) side plate  150  ( 160 ) along straight lines passing through contact points B and C, respectively, and extending along the z axis. 
     Accordingly, pump assembly  100  is held at the first contact point A from the y positive side, and urged toward the y positive side at the second and third points B and C by leaf spring  300 . With this three-point support structure at the points A, B and C, the leaf spring  300  can press the seal block  200  from the y-axis positive side against the first side plate  150  ( 160 ), and thereby unite or bind the pump assembly  100  provisionally. The second side plate  160  and second seal block  200  are bound and united in the same manner as shown in  FIG. 2 . 
     Seal blocks  200  are narrower in the width measured along the x axis than first and second side plates  150  and  160 . Therefore, each of seal blocks  200  is urged stably toward the y negative side with leaf spring  300  including a one-point support portion supporting the y positive side of seal block  200  at point A, and a two-point support portion supporting the y negative side of the first or second side plate  150  or  160  at points B and C. 
     [Drive Shaft and Driven Shaft] 
     Drive shaft  110  is connected with first and second driving gears  130 P and  130 S made of a ferrous material so that they rotates as a unit. Driven shaft  120  is connected with first and second driven gears  140 P and  140 S made of the ferrous material so that they rotates as a unit. As shown in  FIG. 4 , driving shaft  110  extends, along the z axis, from a second (left) end to a first (right) end (z positive end) which is adapted to be connected with the motor not shown in  FIG. 4 . The gears  130 P,  130 S,  140 P and  140 S are spur gears. On each of the first and second sides for the P and S circuits, driving gear  130  ( 130 P,  130 S) and driven gear  140  ( 140 P,  140 S) are engaged with each other in the form of a spur gear set as shown in  FIG. 3 , so that driven shaft  120  is driven by driving shaft  110 . 
     [Center Plate] 
     Center plate  400  is a circular disc-shaped member including a step as best shown in  FIG. 5 . Center plate  400  is an integral member formed by a forming process of uniting a plate (third side plate)  150 ′ ( FIG. 4 ) adapted to be in sliding contact with driving and driven gears  130 P and  140 P, and another plate  160 ′ ( FIG. 4 ) adapted to be in sliding contact with driving and driven gears  130 S and  140 S. 
     Center plate  400  includes a step  410 , a smaller section  420  having a smaller diameter and lying on the z positive side of step  410 , and a larger section  430  having a larger diameter larger than the diameter of smaller section  420 , and lying on the z negative side of step  410 . Center plate  400  further includes a drive shaft hole  401  receiving drive shaft  110  rotatably, and a driven shaft hole  402  receiving driven shaft  120  rotatably. 
     Seal members  34  and  35  are received, respectively, in annular grooves  401 a and  402   a  formed in drive shaft  110  and driven shaft  120  (as shown in  FIG. 4 ). Seal members  34  and  35  seal the clearances around the drive shaft  110  and driven shaft  120 , respectively, and thereby seal off the first and second pumps P 1  and P 2  from each other. 
     The smaller section  420  of center plate  400  includes a z positive side  421  including a sliding surface  421   a  adapted to be in sliding contact with the driving and driven gears  130 P and  140 P (as shown in  FIG. 4  and  FIG. 5 ), and an abutting surface  421   b  formed on the y positive side of the sliding surface  421   a  (as shown in  FIG. 5 ) and adapted to be abut on the first seal block  200  liquid-tightly. 
     Similarly, the larger section  430  of center plate  400  includes a z negative side  431  including a sliding surface  431   a  adapted to be in sliding contact with the drive and driven gears  130 S and  140 S (as shown in  FIG. 4 ), and an abutting surface  431   b  formed on the y positive side of the sliding surface  431   a  and adapted to be abut on the second seal block  200  liquid-tightly. 
     The sliding surfaces  421   a  and  431   a  are formed around the drive shaft hole  401  and driven shaft hole  402  in inner regions of z positive and negative sides  421  and  431  of center plate  400 , respectively.  FIG. 5  shows only the z positive side  421  of the smaller section  420 , and explanation on the sliding surface  431   a  and abutting surface  431   b  of the larger section  430  is omitted since the sliding surfaces  421   a  and  431   a , and the abutting surfaces  421   b  and  431   b  are substantially identical in shape and position. 
     Sliding surface  421   a  is an 8-shaped region including a first annular portion surrounding the drive shaft hole  401 , and a second annular portion surrounding the driven shaft hole  402 . Gears  130 P and  140 P can slide liquid-tightly on the sliding surface  421   a . Between the gear sliding surface  421   a  and the seal block abutting surface  421   b , there are formed sliding surfaces  158  and seal block sliding surfaces  210  and  220 , as shown in  FIG. 5 . 
     Seal block abutting surface  421   b  is a C-shaped region surrounding a suction passage Din. Suction passage Din is defined liquid-tightly by the C-shaped abutting surface  421   b  and an inlet side recessed portion  421   c  which is formed in the 8-shaped sliding surface  421   a  on the y positive side toward the abutting surface  421   b , at the middle of the 8-shaped sliding surface  421   a  in the x direction. 
     An outlet side recessed portion  421   d  is formed on the opposite side (y negative side) of the sliding surface  421   a . The 8-shaped sliding surface  421   a  includes a connecting portion which is formed, along the x axis, between the first annular portion surrounding the drive shaft hole  401  and the second annular portion surrounding the driven shaft hole  402 , and which is located between the inlet side recessed portion  421   c  and outlet side recessed portion  421   d , along the y axis. Outlet side recessed portion  421   d  is located on the outlet or discharge side of the driving and driven gears  130 P and  140 P, and arranged to cause the outlet pressure to flow smoothly to the discharge region Dout 1  formed on the outer side of the sliding surface  421   a.    
     A (smaller) seal ring  31  is fit in an annular groove  422   a  formed in the (cylindrical) circumference  422  of smaller section  420 . Similarly, a (larger) seal ring  32  is fit in an annular groove  432   a  formed in the (cylindrical) circumference  432  of larger section  430 . There are further provided, respectively, in drive and driven shafts  110  and  120 , seal rings  34  and  35  for sealing off the first and second pumps P 1  and P 2  from each other. 
     The circumference  432  of larger section  430  is formed with at least one opening of inlet circuit  20   h  for supplying an inlet pressure to second pump P 2  for the S circuit. The supply of the inlet pressure to first pump P 1  for the P circuit is achieved by inlet circuit  10   h  formed in main housing member  10  on the z positive side of first plate  150  (as shown in  FIG. 3 ). 
     The (external) annular step  410  of center plate  400  includes an annular step surface which faces in the z positive direction (first axial direction) and which abuts on the annular shoulder surface of the (internal) step  13  of main housing member  10  so that the axial movement of center plate  400  in the z positive direction is limited by the shoulder surface of step  13 . Cover member  20  includes cylindrical projecting portion  21  projecting in the z positive direction and terminating at an annular forward end  22 , which abuts on an outer circumference portion  433  of the second side (z negative side)  431  of the larger section  430 . Therefore, center plate  400  is clamped axially between the shoulder surface of step  13  of main housing member  10  and the forward end  22  of cover member  20  so that center plate  400  is unable to move in the axial direction along the z axis. 
     [Side Plates] 
     First and second side plates  150  and  160  are members having the same 8-shaped form as shown in  FIG. 6 . Each of side plates  150  and  160  is formed with a drive shaft hole  153  or  163  and a driven shaft hole  154  or  164 .  FIG. 6  shows only the first side plate  150  and the following explanation is directed mainly to first side plate  150  since first and second side plates  150  and  160  are substantially identical and arranged substantially symmetrical with respect to an imaginary center radial plane (x-y plane) at the middle axially between the first and second side plates  150  and  160 . 
     Discharge region Dout 1  (or Dout 2 ) is formed around side plate  150  (or  160 ), as shown in  FIG. 2  and  FIG. 4 . Side plate  150  ( 160 ) includes a first annular section surrounding drive shaft hole  153  ( 163 ) and a second annular section surrounding driven shaft hole  154  ( 164 ). Side plate  150  ( 160 ) further includes a middle recessed portion  150   b  ( 160   b ) recessed in the y negative direction, from a y positive side  150   a  ( 160   a ), between the first annular section on the x positive side and the second annular section on the x negative side. 
     The middle recessed portion  150   b  ( 160   b ) of side plate  150  ( 160 ) is connected with the inlet or suction circuit  10   i  ( 20   i ) and arranged to supply the operating fluid therethrough. The y positive side  150   a  ( 160   a ) of side plate  150  ( 160 ) includes a driving side sealing curved surface  158   a  ( 168   a ) on the x positive side of the middle recessed portion  150   b  ( 160 ), and a driven side sealing curved surface  158   b  ( 168   b ) on the x negative side of the middle recessed portion  150   b  ( 160   b ). The sealing curved surfaces are used for sealing with the corresponding seal block  200 . 
     First side plate  150  includes a (8-shaped) z negative side surface  159  adapted to be in sliding contact with the first driving and driven gears  130 P and  140 P liquid-tightly. Similarly, second side plate  160  includes a (8-shaped) z positive side surface  169  adapted to be in sliding contact with the second driving and driven gears  130 S and  140 S liquid-tightly. 
     First side plate  150  includes a z positive side surface  155  in which a first seal ring  170  is provided , as shown in  FIG. 4 . Second side plate  160  includes a z negative side surface  165  in which a second seal ring  180  is provided. Each of first and second seal rings  170  and  180  surrounds the drive shaft  110  and driven shaft  120  (as shown in  FIG. 2 ). First seal ring  170  abuts on main housing member  10 . Second seal ring  180  abuts on cover member  20 . 
     Thus, each of the seal rings  170  and  180  surrounds the sliding surfaces between the drive and driven shafts  110  and  120  and the first or second side plate  150  or  160 , and thereby defines the suction region Din (first fluid chamber) sealed off liquid-tightly from the discharge region Dout (second fluid chamber) formed on the outer side of the seal ring  170  or  180 . 
     The side plate  150  ( 160 ) includes two grooves  156  ( 166 ) recessed radially inwards (along the x axis toward the center M, as shown in  FIG. 2 ), respectively, from the x-axis positive and negative end surfaces  157  ( 167 ), at the middle of the width in the z-axis direction (as shown in  FIG. 6 ) (by cutting, for example). 
     Therefore, in the x-y plane, on each of the x-axis positive side and negative side of side plate  150  ( 160 ), the bottom of the groove  156  ( 166 ) and the x-axis end surface  157  ( 167 ) are curved, so as to form a rounded end like a circular arc, with unequal curvatures. The z-axis width of each of the grooves  156  ( 166 ) as measured along the z axis, is greater than the z-axis width of each of metal bands  301  and  302  of the leaf spring  300 . 
     Thus, the grooves  156  and the z positive side surface  151  of first side plate  150  are formed so as to have different curvature, and the grooves  166  and the z negative side surface  161  of second side plate  160  are formed so as to have different curvature. Moreover, the grooves  156  and  166  have the z-axis width greater than the z-axis width of the metal bands  301  and  302 . Thus, the grooves  156  ( 166 ) of side plate  150  ( 160 ) are formed so as to prevent abutment between the leaf spring  300  and the grooves  156  ( 166 ) when leaf spring  300  is fit over pump assembly  100 . 
     Therefore, the leaf spring  300  touches the side plates  150  ( 160 ) only at the contact points B and C with the inner sides of both end portions  321  and  322 , and thereby forms the three-point support structure together with the contact point A of abutment between leaf spring  300  and seal block  200 . The grooves  156  ( 166 ) prevent interference of legs  320  of leaf spring  300  with the x-axis ends  157  ( 167 ) of side plates  150  ( 160 ), and ensure the three-point support structure. 
     [Seal Blocks] 
     The following explanation is directed mainly to the first seal block  200  shown in  FIG. 2  and the first side plate  150 . The second seal block  200  and second side plate  160  are substantially identical to the first seal block  200  and first side plate  160 , and arranged substantially symmetrical with respect to the imaginary center radial plane (x-y plane) at the middle axially between the first and second side plates  150  and  160 . The seal block  200  shown in  FIG. 2  is placed between the abutting surface  12   a  of housing  1  on the y positive side and the side plate  150  ( 160 ) on the y negative side, and designed to achieve sealing. Seal block  200  includes an arched y positive side surface  240  facing in the y positive direction in the form of a convex surface curved like a circular arc and abutting on the abutting surface  12   a  of housing main member  10  for positioning, and a y negative side surface abutting on the side plate  150  ( 160 ). The y negative side surface of seal block  200  includes a driving side arched (concave) sealing surface  210  (shown in  FIGS. 2 and 3 ) and a driven side arched (concave) sealing surface  220  (shown in  FIGS. 2 and 3 ) which are curved like a circular arc for abutting liquid-tightly, respectively, on the driving side arched (convex) sealing surface  158   a  (shown in FIG.  6 )( 168   a  and the driven side arched (convex) sealing surface  158   b  (shown in FIG.  6 )( 168   b ) of the 8-shaped side plate  150  ( 160 ). The mating (concave and convex) arched surfaces of the seal block  200  and the side plate  150  ( 160 ) are cylindrical surfaces having the same curvature. 
     In the pump driving state, the tops ( 131 ) of teeth of the driving gear  130  rotate on the radial outer side of the corresponding sealing surface  158   a  ( 168   a ) of the side plate  150  ( 160 ), and the tops ( 141 ) of teeth of the driven gear  140  rotate on the radial outer side of the corresponding sealing surface  158   b  ( 168   b ) of side plate  150  ( 160 ). 
     Therefore, these sealing surfaces of seal blocks  200  are ground by the tops of the gear teeth so as to form tooth contact surfaces ( 211 ,  221 ). The thus-formed structure can ensure the sealing by reducing the clearance almost to zero while avoiding contact between the sealing surfaces and the gear teeth. 
     Seal block  200  further includes a middle recessed portion  230  which extends, between the sealing surfaces  210  and  220  (as shown in  FIG. 2 ), over the entire axial width along the z axis, and which is recessed in the y positive direction. Together with the middle recessed portion  150   b  ( 160   b ) of the side plate  150  ( 160 ), this middle recessed portion  230  of seal block  200  defines an inlet region Din for introducing the operating fluid (oil) from the suction circuit  10   i  ( 20   i ) to the engaging portions of the driving and driven gears  130  and  140 . 
     The y positive side surface  240  of seal block  200  is curved like a circular arc, as shown in  FIG. 2 . Seal block  200  includes z positive and negative sides  201  ( FIG. 2) and 202  formed with (third and fourth) step portions  251  ( FIG. 2) and 252 , respectively. Each of the step portions  251  and  252  has a convex shape bulging in the y positive direction up to a y positive side end (peak)  253  or  254  which is located on the median plane represented by the II-II straight line, and which abuts on the leaf spring  300  and thereby defines the first contact point A. 
     Seal block  200  further includes a projecting portion  250  projecting in the y positive direction between the step portions  251  and  252 . The metal bands  301  and  302  of leaf spring  300  abut against the seal block  200  at the contact point A. Thus, leaf spring  300  is fit over the projecting portion  250  of seal block  20  and thereby positioned. Moreover, leaf spring  300  abuts against the steps portions  251  and  252  of seal block  200 , and thereby limits the movement of seal block  200  in the y positive direction. 
     The distance between the z positive side surface  155  of first side plate  150  and the z negative side surface  165  of second side plate  160  is set equal to the distance between the seal blocks  200  along the z axis. Therefore, the seal rings  170  and  180  provided in the first and second side plates  150  and  160  abut axially on the seal blocks  200  along the z axis, respectively. Thus, the driving side and driven side sealing surfaces  158   a  ( 168   a ) and  158   b  ( 168   b ) of the side plate  150  ( 160 ) and the sealing surfaces of the seal block  200  are sealed by the seal ring  170  ( 180 ) on the z positive or negative side end surface. 
     [Liquid Pressure Difference] 
     By the operation of the driving and driven gears  130  and  140  in the pump drive state, the operating fluid is sucked from the z negative side of the inlet passage Din, and discharged to from the z positive side. Therefore, the pressure difference is produced in the y negative direction by the pump operation between the higher pressure discharge side formed on the outer side around the pump assembly  100  and the seal block  200  (excepting the abutting surface) and the lower pressure suction side of the abutting surface of the seal block  200 . By this pressure difference, pump assembly  100  is urged in the y positive direction, and seal block  200  is urged in the y negative direction and pressed against pump assembly  100 . Therefore, the pressure difference acts to improve the sealing performance in the abutting surfaces between pump assembly  100  and seal block  200 . 
     [Leaf Spring] 
     Leaf spring  300  shown in  FIG. 7  is a resilient member for provisionary uniting or binding the pump assembly  100 . Leaf spring  300  is bilaterally symmetrical, in the shape and elastic force, with respect to a median plane passing through a middle point A′ located at the middle in the dimension in the x direction. By the use of leaf spring  300 , it is possible to avoid influence of elastic force decrease due to time degradation unlike a coil spring. 
     Leaf spring  300  includes the first and second metal bands  301  and  302  extending side by side (in parallel to each other) so as to describe the letter C from a first end  321  to a second end  322 , and connecting portions  303 ˜ 306  connecting the first and second metal bands  301  and  302  like rungs of a ladder. Connecting portions  305  and  306  extend in the z direction, respectively, at the first and second ends  321  and  322  of leaf spring  300 . Connecting portions  303  and  304  extend in the z direction so that the seal blocks  200  are placed between the connecting portions  303  and  304  in the x direction. 
     An engagement hole  311  is formed by connecting portions  303  and  304  on the y positive side and the first and second metal bands  301  and  302 . The projecting portion  250  of seal block  200  is fit in the engagement hole  311 . Engagement hole  311  facilitates the positioning operation at the time of assemblage. 
     Leaf spring  300  is curved so as to bulge in the y positive direction like a mountain, and to have a vertex at the middle A′ in the x dimension. The vertex point (or points) A′ is located on the II-II straight line as shown in  FIG. 2 , and the leaf spring  300  is designed to deform in the symmetrical manner with respect to the median plane (II-II). A middle portion  310  of leaf spring  300  straddles the seal block  200  and abuts against the seal block  200  so that the point A′ of leaf spring  300  is in point contact with the point A of seal block  200 . In the assembled state, the points A and A′ coincide with each other. 
     On the both sides of the middle portion  310  in the dimension in the x direction, leg portions  320  extend in the y negative direction, respectively, to the first and second ends  321  and  322 , and fit over the side plate  150  ( 160 ). The first and second ends  321  and  322  of leaf spring  300  abut on the side plate  150  ( 16 ) so that contact points B′ and C′ on the inner sides of first and second ends  321  and  322  are in point contact with the contact points B and C of the (first and second) side plate  150  ( 160 ), respectively. 
     The shapes of first and second side plates  150  and  160  are not limited to the illustrated example. Side plates  150  and  160  and leaf springs  300  may be shaped in other forms to produce the urging forces having components in the y positive direction and to prevent interference between the legs  320  and the end surfaces  157  and  167 . 
     [Reduction of Friction and Leak] 
     Driving and driven gears  130  and  140  are identical in construction, between first and second pump P 1  and P 2  for the P and S circuits, and both pumps are driven by one and the same drive shaft  110 . Moreover, the pump chambers Dout 1  and Dout 2  of first and second pumps P 1  and P 2  are formed by the first and second side plates  150  and  160  having the same shape and the first and second seal blocks  200  having the same shape. Therefore, in the normal state, the discharge flow rates and the discharge pressures are equal between the first and second pumps P 1  and P 2 , so that center plate  400  receives forces by the equal discharge pressures on both sides along the z axis, and the forces acting on center plate  400  are balanced. 
     However, when either of the P and S circuits fails or when the pressures of the P and S circuits are controlled at different levels, the pressure on one side of center plate  400  becomes higher than the pressure on the other side, and the balance is lost among the force acting in the z direction on center plate  400 , so that center plate  400  is pushed in the z positive direction or the z negative direction. 
     When, for example, center plate  400  is moved in z positive direction, the center plate  400  pushes the gears  130 P and  140 P axially, and increases the friction. On the other hand, the center plate  400  increases the clearance between center plate  400  and the gears  130 S and  140 S on the z negative side by being moved in the z positive direction away from the gears  130 S and  240 S, and thereby increases the leakage. When center plate  400  is moved in the z negative direction, the same problem arises in the reverse manner. 
     According to the first embodiment, an outer encasing or housing wall structure ( 10 ,  20 ) encases first and second pump sections (P 1 , P 2 ) each including at least one rotating element ( 130 ,  140 ), and includes a surrounding (or circumferential) wall (formed by housing main body  10 ) surrounding the first and second pump sections disposed in a central region, a first end wall (formed by housing main member  10 ) and a second end wall (formed by cover member  20 ). There is further provided a partition or partition wall (formed by center plate  400 ) separating the first and second pump sections liquid-tightly (together with a seal member such as seal members  31 ,  32 ,  34  and  35 ). The partition includes a first abutting surface (or step surface formed by step  410 ) facing in a first axial direction (z positive direction) toward the first end wall, and a second abutting surface (end surface  433 ) facing toward the second end wall ( 20 ) in a second axial direction (z negative direction) opposite to the first axial direction. The first abutting (step) surface ( 410 ) and the second abutting (end) surface ( 433 ) surround a central region in which rotating elements ( 130 ,  140 ) of the first and second pump sections are disposed, and the first and second abutting surfaces ( 410 ,  433 ) may be both annular. The surrounding wall (formed by housing main member  10 ) includes a shoulder surface (formed by step  13 ) which faces in the second axial direction (z negative direction) and which may be annular, and the second end wall (formed by cover member  20 ) includes a projecting end surface ( 22 ) which faces in the first axial direction (z positive direction) and which may be annular. In the assembled state, the (annular) shoulder surface ( 13 ) of the surrounding wall ( 10 ) abuts axially on the (annular) first abutting (step) surface ( 410 ) of the partition ( 400 ), and thereby limits the movement of the partition ( 400 ) in the first axial direction. On the other hand, the (annular) projecting end surface ( 22 ) of the second end wall ( 20 ) abuts axially on the (annular) second abutting surface ( 433 ) of the partition ( 400 ) and thereby limits the movement of the partition ( 400 ) in the second axial direction. This partition structure holds the position of partition ( 400 ) fixed at the predetermined position without regard to the pressure states of the first and second pumps (for the P and S systems), and thereby prevent undesired increase in the friction and leak. 
     The second end wall ( 20 ) is fastened to the surrounding wall ( 10 ) by fastening devices (such as bolts B) extending axially (along the z axis), so that the second end wall ( 20 ) pushes the partition ( 400 ) in the first axial direction (z positive direction). This axial pushing force is applied, through the first abutting surface ( 410 ) of the partition ( 400 ), to the shoulder surface ( 13 ) of the surrounding wall ( 10 ), and the partition ( 400 ) is pushed in the first axial (z positive) direction against the surrounding wall ( 10 ). Furthermore, this pushing force acts to produce a friction force between the first abutting surface ( 410 ) of the partition ( 400 ) and the shoulder surface ( 13 ) of the surrounding wall ( 10 ), and this frictional force acts to restrain rotational movement of the partition ( 400 ) with respect to the surrounding wall ( 10 ). 
     The surrounding wall ( 10 ) receives the pushing force through the first abutting surface ( 410 ) and the shoulder surface ( 13 ). Therefore, this abutment structure can prevent the pushing force from being applied from the second end wall ( 20 ), to the rotating elements ( 130 P,  140 P,  130 S and  140 S) of the first and second pump sections, and thereby eliminates the need for controlling the tightening torque of the fastening devices (B) severely. 
     Effects of First Embodiment  
     (1) A tandem pump apparatus including at least a tandem pump which includes a first pump section (P 1 ) including a first rotating element ( 130 P,  140 P) for producing a first fluid pressure, a second pump section (P 2 ) including a second rotating element ( 130 S,  140 S) for producing a second fluid pressure, a rotating shaft ( 110 ) driving the first and second rotating elements, a housing including a housing wall structure ( 10 ,  20 ). The housing wall structure includes a surrounding wall ( 10 ) surrounding and defining a pump receiving portion or inside cavity ( 12 ) which includes a first discharge region (Dout 1 ) receiving the first rotating element ( 130 P,  140 P) and a second discharge region (Dout 2 ) receiving the second rotating element ( 130 S,  140 S), and a partition ( 400 ) extending in the pump receiving portion ( 12 ) and separating the first and second discharge regions liquid-tightly from each other. The partition ( 400 ) is engaged with the surrounding wall ( 10 ) so that the position of the partition ( 400 ) is determined by the surrounding wall ( 10 ) at least in an axial direction along the axis of the rotating shaft ( 110 ). This tandem pump apparatus can fix the axial position of the partition ( 400 ), and reduce the friction and the leak of the operating fluid. 
     (2) The housing wall structure ( 10 ,  20 ) is arranged to prevent rotational movement of the partition ( 400 ) about the axis of rotating shaft  110 . Therefore, the tandem pump apparatus can fix the partition ( 400 ) securely. 
     (3) The housing wall structure ( 10 ,  20 ) includes a first housing member ( 10 ) including the surrounding wall (and the first end wall) and a second housing member ( 20 ) including the second end wall. Therefore, this structure makes it easier to install a pump assembly ( 100 ) including the first and second pump sections in the housing wall structure. 
     (4) The partition of the first embodiment is in the form of a plate member ( 400 ) which is separate from the first housing member ( 10 ), and which is positioned by being interposed or clamped, at the outer circumferential (or annular) portion ( 433 ), between the first housing member ( 10 ) and the second housing member ( 20 ). Therefore, this tandem pump apparatus can position the plate member ( 400 ) at a position avoiding interference with the rotating elements ( 130 ,  140 ) of the pump sections. 
     (5) The pump receiving portion ( 12 ) includes a first (smaller) portion ( 14 ) and a second (larger) portion ( 15 ) which is greater in the cross sectional size than the first portion, and the surrounding wall ( 10 ) of the first housing member ( 10 ) includes a first wall portion surrounding and defining the first (smaller) portion ( 14 ), a second wall portion surrounding and defining the second (larger) portion, and a step ( 13 ) which is formed between the first and second wall portions and which includes the shoulder surface in the pump receiving portion ( 12 ). The plate member ( 400 ) of the partition is positioned by being clamped between the step ( 13 ) of the first housing member ( 10 ) and the second housing member ( 20 ). Therefore, tandem pump apparatus can determine the position of the partition ( 400 ) relative to the surrounding wall ( 10 ) reliably. In the illustrated example, the pump receiving portion ( 12 ) is substantially circular in the cross section, and the plate member ( 400 ) is clamped between the first and second housing members ( 10 ,  20 ), in an outer annular zone surrounding a center zone in which the first and second pump sections are disposed, so that the plate member ( 400 ) is positioned securely. 
     (6) Each of the first and second pump sections includes a driving gear ( 130 ) drivingly connected with the rotation shaft ( 110 ) and a driven gear ( 140 ) engaged with the driving gear, the first pump section ( 130 P,  140 P) is interposed (axially) between a first side plate ( 150 ) and a first side portion ( 150 ′) of the partition ( 400 ), the second pump section ( 130 S,  140 S) is interposed (axially) between a second side plate ( 160 ) and a second side portion ( 160 ′) of the partition ( 400 ), and the first and second side portions ( 150 ′,  160 ′) are integral parts of the partition. 
     (7) The apparatus comprises the tandem pump (1); a wheel cylinder set including a first subset including at least a first wheel cylinder (W/C) provided for a first wheel of a vehicle and a second subset including at least a second wheel cylinder (W/C) provided for a second wheel of the vehicle; a hydraulic system (HU) including a first subsystem which includes at least a first control valve (IN-V, OUT-V)( and which connects the first pump section (P 1 ) with the first wheel cylinder to increase a first fluid pressure of the first wheel cylinder, and a second subsystem which includes at least a second control valve (IN-V, OUT-V) and which connects the second pump section (P 2 ) with the second wheel cylinder to increase a second fluid pressure of the second wheel cylinder. The apparatus may further comprises a control unit (CU) to control the first fluid pressure to the first wheel cylinder and the second fluid pressure to the second wheel cylinder, respectively, by controlling the first and second pump sections and the first and second control valves. Therefore, the hydraulic brake apparatus can fix the position of the partition ( 400 ) reliably and prevent undesired increase of the friction and leak even if one of the first and second systems becomes abnormal or if the first and second systems require different target pressure levels. 
     (8) A tandem pump comprises volume change preventing means, provided in the pump receiving portion ( 12 ), for preventing volume change of the first and second pump chambers (Dout 1 , Dout 2 ), by being positioned by the housing. This tandem pump can prevent the volume changes in the first and second pump chambers, and thereby reduce the friction and the leak of the operating fluid. The volume change preventing means may include a partition ( 400 ) separating the first and second pump chambers. Alternatively, the volume change preventing means may include a means for preventing the partition ( 400 ) from being moved (at least in the axial direction)(by fluid pressures in the first and second pump chambers). 
     (9) The fluid pressures of the first and second pump chambers (Dout 1 , Dout 2 ) are controlled at different pressure levels, and the partition ( 400 ) is held immovable in the axial direction and in the rotational direction. Therefore, this tandem pump can prevent volume changes in the first and second pump chambers, and thereby reduce the friction and the leak of the operating fluid. 
     Embodiment 2  
     The second embodiment is substantially identical, in the basic construction and arrangement in the hydraulic circuit, to the first embodiment shown in  FIGS. 1˜7 . The second embodiment is different from the first embodiment in that, instead of the fastening devices (bolts B) for fastening cover member  20  to main housing member  10 , the cover member  20  according to the second embodiment is formed with a threaded portion ( 23 ) and screwed into a mating threaded portion ( 16 ) of main housing member  10 . 
       FIG. 8  shows a tandem pump P according to the second embodiment in an axial section (in place of  FIG. 4  of the first embodiment). Cover member  20  of  FIG. 8  includes a threaded portion  23  formed externally in the outer circumference of the cylindrical projecting portion  21 . On the other hand, main housing member  10  includes a threaded portion  16  formed internally in the inside cylindrical surface defining the larger portion  15  of the pump receiving portion  12 . In this example, the externally threaded portion  23  includes a male screw thread while the internally threaded portion  16  includes a corresponding female screw thread. 
     Cover member  20  is fixed to main housing member by screwing the externally threaded portion  23  into the internally threaded portion  16  formed in the surrounding wall formed by main housing member  10 . Cover member  20  abuts on center plate  400  axially with the forward end  22  of cylindrical projecting portion  21 , and thereby determines the position of center plate  400  in the same manner as in the first embodiment, so that the same effects can be obtained. 
     Embodiment 3  
     The third embodiment is substantially identical, in the basic construction and arrangement in the hydraulic circuit, to the first embodiment. The third embodiment is different from the first embodiment in that the center plate  400  is divided into two plates ( 400 P,  400 S). 
       FIG. 9  is a sectional view similar to  FIG. 4 , but showing the tandem pump of the third embodiment.  FIGS. 10 and 11  are perspective views showing, respectively, a first center plate  400   p  on the z positive side, and a second center plate  400 S located on the z negative side of first center plate  400 P. 
     Although seal members  34  and  35  are provided in the outer circumferences of drive and driven shafts  110  and  120  in the first embodiment, the seal members  34  and  35  according to the second embodiment are fit, respectively, in (annular) seal grooves  401   a  and  402   a  formed in the inner circumferences of drive shaft through hole  401 S and driven shaft through hole  402 S of second center plate  400 S. 
     Second center plate  400 S includes a larger disc portion having an annular z positive surface (K), and a center projecting portion projecting in the z positive direction from a central region of the larger disc portion, and having an z positive end surface (K, N). First center plate  400 P includes a smaller disc portion forming the smaller section  420 , an outside flange  410 ′ forming the step  410 , and a center recess which is recessed from the z negative side of first center plate  400 P in the z positive direction and which is adapted to receive the center projecting portion of second center plate  400 S fittingly. Therefore, the center plate  400  is divided into first and second center plates  400 P and  400 S in a parting plane K (shown by a solid line in  FIG. 9 ) composed of an outer annular flat region in which a flat annular z negative surface of the flange  410 ′ of first center plate  400 P abuts on a flat annular z positive surface of the larger disc portion of second center plate  400 S, and a central flat region which extends in an imaginary flat plane N parallel to the x-y plane whereas the outer annular flat region of the parting plane K is located on the z negative side of the flat plane N. In the center flat region of the parting plane K, the flat z positive side surface of the center projecting portion of second center plate  400 S abuts on or confronts the (flat) bottom of the center recess of first center plate  400 P. The center flat region of parting plane K extends around the drive shaft  110  and driven shaft  120 . Thus, parting plane K includes a step K 1  between the outer annular region and the center region. The first and second grooves  401   a  and  402   a  are opened in the center region of parting plane K at  401   b  and  402   b , respective, to the z positive side. The imaginary plane N is located on the z positive side of the step  13  of main housing member  10 , and the center projecting portion of second center plate  400 S extends beyond the axial position of step  31 , to the z positive side. The outer annular flat region of parting plane K is located on the z negative side of the axial position of step  13  whereas the central flat region of parting plane K is located on the z positive side of the axial position of step  13  of main housing member  10 . 
     The z positive side surface of flange  410 ′ of first center plate  400 P abuts on the step  13  of main housing member  10 , and thereby positions first center plate  400 P securely. Therefore, even if first center plate  400 P is pushed in the z positive direction because of leak from the S route discharge circuit  20   h  located on the z negative side of step  410  ( 410 ′), the first center plate  400 P is held immovable at the correct position without increasing the friction with the first driving and driven gears  130 P and  140 P. 
     The parting plane K includes regions forming z positive side  401   a  of  402   b  of seal grooves  401   a  and  402   a . Thus, the seal grooves  401   a  and  402   a  are open in the parting plane K, toward the z positive side, and the seal members  34  and  35  are bared in the z positive side surface of second center plate  400 S. Therefore, seal members  34  and  35  can be fit readily in the respective grooves at the time of the assembly. 
     The (annular) forward end  22  of cover member  20  abuts on the outer annular region  433  of the larger disc portion of second center plate  400 S as in the first embodiment. The flange  410 ′ of first center plate  400 P and the larger disc portion of second center plate  400 S form the larger section of the center plate clamped between the step  13  of main housing member  10  and the cylindrical projecting portion  21  of cover member  20 , as in the first embodiment. Thus, first and second center plates  400 P and  400 S are positioned and fixed immovable as a single unit in the z direction. 
     In the tandem pump according to the first embodiment, the S route discharge circuit  20   h  is formed by drilling in center plate  400 . In the illustrated example of the third embodiment, the S route discharge circuit  20   h  is formed partly by one or more cutout portion  411  formed in the flange  410 ′ of first center plate  400 P. 
     The cutout portion  411  is connected with an oil passage ( 424 ) of second center plate  400 S. This oil passage ( 424 ) is opened by drilling like oil passage  20   h  of the first embodiment, and connected with the suction region Din 2  of second pump P 2 . Thus, this oil passage ( 424 ) and cutout portion  411  form the S route discharge circuit  20   h.    
     It is possible to form one or more cutout portion  411  in first center plate  400 P readily (without the need for adding a special manufacturing step) by forming the shape of the cutout portion in a mold for forming first center plate  400 P at the time of forming the mold (by sintering, for example). 
     In the example of  FIGS. 9-11 , the step  410  and smaller section  420  are formed in first center plate  400 P, and the larger section  430  is formed in second center plate  400 S. Like center plate  400  of the first embodiment, the smaller section  420  of first center plate  400 P includes the z positive side which includes the sliding surface  421   a  and the seal block abutting surface  421   b . The larger section  430  of second center plate  400 S includes the z negative side  431  including the sliding surface  431   a  and the seal block abutting surface  431   b.    
     In the example of  FIGS. 9˜11  according to the third embodiment, the plate member ( 400 ) of the partition includes a first plate member ( 400 P) formed with a through hole ( 401 P) for receiving the rotation shaft ( 110 ), a second plate member ( 400 S) formed with a through hole ( 401 S) for receiving the rotation shaft, and a seal groove ( 401   a ) which is formed (axially) between the first and second plate members and which receives a seal member ( 34 ) surrounding the rotation shaft ( 110 ). Therefore, the third embodiment can provide the same effects as the first embodiment. Furthermore, it becomes easier to form the center plate ( 400 ) with smaller molds for the first and second plate members ( 400 P,  400 S). The seal member ( 34 ) is bared in a parting plane (K) between the first and second plate members, so that it is easier to fit the seal member in the seal groove ( 401   a ). Seal member  35  can be fit easily in the seal groove  402   a  in the same manner. 
     Embodiment 4  
     According to a fourth embodiment, the center plate  400  and main housing member  10  are formed as a single integral member. As in the first, second and third embodiments, the main housing member  10  is made of softer metallic material (aluminum alloy) and the rotating elements of the pump ( 130 ,  140 ) are made of harder metallic material (Fe based material). 
       FIG. 12  shows the tandem pump P according to the fourth embodiment in the form of an axial section. The main housing member  10  shown in  FIG. 12  includes a center partition wall serving as center plate  400 . The center wall is formed with the drive shaft through hole  401  and the driven shaft through hole  402 . The main housing member  10  includes a z positive (first) side surface formed with a first recessed portion  12   d  recessed in the z negative direction from the z positive side surface facing in the z positive direction, and a z negative (second) side surface formed with a second recessed portion  12   c  recessed in the z positive direction from the z negative side surface of the main housing member  10 . The first side plate  150  is disposed in the first recessed portion  12   d , and the second side plate  160  is disposed in the second recessed portion  12   c.    
     The center plate  400  is integral with the main housing member  10 , so that the partition formed by the center plate  400  is stationary relative to main housing member  10 . Therefore, cover member  20  is not required to abut against the center plate  400  unlike the preceding embodiments. In the example of  FIG. 12 , the forward end  22  of the cylindrical projecting portion  21  of cover member  20  is spaced from center plate  400 . Accordingly, it is not necessary to increase the rigidity of the forward end  22  of cover member  20 , and it is possible to reduce the wall thickness of cover member  20 . 
     Since center plate  400  is integral with main housing member  10 , it is not possible to insert the pump assembly  100  from the z negative side into main housing member  10 . Instead, the first and second side plates  150  and  160  are inserted, respectively, from the z positive side and the z negative side. Therefore, there is provided an additional cover member  30  on the z positive side, in addition to the cover member  20  on the z negative side. The additional cover member  30  covers the first recessed portion  12   d  and thereby defines the first discharge region Dout 1  liquid-tightly. Seal members  34  and  35  for sealing the first and second pumps P 1  and P 2  from each other are provided around drive shaft  110  and driven shaft  120  as in the first embodiment. In the fourth embodiment, main housing member  10  includes the surrounding wall and partition, the cover member  20  include the second end wall and the additional cover member  30  includes the first end wall. The projecting portion  21  of cover member  20  is inserted in the second recessed portion  12   c  which is larger in sectional size or diameter than the first recessed portion  12   d  in the example of  FIG. 12 . 
     A first slide member  440  is provided between the partition  400  and the first driving and driven gears  130 P and  140 P in the first recessed portion  12   d . A second slide member  440  is provided between the partition  400  and the second driving and driven gears  130 S and  140 S in the second recessed portion  12   c . Since it is troublesome to form sliding surfaces in the partition  400  of  FIG. 12 , the slide members  440  are used to reduce the friction and improve the sliding contact characteristic without the need for an accurate finishing operation for forming sliding surfaces in center plate  400 . 
     According to the fourth embodiment, the partition ( 400 ) is integral with the housing ( 1 ). Therefore, the fourth embodiment can determine the position of the partition securely, and provide the same effects as in the first embodiment. Moreover, the use of the partition integral with the housing reduces the number of component parts. The fourth embodiment eliminates or reduce the need for increasing the rigidity of the cover member ( 20 ) which is not required to press the partition. In the fourth embodiment, it is possible to eliminate the seal members  31  and  32  to be provided around the center plate  400  (shown in  FIG. 4 ). With the slide members ( 440 ), it is possible to reduce the friction without the need for forming a precisely finished sliding surface in the center plate. 
     Embodiment 5  
     In the illustrated examples of the preceding embodiments, the tandem pump P is a combination of external gear pumps. According to a fifth embodiment, by contrast, the tandem pump P is a combination of internal gear pumps. 
       FIG. 13  shows the tandem pump P according to the fifth embodiment in axial section. This tandem pump P includes a first internal gear pump P 1  including first inner rotor  510 P and a first outer rotor  520 P, and a second internal gear pump P 2  including a second inner rotor  510 S and a second outer rotor  520 S. 
     The first and second inner rotors  510 P and  510 S are connected with and driven by the single common drive shaft  110 . First and second inner rotors  510 P and  510 S are engaged, respectively, with first and second outer rotors  520 P and  520 S, and arranged to produce the respective discharge fluid pressures independently. Unlike the examples of the external gear pumps, the tandem pump P of  FIG. 13  does not include the driven gear ( 120 ). 
     The main housing member  10  of  FIG. 13  includes the pump receiving portion  12  shaped like a hollow cylinder having a bottom as in the illustrated example of the first embodiment. The pump assembly  10  including first pump P 1 , center plate  400  and second pump P 2  is inserted into this pump receiving portion from the z negative side, and the pump receiving portion  12  is closed by the cover member  20  as in the illustrated example of the first embodiment. 
     The center plate  400  is substantially cylindrical, and includes an externally threaded portion  15 a in the outer circumference. On the other hand, the surrounding wall of main housing member  10  includes an internally threaded portion  12   e  in the inside wall of pump receiving portion  12 . Center plate  400  is positioned and fastened in main housing member  10  by screwing the externally threaded portion  15   a  of center plate  400  into the internally threaded portion  12   e  of main housing member  10 . In this example, the step  410  is made smaller. 
     The surrounding wall of main housing member  10  includes a first wall portion surrounding and defining the first (smaller) portion  14  of pump receiving portion  12 , and a second wall portion surrounding and defining the second (larger) portion  15 (which is greater in the cross sectional size than the first portion  14 ). In the example of  FIG. 13 , the internally threaded portion  12   e  formed in the second (larger) portion  15 , so that main housing member  10  can position the center plate  400  securely. 
     In the example of  FIG. 13 , the surrounding wall of main housing member  10  includes a third wall portion surrounding and defining a third portion which is greater in the cross sectional size than the second portion  15 . Correspondingly, the center plate  400  of  FIG. 13  includes a first portion, a second portion which is greater in cross sectional size than the first portion and which is formed with the externally threaded portion  15   a , and a third portion which is greater in cross sectional size than the second portion. The second portion formed with the externally threaded portion  15   a  is located axially between the first portion provided with a seal ring ( 31 ) and the third portion provided with a seal ring ( 32 ). 
     It is possible to employ the internal gear pumps in the first, second, third and fourth embodiments, instead of the external gear pumps. The fixing structure using the externally threaded portion  15   a  and the internally threaded portion  12   e  can be applied to the first through fourth embodiments. 
     This application is based on a prior Japanese Patent Application No. 2008-058970 filed on Mar. 10, 2008. The entire contents of this Japanese Patent Application are hereby incorporated by reference. 
     Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art in light of the above teachings. The scope of the invention is defined with reference to the following claims.