Patent Publication Number: US-2023160321-A1

Title: Bearing and turbocharger

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of International Application No. PCT/JP2021/038388, filed on Oct. 18, 2021, which claims priority to Japanese Patent Application No. 2020-191119, filed on Nov. 17, 2020, the entire contents of which are incorporated by reference herein. 
    
    
     BACKGROUND ART 
     Technical Field 
     The present disclosure relates to a bearing and a turbocharger. The present application claims the benefit of priority based on Japanese Patent Application No. 2020-191119 filed on Nov. 17, 2020, the content of which is incorporated herein. 
     Related Art 
     In various devices, a bearing that pivotally supports a shaft is used. For example, Patent Literature 1 discloses a turbocharger including a bearing that pivotally supports a shaft. Lubricating oil is supplied to a bearing used in a turbocharger or the like. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent No. 5807436 
     SUMMARY 
     Technical Problem 
     As a bearing that pivotally supports a shaft, there is a bearing having a thrust bearing surface that supports a member axially adjacent to the bearing in a thrust direction (that is, a thrust bearing). Lubricating oil supplied to the inside of the bearing is supplied to the thrust bearing surface of the bearing as the shaft rotates. A thrust load (that is, a load in the thrust direction) is supported by an oil film pressure of the lubricating oil supplied to the thrust bearing surface. In such a bearing, it is desired to increase the withstanding load (in other words, the load capacity) in the thrust direction. 
     It is an object of the present disclosure to provide a bearing and a turbocharger capable of increasing the withstanding load of the bearing in the thrust direction. 
     Solution to Problem 
     In order to solve the above problem, a bearing of the present disclosure includes: an annular main body through which a shaft is inserted; a plurality of oil supply grooves included on an inner curved surface of the main body and extending in an axial direction of the main body; a thrust bearing surface included on an end surface of the main body; a plurality of tapered portions included on the thrust bearing surface separated from an outer peripheral edge of the thrust bearing surface at intervals in a circumferential direction of the main body and communicating with the oil supply grooves, the tapered portions each becoming shallower as the tapered portion extends in the circumferential direction; and an oil discharge groove included on the thrust bearing surface and passing through one tapered portion among the plurality of tapered portions, the oil discharge groove connecting an oil supply groove and the outer peripheral edge. 
     A ratio of the flow path cross-sectional area of an opening of the oil discharge groove on the outer peripheral edge side to the area of the thrust bearing surface may be less than or equal to 0.01. 
     A ratio of the flow path cross-sectional area of an opening of the oil discharge groove on the outer peripheral edge side to the area of the thrust bearing surface may be greater than or equal to 0.003. 
     In order to solve the above disadvantage, a turbocharger of the present disclosure includes the bearing described above. 
     Effects of Disclosure 
     According to the present disclosure, it is possible to improve the withstanding load of a bearing in the thrust direction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic cross-sectional view illustrating a turbocharger according to an embodiment of the present disclosure. 
         FIG.  2    is an extracted diagram of an alternate long and short dash line portion of  FIG.  1   . 
         FIG.  3    is a front view illustrating a thrust bearing surface in a bearing according to the embodiment of the present disclosure. 
         FIG.  4    is a cross-sectional view taken along line A-A in  FIG.  3   . 
         FIG.  5    is a diagram illustrating the bearing as viewed from a direction of arrow B in  FIG.  3   . 
         FIG.  6    is a graph illustrating a relationship between the ratio of a flow path cross-sectional area of an opening on an outer peripheral edge side of an oil discharge groove to the area of the thrust bearing surface in the bearing according to the embodiment of the present disclosure and a flow rate of lubricating oil discharged from the bearing. 
         FIG.  7    is a graph illustrating a relationship between the ratio of the flow path cross-sectional area of the opening on the outer peripheral edge side of the oil discharge groove to the area of the thrust bearing surface in the bearing according to the embodiment of the present disclosure and the temperature of an oil film formed on the thrust bearing surface. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present disclosure will be described below by referring to the accompanying drawings. Dimensions, materials, other specific numerical values, and the like illustrated in the embodiment is merely an example for facilitating understanding, and the present disclosure is not limited thereto unless otherwise specified. Note that, in the present specification and the drawings, components having substantially the same function and structure are denoted by the same symbol, and redundant explanations are omitted. Illustration of components not directly related to the present disclosure is omitted. 
       FIG.  1    is a schematic cross-sectional view of a turbocharger TC. In  FIG.  1   , the direction of arrow U is the vertically upward direction, and the direction of arrow D direction is the vertically downward direction. Hereinafter, description is given on the premise that the direction of arrow L illustrated in  FIG.  1    is the left side of the turbocharger TC. Description is given on the premise that the direction of arrow R illustrated in  FIG.  1    is the right side of the turbocharger TC. As illustrated in  FIG.  1   , the turbocharger TC includes a turbocharger main body  1 . The turbocharger main body  1  includes a bearing housing  3 , a turbine housing  5 , and a compressor housing  7 . The turbine housing  5  is connected to the left side of the bearing housing  3  by a fastening mechanism  9 . The compressor housing  7  is connected to the right side of the bearing housing  3  by a fastening bolt  11 . 
     A protrusion  3   a  is formed on the outer curved surface of the bearing housing  3 . The protrusion  3   a  is formed closer to the turbine housing  5 . The protrusion  3   a  protrudes in a radial direction of the bearing housing  3 . A protrusion  5   a  is formed on the outer curved surface of the turbine housing  5 . The protrusion  5   a  is formed closer to the bearing housing  3 . The protrusion  5   a  protrudes in a radial direction of the turbine housing  5 . The bearing housing  3  and the turbine housing  5  are band-fastened to each other by the fastening mechanism  9 . The fastening mechanism  9  is, for example, a G coupling. The fastening mechanism  9  clamps the protrusions  3   a  and  5   a.    
     A bearing hole  3   b  is formed in the bearing housing  3 . The bearing hole  3   b  penetrates through the turbocharger TC in the left-right direction. A bearing  13  is disposed in the bearing hole  3   b . The bearing  13  is a semi-floating bearing. However, as will be described later, the bearing  13  may be a bearing other than a semi-floating bearing. The bearing  13  pivotally supports a shaft  15  in a freely rotatable manner. At the left end of the shaft  15 , a turbine blade wheel  17  is provided. The turbine blade wheel  17  is housed in the turbine housing  5  in a freely rotatable manner. At the right end of the shaft  15 , a compressor impeller  19  is provided. The compressor impeller  19  is housed in the compressor housing  7  in a freely rotatable manner. An oil discharge port  3   c  for discharging lubricating oil scattered from the bearing  13  is formed in a lower portion of the bearing housing  3 . 
     An intake port  21  is formed in the compressor housing  7 . The intake port  21  opens to the right side of the turbocharger TC. The intake port  21  is connected to an air cleaner (not illustrated). Facing surfaces of the bearing housing  3  and the compressor housing  7  form a diffuser flow path  23 . The diffuser flow path  23  pressurizes the air. The diffuser flow path  23  is formed in an annular shape. The diffuser flow path  23  communicates with the intake port  21  via the compressor impeller  19  on an inner side in the radial direction. 
     A compressor scroll flow path  25  is formed in the compressor housing  7 . The compressor scroll flow path  25  is formed in an annular shape. The compressor scroll flow path  25  is positioned on an outer side in the radial direction of the shaft  15  with respect to the diffuser flow path  23 . The compressor scroll flow path  25  communicates with an intake port of an engine (not illustrated) and the diffuser flow path  23 . When the compressor impeller  19  rotates, the air is sucked from the intake port  21  into the compressor housing  7 . The sucked air is pressurized and accelerated in the process of flowing between the blades of the compressor impeller  19 . The pressurized and accelerated air is further pressurized by the diffuser flow path  23  and the compressor scroll flow path  25 . The pressurized air is guided to the intake port of the engine. 
     A discharge port  27  is formed in the turbine housing  5 . The discharge port  27  opens to the left side of the turbocharger TC. The discharge port  27  is connected to an exhaust gas purification device (not illustrated). A communication passage  29  and a turbine scroll flow path  31  are formed in the turbine housing  5 . The turbine scroll flow path  31  is formed in an annular shape. The turbine scroll flow path  31  is positioned, for example, on an outer side in the radial direction of the turbine blade wheel  17  with respect to the communication passage  29 . The turbine scroll flow path  31  communicates with a gas inlet port (not illustrated). Exhaust gas discharged from an exhaust manifold of the engine (not illustrated) is guided to the gas inlet port. The communication passage  29  communicates the turbine scroll flow path  31  with the discharge port  27  via the turbine blade wheel  17 . The exhaust gas guided from the gas inlet port to the turbine scroll flow path  31  is guided to the discharge port  27  via the communication passage  29  and the turbine blade wheel  17 . The exhaust gas guided to the discharge port  27  rotates the turbine blade wheel  17  in the process of flowing therethrough. 
     The turning force of the turbine blade wheel  17  is transmitted to the compressor impeller  19  via the shaft  15 . When the compressor impeller  19  rotates, the air is pressurized as described above. In this manner, the air is guided to the intake port of the engine. 
       FIG.  2    is an extracted diagram of an alternate long and short dash line portion of  FIG.  1   . As illustrated in  FIG.  2   , a bearing structure BS is provided inside the bearing housing  3 . The bearing structure BS includes the bearing hole  3   b , the bearing  13 , and the shaft  15 . 
     An oil passage  3   d  is formed in the bearing housing  3 . Lubricating oil is supplied to the oil passage  3   d . The oil passage  3   d  opens to (communicates with) the bearing hole  3   b . The oil passage  3   d  guides the lubricating oil to the bearing hole  3   b . The lubricating oil flows into the bearing hole  3   b  from the oil passage  3   d.    
     The bearing  13  is disposed in the bearing hole  3   b . The bearing  13  has an annular main body  13   a . An insertion hole  13   b  is formed in the main body  13   a . The insertion hole  13   b  penetrates through the main body  13   a  in the axial direction of the shaft  15 . The axial direction of the shaft  15  intersects with (specifically, orthogonal to) the vertical direction. The shaft  15  is inserted through the insertion hole  13   b . The main body  13   a  extends in a direction intersecting with (specifically, in a direction orthogonal to) the vertical direction. Hereinafter, the axial direction, the radial direction, and the circumferential direction of the bearing  13  (that is, the axial direction, the radial direction, and the circumferential direction of the main body  13   a  and the shaft  15 ) are also simply referred to as the axial direction, the radial direction, and the circumferential direction, respectively. 
     Two radial bearing surfaces  13   d  and  13   e  are formed on an inner curved surface  13   c  of the main body  13   a  (specifically, the insertion hole  13   b ). The two radial bearing surfaces  13   d  and  13   e  are spaced apart in the axial direction. An oil hole  13   f  is formed in the main body  13   a . The oil hole  13   f  penetrates through the main body  13   a  from the inner curved surface  13   c  to an outer curved surface  13   g . The oil hole  13   f  is disposed between the two radial bearing surfaces  13   d  and  13   e . The oil hole  13   f  faces the opening of the oil passage  3   d  in the radial direction of the bearing  13 . 
     The lubricating oil flows from the outer curved surface  13   g  side of the main body  13   a  to the inner curved surface  13   c  side through the oil hole  13   f . The lubricating oil that has flown into the inner curved surface  13   c  side of the main body  13   a  moves along the circumferential direction between the inner curved surface  13   c  and the shaft  15 . The lubricating oil that has flown into the inner curved surface  13   c  side of the main body  13   a  moves between the inner curved surface  13   c  and the shaft  15  along the axial direction (left-right direction in  FIG.  2   ). The lubricating oil is supplied to clearances between the shaft  15  and the two radial bearing surfaces  13   d  and  13   e . The shaft  15  is pivotally supported by the oil film pressure of the lubricating oil. The two radial bearing surfaces  13   d  and  13   e  receive a radial load (that is, a load in the radial direction) of the shaft  15 . 
     A through-hole  13   h  is formed in the main body  13   a . The through-hole  13   h  penetrates from the inner curved surface  13   c  to the outer curved surface  13   g  of the main body  13   a . The through-hole  13   h  is disposed between the two radial bearing surfaces  13   d  and  13   e . The through-hole  13   h  is disposed on the side of the main body  13   a  opposite to the side where the oil hole  13   f  is formed. However, the position of the through-hole  13   h  is not limited thereto and is only required to be different from the position of the oil hole  13   f  in the circumferential direction. 
     A pin hole  3   e  is formed in the bearing housing  3 . The pin hole  3   e  is formed in the bearing hole  3   b  at a position facing the through-hole  13   h . The pin hole  3   e  penetrates through a wall portion forming the bearing hole  3   b . The pin hole  3   e  communicates the internal space and the external space of the bearing hole  3   b . A positioning pin  33  is inserted into the pin hole  3   e . Specifically, the positioning pin  33  is press-fitted into the pin hole  3   e . A tip of the positioning pin  33  is inserted into the through-hole  13   h  of the main body  13   a . The positioning pin  33  restricts the movement of the main body  13   a  in the rotation direction and the axial direction. 
     The shaft  15  includes a large-diameter portion  15   a , a medium-diameter portion  15   b , and a small-diameter portion  15   c . The large-diameter portion  15   a  is positioned closer to the turbine blade wheel  17  (see  FIG.  1   ) than the main body  13   a . The large-diameter portion  15   a  has a cylindrical shape. The outer diameter of the large-diameter portion  15   a  is larger than the inner diameter of the inner curved surface  13   c  (specifically, the radial bearing surface  13   d ) of the main body  13   a . The outer diameter of the large-diameter portion  15   a  is larger than the outer diameter of the outer curved surface  13   g  of the main body  13   a . However, the outer diameter of the large-diameter portion  15   a  may be equal to or smaller than the outer diameter of the outer curved surface  13   g  of the main body  13   a . The large-diameter portion  15   a  faces the main body  13   a  in the axial direction. The large-diameter portion  15   a  has a constant outer diameter. However, the outer diameter of the large-diameter portion  15   a  may not be constant. 
     The medium-diameter portion  15   b  is positioned closer to the compressor impeller  19  (see  FIG.  1   ) side than the large-diameter portion  15   a . The medium-diameter portion  15   b  has a cylindrical shape. The medium-diameter portion  15   b  is inserted into the insertion hole  13   b  of the main body  13   a . Therefore, the medium-diameter portion  15   b  faces the inner curved surface  13   c  of the insertion hole  13   b  in the radial direction. The medium-diameter portion  15   b  has an outer diameter smaller than that of the large-diameter portion  15   a . The outer diameter of the medium-diameter portion  15   b  is smaller than the inner diameters of the radial bearing surfaces  13   d  and  13   e  of the main body  13   a . The medium-diameter portion  15   b  has a constant outer diameter. However, the outer diameter of the medium-diameter portion  15   b  may not be constant. 
     The small-diameter portion  15   c  is positioned closer to the compressor impeller  19  (see  FIG.  1   ) side (that is, on the compressor impeller  19  side with respect to the main body  13   a ) than the medium-diameter portion  15   b  is. The small-diameter portion  15   c  has a cylindrical shape. The small-diameter portion  15   c  has an outer diameter smaller than that of the medium-diameter portion  15   b . The small-diameter portion  15   c  has a constant outer diameter. However, the outer diameter of the small-diameter portion  15   c  may not be constant. 
     An annular oil thrower member  35  is inserted into the small-diameter portion  15   c . The oil thrower member  35  scatters the lubricating oil flowing to the compressor impeller  19  side along the shaft  15  to the outer side in the radial direction. That is, the oil thrower member  35  suppresses leakage of lubricating oil to the compressor impeller  19  side. 
     The oil thrower member  35  has an outer diameter larger than that of the medium-diameter portion  15   b . The outer diameter of the oil thrower member  35  is larger than the inner diameter of the inner curved surface  13   c  of the main body  13   a  (specifically, the radial bearing surface  13   e ). The outer diameter of the oil thrower member  35  is smaller than the outer diameter of the outer curved surface  13   g  of the main body  13   a . However, the outer diameter of the oil thrower member  35  may be equal to or larger than the outer diameter of the outer curved surface  13   g  of the main body  13   a . The oil thrower member  35  faces the main body  13   a  in the axial direction. 
     The main body  13   a  is sandwiched between the oil thrower member  35  and the large-diameter portion  15   a  in the axial direction. Thrust bearing surfaces  13   i  and  13   j  are provided on the end surfaces of the main body  13   a . The thrust bearing surface  13   i  is provided on an end surface of the main body  13   a  on the turbine blade wheel  17  (see  FIG.  1   ) side. The thrust bearing surface  13   j  is provided on an end surface of the main body  13   a  on the compressor impeller  19  (see  FIG.  1   ) side. Lubricating oil is supplied to the thrust bearing surface  13   i  through the inner curved surface  13   c . As a result, lubricating oil is supplied to a clearance between the main body  13   a  and the large-diameter portion  15   a . Lubricating oil is supplied to the thrust bearing surface  13   j  through the inner curved surface  13   c . Lubricating oil is supplied to a clearance between the main body  13   a  and the oil thrower member  35 . 
     When the shaft  15  moves in the axial direction (to the left side in  FIG.  2   ), the load in the thrust direction (axial direction) is supported by the oil film pressure of the lubricating oil supplied to the thrust bearing surface  13   i  (that is, lubricating oil between the main body  13   a  and the large-diameter portion  15   a ). When the shaft  15  moves in the axial direction (rightward in  FIG.  2   ), the load in the thrust direction (axial direction) is supported by the oil film pressure of the lubricating oil supplied to the thrust bearing surface  13   j  (that is, lubricating oil between the main body  13   a  and the oil thrower member  35 ). In this manner, the two thrust bearing surfaces  13   i  and  13   j  receive the thrust load. 
     Damper portions  13   k  and  13   m  are formed on the outer curved surface  13   g  of the main body  13   a . The damper portions  13   k  and  13   m  are separated from each other in the axial direction. The damper portions  13   k  and  13   m  are formed at both ends of the outer curved surface  13   g  in the axial direction. The outer diameters of the damper portions  13   k  and  13   m  are larger than the outer diameter of other portions of the outer curved surface  13   g . Lubricating oil is supplied to clearances between the damper portions  13   k  and  13   m  and the inner curved surface  3   f  of the bearing hole  3   b . The vibration of the shaft  15  is suppressed by the oil film pressure of the lubricating oil. 
       FIG.  3    is a front view illustrating the thrust bearing surface  13   i  in the bearing  13  according to the embodiment.  FIG.  3    is a diagram of the thrust bearing surface  13   i  as viewed from the left side in  FIG.  2   . Note that the thrust bearing surface  13   j  has substantially the same shape as that of the thrust bearing surface  13   i . Therefore, description of the shape of the thrust bearing surface  13   j  is omitted. The shape of the radial bearing surface  13   e  is substantially the same as that of the radial bearing surface  13   d . Therefore, description of the shape of the radial bearing surface  13   e  is omitted. 
     As illustrated in  FIG.  3   , a plurality of arcuate surfaces  37  and a plurality of oil supply grooves  39  are formed on the radial bearing surface  13   d . In the example of  FIG.  3   , the radial bearing surface  13   d  has four arcuate surfaces  37  and four oil supply grooves  39 . However, the number of the arcuate surfaces  37  and the number of the oil supply grooves  39  are not limited thereto and may be other than four. 
     The plurality of arcuate surfaces  37  is separated from the shaft  15  in the radial direction. The plurality of arcuate surfaces  37  is arranged side by side in the circumferential direction. The positions of the centers of curvature of the plurality of arcuate surfaces  37  coincide with each other. That is, the plurality of arcuate surfaces  37  is located on the same cylindrical surface. However, the positions of the centers of curvature of the plurality of arcuate surfaces  37  may be different from each other. An oil supply groove  39  is formed between two arcuate surfaces  37  adjacent to each other in the circumferential direction. The oil supply grooves  39  are formed in the radial bearing surface  13   d  at intervals in the circumferential direction. The oil supply grooves  39  extend in the axial direction. The shape of the flow path cross section of an oil supply groove  39  (that is, the shape in the cross section orthogonal to the axial direction) is a shape in which the width in the circumferential direction becomes narrower as it is closer to the radially outer side (specifically, a triangular shape). However, the shape of a flow path cross section of an oil supply groove  39  may have a polygonal shape (for example, a rectangular shape) other than a triangular shape, a semicircular shape, or the like. 
     An oil supply groove  39  extends from an end of the radial bearing surface  13   d  on a side, where the two radial bearing surfaces  13   d  and  13   e  (see  FIG.  2   ) are close to each other, to an end of the radial bearing surface  13   d  on a side where the two radial bearing surfaces  13   d  and  13   e  are separated from each other. The oil supply grooves  39  are open to the thrust bearing surface  13   i  (that is, an end surface of the main body  13   a  in the axial direction). The oil supply grooves  39  allow the lubricating oil to flow. The oil supply grooves  39  supply the lubricating oil to the radial bearing surface  13   d . The oil supply grooves  39  supply the lubricating oil also to the thrust bearing surface  13   i.    
     The lubricating oil between the shaft  15  and the radial bearing surface  13   d  moves in a rotation direction RD of the shaft  15  as the shaft  15  rotates. At this point, the lubricating oil is compressed between the arcuate surfaces  37  of the radial bearing surface  13   d  and the shaft  15 . The compressed lubricating oil presses the shaft  15  radially inward (that is, in the radial direction) (wedge effect). As a result, the radial load is supported by the radial bearing surface  13   d.    
     As illustrated in  FIG.  3   , a plurality of tapered portions  41  (specifically, tapered portions  41 - 1 ,  41 - 2 ,  41 - 3 , and  41 - 4 ) and a land portion  43  are formed on the thrust bearing surface  13   i . The tapered portions  41  are portions recessed with respect to a plane orthogonal to the axial direction on the thrust bearing surface  13   i . The land portion  43  is a portion (that is, a planar portion orthogonal to the axial direction) of the thrust bearing surface  13   i  where the tapered portions  41  are not formed. In the example of  FIG.  3   , the thrust bearing surface  13   i  has four tapered portions  41 . However, the number of the tapered portions  41  is not limited thereto and may be other than four. 
     The tapered portions  41  are separated from an outer peripheral edge of the thrust bearing surface  13   i . On the thrust bearing surface  13   i , the land portion  43  is included on an outer side of the tapered portions  41  in the radial direction. The tapered portions  41  are connected with the radial bearing surface  13   d . The tapered portions  41  extend in the circumferential direction. The length of a tapered portion  41  in the radial direction is constant. However, the length of a tapered portion  41  in the radial direction may not be constant. 
     The plurality of tapered portions  41  are included at intervals in the circumferential direction of the main body  13   a . The tapered portions  41 - 1 ,  41 - 2 ,  41 - 3 , and  41 - 4  are arranged in this order at equal intervals. However, the tapered portions  41 - 1 ,  41 - 2 ,  41 - 3 , and  41 - 4  may be arranged at unequal intervals. The tapered portions  41 - 1  and  41 - 4  are formed on a vertically upper side (specifically, the upper half in the vertical direction) of the thrust bearing surface  13   i . The tapered portion  41 - 4  is closer to the uppermost portion of the thrust bearing surface  13   i  in the vertical direction than the tapered portion  41 - 1  is. The tapered portions  41 - 2  and  41 - 3  are formed on a vertically lower side (specifically, the lower half in the vertical direction) of the thrust bearing surface  13   i . The tapered portion  41 - 2  is closer to the lowest portion of the thrust bearing surface  13   i  in the vertical direction than the tapered portion  41 - 3  is. 
     A tapered portion  41  communicates with an oil supply groove  39 . Each of the tapered portions  41 - 1 ,  41 - 2 ,  41 - 3 , and  41 - 4  communicates with one oil supply groove  39 . 
       FIG.  4    is a cross-sectional view taken along line A-A in  FIG.  3   . The cross section A-A in  FIG.  3    is a cross section along the circumferential direction of the main body  13   a  passing through the tapered portion  41 - 2 . That is, in  FIG.  4   , illustrated a cross-sectional shape along the circumferential direction of the tapered portion  41 - 2 . As described later, the tapered portion  41 - 2  includes an oil discharge groove  45 . Meanwhile, the tapered portions  41 - 1 ,  41 - 3 , and  41 - 4  do not include the oil discharge groove  45 . The tapered portions  41 - 1 ,  41 - 3 , and  41 - 4  have substantially the same shape as that of the tapered portion  41 - 2  except for the presence or absence of the oil discharge groove  45 . Therefore, description of the shapes of the tapered portions  41 - 1 ,  41 - 3 , and  41 - 4  is omitted. 
     As illustrated in  FIG.  4   , the tapered portion  41  becomes shallower as it extends in the circumferential direction (specifically, the rotation direction RD of the shaft  15 ). The tapered portion  41  is inclined with respect to the circumferential direction at a constant inclination angle. However, the inclination angle of the tapered portion  41  may vary depending on the circumferential position. The lubricating oil supplied to the thrust bearing surface  13   i  moves in the rotation direction RD of the shaft  15  as the shaft  15  rotates. At this point, the lubricating oil is compressed between the tapered portions  41  of the thrust bearing surface  13   i  and the large-diameter portion  15   a  (see  FIG.  2   ). The compressed lubricating oil presses the large-diameter portion  15   a  in the axial direction (that is, in the thrust direction) (wedge effect). As a result, the oil film pressure is likely to be generated, and the withstanding load in the thrust direction by the thrust bearing surface  13   i  increases. 
     As illustrated in  FIGS.  3  and  4   , the thrust bearing surface  13   i  includes the oil discharge groove  45 . The oil discharge groove  45  passes through one tapered portion  41 - 2  among the plurality of tapered portions  41 . The oil discharge groove  45  connects the oil supply groove  39  (specifically, the oil supply groove  39  communicating with the tapered portion  41 - 2 ) and the outer peripheral edge of the thrust bearing surface  13   i . The lubricating oil supplied to the thrust bearing surface  13   i  passes through the oil discharge groove  45  and is discharged from an opening  45   a  (hereinafter also referred to as the opening  45   a  on an outer peripheral edge side of the oil discharge groove  45 ) on the outer peripheral edge side of the thrust bearing surface  13   i  in the oil discharge groove  45 . The oil discharge groove  45  promotes the flow of the lubricating oil in the thrust bearing surface  13   i  by discharging the lubricating oil supplied to the thrust bearing surface  13   i  from the thrust bearing surface  13   i . As a result, an increase in the temperature of the oil film formed on the thrust bearing surface  13   i  is suppressed, and a decrease in the viscosity accompanying the increase in the temperature is suppressed. Therefore, a decrease in the withstanding load in the thrust direction by the thrust bearing surface  13   i  is suppressed. 
     The oil discharge groove  45  extends in the radial direction of the main body  13   a . However, the oil discharge groove  45  may extend in a direction inclined with respect to the radial direction of the main body  13   a . The oil discharge groove  45  is included on the thrust bearing surface  13   i  on the oil discharge port  3   c  (see  FIG.  1   ) side of the bearing housing  3 . As a result, the lubricating oil scatters from the bearing  13  toward the oil discharge port  3   c , and the discharge of the lubricating oil via the bearing housing  3  is smoothly performed. From the viewpoint of smoothly discharging the lubricating oil, for example, when the bearing  13  is viewed in the axial direction of the main body  13   a , it is preferable that the oil discharge port  3   c  (see  FIG.  1   ) of the bearing housing  3  be located on an extension line of the oil discharge groove  45 . 
     The shape of a flow path cross section of the oil discharge groove  45  (that is, the shape in the cross section orthogonal to the extending direction of the oil discharge groove  45 ) is rectangular. However, the flow path cross section of the oil discharge groove  45  may have a polygonal shape (for example, a triangular shape) other than the rectangular shape, a semicircular shape, or others. 
     In the examples of  FIGS.  3  and  4   , the oil discharge groove  45  is separated from an end (specifically, the left end in  FIG.  4   ) of the tapered portion  41 - 2  in the rotation direction RD. However, the positional relationship between the tapered portion  41 - 2  and the oil discharge groove  45  is not limited to the example of  FIGS.  3  and  4   . For example, the oil discharge groove  45  may pass through the end of the tapered portion  41 - 2  in the rotation direction RD. 
     As described above, in the bearing  13  according to the present embodiment, the thrust bearing surface  13   i  includes the plurality of tapered portions  41 . As a result, the oil film pressure is likely to be generated, and the withstanding load in the thrust direction by the thrust bearing surface  13   i  increases. Incidentally, if an oil discharge grooves  45  is included for each of the plurality of tapered portions  41 , the flow of lubricating oil on the thrust bearing surface  13   i  is excessively promoted, and the amount of lubricating oil discharged from the bearing  13  becomes excessively large. As a result, the oil sealing performance of the turbocharger TC decreases. 
     Meanwhile, in the bearing  13  according to the present embodiment, the oil discharge groove  45  is included only for one tapered portion  41 - 2  among the plurality of tapered portions  41 . As a result, the amount of lubricating oil discharged from the bearing  13  is suppressed from becoming excessively large, thereby suppressing deterioration of the oil sealability. Furthermore, with the flow of the lubricating oil on the thrust bearing surface  13   i  promoted, an increase in the temperature of the oil film is suppressed, and a decrease in the viscosity accompanying the increase in the temperature is suppressed. Therefore, a decrease in the withstanding load in the thrust direction by the thrust bearing surface  13   i  is suppressed. As described above, according to the present embodiment, the withstanding load of the bearing  13  in the thrust direction can be improved appropriately. 
     Note that the larger the flow path cross-sectional area of the opening  45   a  on the outer peripheral edge side of the oil discharge groove  45  is, the larger the amount of lubricating oil discharged from the opening  45   a  is. On the other hand, the smaller the flow path cross-sectional area of the opening  45   a  on the outer peripheral edge side of the oil discharge groove  45  is, the smaller the amount of lubricating oil discharged from the opening  45   a  is. Therefore, from the viewpoint of more appropriately achieving both of suppression of a decrease in the oil sealability in the turbocharger TC and suppression of a decrease in the viscosity accompanying an increase in the temperature of the oil film on the thrust bearing surface  13   i , it is preferable to optimize the flow path cross-sectional area of the opening  45   a  on the outer peripheral edge side of the oil discharge groove  45 . 
       FIG.  5    is a diagram illustrating the bearing  13  as viewed from a direction of arrow B in  FIG.  3   . Specifically,  FIG.  5    is a diagram illustrating the tapered portion  41 - 2  as viewed from the radially outer side of the outer peripheral edge of the thrust bearing surface  13   i . In the example of  FIG.  5   , the opening  45   a  on the outer peripheral edge side of the oil discharge groove  45  has a rectangular shape. Therefore, the flow path cross-sectional area of the opening  45   a  on the outer peripheral edge side of the oil discharge groove  45  is determined depending on a width W (that is, the length in the circumferential direction) and a depth H (that is, the axial length) of the opening  45   a . In other words, the flow path cross-sectional area of the opening  45   a  is set by setting the width W and the depth H of the opening  45   a.    
       FIG.  6    is a graph illustrating the relationship between the ratio S 2 /S 1  of a flow path cross-sectional area S 2  of the opening  45   a  on the outer peripheral edge side of the oil discharge groove  45  to an area S 1  of the thrust bearing surface  13   i  in the bearing  13  (specifically, a projection area of the thrust bearing surface  13   i  in the axial direction) according to the present embodiment and a flow rate Q 1  [L/min] of the lubricating oil discharged from the bearing  13 .  FIG.  6    is a graph obtained by numerical analysis simulation. 
     As illustrated in  FIG.  6   , the flow rate Q 1  increases as the ratio S 2 /S 1  increases. As described above, in the present embodiment, since the plurality of tapered portions  41  is included on the thrust bearing surface  13   i , the withstanding load in the thrust direction is increased. Incidentally, as the flow rate Q 1  increases, the flow of the lubricating oil on the thrust bearing surface  13   i  is effectively promoted. Therefore, an increase in the temperature of the oil film and a decrease in the viscosity accompanying the increase in the temperature are effectively suppressed, and the withstanding load in the thrust direction is effectively increased. However, in general, in a thrust bearing, the flow rate Q 1  is required to be smaller than about 0.8 [L/min] (for example, the degree indicated by a broken horizontal line in  FIG.  6   ) in order to ensure the oil sealability. According to the graph of  FIG.  6   , in a case where the ratio S 2 /S 1  is less than or equal to 0.01, the flow rate Q 1  is smaller than about 0.8 [L/min]. That is, it can be seen that in a case where the ratio S 2 /S 1  is less than or equal to 0.01, deterioration of the oil sealability is appropriately suppressed. 
     Here, even in a case where the oil discharge groove  45  is included in each of the plurality of tapered portions  41 , it is conceivable that the flow rate Q 1  of the lubricating oil discharged from the bearing  13  can be adjusted by adjusting the opening  45   a  of each of the oil discharge grooves  45 . However, there is a limit to the minimum value of the amount of the lubricating oil discharged from each of the oil discharge grooves  45 , and in addition, in a case where an oil discharge groove  45  is included at a position different from that of the oil discharge port  3   c  side of the bearing housing  3 , it may deteriorate the oil sealability. Therefore, by setting the number of the oil discharge grooves  45  to one as in the present embodiment, reducing the flow rate Q 1  is appropriately achieved to such an extent that deterioration in the oil sealability is appropriately suppressed. Furthermore, by including the oil discharge groove  45  on the thrust bearing surface  13   i  on the oil discharge port  3   c  side of the bearing housing  3 , the oil sealability can be further improved. 
       FIG.  7    is a graph illustrating the relationship between the ratio S 2 /S 1  of the flow path cross-sectional area S 2  of the opening  45   a  on the outer peripheral edge side of the oil discharge groove  45  to the area S 1  of the thrust bearing surface  13   i  in the bearing  13  according to the present embodiment and the temperature T 1  [° C.] of the oil film formed on the thrust bearing surface  13   i .  FIG.  7    is a graph obtained by numerical analysis simulation. 
     As illustrated in  FIG.  7   , the temperature T 1  decreases as the ratio S 2 /S 1  increases. As described above, the higher the ratio S 2 /S 1  is, the larger the flow rate Q 1  is, and the flow of the lubricating oil on the thrust bearing surface  13   i  is effectively promoted. As a result, an increase in the temperature T 1  of the oil film and a decrease in the viscosity accompanying the increase in the temperature T 1  are effectively suppressed, and the withstanding load in the thrust direction is effectively increased. Incidentally, in general, in a thrust bearing, a temperature T 1  is required to be less than or equal to 170 [° C.] in order to ensure a withstanding load in a thrust direction. According to the graph of  FIG.  7   , in a case where the ratio S 2 /S 1  is greater than or equal to 0.003, the temperature T 1  is less than or equal to 170 [° C.]. That is, in a case where the ratio S 2 /S 1  is greater than or equal to 0.003, it is understood that a decrease in the viscosity of the oil film due to an increase in the temperature is appropriately suppressed and that a decrease in the withstanding load in the thrust direction by the thrust bearing surface  13   i  is appropriately suppressed. 
     As described above, it is particularly preferable that the ratio S 2 /S 1  is within a range between 0.003 and 0.01 from the viewpoint of more appropriately achieving both suppression of a decrease in the oil sealability in the turbocharger TC and suppression of a decrease in the viscosity accompanying an increase in the temperature of the oil film on the thrust bearing surface  13   i.    
     Although the embodiment of the present disclosure has been described with reference to the accompanying drawings, it is naturally understood that the present disclosure is not limited to the above embodiment. It is clear that those skilled in the art can conceive various modifications or variations within the scope described in the claims, and it is understood that they are naturally also within the technical scope of the present disclosure. 
     The example in which the bearing  13  is included in the turbocharger TC has been described above. However, the bearing  13  may be included in a device other than the turbocharger TC (For example, a ship or the like). 
     The example in which the bearing  13  is a semi-floating bearing has been described above. However, the bearing  13  may be a bearing other than the semi-floating bearing as long as it has a thrust bearing surface.