Abstract:
An oil pump assembly includes a plurality of pump chambers constituting a plurality of oil pumps, respectively, a plurality of rotors are fitted into the pump chambers, respectively, with a single driving shaft carrying the plurality of rotors to form a single body. Oil pumped up from an oil pan is supplied under pressure to different associated destinations with the relief valves being arranged compactly to reduce the size of the oil pump assembly. Oil pumps are provided with respective relief valves provided to pass oil discharged from the corresponding pump chambers therethrough and the axes of the relief valves are arranged parallel to the driving shaft. A driving shaft is provided on which a plurality of the inner rotors are carried concentrically and integrally. The rotors are connected to the driving shaft and changed depending on discharge oil pressures, thereby preventing the driving shaft from increasing its diameter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2004-341535 filed on Nov. 26, 2004 and Japanese Patent Application No. 2004-344656 filed on Nov. 29, 2004 the entire contents of which are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to an oil pump assembly integrally composed of a plurality of oil pumps through a common driving shaft, the oil pumps each including a relief valve. In addition, the invention relates to an oil pump structure integrally composed of a plurality of oil pumps having respective different discharge pressures, driven with common driving shaft, and to an oil pump structure combining the oil pumps with a water pump. 
     DESCRIPTION OF BACKGROUND ART 
     One example of a conventional technique relates to a pair of rotors that are carried on a driving shaft coaxially and integrally with a relief valve that is disposed between the pair of rotors and directed toward the driving shaft in a direction perpendicular thereto. See, for example, Japanese Patent Laid-open No. 2000-199413. If it is necessary to feed oil to the components of a power unit at respective different hydraulic pressures, oil pumps are provided at its discharge sides with relief valves having respective different pressures so that oil may be fed to destinations at respective different hydraulic pressures. However, in such an arrangement described above, the plurality of relief valves project in a direction perpendicular to the driving shaft, which increases the sizes of the oil pumps. Consequently, the oil pumps will largely occupy the layout-based restrictive space in the internal of the power unit. 
     One example of a conventional oil pump structure that includes a plurality of oil pumps having the same discharge pressure that are integrally combined with each other through a single driving shaft. See, for example, Japanese Patent Laid-open No. 2000-199413,  FIG. 3 . This example discloses that a plurality of rotors are secured to the driving shaft by means of corresponding connection pins. 
     If one of the plurality of oil pumps is for high pressure and the other is for low pressure, hydraulic pressures applied to the rotors are different from each other, which leads to different torque for driving the rotors. As the torque is increased, it is necessary to increase the diameter of the rotor connection pin. The increased diameter of the pin requires an enlarged pin insertion hole. Thus, an increased diameter of the driving shaft is required which is likely to increase the weight of the oil pump. 
     The present invention intends to solve the problem of the conventional technique described above and provide means for arranging a plurality of relief valves in a compact manner, thereby reducing the size of an oil pump assembly. 
     An embodiment of the present invention provides an oil pump assembly that includes a plurality of pump chambers constituting a plurality of oil pumps, respectively with a plurality of rotors fitted into the pump chambers, respectively. A single driving shaft carries the plurality of rotors to form a single body wherein oil pumped up from an oil pan is supplied under pressure from the plurality of oil pumps to different associated destinations in a power unit. The oil pumps are provided with respective relief valves adapted to pass oil discharged from the corresponding pump chambers therethrough and axes of the relief valves are arranged parallel to the driving shaft. 
     An embodiment of the present invention provides a discharge port for at least one of the relief valves that is disposed within a width of the rotors as viewed from a direction perpendicular to the driving shaft. 
     An embodiment of the present invention provides respective discharge ports of the relief valves that are disposed within a width of the rotors as viewed from a direction perpendicular to the driving shaft with the relief valves being arranged to have most portions of the full lengths that overlap each other. 
     An embodiment of the present invention provides an outer shell of the oil pump assembly that is composed of a pump case body and pump covers shielding lateral sides of the pump cover, the pump chambers are defined between the pump case body and the pump covers to house the rotors in the corresponding pump chambers. The plurality of relief valves are arranged to be inserted in cavities of the pump case body. 
     An embodiment of the present invention provides for a plurality of relief valves that are disposed between a strainer and the oil pumps as viewed from the lateral side of the power unit. 
     An embodiment of the present invention provides for the plurality of relief valves that are arranged parallel to each other along an oil stream line of an oil inflow passage extending from the strainer to the oil pumps. 
     According to an embodiment of the present invention, since the axes of the plurality of relief valves are arranged parallel to the driving shaft, the drive shaft can be brought close to the relief valves, so that the relief valves can be arranged compactly. 
     According to an embodiment of the present invention, since the discharge port of the relief valve is disposed within the width of the rotor as viewed from a direction perpendicular to the driving shaft, the relief valve can be brought close to the oil pump for a compact arrangement. In addition, the discharge oil passage is brought close to the pump suction port to shorten the relief oil passage, thereby simplifying the oil passages. 
     According to an embodiment of the present invention, since the respective discharge ports of the relief valves are disposed within the width of the rotors and the relief valves are arranged to have most portions of the full lengths that overlap each other, the plurality of the relief valves can be further arranged in a compact manner. Note that “to have most portions of the full lengths that overlap each other” means that “a plane perpendicular to the axes of the relief valves are shared by most of the portions of the full lengths of the relief valves”. 
     According to an embodiment of the present invention, since the oil pump assembly is composed of the pump case body and the pump covers shielding both the sides of the pump case body and the plurality of the relief valves are disposed by inserting them into the corresponding cavities of the pump case body, the number of components can be reduced by eliminating members used to support the relief valves. 
     According to an embodiment of the present invention, since the plurality of relief valves are disposed between the strainer and the oil pump as viewed from the lateral side of the power unit, an unused space between the strainer and the oil pumps is used to dispose the relief valves therein, thereby further reducing the size of the power unit. 
     According to an embodiment of the present invention, since the plurality of relief valves are arranged along the oil stream line extending from the strainer to the oil pump, the plurality of relief valves can be further arranged compactly while ensuring sufficient oil passages. 
     The present invention intends to prevent a driving shaft from increasing its diameter particularly for a high pressure pump by changing means for connecting rotors to the driving shaft depending on discharge oil pressures. 
     According to an embodiment of the present invention, the present invention solves the above problem by providing an oil pump assembly structure including a low pressure oil pump which includes a pump chamber, an outer rotor and an inner rotor that is adapted to pump oil in an oil pan and supply under pressure of the oil to components of a power unit. A high pressure oil pump which includes a pump chamber, an outer rotor and an inner rotor is adapted to pump oil in the oil pan and supply the oil under pressure to the other components of the power unit. A driving shaft is provided on which a plurality of the inner rotors are carried concentrically and integrally. The inner rotor of the low pressure oil pump is secured to the driving shaft by means of a retaining pin in such a manner so as to be unable to rotate with respective to the driving shaft. The inner rotor of the high pressure oil pump is fitted to a portion having a plurality of flat surfaces formed in the vicinity of an end of the driving shaft and is secured to the driving shaft in such a manner so as to be unable to rotate with respect to the driving shaft with the driving shaft being driven by receiving power of an internal combustion engine. 
     According to an embodiment of the present invention, oil pressurized in the pump chamber is symmetrically discharged from the rotor to both sides of the rotor, interflows and then is discharged from the oil pump. 
     According to an embodiment of the present invention, a water pump is provided adjacent to the low pressure oil pump of the oil pumps with a driving shaft of the water pump being disposed coaxially with the driving shaft of the oil pumps. A power transmission means is provided which receives power of the internal combustion engine and is located at an end of the oil pump driving shaft on a side of the low pressure oil pump with one of the end of the oil pump driving shaft and an end of the water pump driving shaft being formed in a projecting manner and the other being formed in a recessed manner. Thus, both the pump driving shafts are coupled to each other. 
     According to an embodiment of the present invention, since the inner rotor of the high pressure oil pump is fitted and secured to a portion having a plurality of flat surfaces formed in the vicinity of an end of the driving shaft, the strength of connection between the inner rotor and the driving shaft can be increased. Thus, it is unnecessary to increase the diameter of the driving shaft, thereby reducing the size of the oil pump. In addition, in the case of a flat surface fitting, not only the length of the flat surface can be set more freely but also the rotor having manufacturing or assembling errors can be attached to the driving shaft more freely in terms of the axial position thereof. 
     According to an embodiment of the present invention, since the oil that has passed the rotor is symmetrically discharged from the rotor to both the sides of the rotor, the pressure of the oil discharged to both sides provides a rotor-centering effect, which reduces the contact between the rotor and the pump chamber, thereby suppressing frictional resistance therebetween. Accordingly, a load acting on the driving shaft is reduced, so that the diameter of the driving shaft can be reduced. 
     According to an embodiment of the present invention, the power transmission means is provided at the end of the driving shaft on the side of the low pressure oil pump, the end of the driving shaft of the water pump is disposed coaxially with the end, and both the ends are fitted and coupled to each other. Therefore, a distance between the power transmission means and the projecting-recessed connection portion located at the end of the driving shaft is small, so that torsion acting on the driving shaft is reduced. Accordingly, it is unnecessary to reinforce the projecting-recessed connection portion and reduce the diameter of the driving shaft. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a lateral view of a motorcycle  1  according to an embodiment of the present invention; 
         FIG. 2  is a left side view of a power unit  2  mounted on the vehicle; 
         FIG. 3  is a cross-sectional development view taken along line III-III of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view taken along line IV-IV of  FIG. 2 ; 
         FIG. 5  is a cross-sectional view of the inside of a crankcase  20  as viewed from the right side; 
         FIGS. 6(   a ) and  6 ( b ) are assembly views of the oil pump assembly  90 ; 
         FIGS. 7(   a ),  7 ( b ) and  7 ( c ) are three-side view of a pump case body  95 ; 
         FIGS. 8(   a ),  8 ( b ) and  8 ( c ) are three-side view of a left cover  96 ; 
         FIGS. 9(   a ),  9 ( b ) and  9 ( c ) are three-side view of a right cover  97 ; 
         FIGS. 10(   a ),  10 ( b ),  10 ( c ) and  10 ( d ) are four-side view of a right outer side cover  98 ; 
         FIG. 11  is a cross-sectional view of a low pressure oil pump  90 L; 
         FIG. 12  is a cross-sectional view of a high-pressure oil pump  90 H; 
         FIG. 13  is a cross-sectional view of a low pressure relief valve  107 ; 
         FIG. 14  depicts the horizontal cross-section of the oil pump assembly  90  and discharge passages as viewed from above; and 
         FIG. 15  depicts the horizontal cross-section of the oil pump assembly  90 , the water pump  143  and an oil passage subsequent to the oil passage L 3  of the low pressure oil pump, as viewed from above. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a lateral view of a motorcycle  1  provided with a power unit according to an embodiment of the present invention. The motorcycle  1  has a pair of main frames  4  and a pair of subframes  5 . The pair of main frames  4  is continuous to a head pipe  3  and slants rearwardly and downwardly. The pair of subframes  5  extends downwardly from the lower portion of the head pipe  4 , bend rearwardly, and then the subframes  5  are joined at its ends to the rear ends of the main frames  4 , respectively. 
     The power unit  2  is integrally composed of an internal combustion engine  6  and a transmission  7  that are mounted in an almost-triangular space, as viewed laterally and as defined by the main frames  4  and the subframes  5 . The head pipe  3  rotatably supports a front fork  8 , which has an upper end to which a steering handlebar  9  is attached and a lower end which rotatably supports a front wheel  10 . The rear portions of the main frames  4  pivotally support the front ends of the pair of rear forks  11  so as to swing the rear forks  11  in an up-and-down direction. Rear shock absorbers (not shown) are attached between the middle portions of the rear forks  11  and the rear ends of the main frames  4 , respectively. A rear wheel  12  is rotatably supported by the rear ends of the rear forks  11 . 
     The internal combustion engine  6  mentioned above is a water-cooled V-type 2-cylinder internal combustion engine. The cylinders take a V-shape in the back-and-forth direction. A crankshaft of the engine  6  is disposed to be perpendicular to the advancing direction of the vehicle, that is, to extend in the lateral direction of the vehicle. A transmission shaft of the transmission  7  is parallel to the crankshaft mentioned above. A rear wheel driving shaft (not shown) is coupled to a connection shaft  85 , see  FIG. 2 , perpendicular to the output shaft of the transmission and extends rearwardly of the vehicle and reaches the rear wheel  12  for driving the rear wheel  12 . 
     An exhaust pipe  13  communicates with exhaust ports which are respectively disposed in the two cylinders at positions in the back and forth direction of the vehicle. The exhaust pipe  13  extends forward of the engine  6 , goes around below the transmission  7  and reaches the rear of the vehicle, at which it is connected to an exhaust muffler  14 . A fuel tank  17  is attached onto the body frame  4  and a seat  18  is attached to a portion in rear of the fuel tank  17 . The engine  6  is of a water-cooled type. Cooling water that has risen in temperature in the process of cooling the cylinders and oil is cooled by a radiator  19  attached to the front of the subframes  5 . 
       FIG. 2  is a left-hand lateral view of the power unit  2  mounted on the motorcycle. Arrow F indicates the front of the vehicle when the power unit is mounted thereon. The same holds true for the other figures. Since the front cylinder  24 F and the rear cylinder  24 R have the same configuration, the cross-section of the rear cylinder  24 R only is depicted. The crankcase portion is shown with a left-hand crankcase cover removed so as to indicate the respective internal positions of the primary rotary shafts, gears and sprockets. 
       FIG. 3  is a cross-sectional development view taken along line III-III of  FIG. 2 .  FIG. 3  develops the section including the rear cylinder  24 R, the crankshaft  30  and a transmission shaft  66  of a static hydraulic type continuously variable transmission  55 . The rear cylinder  24 R is adapted to hold a piston  33  connected to a left side crankpin  31 . 
     Referring to  FIGS. 2 and 3 , the main outer shells of the power unit  2  include a left crankcase  20 , a right crankcase  21 , a left crankcase cover  22 , and a right crankcase cover  23 , as well as a cylinder block  25 , a cylinder head  26  and a cylinder head cover  27  provided for each of the front cylinder  24 F and the rear cylinder  24 R. 
     In  FIG. 3 , the crankshaft  30  is journaled by a left bearing  28  and a right bearing  29  held by the left crankcase  20  and the right crankcase  21 , respectively. A connecting rod  32  and the piston  33  are connected to a left crankpin  31  of the crankshaft  30 . The piston  33  is slidably held within a cylinder bore  34  of the cylinder block  25 . A combustion chamber  35  is formed at a portion of the cylinder head  26  that faces the piston  33 . An ignition plug  36  is placed to pierce a wall body of the cylinder head  26  in such a manner that its top faces the combustion chamber  35  and its rear end is exposed to the exterior. 
     Referring to  FIG. 2 , an exhaust port  40  and an intake port  41  are each continuous with the combustion chamber  35 . The exhaust port  40  of the front cylinder  24 F extends forward, whereas that of the rear cylinder  24 R extends rearwardly. The intake port  41  of any of the cylinders extends upwardly in a space defined between both the cylinders. An exhaust valve  42  is provided at the exhaust port  40  and an intake valve  43  is provided at the intake port  41 . A camshaft  44  is placed in the cylinder head cover  27 . An exhaust rocker arm shaft  45  and an intake rocker arm shaft  46  are provided above the camshaft  44 . An exhaust rocker arm  47  and an intake rocker arm  48  attached to the respective arm shafts are driven by cams  44   a  and  44   b  of the camshaft  44  to push the stem tops of the exhaust valve  42  and the intake valve  43 , respectively, thereby drivingly opening and closing the corresponding valves. Referring to  FIG. 3 , the camshaft  44  is driven by a camshaft driving chain  51  wound around a camshaft driven sprocket  49  provided at the end of the camshaft  44  and a camshaft driving sprocket  50  provided at the crankshaft  30 . 
     In  FIG. 3 , the static hydraulic continuously variable transmission  55  is placed in rear of the crankshaft  30 . This transmission is an apparatus integrally combining a centrifugal governor  56 , a swash plate type hydraulic pump  57  and the swash plate type hydraulic motor  58  through the transmission shaft  66 . A crankshaft output gear  37  attached to the left end of the crankshaft  30  is a gear functioning integrally with a cam type torque damper  38  adjacent thereto. The gear  37  has a meshing engagement with a transmission input gear  60  integrally joined to the casing  61  of the swash plate type hydraulic pump  57 . 
     A crankshaft output gear  37  and a cam type torque damper  38  are carried on a collar spline-joined to the crankshaft  30 . The crankshaft output gear  37  is rotatably fitted onto the collar  155  and is formed on its side surface with a recessed cam  37   a  having an arc-shaped recessed surface. A lifter  156  is axially movably fitted to a spline formed on the outer circumference of the collar  155  and is formed at its end surface with a projecting cam  156   a  having an arc-shaped projecting surface. The projecting cam  156   a  has a meshing engagement with the recessed cam  37   a . A spring holder  157  is fastened to an end of the collar  155  by means of a spline and a cotter. Disc springs are placed between the spring holder  157  and the lifter  156  so as to urge the projecting cam  156   a  toward the recessed cam  37   a.    
     In a case of a steady speed operation, the torque of the crankshaft  30  is transmitted through the collar  155 , lifter  156 , projecting cam  156 , recessed cam  37   a  and crankshaft output gear  37  in this order. The crankshaft output gear  37  is rotated together with the crankshaft  30 . If excessive torque is supplied to the crankshaft  30 , while sliding on the cam surface of the recessed cam  37   a  in a circumferential direction, the projecting cam  156   a  axially moves against the urging force of the disc springs  158  to absorb the excessive torque, thereby alleviating a shock. 
     The crankshaft output gear  37  is a backlash reduction gear and is composed of a main gear  160  having a thick center portion, a thin sub-gear  161  which is carried by the main gear  160  in such a manner as to be coaxially rotatable with respect to the main gear  160 , and a coil spring  162  which circumferentially urges the sub-gear  161  to the main gear  160 . When a backlash reduction gear comes into a meshing engagement with an ordinary gear, a sub-gear is urged circumferentially to move for filling in a backlash gap defined between a main gear and the ordinary gear. This eliminates backlash and thereby provides reduced noise and quietness. In the present embodiment, the meshing engagement between the crankshaft output gear  37  and the transmission input gear  60  does not make any noise. 
     A casing  62  of the centrifugal governor clutch  56  is integrally joined to a casing  61  of the swash plate type hydraulic pump  57 . If the rotating speed reaches or exceeds a certain level, a centrifugal weight  63  (e.g., a steel roller, a steel ball or the like) housed in the casing  62  of the centrifugal governor clutch  56  pushes a moving member  64 , which moves a hydraulic circuit switching rod  65  coupled to the moving member  64 , in the transmission shaft  66 . Thus, the hydraulic circuit switching rod  65  closes an oil passage adapted to circulate oil discharged from the swash plate type hydraulic pump  57 , thereby causing a switching to discharge the oil from the hydraulic pump  57  to flow toward the swash plate type hydraulic motor  58 . 
     The swash plate type hydraulic pump  57  and the swash plate type hydraulic motor  58  are connected to each other at a speed change ratio corresponding to the inclination of a swash plate  67  included in the hydraulic motor  58 . The rotating force thus changed is taken out of the transmission output gear  68  fixed to the transmission shaft  66  integral with the output part of the hydraulic motor  58 . An angle of inclination of the swash plate  67  included in the hydraulic motor  58  is changed by a swash plate driving mechanism (not shown) that is driven by an electric motor. 
       FIG. 4  is a cross-sectional view taken along line IV-IV of  FIG. 2  that depicts a power transmission path extending from the transmission shaft  66  to the connection shaft  85 . A clutch shaft  76  is journaled, parallel to the transmission shaft  66 , by the right crankcase  21  and the right crankcase cover  23  through ball bearings. An output shaft  80  is journaled, parallel to the clutch shaft  76 , by the left crankcase  20  and the right crankcase  21  through ball bearings. The connection shaft  85  is journaled, perpendicularly to the output shaft  80 , by a connection shaft support portion  84  placed near the left end of the output shaft  80 . The connection shaft support portion  84  is attached to a portion outside the left crankcase  20 . See, also  FIG. 2 . 
     A gear  77  is loosely fitted to the clutch shaft  76  in a rotatable manner with respect to the axis thereof. The gear  77  has a meshing engagement with the transmission output gear  68  fitted to the transmission shaft  66 . Adjacent to the gear  77 , a sliding member  78  having engagement gear teeth  78   a  is loosely fitted to the clutch shaft  76  in an axially slidable manner. The clutch shaft  76 , gear  77  and sliding member  78  constitute an electrically-driven or manually-operated mechanical clutch  75  which can connect and disconnect power transmission. The sliding member  78  is allowed to slide along the clutch shaft  76  to establish a meshing engagement between the meshing teeth  78   a  and an engagement portion of the gear  77 , whereby the clutch is engaged, to thereby provide a driving state. The sliding member  78  is moved to disengage the engagement teeth  78   a  from the gear  77 , whereby the clutch is disengaged, to provid a neutral state. 
     Adjacent to the gear  77 , a gear  79  is fitted to the clutch shaft  76  on a side opposite to the sliding member  78  with respect to the gear  77 . A gear  81  is fitted to the right end of the output shaft  80 . The gear  81  has a meshing engagement with the gear  79  carried on the clutch shaft  76 . A bevel gear  82  is formed integrally with the other end of the output shaft  80 . A bevel gear  86  is formed integrally with the front end of the connection shaft  85  and has a meshing engagement with the bevel gear  82  carried on the output shaft  80 . The rear end of the connection shaft  85  is formed with a spline  85   a  so as to be connected to the rear wheel driving shaft (not shown). Such shafts and gears transmit the rotating power of the static hydraulic type continuously variable transmission  55  to the rear wheel driving shaft. 
       FIG. 5  is a cross-sectional view of the inside of the crankcase  20  as viewed from the right side. This view depicts the locations of the crankshaft  30 , transmission shaft  66  and output shaft  80  and the cross-section of the swash plate  67  of the hydraulic motor  58 . An electric motor  74  drives the swash plate as is shown at an upper portion in  FIG. 5 . The electric motor  74  drives the swash plate  67  through a group of reduction gears (not shown). An oil pump assembly  90  is placed at the lower portion of the crankcase  20  and an oil strainer  91  connected to the oil pump assembly  90  is placed in an oil pan  92 . An oil cooler  139  and a low pressure oil filter  140  are attached to a portion rearwardly of the left crankcase  20 . 
       FIGS. 6(   a ) and  6 ( b ) are an assembly view of the oil pump assembly  90 .  FIG. 6(   a ) is a right lateral view of the oil pump assembly  90  and  FIG. 6(   b ) is a cross-sectional view taken along line  6 - 6  of  FIG. 6(   a ). The oil pump assembly  90  is integrally composed of a left side low pressure oil pump  90 L and a right side high pressure oil pump  90 H. An oil pump outer shell  94  is composed of a center pump case body  95 , a left cover  96 , a right cover  97  and a right outer cover  98 . An oil pump shaft  100  is attached for rotation to pass through the pump case body  95 , left cover  96  and right cover  97 . A low pressure oil pump inner rotor  101  is secured to the oil pump shaft  100  by means of a retaining pin  103  on the left side of the center of the oil pump. A low pressure oil pump outer rotor  102  is attached to the outer circumference of the inner rotor  101  in such a manner that the outer rotor  102  and the inner rotor  101  are engaged with each other. A retaining pin  103  is inserted into a retaining pin insertion hole  100   a  of the oil pump shaft  100 . An oil pump shaft insertion hole  101   a  of a low pressure oil pump inner rotor  101  is formed with a retaining recessed portion  101  with which the retaining pin  103  is engaged. 
     A high pressure oil pump inner rotor  104  is fixedly fitted to the flat surface retaining portion  100   b  of the oil pump  100  on the right side of the center of the oil pump. A high pressure oil pump outer rotor  105  is attached to the outer circumference of the inner rotor  104  in such a manner that the outer rotor  105  and the inner rotor  104  are engaged with each other. The flat surface retaining portion  100   b  consists of four sub-portions formed by removing corresponding flat surfaces on the oil pump shaft  100 . An oil pump shaft insertion hole  104   a  of the high pressure oil pump inner rotor  104  is a through hole having the same cross-section as that of a portion corresponding to the flat surface retaining portion  100   b  of the oil pump shaft  100 , that is, being fitted to the flat surface retaining portion  100   b.    
     A low pressure relief valve  107  and a high pressure relief valve  108  are attached to the lower portion of the oil pump assembly  90  with functions of maintaining the discharge pressures of the low pressure oil pump  90 L and the high pressure oil pump  90 H at predetermined pressures, respectively. 
     The oil pump shaft  100  is formed at its almost-left end with a spline  100   c  adapted to attach a pump driven sprocket described later thereto. In addition, the oil pump shaft is formed at its left end with a flat projection  100   d  adapted to connect with a water pump described later. 
       FIGS. 7(   a ) through  10 ( d ) depict the pump case body  95 , the left cover  96 , the right cover  97  and the right outer cover  98  constituting the outer shell  94  of the oil pump assembly  90 . A description will be hereinafter made of portions of each of the components mentioned above. In the description, symbols denoting the portions are lower-case alphabetical characters, which are suffixed to reference numerals denoting the components constituting the oil pump assembly, thus representing each of the portions belonging to corresponding one of the components with: 
     “x”: a portion belonging to the pump case body  95   
     “y”: a portion belonging to the left cover  96   
     “z”: a portion belonging to the right cover  97   
     “w”: a portion belonging to the right outer cover  98   
     Thus, the relationship between the whole and portions can be cleared. For example, the oil pump shaft insertion hole  113  represents the whole composed of a portion  113   x  belonging to the pump case body  95 , a portion  113   y  belonging to the left cover  96 , a portion  113   z  belonging to the right cover  97  and a portion  113   w  belonging to the right outside cover  98 . Incidentally,  FIGS. 6(   a ) and  6 ( b ) do not describe reference numeral “ 113 ” denoting the entire oil pump shaft insertion hole for simplification of the drawing of the oil pump assembly. The same holds true for the other reference numerals. 
       FIGS. 7(   a ),  7 ( b ) and  7 ( c ) are three-side view of the pump case body  95 .  FIG. 7(   a ) is a right side view,  FIG. 7(   b ) is a cross-sectional view taken along line  7 - 7  of  FIG. 7(   a ) and  FIG. 7(   c ) is a left side view. In  FIGS. 7(   a ) and  7 ( c ), an oil inflow hole  110   x  is bored in the lower portion of the pump case body  95  and an oil inflow passage  111   x  continuous with the oil inflow hole  10   x  extends upwardly. A partition wall  112   x  extending longitudinally is formed at the center portion of the pump case body  95 . An oil pump shaft insertion hole  113   x  is bored in the partition wall  112   x  so as to pass therethrough. A low pressure pump chamber  114   x  is provided on the left side in  FIG. 7(   b ). Referring to  FIG. 7(   c ), a low pressure oil discharge passage  115   x  continuous with the low pressure pump chamber  114   x  is provided on a side opposite to the oil inflow passage  111   x  with respect to the partition wall  112   x  so as to extend downwardly. A low pressure relief valve attachment hole  117   x  is bored on the left side in  FIG. 7(   b ) so as to communicate with a low pressure relief valve attachment space  118   x  placed on the right side in  FIG. 7(   b ). Referring to  FIG. 7(   a ), a high pressure oil discharge passage  121   x  is provided on a side opposite to the oil inflow passage  111   x  with respect to the partition wall  112   x . A high pressure relief valve attachment space  124   x  is provided on a side opposite to the high pressure oil discharge passage  121   x  so as to be partitioned by the partition wall  112   x . In addition, the high pressure relief valve attachment space  124   x  communicates with the low pressure relief valve attachment space  118   x  as shown in  FIG. 7(   b ). The relief valve attachment spaces  118   x  and  124   x  each communicate with the oil inflow passage  111   x  as shown in  FIG. 7(   a ). This is intended to return excessive oil discharged from the relief valve to the oil inflow passage  111   x . Assembly screw insertion holes  126   x  are provided at a peripheral portion. 
       FIGS. 8(   a ),  8 ( b ) and  8 ( c ) are three-side view of the left cover  96 .  FIG. 8(   a ) is a right side view,  FIG. 8(   b ) is a cross-sectional view taken along line  8 - 8  of  FIG. 8(   c ) and  FIG. 8(   c ) is a left side view. In  FIG. 8(   a ), a partition wall  112   y  is provided in the center portion, an oil inflow passage  111   y  is provided on one side of the partition wall  112   y  and a low pressure discharge passage  115   y  is provided on the other side. These are adapted to cover the side faces of the partition wall  112   x , oil inflow passage  111   x  and low pressure oil discharge passage  115   x  included in the pump case body  95 . See,  FIG. 7 . An oil pump shaft insertion hole  113   y  is provided to pass through the partition wall  112   y . A low pressure oil relief passage  116   y  that is continuous with the lower pressure oil discharge passage  115   y  is provided at the right lower portion, and communicates with the low pressure relief valve attachment hole  117   x  of the pump case body  95  shown in  FIG. 7 . A lower pressure discharge hole  119   y  is provided at the connection portion of the low pressure oil discharge passage  115   y  and the low pressure oil relief passage  116   y  so as to open on the left side of the oil pump assembly  90 . 
       FIGS. 9(   a ),  9 ( b ) and  9 ( c ) are three-side view of the right cover  97 .  FIG. 9(   a ) is a right side view,  FIG. 9(   b ) is a cross-sectional view taken along line  9 - 9  of  FIG. 9(   c ) and  FIG. 9(   c ) is a left side view. In  FIG. 9(   b ), a high pressure pump chamber  120   z  is provided on the left side. In  FIG. 9(   c ), a partition wall  112   z  is provided at the center portion on one side of which an oil inflow passage  111   z  is provided and on the other side of which a high pressure oil discharge passage  121   z  is provided. These are adapted to cover the side faces of the partition wall  112   x , oil inflow passage  111   x , and high pressure discharge passage  121   x  included in the pump case body  95 . See,  FIG. 7 . An oil pump shaft insertion hole  113   z  is provided to pass through the partition wall  112   z . A high pressure relief valve attachment hole  123   z  can be seen in  FIGS. 9(   a ),  9 ( b ) and  9 ( c ). A high pressure relief valve attachment space  124   z  and an end of a low pressure relief valve attachment space  118   z  can be seen in  FIGS. 9(   b ) and  9 ( c ). A high pressure discharge hole  125   z  continuous with the high pressure discharge passage  121   z  can be seen in  FIGS. 9(   a ) and  9 ( c ). The high pressure discharge hole  125   z  communicates with a corresponding part of the right outer cover  98 . A high pressure oil relief passage  122   z  is placed between the high pressure discharge hole  125   z  and the high pressure relief valve attachment hole  123   z , and is shield with the right outer cover  98 . 
       FIGS. 10(   a ),  10 ( b ),  10 ( c ) and  10 ( d ) are four-side view of the right outer cover  98 .  FIG. 10(   a ) is a right side view,  FIG. 10(   b ) is a cross-sectional view taken along line  10 - 10  of  FIG. 10(   a ),  FIG. 10(   c ) is a left side view and  FIG. 10(   d ) is a cross-sectional view taken along line  11 - 11  of  FIG. 10(   a ). A high-pressure oil relief passage  122   w  running on the left surface of the right outer cover  98  can be seen in  FIGS. 10(   c ) and  10 ( d ). This relief passage  122   w  is an oil passage adapted to connect the high pressure oil discharge hole  125   z  and the high pressure relief valve attachment hole  123   z  included in the right cover ( FIG. 9) . The high pressure discharge hole  125   w  of the right outer cover  98  communicates concentrically with the high pressure oil discharge hole  125   z  of the right cover  97  ( FIG. 9 ). Assembly screw insertion holes  126   z  are provided at a peripheral portion. 
       FIG. 11  is a cross-sectional view of the low pressure oil pump  90 L, also depicting a section of the low pressure relief valve  107 . The inner rotor  101  and outer rotor  102  of the low pressure oil pump  90 L are housed in the low pressure pump chamber  114   x , see  FIG. 7 , formed in the pump case body  95 . The inner rotor  101  of the low pressure oil pump  90 L is retained by a retaining pin  103  fitted into the retaining pin insertion hole  100   a  of the oil pump shaft  100 . The oil pump shaft insertion hole  101   a  of the inner rotor  101  is formed with a retaining recess  101   b  in which the retaining pin  103  is fitted. The cross-section of the low pressure relief valve  107  is taken along the position of a stopper pin (described later). 
       FIG. 12  is a cross-sectional view of the high pressure oil pump  90 H, also depicting a section of the high pressure relief valve  108 . The inner rotor  104  and outer rotor  105  of the high pressure oil pump  90 H are housed in the high pressure pump chamber  120   z , see  FIG. 9 , formed in the right cover  97 . The inner rotor  104  of the high pressure oil pump  90 H is retained in place by the flat surface retaining part  100   b  formed in the oil pump shaft  100  and the oil pump shaft insertion hole  104   a  having the same cross-section as that of the flat surface retaining part  100   b . The cross-section of the high pressure relief valve  108  is taken along the position of the excessive oil discharge port. 
       FIG. 13  is a cross-sectional view of the low pressure relief valve  107 . The valve  107  includes a cylindrical case  130 , an inner cylinder  131 , a C-shaped clip  132 , a washer  133 , a coil spring  134 , and a stopper pin  135 . The cylindrical case  130  is provided with opposite opening ends. The inner cylinder  131  is slidably fitted in the cylindrical case  130 , and has one end opened and the other end closed. The C-shaped clip  132  is fitted to the annular recess at the inside end of the cylindrical case. The washer  133  is in contact with the inside of the clip  132 . The coil spring  134  is compressively placed between the washer  133  and the inside of the closed end of the inner cylinder  131 . The stopper pin  135  is fitted into the through hole at the end of the cylinder case  130  in order to prevent the inner cylinder  131  from coming off. A pair of excessive oil discharge ports  136  is provided at the respective side surfaces of the cylindrical case. An O-ring attachment groove  130   a  is provided to surround a side surface of the cylindrical case  130  at a portion between the stopper pin  135  and the excessive oil discharge ports  136 . An end located on a side of the stopper pin  135  of the low pressure relief valve  107  provides a pressurizing end space  137 . 
     The low pressure relief valve  107  is inserted into the low pressure relief valve attachment hole  117   x  and is attached in the low pressure relief valve attachment space  118   x . Pressure in the low pressure oil discharge passages  115   x ,  115   y  is applied to the pressurizing end  137  of the low pressure relief valve  107  through the oil relief passage  116   y . As the pressure is increased, the inner cylinder  131  is moved against the urging force of the coil spring  134 . When the pressure exceeds a predetermined value, the pressurizing end space  137  communicates with the excessive oil discharge port  136 , so that excessive oil is discharged to the oil inflow passage  111   x . Thus, the upper limit value of the pressure in the low pressure discharge passages  115   x ,  115   y  is maintained at a fixed value. The above description is made of the low pressure relief valve  107 . Since the configuration and function of the high pressure relief valve  108  is almost the same as those of the low pressure relief valve  107  except that the high pressure relief valve  108  uses a stronger coil spring than the low pressure relief valve, the description is omitted for the high pressure relief valve  108 . 
       FIG. 14  depicts the horizontal cross-section of the oil pump assembly  90  and the discharge passages as viewed from above. The oil inflow port  110   x  connected to the oil strainer  91  can be seen at the lower portion of the oil inflow passage  111 . The oil in the low pressure oil discharge passages  115   x ,  115   y  of the low pressure oil pump  90 L interflows and is discharged from the left side low pressure oil discharge hole  119   y . Then, the oil flows through the oil passage L 1  extending rearwardly, and moves toward the oil cooler  139  and the low pressure oil filter  140 . The oil that has cooled by the oil cooler and is purified by the low pressure oil filter and passes through the outflow oil passage L 2  communicating with an oil outflow pipe  140   a  placed at the center portion of the low pressure oil filter moves to the right in  FIG. 14  via the oil passage L 3 . 
     The oil in the high pressure discharge passages  121   x ,  121   z  of the high pressure oil pump  90 H interflows and is discharged from the right side high pressure oil discharge hole  125   w  to enter the high pressure oil filter  141  for purification. Then, the purified oil flows out from the outflow oil passage H 1  communicating with the center portion of the high pressure oil filter and moves rearwardly through the oil passage H 2 . In addition, on the midway, the purified oil changes its direction to the right in  FIG. 14  through the passage H 3  and moves upwardly through a passage not shown and is used to lubricate the static hydraulic type continuously variable transmission  55 . 
       FIG. 15  depicts the horizontal cross-section of the oil pump assembly  90 , the water pump  143  and an oil passage subsequent to the oil passage L 3  of the low pressure oil pump, as viewed from above. The oil passage L 3  of the low pressure oil pump shown in  FIG. 14  further extends to the right, and connects with an oil passage L 4  at the midway thereof and then the oil passage L 4  extends forwardly. The oil passage L 4  connects at its end with a right rising oil passage L 5  so that oil moves upwardly to lubricate the right bearing  29  ( FIG. 3 ) of the crankshaft. The right rising oil passage L 5  connects at its midway with an oil gallery L 6 , which is connected to a left rising oil passage L 7 , so that oil moves upwardly to lubricate the left bearing  28 . See,  FIG. 3 .  FIG. 5  shows some of the oil passages described above. 
     The water pump  143  is mainly composed of a case  144 , a cover  145 , a vane wheel  146  and a water pump shaft  147 . The vane wheel  146  is housed in the case  146  in such a manner so as to be secured to the end of the water pump shaft  147  therein. The water pump shaft  147  is journaled in the case  144  via ball bearings  148 . The case  144  together with the cover  145  is attached to the left crankcase  20  with bolts  149 . A flat slit-like recess  147   a  is formed at the end of the water pump shaft  147  outside the case so as to suitably receive the flat projection  100   d  formed at the end of the oil pump shaft  100 . Thus, the water pump  143  operates simultaneously with the oil pump assembly  90 . 
     The oil pump shaft  100  is formed at its end with the spline  100   c  to which the oil pump shaft driven sprocket  150  is fitted. The oil pump shaft driving sprocket  151 , see  FIG. 3 , is carried on the crankshaft  30  at a position corresponding to the sprocket  150 . The pump driving chain  152 , see  FIG. 2 , is wound around both the sprockets. When the crankshaft  30  is rotated, the oil pump shaft  100  is rotated through the chain  152 , and the water pump shaft  147  is rotated simultaneously therewith. Water discharged from the water pump  143  is used to cool the cylinder  25  and the cylinder head  26 . The water that has passed the high temperature portions is cooled by the radiator  19 , see  FIG. 1 , and then again returns to the water pump  143 . 
     As descried above in detail, the oil pump assembly of the present invention has the following effects: 
     Since the respective axes of the plurality of relief valves are disposed parallel to the driving shaft, the drive shaft can be brought close to the relief valves, so that the relief valves can be arranged compactly. 
     (2) Since the discharge port of the relief valve is disposed in the width of the rotor as viewed from a direction perpendicular to the driving shaft, the relief valve can be brought close to the oil pump for a compact arrangement. In addition, the discharge oil passage is brought close to the pump suction port to shorten the relief oil passage, thereby simplifying the oil passage. 
     (3) Since the discharge ports of the relief valves are disposed in the width of the rotors and the relief valves are arranged to have most portions of the full lengths that overlap each other, the plurality of the relief valves can be further arranged compactly. Note that “to have most portions of the full lengths that overlap each other” means that “a plane perpendicular to the axes of the relief valves are shared by most parts of the full lengths of the relief valves.” 
     (4) Since the oil pump assembly is composed of the pump case body and the pump covers shielding both the opposite sides of the pump case body and the plurality of the relief valves are disposed by inserting them into the corresponding cavities of the pump case body, the number of components can be reduced by eliminating members used to support the relief valves. 
     (5) Since the plurality of relief valves are disposed between the strainer and the oil pump as viewed from the lateral side of the power unit, an unused space between the strainer and the oil pumps is used to dispose the relief valves therein, thereby further reducing the size of the power unit. 
     (6) Since the plurality of relief valves are arranged along the oil stream line extending from the strainer to the oil pump, the plurality of relief valves can be further arranged compactly while ensuring sufficient oil passages. 
     (7) Since the power transmission portion between the crankshaft and the transmission uses the cam type torque damper and the backlash reduction gears, shocks can be reduced and the meshing engagement between the gears is quiet. 
     As descried above in detail, the oil pump structure of the present invention has the following effects: 
     Since the inner rotor of the high pressure oil pump is fitted and secured to a portion having a plurality of flat surfaces formed in the vicinity of an end of the driving shaft, it is unnecessary to provide a retaining recess which accommodates retaining pins. Further, the strength of connection between the inner rotor and the driving shaft can be increased and it is unnecessary to increase the diameter of the driving shaft, thereby reducing the size of the oil pump. In addition, in the case of the flat surface fitting compared to the retaining pins, not only the length of the flat surface can be set more freely but also the rotor having manufacturing or assembling errors can be attached to the driving shaft more freely in terms of the axial position thereof. 
     Since the oil that has passed the rotor is symmetrically discharged from the rotor to both sides of the rotor, the pressure of the oil discharged to both sides provides a rotor-centering effect, which reduces the contact between the rotor and the pump chamber, thereby suppressing frictional resistance therebetween. Accordingly, load acting on the driving shaft is reduced, so that the diameter of the driving shaft can be reduced. 
     The power transmission means is provided at the end of the driving shaft on the side of the low pressure oil pump, the end of the driving shaft of the water pump is disposed coaxially with the end, and both ends are fitted and coupled to each other. Therefore, a distance between the power transmission means and the projecting-recessed connection portion located at the end of the driving shaft is small, so that torsion acting on the driving shaft is reduced. Accordingly, it is unnecessary to reinforce the projecting-recessed connection portion and reduce the diameter of the driving shaft. 
     In the present embodiment, the inner rotors are connected to the oil pump shaft in such a manner that the retaining pin is used to connect the inner rotor of the low pressure oil pump to the oil pump shaft and the flat retaining portion is used to fit and connect the inner rotor of the high pressure oil pump to the oil pump shaft. Therefore, after the inner rotors of the high and low pressure oil pumps are built in the outer shell of the oil pumps, the oil pump shaft is inserted into the rotor center hole from the side of the low pressure oil pump, thereby connecting the oil pump shaft to each rotor. Thus, the assembling work can be facilitated. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.