Patent Publication Number: US-11384645-B2

Title: Turbine wheel

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a turbine wheel of a gas turbine, and in particular relates to a turbine wheel including a balance weight. 
     2. Description of the Related Art 
     A gas turbine generally includes: a compressor that compresses air to generate compressed air; a combustor that mixes the compressed air from the compressor with fuel, and combusts the mixture to generate a combustion gas; and a turbine that obtains shaft power by the combustion gas from the combustor. The turbine includes a turbine rotor that converts the kinetic energy of the combustion gas into rotational power. In the turbine, it is necessary to adjust the balance of the turbine rotor in order to reduce vibrations during its rotation. Examples of the method of adjusting the balance of the turbine rotor include a method in which a portion of a component of the turbine rotor is machined, and a method in which a balance weight is attached to a component of the turbine rotor. 
     In the technology of adjusting the balance of the turbine rotor by attaching the balance weight, typically, at least one balance weight is arranged at an appropriate position, in the circumferential direction, in an annular dovetail groove provided at the wall surface of the turbine wheel (see JP 48-064601 U1 (1971), for example). JP 48-64601 U1 discloses that a balance weight attachment for turbine wheels is formed such that it can be inserted in any position in a dovetail-shaped annular groove formed on a turbine wheel without providing an access slot. The balance weight is retained in the annular groove of the turbine wheel with a projection of one side of its body portion abutted against one side of the annular groove when fastening means is inserted in an oblique passageway that is opened on the other side of the body portion and loads against the other side of the annular groove. 
     Meanwhile, since a gas turbine obtains shaft power of a turbine rotor from a high-temperature and high-pressure combustion gas, it is necessary to cool each part of the turbine rotor such as turbine wheels or turbine rotor blades by cooling air, and to suppress a temperature increase of each part. In the gas turbine, typically, compressed air bled from a compressor is used as the cooling air. In this case, increasing the flow rate of the cooling air means increasing the flow rate of the compressed air bled from the compressor. Accordingly, if the flow rate of the cooling air is increased, the flow rate of the combustion gas to drive the turbine rotor decreases by a corresponding amount, and thus the overall efficiency of the gas turbine deteriorates. 
     One of the effective means for attaining high efficiency of a gas turbine is to reduce cooling air used to cool each part of a turbine rotor. In this case, the ambient temperature in a wheel space formed in front and rear of the turbine wheel in the axial direction increases. In view of this, it has been proposed to change the material of a turbine wheel to a Ni based alloy that is more heat-resistant than conventionally used 12Cr steel materials. It should be noted however that there is a concern that cracks due to the residual tensile stress occur if parts formed of a Ni based alloy material are used in a high temperature environment in a state in which they are receiving a residual tensile stress. 
     In the technology described in JP 48-064601 U1, the balance weight is retained in the annular groove of the turbine wheel with the projection of the balance weight abutted against the one side of the annular groove when the fastening means is inserted in the oblique passageway of the balance weight and loads against the other side of the annular groove. In the technology of retaining the balance weight in the annular groove in this manner, an opening edge portion of the annular groove of the turbine wheel is crimped in some cases in order to inhibit a circumferential shift of the balance weight along the annular groove. In this case, a residual tensile stress is generated in and around the crimped portion of the turbine wheel. 
     In a case where not a 12Cr steel material, but a Ni based alloy material is applied to a turbine wheel for which a method, like the one mentioned above, of inhibiting the shift of a balance weight by crimping a portion of the turbine wheel is employed, there is a concern over occurrences of cracks in the turbine wheel due to a residual tensile stress generated by the crimping. 
     The present invention has been made in order to solve the problems described above, and an object of the present invention is to provide a turbine wheel that can suppress a residual tensile stress caused in a turbine wheel by fixing a balance weight. 
     SUMMARY OF THE INVENTION 
     The present application includes a plurality of means for solving the problems described above, and one example thereof is a turbine wheel provided with a groove having a bottom surface extending circumferentially and a pair of side wall surfaces forming an opening. The turbine wheel including: a balance weight that is arranged in the groove, is configured to be insertable from any circumferential position of the opening of the groove, and has a through-hole opened toward one of the pair of side wall surfaces of the groove; and a retaining member that contacts a portion of the one of the pair of side wall surfaces of the groove in a state of being inserted in the through-hole of the balance weight, to thereby cause the balance weight to abut against other one of the pair of side wall surfaces of the groove and be retained in the groove. The groove has a plurality of engagement recesses provided at intervals in a circumferential direction at the bottom surface; or an engagement protrusion fitted to one of a plurality of fitting recesses provided at intervals in the circumferential direction at the bottom surface and protruding from the bottom surface. The balance weight has an engagement protrusion that engages with one of the engagement recesses of the groove to restrict a circumferential shift of the balance weight in the groove or an engagement groove that engages with the engagement protrusion of the groove to restrict a circumferential shift of the balance weight in the groove. 
     According to the present invention, since the engagement protrusion or the engagement groove of the balance weight engages with the engagement recess or the engagement protrusion in the groove of the turbine wheel, the circumferential shift of the balance weight within the groove is restricted, and thus it becomes unnecessary to crimp the turbine wheel in order to fix the balance weight. Accordingly, it is possible to suppress a residual tensile stress caused in the turbine wheel by fixing the balance weight. 
     Problems, configurations and effects other than those described above become apparent from the following explanation of embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional diagram illustrating a gas turbine including a turbine wheel according to a first embodiment of the present invention in a state in which a lower half section is omitted therefrom; 
         FIG. 2  is an enlarged cross-sectional diagram illustrating a portion of a turbine rotor including the turbine wheel according to the first embodiment of the present invention illustrated in  FIG. 1 ; 
         FIG. 3  is an enlarged view of an attachment structure of a balance weight of the turbine wheel according to the first embodiment of the present invention as seen in the axial direction; 
         FIG. 4  is a cross-sectional diagram illustrating the fixed state of the balance weight in a groove of the turbine wheel according to the first embodiment of the present invention illustrated in  FIG. 3  as seen in the direction of arrows IV-IV; 
         FIG. 5  is a cross-sectional diagram of the groove of the turbine wheel according to the first embodiment of the present invention illustrated in  FIG. 3  as seen in the direction of arrows V-V; 
         FIG. 6  is a cross-sectional diagram of the balance weight of the turbine wheel according to the first embodiment of the present invention; 
         FIG. 7  is a diagram of the balance weight of the turbine wheel according to the first embodiment of the present invention illustrated in  FIG. 6  as seen in the direction of an arrow VII; 
         FIG. 8  is a front view illustrating a retaining member of the turbine wheel according to the first embodiment of the present invention; 
         FIG. 9  is an explanatory diagram illustrating an example of the method of insertion of the balance weight into the groove in the turbine wheel according to the first embodiment of the present invention; 
         FIG. 10  is a cross-sectional diagram illustrating a balance weight of a turbine wheel according to a second embodiment of the present invention; 
         FIG. 11  is a diagram of the balance weight of the turbine wheel according to the second embodiment of the present invention illustrated in  FIG. 10  as seen in the direction of an arrow XI; 
         FIG. 12  is a cross-sectional diagram illustrating a groove of a turbine wheel according to a third embodiment of the present invention; 
         FIG. 13  is a cross-sectional diagram illustrating a balance weight of the turbine wheel according to the third embodiment of the present invention; 
         FIG. 14  is a diagram of the balance weight of the turbine wheel according to the third embodiment of the present invention illustrated in  FIG. 13  as seen in the direction of an arrow XIV; and 
         FIG. 15  is an explanatory diagram illustrating an example of the method of insertion of the balance weight into the groove in the turbine wheel according to the third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of a turbine wheel according to the present invention are explained by using the drawings. 
     First Embodiment 
     First, the configuration of a gas turbine including a turbine wheel according to a first embodiment of the present invention is explained by using  FIG. 1  and  FIG. 2 .  FIG. 1  is a cross-sectional diagram illustrating the gas turbine including the turbine wheel according to the first embodiment of the present invention in a state in which a lower half section is omitted therefrom.  FIG. 2  is an enlarged cross-sectional diagram illustrating a portion of a turbine rotor including the turbine wheel according to the first embodiment of the present invention illustrated in  FIG. 1 . 
     In  FIG. 1 , the gas turbine includes a compressor  1 , a combustor  2  and a turbine  3 . The compressor  1  compresses air taken in to generate compressed air. The combustor  2  mixes the compressed air generated by the compressor  1  with fuel from a fuel system (not illustrated), and combusts the mixture to generate a combustion gas. The gas turbine has a multi-can type combustor, for example, and in the multi-can type, a plurality of combustors  2  are annularly arranged at intervals. The turbine  3  is rotation-driven by the high temperature and high-pressure combustion gas generated at the combustor  2  to drive the compressor  1  and a load (a driven device such as a generator, a pump, and a process compressor) which is not illustrated. The turbine  3  is supplied with compressed air bled from the compressor  1  as cooling air to cool components of the turbine  3 . 
     The compressor  1  includes: a compressor rotor  10  that is rotation-driven by the turbine  3 ; and a compressor casing  15  that houses the compressor rotor  10  such that compressor rotor  10  can rotate therein. The compressor  1  is an axial compressor, for example. The compressor rotor  10  includes: a plurality of disc-like compressor wheels  11  stacked in the axial direction; and a plurality of compressor rotor blades  12  that are coupled to an outer circumferential edge portion of each compressor wheel  11 . In the compressor rotor  10 , the plurality of compressor rotor blades  12  arrayed annularly at the outer circumferential edge portion of each compressor wheel  11  form one compressor rotor blade row. 
     A plurality of compressor stator blades  16  are arrayed annularly downstream side of a working fluid from each compressor rotor blade row. The plurality of compressor stator blades  16  arrayed annularly form one compressor stator blade row. The compressor stator blade rows are fixed inside the compressor casing  15 . In the compressor  1 , each compressor rotor blade row, and each compressor stator blade row located immediately downstream of the compressor rotor blade row form one stage. 
     The turbine  3  includes: a turbine rotor  30  that is rotation-driven by the combustion gas from the combustor  2 ; and a turbine casing  35  that houses the turbine rotor  30  such that the turbine rotor  30  can rotate therein. The turbine  3  is an axial turbine. A flow passage P through which the combustion gas flows is formed between the turbine rotor  30  and the turbine casing  35 . 
     As illustrated in  FIG. 1  and  FIG. 2 , the turbine rotor  30  is built by alternately stacking, in the axial direction, a plurality of disc-like turbine wheels  40  having a plurality of turbine rotor blades  31  coupled thereto circumferentially at an outer circumferential edge portion, and a plurality of disc-like spacers  32 . The stacked turbine wheels  40  and spacers  32  are fixed by stacking bolts  33 . In the turbine rotor  30 , a plurality of turbine rotor blades  31  arrayed annularly at an outer circumferential edge portion of each turbine wheel  40  form one turbine rotor blade row. Each turbine rotor blade row is disposed in the flow passage P. 
     A plurality of turbine stator blades  36  are arrayed annularly upstream of the working fluid from each turbine rotor blade row. The plurality of turbine stator blades  36  arrayed annularly form one turbine stator blade row. The turbine stator blade rows are fixed inside the turbine casing  35 , and are disposed in the flow passage P. In the turbine  3 , each turbine stator blade row, and each turbine rotor blade row located immediately downstream of the turbine stator blade row form one stage. 
     The turbine rotor  30  is connected to the compressor rotor  10  via an intermediate shaft  38 . The turbine casing  35  is connected to the compressor casing  15 . 
     Next, the configuration and structure of the turbine wheel according to the first embodiment of the present invention are explained by using  FIG. 2  to  FIG. 8 .  FIG. 3  is an enlarged view of an attachment structure of a balance weight of the turbine wheel according to the first embodiment of the present invention as seen in the axial direction.  FIG. 4  is a cross-sectional diagram illustrating the fixed state of the balance weight in a groove of the turbine wheel according to the first embodiment of the present invention illustrated in  FIG. 3  as seen in the direction of arrows IV-IV.  FIG. 5  is a cross-sectional diagram of the groove of the turbine wheel according to the first embodiment of the present invention illustrated in  FIG. 3  as seen in the direction of arrows V-V.  FIG. 6  is a cross-sectional diagram of the balance weight of the turbine wheel according to the first embodiment of the present invention.  FIG. 7  is a diagram of the balance weight of the turbine wheel according to the first embodiment of the present invention illustrated in  FIG. 6  as seen in the direction of an arrow VII.  FIG. 8  is a front view illustrating a retaining member of the turbine wheel according to the first embodiment of the present invention. 
     In  FIG. 2  and  FIG. 3 , the turbine wheels  40  are formed with a Ni based alloy as their base material. An annular thicker portion  41  at an intermediate section in a radial direction R of a turbine wheel  40  is provided with bolt holes  43  that penetrates the thicker portion  41  in an axial direction A (the thickness direction of the turbine wheel  40 ). The bolt holes  43  are provided at predetermined intervals in a circumferential direction C. A stacking bolt  33  is inserted into each bolt hole  43 . 
     In addition, as illustrated in  FIG. 3 , on the end surface of the thicker portion  41  of the turbine wheel  40  in the axial direction A, a groove  50  is formed such that it extends in the circumferential direction C of the turbine wheel  40 . The groove  50  intermittently extends over the entire circumference of the turbine wheel  40  such that the bolt holes  43  are sandwiched between parts of the groove  50 , for example. A balance weight  60  is arranged in the groove  50  for the balance adjustment of the turbine rotor  30  (see  FIG. 2 ). A plurality of balance weights  60  are arranged in the groove  50  as necessary in some cases. The balance weight  60  is retained in the groove  50  by a retaining screw member  80  as a retaining member. 
     As illustrated in  FIG. 4  and  FIG. 5 , the groove  50  is formed such that the width (the length in the upward/downward direction or the radial direction R in  FIG. 4  and  FIG. 5 ) of a bottom surface  51  is larger than the width (the length in the upward/downward direction or the radial direction R in  FIG. 4  and  FIG. 5 ) of an opening  58 , and is formed like a dovetail groove, for example. The groove  50  is formed such that the width of the bottom surface  51  and the width of the opening  58  are each approximately the same in the circumferential direction C, for example. 
     The groove  50  has the flat bottom surface  51  that is approximately parallel to the end surface, in the axial direction A, of the thicker portion  41  of the turbine wheel  40 , and a first side wall surface  52  and a second side wall surface  53  as a pair of side wall surfaces that form the opening  58  and is closer to each other on a direction away from the bottom surface  51  (leftward direction in  FIG. 4  and  FIG. 5 ). The first side wall surface  52  is inclined such that it is gradually positioned radially outward Ro as it comes from the side where the bottom surface  51  is located toward the side where the opening  58  is located. On the other hand, the second side wall surface  53  is inclined such that it is gradually positioned radially inward Ri as it comes from the side where the bottom surface  51  is located toward the side where the opening  58  is located, and is positioned radially outward Ro relative to the first side wall surface  52 . 
     A first corner portion  54  between the first side wall surface  52  and the bottom surface  51  is formed as a concave curved surface. The concave curved surface of the first corner portion  54  has a predetermined radius of curvature in its cross-sectional shape, for example. Similarly to the first corner portion  54 , a second corner portion  55  between the second side wall surface  53  and the bottom surface  51  is formed as a concave curved surface having a predetermined radius of curvature in its cross-sectional shape. 
     As illustrated in  FIG. 3  to  FIG. 5 , a plurality of engagement recesses  56  are provided at intervals in the circumferential direction C on the bottom surface  51  of the groove  50 . The engagement recesses  56  are configured to engage with engagement protrusion  71 , which is mentioned below, of the balance weight  60 , and have the function of restricting the shift of the balance weight  60  in the groove  50  in the circumferential direction C (the extending direction of the groove  50 ). The engagement recesses  56  are formed as grooves (engagement grooves) that extend in the groove widthwise direction of the groove  50  (in the upward/downward direction or the radial direction R in  FIG. 4  and  FIG. 5 ), for example. As illustrated in  FIG. 5 , in a meridional cross-section of the turbine wheel  40  including an engagement recess  56 , a length Lg from an opening edge  58   b , which is on a side where the second side wall surface  53  is located, of the opening  58  of the groove  50  to an end section  59   a , which is on a side where a first side wall surface  52  is located, of an opening edge  59  of the engagement recess  56  in the groove  50  is set to a predetermined length. 
     In  FIG. 3  and  FIG. 4 , the balance weight  60  is formed such that it is insertable from any position, in the circumferential direction C, of the opening  58  of the groove  50  of the turbine wheel  40 . In addition, the balance weight  60  is formed such that it abuts against the second side wall surface  53  of the groove  50 , and engages with the engagement recess  56  of the groove  50 . 
     Specifically, as illustrated in  FIG. 4 , the balance weight  60  includes a body section  61  to be arranged between the first side wall surface  52  and second side wall surface  53  of the groove  50 , and an engagement protrusion  71  formed integrally with the body section  61 . The body section  61  is a portion to abut against the second side wall surface  53  of the groove  50 , and has the function of restricting the shift of the balance weight  60  in the groove  50  in the radial direction R (the groove widthwise direction of the groove  50 ). The engagement protrusion  71  is a portion to engage with any one of the engagement recesses  56  of the groove  50 , and has the function of restricting the shift of the balance weight  60  in the groove  50  in the circumferential direction C (the extending direction of the groove  50 ). 
     A side portion of the body section  61  on a side where the second side wall surface  53  of the groove  50  is located is formed in a shape that is approximately complementary to the groove shape of the groove  50 , and has a shape that can make surface contact with (abut against) the second side wall surface  53  of the groove  50 . In addition, the side portion on the second side wall surface  53  side of the body section  61  is shaped such that a portion corresponding to a corner portion on a side where the second corner portion  55  of the groove  50  is located is cut out, and has a shape that does not inhibit the insertion of the balance weight  60  through the opening  58  of the groove  50 . In addition, a side portion of the body section  61  on a side where the first side wall surface  52  is located is formed not in a shape complementary to the groove shape of the groove  50 , but in a shape that creates a gap between itself and the first side wall surface  52 , and is shaped such that a portion corresponding to a corner portion on a side where the first corner portion  54  of the groove  50  is located is cut out. That is, the side portion on the first side wall surface  52  side of the body section  61  has a shape that does not inhibit the insertion of the balance weight  60  through the opening  58  of the groove  50 . 
     More specifically, as illustrated for example in  FIGS. 4, 6 and 7 , the body section  61  has a rear surface  62  that faces the bottom surface  51  of the groove  50 , a front surface  63  that is positioned on the side opposite to the rear surface  62  and faces the opening  58  of the groove  50 , a first side surface  64  that is connected to the rear surface  62  and the front surface  63  and faces the first side wall surface  52  of the groove  50 , a second side surface  65  that is connected to the rear surface  62  and the front surface  63 , positioned on the side opposite to the first side surface  64 , and faces the second side wall surface  53  of the groove  50 , and a pair of circumferential side surfaces  66  that are connected to the rear surface  62  and the front surface  63 , are connected to the first side surface  64  and the second side surface  65 , and face the circumferential direction C of the groove  50 . 
     The front surface  63  and the rear surface  62  are formed such that they become approximately parallel to each other. As illustrated in  FIG. 4 , a length Lw 1  (see  FIG. 6 ) from a ridge E 1 , which is on a side where the first side surface  64  is located, of the front surface  63  to a ridge, which is on a side where the second side surface  65  is located, of the front surface  63  is set such that it is slightly shorter than the width of the opening  58  of the groove  50 . 
     As illustrated in  FIG. 4  and  FIG. 6 , the first side surface  64  includes: a perpendicular surface  64   a  that is substantially perpendicularly connected to the front surface  63 ; and a first inclined surface  64   b  that extends from the perpendicular surface  64   a  and is connected to the rear surface  62  while being inclined in a direction toward the second side surface  65 . This configuration of the first side surface  64  allows the balance weight  60  to be inserted into the groove  50  without making the first side surface  64  contact an opening edge on the first side wall surface  52  side of the groove  50 . 
     The second side surface  65  includes: an abutting surface  65   a  that extends from the front surface  63  toward the rear surface  62  while being inclined in a direction away from the first side surface  64 ; and a second inclined surface  65   b  that extends from the abutting surface  65   a  and is connected to the rear surface  62  while being inclined in a direction toward the first side surface  64 . The abutting surface  65   a  is formed such that its angle of inclination is approximately the same as the angle of inclination of the second side wall surface  53  of the groove  50 , and it is possible for the abutting surface  65   a  to make surface contact with the second side wall surface  53 . 
     As illustrated in  FIG. 7 , the pair of circumferential side surfaces  66  are formed such that they are substantially perpendicular to the bottom surface  62  and the front surface  63 , and are approximately parallel to each other. For example, the pair of circumferential side surfaces  66  are portions to serve as a portion to be gripped by an operator when the operator inserts the balance weight  60  into the groove  50 . 
     As illustrated in  FIGS. 4, 6, and 7 , the engagement protrusion  71  of the balance weight  60  is formed such that it protrudes from the rear surface  62  of the body section  61 , and forms a shape that is generally complementary to the engagement recess  56  of the groove  50 . The engagement protrusion  71  is formed as a projecting section that extends in a direction (the groove widthwise direction of the groove  50 ) connecting the side where the first side surface  64  is located and the side where the second side surface  65  is located, for example. 
     The balance weight  60  is provided with a through-hole  68  that penetrates the body section  61 , and that is opened toward the first side wall surface  52  of the groove  50 . The through-hole  68  is opened at the front surface  63  of the body section  61  and at the first inclined surface  64   b  of the first side surface  64 , for example. The through-hole  68  is provided with a female thread portion, for example. As illustrated in  FIG. 4 , the retaining screw member  80  as the retaining member is disposed in a screwed (inserted) state in the through-hole  68  having the female thread portion. 
     In addition, the balance weight  60  is formed such that a length Lw 2  (see  FIG. 6 ) from the ridge E 1 , which is located between the front surface  63  and the second side surface  65 , of the body section  61  to an end portion E 2 , which is on a side where the first side surface  64  is located, of a tip surface  71   a  of the engagement protrusion  71  is shorter than the length Lg (see  FIG. 5 ) from the opening edge  58   b  on the second side wall surface  53  side of the opening  58  of the groove  50  to the end portion  59   a  on the first side wall surface  52  side of the opening edge  59  of the engagement recess  56  (see  FIG. 9  mentioned below also). This allows the balance weight  60  to be inserted into the groove  50  without making the engagement protrusion  71  contact the opening edge  59  of the engagement recess  56  of the groove  50 . 
     Note that, for example, the length between the pair of circumferential side surfaces  66  of the balance weight  60  may vary. In this case, it is possible to ensure balance weights having different weights. 
     As illustrated in  FIG. 4 , the retaining screw member  80  contacts the first corner portion  54  of the first side wall surface  52  of the groove  50  in a state of being inserted in the through-hole  68  of the balance weight  60 , thereby causing the second side surface  65  (the abutting surface  65   a ) of the body section  61  of the balance weight  60  to abut against the second side wall surface  53  of the groove  50  and the balance weight  60  to be retained in the groove  50 . As illustrated in  FIGS. 4 and 8 , the retaining screw member  80  includes: a body section  81  having a male thread portion; and a tip section  82  that is formed integrally on one side of the body section  81  and has a curved surface. The tip section  82  is formed such that it makes line contact with a part of the concave curved surface of the first corner portion  54  of the groove  50 . For example, a shape profile of the tip section  82  in a meridional plane cross-section has a convex curved shape having a radius of curvature approximately the same as the radius of curvature of the cross-sectional shape of the concave curved surface of the first corner portion  54 . 
     Next, an example of the procedure of attachment of the balance weight into the groove in the turbine wheel according to the first embodiment of the present invention is explained by using  FIGS. 4 and 9 .  FIG. 9  is an explanatory diagram illustrating an example of the method of insertion of the balance weight into the groove in the turbine wheel according to the first embodiment of the present invention. 
     First, as illustrated in  FIG. 9 , the ridge E 1  between the front surface  63  and the second side surface  65  of the body section  61  of the balance weight  60  is caused to contact on the opening edge  58   b  on the second side wall surface  53  side of the opening  58  of the groove  50 . In this state, the balance weight  60  is turned about the ridge E 1  as the turning axis toward the bottom surface  51  of the groove  50 . At this time, the engagement protrusion  71  of the balance weight  60  relatively shifts along the engagement recess  56  of the groove  50 . Thereby, the body section  61  of the balance weight  60  is arranged between the first side wall surface  52  and second side wall surface  53  of the groove  50 , and the engagement protrusion  71  of the balance weight  60  is arranged in the engagement recess  56  of the groove  50 . 
     In the present embodiment, the length Lw 2  of the balance weight  60  from the ridge E 1  to the end portion E 2  on the first side surface  64  side of the tip surface  71   a  of the engagement protrusion  71  is set shorter than the length Lg of the groove  50  from the opening edge  58   b  on the second side wall surface  53  side of the opening  58  to the end portion  59   a  on the first side wall surface  52  side of the opening edge  59  of the engagement recess  56 . Accordingly, it is possible to insert the balance weight  60  into the groove  50  without making the engagement protrusion  71  of the balance weight  60  contact the opening edge  59  of the engagement recess  56  of the groove  50 . 
     Next, as illustrated in  FIG. 4 , the retaining screw member  80  is screwed (inserted) into the through-hole  68  of the balance weight  60  in which the female thread portion is formed, and the tip section  82  of the retaining screw member  80  is pressed against the concave curved surface of the first corner portion  54  of the groove  50 . By further screwing the retaining screw member  80  into the through-hole  68 , the balance weight  60  shifts toward the second side wall surface  53  of the groove  50  along the retaining screw member  80 . Eventually, the abutting surface  65   a  of the second side surface  65  of the balance weight  60  makes surface contact with the second side wall surface  53  of the groove  50 . 
     In this manner, in the present embodiment, the retaining screw member  80  contacts the first corner portion  54  on the first side wall surface  52  side of the groove  50  in a state of being inserted in the through-hole  68  of the balance weight  60 , thereby causing the abutting surface  65   a  of the balance weight  60  to make surface contact with (abut against) the second side wall surface  53  of the groove  50 . As a result, the shift of the balance weight  60  in the radial direction R (in the groove widthwise direction of the groove  50 ) within the groove  50  is restricted, and the balance weight  60  is retained in the groove  50 . In addition, the engagement protrusion  71  of the balance weight  60  engages with the engagement recess  56  of the groove  50 , thereby restricting the shift of the balance weight  60  within the groove  50  in the circumferential direction C (in the extending direction of the groove  50 ). Accordingly, it is possible to fix the balance weight  60  in the groove  50  of the turbine wheel  40  without crimping the turbine wheel  40 . 
     As mentioned above, according to the first embodiment of the turbine wheel according to the present invention, the engagement protrusion  71  of the balance weight  60  engages with the engagement recess  56  of the groove  50  of the turbine wheel  40 , thereby restricting the shift of the balance weight  60  in the circumferential direction C within the groove  50 . In this way, the shift of the balance weight  60  is restricted also by the engagement protrusion  71  in addition to fixation by the retaining screw member  80 , and therefore the balance weight  60  can be firmly fixed. Thus, it becomes unnecessary to crimp the turbine wheel  40  in order to fix the balance weight  60 . Accordingly, it is possible to suppress a residual tensile stress caused in the turbine wheel  40  by fixing the balance weight  60 . 
     In addition, according to the present embodiment, the length Lw 2  from the ridge E 1 , which is located between the front surface  63  and the second side surface  65 , of the body section  61  to the end portion E 2 , which is closer to the first side surface  64 , of the tip surface  71   a  of the engagement protrusion  71  in the balance weight  60  is set shorter than the length Lg from the opening edge  58   b , which is closer to the second side wall surface  53 , of the opening  58  to the end portion  59   a , which is closer to the first side wall surface  52 , of the opening edge  59  of the engagement recess  56  in the groove  50 , and thus it is possible to insert the balance weight  60  into the groove  50  from any position, in the circumferential direction C, of the opening  58  of the groove  50  of the turbine wheel  40 . 
     Furthermore, according to the present embodiment, the body section  61  and engagement protrusion  71  of the balance weight  60  are formed integrally, and thus the attachment of the balance weight  60  in the groove  50  is easy as compared with a configuration in which a body section and an engagement protrusion of a balance weight are separate members. That is, the integral structure of the body section  61  and engagement protrusion  71  of the balance weight  60  does not require assembly work of the balance weight  60  itself. As a result, the integral structure can avoid the falling of an engagement protrusion  71  from a body section  61 , which may occur in a case where a body section  61  and an engagement protrusion  71  are separate members. 
     In addition, according to the present embodiment, the first corner portion  54  of the groove  50  is formed as a concave curved surface, and the tip section  82  of the retaining screw member  80  is formed such that it makes line contact with a portion of the concave curved surface of the first corner portion  54  of the groove  50 . Accordingly, it is possible to suppress a residual tensile stress caused in the portion of the first corner portion  54  of the groove  50  with which the retaining screw member  80  makes contact. 
     Second Embodiment 
     Next, a turbine wheel of a second embodiment according to the present invention is explained by using  FIGS. 10 and 11 .  FIG. 10  is a cross-sectional diagram illustrating a balance weight of the turbine wheel according to the second embodiment of the present invention.  FIG. 11  is a diagram of the balance weight of the turbine wheel according to the second embodiment of the present invention illustrated in  FIG. 10  as seen in the direction of an arrow XI. Note that since the reference characters in  FIGS. 10 and 11  that are the same as reference characters illustrated in  FIGS. 1 to 9  denote similar portions, detailed explanations thereof are omitted. 
     While the body section  61  and engagement protrusion  71  of the balance weight  60  in the first embodiment are formed integrally (see  FIG. 6 ), the turbine wheel according to the second embodiment of the present invention illustrated in  FIGS. 10 and 11  has a configuration including a body section  61 A and an engagement protrusion  72  of a balance weight  60 A as separate members. 
     Specifically, the balance weight  60 A includes: the body section  61 A having the through-hole  68  and a fitting recess  69 ; and a pin  72  attached to the fitting recess  69  of the body section  61 A by being fit thereto. Similarly to the body section  61  of the balance weight  60  of the first embodiment, the body section  61 A has the rear surface  62 , the front surface  63 , the first side surface  64 , the second side surface  65  and the pair of circumferential side surfaces  66 . Similarly to the first embodiment, the first side surface  64  includes the perpendicular surface  64   a  and the first inclined surface  64   b . Similarly to the first embodiment, the second side surface  65  includes the abutting surface  65   a  and the second inclined surface  65   b . The fitting recess  69  is provided in an approximately middle portion of the rear surface  62 . The fitting recess  69  has a circular cross-section shape, for example. The pin  72  is a member separate from the body section  61 A, and functions as an engagement protrusion to engage with any one of the engagement recesses  56  of the groove  50 . The pin  72  has a circular transverse cross-section shape, for example. 
     The balance weight  60 A is formed such that a length Lw 3  from the ridge E 1 , which is located between the front surface  63  and the second side surface  65 , of the body section  61 A to an end portion E 3 , which is on a side where the first side surface  64  is located, of the tip surface  72   a  of the pin  72  as the engagement protrusion is shorter than the length Lg (see  FIG. 5 ) from the opening edge  58   b  on the second side wall surface  53  side of the opening  58  of the groove  50  to the end portion  59   a  on the first side wall surface  52  side of the opening edge  59  of the engagement recess  56 . This allows the balance weight  60 A to be inserted into the groove  50  without making the pin  72  as the engagement protrusion contact the opening edge  59  of the engagement recess  56  of the groove  50 . 
     According to the second embodiment of the turbine wheel according to the present invention mentioned above, similarly to the first embodiment mentioned before, the pin  72  as the engagement protrusion of the balance weight  60 A engages with the engagement recess  56  of the groove  50  of the turbine wheel  40 , thereby restricting the shift of the balance weight  60 A in the circumferential direction C within the groove  50 . As a result, it becomes unnecessary to crimp the turbine wheel  40  in order to fix the balance weight  60 A. Accordingly, it is possible to suppress a residual tensile stress caused in the turbine wheel  40  by fixing the balance weight  60 A. 
     Third Embodiment 
     Next, the configuration and structure of a turbine wheel according to a third embodiment of the present invention are explained by using  FIGS. 12 to 14 .  FIG. 12  is a cross-sectional diagram illustrating a groove of the turbine wheel according to the third embodiment of the present invention.  FIG. 13  is a cross-sectional diagram illustrating a balance weight of the turbine wheel according to the third embodiment of the present invention.  FIG. 14  is a diagram of the balance weight of the turbine wheel according to the third embodiment of the present invention illustrated in  FIG. 13  as seen in the direction of an arrow XIV. Note that since the reference characters in  FIGS. 12 to 14  that are the same as reference characters illustrated in  FIGS. 1 to 11  denote similar portions, detailed explanations thereof are omitted. 
     A difference of the third embodiment of the turbine wheel according to the present invention illustrated in  FIGS. 12 to 14  from the first embodiment is that the recessed shape and the projecting shape in engagement between the groove and the balance weight in the turbine wheel  40  are interchanged. That is, in the first embodiment, the engagement protrusion  71  of the balance weight  60  engages with the engagement recess  56  of the groove  50  of the turbine wheel  40 , thereby restricting the shift of the balance weight  60  in the circumferential direction C within the groove  50  (see  FIG. 4 ). On the other hand, in the third embodiment, an engagement groove  69 B of a balance weight  60 B engages with a pin  57  as an engagement protrusion of a groove  50 B, thereby restricting the shift of the balance weight  60 B in the circumferential direction C within the groove  50 B. 
     Specifically, as illustrated in  FIG. 12 , the bottom surface  51  of the groove  50 B is provided with a plurality of fitting recesses  56 B at intervals in the circumferential direction C. A pin  57  can be fit to and fixed to each fitting recess  56 B. The pin  57  protrudes from the bottom surface  51  of the groove  50 B, engages with the engagement groove  69 B of the balance weight  60 B, and functions as an engagement protrusion that restricts the shift of the balance weight  60 B in the circumferential direction C within the groove  50 B. The pin  57  may be fit only to a fitting recess  56 B corresponding to the attachment position of the balance weight  60 B among the plurality of fitting recesses  56 B of the groove  50 B. 
     As illustrated in  FIGS. 13 and 14 , in the balance weight  60 B, the rear surface  62  of a body section  61 B is provided with the engagement groove  69 B. The engagement groove  69 B extends toward the first side surface  64  from the end edge closer to the second side surface  65  to the position of a middle portion, and is opened at the rear surface  62  and the second side surface  65 . The engagement groove  69 B engages with the pin  57  fitted to the fitting recess  56 B of the groove  50 B, and has the function of restricting the shift of the balance weight  60 B in the circumferential direction C within the groove  50 B. 
     Next, an example of the procedure of attachment of the balance weight into the groove in the turbine wheel according to the third embodiment of the present invention is explained by using  FIG. 15 .  FIG. 15  is an explanatory diagram illustrating an example of the method of insertion of the balance weight into the groove in the turbine wheel according to the third embodiment of the present invention. 
     As illustrated in  FIG. 15 , the ridge E 1  of the body section  61 B of the balance weight  60 B, which is located between the front surface  63  and the second side surface  65 , is caused to contact on the opening edge  58   b , closer to the second side wall surface  53 , of the opening  58  of the groove  50 B. In this state, the balance weight  60 B is turned toward the bottom surface  51  of the groove  50 B about the ridge E 1  as the turning axis. 
     In the present embodiment, the pin  57  fitted to the fitting recess  56 B of the groove  50 B relatively shifts along the engagement groove  69 B of the body section  61 B of the balance weight  60 B. Thereby, the balance weight  60 B is inserted into the groove  50 B without making the second side surface  65  and rear surface  62  of the balance weight  60 B contact the pin  57  as the engagement protrusion of the groove  50 B. 
     Similarly to the first embodiment, in the present embodiment also, the retaining screw member  80  (see  FIG. 4 ) contacts the first corner portion  54  of the groove  50 B closer to the first side wall surface  52  in a state of being inserted in the through-hole  68  of the balance weight  60 B, thereby causing the abutting surface  65   a  of the balance weight  60 B to make surface contact with the second side wall surface  53  of the groove  50 B. As a result, the shift of the balance weight  60 B in the radial direction R (in the groove widthwise direction of the groove  50 ) within the groove  50 B is restricted, and the balance weight  60 B is retained in the groove  50 B. In addition, the engagement groove  69 B of the balance weight  60 B engages with the pin  57  fitted to the fitting recess  56 B of the groove  50 B, thereby restricting the shift of the balance weight  60 B in the circumferential direction C (in the extending direction of the groove  50 B) within the groove  50 B. Accordingly, it is possible to fix the balance weight  60 B in the groove  50 B without crimping the turbine wheel  40 . 
     According to the third embodiment of the turbine wheel according to the present invention mentioned above, since the engagement groove  69 B of the balance weight  60 B engages with the pin  57  as the engagement protrusion of the groove  50 B of the turbine wheel  40 , the shift of the balance weight  60 B in the circumferential direction C within the groove  50 B is restricted, and thus it becomes unnecessary to crimp the turbine wheel  40  in order to fix the balance weight  60 B. Accordingly, it is possible to suppress a residual tensile stress caused in the turbine wheel  40  by fixing the balance weight  60 B. 
     Other Embodiments 
     Note that the present invention is not limited to the first to third embodiments mentioned above, but includes various modification examples. The embodiments described above are explained in detail in order to explain the present invention in an easy-to-understand manner, and the present invention is not necessarily limited to embodiments including all the configurations explained. For example, some of the configurations of an embodiment can be replaced with configurations of another embodiment, and configurations of an embodiment can also be added to the configurations of another embodiment. In addition, some of the configurations of individual embodiments can have other additional configurations, or can be removed or replaced with other configurations. 
     For example, in the first embodiment mentioned above, the engagement protrusion  71  of the balance weight  60  is formed as a projecting section that extends in the direction connecting the side where the first side surface  64  is located and the side where the second side surface  65  is located (in the groove widthwise direction of the groove  50 ). However, the engagement protrusion  71  may have any shape as long as the engagement protrusion  71  engages with the engagement recess  56  of the groove  50  of the turbine wheel  40 , and thereby restricts the shift of the balance weight  60  in the circumferential direction C. It is also possible to form the cross-sectional shape of the engagement protrusion  71  in a circular, rectangular or polygonal shape, for example. 
     In addition, in the first and second embodiments mentioned above, the engagement recess  56  is formed as a groove (engagement groove) that extends in the groove widthwise direction of the groove  50 . However, the engagement recess  56  may have any shape as long as the engagement recess  56  engages with the engagement protrusion  71  of the balance weight  60  or the pin  72  of the balance weight  60 A, and thereby can restrict the shift of the balance weights  60  and  60 A in the circumferential direction C.