Patent Publication Number: US-2007122264-A1

Title: Pump

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims priority to Japanese Patent Application numbers 2005-341956, 2006-2734 and 2006-126268, the contents of which are hereby incorporated by reference into the present application.  
     BACKGROUND OF THE INVENTION 1. Field of the Invention  
      The present invention relates to a pump for drawing in a fluid such as fuel etc., increasing pressure thereof, and discharging the pressurized fuel.  
      2. Description of the Related Art  
      This type of pump normally includes a substantially disk-shaped impeller and a casing that houses the impeller so that the impeller can rotate. On both the front and reverse surfaces of the impeller, a group of concavities is formed. Concavities forming each group are repeated in the circumferential direction. Circular arc-shaped grooves extending from an upstream end to a downstream end are formed in the two casing internal surfaces in the area in opposition to the groups of concavities on the impeller. The pump flow path is formed by the. groups of concavities on the impeller and the circular arc-shaped grooves on the casing. An intake hole is formed in the casing. The intake hole links the upstream end of the pump flow path with the outside of the casing. An discharge hole is formed in the casing. The discharge hole links the downstream end of the pump flow path with the outside of the casing. When the impeller rotates within the casing, fluid is drawn from the intake hole into the pump flow path. Fluid that is drawn into the pump flow path is pressurized while flowing to the downstream end of the pump flow path. The pressurized fluid is expelled outside the casing from the discharge hole.  
      In this pump, the pressure acting on the front and reverse surfaces of the impeller tends to be non-uniform. Also, the difference in pressure acting on the front and reverse surfaces of the impeller tends to be non-uniform according to position in the circumferential direction. For example, fluid is drawn into the pump flow path within the casing from the intake hole, is pressurized within the pump flow path, and is expelled from the discharge hole. At the upstream end of the pump flow path connecting with the intake hole, fuel is drawn in from one surface of the impeller, but fuel is not drawn in from the other surface. Also, at the downstream end of the pump flow path connected to the discharge hole, fuel is expelled from one surface of the impeller, but fuel is not expelled from the other surface of the impeller. As a result, near the intake hole and discharge hole the difference in pressure acting on the front and reverse surfaces of the impeller becomes larger. If the difference in pressure acting on the impeller varies with position in the circumferential direction the impeller will incline with respect to the axis of the impeller, and contact between impeller and casing will occur. If the impeller continues rotating in this condition, the performance of the pump will be reduced by friction losses and wear.  
      Therefore, in the pump disclosed in Japanese Laid-open Patent Publication No.6-280776, depression-shaped grooves in a U-shape are formed in both the front and reverse surfaces of the impeller. In this pump, when the impeller rotates, fluid flows into these depression-shaped grooves. When the fluid that has flowed into the depression-shaped grooves is discharged from the depression-shaped grooves, a component of velocity in the axial direction of the impeller is produced. Therefore, the fluid that is discharged from the depression-shaped grooves presses the casing internal surface in the axial direction. In this way, a force is generated that acts in the direction to increase the clearance between the impeller and the casing internal surface, contact between the impeller and the casing is prevented, and the pump efficiency is improved. However, it is difficult to prevent inclination of the impeller by simply forming depression-shaped grooves in the impeller, and it was not possible to prevent sufficiently contact between the impeller and the casing.  
     BRIEF SUMMARY OF THE INVENTION  
      It is, accordingly, an object of the present teachings to provide a pump capable of effectively suppressing contact between the impeller and the casing.  
      In one aspect of the present teachings, a pump may comprise a casing, and a substantially disk-shaped impeller that rotates within the casing. A group of concavities may be formed on both front and reverse surfaces of the impeller. Concavities forming each group may be repeated in a circumferential direction of the impeller. A first groove may be formed on a first casing internal surface in opposition to the front surface of the impeller. The first groove may extend from an upstream end to a downstream end in an area facing one of the groups of concavities of the impeller. A second groove may be formed on a second casing internal surface in opposition to the reverse surface of the impeller. The second groove may extend from an upstream end to a downstream end in an area facing the other of the groups of concavities of the impeller. An intake hole and a discharge hole may be formed in the casing. The intake hole may link the upstream end of one of the first groove and the second groove with the outside of the casing, and the discharge hole may link the downstream end of the other of the first groove and the second groove with the outside of the casing. Therefore, when the impeller rotates, fluid is drawn into the casing from the intake hole. The fluid drawn into the casing is pressurized by the impeller, and is expelled outside the casing from the discharge hole.  
      In one aspect of the present teachings, a group of depression-shaped grooves may be formed in at least the first and second casing internal surfaces. Each of depression-shaped grooves may extend from the center towards the outer periphery while shifting in the direction of rotation of the impeller. The group of depression-shaped grooves may be asymmetrically formed with respect to the axis of rotation of the impeller in accordance with the position of the first and second grooves.  
      In this pump, when the impeller rotates, fluid in the clearance between the impeller and the casing is drawn into the group of depression-shaped grooves, and propelled from the center towards the outer periphery. This direction is the same as the direction of the centrifugal force acting on the fluid in the clearance between the impeller and the casing. Therefore, when the impeller rotates, a force that propels the fluid within the group of depression-shaped grooves from the center towards the outer periphery is efficiently generated. The fluid that is propelled from the center towards the outer periphery within the group of depression-shaped grooves presses on the surface of the casing in opposition to the impeller, and generates an effective lift force (i.e., a force acting in the direction to increase the clearance between the impeller and the casing internal surface) on the impeller.  
      Also, the groups of depression-shaped grooves are formed asymmetrically in accordance with the position of the first groove and the second groove formed on the casing internal surfaces. By forming the group of depression-shaped grooves asymmetrically, it is possible to vary the magnitude of the lift force generated on the impeller according to the area on the impeller. By forming the group of depression-shaped grooves asymmetrically according to the position of the first groove and the second groove, it is possible to increase the lift force generated on the impeller in the areas where the pressure difference is large, and to decrease the lift force generated on the impeller in the areas where the pressure difference is small. In this way, it is possible to eliminate the non-uniformity in the pressure difference between the front and reverse surfaces of the impeller. In this way, it is possible to suppress the inclination of the impeller with respect to the axis of the impeller, and suppress contact between the impeller and the casing.  
      Forming the groups of depression-shaped grooves asymmetrically may be achieved, for example, by forming areas where the adjacent grooves are closely spaced and areas where the grooves are more widely spaced, or forming areas where the length of grooves is long and areas where the length of grooves is short, or forming areas where the depth of grooves is deep and areas where the depth of grooves is shallow, or forming areas where the width of grooves is wide and areas where the width of grooves is narrow, or forming areas where there are grooves and areas where there are none.  
      In the above pump, it is preferred that in the area near the discharge hole in the casing internal surface and/or in the area near the intake hole in the casing internal surface, the lift forces generated on the impeller by the group of depression-shaped grooves are greater than in other areas.  
      Fluid is drawn into the casing from the intake hole, pressurized within the casing and expelled from the discharge hole. Therefore, near the intake hole and the discharge hole the difference in pressure acting on the front and reverse surfaces of the impeller tends to be large. If the group of depression-shaped grooves is formed so that the lift force generated on the impeller in the area near the intake hole and/or the discharge hole is larger than in other areas the pressure difference that varies with the area can be effectively cancelled out.  
      Alternatively, the group of depression-shaped grooves that generates lift forces on the impeller may be formed in the area near the discharge hole and/or the area near the intake hole only. According to this configuration, lift forces act only on the impeller in the area near the discharge hole and/or in the area near the intake hole, so it is possible to effectively cancel out the pressure difference that varies with the area.  
      It is preferably that each depression-shaped groove comprising the group of depression-shaped grooves extends from the center of the impeller towards the outer periphery in a spiral shape. By forming the grooves in a spiral shape from the center towards the outer periphery it is possible to smoothly draw the fluid into the grooves.  
      In another aspect of the present teachings, in at least one surface from among the front and reverse surfaces of the impeller and the first and second casing internal surfaces, depression-shaped grooves may be formed so that fluid in the clearance between the impeller and the casing is pressurized and a force is generated in the direction that increases the clearance between the impeller and the casing when the impeller rotates. Also, the clearance between the surface on which the depression-shaped grooves are formed and the surface in opposition thereto when the impeller is not inclined with respect to the casing may increase from the center of the impeller towards the outer periphery of the impeller.  
      In this pump, the clearance between the surface on which the depression-shaped grooves are formed and the surface in opposition to this surface increases towards the outer periphery of the impeller. Therefore, even if the impeller inclines slightly, the outer periphery of the impeller and the casing internal surface will not contact. On the other hand at the center of the impeller the clearance between the surface on which the depression-shaped grooves are formed and the surface in opposition to this surface is small. The lift force (i.e., force acting in the direction to increase the clearance between the impeller and the casing) generated by the fluid within the depression-shaped grooves increases the smaller the clearance. Therefore, it is possible to increase the lift force generated by the fluid within the depression-shaped grooves In this way, it is possible to prevent large inclination of the impeller, and contact of the impeller and casing can be suppressed.  
      In this pump, it is preferred that one surface from among the surface in which the depression-shaped grooves is formed and the surface in opposition thereto is formed as a flat plane, and the other surface is formed in a tapered shape so that the clearance with the impeller increases from the center of the impeller towards the outer periphery of the impeller. According to this configuration, one surface from among the surface in which the depression shaped grooves are formed and the surface that is in opposition to this surface is formed as a flat plane, so processing of this plane is simple.  
      It is preferred that the depression-shaped grooves extend from the center of the impeller towards the outer periphery in a spiral shape. Also, the depression-shaped grooves may be formed on the impeller or on the casing.  
      In another aspect of the present teachings, an intake hole and a discharge hole may be formed in the casing. The intake hole links the upstream end of a pump flow path formed by the groups of concavities, the first groove, and the second groove with the outside of the casing. The discharge hole links the downstream end of the pump flow path with the outside of the casing. Depression-shaped grooves sealed from the pump flow path may be formed on at least one surface from among the intake hole side impeller surface and the intake hole side casing internal surface, and depression-shaped grooves sealed from the pump flow path may be formed in neither the discharge hole side impeller surface nor the discharge hole side casing internal surface.  
      In this type of pump, the force due to the pressure difference of the fluid in the pump flow path acts in a direction to press the impeller towards the intake hole side casing internal surface. In other words, the fluid drawn into the pump flow path is pressurized as it flows from the upstream side (i.e., intake hole side) of the pump flow path to the downstream side (i.e., discharge hole side). Therefore, the fluid in the pump flow path is at a higher pressure in the discharge hole side than the intake hole side Therefore, the surface of the discharge hole side of the impeller is subjected to a higher pressure than the surface of the intake hole side, so the impeller is subject to a force in the direction of the intake hole side casing internal surface.  
      In this pump, depression-shaped grooves are not formed on the discharge hole side of the impeller surface or the discharge hole side of the casing internal surface. Therefore, a lift force is not generated on the discharge hole side impeller surface; a lift force is only generated on the intake hole side impeller surface. The lift force acting on the intake hole side impeller surface acts in a direction to cancel out the force (i.e., force acting in the direction to press the impeller towards the intake hole side casing internal surface) generated by the difference in pressure of the fluid in the casing. In this way, pressing the impeller towards and contact with the intake hole side casing internal surface, is suppressed.  
      In this pump, it is preferred that a projecting portion is formed in the discharge hole side casing internal surface forming a loop in the circumferential direction of the impeller. If a projecting portion is formed in the discharge hole side casing internal surface, even if there is contact between the discharge hole side casing internal surface and the impeller, the discharge hole side casing internal surface and the impeller only contact locally. Therefore, it is possible to reduce the friction losses when the impeller and casing contact.  
      Further, it is preferred that the projecting portion is positioned to the inside of the pump flow path. By providing the projecting portion to the inside of the pump flow path it is possible to suppress leakage of fluid from the pump flow path that passes the projecting portion and flows into the clearance on the discharge hole side. In this way, it is possible to efficiently pressurize of the fluid within the casing, and the pump performance can be improved.  
      Also, this pump may further include a motor chamber provided on the outside of the casing and a motor housed within the motor housing. The motor may have a shaft that rotates. In this case, it is preferred that the discharge hole links the pump flow path and the motor chamber and a through hole that is penetrated by the motor shaft are formed in the casing, and one end of the motor shaft is fitted to the impeller.  
      According to this configuration, the impeller is subject to a force in the direction of the intake bole side casing internal surface as a result of high pressure fluid that flows backwards from the motor chamber into the casing via the gap between the shaft and the through hole. As a result, contact between the impeller and the discharge hole side casing internal surface is suppressed. Also, even if the impeller is subject to a force in the direction of the intake hole side casing internal surface, the lift force generated by the depression-shaped grooves acts in the direction to cancel out that force, so contact between the impeller and the intake hole side casing internal surface is suppressed.  
      In another aspect of the present teachings, an intake hole and a discharge hole may be formed in the casing. The intake hole may link the upstream end of a pump flow path formed by the groups of concavities, the first groove, and the second groove with the outside of the casing. The discharge hole may link the downstream end of the pump flow path with the outside of the casing. Intake hole side depression-shaped grooves may be formed on at least one surface from among the intake hole side impeller surface and the intake hole side casing internal surface so that when the impeller rotates fluid is pressurized and a lift force is generated that acts in the direction to increase the clearance between the intake hole side impeller surface and the intake hole side casing internal surface. Discharge hole side depression-shaped grooves may be formed on at least one surface from among the discharge hole side impeller surface and the discharge hole side casing internal surface so that when the impeller rotates fluid is pressurized and a lift force is generated that acts in the direction to increase the clearance between the discharge hole side impeller surface and the discharge hole side casing internal surface. The number and/or shape of the discharge hole side depression-shaped grooves are preferably determined so that the lift force generated is smaller than the lift force generated by the intake hole side depression-shaped grooves, in accordance with the number and/or shape of the intake hole side depression-shaped grooves.  
      According to this pump also, it is possible to suppress pressing the impeller towards and contact with the intake hole side casing internal surface.  
      These aspects and features may be utilized singularly or, in combination, in order to make improved pump. In addition, other objects, features and advantages of the present teachings will be readily understood after reading the following detailed description together with the accompanying drawings and claims. Of course, the additional features and aspects disclosed herein also may be utilized singularly or, in combination with the above-described aspect and features.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a vertical section through a pump according to a first representative embodiment of the present teachings.  
       FIG. 2  is a section through the line II-II in  FIG. 1 .  
       FIG. 3  is a section on the line III-III in  FIG. 1 .  
       FIG. 4  is a view corresponding to the section through the line II-II in  FIG. 1  for a pump according to a second representative embodiment of the present teachings.  
       FIG. 5  is a view corresponding to the section through the line III-III in  FIG. 1  for a pump according to a second representative embodiment of the present teachings.  
       FIG. 6  is a diagram to explain an example of the depression-shaped grooves formed on the pump casing.  
       FIG. 7  is a diagram to explain another example of the depression-shaped grooves formed on the pump casing.  
       FIG. 8  is a section on the line VIII-VIII in  FIG. 7 .  
       FIG. 9  is a section on the line IX-IX in  FIG. 7 .  
       FIG. 10  is a vertical section through a pump according to a third representative embodiment of the present teachings.  
       FIG. 11  is a plan (of the discharge hole side surface) of the impeller of the pump shown in  FIG. 10 .  
       FIG. 12  is a view to explain the shape of the grooves formed in the discharge hole side surface of the impeller shown in  FIG. 11 .  
       FIG. 13  is a section showing an enlargement of the pump according to the third representative embodiment.  
       FIG. 14  is a vertical section through a pump according to a fourth representative embodiment of the present teachings.  
       FIG. 15  is a plan view of the impeller of the fourth representative embodiment viewed from the bottom.  
       FIG. 16  is a diagram to explain the shape of the grooves formed in the bottom surface of the impeller shown in  FIG. 15 .  
       FIG. 17  is a section showing an enlargement of the pump according to the fourth representative embodiment of the present teachings.  
       FIG. 18  is a section showing an enlargement of the Dump according to a fifth representative embodiment of the present teachings.  
       FIG. 19  is a plan of the impeller according to the fifth representative embodiment viewed from the bottom.  
       FIG. 20  is a plan showing another example of the depression-shaped grooves formed in the top surface of the impeller.  
       FIG. 21  is a plan showing another example of the depression-shaped grooves formed in the top surface of the impeller.  
       FIG. 22  is a plan showing another example of the depression-shaped grooves formed in the top surface of the impeller.  
       FIG. 23  shows the schematic relationship between the direction of the depression-shaped grooves and the direction of flow of the fuel within the clearance. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     First Representative Embodiment  
      A Wesco pump  10  according to a first representative embodiment of the present teachings is explained with reference to the drawings. The Wesco pump  10  may be used as fuel pump for an automobile. The Wesco pump  10  may be utilized within a fuel tank, being utilized for supplying fuel to an engine of the automobile.  
      As shown in  FIG. 1 , the Wesco pump  10  includes a motor unit  12  and a pump unit  14 . The motor unit  12  has a rotor  18 . The rotor  18  includes a shaft  20 , a laminated iron core  22  fixed to the shaft  20 , a coil (not shown in the drawings) wound around the laminated iron core  22 , and a commutator  24  connected to the ends of the coil. The shaft  20  is rotatably supported by a housing  16  via bearings  26 ,  28 . A permanent magnet  30  is fixed to the inside of the housing  16  so as to surround the rotor  18 . Terminals which are not shown on the drawings are provided on a top cover  32  attached to the top of the housing  16 , to supply electricity to the motor unit  12 . The coil is activated via a brush  34  and the commutator  24 , to rotate the rotor  18  and shaft  20 .  
      The lower part of the housing  16  houses the pump unit  14 . The pump unit  14  includes a substantially disk-shaped impeller  36 . On the top surface of the impeller  36 , a group of concavities  3   a  is formed along the outer periphery. On the bottom surface of the impeller  36 , a group of concavities  3   b  is formed along the outer periphery. A through hole is formed in the center of the impeller  36 , connected to the shaft  20  so as to prevent relative rotation.  
      A pump casing  39  that houses the impeller  36  includes a pump cover  38  and a pump body  40 .  
      As shown in  FIG. 2 , a groove  38   a  is formed in the pump cover  38  in the area in opposition to the group of concavities  36   a . The groove  38   a  is formed in an approximately C-shape stretching from the upstream end to the downstream end along the direction of rotation of the impeller  36 . An discharge hole  50  is formed in the pump cover  38  from the downstream end of the groove  38   a  to the top surface of the pump cover  38 . The discharge hole  50  links the interior of the pump casing  39  with the exterior (i.e., the internal space of the motor unit  12 ). A first pump flow path  44  is formed by the group of concavities  36   a  and the groove  38   a . A group of depression-shaped grooves  38   b ,  38   b , . . . is provided on the bottom surface of the pump cover  38  centralized in the radial direction. The group of depression-shaped grooves  38   b ,  38   b , . . . is described later.  
      As shown in  FIG. 3 , a groove  40   a  is formed in the pump body  40  in the area in opposition to the group of concavities  36   b . Similar to the groove  38   a , the groove  40   a  is formed in an approximate C-shape stretching from the upstream end to the downstream end along the direction of rotation of the impeller  36 . An intake hole  42  is formed in the pump body  40  from the bottom surface of the pump body  40  to the upstream end of the groove  40   a  The intake hole  42  links the interior of the pump casing  39  and the exterior (i.e., exterior of the Wesco pump  10 ). A second pump flow path  46  is formed by the group of concavities  38   b  and the groove  40   a . A group of depression-shaped grooves  40   b ,  40   b , . . . is provided in the top surface of the pump body  40  centralized in the radial direction. The group of depression-shaped grooves  40   b ,  40   b , . . . is described later.  
      When the impeller  36  rotates within the pump casing  39 , fuel is drawn in from the intake hole  42  into the pump unit  14  and is led to the pump flow paths  44 ,  46 . The fuel is pressurized while flowing along the fuel flow paths  44 ,  46 , and is propelled from the discharge hole  50  towards the motor unit  12 . The fuel propelled towards the motor unit  12  passes the motor unit  12 , and is expelled to the outside from a discharge port  48  formed in the top cover  32 .  
      As shown in  FIG. 2 , the group of depression-shaped grooves  38   b ,  38   b , . . . formed in the pump cover  38  all have the same shape and size. The depression-shaped grooves  38   b  extend from near the center towards the periphery in a curved shape (spiral shape). The ends of the depression-shaped grooves  38   b  near the periphery are shifted in the direction of rotation of the impeller  36  (in the direction of the arrow A) relative to the ends near the center.  
      The interval between adjacent depression-shaped grooves  38   b  varies depending on the area in which the grooves are formed. The depression-shaped grooves  38   b ,  38   b , . . . formed in the area B with the discharge hole  50  in the center (one end of the area B extends to the upstream end of the groove  38   a ) are formed more closely spaced than the depression-shaped grooves  38   b ,  38   b , . . . formed in the area C which is the area outside area B. A distance D is provided between the outer periphery end of the depression-shaped grooves  38   b  and the inner edge of the groove  38   a . In other words, a flat plane doughnut shape of width D is formed between the outer periphery ends of the depression-shaped grooves  38   b ,  38   b , . . . and the inner edge of the groove  38   a . The depression-shaped grooves  38   b ,  38   b , . . . and the groove  38   a  are sealed by this flat plane.  
      As shown in  FIG. 3 , the group of depression-shaped grooves  40   b ,  40   b , . . . formed in the pump body  40  all have the same shape and size. The depression-shaped grooves  40   b  extend from the near the center towards the periphery in a curved shape (spiral shape). The ends of the depression-shaped grooves  40   b  near the periphery are shifted in the direction of rotation of the impeller  36  (in the direction of the arrow E) relative to the end near the center.  
      The interval between adjacent depression-shaped grooves  40   b  varies depending on the area in which the grooves are formed. The depression-shaped grooves  40   b ,  40   b , . . . formed in the area F with the downstream end of the groove  40   a  in the center (one end of the area F extends to the intake hole  42 ) are formed more closely spaced than the depression-shaped grooves  40   b ,  40   b , . . . formed in the area G which is the area outside area F. A distance H is provided between the outer periphery end of the depression-shaped grooves  40   b  and the inner edge of the groove  40   a . In other words, a flat plane doughnut shape of width H is formed between the outer periphery ends of the depression-shaped grooves  40   b ,  40   b , . . . and the inner edge of the groove  40   a.    
      In the Wesco pump  10  according to the present representative embodiment, a group of depression-shaped grooves  38   b ,  38   b . . . and a group of depression-shaped grooves  40   b ,  40   b , . . . are formed in the pump cover  38  and the pump body  40  respectively. When the impeller  36  rotates the fuel in the clearance between the impeller  36  and the pump casing  39  is drawn into the group of depression-shaped grooves  38   b ,  38   b , . . . and the group of depression-shaped grooves  40   b ,  40   b . . . The outer ends of the depression-shaped grooves  38   b ,  40   b  are shifted in the direction of rotation of the impeller  36  relative to the inner ends. Therefore, when the impeller  36  rotates, the direction of the viscous forces that draws the fuel into the depression-shaped grooves  38   b ,  40   b  acts from the center of the impeller  36  towards the Outer periphery. This direction is the same as the direction of the centrifugal force acting on the fuel within the clearance between the impeller  36  and the pump casing  39  when the impeller  36  rotates.  
      Therefore, when the impeller  36  rotates a force is generated that efficiently propels the fuel within the depression-shaped grooves  38   b ,  40   b  in the direction from the center towards the periphery. The impeller  36  is pressed from both the top and bottom surfaces by the fuel propelled from the center towards the outer periphery within the group of depression-shaped grooves  38   b ,  38   b , . . . and the group of depression-shaped grooves  40   b ,  40   b , . . . , so the impeller  36  is maintained between the pump cover  38  and the pump body  40 .  
      Also, in the pump cover  38  the group of depression-shaped grooves  38   b ,  38   b , . . . is closely spaced near the discharge hole  50  (area B), and elsewhere (area C) the group of depression-shaped grooves  38   b ,  38   b , . . . is more widely spaced. In the pump body  40  the group of depression-shaped grooves  40   b ,  40   b , . . . is closely spaced near the downstream end of the groove  40   a  (area F), and elsewhere (area G) the group of depression-shaped grooves  40   b ,  40   b , . . . is more widely spaced. In areas B and F where the grooves are closely spaced, more fuel is propelled from the center to the outer periphery, so the forces acting on the impeller  36  are greater, and the difference in pressure on the top and bottom surfaces of the impeller  36  is cancelled out. In this way it is possible to suppress the inclination of the impeller  36  with respect to the axis, and contact between the impeller  36  and the pump casing  39  can be suppressed. Also, in the areas C and G where the grooves are widely spaced, sealing can be maintained as a result of the large flat surface. As a result inclination of the impeller  36  can be suppressed, and leakage of fuel from the pump flow paths can be reduced.  
      In this way, according to the Wesco pump  10  of the present representative embodiment friction losses and wear can be reduced, and leakage of fuel from within the pump flow paths can be reduced. Therefore the performance of the pump can be effectively improved.  
     Second Representative Embodiment  
      A Wesco pump according to a second representative embodiment is explained with reference to the drawings. The Wesco pump according to the second representative embodiment is constituted similar to the Wesco pump  10  according to the first representative embodiment, and differs from the first representative embodiment only in the configuration of the groups of depression-shaped grooves formed on the pump casing. Here only the points of difference between the second representative embodiment and the first representative embodiment are explained, explanation of common points is omitted.  
      As shown in  FIG. 4 , a group of depression-shaped grooves  68   b ,  68   b , . . . formed on a pump cover  68  extends from the center towards the periphery in curved lines. The ends of the depression-shaped grooves  68   b  near the periphery are shifted in the direction of rotation of the impeller  36  (the direction of arrow J) relative to the ends near the center. The spacings between adjacent depression-shaped grooves  68   b ,  68   b . . . are all equal.  
      There are two types of shape for the depression-shaped grooves  68   b ,  68   b , . . . The length of the group of depression-shaped grooves  68   b   1 ,  68   b   1 , . . . formed in the area K with an discharge hole  50   a  at the center (one end of area K extends to the upstream end of a groove  68   a ), is longer than the length of the group of depression-shaped grooves  68   b   2 ,  68   b   2 , . . . formed in the area L, the area other than the area K, so the ends near the outer periphery are positioned closer to the periphery. In other words, the distance M between the ends of the depression-shaped grooves  68   b   1  near the periphery and the inner edge of the groove  68   a  is shorter than the distance N between the ends of the depression-shaped grooves  68   b   2  near the periphery and the inner edge of the groove  68   a . A flat surface is formed in the area between the ends of the group of depression-shaped grooves  68   b ,  68   b , . . . near the periphery and the inner edge of the groove  68   a.    
      As shown in  FIG. 5 , a group of depression-shaped grooves  70   b ,  70   b , . . . formed on a pump cover  70  extends from near the center towards the periphery in curved lines. The ends of the depression-shaped grooves  70   b  near the periphery are shifted in the direction of rotation of the impeller  36  (the direction of arrow P) relative to the ends near the center. The spacings between adjacent depression-shaped grooves  70   b ,  70   b , . . . are all equal.  
      There arc two types of shape for the depression-shaped grooves  70   b ,  70   b , . . . The length of the group of depression-shaped grooves  70   b   1 ,  70   b   1 , . . . formed in the area Q with the downstream end of a groove  70   a  in the center (one end of the area Q extends to an intake hole  40   a ) is longer than the length of the group of depression-shaped grooves  70   b   2 ,  70   b   2 , . . . formed in the area R, the area other than the area Q, so the ends near the outer periphery are positioned closer to the periphery. In other words, the distance S between the ends of the depression-shaped grooves  70   b   1  near the periphery and the inner edge of the groove  70   a  is shorter than the distance T between the ends of the depression-shaped grooves  70   b   2  near the periphery and the inner edge of the groove  70   a . A flat surface is formed in the area between the ends of the depression-shaped group of grooves  70   b ,  70   b , . . . near the periphery and the inner edge of the groove  70   a.    
      In the Wesco pump according to the second representative embodiment, as for the Wesco pump  10  according to the first representative embodiment, the group of depression-shaped grooves  68   b ,  68   b , . . . and the group of depression-shaped grooves  70   b ,  70   b , . . . are formed in the pump cover  68  and the pump body  70  respectively. In the pump cover  68 , the group of depression-shaped grooves  68   b   1 ,  68   b   1 , . . . near the discharge hole  50   a  (area K) are long, and the depression-shaped group of grooves  68   b   2 ,  68   b   2 , . . . formed in the other area (area L) are short. In the pump body  70  the group of depression-shaped grooves  70   b   1 ,  70   b   1 , . . . near the downstream end of the groove  70   a  (area Q) are long, and the group of depression-shaped grooves  70   b   2 ,  70   b   2 , . . . in the other area (area R) are short. In areas K and Q where the grooves are formed longer, more fuel is propelled from the center towards the periphery, so the difference in pressure applied to the top and bottom surfaces of the impeller  36  is cancelled out. In this way it is possible to suppress the inclination of the impeller  36  with respect to the axis, and contact between the impeller  36  and the pump casing  68 ,  70  can be suppressed. Also, in the areas L and R where the grooves are formed short, sealing can be maintained as a result of the large flat surface. As a result, leakage of fuel from the pump flow paths can be reduced.  
      In both the first and second representative embodiments described above, depression-shaped grooves  38   b ,  40   b ,  68   b ,  70   b  are formed in both the areas B, F, K, Q near the intake hole and discharge hole and in the other areas C, G, L, R. However, the present teachings are not limited to this type of configuration. For example, depression-shaped grooves may be formed in the areas near the intake hole and discharge hole to generate forces to cancel out the difference in pressure on the top and bottom surfaces of the impeller as much as possible, but in the other areas the depression-shaped grooves may be omitted. This is because in the areas apart from the areas near the intake hole and discharge hole, the difference in pressure applied to the top and bottom surfaces is very small. By forming groups of depression-shaped grooves only near the intake hole and discharge hole the sealing is further improved, and leakage of fuel from the pump flow paths is effectively reduced.  
      Also, it is possible to obtain similar effects by varying the shape of the depression-shaped grooves (for example, groove width, groove depth, inflow angle) (see Table 1). For example, as shown in  FIG. 6 , in area B where the difference in pressure on the top and bottom surface of the impeller is large the width of the depression-shaped groove  88   b  can be increased, and in area C where the pressure difference is small the width of the depression-shaped groove  88   b  can be decreased. Or, as shown in  FIGS. 7 through 9 , in area B where the difference in pressure on the top and bottom surface of the impeller is large, the depth t 2  of the depression-shaped groove  98   b  can be increased (see  FIG. 9 ), and in area C where the pressure difference is small the depth t 1  of the depression-shaped groove  98   b  can be decreased (see  FIG. 8 ). Furthermore, in the area where the difference in pressure of the top and bottom surfaces of the impeller is large the inflow angle can be made an acute angle, and in the area where the difference in pressure of the top and bottom surfaces of the impeller is small the inflow angle can be made an obtuse angle.  
                           TABLE 1                                   Large pressure   Small pressure           difference   difference                                                        Number of grooves   Many   Few           Groove length   Long   Short           Groove width   Large   Small           Groove depth   Deep   Shallow           Inflow angle   Acute angle   Obtuse angle                      
 
      Also, in the first and second representative embodiments, depression-shaped grooves are formed in both the pump cover and pump body, but depression-shaped grooves may be formed in either one of the pump cover or the pump body. This is because depending on the type of fluid pressurized by the Wesco pump and the configuration of the intake hole and discharge hole, and the like, forming the depression-shaped grooves in only one of either the pump cover or pump body can suppress the inclination of the impeller.  
      It is possible to select the number, length, cross-sectional shape of the depression-shaped grooves as appropriate.  
     Third Representative Embodiment  
      A Wesco pump  110  according to a third representative embodiment of the present teachings is explained with reference to the drawings. The Wesco pump  110  according to the third representative embodiment has a configuration that is substantially similar to the configuration of the Wesco pump  10  in the first representative embodiment. However, the third representative embodiment differs from the Wesco pump  10  according to the first representative embodiment in that groups of depression-shaped grooves are formed on the impeller, and the clearance between the impeller and the pump casing varies in the radial direction Here the points of difference with the first representative embodiment are explained in detail and the points in common with the first representative embodiment are omitted.  
      As shown in  FIG. 10 , the Wesco pump  110  includes a motor unit  112  and a pump unit  114 . The motor unit  112  has the same configuration as the motor unit  12  of the Wesco pump  10  according to the first representative embodiment. The pump unit  114  includes a substantially disk-shaped impeller  136  and a pump casing  139  that houses the impeller  136 .  
      As shown in  FIG. 11 , a D-shaped through hole  138   f  is formed in the center of the impeller  136 . The through hole  138   f  is fitted to the bottom end of the shaft  120 . Therefore, the impeller  136  can move in the axial direction of the shaft  120 , but cannot rotate relative to the shaft  120 . Thus, when the shaft  120  rotates the impeller  136  also rotates.  
      The top and bottom surfaces of the impeller  136  are formed as planes substantially perpendicular to the shaft  120 . On the top surface of the impeller  136 , a group of concavities  136   a ,  136   a , . . . is formed along the periphery, and a group of depression-shaped grooves  136   c ,  136   c , . . . is provided in the central part in the radial direction of the impeller  136 . On the bottom surface of the impeller  136 , a group of concavities  136   b ,  136   b , . . . is formed along the periphery, and a group of depression-shaped grooves  136   d ,  136   d , . . . is provided in the central part in the radial direction. Each of the group of concavities  136   a ,  136   a , . . . formed in the top surface of the impeller  136  and each of the group of concavities  136   b ,  136   b , . . . formed in the bottom surface are linked at the bottom of the concavities.  
      As shown in  FIGS. 11 and 12 , the depression-shaped grooves  136   c  formed in the top surface of the impeller  136  extend from their end  137   c  near the center to their end  137   a  near the periphery in a curved shape (spiral shape). Also, a distance A is provided between the end  137   a  of the depression-shaped grooves  136   c  and the concavities  136   a . In other words, a flat plane is formed between the ends  137   a ,  137   a , . . . of the group of depression-shaped grooves  136   c ,  136   c , . . . near the periphery and the group of concavities  136   a ,  136   a , . . . Furthermore, a flat plane is also formed between the group of concavities  136   a ,  136   a , . . . and surface of the periphery  136   e  of the impeller  136 .  
      Although not shown on the drawings, the depression-shaped grooves  136   d  formed in the bottom surface of the impeller  136  are configured in the same way as the depression-shaped grooves  136   c  on the top surface as described above. Also, a flat plane is formed between the ends of the outer periphery of the depression-shaped group of grooves  136   d  and the group of concavities  136   b . Furthermore, a flat plane is also formed between the group of concavities  136   b ,  136   b , . . . and surface of the periphery  136   e  of the impeller  136 .  
      The pump casing  139  includes a pump cover  138  and a pump body  140 . A taper is formed on the casing surface  138   b  of the pump cover  138  so that the clearance with the impeller  136  increases from the center of the impeller  136  towards the periphery of the impeller  136 . A groove  138   a  is formed in the casing surface  138   b  in opposition to the group of concavities  136   a  provided in the top surface of the impeller  136 . A taper is also formed on the casing surface  140   b  of the pump body  140  so that the clearance with the impeller  136  increases from the center of the impeller  136  towards the periphery of the impeller  136 . A groove  140   a  is formed in the casing surface  140   b  in opposition to the group of concavities  136   b  provided in the bottom surface of the impeller  136 . The grooves  138   a  and  140   a  are formed in an approximate C-shape from the upstream end to the downstream end along the direction of rotation of the impeller  136 . The upstream end of the groove  140   a  is formed to that it links with the intake hole  142  in the pump body  140 . The downstream end of the groove  138   a  is formed to that it links with the discharge hole  150  in the pump cover  138 . A first pump flow path  144  is formed by the group of concavities  136   a  formed in the top surface of the impeller  136  and the groove  138   a  formed in the pump cover  138 . A second pump flow path  146  is formed by the group of concavities  136   b  formed in the bottom surface of the impeller  136  and the groove  140   a  formed in the pump body  140 . In  FIGS. 10 and 13 , the taper angle on the casing surface  138   b  and the casing surface  140   b  has been magnified for ease of viewing. In reality the taper angle of the casing surface  138   b  and the casing surface  140   b  is very small.  
      When the impeller  136  rotates within the pump casing  139 , fuel is drawn into the pump unit  114  from the intake hole  142  and is led into the pump flow paths  144 ,  146 . The fuel that is pressurized while flowing through the pump flow paths  144 ,  146  is propelled from the discharge hole  150  towards the motor unit  112 . The fuel that is propelled towards the motor unit  112  passes the motor unit  112 , and is propelled to the outside from a discharge port  148  formed in a top cover  132 .  
      When the impeller  136  rotates, the fuel in the clearance between the impeller  136  and the pump casing  138 ,  140  is drawn into the group of depression-shaped grooves  136   c ,  136   c , . . . and the group of depression-shaped grooves  136   d ,  136   d , . . . The fuel that is drawn into the depression-shaped grooves  136   c ,  136   c , . . . is guided by the wall  137   b  on one side of the depression-shaped grooves  136   c ,  136   c , . . . , and flows towards the end  137   a  near the outer periphery of the depression-shaped grooves  136   c ,  136   c , . . . (refer to  FIG. 12 ). Likewise on the bottom surface of the impeller  136 , the fuel is drawn into the group of depression-shaped grooves  136   d ,  136   d . . . , and flows within the depression-shaped grooves  136   d ,  136   d , . . . towards the ends near the outer periphery. The fuel that is propelled from the center towards the outer periphery within the depression-shaped grooves  136   c ,  136   c , . . . and the depression-shaped grooves  136   d ,  136   d , . . . pressurizes the casing surface  136   b  and the casing surface  140   b , and generates a lift force on the impeller  136  (i.e., a force in the direction that increases the clearance with the casing surface  138   b  and with the casing surface  140   b ). Contact between the impeller  136  and the casing surface  138   b  or the casing surface  140   b  is prevented by these lift forces. The lift force acting on the impeller  136  increases as the clearance between the impeller  136  and the pump casing  138 ,  140  decreases. In the Wesco pump  110  according to the present representative embodiment, the casing surfaces  136   b ,  140   b  are formed with a taper so that the clearance with the impeller  136  increases from the center of the impeller  136  towards the outer periphery of the impeller  136 . In other words, in the locations where the group of depression-shaped grooves  136   c ,  136   c , . . . and the group of depression-shaped grooves  136   d ,  136   d , . . . are formed the clearance between the impeller  136  and the pump casing  138 ,  140  is small. Therefore, a larger lift force acts on the impeller  136 . In this way, it is possible to further reduce friction losses and wear.  
      The following is a detailed explanation of the lift force that is generated by the groups of depression-shaped grooves  136   c ,  136   c , . . . and  136   d ,  136   d , . . . when the impeller  136  rotates. As stated above, when the impeller  136  rotates fuel is led from the intake hole  142  into the pump flow paths  144 ,  146 , and the fuel is pressurized as it flows in the pump flow paths  144 ,  146 . Therefore, the further upstream in the pump flow paths  144 ,  146  the lower the fuel pressure, and the further downstream in the pump flow paths  144 ,  146  the higher the fuel pressure. Also, the pump flow paths  144 ,  146  are formed on the top and bottom surfaces of the impeller  136 , so the impeller  136  is subject to a force in the thrust direction as a result of the pressure difference of the fuel flowing in the first pump flow path  144  and the second pump flow path  146 . The pressure difference of the fuel flowing in the first pump flow path  144  and the second pump flow path  146  varies according to position in the circumferential direction of the impeller  136 . Therefore, the impeller  136  is subject to non-uniform forces, so the impeller  136  inclines a very small amount On the other hand, the casing surfaces  138   b ,  140   b  of the pump casing  139  are formed with a taper so that the clearance with the impeller  136  increases from the center of the impeller  136  towards the outer periphery of the impeller  136 . Therefore, even though the impeller  136  inclines slightly, the periphery of the impeller  136  does not contact the casing surfaces  138   b ,  140   b  (refer to  FIG. 13 ). Also, if the impeller  136  inclines slightly, part of the group of depression-shaped grooves  136   c ,  136   c , . . . (the part on the right hand side in  FIG. 13 ) approaches the casing surface  138   b , and part of the group of depression-shaped grooves  136   d ,  136   d , . . . (the part on the left hand side in  FIG. 13 ) approaches the casing surface  140   b . Then at the position where they approach, the pressure of the fuel in the group of depression-shaped grooves  136   c ,  136   c , . . . and the group of depression-shaped grooves  136   d ,  136   d , . . . increases, so the pressure on the casing surface  138   b  and the casing surface  140   b  increases. This increased pressure acts in a direction to prevent inclination of the impeller  136 , so the impeller  136  returns to a horizontal attitude. Therefore, even if the impeller  136  inclines slightly, the impeller  136  tends to return to the horizontal as a result of the lift forces generated by the group of depression-shaped grooves  136   c ,  136   d . Therefore, contact between the impeller  136  and the casing surfaces  138   b ,  140   b  is prevented, and friction losses and wear can be reduced.  
      According to the Wesco pump  110  of the present representative embodiment, the lift force of the impeller  136  is increased, so it is possible to suppress friction losses and wear. Also, even if the impeller  136  inclines slightly, contact between the periphery of the impeller  136  and the casing surfaces  138   b ,  140   b  can be prevented. Also, forces that tend to restore the impeller  136  to the horizontal act on the impeller  136  as a result of the lift forces generated by the depression-shaped grooves  136   c ,  136   d . In this way it is possible to effectively improve the performance of the pump.  
      Also, in the Wesco pump  110  according to the present representative embodiment, the group of depression-shaped grooves  136   c ,  136   c , . . . and the group of depression-shaped grooves  136   d ,  136   d , . . . are formed on the impeller  136  that rotates, in order to generate lift forces on the impeller  136 . Therefore, in addition to centrifugal forces and viscous forces, inertial forces also act on the fuel within the group of depression-shaped grooves  136   c ,  136   c , . . . and the group of depression-shaped grooves  136   d ,  136   d , . . . As a result of the synergistic effect of these forces, it is possible to generate more effective lift forces.  
      Also, in the Wesco pump  110  according to the present representative embodiment, the groups of depression-shaped grooves  136   c ,  136   d  extend from near the center of the impeller  136  towards the outer periphery of the impeller  136  in a curved shape (spiral shape). Therefore, fuel drawn in can more effectively flow towards the periphery, and a greater lift force can be obtained.  
      In the Wesco pump  110  described above, the group of depression-shaped grooves  136   c ,  136   d  formed in the impeller  136  extend from near the center of the impeller  136  towards the outer periphery in a curved shape. However, the present teachings are not limited to this form. The number, length, cross-sectional shape, and the like of the depression-shaped grooves formed on the impeller may be selected as appropriate. Also, the groups of depression-shaped grooves  136   c ,  136   d  may be formed on the casing surfaces  138   b ,  140   b.    
      Also, in the present representative embodiment, the casing surfaces  138   b ,  140   b  are formed in a tapered shape so that the clearance with the impeller  136  increases from near the center of the impeller  136  towards the outer periphery. However, the present teachings are not limited to this form. For example, the top and bottom surfaces of the impeller  136  may be formed in a taper so that the clearance with the casing surfaces  138   b ,  140   b  increases from near the center of the impeller  136  towards the outer periphery.  
     Fourth Representative Embodiment  
      A Wesco pump  210  according to a fourth representative embodiment is explained with reference to the drawings. The Wesco pump  210  according to the fourth representative embodiment is substantially similar to the Wesco pump  10  according to the first representative embodiment. However, the Wesco pump in the fourth representative embodiment differs from the Wesco pump  10  in the first representative embodiment in that a group of depression-shaped grooves is formed only in the bottom surface of the impeller, and the clearance between the top surface of the impeller and the pump casing varies in the radial direction. Here the points of difference with the first representative embodiment are explained in detail, and the explanation of the points in common with the first representative embodiment are omitted.  
      As shown in  FIG. 14 , the Wesco pump  210  includes a motor unit  212  and a pump unit  214 . The motor unit  212  is configured in the same way as the motor unit  12  of the Wesco pump  10  according to the first representative embodiment. The pump unit  214  includes a substantially disk-shaped impeller  236  and a pump casing  239  that houses the impeller  236 .  
      The top and bottom surfaces of the impeller  236  are formed in a plane shape substantially normal to a shaft  220 . In the top surface of the impeller  236 , a group of concavities  236   b ,  236   b , . . . is provided continuously in the radial direction along the outer periphery. In the bottom surface of the impeller  236 , a group of concavities  236   a ,  236   a , . . . is provided continuously in the radial direction along the outer periphery, and a group of depression-shaped grooves  236   c ,  236   c , . . . is provided to the inside of the group of concavities  236   a ,  236   a , . . . extending from near the center of the impeller  236  towards the outer periphery. The group of concavities  236   b ,  236   b , . . . formed in the top surface of the impeller  236  and the group of concavities  236   a ,  236   a , . . . formed in the bottom surface are linked at the bottom of the concavities.  
      As shown in  FIGS. 15 and 16 , the depression-shaped grooves  236   c  formed in the bottom surface of the impeller  236  extend from their end  237   c  near the center to their end  237   a  near the periphery in a curved shape (spiral shape). Also, a distance A is provided between the end  237   a  of the depression-shaped grooves  236   c  near the periphery and the concavities  236   a . In other words, a flat plane is formed between the ends  237   a ,  237   a , . . . of the group of depression-shaped grooves  236   c ,  236   c , . . . near the periphery and the group of concavities  236   a ,  236   a , . . . Furthermore, a flat plane is also formed between the group of concavities  236   a ,  236   a , . . . and surface of the periphery  236   e  of the impeller  236 .  
      The pump casing  239  includes a pump cover  238  and a pump body  240 . A casing surface  240   b  of the pump body  240  is formed in a plane shape parallel to the bottom surface of the impeller  236 . A groove  240   a  is formed in the casing surface  240   b  in opposition to the group of concavities  236   a ,  236   a , . . . provided in the bottom surface of the impeller  236 . A casing surface  238   b  of the pump cover  238  is formed so that a part of the casing surface  238   b  is closest to the impeller  236 , as shown in  FIG. 17 . The part (projecting portion  238   c ) that is closest to the impeller  236  is formed as a continuous loop in the circumferential direction. A groove  238   a  is formed in the casing surface  238   b  in opposition to the group of concavities  236   b ,  236   b , . . . provided in the top surface of the impeller  236 . The grooves  238   a  and  240   a  are formed in an approximate C-shape from the upstream end to the downstream end along the direction of rotation of the impeller  236 . The upstream end of the groove  240   a  is formed to that it links with an intake hole  42  in the pump body  240  The downstream end of the groove  238   a  is formed to that it links with a discharge hole  250  formed in the pump cover  238 . A first pump flow path  244  is formed by the group of concavities  236   b  formed in the top surface of the impeller  236  and the groove  238   a  formed in the pump cover  238 . A second pump flow path  246  is formed by the group of concavities  236   a  formed in the bottom surface of the impeller  236  and the groove  240   a  formed in the pump body  240 .  
      When the impeller  236  rotates within the pump casing  239 , fuel is drawn into the pump unit  214  from the intake hole  242 . Fuel drawn into the pump unit  214  flows from the upstream side to the downstream side of the pump flow paths  244 ,  246 . Also, while the fuel is flowing in the pump flow paths  244 ,  246 , the fuel pressure is increased. When the fuel flowing in the pump flow paths  244 ,  246  reaches the downstream end of the pump flow path  244 , the fuel is expelled from the discharge hole  250  to the motor unit  212 . The fuel that is propelled towards the motor unit  212  passes the motor unit  212 , and is propelled to the outside from an discharge port  248 .  
      Here, the forces acting on the impeller  236  when the impeller  236  rotates are explained. As stated above, when the impeller  236  rotates, the fuel is pressurized as it flows from the upstream side to the downstream side of the pump flow paths  244 ,  246 . While the impeller  236  is rotating, the pressure of the fuel in the pump flow path  244  becomes higher than the pressure of the fuel in the pump flow path  246 . The pump flow paths  244 ,  246  are formed in the top and bottom surfaces of the impeller  236 , so the impeller  236  is subject to a force as a result of the difference in pressure of the fuel flowing in the first pump flow path  244  and the second pump flow path  246 . In other words, as a result of the difference in pressure of the fuel flowing in the pump flow paths  244 ,  246 , the impeller  236  is subject to a force that presses the impeller  236  towards the casing surface  240   b.    
      Also, a minute amount of the fuel expelled from the pump unit  214  into the motor unit  212  flows into the clearance between the top surface of the impeller  236  and the casing surface  238   b  through the gap between the shaft  220  and a bearing  228 . The pressure of the minute amount of fuel that has flowed into this clearance is high, so a force is applied to the impeller  236  in the direction of the casing surface  240   b  as a result of the pressure of this fuel.  
      Also, a minute amount of fuel flows into the clearance between the bottom surface of the impeller  236  and the casing surface  240   b . Fuel that has flown into the clearance is drawn into the group of depression-shaped grooves  236   c ,  236   c , . . . The fuel drawn into the depression-shaped grooves  236   c ,  236   c , . . . is guided by one wall  237   b  of the depression-shaped grooves  236   c ,  236   c , . . . and flows towards the end  237   a  of the depression-shaped grooves  236   c ,  236   c , . . . near the outer periphery (refer to  FIG. 16 ). The fuel within the depression-shaped grooves  236   c ,  236   c , . . . that is propelled from near the center towards the outer periphery presses against the casing surface  240   b , generating a lift force on the impeller  236  (i.e., a force acting in the direction to increase the clearance between the impeller  236  and the casing surface  240   b ). On the other hand, depression-shaped grooves are not formed on the top surface of the impeller  236 , so no lift force is generated between the top surface of the impeller  236  and the casing surface  238   b.    
      In this way, a force as a result of the pressure difference of the fuel in pump flow paths  244 ,  246 , a force as a result of the pressure of fuel that has flowed upstream from the motor unit  212  to the pump unit  214  through the gap between the shaft  220  and the bearing  228 , and a force due to the group of depression-shaped grooves  236   c ,  236   c , . . . act on the impeller  236 . The force due to the pressure difference of the fuel and the force due to the pressure of the fuel that has flowed upstream act in a direction that presses the impeller  236  towards the casing surface  240   b . The lift force due to the group of depression-shaped grooves  236   c ,  236   c , . . . acts in a direction to cancel out the forces pressing the impeller  236  towards the casing surface  240   b . Therefore, the impeller  236  can rotate without being pressed towards the casing surface  240   b . In this way, contact of the impeller  236  with the casing surface  240   b  is suppressed, and friction losses and wear can be reduced.  
      As explained above, in the Wesco pump  210 , depression-shaped grooves  236   c ,  236   c , . . . are formed in the bottom surface of the impeller  236 , and depression-shaped grooves are not formed in the top surface of the impeller  236  and the casing surface  238   b . Therefore, it is possible to cancel out the forces acting to press the impeller  236  towards the casing surface  240   b  by the lift forces generated by the depression-shaped grooves  236   c ,  236   c , . . . In this way, pressing of the impeller  236  towards the casing surface  240   b  and contact with the casing surface  240   b  can be suppressed.  
      Also, in the Wesco pump  210 , a projecting portion  238   c  is formed in the casing surface  238   b  to the inside of the group of concavities  236   b ,  236   b , . . . as a continuous loop in the circumferential direction of the impeller  236 . At the projecting portion  238   c , the clearance with the impeller  236  is smaller than in other parts, so the flow of fuel leaking from the pump flow path past the projecting portion  238   c  into the clearance on the discharge hole side is suppressed. Therefore, the quantity of fuel leaking from the pump fuel path  244  can be reduced. In this way, the fuel within the casing can be efficiently pressurized, and high pump performance can be achieved.  
      Also, even if the force acting on the impeller  236  toward the casing surface  238   b  increases due to fluctuations of the fuel pressure within the pump flow paths  244 ,  246 , contact of the impeller  236  with the casing surface  240   b  is suppressed by the pressure of the fuel that has flowed from the motor unit  212  to the pump unit  214  through the gap between the shaft  220  and the bearing  228 . Also, even assuming the impeller  236  and the casing surface  238   b  contacted, the impeller  236  will just contact the projecting portion  238   c , so it is possible to minimize and suppress the friction losses when the impeller and the casing contact.  
     Fifth Representative Embodiment  
      In the Wesco pump  210  according to the fourth representative embodiment as described above, depression-shaped grooves are only formed on the bottom surface of the impeller  236 , but depression-shaped grooves may also be formed in the top surface of the impeller. The following is a description of a Wesco pump  310  according to a fifth representative embodiment, in which depression-shaped grooves are formed in the top surface of the impeller. The explanation is either omitted or simplified for parts that overlap with the fourth representative embodiment.  
      The Wesco pump  310  according to the fifth representative embodiment also includes a motor unit and a pump unit  314 . The motor unit has the same configuration as the Wesco pump  10  according to the first representative embodiment. The pump unit  314  includes a substantially disk-shaped impeller  336  and a pump casing  339  that houses the impeller  336 .  
      The configuration of the impeller  336  is substantially similar to the impeller  236  according to the fourth representative embodiment. That is, a group of concavities  336   b ,  336   b , . . . is formed in the top surface of the impeller  336 . In the bottom surface of the impeller  336 , a group of concavities  336   a ,  336   a , . . . and a group of depression-shaped grooves  336   c ,  336   c , . . . are formed.  
      Also, a group of depression-shaped grooves  336   d ,  336   d , . . . is formed in the top surface of the impeller  336 . As shown in  FIG. 19 , the depression-shaped grooves  336   d  are formed in the same shape as the depression-shaped grooves  336   c  formed in the bottom surface of the impeller  336 . That is, the depression-shaped grooves  336   d  extend from an end near the center to an end towards the periphery in a curved shape (spiral shape) However, the number of depression-shaped grooves  336   d  is fewer than the number of depression-shaped grooves  336   c  (refer to  FIGS. 15 and 19 ).  
      The pump casing  339  includes a pump cover  338  and a pump body  340 . A casing surface  340   b , groove  340   a , and intake hole  342  of the pump body  340  are formed in the same way as those of the pump body  240  according to the fourth representative embodiment. The casing surface  338   b  of the pump cover  338  is formed in a plane shape parallel to the top surface of the impeller  336 , as shown in  FIG. 18 . Also, the groove  338   a  and the discharge hole  350  of the pump cover  338  are formed in the same way as the pump cover  238  according to the fourth representative embodiment. A first pump flow path  344  is formed by the group of concavities  336   b  formed in the top surface of the impeller  336  and the groove  338   a  formed in the pump cover  338 . A second pump flow path  346  is formed by the group of concavities  338   a  formed in the bottom surface of the impeller  336  and the groove  340   a  formed in the pump body  340 .  
      Here the forces acting on the impeller  336  when the impeller  336  rotates are explained. A force due to the pressure difference of the fuel flowing in the first pump flow path  344  and the second pump flow path  346  acts on the impeller  336 , as for the fourth representative embodiment. Also, a force acts as a result of fuel that flows from the motor unit into the clearance between the top surface of the impeller  336  and the casing surface  338   b  through the gap between the shaft and the bearing. These forces act in a direction that presses the impeller  336  towards the casing surface  340   b.    
      Also, the group of depression-shaped grooves  336   c ,  338   c , . . . formed in the bottom surface of the impeller  336  generates a lift force B. The lift force B acts in a direction to increase the clearance between the bottom surface of the impeller  336  and the casing surface  340   b.    
      Furthermore, the group of depression-shaped grooves  336   d ,  336   d , . . . formed in the top surface of the impeller  336  generate a lift force C acting in a direction to increase the clearance between the top surface of the impeller  336  and the casing surface  338   b . As stated above, the number of grooves in the group of depression-shaped grooves  336   d ,  336   d , . . . is fewer than the number of grooves in the group of depression-shaped grooves  336   c ,  336   c , . . . Therefore, the lift force C is smaller than the lift force B.  
      In this way, when the impeller  336  rotates, a force due to the pressure difference of the fuel flowing in the pump flow paths  344 ,  346 , a force due to the pressure of the fuel that has flowed from the motor unit into the pump casing  339 , the lift force B, and the lift force C act on the impeller  336 . The force due to the pressure difference of the fuel, the pressure of the fuel that has flowed upstream, and the lift force C act in a direction to press the impeller  336  towards the casing surface  340   b . The lift force B acts in a direction to cancel out these forces. The lift force B is larger than the lift force C, so the force obtained by subtracting the force C from the force B can act to cancel the force due to the pressure difference of the fuel and force due to the pressure of the fuel that has flowed upstream. Therefore, contact of the impeller  336  with the casing surface  340   b  can be suppressed, and the impeller  336  can rotate smoothly. In this way, it is possible to improve the efficiency of the pump.  
      Also, if the impeller  336  is pressed against the casing surface  338   b  as a result of fluctuations in fuel pressure, the clearance between the top surface of the impeller  336  and the casing surface  338   b  is reduced. Then the fuel in this clearance is compressed, and the lift force C increases. The impeller  336  is pressed by the increased lift force C to return to the original position. Therefore, contact between the impeller  336  and the casing surface  338   b  is suppressed.  
      In the fifth representative embodiment as described above, by making the number of grooves in the group of depression-shaped grooves  336   d ,  336   d , . . . fewer than the number of grooves in the group of depression-shaped grooves  336   c ,  336   c , . . . the magnitude of the lift force C (i.e., the force pressing the impeller downwards) was made smaller than the lift force B (i.e., the force pressing the impeller upwards). However, the present teachings are not limited to this form. For example, as shown in  FIG. 20 , the length of the depression-shaped grooves  436   d  in the top surface of the impeller may be made shorter than the length of the depression-shaped grooves in the bottom surface of the impeller. Also, as shown in  FIG. 21 , the width of the depression-shaped grooves  536   d  in the top surface of the impeller may be made smaller than the width of the depression-shaped grooves in the bottom surface of the impeller. Or, as shown in  FIG. 22 , the inflow angle (in other words, the angle θ formed between the depression-shaped grooves and the direction of flow of fuel within the clearance (refer to  FIG. 23 )) of the depression-shaped groove  636   d  in the top surface of the impeller may be made larger than the inflow angle of the depression-shaped grooves in the bottom surface of the impeller. Also, the depth of the depression-shaped grooves in the top surface of the impeller may be made shallower than the depth of the depression-shaped grooves in the bottom surface of the impeller. In this way, by determining the shape of the depression-shaped grooves on the top surface of the impeller in accordance with the shape of the depression-shaped grooves on the bottom surface of the impeller, it is possible to make the lift force C smaller than the lift force B.  
      Also, in each of the representative embodiments described above, the depression-shaped grooves formed in either the impeller or in the pump casing extend from near the center of the impeller towards the outer periphery in a curved shape (spiral shape). However, the present teachings are not limited to this form. The number, length, cross-sectional shape of the depression-shaped grooves formed in the impeller or in the pump casing may be appropriately designed.  
      Finally, although the preferred embodiments have been described in detail, the present embodiments arc for illustrative purpose only and not restrictive. It is to be understood that various changes and modifications may be made without departing from the spirit or scope of the appended claims. In addition, the additional features and aspects disclosed herein also may be utilized singularly or in combination with the above aspects and features.