Patent Publication Number: US-7223077-B2

Title: Structure for connecting compressor wheel and shaft

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a structure for connecting a compressor wheel and a shaft. 
     2. Description of the Related Art 
     As means for compressing air to increase the amount of intake air to an engine, a compressor of a turbo machine which rotates a turbine wheel and a shaft by utilizing energy of exhaust gas and drives a centrifugal type compressor wheel connected with the shaft is known as a turbo charger. 
       FIG. 11  is a sectional side view of a turbo charger  111  according to the related prior art. The turbo charger  111  includes an exhaust-side unit  112  for gaining rotational energy from the exhaust gas of an engine and an intake-side unit  113  for compressing air by the rotational energy and supplying the compressed air to the engine. 
     A turbine wheel  114  receives energy from the exhaust gas flowing thereto from an exhaust inflow passage  119  and rotates by the energy. A centrifugal type compressor wheel  116  for compressing air via a shaft  123  is fitted to the shaft  123  on a side opposite to the turbine wheel  114 , i.e., the tip of the shaft  123 . 
     A fitting hole  125  penetrates through a center of the compressor wheel  116 . The shaft  123  is fitted into the fitting hole  125  by slight clearance fit or close fit. The compressor wheel  116  is fixed to the shaft  123  by fastening a fitting nut  126  to a male screw  140  formed at the tip of the shaft  123 . 
       FIG. 12  is a sectional side view of the compressor wheel  116  according to the related art. A main body  129  of the compressor wheel  116  includes an inlet-side disk portion  129 A and a back-side disk portion  129 B. A plurality of vanes  118  are arranged outside the main body  129 , and the fitting hole  125  penetrates through the center of the main body  129 . 
     The compressor wheel  116  is produced from a casting such as an aluminum alloy or other material so as to be light-weight. Since the rotating speed of the compressor wheel  116  reaches values as high as tens of thousands rpm, extremely high tensile stress is applied on the compressor wheel  116  in its radial direction due to centrifugal force generated by the high rotating speed and thus the compressor wheel  116  may be broken in some cases. 
     It is known that the breakage of this type is likely to develop particularly in the inner wall of the fitting hole  125  starting therefrom. More specifically, it has been clarified that the breakage of the inner wall of the fitting hole  125  formed on the compressor wheel  116  occurs particularly in the vicinity of a maximum outer diameter  130  where the outer diameter of the compressor wheel  116  reaches a maximum in an axial direction of a rotational axis of the compressor wheel  116 . 
     In order to solve this problem, a technology described in Patent Reference No. JP-T-5-504178 (the term “JP-T” as used herein means a published Japanese translation of a PCT patent application. pp. 3 to 5,  FIGS. 1 and 2 ), for example, is utilized. 
       FIG. 13  is a cross-sectional view of a compressor wheel  216  according to the patent reference. A fitting hole penetrating through the compressor wheel  216  is not provided but a fitting opening  242  having a female screw is formed at a lower region of the compressor wheel  216 . A male screw is provided at a tip  254  of a shaft  223 . The shaft  223  and the compressor wheel  216  are coupled with each other by screwing the tip  254  into the fitting opening  242 . 
     However, since the fitting opening is also provided in the vicinity of the maximum outer diameter where the outer diameter of the compressor wheel reaches a maximum in the axial direction of the rotational axis of the compressor wheel in the related art shown in the patent reference, there is a possibility of breakage starting from a region around the maximum outer diameter when the rotating speed is increased. 
     Particularly when an engine equipped with the turbo charger using the compressor wheel is employed in working machines such as construction machines, a high load condition such as a loading operation (a high rotating speed of the engine) and an almost no load condition (a low rotating speed of the engine) are alternately repeated at short intervals. 
     As a result, the stress amplitude applied to the compressor wheel increases and the breakage is more likely to occur. 
     Recently, a technology called “EGR” (Exhaust Gas recirculation) has been executed as measures for the reduction of nitrogen oxides (NOx) contained in exhaust gas of Diesel engines. In this method, a part of exhaust gas discharged from an engine is returned to an intake system of the engine for re-circulation. 
     For accomplishing EGR, it is necessary to achieve a higher pressure ratio of the turbo charger so as to secure combustion air from a capacity of fresh air within a cylinder which capacity is reduced by the amount of the re-circulated exhaust gas, and thus the rotating speed at which the compressor wheel is rotated needs to be increased. However, the related art is not sufficient to overcome the above problem and it is thus desired to develop a compressor wheel having higher durability. 
     SUMMARY OF THE INVENTION 
     In view of the problems described above, it is an object of the invention to provide a structure for connecting a compressor wheel and a shaft, which is not easily broken at high rotating speed. 
     In order to achieve the above object, a connecting structure according to the present invention includes a compressor wheel, a shaft and a sleeve, wherein: the compressor wheel has a male screw formed on an outer surface of a projection provided at the center of a rear surface of the compressor wheel; the shaft has a male screw provided at one end thereof; the sleeve has a female screw provided at each end thereof and connects the compressor wheel and the shaft; and an engagement portion is provided between the compressor wheel and the shaft. 
     An engagement portion may be provided between the compressor wheel and the sleeve. 
     An engagement portion may be provided between the shaft and the sleeve. 
     An engagement portion engaging with the sleeve may be provided on each of the compressor wheel and the shaft. 
     A plate made from material having higher strength than the material of the compressor wheel may be provided, and the compressor wheel and the sleeve may be fastened with the plate interposed between the tip end surface of the male screw of the compressor wheel and the root end surface of the female screw of the sleeve. 
     The male screw and the female screw may be right-handed screws when the compressor wheel rotates counterclockwise and may be left-handed when the compressor wheel rotates clockwise as viewed from an inlet of the compressor wheel. 
     In this structure, a fitting hole or fitting opening for connecting the compressor wheel to the shaft is not required to be formed on the compressor wheel main body. Also, the concentricity between the compressor wheel and the shaft can be secured by the engagement portion formed therebetween. Accordingly, stress applied to the compressor wheel is decreased and the occurrence of breakage is reduced even if the compressor wheel is rotated at high speed. Furthermore, the structure in which the female screws are formed on the sleeve enlarges the screw size and thus increases the strength of the connection. In this specification, we use properly “hole” and “opening”. “hole” means a through-hole”. On the other hand, “opening” has a bottom. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a turbo charger in a first embodiment according to the invention. 
         FIG. 2  is a side view of a compressor wheel in the first embodiment. 
         FIG. 3  is a cross-sectional view of  FIG. 2 . 
         FIG. 4  illustrates a P area of  FIG. 1  in detail. 
         FIG. 5  is a flowchart showing processes for attaching the compressor wheel of the first embodiment. 
         FIG. 6  is a graph showing a general relationship between an inside diameter of a fitting hole and a magnitude of stress in the related art. 
         FIG. 7  illustrates a second embodiment according to the invention in detail. 
         FIGS. 8A and 8B  each illustrate a third embodiment according to the invention in detail. 
         FIG. 9  illustrates a fourth embodiment according to the invention in detail. 
         FIG. 10  illustrates a fifth embodiment according to the invention in detail. 
         FIG. 11  is a sectional side view of a prevailing type of a turbo charger in the related art. 
         FIG. 12  is a sectional side view of a prevailing type of a compressor wheel in the related art. 
         FIG. 13  is a cross-sectional view of a prevailing type of a compressor wheel in the related art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the invention will be hereinafter described in detail with reference to the accompanying drawings. 
     Referring to  FIG. 1 , a turbo charger  11  includes an exhaust-side unit  12  for gaining rotational energy from exhaust gas of an engine, and an intake-side unit  13  for compressing air by the rotational energy and supplying the compressed air to the engine. The exhaust-side unit  12  of the turbo charger  11  has an exhaust-side housing  15  and a turbine wheel  14  which has a plurality of vanes and is supported by a shaft  23 . 
     The exhaust-side housing  15  has an exhaust inflow passage  19  for supplying exhaust gas to the turbine wheel  14 . The exhaust inflow passage  19  having an annular shape encompasses the outer diameter of the turbine wheel  14 , and is connected to an engine exhaust flow passage through which the exhaust gas discharged from the engine (not shown) flows. 
     The exhaust-side housing  15  has an exhaust outflow port  21  for discharging the exhaust gas which has already released energy for the turbine wheel  14 . The exhaust outflow port  21  is substantially cylindrical and concentric with the rotational center of the turbine wheel  14 . An opening on the side opposite to the exhaust outflow port  21  is closed by an exhaust-side inner plate  22 . 
     The shaft  23  is formed integrally with the turbine wheel  14 . The shaft  23  penetrates through the exhaust-side inner plate  22  and is rotatably supported by a bearing  24 . The turbine wheel  14  is generally made from a nickel-base super-alloy, while the shaft  23  is generally made from alloy steel or carbon steel. 
     A compressor wheel  16  is accommodated inside an intake-side housing  17 . The intake-side housing  17  has an intake inflow port  27  for taking air into the compressor wheel  16 . The intake inflow port  27  is substantially cylindrical and concentric with the rotational center of the compressor wheel  16 . An opening on the side opposite to the intake inflow port  27  is closed by an intake-side inner plate  55 . 
     Air having received velocity energy from the compressor wheel  16  is sent to a diffuser  56  where the velocity energy is converted into pressure energy. Then, the air passes through an intake exhaust passage  28  which is annular and encompasses the outer diameter of the compressor wheel  16 , and is supplied to an air supply port of the engine (not shown). 
     The vanes  18  are constituted by full vanes  18 A having a large width in an axial direction of the vane and intermediate vanes  18 B whose vane inlet starts from an intermediate part of the full vanes  18 A in the axial direction. The full vanes  18 A and the intermediate vanes  18 B are alternately disposed. 
     As illustrated in  FIGS. 2 and 3 , a main body  29  of the compressor wheel  16  of the invention is solid and has no fitting hole or fitting opening. 
     A cylindrical portion  43  is formed integrally with the rearmost region of a rear-side disk portion  29 B with its center aligned with that of the main body  29 . A wheel male screw  44  having a smaller diameter than that of the cylindrical portion  43  is formed integrally with the cylindrical portion  43  at the lower end thereof. The wheel male screw  44  has an engagement opening  44 H for securing the concentricity with the shaft  23 . 
     A nut-shaped portion  16 N is provided on the outer diameter of a wheel inlet  35  of the compressor wheel  16 . The nut-shaped portion  16 N has a clamping region to which clamping torque is applied. The clamping region may be nut-shaped or have two parallel surfaces, for example, which can be clamped by a spanner or the like. 
       FIG. 4  illustrates a P area of  FIG. 1  in detail. A shaft cylindrical portion  60  which is cylindrical and concentric with the shaft  23  is provided on the tip of the shaft  23  fixed to the turbine wheel  14 . 
     A shaft male screw  46  is further provided on the tip of the shaft cylindrical portion  60 . As the shaft male screw  46  and the wheel male screw  44  have the same screw size, the outside diameters of those screws  44  and  46  are also the same. An engagement cylindrical portion  23 H which is precisely machined to be cylindrical and concentric with the shaft  23  is provided at the tip of the shaft  23 . The engagement cylindrical portion  23 H is so sized as to be inserted into the engagement opening  44 H of the wheel male screw  44  by slight clearance fit or close fit. 
     As illustrated in  FIGS. 1 and 4 , a flange  49 F for receiving a thrust bearing  48  is provided on a cylindrical portion  49 E of a sleeve  49 , and a seal groove  50  is formed on the entire circumference of the middle part of the outer surface of the sleeve  49  in the axial direction of the rotational axis of the sleeve  49 . A shaft-side female screw  53  engaging with the shaft male screw  46  is provided on an inner surface  58  of the sleeve  49  facing to the shaft  23 , while a wheel-side female screw  52  engaging with the wheel male screw  44  is provided on the inner surface  58  of the sleeve  49  facing to the compressor wheel  16 . 
     As the shaft male screw  46  and the wheel male screw  44  have the same screw size, the shaft-side female screw  53  and the wheel-side female screw  52  of the sleeve  49  also have the same size. Thus, the female screws provided on the inner surface of the sleeve  49  can be easily formed by a single process, and the accuracy of concentricity between the shaft-side female screw  53  and the wheel-side female screw  52  can be increased. 
     As illustrated in  FIGS. 1 and 4 , the shaft male screw  46  and the wheel male screw  44  are connected via the sleeve  49  having the female screws  52  and  53 . 
     As illustrated in  FIG. 4 , the engagement cylindrical portion  23 H of the shaft  23  is inserted into the engagement opening  44 H of the compressor wheel  16  by slight clearance fit or close fit. The inner surface  58  of the sleeve  49  at an end facing to the compressor wheel  16  provides a spigot joint to be connected with the cylindrical portion  43  formed on the rear of the compressor wheel  16 . A wheel engagement cylindrical portion  44 H which is precisely machined to be cylindrical and concentric with the wheel male screw  44  is provided at the tip of the wheel male screw  44 . A wheel engagement opening  57  is formed on the end inside diameter of the sleeve  49  facing to the compressor wheel  16 . A wheel engagement cylindrical portion  43 H is provided at the end of the cylindrical portion  43  of the wheel  16 . 
     The wheel engagement cylindrical portion  43 H is so sized as to be inserted into the wheel engagement opening  57  by slight clearance fit. Thus, the concentricity between the compressor wheel  16  and the shaft  23  can be secured. 
     An outer surface  61  of the cylindrical portion  49 E of the sleeve  49  facing to the compressor wheel  16  is processed to have two parallel surfaces or to be nut-shaped (not shown) for example, so as to be clamped by a spanner or the like. 
     A seal ring  51  made from FC material or others is fitted to the seal groove  50  of the sleeve  49 . When force is applied to the seal ring  51  in such a manner as to decrease the diameter of the seal ring  51 , the outer diameter thereof is fitted to the inner surface of the intake-side inner plate  55  while tightly contacting therewith. 
       FIG. 5  shows processes for attaching the compressor wheel  16  to the shaft  23 . 
     First, a disk-shaped thrust collar  47  having a round hole at its center is fitted to the shaft  23  supported by the bearing  24  (Step S 11 ). 
     Next, the thrust bearing  48  is fitted to a bearing housing  45  (Step S 12 ). An oil passage  56  through which lubricant oil flows is formed on the thrust bearing  48 . The lubricant oil lubricates the contact surfaces of the rotating sleeve  49  and the thrust collar  47  and the non-rotating thrust bearing  48 . 
     The sleeve  49  is screwed to the shaft  23  (Step S 13 ). In this step, the sleeve  49  is screwed to the shaft male screw  46  while clamping the outer diameter  61  of the sleeve  49  which is processed to be nut-shaped by a spanner or the like. 
     Then, the intake-side inner plate  55  is fixed to the bearing housing  45  (Step S 14 ). Through this step, the thrust bearing  48  is sandwiched between the bearing housing  45  and the intake-side inner plate  55  as the non-rotating members and fixed therebetween, whereby the sleeve  49  and the thrust collar  47  come to rotate with the shaft  23  as one piece. 
     As a result, the thrust bearing  48  fixed to the non-rotating members in Step S 13  is sandwiched between the thrust collar  47  and the sleeve  49  as the rotating members which rotate with the shaft  23  as one piece. Accordingly, force generated in the thrust direction of the shaft  23  during rotation is received by the thrust bearing  48 , and the position of the rotational axis in the axial direction is thus restricted. 
     When the intake-side inner plate  55  is fixed to the bearing housing  45  in Step S 14 , the outer diameter of the seal ring  51  comes into tight contact with the inner surface of the intake-side inner plate  55 . This structure prevents the oil for lubricating the bearing  24  and the thrust bearing  48  from flowing out toward a space at the back of the compressor wheel  16 , i.e., a so-called “back chamber”. 
     Next, the compressor wheel  16  is screwed into the sleeve  49  (Step S 15 ). In this step, the nut-shaped portion  16 N at the wheel inlet  35  of the compressor wheel  16  and the nut-shaped portion  14 N of the turbine wheel  14  are clamped by a spanner or the like and screwed to each other as illustrated in  FIG. 1 . Simultaneously, the engagement cylindrical portion  23 H of the shaft  23  is inserted into the engagement opening  44 H of the compressor wheel  16  by slight clearance fit or close fit. Through this step, the compressor wheel  16  and the shaft  23  are connected with each other. 
     According to the invention as described above, the wheel male screw  44  is provided at the small diameter position of the compressor wheel  16 . The wheel male screw  44  and the shaft male screw  46  formed at the tip of the shaft  23  are connected with each other via the sleeve  49  having the female screw  52  on one side and the female screw  53  on the other side. 
     Since the compressor wheel  16  and the shaft  23  are coupled with each other without the fitting hole  125  and the fitting opening  242  included in the related art, the compressor wheel  16  can be made solid. Thus, the stress applied to the compressor wheel  16  is decreased and the occurrence of the breakage is reduced even if rotated at high speed. 
     The reason for this effect is described with reference to  FIG. 6 .  FIG. 6  is a graph showing the relationship between an inside diameter φ of the fitting hole of the compressor wheel and stress T applied to the compressor wheel in a maximum outer diameter where the outer diameter of the compressor wheel reaches a maximum in the axial direction of the rotational axis of the compressor wheel in the related art. As shown in  FIG. 6 , the stress T is small when the inside diameter of the fitting hole is zero, and the stress T is extremely large when the inside diameter is excessively small. When the inside diameter is a certain value D or larger, the stress T increases as the inside diameter of the fitting hole becomes larger. 
     Accordingly, unlike the related art, the stress applied is reduced in the invention where a solid component having no fitting hole is employed. 
     Next, a second embodiment is herein described. The second embodiment is different from the first embodiment in the structure of the P area. Similar reference numerals are given to similar components to those in the first embodiment, and description associated therewith is omitted. 
     As illustrated in  FIG. 7 , the screw size of a wheel male screw  44 A is larger than that of the shaft male screw  46 . A wheel engagement cylindrical portion  44 JH which is precisely machined to be cylindrical and concentric with the wheel male screw  44 A is provided at the tip of the wheel male screw  44 A. A sleeve engagement opening  49 JH is formed between the shaft-side female screw  53  and a wheel-side female screw  52 A of a sleeve  49 A. The wheel engagement cylindrical portion  44 JH is so sized as to be inserted into the wheel engagement opening  49 JH of the sleeve  49 A by slight clearance fit. 
     The shaft-side female screw  53  engaging with the shaft male screw  46  is formed on an inner surface  58 A of the sleeve  49 A facing to the shaft  23 . The wheel-side female screw  52 A engaging with the wheel male screw  44 A is formed on the inner surface  58 A of the sleeve  49 A facing to a compressor wheel  16 A. The screw size of the wheel male screw  44 A is larger than that of the shaft male screw  46 . 
     The shaft male screw  46  and the wheel male screw  44 A are connected with each other via the sleeve  49 A having the wheel-side female screw  52 A and the shaft-side female screw  53 . 
     The engagement cylindrical portion  23 H of the shaft  23  is inserted into the engagement opening  44 H of the compressor wheel  16 A by slight clearance fit or close fit. The wheel engagement cylindrical portion  44 JH is inserted into the wheel engagement opening  49 JH of the sleeve  49 A by slight clearance fit. Thus, the concentricity between the compressor wheel  16 A and the shaft  23  can be sufficiently secured. The wheel engagement cylindrical portion  44 JH may be provided at the tip outer surface of the wheel male screw  44 A, or at the root end outer surface of the wheel male screw  44 A. 
     The compressor wheel  16 A and the wheel male screw  44 A are made from an aluminum alloy casting or other material, while the shaft  23  and the shaft male screw  46  are made from hard material such as iron or iron alloy. The diameter of the wheel male screw  44 A formed integrally with the compressor wheel  16 A is larger than the diameter of the shaft male screw  46  formed at the tip of the shaft  23 . Since the diameter of the aluminum alloy casting having lower strength is larger, the possibility that either the compressor wheel or the shaft is particularly easy to break is reduced. 
     Next, a third embodiment is herein described. The third embodiment is different from the first embodiment also in the structure in the P area. Similar reference numerals are given to similar components to those in the first embodiment, and description associated therewith is omitted. 
     As illustrated in  FIG. 8A , the shaft cylindrical portion  60  which is processed to be cylindrical and concentric with a shaft  23 B is provided at the tip of the shaft  23 B. 
     A shaft male screw  46 B is formed at a position closer to the tip from the shaft cylindrical portion  60 . The screw size of a wheel male screw  44 B is larger than that of the shaft male screw  46 B, and thus the outside diameter of the wheel male screw  44 B is larger than that of the shaft male screw  46 B. A shaft engagement cylindrical portion  23 JH which is precisely machined to be cylindrical and concentric with the shaft  23 B is provided at a position closer to the tip from the shaft male screw  46 B. 
     A shaft engagement opening  49 SH is formed between a shaft-side female screw  53 B and a wheel-side female screw  52 B of a sleeve  49 B. The shaft engagement cylindrical portion  23 JH is so sized as to be inserted into the shaft engagement opening  49 SH of the sleeve  49 B by slight clearance fit. An engagement cylindrical portion  23 BH which is precisely machined to be cylindrical and concentric with the shaft  23 B is provided at the tip of the shaft  23 B. The cylindrical portion  23 BH is so sized as to be inserted into an engagement opening  44 BH of the wheel male screw  44 B by slight clearance fit or close fit. 
     The shaft engagement cylindrical portion  23 JH may be provided at the tip outer surface of the shaft male screw  46 B, or at the root end of the shaft male screw  46 B as illustrated in  FIG. 8B . 
     As illustrated in  FIG. 8A , a shaft-side female screw  53 B engaging with the shaft male screw  46 B is provided on an inner surface  58 B of the sleeve  49 B facing to the shaft  23 B, while a wheel-side female screw  52 B engaging with the wheel male screw  44 B on the inner surface  58 B of the sleeve  49 B facing to the compressor wheel  16 B. Since the screw size of the wheel male screw  44 B is larger than that of the shaft male screw  46 B, the screw size of the wheel-side female screw  52 B is larger than that of the shaft-side female screw  53 B. 
     The shaft male screw  46 B and the wheel male screw  44 B are connected with each other via the sleeve  49 B having the wheel-side female screw  52 B and the shaft-side female screw  53 B. 
     The engagement cylindrical portion  23 BH of the shaft  23 B is inserted into the engagement opening  44 BH of the compressor wheel  16 B by slight clearance fit or close fit. The shaft engagement cylindrical portion  23 JH is inserted into the shaft engagement opening  49 SH of the sleeve  49 B by slight clearance fit. Thus, the concentricity between the compressor wheel  16 B and the shaft  23 B can be sufficiently secured. 
     Next, a fourth embodiment is herein described. The fourth embodiment is an example in which the engagement part between the sleeve and the wheel in the second embodiment is added to the third embodiment. Similar reference numerals are given to similar components to those in the second and third embodiments, and description associated therewith is omitted. 
     As illustrated in  FIG. 9 , the shaft  23 B includes the shaft cylindrical portion  60 , the shaft male screw  46 B, and the shaft engagement cylindrical portion  23 JH. A sleeve  49 C has the shaft engagement opening  49 SH. The shaft engagement cylindrical portion  23 JH is so sized as to be inserted into the shaft engagement opening  49 SH of the sleeve  49 C by slight clearance fit. The engagement cylindrical portion  23 BH is provided at the tip of the shaft  23 B. The engagement cylindrical portion  23 BH is so sized as to be inserted into the engagement opening  44 H of the wheel male screw  44 A by slight clearance fit or close fit. 
     The wheel engagement cylindrical portion  44 JH which is precisely machined to be cylindrical and concentric with the wheel male screw  44 A is provided at the tip of the wheel male screw  44 A. A sleeve engagement opening  49 JHC is formed between a shaft-side female screw  53 C and a wheel-side female screw  52 C of the sleeve  49 C. The wheel engagement cylindrical portion  44 JH is so sized as to be inserted into the wheel engagement opening  49 JHC of the sleeve  49 C by slight clearance fit. 
     The shaft-side female screw  53 C engaging with the shaft male screw  46 B is provided on an inner surface  58 C of the sleeve  49 C facing to the shaft  23 B, while the wheel-side female screw  52 C engaging with the wheel-side male screw  44 A is provided on the inner surface  58 C of the sleeve  49 C facing to the compressor wheel  16 A. 
     The shaft male screw  46 B and the wheel male screw  44 A are connected with each other via the sleeve  49 C having the wheel-side female screw  52 C and the shaft-side female screw  53 C. 
     The engagement cylindrical portion  23 BH of the shaft  23 B is inserted into the engagement opening  44 H of the compressor wheel  16 A by slight clearance fit or close fit. The shaft engagement cylindrical portion  23 JH is inserted into the shaft engagement opening  49 SH of the sleeve  49 C by slight clearance fit. The wheel engagement cylindrical portion  44 JH is inserted into the wheel engagement opening  49 JHC of the sleeve  49 C by slight clearance fit. Thus, the concentricity between the compressor wheel  16 A and the shaft  23 B can be sufficiently secured. 
     Next, a fifth embodiment is herein described. The fifth embodiment is a different example in which a plate  70  is added to the third embodiment. 
     As illustrated in  FIG. 10 , a shaft  23 D includes the shaft cylindrical portion  60 , a shaft male screw  46 D, and a shaft engagement cylindrical portion  23 JHD. A sleeve  49 D has a shaft engagement opening  49 SHD. An engagement cylindrical portion  23 DH is provided at the tip of the shaft  23 D and is so sized as to be inserted into an engagement opening  44 DH of a wheel male screw  44 D by slight clearance fit or close fit. The sleeve  49 D has a shaft-side female screw  53 D and a wheel-side female screw  52 D. 
     An end surface  43 DT of a cylindrical portion  43 D of a compressor wheel  16 D and an end surface  49 DT of the sleeve  49 D are so sized as to have a clearance between each other when the compressor wheel  16 D and the shaft  23 D are tightened. An end surface  44 DT of the wheel male screw  44 D and a stepped portion  49 DD of the sleeve  49 D are tightened with a washer-shaped plate  70  interposed therebetween. The plate  70  is made from a material harder than the material of the compressor wheel  16 D. Since the end surface of the compressor wheel  16 D which is pressed when fastening torque is applied for the attachment of the compressor wheel  16 D has a wide area, the surface pressure can be decreased. 
     According to the invention, the shafts  23 ,  23 B and  23 D have the shaft male screws  46 ,  46 B and  46 D to which the sleeves  49 ,  49 A,  49 B,  49 C and  49 D having the female screws  52 ,  52 A,  52 B,  52 C,  52 D,  53 ,  53 B,  53 C and  53 D are screwed. This structure allows the screw diameter of the wheel-side female screw  52 D to be larger than that of an example where a female screw is provided on a shaft, thereby increasing the fastening strength. 
     Since the seal groove  50  is formed on the outer surface of the sleeves  49 ,  49 A,  49 B,  49 C and  49 D, oil can be sealed by a compact structure. 
     The male screws  44 ,  44 A,  44 B and  44 D of the compressor wheels  16 ,  16 A,  16 B and  16 D, the shaft male screws  46 ,  46 B,  46 D, and the female screws  52 ,  52 A,  52 B,  52 C,  52 D,  53 ,  53 B,  53 C and  53 D of the sleeves  49 ,  49 A,  49 B,  49 C and  49 D are right-handed screws when the compressor wheels  16 ,  16 A,  16 B and  16 D rotate counterclockwise and are left-handed screws when the compressor wheels  16 ,  16 A,  16 B and  16 D rotate clockwise as viewed from the intake inflow port  27  as the inlet of the compressor wheels  16 ,  16 A,  16 B and  16 D. Since the rotational torque produced due to inertial force generated when the compressor wheels  16 ,  16 A,  16 B and  16 D are rapidly accelerated for rotation is applied in a direction where the screws are tightened, loosening of the screws is prevented. 
     While only an example of a turbo charger to which the invention is applied has been described, the invention is applicable to other turbo machines such as micro gas turbines and engine-driven superchargers.