Patent Publication Number: US-9835152-B2

Title: Fluid pump

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and incorporates herein by reference Japanese Patent Application No. 2015-82664 filed on Apr. 14, 2015. 
     TECHNICAL FIELD 
     The present disclosure relates to a fluid pump that draws and discharges fluid by changing a volume of respective pump chambers formed between external teeth of an inner rotor and internal teeth of an outer rotor. 
     BACKGROUND 
     A previously proposed fluid pump has a rotatable shaft, an inner rotor, an outer rotor, and a pump housing. The inner rotor has a main body, to which the rotatable shaft is coupled, and external teeth, which are formed in an outer peripheral portion of the main body. The outer rotor has internal teeth for meshing with the external teeth. When the inner rotor is rotated by rotating the rotatable shaft, a rotational force of the inner rotor is transmitted from the external teeth to the internal teeth. Thereby, the outer rotor is also rotated. When the inner rotor and the outer rotor are rotated, the volume of the respective pump chambers, which are formed between the external teeth and the internal teeth, changes. In response to increasing of the volume of the pump chamber, the fluid is drawn into the pump chamber. Thereafter, in response to decreasing of the volume of the pump chamber, the fluid is compressed in the pump chamber and is discharged from the pump chamber (see, for example, JP2013-60901A). 
     In general, when the temperature of the fluid is decreased, viscosity of the fluid is increased. Particularly, in a case where the fluid is light oil (diesel fuel), a wax component (paraffin) of the light oil is solidified to cause very high viscosity of the light oil at the low temperature (e.g., low winter temperatures). In the case where the viscosity of the fluid is increased, a repulsive force, which is applied from the fluid to the inner rotor, is increased. Thereby, a force (tilting force), which is applied from the fluid to the inner rotor in a direction for tilting the inner rotor, is increased. Thereby, a slide resistance between a radial bearing, which rotatably and slidably supports the rotatable shaft, and the rotatable shaft is increased to cause an increase in the energy loss or generation of damage at a sliding portion between the radial bearing and the rotatable shaft. 
     With respect to the above point, the inventors of the present application have studied a structure for coupling the inner rotor to the rotatable shaft through a joint member rather than directly coupling the inner rotor to the rotatable shaft. With this structure, the above-described tilting force can be absorbed through resilient deformation of the joint member, and thereby the slide resistance between the radial bearing and the rotatable shaft can be reduced. 
     However, the inventors of the present application have noticed that the above-described coupling structure poses the following new disadvantage. The pump housing has a rotor receiving chamber, which receives the inner and outer rotors. In the case of the above coupling structure, a joint chamber, which receives the joint member, is required separately from the rotor receiving chamber. A joint receiving chamber side surface of the main body of the inner rotor receives a pressure in the axial direction from the fluid in the joint receiving chamber. Thereby, a surface of the inner rotor, which is perpendicular to the axial direction and is located on an axial side opposite from the joint receiving chamber, is urged against an inner wall surface of the pump housing to cause an increase in the slide resistance of the inner rotor. 
     That is, in the case where the above-described coupling structure is used, although the tilting force can be absorbed by the joint member, the joint receiving chamber is required. Therefore, the increase in the slide resistance of the inner rotor becomes a new disadvantage. 
     SUMMARY 
     The present disclosure is made in view of the above disadvantage. According to the present disclosure, there is provided a fluid pump that includes a rotatable shaft, an inner rotor, a joint member, an outer rotor, a pump housing, an external tooth slide surface and a main body slide surface. The inner rotor includes a main body and a plurality of external teeth. The main body has a through-hole, through which the rotatable shaft is inserted. The plurality of external teeth is formed in an outer peripheral portion of the main body. The joint member is placed on an axial side of the inner rotor and couples between the inner rotor and the rotatable shaft to transmit a rotational torque of the rotatable shaft to the inner rotor. The outer rotor has a plurality of internal teeth for meshing with the plurality of external teeth. The pump housing forms a rotor receiving chamber and a joint receiving chamber. The rotor receiving chamber receives the outer rotor and the inner rotor. The joint receiving chamber receives the joint member. The pump housing also forms a plurality of pump chambers between the plurality of internal teeth and the plurality of external teeth. Each of the plurality of pump chambers draws and compresses fluid by changing a volume of the pump chamber. The external tooth slide surface and the main body slide surface are formed in a portion of an inside wall surface of the pump housing located on an opposite side of the inner rotor, which is opposite from the joint member in an axial direction. The plurality of external teeth of the inner rotor is slidable relative to the external tooth slide surface while the main body of the inner rotor is slidable relative to the main body slide surface, and a surface roughness of the main body slide surface is higher than a surface roughness of the external tooth slide surface. 
     According to the present disclosure, there is also provided a fluid pump that includes a rotatable shaft, an inner rotor, a joint member, an outer rotor, a pump housing, an external tooth slide surface and a main body slide surface. The inner rotor includes a main body and a plurality of external teeth. The main body has a through-hole, through which the rotatable shaft is inserted. The plurality of external teeth is formed in an outer peripheral portion of the main body. The joint member is placed on an axial side of the inner rotor and couples between the inner rotor and the rotatable shaft to transmit a rotational torque of the rotatable shaft to the inner rotor. The outer rotor has a plurality of internal teeth for meshing with the plurality of external teeth. The pump housing forms a rotor receiving chamber and a joint receiving chamber. The rotor receiving chamber receives the outer rotor and the inner rotor. The joint receiving chamber receives the joint member. The pump housing also forms a plurality of pump chambers between the plurality of internal teeth and the plurality of external teeth. 
     Each of the plurality of pump chambers draws and compresses fluid by changing a volume of the pump chamber. The external tooth slide surface and the main body slide surface are formed in a portion of an inside wall surface of the pump housing located on an opposite side of the inner rotor, which is opposite from the joint member in an axial direction. The plurality of external teeth of the inner rotor is slidable relative to the external tooth slide surface while the main body of the inner rotor is slidable relative to the main body slide surface, and a surface roughness of a rotor side main body slide surface of the main body, which is slidable relative to the main body slide surface, is higher than a surface roughness of a rotor side external tooth slide surface of the plurality of external teeth, which is slidable relative to the external tooth slide surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a partial cross-sectional view indicating a fuel pump according to a first embodiment of the present disclosure; 
         FIG. 2  is a cross-sectional view taken along line II-II in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along line III-III in  FIG. 1 ; 
         FIG. 4  is a cross-sectional view taken along line IV-IV in  FIG. 1 ; 
         FIG. 5  is a partial enlarged view of  FIG. 1 ; 
         FIG. 6  is a plan view of a pump casing of the first embodiment seen from a rotor receiving chamber side; 
         FIG. 7  is a cross-sectional view taken along line VII-VII in  FIG. 6 ; 
         FIG. 8  is a schematic cross sectional view for describing an axial dimension of the rotor receiving chamber in a state before surface treatment of the pump casing; 
         FIG. 9  is a schematic cross sectional view for describing an axial dimension of the rotor receiving chamber in a case where the pump casing is processed by electrical discharge machining; 
         FIG. 10  is a schematic cross sectional view for describing an axial dimension of the rotor receiving chamber in a case where the pump casing is processed by shot blasting; and 
         FIG. 11  is a plan view of an inner rotor of a fuel pump according to a second embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of a fluid pump according to the present disclosure will be described with reference to the accompanying drawings. 
     First Embodiment 
     The fluid pump of the present embodiment is installed in a vehicle. A subject fluid to be pumped with the fluid pump is liquid fuel used for combustion in an internal combustion engine. Specifically, in the present embodiment, light oil (diesel fuel), which is used for combustion in a compression self-ignition internal combustion engine, is used as the subject fluid to be pumped. The fluid pump is received in an inside of a fuel tank. 
     As shown in  FIG. 1 , the fluid pump  101  of the present embodiment is a rotary internal gear pump of a positive displacement type. The fluid pump  101  includes a pump body  102 , a pump main body  103 , an electric motor  104  and a side cover  105 . The pump main body  103  and the electric motor  104  are received in an inside of the pump body  102 , which is shaped into a cylindrical tubular form, such that the pump main body  103  and the electric motor  104  are arranged one after another in an axial direction. The side cover  105  is installed to an opening of one of two axially opposite end parts of the pump body  102 , which is located on the electric motor  104  side. 
     The side cover  105  includes an electric connector  105   a , which supplies an electric power to the electric motor  104 , and a discharge port  105   b , through which fuel is discharged from the fluid pump  101 . In the fluid pump  101 , a rotatable shaft  104   a  of the electric motor  104  is rotated when the electric power is supplied from an external circuit through the electric connector  105   a . Thus, an outer rotor  130  and an inner rotor  120  of the pump main body  103  are rotated by a drive force of the rotatable shaft  104   a  of the electric motor  104 , and thereby fuel is drawn into and compressed in the fluid pump  101  and is then discharged from the fluid pump  101  through the discharge port  105   b . The fluid pump  101  pumps the light oil, which has the higher viscosity in comparison to gasoline, as the fuel. 
     In the present embodiment, the electric motor  104  is an inner rotor brushless motor and includes magnets  104   b , which form four magnetic poles, and coils  104   c , which are installed in six slots. For example, at a start preparation time (e.g., a time of turning on of an ignition switch of the vehicle), a positioning control operation of the electric motor  104  is executed to rotate the rotatable shaft  104   a  toward a drive rotation side or a counter-drive rotation side (the counter-drive rotation side being opposite from the drive rotation side). Thereafter, the electric motor  104  executes a drive control operation, which rotates the rotatable shaft  104   a  from the position, at which the rotatable shaft  104   a  is positioned in the positioning control operation, toward the drive rotation side. 
     Here, the drive rotation side is a positive direction side of a rotational direction Ri of the inner rotor  120  in a circumferential direction of the inner rotor  120 . The counter-drive rotation side is a negative direction side of the rotational direction Ri of the inner rotor  120 , which is opposite from the positive direction side. 
     Hereinafter, the pump main body  103  will be described in detail. The pump main body  103  includes a pump housing  110 , the inner rotor  120 , the outer rotor  130  and a joint member  160 . The pump housing  110  includes a pump cover  112  and a pump casing  116 , which are placed one after another in the axial direction. 
     The pump cover  112  is made of metal and is shaped into a circular disk form. The pump cover  112  axially projects outward from the end part of the pump body  102 , which is located on the side of the electric motor  104  that is opposite from the side cover  105 . 
     In order to draw the fuel from an outside of the fluid pump  101 , the pump cover  112  shown in  FIGS. 1, 2 and 5  has a suction passage  112   a , which is formed as a cylindrical hole, and a suction groove  113 , which is shaped into an arcuate form. The suction groove  113  is axially grooved, i.e., formed in an inside wall surface of the pump cover  112  and opens on the pump casing  116  side of the pump cover  112 . The suction passage  112   a  opens in a groove bottom portion  113   e  of the suction groove  113  at a predetermined area, so that the suction groove  113  is communicated with the suction passage  112   a . A communicating portion of the suction groove  113 , which is communicated with the suction passage  112   a , extends through the pump cover  112  in the axial direction. A non-communicating portion of the suction groove  113 , which is not directly communicated with the suction passage  112   a , is shaped into a cup form having a bottom. As shown in  FIG. 2 , the suction groove  113  has a circumferential extent, which is less than one half (less than 180 degrees) of an entire circumference of the inner rotor  120  in the rotational direction Ri (also see  FIG. 4 ). The suction groove  113  extends from a start end part  113   c  to a terminal end part  113   d  in the rotational direction Ri, Ro such that a radial extent (hereinafter referred to as a width) of the suction groove  113 , which is measured in a radial direction of the rotational axis, progressively increases in the rotational direction Ri, Ro from the start end part  113   c  to the terminal end part  113   d.    
     Furthermore, the pump cover  112  forms a joint receiving chamber  110   b  at an area that is opposed to the inner rotor  120  along a central axis (hereinafter referred to as an inner central axis) Ci of the inner rotor  120 . The joint receiving chamber  110   b  is shaped into a recessed hole. A main body  162  of the joint member  160  is rotatably installed in the joint receiving chamber  110   b.    
     The pump casing  116  shown in  FIGS. 1 and 3-5  is made of metal and is shaped into a cylindrical tubular form having a bottom. An opening portion  116   a  of the pump casing  116  is covered with the pump cover  112  such that an entire circumferential extent of the opening portion  116   a  is tightly closed by the pump cover  112 . As shown particularly in  FIGS. 1 and 4 , an inner peripheral portion  116   b  of the pump casing  116  is formed as a cylindrical hole that is eccentric relative to the inner central axis Ci of the inner rotor  120 . 
     The pump casing  116  forms a discharge passage  117 , which is formed as an arcuate hole, to discharge the fuel from the discharge port  105   b  through a high pressure passage  106  defined between the pump body  102  and the electric motor  104 . The discharge passage  117  axially extends through a recessed bottom portion  116   c  of the pump casing  116 . Particularly, as shown in  FIG. 3 , the discharge passage  117  has a circumferential extent, which is less than one half (i.e., less than 180 degrees) of the entire circumference of the inner rotor  120  in the rotational direction Ri. A radial extent (hereinafter referred to as a width) of the discharge passage  117 , which is measured in the radial direction, progressively decreases in the rotational direction Ri, Ro from a start end part  117   c  to an terminal end part  117   d.    
     Furthermore, the pump casing  116  includes a reinforcing rib  116   d  in the discharge passage  117 . The reinforcing rib  116   d  is formed integrally with the pump casing  116  such that the reinforcing rib  116   d  extends across the discharge passage  117  in a crossing direction, which crosses the rotational direction Ri of the inner rotor  120 , and thereby the reinforcing rib  116   d  reinforces the pump casing  116 . 
     An opposing suction groove  118  shown in  FIG. 3  is formed in the recessed bottom portion  116   c  of the pump casing  116  at a corresponding area that is opposed to the suction groove  113  in the axial direction while pump chambers  140  (described later in detail) are interposed between the opposing suction groove  118  and the suction groove  113  in the axial direction. The opposing suction groove  118  is an arcuate groove that corresponds to a shape, which is produced by projecting the suction groove  113  onto the pump casing  116  in the axial direction. In this way, in the pump casing  116 , the discharge passage  117  is formed to be symmetric to the opposing suction groove  118  with respect to the symmetry axis located between the discharge passage  117  and the opposing suction groove  118 . As shown particularly in  FIG. 2 , an opposing discharge groove  114  is formed in the pump cover  112  at a corresponding area that is opposed to the discharge passage  117  in the axial direction while the pump chambers  140  are interposed between the opposing discharge groove  114  and the discharge passage  117  in the axial direction. The opposing discharge groove  114  is formed as an arcuate groove that is shaped to correspond with a shape, which is produced by projecting the discharge passage  117  onto the pump cover  112  in the axial direction. In this way, in the pump cover  112 , the suction groove  113  is formed to be symmetric to the opposing discharge groove  114  with respect to the symmetry axis located between the suction groove  113  and the opposing discharge groove  114 . An outline (contour) of the suction groove  113 , an outline (contour) of the opposing discharge groove  114 , an outline (contour) of the discharge passage  117 , and an outline (contour) of the opposing suction groove  118  are shaped to extend in parallel with a rotational path of the external teeth  122  and a rotational path of the internal teeth  132   a.    
     As shown in  FIG. 1 , a radial bearing  150  is securely fitted to the recessed bottom portion  116   c  of the pump casing  116  along the inner central axis Ci to radially support the rotatable shaft  104   a  of the electric motor  104  in a manner that enables rotation of the rotatable shaft  104   a . Furthermore, a thrust bearing  152  is securely fitted to the pump cover  112  along the inner central axis Ci to axially support the rotatable shaft  104   a  in a manner that enables the rotation of the rotatable shaft  104   a.    
     As shown in  FIGS. 1, 4 and 5 , a rotor receiving chamber  110   a , which receives the inner rotor  120  and the outer rotor  130 , is formed by the recessed bottom portion  116   c  and the inner peripheral portion  116   b  of the pump casing  116  and the pump cover  112 . The inner rotor  120 , which is indicated in  FIGS. 1 and 4 , is centered at the inner central axis Ci and is thereby coaxial with the rotatable shaft  104   a  (i.e., coaxial with a rotational axis of the rotatable shaft  104   a ), so that the inner rotor  120  is eccentrically placed in the rotor receiving chamber  110   a . A through-hole  126 , which receives the radial bearing  150 , is formed in a main body  121  of the inner rotor  120 . When the inner rotor  120  is rotated, an inner wall surface of the through-hole  126  is slid along a cylindrical outer peripheral surface  150   o  of the radial bearing  150 . Thereby, the inner rotor  120  is radially supported by the radial bearing  150  in a rotatable member. Furthermore, two slide surfaces  125  of the inner rotor  120 , which are respectively formed at two opposed axial ends of the inner rotor  120 , are supported by the recessed bottom portion  116   c  of the pump casing  116  and the pump cover  112 , respectively, in a manner that enables rotation of the inner rotor  120 . 
     The inner rotor  120  has a plurality of insertion holes  127  that extend in the axial direction at a corresponding area of the inner rotor  120 , which is opposed to the joint receiving chamber  110   b . In the present embodiment, the number of the insertion holes  127  is five, and these insertion holes  127  are arranged one after another at equal intervals in the circumferential direction along the rotational direction Ri. The insertion holes  127  extend through the inner rotor  120  from the joint receiving chamber  110   b  side to the recessed bottom portion  116   c  side in the axial direction. Legs (projections)  164  of the joint member  160  are inserted into the insertion holes  127 , respectively, so that the drive force of the rotatable shaft  104   a  is transmitted to the inner rotor  120  through the joint member  160 . Thereby, the inner rotor  120  is rotated in the circumferential direction about the inner central axis Ci in response to the rotation of the rotatable shaft  104   a  of the electric motor  104  while the slide surfaces  125  of the inner rotor  120  are slid along the recessed bottom portion  116   c  and the pump cover  112 , respectively. 
     The inner rotor  120  includes a plurality of external teeth  122 , which are formed in an outer peripheral portion  124  of the inner rotor  120  and are arranged one after another at equal intervals in the circumferential direction along the rotational direction Ri. Each of the external teeth  122  can axially oppose the suction groove  113 , the discharge passage  117 , the opposing discharge groove  114  and the opposing suction groove  118  in response to the rotation of the inner rotor  120 . Thereby, it is possible to limit sticking of the inner rotor  120  to the recessed bottom portion  116   c  and the pump cover  112 . 
     As shown in  FIGS. 1, 4 and 5 , the outer rotor  130  is eccentric to the inner central axis Ci of the inner rotor  120 , so that the outer rotor  130  is coaxially received in the rotor receiving chamber  110   a . In this way, the inner rotor  120  is eccentric to, i.e., is decentered from the outer rotor  130  in an eccentric direction De, which is the radial direction. An outer peripheral portion  134  of the outer rotor  130  is radially supported by the inner peripheral portion  116   b  of the pump casing  116  in a manner that enables rotation of the outer rotor  130 . Furthermore, the outer peripheral portion  134  of the outer rotor  130  is axially supported by the recessed bottom portion  116   c  of the pump casing  116  and the pump cover  112  in a manner that enables the rotation of the outer rotor  130 . The outer rotor  130  is rotatable in the rotational direction (certain rotational direction) Ro about an outer central axis Co, which is eccentric to the inner central axis Ci. 
     The outer rotor  130  has a plurality of internal teeth  132   a  for meshing with the external teeth  122  of the inner rotor  120 . The internal teeth  132   a  are formed in an inner peripheral portion  132  of the outer rotor  130  and are arranged one after another at equal intervals in the rotational direction Ro. Each of the internal teeth  132   a  can axially oppose the suction groove  113 , the discharge passage  117 , the opposing discharge groove  114  and the opposing suction groove  118  in response to the rotation of the outer rotor  130 . Thereby, it is possible to limit sticking of the outer rotor  130  to the recessed bottom portion  116   c  and the pump cover  112 . 
     A fuel pressure (discharge pressure) in an inside of the discharge passage  117  is axially exerted against the inner rotor  120  and the outer rotor  130  toward the suction passage  112   a . A fuel pressure in the opposing discharge groove  114  is also the discharge pressure and is axially exerted against the inner rotor  120  and the outer rotor  130  toward the electric motor  104  side. Since the opposing discharge groove  114  is axially opposed to the discharge passage  117 , the fuel pressure of the opposing discharge groove  114  and the fuel pressure of the discharge passage  117  are balanced with each other. Therefore, it is possible to limit tilting of the inner rotor  120  and the outer rotor  130 , which would be otherwise caused by the discharge pressure. 
     Similarly, since the opposing suction groove  118  is axially opposed to the suction groove  113 , the fuel pressure (the suction pressure) of the opposing suction groove  118  and the fuel pressure (the suction pressure) of the suction groove  113  are balanced with each other. Therefore, it is possible to limit tilting of the inner rotor  120  and the outer rotor  130 , which would be otherwise caused by the suction pressure. 
     The external teeth  122  and the internal teeth  132   a  are shaped to have a trochoid tooth profile. The number of the internal teeth  132   a  is set to be larger than the number of the external teeth  122  by one. The inner rotor  120  is meshed with the outer rotor  130  due to the eccentricity in the eccentric direction De. In this way, the pump chambers  140  are radially formed between the internal teeth  132   a  and the external teeth  122  in the rotor receiving chamber  110   a . A volume of each pump chamber  140  is increased and decreased through the rotation of the outer rotor  130  and the rotation of the inner rotor  120 . 
     The volume of each of opposing ones of the pump chambers  140 , which are axially opposed to and communicated with the suction groove  113  and the opposing suction groove  118 , is increased in response to the rotation of the inner rotor  120  and the rotation of the outer rotor  130 . Thereby, the fuel is drawn from the suction passage  112   a  into the corresponding pump chambers  140  through the suction groove  113 . At this time, since the width (radial extent) of the suction groove  113  progressively increases from the start end part  113   c  to the terminal end part  113   d  in the rotational direction Ri, Ro (also see  FIG. 2 ), the amount of fuel drawn into the pump chamber  140  through the suction groove  113  corresponds to the amount of increase in the volume of the pump chamber  140 . The corresponding ones of the pump chambers  140 , each of which draws the fuel by increasing its volume in the above-described manner, are referred to as negative pressure portions (or negatively pressurized pump chambers)  140 L. 
     The volume of each of opposing ones of the pump chambers  140 , which are axially opposed to and communicated with the discharge passage  117  and the opposing discharge groove  114 , is decreased in response to the rotation of the inner rotor  120  and the rotation of the outer rotor  130 . Therefore, simultaneously with the suctioning function discussed above, the fuel is discharged from the corresponding pump chamber  140  into the high pressure passage  106  through the discharge passage  117 . At this time, since the width (radial extent) of the discharge passage  117  progressively decreases from the start end part  117   c  to the terminal end part  117   d  in the rotational direction Ri, Ro (also see  FIG. 3 ), the amount of fuel discharged from the pump chamber  140  through the discharge passage  117  corresponds to the amount of decrease in the volume of the pump chamber  140 . The corresponding ones of the pump chambers  140 , each of which compresses the fuel by decreasing its volume in the above-described manner, are referred to as high pressure portions (or highly pressurized pump chambers or positively pressurized pump chambers)  140 H. 
     The joint member  160  is made of synthetic resin, such as poly phenylene sulfide (PPS). The joint member  160  relays the rotatable shaft  104   a  to the inner rotor  120  to rotate the inner rotor  120  in the circumferential direction. The joint member  160  includes the main body  162  and the legs  164 . 
     The main body  162  is installed in the joint receiving chamber  110   b , which is formed in the pump cover  112 . A fitting hole  162   a  is formed in a center of the main body  162 , and thereby the main body  162  is shaped into a circular ring form. When the rotatable shaft  104   a  is fitted into the fitting hole  162   a , the main body  162  is securely fitted to the rotatable shaft  104   a  to rotate integrally with the rotatable shaft  104   a.    
     The number of the legs  164  corresponds to the number of the insertion holes  127  of the inner rotor  120 . Specifically, in order to reduce or minimize the influence of the torque ripple of the electric motor  104 , the number of the legs  164  is different from the number of the magnetic poles and the number of the slots of the electric motor  104  and is thereby set to five (5), which is a prime number, in the present embodiment. The legs  164  axially extend from a plurality of locations (five locations in the present embodiment), respectively, on a radially outer side of the fitting hole  162   a , which is a fitting location of the main body  162 . The legs  164  are arranged one after another at equal intervals in the circumferential direction. Each leg  164  is resiliently deformable because of the resilient material and the axially elongated shape of the leg  164 . When the rotatable shaft  104   a  is rotated, each leg  164  is flexed through the resilient deformation thereof in conformity with the corresponding insertion hole  127 . Thereby, the leg  164  contacts an inner wall of the insertion hole  127  while absorbing circumferential dimensional errors of the insertion hole  127  and the leg  164  generated at the manufacturing. In this way, the joint member  160  transmits the drive force of the rotatable shaft  104   a  to the inner rotor  120  through the legs  164 . 
     As shown in  FIG. 5 , the radial bearing  150  is shaped into a cylindrical tubular form. The radial bearing  150  is made of metal and is coated with resin. The rotatable shaft  104   a  is inserted into the inside of the radial bearing  150  such that a cylindrical inner peripheral surface  150   i  of the radial bearing  150  rotatably and slidably supports the rotatable shaft  104   a . A portion of the radial bearing  150  is securely press fitted into a through-hole  116   e  of the pump casing  116 . The radial bearing  150  is non-rotatably fixed to the pump casing  116  through this pressing fitting. Another portion of the radial bearing  150  is inserted into an inside of a cylindrical hole of the inner rotor  120 , such that the cylindrical outer peripheral surface  150   o  of the radial bearing  150  rotably and slidably supports the inner rotor  120 . 
     The high pressure fuel of the high pressure passage  106  penetrates into an area (slide surface) between the cylindrical inner peripheral surface  150   i  of the radial bearing  150  and the outer peripheral surface of the rotatable shaft  104   a  and thereafter leaks from this area (slide surface) into the joint receiving chamber  110   b  after dropping of the pressure of the high pressure fuel in this area (slide surface). Therefore, the joint receiving chamber  110   b  accumulates the fuel (intermediate pressure fuel) that has the pressure, which is lower than the pressure of the high pressure fuel of the high pressure passage  106  and is higher than the pressure of the fuel (suction fuel) of the suction passage  112   a.    
     As shown in  FIGS. 4 and 5 , a first groove  1201  is formed in a surface of the inner rotor  120 , which is axially opposed to the pump casing  116 . The first groove  1201  is shaped into a ring form (annular form) and circumferentially extends about the radial bearing  150 . Furthermore, a second groove  1202  is formed in an opposite surface of the inner rotor  120 , which is axially opposite from the pump casing  116 . The second groove  1202  is shaped into a ring form (annular form) and circumferentially extends about the radial bearing  150 . An outer diameter of the second groove  1202  is the same as an outer diameter of the first groove  1201 . 
     The high pressure fuel of the discharge passage  117  penetrates into an area (slide surface) between the inner rotor  120  and the pump casing  116  and thereafter leaks form this area (slide surface) into the first groove  1201  after dropping of the pressure of the high pressure fuel in this area (slide surface). Therefore, the first groove  1201  accumulates the fuel (intermediate pressure fuel) that has the pressure, which is lower than the pressure of the high pressure fuel of the high pressure passage  106  and is higher than the pressure of the fuel (suction fuel) of the suction passage  112   a . The second groove  1202  is filled with the intermediate pressure fuel of the joint receiving chamber  110   b . Since both of the first groove  1201  and the second groove  1202  are shaped into the ring form and have the same outer diameter, the pressure (the intermediate pressure) of the fuel accumulated in the first groove  1201  and the pressure (the intermediate pressure) of the fuel accumulated in the second groove  1202  are balanced with each other. Therefore, it is possible to limit tilting of the inner rotor  120 , which would be otherwise caused by the intermediate pressure fuel. 
     Next, with reference to  FIGS. 6 and 7 , the structure of the pump casing  116  will be described in detail. 
     A slide surface of the recessed bottom portion  116   c  of the pump casing  116 , which is slidable relative to the inner rotor  120 , includes an external tooth slide surface  116   c   2  and a main body slide surface  116   c   1 . The external teeth  122  of the inner rotor  120  are slidable relative to the external tooth slide surface  116   c   2 . The main body  121  of the inner rotor  120  is slidable relative to the main body slide surface  116   c   1 . A dotted area of  FIGS. 6 and 7  indicates the main body slide surface  116   c   1 . Another slide surface of the recessed bottom portion  116   c , which is slidable relative to the internal teeth  132   a  of the outer rotor  130 , is referred to as an internal tooth slide surface  116   c   3 . A surface of the recessed bottom portion  116   c , which is opposed to the first groove  1201  of the inner rotor  120 , is referred to as a groove opposing surface  116   c   4 . 
     The opposing suction groove  118  and the discharge passage  117  are formed in a rotational path range of the external tooth slide surface  116   c   2  in the recessed bottom portion  116   c . Therefore, each corresponding portion of the recessed bottom portion  116   c , which is circumferentially located between the opposing suction groove  118  and the discharge passage  117 , serves as the external tooth slide surface  116   c   2 . 
     The groove opposing surface  116   c   4  is formed in an annular region, which circumferentially extends along a peripheral edge of the through-hole  116   e . The groove opposing surface  116   c   4  is not slidable relative to the inner rotor  120 . The main body slide surface  116   c   1  is formed in an annular range, which is radially located between the rotational path range of the external tooth slide surface  116   c   2  and the groove opposing surface  116   c   4 . In other words, the main body slide surface  116   c   1  is located in the range, which is on the radially inner side of the opposing suction groove  118  and the discharge passage  117  in the radial direction of the rotational axis and is on the radially outer side of the first groove  1201  in the radial direction of the rotational axis. The main body slide surface  116   c   1 , the external tooth slide surface  116   c   2 , the internal tooth slide surface  116   c   3 , and the groove opposing surface  116   c   4  are placed on a common plane. 
     The recessed bottom portion  116   c  is processed through a surface treatment such that a surface roughness of the main body slide surface  116   c   1  is higher than a surface roughness of the external tooth slide surface  116   c   2 . Specifically, first, all of the main body slide surface  116   c   1 , the external tooth slide surface  116   c   2 , the internal tooth slide surface  116   c   3  and the groove opposing surface  116   c   4  are cut with a lath (a cutting process). Thereafter, the main body slide surface  116   c   1  and the groove opposing surface  116   c   4  are processed by electrical discharge machining (an electrical discharge machining process). In this electrical discharge machining process, the external tooth slide surface  116   c   2  and the internal tooth slide surface  116   c   3  are not processed by the electrical discharge machining. 
     For example, at the time of processing the recessed bottom portion  116   c  with an electrode E, which is shaped into a circular disk form and is indicated by a dot-dash line in  FIG. 7 , an outer diameter of the electrode E is set to be the same as a diameter of the main body slide surface  116   c   1 . Specifically, a radial location of an outer peripheral surface (a radially outer end surface) Ea of the electrode E is set to coincide with a radial location of an outer peripheral edge of the main body slide surface  116   c   1 . In this way, the main body slide surface  116   c   1  can be processed by the electrical discharge machining without processing the external tooth slide surface  116   c   2  by the electrical discharge machining. Now, a procedure of the electrical discharge machining process will be described. First of all, the electrode E is placed to contact the recessed bottom portion  116   c . Next, the electrode E is spaced away from the recessed bottom portion  116   c  by a predetermined distance to place the electrode E in a state shown in  FIG. 7 . Then, a voltage is applied to the electrode E to generate electrical discharges (sparks) between the pump casing  116  and the electrode E. Thereby, a portion of the recessed bottom portion  116   c , which is opposed to the electrode E, i.e., the main body slide surface  116   c   1  and the groove opposing surface  116   c   4  are processed by the electrical discharge machining. However, the other portion of the recessed bottom portion  116   c , which is not opposed to the electrode E, i.e., the external tooth slide surface  116   c   2  and the internal tooth slide surface  116   c   3  are not processed by the electrical discharge machining. 
     Next, there will be described the technical significance of processing the main body slide surface  116   c   1  by the electrical discharge machining without processing the external tooth slide surface  116   c   2  by the electrical discharge machining. 
     As shown in  FIG. 8 , in the state before execution of the electrical discharge machining process, the pump casing  116  and the pump cover  112  are cut with the lath in the cutting process such that an axial dimension L of the rotor receiving chamber  110   a  is within a predetermined dimensional tolerance. Specifically, the contact surface  116   f  (see  FIG. 5 ) of the pump casing  116 , which contacts the pump cover  112 , a top surface  112   b  of the pump cover  112 , the recessed bottom portion  116   c  are cut such that a surface roughness is within a first predetermined value Ra 1 . A value, which is defined by, for example, arithmetic mean deviation of the profile, is used as the first predetermined value Ra 1 . 
     The first predetermined value Ra 1  is set such that a required sealing performance is achieved between the contact surface  116   f  of the pump casing  116  and the top surface  112   b  of the pump cover  112 . Furthermore, the first predetermined value Ra 1  is also set such that the sufficient sealing performance is achieved between the external tooth slide surface  116   c   2  and the external teeth  122  and also between the internal tooth slide surface  116   c   3  and the internal teeth  132   a.    
     With reference to  FIG. 9 , for example, a clearance distance (discharge distance) between the recessed bottom portion  116   c  and the electrode E, a discharge electric power, a discharge frequency, and a discharge time period are set such that the surface roughness of the processed surface, which is processed through the electrical discharge machining, becomes higher than the surface roughness of the unprocessed surface, which is not processed through the electrical discharge machining. In other words, the main body slide surface  116   c   1  is processed by the electrical discharge machining such that the surface roughness of the main body slide surface  116   c   1  becomes equal to or larger than a second predetermined value Ra 2 . The second predetermined value Ra 2  is set to be a value that is larger than the first predetermined value Ra 1 . 
     With this setting, the surface roughness of the main body slide surface  116   c   1  becomes higher than the surface roughness of the external tooth slide surface  116   c   2 . That is, the surface roughness of the external tooth slide surface  116   c   2  becomes less than the first predetermined value Rat and the surface roughness of the main body slide surface  116   c   1  becomes equal to or larger than the second predetermined value Ra 2 , and a large number of grooves Pa are formed in the main body slide surface  116   c   1 . In the case where the electrical discharge machining process is executed, a surface roughness profile of the processed surface, which is processed by the electrical discharge machining process, is formed such that the grooves Pa are formed in the processed surface without substantially generating protrusions from the location of the unprocessed surface, which is the surface before the execution of the electrical discharge machining (see  FIG. 9 ). 
     Therefore, in a case where an axial location of a maximum peak height Rp of the roughness profile (more specifically, an axial location of a top end of the peak having the maximum peak height Rp) is defined as a maximum peak location in each of the main body slide surface  116   c   1  and the external tooth slide surface  116   c   2 , the maximum peak location (see a reference sign P 2  in  FIG. 9 ) of the main body slide surface  116   c   1  is the same as the maximum peak location of the external tooth slide surface  116   c   2 . Therefore, the axial dimension L does not substantially change between the time before the execution of the electrical discharge machining process and the time after the execution of the electrical discharge machining process. Furthermore, in a case where an axial location of a maximum valley depth Rv of the roughness profile (more specifically, an axial location of a bottom end of the valley having the maximum valley depth Rv) is defined as a maximum valley location in each of the main body slide surface  116   c   1  and the external tooth slide surface  116   c   2 , the maximum valley location (see a reference sign P 1  in  FIG. 9 ) of the main body slide surface  116   c   1  is spaced further away from the inner rotor  120  in comparison to the maximum valley location of the external tooth slide surface  116   c   2 . Thereby, the grooves Pa are formed. 
     In contrast, in a case where a shot blasting process, which is a mechanical process, is used in place of the electrical discharge machining process, although the surface roughness, which is produced by the shot blasting process, may be the same as the surface roughness, which is produced by the electrical discharge machining, the surface roughness profile, which is produced by the shot blasting process, differs from the surface roughness profile, which is produced by the electrical discharge machining process as follows. That is, in the case of the shot blasting process, although the grooves Pa are formed, the surface roughness profile includes portions (protrusions Pb), which protrude from the location of the unprocessed surface that is the surface before the execution of the shot blasting process. The shot blasting process is a process of forcefully propelling blast media, which includes abrasive particles, against the subject surface to roughen the subject surface. In the subject surface, spots, against which the media collide, are depressed to form the grooves Pa. However, these spots are plastically deformed to form the grooves Pa. Therefore, each surrounding area, which surrounds the corresponding spot, is bulged. 
     In such a case, as indicated by a dot-dash line in  FIG. 10 , the maximum peak location (see the reference sign P 2  in  FIG. 10 ) of the main body slide surface  116   c   1  is placed to be closer to the inner rotor  120  in comparison to the maximum peak location of the external tooth slide surface  116   c   2 . Therefore, the axial dimension L after the time of executing the shot blasting process is reduced in comparison to the axial dimension L before the time of executing shot blasting process. Thus, the axial dimension L substantially changes between the time before the execution of the shot blasting process and the time after the execution of the shot blasting process. 
     Now, advantages of the present embodiment will be described. 
     When the temperature of the fuel is low to have the high viscosity, the tilting force is applied to the inner rotor  120 . With respect to the above-described disadvantage, according to the present embodiment, the inner rotor  120  is coupled to the rotatable shaft  104   a  through the joint member  160 , so that the above-described tilting force is absorbed through the resilient deformation of the joint member  160 , and thereby the slide resistance between the radial bearing  150  and the rotatable shaft  104   a  is reduced. 
     Furthermore, according to the present embodiment, the surface roughness of the main body slide surface  116   c   1  is higher than the surface roughness of the external tooth slide surface  116   c   2 . Therefore, since the external tooth slide surface  116   c   2  has the low surface roughness, it is possible to have the sufficient sealing performance between the external teeth  122  of the inner rotor  120  and the external tooth slide surface  116   c   2 . The main body slide surface  116   c   1  has the high surface roughness, so that the fuel can penetrate into the area (the grooves Pa) between the main body  121  of the inner rotor  120  and the main body slide surface  116   c   1  to implement the lubricating function. Therefore, even when the main body  121  of the inner rotor  120  is urged against the main body slide surface  116   c   1  of the pump casing  116  due to the formation of the joint receiving chamber  110   b , the lubricating function is implemented to sufficiently reduce the slide resistance. 
     Thereby, according to the present embodiment, there is implemented the structure, which can absorb the tilting force with the joint member  160  and can sufficiently reduce the slide resistance of the inner rotor  120 . 
     Furthermore, in the present embodiment, the main body slide surface  116   c   1  is processed by the electrical discharge machining, so that the surface roughness of the main body slide surface  116   c   1  becomes higher than the surface roughness of the external tooth slide surface  116   c   2 . In this way, the grooves Pa are formed while limiting the generation of the protrusions Pb shown in  FIG. 10 . Thus, the decrease of the axial dimension L can be limited by the electrical discharge machining, and thereby the rotor receiving chamber  110   a  having the high dimensional accuracy can be provided. 
     Furthermore, in the present embodiment, in the case where the axial location of the maximum peak height Rp of the roughness profile (more specifically, the axial location of the top end of the peak having the maximum peak height Rp) is defined as the maximum peak location in each of the main body slide surface  116   c   1  and the external tooth slide surface  116   c   2 , the maximum peak location of the main body slide surface  116   c   1  is the same as the maximum peak location of the external tooth slide surface  116   c   2 . Therefore, the axial location of the main body slide surface  116   c   1  can be set to be the same as the axial location of the external tooth slide surface  116   c   2 . Thus, the excessive increase of the slide resistance of the main body  121  and the external teeth  122  can be limited, and the sufficient sealing performance can be obtained. 
     Second Embodiment 
     In the first embodiment, the surface roughness of the portion of the slide surface of the pump casing  116  is increased to implement the lubricating function. Thereby, the provision of the joint member  160  and the decrease of the slide resistance of the inner rotor  120  are both achieved. In the present embodiment, a surface roughness of a portion of the inner rotor  120  is increased to implement the lubricating function. 
     As shown in  FIG. 11 , similar to the first embodiment, the inner rotor  120  has the main body  121  and the external teeth  122 . Similar to the first embodiment, the first groove  1201  and the insertion holes  127  are formed in the main body  121 . The slide surface  125  of the inner rotor  120 , which is slidable relative to the recessed bottom portion  116   c  of the pump casing  116 , is divided into an external tooth slide surface  122   a , which is formed by the external teeth  122 , and a main body slide surface  121   a , which is formed by the main body  121 . The external tooth slide surface  122   a  serves as a rotor side external tooth slide surface of the present disclosure, and the main body slide surface  121   a  serves as a rotor side main body slide surface of the present disclosure. The main body slide surface  121   a  is a dotted area of  FIG. 11  and is located between the first groove  1201  and the external teeth  122  in the radial direction. 
     The external tooth slide surface  122   a  and the main body slide surface  121   a  are located in a common plane. The slide surface  125  is processed through a surface treatment such that a surface roughness of the main body slide surface  121   a  is higher than a surface roughness of the external tooth slide surface  122   a . Specifically, first, all of the main body slide surface  121   a  and the external tooth slide surface  122   a  are cut with a lath (a cutting process). Thereafter, the main body slide surface  121   a  is processed by the electrical discharge machining (an electrical discharge machining process). In this electrical discharge machining process, the external tooth slide surface  122   a  is not processed by the electrical discharge machining. For example, the main body slide surface  121   a  can be processed by the electrical discharge machining without processing the external tooth slide surface  122   a  by executing the electrical discharge machining process with an electrode having a shape that corresponds to the main body slide surface  121   a.    
     Thereby, according to the present embodiment, the surface roughness of the main body slide surface  121   a  is higher than the surface roughness of the external tooth slide surface  122   a . Thus, since the surface roughness of the external tooth slide surface  122   a  is small, it is possible to implement the sufficient sealing performance between the external tooth slide surface  116   c   2  of the pump casing  116  and the external tooth slide surface  122   a  of the inner rotor  120 . Furthermore, the main body slide surface  121   a  has the high surface roughness, so that the fuel can penetrate into the area (the grooves) between the main body slide surface  116   c   1  of the pump casing  116  and the main body slide surface  121   a  of the inner rotor  120  to implement the lubricating function. Therefore, even when the main body  121  of the inner rotor  120  is urged against the main body slide surface  121   a  of the pump casing  116  due to the formation of the joint receiving chamber  110   b , the lubricating function is implemented to sufficiently reduce the slide resistance. 
     Thereby, according to the present embodiment, there is implemented the structure, which can absorb the tilting force with the joint member  160  and can sufficiently reduce the slide resistance of the inner rotor  120 . 
     Furthermore, in the present embodiment, the main body slide surface  121   a  is processed by the electrical discharge machining, so that the surface roughness of the main body slide surface  121   a  becomes higher than the surface roughness of the external tooth slide surface  122   a . In this way, the grooves Pa shown in  FIG. 9  are formed in the inner rotor  120  while limiting the generation of the protrusions Pb shown in  FIG. 10 . Thus, the decrease of the axial dimension L can be limited by the electrical discharge machining, and thereby the rotor receiving chamber  110   a  having the high dimensional accuracy can be provided. 
     Other Embodiments 
     The present disclosure has been described with respect to the above embodiments. However, the present disclosure is not limited to the above embodiments, and the above embodiments may be modified in various ways within a principal of the present disclosure. 
     In the embodiment shown in  FIG. 7 , the electrode E, which is shaped into the circular disk form, is used for the electrical discharge machining process. Alternative to this electrode E, an electrode, which is shaped into a ring form (annular form) having a through-hole at the center thereof, may be used. In the case where the electrode E is shaped into the circular disk form shown in  FIG. 7 , the groove opposing surface  116   c   4  is also processed by the electrical discharge machining in addition to the main body slide surface  116   c   1 . However, the groove opposing surface  116   c   4 , which does not need to be processed by the electrical discharge machining, is also processed by the electrical discharge machining, and thereby the electrical discharges (sparks) are also applied to the through-hole  116   e , which does not require the electrical discharges (sparks). In contrast, in the case where the electrode, which is shaped into the ring form, is used, the electrical discharges (sparks) are not applied to the through-hole  116   e . Furthermore, when the through-hole of the electrode, which is shaped into the ring form, is positioned at the boundary between the main body slide surface  116   c   1  and the groove opposing surface  116   c   4 , it is possible to avoid the processing of the groove opposing surface  116   c   4  by the electrical discharge machining. Therefore, the electric power consumption can be reduced. 
     In each of the above embodiments, the surface roughness of the portion of the slide surface of the pump casing  116  or the surface roughness of the portion of the slide surface of the inner rotor  20  is increased by the electrical discharge machining. However, the present disclosure is not limited to this electrical discharge machining. For example, the surface roughness of the portion of the slide surface of the pump casing  116  or the surface roughness of the portion of the slide surface of the inner rotor  20  may be increased by, for example, the shot blasting of  FIG. 10 . Here, it should be noted that in the case of the electrical discharge machining, the entire subject surface is cut with the lath and is thereafter processed by the electrical discharge machining. However, in the case of the shot blasting, it is desirable that the entire subject surface is processed by the shot blasting and is thereafter cut with the lath. With this procedure, it is possible to limit the change of the axial dimension L by the protrusions Pb generated by the short blasting. Furthermore, the method of increasing the surface roughness of the portion of the slide surface may be, for example, magnetic fluid polishing, electropolishing, or corrosion with etching agent besides the electrical discharge machining or the shot blasting. 
     In the embodiment shown in  FIG. 4 , the external teeth  122  and the internal teeth  132   a  are shaped to have the trochoid tooth profile. Alternatively, the external teeth  122  and the internal teeth  132   a  may be shaped to have any other suitable type of tooth profile, such as a cycloid tooth profile or a profile of a combination of various curved lines. 
     The subject fluid to be pumped with the fluid pump  101  is not limited to the light oil (diesel fuel) and may be any other liquid fuel, such as gasoline or alcohol. Furthermore, the subject fluid to be pumped with the fluid pump  101  is not limited to the fuel and may be liquid, such as hydraulic oil used in a hydraulic actuator or any of various lubricant oils. The fluid pump  101  is not limited to the fluid pump installed in the vehicle. 
     In the embodiment shown in  FIG. 1 , the present disclosure is implemented in the fluid pump  101  that has the pump main body  103  and the electric motor  104 , which are integrated together. However, the electric motor  104  may not be provided in the fluid pump  101  of the present disclosure, and the electric motor  104  may be formed separately from the rest of the fluid pump  101 . In the embodiment shown in  FIG. 1 , the inner rotor  120  is driven by the electric motor  104 . Alternatively, the inner rotor  120  may be driven to rotate by a portion of a drive force for driving the vehicle, such as a drive force of a crankshaft of an internal combustion engine of the vehicle. 
     In the embodiment shown in  FIG. 1 , the discharge passage  117  is located on the opposite side of the pump housing  110 , which is opposite from the suction passage  112   a  in the axial direction. Alternatively, the discharge passage  117  and the suction passage  112   a  may be placed on the same axial side of the pump housing  110 .