Patent Publication Number: US-9404496-B2

Title: Oil return passage structure for oil pump

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a configuration of an oil pump that can achieve a size reduction of the entire pump, reduction in wear of the rotor during operation and that can also achieve longer pump life and reduction in production cost. 
     2. Description of the Related Art 
     There are, as conventionally known, internal gear oil pumps with a relief valve. Japanese Patent Application Laid-open No. S63-246482 discloses a specific configuration of such an oil pump (see  FIGS. 8A and 8B ). The pump according to Japanese Patent Application Laid-open No. S63-246482 has in general a configuration, in which a circular recess  6  in which inner and outer rotors are arranged has a smooth cover attachment surface  22  therearound to attach a cover  24 , and a plurality of bolt holes  23  drilled at suitable locations for fastening the cover  24 . 
     An oil return passage  26  is formed in the cover attachment surface  22  in the form of a groove from near a discharge chamber  11  toward an inlet chamber  10 . One end of this oil return passage  26  opens to an inlet passage  12 , while the other end extends as far as to a portion adjacent the discharge chamber  11 . The cover attachment surface  22  is thus divided into a pump chamber-side portion  22   a  that surrounds the circular recess  6 , and an outer portion  22   b.    
     A side hole  27   a , which is drilled in a middle position of a relief passage  27  that opens to an outlet passage  14 , opens to the oil return passage  26 . A known relief valve  28  is mounted in the relief passage  27 , so that lubricating oil under excess pressure is discharged into the oil return passage  26  through the side hole  27   a  to flow back to the inlet chamber  10  when the pressure of discharged oil exceeds a predetermined value. 
     SUMMARY OF THE INVENTION 
     According to Japanese Patent Application Laid-open No. S63-246482, the pump chamber-side portion  22   a  is provided between the oil return passage  26  and the circular recess  6  so as to separate the oil return passage  26  and the circular recess  6 . Accordingly, the pump casing 5 is increased in size radially outward by the width of the pump chamber-side portion  22   a.    
     The oil return passage  26  is formed independently of and located away from the circular recess  6 . The pump casing 5 has a complex shape because of such a configuration, which causes high production cost. The flow path of the relief oil is long since the oil return passage  26  is formed at a position away from the circular recess  6 , because of which the relief oil may not flow smoothly and it is highly likely that the pressure relief action may not be performed properly. 
     The technical solutions (objects) of the present invention are to achieve: efficient return of relief oil to the inlet side by a relief valve to ensure a favorable pressure relief action; retardation of wear of the rotor mounted in the pump body to increase pump life; a very compact design; and simple production. 
     Through vigorous research, the inventors have achieved the above objects by providing an oil pump, which, according to a first aspect of the present invention, includes: a pump body; a pump cover; an outer rotor; and an inner rotor, the pump body including a rotor chamber having an inner circumferential support wall on an inner circumferential side, a first inlet port and a first outlet port formed in the rotor chamber, an inlet passage communicating with the first inlet port, an outlet passage communicating with the first outlet port, a relief valve allowing oil to flow from the outlet passage to the inlet passage by relieving pressure, a relief chamber formed on a discharge side of the relief valve, and a first oil return passage formed from the relief chamber to the inlet passage; the pump cover including a second inlet port and a second outlet port, and a second oil return passage facing and communicating with the first oil return passage; the outer rotor being supported by the inner circumferential support wall of the rotor chamber; and the inner rotor being arranged on an inner circumferential side of the outer rotor. The first oil return passage is formed in the inner circumferential support wall as a groove-like recess that opens along an outer circumferential surface of the outer rotor. A support protrusion is formed near a portion where the second oil return passage is formed in the pump cover to support a front surface, in a radial direction, of the outer rotor. 
     According to a second aspect of the present invention, in the oil pump according to the first aspect, each of the first oil return passage and the second oil return passage is formed at and around a symmetric point of a maximum partition part located between a trailing end of the inlet port and a leading end of the outlet port relative to a center point of the rotor chamber, whereby the above objects were achieved. 
     According to a third aspect of the present invention, in the oil pump according to the first aspect, the first oil return passage is formed at an upper end portion in a depth direction of the inner circumferential support wall and opened in a surface portion of the rotor chamber, whereby the above objects were achieved. 
     According to a fourth aspect of the present invention, in the oil pump according to the third aspect, the first oil return passage is formed to a depth from a surface of the rotor chamber less than half a thickness in an axial direction of the outer rotor, whereby the above objects were achieved. 
     Through vigorous research, the inventors have achieved the above objects by providing an oil pump, which, according to a fifth aspect of the present invention, includes: a pump body; a pump cover; an outer rotor; and an inner rotor, the pump body including a rotor chamber having an inner circumferential support wall on an inner side, a first inlet port and a first outlet port formed in the rotor chamber, an inlet passage communicating with the first inlet port, an outlet passage communicating with the first outlet port, a relief valve allowing oil to flow from the outlet passage to the inlet passage by relieving pressure, a relief chamber formed on a discharge side of the relief valve, and a first oil return passage formed from the relief chamber to the inlet passage; the pump cover including a second inlet port and a second outlet port, and a second oil return passage facing and communicating with the first oil return passage; the outer rotor being supported by the inner circumferential support wall of the rotor chamber; and the inner rotor being arranged on an inner circumferential side of the outer rotor. The first oil return passage is formed as a gap extending to a same depth in an axial direction as a depth of the rotor chamber between a body wall portion, located between the relief chamber and the inlet passage, and an outer circumferential surface of the outer rotor. A support protrusion is formed near a portion where the second oil return passage is formed in the pump cover to support a front surface, in a radial direction, of the outer rotor. 
     According to a sixth aspect of the present invention, in the oil pump according to the first aspect, the support protrusion is sandwiched between the second inlet port on a radially inner side and the second oil return passage on a radially outer side, and formed as an independent protrusion, whereby the above objects were achieved. According to a seventh aspect of the present invention, in the oil pump according to the fifth aspect, the support protrusion is sandwiched between the second inlet port on a radially inner side and the second oil return passage on a radially outer side, and formed as an independent protrusion, whereby the above objects were achieved. According to an eighth aspect of the present invention, in the oil pump according to the first aspect, the first oil return passage is formed by a gap formed in an upper portion of the inner circumferential support wall and by a deep groove formed on a radially outer side of the inner circumferential support wall in close proximity thereto, so as to communicate the relief chamber with the inlet passage, the deep groove communicating with the gap, whereby the above objects were achieved. 
     According to the present invention, the first oil return passage on the pump body side is formed in the inner circumferential support wall from the relief chamber to the inlet passage as a groove-like recess that opens along an outer circumferential surface of the outer rotor. According to this configuration, in the first return oil passage, the outer circumferential surface of the outer rotor forms part of the wall of the oil return passage. 
     Therefore, the first oil return passage of the present invention is not a separate groove-like recess formed at a position away from the rotor chamber of the pump body as seen in conventional pumps, but rather, it forms a groove together with the outer circumferential surface of the outer rotor. Accordingly, the oil pump of the present invention can be made smaller and more lightweight than conventional counterparts. 
     Moreover, a second oil return passage is formed in the pump cover such as to face and communicate with the first oil return passage of the oil pump body, so that the overall cross-sectional area of the oil return passage is the sum of the cross-sectional areas of the first oil return passage of the pump body and the second oil return passage of the pump cover. 
     As the oil return passage formed in the pump body and pump cover has a sufficient and necessary cross-sectional area that is the sum of the cross-sectional areas of both first and second oil return passages, and as the first oil return passage is formed to open along the outer circumferential surface of the outer rotor, with the first and second oil return passages, the oil pump is kept compact and the radial dimension of the oil pump body is minimized. 
     Moreover, the portion of the inner circumferential support wall of the rotor chamber where the first oil return passage is formed does not contact the outer circumferential surface of the outer rotor. Therefore, the area of surface where the rotor chamber and the outer rotor substantially contact each other is reduced, and the smaller contact area leads to lower friction resistance, whereby drive loss is reduced and fuel economy is increased. 
     The support protrusion formed with the second oil return passage of the oil pump cover partially supports the front surface of a portion at the distal end in the radial direction of the outer rotor, as well as restricts axial displacement of the outer rotor. As the support protrusion supports the front surface in the radial direction of the outer rotor, the outer rotor is unlikely to tilt inside the rotor chamber, and thus the outer rotor is prevented from tilting and abutting the inner circumferential support wall of the oil pump body obliquely, and possible damage to the outer rotor is prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a partially sectional front view of a first embodiment of the present invention,  FIG. 1B  is a cross-sectional view as seen from the direction of arrows Y1-Y1 in  FIG. 1A , and  FIG. 1C  is an enlarged view of part α in  FIG. 1B ; 
         FIG. 2A  is a partially sectional front view of a pump body in the first embodiment, and  FIG. 2B  is a cross-sectional view as seen from the direction of arrows Y2-Y2 in  FIG. 2A ; 
         FIG. 3A  is a front view of a pump cover, and  FIG. 3B  is a cross-sectional view as seen from the direction of arrows Y3-Y3 in  FIG. 3A ; 
         FIG. 4A  is a longitudinal cross-sectional front view of a pressure relief action in the first embodiment,  FIG. 4B  is an enlarged view of part β in  FIG. 4A , and  FIG. 4C  is an enlarged view of part γ in  FIG. 4A ; 
         FIG. 5A  is an enlarged view as seen from the direction of arrows Y4-Y4 in  FIG. 4B ,  FIG. 5B  is an enlarged longitudinal cross-sectional side view of essential parts illustrating how forces act to resist tilting of the outer rotor, and  FIG. 5C  is an enlarged longitudinal cross-sectional side view of essential parts illustrating how forces act in the pump cover to resist tilting of the outer rotor; 
         FIG. 6A  is a partially sectional front view of a second embodiment of the present invention,  FIG. 6B  is an enlarged view of part  8  in  FIG. 6A , and  FIG. 6C  is a cross-sectional view as seen from the direction of arrows Y5-Y5 in  FIG. 6B ; 
         FIG. 7A  is a partially sectional front view of a third embodiment of the present invention,  FIG. 7B  is an enlarged view of part s in  FIG. 7A , and  FIG. 7C  is a cross-sectional view as seen from the direction of arrows Y6-Y6 in  FIG. 7B ; and 
         FIGS. 8A-8B  illustrate a conventional configuration of an internal gear oil pump with a relief valve. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. The oil pump according to the present invention is generally comprised of a pump body A, a pump cover B, an outer rotor  91 , and an inner rotor  92  (see  FIG. 1 ). The pump body A is comprised of a rotor chamber  11 , a first inlet port  14 , a first outlet port  15 , and a relief valve  2  (see  FIG. 2 ). 
     The rotor chamber  11  is made up of an inner circumferential support wall  11   a  and a bottom  11   b . The pump body A has a body wall portion  1   a  at the outer periphery. The distal end of the body wall portion  1   a  is formed flat. Suitably spaced bolt holes  1   b  are formed in the body wall portion  1   a  for fixedly attaching the body to the pump cover B to be described later with fastening means such as bolts. 
     The outer rotor  91  and inner rotor  92  are trochoid or substantially trochoid gears. The outer rotor  91  has a plurality of inner teeth  91   g  formed on the inner periphery, while the inner rotor  92  has a plurality of outer teeth  92   g . The inner rotor  92  has one fewer number of outer teeth  92   g  than the number of inner teeth  91   g  of the outer rotor  91 , so that there are formed a plurality of interteeth spaces S between the inner teeth  91   g  of the outer rotor  91  and the outer teeth  92   g  of the inner rotor  92 . 
     A shaft hole  12  is formed in the bottom  11   b  of the rotor chamber  11  for a drive shaft  8  to pass through (see  FIG. 1 ). Also formed in the bottom  11   b  are the first inlet port  14  and the first outlet port  15 . Between the trailing end  14   t  of the first inlet port  14  and the leading end  15   f  of the first outlet port  15  is formed a maximum partition part  16 , while, between the trailing end  15   t  of the first outlet port  15  and the leading end  14   f  of the first inlet port  14  is formed a minimum partition part  17  (see  FIG. 2 ). 
     A first inlet passage  14   a  communicates with the first inlet port  14 . The first inlet passage  14   a  communicates with the outside of the pump body A and allows oil to flow in from a lubrication circuit outside the pump body A. A first outlet passage  15   a  communicates with the first outlet port  15 . The first inlet outlet passage  15   a  allows oil to flow out to the lubrication circuit outside the pump body A. 
     The inner circumferential support wall  11   a  of the rotor chamber  11  is a portion that holds and rotatably supports the outer rotor  91 . The inner circumferential support wall  11   a  forms a cylindrical inner wall surface, which is non-continuous at portions where it intersects with the first inlet port  14  and the first outlet port  15  (see  FIG. 2A ). Namely, the inner circumferential support wall  11   a  of the rotor chamber  11  is formed from a plurality of wall parts, which hold the outer circumferential surface  91   a  of the outer rotor  91  (see  FIG. 4A ). 
     The relief valve  2  is provided between the first inlet port  14  and the first outlet port  15 , and serves to return oil from the first outlet port  15  side to the first inlet port  14  side when the pressure of discharged oil exceeds a predetermined value. A valve member passage  21   a  is formed inside a valve housing  21 , and a relief passage  21   b  is formed at one end in the longitudinal direction of the valve member passage  21   a  to communicate with the first Wet outlet passage  15   a . Part of the oil flowing through the first outlet passage  15   a  enters the valve member passage  21   a  through the relief passage  21   b  as relief oil. 
     A relief drain hole  21   c  is formed in the valve housing  21 , so that the valve member passage  21   a  inside the valve housing  21  communicates with the outside. The relief drain hole  21   c  is opened and closed by a valve member  22  to be described later. The relief drain hole  21   c  is opened to relieve pressure (see  FIG. 4A ). 
     The valve member  22  and a resilient member  23  are arranged inside the valve member passage  21   a  such that the resilient member  23  resiliently presses the valve member  22  to close the relief passage  21   b . More specifically, a coil spring is used as the resilient member  23 . A relief chamber  18  is formed around a portion where the relief drain hole  21   c  is formed in the valve housing  21  (see  FIG. 1A ,  FIG. 2A ,  FIG. 4A , and others). 
     The relief chamber  18  is a cavity (space) that communicates the relief drain hole  21   c  with the first inlet port  14 . The relief chamber  18  serves to deliver the oil drained from the relief drain hole  21   c  into the first inlet port  14 . 
     Next, a first oil return passage  3  in the first embodiment of the present invention will be described. The first oil return passage  3  is formed in a suitable region of the inner circumferential support wall  11   a  of the rotor chamber  11 . The first oil return passage  3  is formed at a location opposite from the maximum partition part  16 , with the rotation center Pa of the outer rotor  91  being in the middle as a center point, i.e., at a symmetrical point (see  FIG. 2A ). This location includes the surrounding region. The first oil return passage  3  is formed in the inner circumferential support wall  11   a  between the relief chamber  18  and the first inlet passage  14   a.    
     The first oil return passage  3  is formed as a substantially arcuate recess extending along the circumferential direction of the rotor chamber  11  in a suitable region of the inner circumferential support wall  11   a  (see  FIG. 2 ). The first oil return passage  3  is formed to have a substantially L-shaped cross-sectional shape in a section orthogonal to the circumferential direction from the upper end face to the inner side face of the inner circumferential support wall  11   a . The corner of the first oil return passage  3  with a substantially L-shaped cross-sectional shape may either be rounded or orthogonal. 
     The inner circumferential support wall  11   a  is shaped like the rest thereof below the first oil return passage  3  in the depth direction so as to support the outer circumferential surface  91   a  of the outer rotor  91  housed in the rotor chamber  11  (see  FIGS. 1B and 1C  and  FIG. 2B ). Therefore, the outer rotor  91  is prevented from moving in radial directions by parts of the inner circumferential support wall  11   a  supporting the outer circumferential surface  91   a  of the outer rotor  91 . As radial rocking movement of the outer rotor  91  is reduced, knocking noise produced by the outer rotor  91  colliding the rotor chamber  11 , or damage to the outer rotor  91 , can be reduced. 
     Part of the outer circumferential surface  91   a  of the outer rotor  91  that passes the region of the first oil return passage  3  forms the substantially groove-like recess together with the first oil return passage  3 . The first oil return passage  3  is a fluid passage that communicates the relief chamber  18  with the first inlet passage  14   a  and allows the relief oil to return from the relief chamber  18  back to the first inlet passage  14   a  through the first oil return passage  3  (see  FIG. 2A ). 
     The relief oil flowing through the first oil return passage  3  thus makes direct contact with the outer circumferential surface  91   a  of the outer rotor  91 , so that, as the outer rotor  91  rotates inside the rotor chamber  11 , oil can be distributed between the outer circumferential surface  91   a  of the outer rotor  91  and the inner circumferential support wall  11   a  (see  FIG. 4A  and  FIG. 4B ). 
     Since the first oil return passage  3  is formed along the outer circumferential surface  91   a  of the outer rotor  91 , the pump body A can be made smaller as compared to the conventional pump that has the oil passage at a position away from the rotor chamber  11 . The contact area between the inner circumferential support wall  11   a  and the outer circumferential surface  91   a  of the outer rotor  91  is reduced in the region where the first oil return passage  3  is formed (see  FIGS. 1B and 1C ), so that the friction resistance between the outer rotor  91  and the rotor chamber  11  is reduced. Drive loss is accordingly reduced, and fuel economy is improved. 
     Moreover, since the first oil return passage  3  is located on the opposite side from the maximum partition part  16  between the trailing end  14   t  of the first inlet port  14  and the leading end  15   f  of the first outlet port  15 , with the rotation center Pa of the outer rotor  91  being in the middle (at the symmetric point), oil that flows from the relief chamber  18  back to the first inlet passage  14   a  passes through the first oil return passage  3  (see  FIG. 4 ). 
     Since the pressure of oil flowing through the first oil return passage  3  is negative, the outer rotor  91  is pulled from the side of the maximum partition part  16  toward the first oil return passage  3  by the force of negative pressure f (see  FIG. 4B ). The direction in which the outer rotor  91  is pulled by the force of negative pressure f is indicated by arrow Q in  FIG. 4A  and  FIG. 4C . 
     Therefore, the tip clearance t between the inner teeth of the outer rotor  91  and the outer teeth of the inner rotor  92  on the maximum partition part  16  (see  FIG. 4C ) is reduced. That is, the seal tightness of the interteeth spaces S between the outer rotor  91  and the inner rotor  92  on the maximum partition part  16  is increased, so that leakage from the outlet side to the inlet side is reduced, and the volume efficiency (ratio of actual discharge to theoretical discharge) can be increased. 
     Moreover, the oil flowing through the first oil return passage  3  can be delivered to the gap between the inner circumferential support wall  11   a  of the rotor chamber  11  and the outer circumferential surface  91   a  of the outer rotor  91  and serves as lubricating oil to allow smooth rotation of the outer rotor  91  (see  FIG. 5A ). 
     Next, the relationship between the depth of the first oil return passage  3  and the length in the thickness direction of the outer rotor  91  will be explained. One half the length in the depth direction of the rotor chamber  11  is denoted as Db, while the length in the depth direction of the first oil return passage  3  is denoted as Da (see  FIG. 5B ). The imaginary line L in the drawing indicates the centerline in the thickness direction of the outer rotor. The depth direction of the rotor chamber  11  and the thickness direction of the outer rotor  91  are the same. The depth Da of the first oil return passage  3  is set smaller than half the length in the depth direction Db of the rotor chamber  11 . 
     Namely, Db&gt;Da. 
     Therefore, in the region where the first oil return passage  3  is formed, the inner circumferential support wall  11   a  extends from the bottom  11   b  of the rotor chamber  11  in the height direction to a point beyond half the depth of the rotor chamber  11 . Accordingly, even if there is created a rotational force M that causes the outer rotor  91  to swing and tilt relative to the rotor chamber  11  around the contact point P1 between the lower end in the depth direction of the first oil return passage  3  and the outer circumferential surface  91   a  of the outer rotor  91 , the outer circumferential surface  91   a  of the outer rotor  91  is supported by part of the inner circumferential support wall  11   a  up to a point higher than half the thickness of the outer rotor. 
     That is, the outer rotor  91  is supported by the inner circumferential support wall  11   a  over a range that extends beyond the center of gravity in the axial direction of the outer circumferential surface  91   a  (midpoint of the thickness of the outer rotor  91 ). Therefore, the reaction force F1 from the contact point P1 against the outer rotor  91  abutting the contact point P1 acts on a point higher than the midpoint of the thickness of the outer rotor  91  (see  FIG. 5B ). This configuration makes it difficult for the outer rotor  91  to tilt inside the rotor chamber  11  and thus the outer rotor  91  is prevented from abutting the inner circumferential support wall  11   a  obliquely, and possible damage to the outer rotor  91  is reduced. 
     Next, the pump cover B will be described. The pump cover B is formed in a shape substantially the same as but symmetric to the opening shape on the front side of the pump body A (see  FIG. 3A ).  FIG. 3A  is a front view of the pump cover B. The front side of the pump cover B here is the side that faces the front opening of the pump body A (see  FIG. 1B ). 
     The pump cover B has parts corresponding to the first inlet port  14 , first inlet passage  14   a , first outlet port  15 , first outlet passage  15   a , the first oil return passage  3  and others of the pump body A as will be shown below, being formed at corresponding locations. The pump cover B has a cover wall portion  4   a , in which bolt holes  4   b  are formed with suitable spacing. In the pump cover B are formed a shaft hole  42 , a discharge port  43 , a second inlet port  44 , a second inlet passage  44   a , a second outlet port  45 , a second outlet passage  45   a , and a second oil return passage  5 . 
     The second inlet port  44 , second inlet passage  44   a , second outlet port  45 , and second outlet passage  45   a  of the pump cover B are located correspondingly to the first inlet port  14 , first inlet passage  14   a , first outlet port  15 , and first outlet passage  15   a , of the pump body A, so that, with the pump cover B being attached to the pump body A, their positions match each other. 
     The second oil return passage  5  is located at a position where it will face and communicate with the first oil return passage  3  of the pump body A when the pump cover B is attached to the pump body A (see  FIG. 1B ,  FIG. 1C , and  FIG. 3B ). Thus the overall cross-sectional area of the oil return passage in the present invention is the sum of the cross-sectional area of the second oil return passage  5  and that of the first oil return passage  3 . 
     The oil return passage formed in the pump body A and pump cover B has a sufficient and necessary cross-sectional area that is the sum of the cross-sectional areas of both first and second oil return passages  3  and  5 , and the first oil return passage  3  is formed to open along the outer circumferential surface  91   a  of the outer rotor  91 . Accordingly, with the first and second oil return passages  3  and  5 , a large amount of relief oil can be conveyed, while the oil pump is kept compact and the radial dimension of the pump body A is minimized. The pressure of oil flowing through the second oil return passage  5  is negative. 
     A support protrusion  6  is formed between the second oil return passage  5  and second inlet port  44  (see  FIG. 1B ,  FIG. 1C , and  FIG. 3 ). More specifically, the support protrusion  6  is sandwiched between the second inlet port  44  on the radially inner side, and the second oil return passage  5  on the radially outer side, and formed as an independent protrusion. The distal end of the support protrusion  6  is formed flat (see  FIG. 3B ). The support protrusion  6  is formed substantially arcuate along the longitudinal direction of the second oil return passage  5 . 
     The support protrusion  6  partially and slidably supports a front surface  91   b  at the distal end in the radial direction of the outer rotor  91 , with the pump cover B fitted on the pump body A (see  FIGS. 1C, 5B and 5C ). Therefore, the support protrusion  6  is formed on the same plane as that of the cover wall portion  4   a  of the pump cover B. 
     The radial front surface  91   b  of the outer rotor  91  is thus supported by the support protrusion  6 , so that the outer rotor  91  is unlikely to tilt inside the rotor chamber  11  (see  FIG. 5C ). Even if a force F2 is generated that causes the outer rotor  91  to tilt obliquely relative to the radial direction inside the rotor chamber  11 , a reaction force F3 will act against the support protrusion  6  pressing down the front surface  91   b  of the outer rotor  91 , so that the outer rotor is prevented from abutting the inner circumferential surface of the oil pump body obliquely, and thus possible damage to the outer rotor  91  is prevented. 
     In a second embodiment of the present invention, the first oil return passage  3  is not formed in the inner circumferential support wall  11   a  of the rotor chamber  11  but on the inner side of the body wall portion  1   a  (see  FIG. 6 ). In this embodiment, the first oil return passage  3  extends axially all along the outer circumferential surface  91   a  of the outer rotor  91 . 
     Therefore, the outer circumferential surface  91   a  of the outer rotor  91  passing the region where the first oil return passage  3  is formed does not make contact with the inner circumferential support wall  11   a . The first oil return passage  3  has a large volume so that it can deliver a large amount of relief oil from the relief chamber  18  to the inlet passage  14   a . A shallow relief chamber  18  may be formed in the pump cover B at a position corresponding to that of the relief chamber  18  of the pump body A and with substantially the same shape (see  FIG. 3A ). 
     Next, a first oil return passage  3  in a third embodiment of the present invention will be described. The first oil return passage  3  of the third embodiment is substantially an embodiment of a narrower concept of the first embodiment described in the foregoing. The first oil return passage  3  of the first embodiment is formed as a groove-like recess in the inner circumferential support wall  11   a  and opens along the outer circumferential surface  91   a  of the outer rotor  91 . In contrast, the first oil return passage  3  of the third embodiment is made up of two parts, a gap  31  and a deep groove  32 . The gap  31  and the deep groove  32  both extend between the relief chamber  18  and the inlet passage  14   a  and communicate with each other. 
     The gap  31  is formed by cutting away an upper portion of the inner circumferential support wall  11   a  along the circumferential direction of the wall  11   a  (see  FIG. 7C ). In other words, the upper end of the inner circumferential support wall  11   a  is lower in the region where the first oil return passage  3  is formed than other portions of the inner circumferential support wall  11   a . The top of the inner circumferential support wall  11   a  where the gap  31  is formed is flat, and the height is constant. The gap  31  formed above the inner circumferential support wall  11   a  opens along the outer circumferential surface  91   a  of the outer rotor  91  (see  FIG. 7C ). 
     The deep groove  32  is formed on a radially outer side of the inner circumferential support wall  11   a  in close proximity thereto (see  FIG. 7B  and  FIG. 7C ). The deep groove  32  is a fluid passage that is arcuate similarly to the inner circumferential support wall  11   a . The deep groove  32  is formed in communication with and between the relief chamber  18  and the inlet passage  14   a  as mentioned above, the upper part of the deep groove  32  communicating with the gap  31  (see  FIG. 7C ). 
     The deep groove  32  has a rectangular cross-sectional shape, and its bottom may be deeper, or shallower than, or equal to the bottom of the rotor chamber  11 . The deep groove  32  should preferably be located closest possible to the inner circumferential support wall  11   a . The first oil return passage  3  formed by such deep groove  32  and gap  31  has a substantially inverted L-shaped cross-sectional shape in a section orthogonal to the circumferential direction of the inner circumferential support wall  11   a  (see  FIG. 7C ). 
     Part of the inner circumferential support wall  11   a  stands as an upright wall portion beside the deep groove  32 . In the third embodiment, in this way, the gap  31  that forms part of the first oil return passage  3  extends along the circumferential direction of the inner circumferential support wall  11   a , so that the first oil return passage  3  is open along the outer circumferential surface  91   a  of the outer rotor  91  through the gap  31  (see  FIG. 7A  and  FIG. 7B ). 
     According to the third embodiment, the first oil return passage  3  formed by the gap  31  and the deep groove  32  can return a large amount of relief oil from the relief chamber  18  to the inlet passage  14   a , so that the pressure relief action can be performed most favorably. The gap  31  allows part of the oil being returned to be distributed between the inner circumferential support wall  11   a  below the gap  31  and the outer circumferential surface  91   a  of the outer rotor  91 , so that the outer rotor  91  can rotate very smoothly. 
     Similarly to the first and second embodiments, the first oil return passage  3  in the third embodiment should preferably be formed at or around a location opposite from the maximum partition part  16 , with the rotation center Pa of the outer rotor  91  being in the middle as a center point, i.e., at a symmetric point. 
     According to the second aspect of the invention, the first oil return passage is located opposite from the maximum partition part between the trailing end of the first inlet port and the leading end of the first outlet port, with the rotation center of the outer rotor being in the middle. The second oil return passage of the pump cover is positioned opposite the first oil return passage of the pump body and in communication with the first oil return passage. Namely, each of the first and second the oil return passages is located at or around a symmetric point of the maximum partition part relative to the rotation center of the outer rotor as the point of symmetry. 
     Relief oil flowing back from the relief chamber to the inlet passage flows through the first and second oil return passages formed at such a position. Since a negative pressure is created by the relief oil flowing through the first and second oil return passages, the outer rotor is pulled from the maximum partition part toward the oil return passage. 
     The tip clearance between the inner rotor and the outer rotor is reduced on the maximum partition part, or both rotors almost abut each other, so that airtight interteeth spaces are formed between the outer rotor and the inner rotor. Leakage to the inlet side is thus reduced, and the volume efficiency (actual discharge to theoretical discharge) can be improved. 
     According to the third aspect of the invention, the first oil return passage is formed at an upper end portion in the depth direction of the inner circumferential support wall and opened to a surface portion of the rotor chamber. It is therefore provided as a recess in the thickness direction of the outer rotor, with a support portion that partially supports the outer circumference of the outer rotor. That is, the inner circumferential support wall exists in the region of the rotor chamber where the first oil return passage is formed. 
     Since the outer circumferential surface of the outer rotor is supported by the remaining inner circumferential support wall in the region where the first oil return passage is formed, the outer rotor is prevented from moving in radial directions. As radial rocking movement of the outer rotor is reduced, knocking noise produced by the outer rotor colliding the pump body or inner circumferential support wall, or damage to the outer rotor, can be reduced. Since the first oil return passage is formed at the upper end portion in the depth direction of the inner circumferential support wall and opened to a surface portion of the rotor chamber, it can be formed by casting in which the casting with holes is removed from the mold, i.e., there is no need of post-processing such as machining or welding but the groove can be formed from the beginning by casting, so that the production cost can be reduced. Other effects of the present invention as described herein are likewise achieved. 
     According to the fourth aspect of the invention, the first oil return passage is formed to a depth from the surface of the rotor chamber less than half the thickness in the axial direction of the outer rotor. That is, the outer rotor is supported by the inner circumferential support wall at the center of gravity in the axial direction of the outer circumferential surface (midpoint of the thickness of the outer rotor), so that it is difficult for the outer rotor to tilt, and thus the outer rotor is prevented from tilting and abutting the inner circumferential support wall of the oil pump body obliquely, and possible damage to the outer rotor is reduced. 
     According to the fifth aspect of the invention, the first oil return passage is formed as a gap between a body wall portion located between the relief chamber and the inlet passage and the outer circumferential surface of the outer rotor. As there is no inner circumferential support wall in the region where the oil return passage is formed in the rotor chamber, the outer circumferential surface of the outer rotor does not contact the inner circumferential support wall there, so that friction resistance is reduced, whereby drive loss is reduced and fuel economy is improved. The oil return passage has a large volume so that it can deliver a large amount of relief oil from the relief chamber to the inlet passage and ensure a favorable pressure relief action. Moreover, the shape of the pump body is made simple, so that molds for casting the pump body can be made simple. 
     In the sixth and seventh aspects of the invention, the support protrusion is sandwiched between the second inlet port on the radially inner side and the second oil return passage on the radially outer side, and formed as an independent protrusion. As described above, the support protrusion restricts axial displacement of the outer rotor, and as it is formed as an independent protrusion, it supports the front surface of a portion at the distal end in the radial direction of the outer rotor in a minimum area, so that it allows oil to flow sufficiently around itself, and ensures even smoother rotation of the outer rotor. 
     According to the eighth aspect of the invention, the first oil return passage is formed as a gap formed in an upper portion of the inner circumferential support wall and a deep groove formed on the radially outer side of the inner circumferential support wall in close proximity thereto, such as to communicate the relief chamber with the inlet passage. The deep groove communicates with the gap so that the gap and the deep groove together can return a large amount of relief oil from the relief chamber to the inlet passage, whereby the pressure relief action can be performed most favorably. The gap allows part of the oil being returned to be distributed between the inner circumferential support wall below the gap and the outer circumferential surface of the outer rotor, so that the outer rotor can rotate very smoothly.