Patent Publication Number: US-9903379-B2

Title: Variable nozzle unit and variable geometry system turbocharger

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of International Application No. PCT/JP2013/073265, filed on Aug. 30, 2013, which claims priority to Japanese Patent Application No. 2012-201268, filed on Sep. 13, 2012, the entire contents of which are incorporated by references herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a variable nozzle unit which can alter a passage area for (a flow rate of) gas such as exhaust gas to be supplied to a turbine impeller in turbo rotating machinery such as a variable geometry system turbocharger or a gas turbine, and relates to a variable geometry system turbocharger. 
     2. Description of the Related Art 
     In recent years, various variable nozzle units for use in variable geometry system turbochargers have been developed. A general configuration of a conventional variable nozzle unit will be described below. 
     In a housing of a variable geometry system turbocharger, a shroud ring as a first base ring is provided concentrically with a turbine impeller. A plurality of first supporting holes are formed in the shroud ring at equal intervals in the circumferential direction of the shroud ring. Moreover, a nozzle ring as a second base ring is provided at a position away from and facing the shroud ring in the axial direction of the turbine impeller integrally and concentrically with the shroud ring. A plurality of second supporting holes are formed in the nozzle ring at equal intervals in the circumferential direction of the nozzle ring in such a manner as to match the plurality of first supporting holes of the shroud ring. 
     A plurality of variable nozzles are disposed between a facing surface of the shroud ring and a facing surface of the nozzle ring at equal intervals in the circumferential direction of the shroud ring (nozzle ring). Each variable nozzle is rotatable in both the forward and reverse directions about an axis parallel to the turbine impeller. Moreover, a first nozzle shaft is integrally formed on a side surface of each variable nozzle on one side in the axial direction. Each first nozzle shaft is rotatably supported by the corresponding supporting hole of the shroud ring. Further, a second nozzle shaft is formed on a side surface of each variable nozzle on the other side in the axial direction integrally and concentrically with the first nozzle shaft. Each second nozzle shaft is rotatably supported by the corresponding second supporting hole of the nozzle ring. 
     A link mechanism for synchronously rotating the plurality of variable nozzles in the forward and reverse directions is provided on the opposite side of the shroud ring from the facing surface. Synchronously rotating the plurality of variable nozzles in the forward direction (opening direction) increases the passage area of exhaust gas to be supplied to the turbine impeller. Synchronously rotating the plurality of variable nozzles in the reverse direction (closing direction) decreases the passage area of the exhaust gas. 
     A nozzle supporting structure for supporting the variable nozzle will be described below. 
     The inner surface of the first supporting hole of the shroud ring has on one side in the axial direction a first bearing portion by which the first nozzle shaft of the variable nozzle is rotatably supported. The inner surface of the second supporting hole of the nozzle ring has a second bearing portion by which the second nozzle shaft of the variable nozzle is rotatably supported. In other words, the variable nozzle is supported on both sides from both sides of the variable nozzle in the axial direction by the first bearing portion and the second bearing portion. The fitting clearance between the first bearing portion and the first nozzle shaft and the fitting clearance between the second bearing portion and the second nozzle shaft are set to the same value to an accuracy of several tens of micrometers. 
     Meanwhile, in some conventional variable nozzle units, the plurality of second supporting holes are omitted from the nozzle ring, and the second nozzle shafts are omitted from the variable nozzles. In such a case, the inner surface of the first supporting hole of the shroud ring has on both sides in the axial direction two first bearing portions by which the first nozzle shaft of the variable nozzle is rotatably supported. In other words, the variable nozzle is supported on one side from one side of the variable nozzle in the axial direction by the two first bearing portions. The fitting clearance between one of the two first bearing portions and the first nozzle shaft and the fitting clearance between the other of the two first bearing portions and the first nozzle shaft are set to the same value to an accuracy of several tens of micrometers. 
     It should be noted that conventional techniques relating to the present invention are disclosed in Japanese Patent Application Laid-Open Publications Nos. 2012-102660 and 2010-71142. 
     SUMMARY OF THE INVENTION 
     In a variable nozzle unit of a type in which a nozzle is supported on both sides, the inclination of the axis of the variable nozzle with respect to the axis of the first supporting hole of the shroud ring during the operation of the variable geometry system turbocharger can be smaller than in a variable nozzle unit of a type in which a nozzle is supported on one side. However, the first bearing portion and the second bearing portion need to be respectively formed in the shroud ring and the nozzle ring separately prepared. This makes it difficult to sufficiently ensure the accuracy of the relative position between a hole constituting the first bearing portion and a hole constituting the second bearing portion. Moreover, before the nozzle ring is attached to the shroud ring, the variable nozzle is supported by only one first bearing portion. In this state, the axis of the variable nozzle is prone to incline with respect to the axis of the first supporting hole of the shroud ring. Accordingly, a special jig is needed when the nozzle ring is attached to the shroud ring, and the assembly work of the variable nozzle unit becomes complicated. 
     On the other hand, in a variable nozzle unit of a type in which a nozzle is supported on one side, the variable nozzle is supported by the two first bearing portions in a stabler state before the nozzle ring is attached to the shroud ring, than in a variable nozzle unit of a type in which a nozzle is supported on both sides. However, the inclination of the axis of the variable nozzle with respect to the axis of the supporting hole of the shroud ring during the operation of the variable geometry system turbocharger tends to be large. Accordingly, during the operation of the variable geometry system turbocharger, as wear between the first bearing portion on the side closer to the side surface of the variable nozzle and the first nozzle shaft proceeds, the non-smooth movement of the variable nozzle occurs, and may often become likely to cause the malfunction of the variable nozzle unit. 
     In other words, there is a problem that it is difficult to improve the efficiency of the assembly work of the variable nozzle unit while stabilizing the operation of the variable nozzle unit by reducing the non-smooth movement of the variable nozzle during the operation of the variable geometry system turbocharger. It should be noted that the above-described problem also occurs in a variable nozzle unit used in turbo rotating machinery such as a gas turbine. 
     An object of the present invention is to provide a variable nozzle unit and a variable geometry system turbocharger which can improve the working efficiency of assembling the variable nozzle unit while stabilizing the operation of the variable nozzle unit. 
     A first aspect of the present invention is a variable nozzle unit configured to alter a passage area of gas to be supplied to a turbine impeller of turbo rotating machinery, the variable nozzle unit including: a first base ring provided in a housing of the turbo rotating machinery concentrically with the turbine impeller, the first base ring including a plurality of first supporting holes formed in a circumferential direction thereof; a second base ring provided at a position away from and facing the first base ring in an axial direction of the turbine impeller integrally and concentrically with the first base ring, the second base ring including a plurality of second supporting holes formed in a circumferential direction thereof in such a manner as to match the plurality of the first supporting holes of the first base ring; a plurality of variable nozzles disposed between a facing surface of the first base ring and a facing surface of the second base ring in a circumferential direction of the first and second base rings, each variable nozzle being rotatable in both of forward and reverse directions about an axis parallel to an axis of the turbine impeller and including a first nozzle shaft integrally formed on a side surface thereof on one side in the axial direction and rotatably supported by the corresponding first supporting hole of the first base ring and a second nozzle shaft formed on a side surface thereof on another side in the axial direction integrally and concentrically with the first nozzle shaft and rotatably supported by the corresponding second supporting hole of the second base ring; and a link mechanism configured to synchronously rotating the plurality of variable nozzles in the forward and reverse directions, wherein an inner surface of each first supporting hole of the first base ring includes on both sides in the axial direction two first bearing portions by which the first nozzle shaft of the variable nozzle is rotatably supported, an inner surface of each second supporting hole of the second base ring includes a second bearing portion by which the second nozzle shaft of the variable nozzle is rotatably supported, and a fitting clearance between the second bearing portion and the second nozzle shaft of the variable nozzle is set larger than a fitting clearance between each of the first bearing portions and the first nozzle shaft of the variable nozzle. 
     In the specification and claims of the present application, the meaning of “turbo rotating machinery” includes a variable geometry system turbocharger and a gas turbine, the meaning of “provided” includes provided indirectly with the interposition of another member as well as provided directly, and the meaning of “disposed” includes disposed indirectly with the interposition of another member as well as disposed directly. 
     A second aspect of the present invention is a variable geometry system turbocharger for turbocharging air to be supplied to an engine side using pressure energy of gas from the engine, the variable geometry system turbocharger including the variable nozzle unit according to the first aspect. 
     The present invention can provide a variable nozzle unit and a variable geometry system turbocharger which can improve the working efficiency of assembling the variable nozzle unit while stabilizing the operation of the variable nozzle unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a cross-sectional view showing a characteristic portion of a variable nozzle unit according to an embodiment of the present invention. 
         FIG. 1B  is a view showing a state of the variable nozzle unit before a nozzle ring is attached to a shroud ring. 
         FIG. 2  is an enlarged view of a portion indicated by arrow II in  FIG. 3 . 
         FIG. 3  is a front cross-sectional view of a variable geometry system turbocharger according to the embodiment of the present invention. 
         FIG. 4A  is a cross-sectional view showing part of a variable nozzle unit according to comparative example 1 
         FIG. 4B  is a view showing a state of the variable nozzle unit before a nozzle ring is attached to a shroud ring. 
         FIG. 5A  is a cross-sectional view showing part of a variable nozzle unit according to comparative example 2 
         FIG. 5B  is a view showing a state of the variable nozzle unit before a nozzle ring is attached to a shroud ring. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described with reference to  FIGS. 1 to 3 . It should be noted that as shown in the drawings, “L” indicates the left direction, and “R” indicates the right direction. 
     As shown in  FIG. 3 , a variable geometry system turbocharger  1  according to an embodiment of the present invention turbocharges (compresses) air to be supplied to an engine using the pressure energy of exhaust gas from the engine (not shown). 
     The variable geometry system turbocharger  1  includes a bearing housing  3 . A plurality of radial bearings  5  and a plurality of thrust bearings  7  are provided in the bearing housing  3 . Moreover, a rotor shaft (turbine shaft)  9  extending in the lateral direction is rotatably provided through the plurality of bearings  5  and  7 . In other words, the rotor shaft  9  is rotatably provided in the bearing housing  3  with the plurality of bearings  5  and  7  interposed therebetween. 
     A compressor housing  11  is provided to the right of the bearing housing  3 . A compressor impeller  13  configured to compress air using centrifugal force is provided in the compressor housing  11  to be rotatable about the axis thereof (in other words, the axis of the rotor shaft  9 ). Moreover, the compressor impeller  13  includes a compressor disk (compressor wheel)  15  integrally coupled to a right end portion of the rotor shaft  9  and a plurality of compressor blades  17  provided on an outer peripheral surface of the compressor disk  15  at equal intervals in the circumferential direction of the compressor disk  15 . 
     The compressor housing  11  has an air inlet port  19  for introducing air, which is formed on an entrance side (right side of the compressor housing  11 ) of the compressor impeller  13 . The air inlet port  19  can be connected to an air cleaner (not shown) for cleaning air. Moreover, an annular diffuser passage  21  configured to increase the pressure of compressed air is formed on an exit side of the compressor impeller  13  between the bearing housing  3  and the compressor housing  11 . The diffuser passage  21  communicates with the air inlet port  19 . Further, a volute-shaped compressor scroll passage  23  is formed in the compressor housing  11 . The compressor scroll passage  23  communicates with the diffuser passage  21 . Further, an air discharge port  25  for discharging compressed air is formed at an appropriate position on the compressor housing  11 . The air discharge port  25  communicates with the compressor scroll passage  23 . The air discharge port  25  can be connected to an intake manifold (not shown) of the engine. 
     As shown in  FIGS. 2 and 3 , a turbine housing  27  is provided to the left of the bearing housing  3 . The turbine housing  27  includes a turbine housing body  29  provided to the left of the bearing housing  3  and a housing cover  31  provided to the left of the turbine housing body  29 . Moreover, to generate turning force (rotating torque) using the pressure energy of exhaust gas, a turbine impeller  33  is provided in the turbine housing  27  to be rotatable about the axis thereof (the axis of the turbine impeller  33  or the axis of the rotor shaft  9 ). The turbine impeller  33  includes a turbine disk (turbine wheel)  35  provided integrally with a left end portion of the rotor shaft  9  and a plurality of turbine blades  37  provided on the outer peripheral surface of the turbine disk  35  at equal intervals in the circumferential direction of the turbine disk  35 . 
     A gas inlet port  39  for introducing exhaust gas is formed at an appropriate position on the turbine housing  27  (turbine housing body  29 ). The gas inlet port  39  can be connected to an exhaust manifold (not shown) of the engine. Moreover, a volute-shaped turbine scroll passage  41  is formed in the turbine housing  27  (turbine housing body  29 ). The turbine scroll passage  41  communicates with the gas inlet port  39 . Further, a gas discharge port  43  for discharging exhaust gas is formed on an exit side of the turbine impeller  33  (left side of the turbine housing  27 ) in the turbine housing  27  (housing cover  31 ). The gas discharge port  43  can be connected to an exhaust gas cleaner (not shown) for cleaning exhaust gas. 
     A variable nozzle unit  45  which alters the passage area (flow rate) of exhaust gas to be supplied to the turbine impeller  33  side is disposed in the turbine housing  27 . The configuration of the variable nozzle unit  45  will be described below. 
     As shown in  FIG. 2 , a shroud ring  47  as a first ring base is provided in the turbine housing  27  concentrically with the turbine impeller  33 . The shroud ring  47  covers the outer edges of the plurality of turbine blades  37 . Moreover, a plurality of first supporting holes  49  are formed to pass through the shroud ring  47  and are equally spaced in the circumferential direction of the shroud ring  47  (or turbine impeller  33 ). 
     A nozzle ring  51  as a second base ring is provided at a position away from and facing the shroud ring  47  in the axial direction (lateral direction) of the turbine impeller  33  integrally and concentrically with the shroud ring  47  with a plurality of connecting pins  53  interposed therebetween. Moreover, a plurality of second supporting holes  55  are formed to pass through the nozzle ring  51  and are equally spaced in the circumferential direction of the nozzle ring  51  (or turbine impeller  33 ) in such a manner as to match the plurality of first supporting holes  49  of the shroud ring  47 . A left end portion of each connecting pin  53  is integrally coupled to the shroud ring  47  with a screw. A right end portion of each connecting pin  53  is integrally coupled to the nozzle ring  51  by staking. The plurality of connecting pins  53  have the function of setting the distance between the facing surface of the shroud ring  47  and the facing surface of the nozzle ring  51 . Means for coupling the connecting pin  53  to the shroud ring  47  and the nozzle ring  51  is not limited to the above-described one. Coupling these components together may be achieved by, for example, welding. 
     A plurality of variable nozzles  57  are disposed between the facing surface of the shroud ring  47  and the facing surface of the nozzle ring  51  at equal intervals in the circumferential direction of the shroud ring  47  and the nozzle ring  51  (or in the circumferential direction of the turbine impeller  33 ). Each variable nozzle  57  is rotatable in the forward and reverse directions (opening and closing directions) about an axis parallel to the axis of the turbine impeller  33 . Moreover, a first nozzle shaft  59  is integrally formed on a left side surface (side surface on one side in the axial direction) of each variable nozzle  57 . The first nozzle shaft  59  of each variable nozzle  57  is rotatably supported by the corresponding first supporting hole  49  of the shroud ring  47 . Further, a second nozzle shaft  61  is formed on a right side surface (side surface on another side in the axial direction) of each variable nozzle  57  integrally and concentrically with the first nozzle shaft  59 . The second nozzle shaft  61  of each variable nozzle  57  is rotatably supported by the corresponding second supporting hole  55  of the nozzle ring  51 . 
     It should be noted that the distances between adjacent variable nozzles  57  need not be equal to each other in consideration of shapes and aerodynamic effects of individual variable nozzles. In such a case, the distances between the first supporting holes  49  of the shroud ring  47  and the distances between the second supporting holes  55  of the nozzle ring  51  are also set in such a manner as to match the distances between the variable nozzles  57 . 
     An annular link chamber  63  is delimited and formed on the opposite side of the shroud ring  47  from the facing surface. A link mechanism (synchronization mechanism)  65  for synchronously rotating the plurality of variable nozzles  57  in the forward and reverse directions (opening and closing directions) is disposed in the link chamber  63 . The link mechanism  65  is linked and coupled to the first nozzle shafts  59  of the plurality of variable nozzles  57 . Moreover, the link mechanism  65  has a known configuration such as shown in the aforementioned Patent Literature 1 and 2. The link mechanism  65  is connected through a power transmission mechanism  67  to a motor or a rotating actuator (not shown) such as a cylinder for rotating the plurality of variable nozzles  57  in the opening and closing directions. 
     A nozzle supporting structure for supporting the variable nozzle  57  at both ends will be described below. 
     As shown in  FIG. 1A , the first nozzle shaft  59  of the variable nozzle  57  has on right and left sides (both sides in the axial direction) two large-diameter portions  59   a  and  59   b  having diameters larger than a reference outside diameter (outside diameter of an intermediate portion of the first nozzle shaft  59 ). The large-diameter portions  59   a  and  59   b  are rotatably supported by portions of an inner surface of the first supporting hole  49  of the shroud ring  47 . In other words, the inner surface of the first supporting hole  49  has on right and left sides thereof two first bearing portions  49   a  and  49   b  (portions contacting the large-diameter portions  59   a  and  59   b ) by which the first nozzle shaft  59  of the variable nozzle  57  is rotatably supported. 
     The outside diameter of the large-diameter portion  59   a  and the outside diameter of the large-diameter portion  59   b  are set to the same value. The inside diameter of the first bearing portion  49   a  and the inside diameter of the first bearing portion  49   b  are set to the same value. The fitting clearance between the first bearing portion  49   a  and the large-diameter portion  59   a  and the fitting clearance between the first bearing portion  49   b  and the large-diameter portion  59   b  are set to the same value to an accuracy of several tens of micrometers. 
     The second nozzle shaft  61  of the variable nozzle  57  has, in a portion other than a proximal end portion, a large-diameter portion  61   a  having a diameter larger than a reference outside diameter (outside diameter of the proximal end portion of the second nozzle shaft  61 ). The large-diameter portion  61   a  is rotatably supported by a portion of an inner surface of the second supporting hole  55  of the nozzle ring  51 . In other words, the inner surface of the second supporting hole  55  has a second bearing portion  55   a  (portion contacting the large-diameter portion  61   a ) by which the second nozzle shaft  61  of the variable nozzle  57  is rotatably supported. 
     The inside diameter of the second bearing portion  55   a  is set to the same value as the inside diameters of the first bearing portions  49   a  and  49   b . The outside diameter of the large-diameter portion  61   a  is set smaller than the outside diameters of the large-diameter portions  59   a  and  59   b . The fitting clearance between the second bearing portion  55   a  and the large-diameter portion  61   a  is set with an accuracy of several hundred micrometers. In other words, the fitting clearance between the second bearing portion  55   a  and the large-diameter portion  61   a  is set larger than the fitting clearances between the large-diameter portions  59   a  and  59   b  and the first bearing portions  49   a  and  49   b . It should be noted that the following may be employed: the outside diameter of the large-diameter portion  61   a  is set to the same value as the outside diameters of the large-diameter portions  59   a  and  59   b , and the inside diameter of the second bearing portion  55   a  is set larger than the inside diameters of the first bearing portions  49   a  and  49   b.    
     Further, in an early stage of the use of the unit (early stage of the use of the variable nozzle unit  45 ), the variable nozzle  57  is supported on one side from the left side (one side in the axial direction) of variable nozzle  57  by the two first bearing portions  49   a  and  49   b . As wear between the first bearing portion  49   b  on the right side (on the other side in the axial direction) and the large-diameter portion  59   b  proceeds, the angle of inclination of the axis of the variable nozzle  57  with respect to the axis of the first supporting hole  49  of the shroud ring  47  increases. In a further advanced stage of the wear, the large-diameter portion  61   a  of the second nozzle shaft  61  comes in contact with the second bearing portion  55   a . Finally, the variable nozzle  57  is supported on both sides from both the right and left sides of the variable nozzle  57  (both sides thereof in the axial direction) by the first bearing portion  49   a  on the left side (one side in the axial direction) and the second bearing portion  55   a . In a state in which the variable nozzle  57  is supported on both sides by the left-side first bearing portion  49   a  and the second bearing portion  55   a , the angle of inclination of the axis of the variable nozzle  57  with respect to the axis of the first supporting hole  49  of the shroud ring  47  is set to an angle equal to or less than a reference allowable angle of inclination. It should be noted that the reference allowable angle of inclination is an angle found in advance by testing so that the non-smooth movement of the variable nozzle  57  may be reduced. 
     Next, functions and effects of the embodiment of the present invention will be described. 
     Exhaust gas introduced through the gas inlet port  39  flows from the entrance side to the exit side of the turbine impeller  33  through the turbine scroll passage  41 . The flow of the exhaust gas causes the pressure energy of the exhaust gas to generate turning force (rotating torque), which can cause the rotor shaft  9  and the compressor impeller  13  to rotate integrally with the turbine impeller  33 . This rotation compresses air introduced through the air inlet port  19 , and allows the compressed air to be discharged from the air discharge port  25  through the diffuser passage  21  and the compressor scroll passage  23 . In other words, air to be supplied to the engine can be turbocharged (compressed). 
     During the operation of the variable geometry system turbocharger  1 , when the number of revolutions of the engine is in a high revolution region and the flow rate of exhaust gas is high, the actuation of the link mechanism  65  by the rotating actuator causes the plurality of variable nozzles  57  to synchronously rotate in the forward direction (opening direction). As a result, the gas passage area (area of the throat of the variable nozzle  57 ) of exhaust gas to be supplied to the turbine impeller  33  side increases, and the amount of exhaust gas to be supplied increases. On the other hand, when the number of revolutions of the engine is in a low revolution region and the flow rate of exhaust gas is low, the actuation of the link mechanism  65  by the rotating actuator causes the plurality of variable nozzles  57  to synchronously rotate in the reverse direction (closing direction). As a result, the gas passage area of exhaust gas to be supplied to the turbine impeller  33  side decreases, the velocity of flow of exhaust gas increases, and the amount of work produced by the turbine impeller  33  is sufficiently ensured. Thus, irrespective of whether the flow rate of exhaust gas is high or low, the turbine impeller  33  can sufficiently and stably generate turning force (general function of the variable geometry system turbocharger  1 ). 
     The fitting clearance between the second bearing portion  55   a  and the large-diameter portion  61   a  is set larger than the fitting clearances between the large-diameter portions  59   a  and  59   b  and the first bearing portions  49   a  and  49   b . Accordingly, as wear between the right-side first bearing portion  49   b  and the large-diameter portion  59   b  proceeds, the variable nozzle  57  comes to be supported on both sides from both the right and left sides of the variable nozzle  57  by the left-side first bearing portion  49   a  and the second bearing portion  55   a . Thus, the inclination (tilting) of the axis of the variable nozzle  57  with respect to the axis of the first supporting hole  49  of the shroud ring  47  during the operation of the variable geometry system turbocharger  1  can be reduced. 
     The inner surface of each first supporting hole  49  of the shroud ring  47  has the two first bearing portions  49   a  and  49   b  at the right and left ends thereof. In other words, the two first bearing portions  49   a  and  49   b  are formed in the shroud ring  47  as a single component. Accordingly, the accuracy of the relative position between the respective holes constituting the two first bearing portions  49   a  and  49   b  can be sufficiently ensured. Moreover, before the nozzle ring  51  is attached to the shroud ring  47 , the variable nozzle  57  can be supported by the two first bearing portions  49   a  and  49   b  in a stable state as shown in  FIG. 1B . 
     The fitting clearance between the second bearing portion  55   a  and the second nozzle shaft  61  of the variable nozzle  57  is set larger than the fitting clearance between each of the first bearing portions  49   a  and  49   b  and the first nozzle shaft  59  of the variable nozzle  57 . Accordingly, when the nozzle ring  51  is attached to the shroud ring  47 , the difference between the two fitting clearances can absorb position errors (installation errors) between respective holes of the first bearing portions  49   a  and  49   b  and the second bearing portion  55   a  (function specific to the variable geometry system turbocharger  1 ). 
     Accordingly, according to the embodiment of the present invention, the inclination of the axis of the variable nozzle  57  with respect to the axis of the first supporting hole  49  of the shroud ring  47  during the operation of the variable geometry system turbocharger  1  can be reduced. Moreover, during the operation of the variable geometry system turbocharger  1 , the operation of the variable nozzle unit  45  can be stabilized by reducing the non-smooth movement of the variable nozzle  57 . 
     Moreover, before the nozzle ring  51  is attached to the shroud ring  47 , the variable nozzle  57  is supported by the two first bearing portions  49   a  and  49   b  in a stable state. Moreover, when the nozzle ring  51  is attached to the shroud ring  47 , the difference between the two fitting clearances can absorb position errors between respective holes of the first bearing portions  49   a  and  49   b  and the second bearing portion  55   a . Accordingly, the nozzle ring  51  can be attached to the shroud ring  47  without using a special jig, and the efficiency of assembly work of the variable nozzle unit  45  can be sufficiently improved. 
     The present invention is not limited to the description of the above-described embodiment, and can be carried out in various aspects, for example, as described below. Specifically, instead of employing the shroud ring  47  and the nozzle ring  51  as the first base ring and the second base ring, respectively, the nozzle ring  51  and the shroud ring  47  may be employed as the first base ring and the second base ring, respectively. In that case, a link mechanism (not shown) similar to the link mechanism  65  is provided in the link chamber (not shown) formed on the opposite side of the nozzle ring  51  from the facing surface. The scope of rights covered by the present invention is not limited to these embodiments. The scope of rights of the present invention also covers, for example, the case where a variable nozzle unit (not shown) having a configuration similar to that of the variable nozzle unit  45  is applied to turbo rotating machinery (not shown) such as a gas turbine (not shown) other than the variable geometry system turbocharger  1 . 
     COMPARATIVE EXAMPLES 
     Comparative examples of the present invention will be described with reference to  FIGS. 4 and 5 . It should be noted that as shown in the drawings, “L” indicates the left direction, and “R” indicates the right direction. 
     As shown in  FIG. 4A , a variable nozzle unit  69  according to comparative example 1 corresponds to a conventional variable nozzle unit of a type in which a nozzle is supported on both sides. The variable nozzle unit  69  has a configuration similar to that of the variable nozzle unit  45  (see  FIG. 1 ) according to the above-described embodiment of the present invention. In the following description, in the configuration of the variable nozzle unit  69  according to comparative example 1, only points different from those of the variable nozzle unit  45  will be described. It should be noted that components of the variable nozzle unit  69  according to comparative example 1 which correspond to components of the variable nozzle unit  45  are denoted by the same reference numerals in the drawings. 
     The first nozzle shaft  59  of the variable nozzle  57  has on only a left side thereof a large-diameter portion  59   a  having a diameter larger than a reference outside diameter (outside diameter of an intermediate portion of the first nozzle shaft  59 ). In other words, the inner surface of the first supporting hole  49  of the shroud ring  47  has on only a left side thereof a first bearing portion  49   a  by which the first nozzle shaft  59  of the variable nozzle  57  is rotatably supported. Specifically, the variable nozzle  57  is supported on both sides from both sides of the variable nozzle  57  in the axial direction by the first bearing portion  49   a  and the second bearing portion  55   a . It should be noted that as shown in  FIG. 4B , before the nozzle ring  51  is attached to the shroud ring  47 , the variable nozzle  57  is supported by only one first bearing portion  49   a.    
     The inside diameter of the second bearing portion  55   a  is set to the same value as the inside diameter of the first bearing portion  49   a . The outside diameter of the large-diameter portion  61   a  is set to the same value as the outside diameter of the large-diameter portion  59   a . The fitting clearance between the second bearing portion  55   a  and the large-diameter portion  61   a  and the fitting clearance between the first bearing portion  49   a  and the large-diameter portion  59   a  are set to the same value to an accuracy of several tens of micrometers. The bearing span between the first bearing portion  49   a  and the second bearing portion  55   a  is denoted by L 1 . It is assumed that wear occurs between the second bearing portion  55   a  and the large-diameter portion  61   a , and the distance therebetween becomes X 1 . This wear is prone to occur when the variable nozzle  57  is subjected to a bending load due to, for example, pulsating pressure of exhaust gas or the like. In that case, the axis of the variable nozzle  57  inclines with respect to the axis of the first supporting hole  49  of the shroud ring  47  by θ 1  (θ 1 =tan −1 (X 1 /L 1 )). 
     As shown in  FIG. 5A , the variable nozzle unit  71  according to comparative example 2 corresponds to a conventional variable nozzle unit of a type in which a nozzle is supported on one side. The variable nozzle unit  71  has a configuration similar to that of the variable nozzle unit  45  according to the above-described embodiment of the present invention. In the following description, in the configuration of the variable nozzle unit  71  according to comparative example 2, only points different from those of the variable nozzle unit  45  will be described. It should be noted that components of the variable nozzle unit  71  according to comparative example 2 which correspond to components of the variable nozzle unit  45  are denoted by the same reference numerals in the drawings. 
     In the variable nozzle unit  71 , the plurality of second supporting holes  55  (see  FIGS. 1 and 2 ) of the nozzle ring  51  are omitted. Accordingly, the second nozzle shafts  61  (see  FIG. 1 ) are omitted from the variable nozzles  57 . In other words, the variable nozzle  57  is supported on one side from one side of the variable nozzle  57  in the axial direction by the two first bearing portions  49   a  and  49   b . It should be noted that as shown in  FIG. 5B , before the nozzle ring  51  is attached to the shroud ring  47 , the variable nozzle  57  is supported by the two first bearing portions  49   a  and  49   b  in a stable state. 
     The bearing span between the two first bearing portions  49   a  and  49   b  is denoted by L 2  (L 2 &lt;L 1 ). It is assumed that of the two first bearing portions  49   a  and  49   b , wear occurs between the first bearing portion  49   b , which is closer to a side surface of the variable nozzle  57 , and the large-diameter portion  59   b , and the distance therebetween becomes X 2 . This wear is prone to occur when the variable nozzle  57  is subjected to a bending load due to pulsating pressure of exhaust gas or the like. In that case, the axis of the variable nozzle  57  inclines with respect to the axis of the first supporting hole  49  of the shroud ring  47  by θ 2  (θ 2 =tan −1 (X 2 /L 2 )). Moreover, if X 2  is equal to the amount of wear X 1 , the angle of inclination θ 2  is larger than the angle of inclination θ 1 . In other words, in the case where the variable nozzle  57  is supported on one side, the inclination (tilting) of the axis of the variable nozzle  57  with respect to the axis of the first supporting hole  49  of the shroud ring  47  during the operation of the variable geometry system turbocharger  1  (see  FIG. 1 ) can become larger than in the case where the variable nozzle  57  is supported on both sides. 
     The present invention is applicable to a variable nozzle unit and a variable geometry system turbocharger which can improve the working efficiency of assembling the variable nozzle unit while stabilizing the operation of the variable nozzle unit.