Abstract:
It is intended, with respect to a first floating bush bearing disposed on a compressor housing side and a second floating bush bearing disposed on a turbine housing side, to improve oscillation stability when a turbine rotor revolves at a high speed, reduce the number of parts, and facilitate processing and assemblability. Provided are a bearing housing ( 13 ) for a turbocharger ( 1 ), a rotor shaft ( 16 ) passing through the bearing housing ( 13 ), a first floating bush bearing ( 17 ) interposed between the bearing housing ( 13 ) and the rotor shaft ( 16 ) and disposed on a compressor housing ( 12 ) side and comprising a first floating bush, and a second floating bush bearing ( 18 ) that has the same shape as the first floating bush bearing ( 17 ) and is disposed on a turbine housing ( 11 ) side and comprises a second floating bush ( 20 ). The inner bearing width (Li) of the first and second floating bushes is formed to be smaller than the outer bearing width (Lo) by the same proportion in both bushes.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates a floating bush bearing which is suitable for a rotary machine having a high-speed rotation shaft, such as a turbocharger. 
       BACKGROUND ART 
       [0002]    A floating bush bearing includes a floating bush rotatably provided in a space between a rotation shaft and a bearing housing. The floating bush bearing is configured so as to supply pressurized lubricating oil from the bearing housing to a space between an inner peripheral surface of the bearing housing and an outer peripheral surface of the floating bush and also to a space between an inner peripheral surface of the floating bush and the rotation shaft via an oil supply path provided in the floating bush in the radial direction. 
         [0003]    The floating bush bearing is configured to support the rotation shaft stably while suppressing its oscillation by a damping effect of the oil film of the lubricating oil formed in these spaces and also to prevent seizure. 
         [0004]      FIG. 7  is a detailed view of a floating bush bearing for rotatably supporting a rotation shaft of a high-speed rotary machine such as a turbocharger. 
         [0005]    In  FIG. 7 , a turbocharger  0100  is formed by a pair of vane wheels  0102   a  and  0102   b  and a rotor shaft  0104  for integrally connecting the pair of vane wheels  0102   a  and  0102   b.  A floating bush bearing  0110  is formed by a bearing housing  0112  and a floating bush  0114  placed between the bearing housing  0112  and the rotor shaft  0104 . The floating bush  0114  rotatably supports the rotor shaft  0104 . 
         [0006]    Oil passages  0116  and  0118  are provided in the bearing housing  0112  and the floating bush  0114 , respectively. The lubricating oil is supplied from the bearing housing  0112  through the oil passages  0116  and  0118  to the space between the inner peripheral surface of the bearing housing  0112  and the outer peripheral surface of the floating bush  0114  and the space between the inner peripheral surface of the floating bush  0114  and the rotor shaft  0104 . 
         [0007]    The floating bush  0114  is rotatably arranged and is configured to co-rotate with the rotor shaft  0104  at a speed lower than the rotor shaft  0104  due to sliding resistance against the rotor shaft  0104 . 
         [0008]    The oil film composed of the lubricating oil is formed in these spaces so as to prevent seizure occurring at a sliding face in the spaces and damage (wear). 
         [0009]    In JP 2009-156333 A (Patent Document 1), the width of the inner periphery of the floating bush on the compressor side, Lc is set smaller than the width of the inner periphery of the floating bush on the turbine side, Lt (Lc&lt;Lt). Hence, a spring constant by an oil film on the floating bush bearing on the compressor side is smaller than a sprint constant by an oil film on the floating bush bearing on the turbine side. As a result, without an increase of an eccentricity ratio of the rotor shaft relative to the floating bush bearing on the turbine side subjected to a large bearing load, equilibrium is maintained between the floating bush bearing on the turbine side and the floating bush bearing on the compressor side and natural frequency caused by self-excited oscillation in a high frequency range decreases, thereby achieving noise reduction. 
         [0010]    In JP 2007-46642A (Patent Document 2), a spiral groove is provided in at least one of an inner peripheral surface or an outer peripheral surface of a fully floating bearing and that, when the fully floating bearing is rotated, distribution of the pressure loaded on the inner or outer peripheral surface from fluid (lubricating oil) changes along the axial direction by the spiral groove formed in at least one of the inner peripheral surface or the outer peripheral surface. This change makes it difficult for the fully floating bearing to be in a stable state, thereby suppressing self-excited oscillation. 
       CITATION LIST  
     Patent Literature 
       [0011]    [Patent Document 1]JP 2009-156333 A 
         [0012]    [Patent Document 2]JP 2007-46642 A 
       SUMMARY 
     Technical Problem  
       [0013]    In Patent Document 1, the width of the inner periphery of the floating bush on the compressor side, Lc is set smaller than the width of the inner periphery of the floating bush on the turbine side, Lt (Lc&lt;Lt) so as to reduce the natural frequency caused by self-excited oscillation in the high frequency range decreases, thereby achieving noise reduction. However, the floating bush bearing on the compressor side and the floating bush bearing on the turbine side have different shapes and thus, the number of components increases. This results in increase in a management cost and a higher risk of misassembling. 
         [0014]    In Patent Document 2, the spiral groove is provided in at least one of the inner and outer peripheral surfaces of the fully floating bearing. Therefore, it is a complicated work to form the groove and, in particular, the groove in the inner peripheral surface which has small inner diameter where the rotor shaft of the turbine rotor is fitted and the machining man hour increases. This results in higher component cost. 
         [0015]    In view of the above issues, an object of the present invention is to improve oscillation stability when a turbine rotor revolves at a high speed, reduce the number of parts, and facilitate processing and assemblability by configuring a first floating bush bearing disposed on a compressor housing side and a second floating bush bearing disposed on a turbine housing side to have substantially the same shape and reducing the width of the inner peripheral width Li relative to the outer peripheral width Lo of each of the first and second floating bushes at the same ratio. 
       Solution to Problem  
       [0016]    To achieve the object of the present invention, a bearing device for a turbocharger according to the present invention, comprises: 
         [0017]    a bearing housing interposed between a turbine housing and a compressor housing of a turbocharger for pressurizing supply air to a combustion chamber of an internal combustion engine; 
         [0018]    a rotor shaft of a turbine rotor arranged to pass through the bearing housing; and 
         [0019]    a first floating bush bearing interposed between the bearing housing and the rotor shaft, comprising a first floating bush and being disposed on a compressor housing side, the first floating bush having an oil supply hole which interconnects an outer peripheral surface and an inner peripheral surface and being configured to supply lubricating oil from the bearing housing to the outer peripheral surface and the inner peripheral surface; 
         [0020]    a second floating bush bearing having the same configuration as the first floating bush bearing, comprising a second floating bush and being disposed on a turbine housing side, and 
         [0021]    each of the first and second floating bushes is configured so that an inner bearing width Li is formed smaller than an outer bearing width Lo at the same ratio. 
         [0022]    According to the present invention, each of the first and second bushes is configured so that the inner bearing width Li is smaller than the outer bearing width Lo to reduce the co-rotation amount of the floating bush during the high-speed rotation of the rotor shaft and improve the oscillation stability during the high-speed rotation, thereby achieving the noise reduction effect. 
         [0023]    Further, by configuring the first and second bushes so that the first and second bushes have the same ratio of the inner bearing width to the outer bearing width, viscosity resistance of the lubricating oil which occurs at the first and second bushes is the same, thereby achieving stable support of the rotor shaft. 
         [0024]    As the first and second floating bushes have the same configuration, it is possible to reduce the number of parts, and facilitate processing and assemblability. As a result, the risk of misassembling can be lowered and stable processing quality and cost reduction can be achieved. 
         [0025]    By configuring the outer bearing width larger than the inner bearing width, it is possible to maintain the damping effect on the floating bush bearing and the housing. 
         [0026]    It is preferable in the present invention that a ratio Li/Lo of the outer bearing width Lo to the inner bearing width Li is set to satisfy a range of Lx/Lo&lt;Li/Lo&lt;0.5 where Lx is a minimum inner bearing width including a diameter φ of the oil supply hole (oil supply path). 
         [0027]    In this configuration, it is possible to maintain the co-rotation amount of each floating bush at an appropriate rotation speed with respect to the rotation speed of the rotor shaft during high-speed rotation of the rotor shaft, and improve the rotation oscillation stability so as to obtain the noise reduction effect. 
         [0028]    Further, if the minimum width is set smaller than the diameter φ of the oil supply hole, a tip of the oil supply path is removed to form an inclined surface of a cone shape. This causes the lubricating oil to flow directly in the axial direction of the rotor shaft from the oil supply hole φ. This minimizes the damping effect of the lubricating oil on the floating bush bearing and the rotor shaft. 
         [0029]    It is preferable in the present invention that the outer peripheral surface of the floating bush bearing has a low friction treatment. 
         [0030]    It is also preferable in the present invention that the low friction treatment includes Diamond Like Carbon coating or molybdenum disulfide coating on the outer peripheral surface of the floating bush. 
         [0031]    It is preferable in the present invention that the low friction treatment includes surface texture (dimples or micro recesses) formed on the outer peripheral surface of the floating bush. 
         [0032]    In such configurations, by providing the low friction treatment on the outer peripheral surface of the floating bush bearing to reduce the friction resistance between an outer peripheral bearing surface of the floating bush and the bearing housing, it is possible to start rotation of the floating bush at an early stage in the low-speed rotational range of the rotor shaft so as to reduce rotation resistance of the rotor shaft. 
         [0033]    It is preferable in the present invention that the low friction treatment includes a groove circumferentially arranged on the outer peripheral surface of the floating bush, the groove communicating with the oil supply hole. 
         [0034]    The lubricating oil fed from the lubricating oil path flows to a lower part of the floating bush via an edge groove, thereby filling a space between the floating bush and the bearing housing. As a result, the friction resistance at the start of rotation of can be suppressed. 
       ADVANTAGEOUS EFFECTS  
       [0035]    According to the present invention, by configuring each of the first and second bushes so that the inner bearing width is smaller than the outer bearing width, it is possible to maintain the co-rotation amount of each floating bush at an appropriate rotation speed with respect to the rotation speed of the rotor shaft during high-speed rotation of the rotor shaft, and improve the rotation oscillation stability so as to achieve the noise reduction effect. 
         [0036]    Further, by forming the outer bearing width larger than the inner bearing width, it is possible to maintain the damping effect on the floating bush bearing and the housing. 
         [0037]    Furthermore, as the first and second floating bushes have the same configuration, it is possible to reduce the number of parts, and facilitate processing and assemblability. As a result, the risk of misassembling can be lowered and stable processing quality and cost reduction can be achieved. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0038]      FIG. 1  is an enlarged cross-sectional view of a bearing device for a turbocharger according to embodiment of the present invention. 
           [0039]      FIG. 2  is an enlarged cross-sectional view of a turbine rotor shaft and a floating bush according to an embodiment. 
           [0040]      FIG. 3  is a characteristic diagram of a damping ratio indicating oscillation stability with respect to an inner/outer width ratio of the bush according to an embodiment. 
           [0041]      FIG. 4A  illustrates a low friction treatment provided on an outer peripheral part of the floating bush according to a first embodiment. 
           [0042]      FIG. 4B  illustrates a low friction treatment provided on an outer peripheral part of the floating bush according to a second embodiment. 
           [0043]      FIG. 4C  illustrates a low friction treatment provided on an outer peripheral part of the floating bush according to a third embodiment in (C1) and shows a longitudinal cross-sectional view of (C1) in the radial direction in (C2). 
           [0044]      FIG. 4D  illustrates a low friction treatment provided on an outer peripheral part of the floating bush according to a fourth embodiment in (D1) and shows a longitudinal cross-sectional view of (D1) in the radial direction in (D2). 
           [0045]      FIG. 5  is an illustration of related art. 
       
    
    
     DETAILED DESCRIPTION 
       [0046]      FIG. 1  illustrates a floating bush bearing which rotatably supports a high-speed turbocharger according to an embodiment of the present invention. In  FIG. 1 , a turbocharger  1  is formed by an exhaust turbine  14  driven by exhaust gas of an internal combustion engine (hereinafter described as “engine”), a turbine housing  11  covering the exhaust turbine  14 , a compressor impeller  15  for pressurizing intake air to the engine by a driving force of the exhaust turbine  14 , a compressor housing  12  for covering the compressor impeller  15 , a rotor shaft  16  for integrally connecting the exhaust turbine  14  and the compressor impeller  15 , a bearing housing  15  interposed between the turbine housing  11  and the compressor housing  12  and rotatably supporting the rotor shaft  16 , a thrust bearing  16   b  receiving a thrust load of the rotor shaft  16 , and a sleeve  16   c  for positioning the compressor impeller  15  when fastening by a fastening member (not shown) from the compressor impeller  15  side. 
         [0047]    In the bearing housing  13 , a first floating bush bearing  17  and a second floating bush bearing  18  are arranged. The first floating bush bearing  17  is configured to rotatably support a compressor impeller side of the rotor shaft  16 . The second floating bush bearing  18  is configured to rotatably support an exhaust turbine  14  side of the rotor shaft  16 . 
         [0048]    The first floating bush bearing  17  is formed by a bearing portion  13   d  formed in the bearing housing  13  and a first floating bush  19  which is fitted in the bearing portion  13   d  and in which the rotor shaft  16  is arranged. 
         [0049]    The first floating bush  19  is formed into an annular shape and an oil supply path  19   a  is arranged to penetrate the first floating bush  19  from an outer peripheral surface to an inner peripheral surface. 
         [0050]    The second floating bush bearing  48  is formed by a bearing portion  13   d  formed in the bearing housing  13  and a second floating bush  20  which is fitted in the bearing portion  13   d  and in which the rotor shaft  16  is arranged. 
         [0051]    The second floating bush  20  is formed into an annular shape and an oil supply path  20   a  is arranged to penetrate the second floating bush  20  from an outer peripheral surface to an inner peripheral surface. 
         [0052]    A pipe (not shown) is connected to a connection port  13   a  of the bearing housing  13  so that lubricating oil is supplied to the floating bush bearings  17  and  18 . 
         [0053]    A compressor-side lubricating oil passage  13   c  is an oil passage extending linearly and obliquely with respect to the first floating bush bearing  17  from the connection port  13   a.  The lubricating oil pumped to the first floating bush bearing  17  side is supplied to the first floating bush bearing  17  and between the first floating bush bearing  17  and the thrust bearing  16   b  so as to lubricate and cool these parts. 
         [0054]    A turbine-side lubricating oil passage  13   b  is an oil passage extending linearly and obliquely with respect to the second floating bush bearing  18  from the connection port  13   a.  The lubricating oil pumped to the second floating bush bearing  18  side is supplied to the second floating bush bearing  18  and an increased diameter portion  16   d  of the rotor shaft  16  on the exhaust turbine  14  side so as to lubricate and cool these parts. 
         [0055]    Next, the lubricating oil supplied to the first floating bush bearing  17  fills a gap between the bearing portion  13   d  and the first floating bush  19 , passes through the oil supply path  19   a  penetrating the first floating bush  19  from the outer peripheral side to the inner peripheral side, and is finally supplied to between the first floating bush  19  and a shaft support part  16   a  of the rotor shaft  16 . 
         [0056]    Further, the lubricating oil supplied to the second floating bush bearing  18  fills a gap between the bearing portion  13   d  and the second floating bush  20 , passes through the oil supply path  20   a  penetrating the second floating bush  20  from the outer peripheral side to the inner peripheral side, and is finally supplied to between the second floating bush  20  and the shaft support part  16   a  of the rotor shaft  16 . 
         [0057]    Each of the floating bushes  19  and  29  and the rotor shaft  16  is configured to be supported in a floating manner to the bearing housing  13  by means of an oil film composed of the lubricating oil. 
         [0058]    The first floating bush bearing  17  (including the first floating bush  19 ) and the second floating bush bearing  18  (including the second floating bush  20 ) are explained above to avoid confusion, although they have the same configuration. 
         [0059]    Hereinafter, “first floating bush bearing  17 ” is described as “floating bush bearing  17 ”, and “second floating bush  19 ” is described as “floating bush  19 ”. 
         [0060]    The floating bush bearing  17  of the present invention is illustrated in  FIG. 2 . The configuration of the floating bush bearing  17  is explained in reference to  FIG. 2 . 
         [0061]    The floating bush  19  forming the floating bush bearing  17  is arranged to have a gap a so that an oil film is formed between the bearing portion  13   d  of the bearing housing  13  and an outer peripheral surface  19   b  of the floating bush  19 . 
         [0062]    The rotor shaft  16  is loosely fitted to the floating bush  19  with a gap  6  so that an oil film is formed between an inner peripheral surface  19   c  of the floating bush  19  and the shaft support part  16   a  of the rotor shaft  16 . 
         [0063]    To these gaps a,  6 , the lubricating oil is fed through the compressor-side lubricating oil passage  13   c  within the bearing housing  13  (similarly to the turbine-side lubricating oil passage  13   b ). 
         [0064]    When filling the gap a, the lubricating oil passes through the oil supply path  19   a  of the floating bush  19 , filling the gap B between the inner peripheral surface  19   c  of the floating bush  19  and the shaft support part  16   a,  thereby supporting the rotor shaft  16  in a floating state with the oil film composed of the lubricating oil. 
         [0065]    The floating bush  19  is configured so that an inner bearing width Li of an inner peripheral surface of the floating bush  19  is formed smaller than an outer bearing width Lo of an outer peripheral surface of the floating bush  19 . The inner bearing width Li of the inner peripheral surface  19   c  of the floating bush  19  is formed by trimming both ends of the floating bush  19  inwardly into a cone shape. In this embodiment, both ends are cut by the same amount to be symmetrical with respect to the oil supply path  19   a.    
         [0066]    With this symmetrical configuration, it is possible to uniformly maintain a gap between the shaft support part  16   a  and a width-directional portion of the inner peripheral surface of the floating bush  19 , which is symmetrical about the oil supply path  19   a,  thereby improving support stability. 
         [0067]    In this embodiment, Lx is the minimum inner bearing width including a diameter φ of the oil supply path  19   a  (a lubricating oil path diameter φ). 
         [0068]    The minimum inner bearing width Lx is the minimum bearing width required to form an oil film thereon for supporting the shaft support part  16   a  by the inner peripheral surface  19   c  of the floating bush  19 . 
         [0069]    Lx (the minimum inner bearing width Lx=minimum contact width+diameter φ of oil supply path  19   a ) is influenced by weights of the exhaust turbine  14 , the compressor impeller  15  and the rotor shaft  16 , rotation balance, etc. Therefore, Lx may be determined in accordance with specifications of the turbocharger  1 . 
         [0070]      FIG. 3  shows calculation of a damping ratio based on experiment results. The damping ratio % indicates support stability of the rotor shaft  16  with respect to an inner/outer width ratio Li/Lo of the floating bush  19 . 
         [0071]    The inner/outer width ratio Li/Lo is shown on the horizontal axis while the damping ratio % is shown on the longitudinal axis as a parameter defining the damping characteristic of the oil film. The higher damping ratio means better support stability of the rotation shaft  16 . 
         [0072]    The support of the rotor shaft  16  by the floating bush  19  tends to be unstable when the floating bush  19  is co-rotated at high speed. To prevent co-rotation of the floating bush  19  at high speed, the inner bearing width Li of the floating bush  19  is set smaller than the outer bearing width Lo of the floating bush  19  (Test Case 1 of  FIG. 3 ). 
         [0073]    To achieve performance improvement of the turbocharger such as durability improvement, supercharging pressure enhancement and oscillation noise reduction, an optimum range of the inner/outer width ratio Li/Lo is determined from this experiment result. 
         [0074]    More specifically, as shown in  FIG. 3 , the inner/outer width ratio Li/Lo of the floating bush  19  is Lx/Lo&lt;Li/Lo&lt;0.5. 
         [0075]    Lx is the minimum inner bearing width=minimum contact width+diameter φ of oil supply path  19   a,  as described above. 
         [0076]    When Li/Lo&gt;0.5 is the upper limit of this range, rotation of the rotor shaft  16  at high speed causes co-rotation of the floating bush  19  at increased speed, and the outer peripheral surface  19   b  of the floating bush  19  mainly becomes a sliding surface. 
         [0077]    Thus, the support of the rotor shaft  16  tends to become unstable, and a noise and rotation resistance of the rotor shaft  16  becomes large. 
         [0078]    When the ratio Li/Lo is 0.5, the damping ratio is improved from 2% of the conventional case to 3.5%, which results in the significantly improved damping ratio. Meanwhile, on the lower limit side, the bearing width Li of the inner peripheral surface  19   c  is set to the minimum inner bearing width Lx required to provide the hole diameter φ of the oil supply path  19   a  and to form an oil film. 
         [0079]    In this embodiment, as the floating bush  19  is formed to satisfy Lx/Lo&lt;Li/Lo&lt;0.5, the co-rotation amount of the floating bush  19  can be optimized in the high-speed rotational range of the rotor shaft  16 . 
         [0080]    Therefore, during high-speed rotation of the rotor  16 , by optimizing the co-rotation force applied to the floating bush  19  and reducing (optimizing) the co-rotation amount (the rotation amount) of the floating bush  19 , the oil film formed between the inner peripheral surface  19   c  of the floating bush  19  and the rotor shaft  16  becomes the sliding surface for supporting rotation of the rotor shaft  16 . As a result, the rotation sliding resistance and oscillation of the rotor shaft  16  can increase, and oscillation stability of the rotor shaft  16  can be improved. 
         [0081]    Further, as the first floating bush bearing  17  and the second floating bush bearing  18  have the same configuration, viscosity resistance generated at the first and the second floating bushes is the same. As a result, the support of rotor shat  16  becomes stable and the risk of misassembling is reduce. This achieves improved quality and reduced production cost. 
         [0082]    This also has a performance improvement effect, such as durability improvement, supercharging pressure enhancement and oscillation noise reduction of the turbocharger  1 . 
         [0083]    The ratio of the inner bearing width Li to the outer bearing width Lo of the floating bush  19  is set to Lx/Lo&lt;Li/Lo&lt;0.5 to achieve stable support of the rotor shaft  16  during the high speed rotation. To achieve performance improvement in the entire rotation range of the turbocharger, it is necessary to improve the supercharging pressure during the low speed rotation. During the low speed rotation of the turbocharger  1 , it is advantageous to cause the floating bush  19  to co-rotate at an early stage and thus, it is necessary to improve starting performance of the turbocharger  1  by reducing the rotation resistance of the rotor shaft  16 . 
         [0084]      FIG. 4A  illustrates a low friction treatment provided on an outer peripheral part of the floating bush according to a first embodiment.  FIG. 4A  shows an enlarged cross-sectional view of the floating bush  19  provided with the low friction treatment on the outer peripheral part to cause co-rotation of the floating bush  19  at an early stage in the low-speed rotational range of the rotor shaft  16 . 
         [0085]    On the outer peripheral surface of the floating bush  19 , DLC (Diamond Like Carbon) coating  19   d  is provided as the low friction treatment. 
         [0086]    The DLC used here may be metal-containing DLC (WC: tungsten carbide, Si: silicon), hydrogen containing DLC, or the like. 
         [0087]    The DLC coating is normally formed by, but not particularly limited to, chemical vapor deposition (CVD) 
         [0088]    The DLC film is hard and fragile and thus, to improve adhesion of the film to a base material (the floating bush), a substrate material may be provided between the base material (the floating bush) and the DLC film. The substrate material may be a single layer of or multiple layers of combination of chromium (Cr), chromium nitride 
         [0089]    (CrN), titanium (Ti), titanium nitride (TiN), tungsten carbide (WC), tungsten (W), nickel (Ni), copper (Cu), iron (Fe), iron nitride (FeN), silicon (Si), silicon carbide (Sic), etc. 
         [0090]    By composing the outer peripheral surface of the floating bush  19  of the low friction treatment  19   d  (e.g. DLC coating), it is possible to reduce sliding resistance between the outer peripheral surface and the oil film intervening between the bearing portion  13   d  and the floating bush  19 . 
         [0091]    Consequently, by starting co-rotation of the floating bush  19  at a stage where the rotation speed of the rotor shaft  16  is relatively low (the early stage), the rotation resistance of the floating bush  19  and the rotor shaft  16  is reduced even in the low-speed rotation range of the turbocharger  1  to obtain the effect of improving the supercharging pressure of the turbocharger  1 . 
         [0092]      FIG. 4B  illustrates a low friction treatment of a second embodiment. In the second embodiment, dimples  27   s  are provided, as the low friction treatment in the outer peripheral surface of a floating bush  27 . The lubricant oil is retained in depressions of the dimples  27   a,  thereby reducing sliding resistance between the outer peripheral surface of the floating bush  27  and the oil film which intervenes between the bearing portion  13   d  and the floating bush  27 . 
         [0093]    The dimples  27   a  may be formed, but not limit to, by fine particle shot peening. 
         [0094]    In  FIG. 4A , the dimples  27   a  have hemispheric shapes. This is, however, not restrictive and the dimples may have any shape such as a triangular pyramid as long as the shape allows the lubricating oil to retained in the dimples and achieves reduction of the friction coefficient. 
         [0095]      FIG. 4C  illustrates a low friction treatment of a third embodiment in (C1).  FIG. 4C  shows in (C2) a longitudinal cross-sectional view of (C1) in the radial direction. In a floating bush  30 , a center groove  30   d  is provided in an outer peripheral surface  30   a  of the floating bush  30  over the entire circumference of outer peripheral surface  30   a  approximately at the center of the outer peripheral surface  30   a  in the bearing width direction. The center groove  30   d  communicates with an oil supply hole  30   c  which penetrates the floating bush  30  from the outer peripheral surface  30   a  to the inner peripheral surface  30   b.    
         [0096]    As illustrated in (C2) of  FIG. 4C , the lubricating oil fed from the lubricating oil path  13   c  flows to a lower part of the floating bush  30  via the center groove  30   d,  thereby filling the space between the floating bush  30  and the bearing portion  13   d. As a result, the friction resistance at the start of rotation of the floating bush  30  can be reduced.    
         [0097]      FIG. 4D  illustrates a low friction treatment of a fourth embodiment in (D1).  FIG. 4D  shows in (D2) a longitudinal cross-sectional view of (D1) in the radial direction. In a floating bush  35 , an oil supply hole  35   c  is provided approximately at the center of the outer peripheral surface  35   a  of the floating bush  35  in the bearing width direction. The oil supply hole  35   c  penetrates the floating bush  35  from the outer peripheral surface  30   a  to the inner peripheral surface  30   b.    
         [0098]    On both sides of the oil supply hole  35   c  of the outer peripheral surface  35   c  in the bearing width direction, a pair of edge grooves  35   d,    35   d  is formed over the entire circumference of the outer peripheral surface  35   a.  In each these edge grooves  35   d,    35   d,  a transverse groove  35   e  is provided for each oil supply hole  35   c  so that the edge groove  35   d  communicates with the oil supply hole  35   c.    
         [0099]    As illustrated in (D2) of  FIG. 4D , the lubricating oil fed from the lubricating oil path  13   c  flows to a lower part of the floating bush  35  (as indicated by arrows) via the edge grooves  35   d,    35   d,  thereby filling the space between the floating bush  35  and the bearing portion  13   d.  As a result, the friction resistance during rotation start of the floating bush  35  can be suppressed. 
         [0100]    According to the present invention, in each floating bush  19  ( 20 ), the inner bearing width is set smaller than the outer bearing width to reduce the co-rotation amount of the floating bush during the high-speed rotation and improve the oscillation stability, thereby achieving the noise reduction effect. Further, by applying the low friction treatment to the outer peripheral part of the floating bush  19  ( 20 ), the sliding resistance during the low-speed rotation of the rotor shaft  16  can be improved. 
         [0101]    As the floating bushes  19  ( 20 ) have the same ratio of the inner bearing width Li to the outer bearing width Lo, viscosity resistance of the lubricating oil is the same for the floating bushes  19  ( 20 ). As a result, the rotor shaft  16  can be stably support, and misassembling of the floating bushes  19  ( 20 ) can be eliminated, which achieves the cost reduction. 
         [0102]    Further, by setting the ratio Li/Lo of the inner bearing width Li to the outer bearing width Lo to Lx/Lo&lt;Li/Lo&lt;0.5, the co-rotation amount of the floating bush  19  ( 20 ) is optimized with respect to the rotation speed of the rotor shaft  16 . Thus, the co-rotation amount of each floating bush can be lowered during the high-speed rotation of the rotor shaft  16 , and with improved oscillation stability, it is possible to obtain the noise reduction effect. 
       INDUSTRIAL APPLICABILITY  
       [0103]    The above floating bearing is suitable as a bearing for a high-speed rotation shaft, and is suitable for use in a rotary machine, such as a turbocharger, revolving at high speed, so as to maintain favorable supply of the lubricant oil to the inner peripheral surface of the floating bush at low cost and to lower the noise and rotation resistance by stably supporting the rotation shaft. 
       REFERENCE SIGNS LIST  
       [0000]    
       
           1  Turbocharger 
           11  Turbine housing 
           12  Compressor housing 
           13  Bearing housing 
           13   a  Connection port 
           13   b,    13   c  Lubricating oil path 
           13   d  Bearing portion 
           14  Exhaust turbine 
           15  Compressor impeller 
           16  Rotor shaft 
           16   a  Support part 
           17  First floating bush bearing (floating bush bearing) 
           18  Second floating bush bearing 
           19 ,  27 ,  30 ,  35  First floating bush (floating bush) 
           19   a  Lubricating oil path 
           19   b,    27   b,    30   c,    35   c  Outer peripheral surface 
           19   c  Inner peripheral surface 
           19   d  DLC coating 
           20  Second floating bush 
           30   d  Low friction treatment 
         Lo Outer bearing width 
         Li Inner bearing width