Abstract:
The invention provides a compressor of a turbo machine and its compressor wheel that do not easily undergo breaking even when rotated at a high number of revolutions. The compressor of the turbo machine includes a male screw portion integrally formed on a main body portion of the compressor wheel and a male screw portion disposed at a distal end of a shaft for driving the compressor wheel that are coupled with each other through a sleeve equipped at one of the ends thereof with a female screw portion capable of meshing with the male screw portion of the compressor wheel and at the other end with a female screw portion capable of meshing with the male screw portion of the shaft.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to a compressor of a turbo machine and its compressor wheel.  
         [0003]     2. Description of the Related Art  
         [0004]     A compressor of a turbo machine of the type that is rotated by a turbine wheel via a shaft by utilizing energy of an exhaust gas as means for increasing an intake amount of an engine by compressing air and drives a centrifugal type compressor wheel coupled with a shaft is known as a turbo charger.  
         [0005]      FIG. 7  is a sectional side view of the turbo charger  11  according to the prior art. The turbo charger  11  includes an exhaust-side unit  12  for taking out energy of revolution from the exhaust gas of an engine and an intake-side unit  13  for compressing air by this energy of revolution and sending compressed air into the engine.  
         [0006]     A turbine wheel  14  is imparted with energy and is rotated by the exhaust gas inflowing from an exhaust inflow passage  19 . A centrifugal type compressor wheel  16  for compressing air is fitted on the opposite side to the turbine wheel  14  of a shaft  23  (hereinafter called a “distal end side of the shaft 23”) through the shaft  23 . A fitting hole  25  penetrates through a center of the compressor wheel  16 . The shaft  23  is fitted into the fitting hole  25  with slight loose fit or tight fit. The compressor wheel  16  is fixed to the shaft  23  as a nut  26  is fastened to a male screw portion  40  formed at a distal end of the shaft  23 .  
         [0007]      FIG. 8  is a sectional side view of the compressor wheel  16  shown in  FIG. 7 . A main body portion  29  of the compressor wheel  16  includes an inlet-side disk portion  29 A and a back surface-side disk portion  29 B. A plurality of vane portions  18  are arranged on the outside of the main body portion  29  and the fitting hole  25  penetrates through the center of the main body portion  29 .  
         [0008]     To accomplish lightweight, the compressor wheel  16  is produced from a casting such as an aluminum alloy. Because rotating speed of the compressor wheel  16  reaches high values of dozens of thousands of rounds per minute (rpm), centrifugal force resulting from the high-speed revolution imparts an extremely high tensile strength in a radial direction and sometimes invites breaking of the compressor wheel  16 . It is known that this breaking is likely to particularly develop in the inner wall of the fitting hole  25  as the starting point.  
         [0009]     To solve this problem, a technology described in Patent Reference (JP-A-5-504178), for example, is known.  
         [0010]      FIG. 9  is a sectional view of a compressor wheel  16  according to this Patent Reference. A fitting hole penetrating through the compressor wheel  16  is not disposed but a fitting hole  42  having a female screw is formed at its lower part. A male screw is formed at a distal end  54  of a shaft  23 . A male screw is formed at the distal end  54  of the shaft  23 . The shaft  23  and the compressor wheel  16  are coupled with each other as the distal end  54  is screwed into the fitting hole  42 .  
         [0011]     However, the prior art technology shown in  FIG. 8  is not free from the following problem. In other words, it has been confirmed that breaking of the inner wall of the fitting hole  25  in the compressor wheel  16  occurs particularly frequently in the proximity of the maximum outer circumferential portion  30  at which the outer circumferential portion of the compressor wheel  16  becomes maximal in an axial direction.  
         [0012]     According to the prior art shown in  FIG. 9 , the fitting hole  42  is disposed in the proximity of the maximum outer circumferential portion  30  in the axial direction. Therefore, when rotating speed is increased, breaking may occur from near the maximum outer circumferential portion  30 .  
         [0013]     Particularly when an engine equipped with the turbo charger  11  using the compressor wheel  16  is used for work machines such as construction machines, a high load state such as a loading operation (that is, a high rotating speed of the turbocharger) and a state almost free from the load (that is, a low rotating speed) are repeated within short time intervals. As a result, the stress amplitude applied to the compressor wheel  16  becomes high and breaking is more likely to occur.  
         [0014]     A technology called “EGR (Exhaust Gas Recirculation)” has been adopted in recent years as a counter-measure for reducing nitrogen oxides (NOx) contained in an exhaust gas of a Diesel engine. This technology returns a part of the exhaust gas emitted from the engine to an intake system of the engine for re-circulation. To accomplish EGR, it is necessary to secure fresh air for combustion capacity in a cylinder where the quantity of fresh air becomes smaller by the re-circulation amount of the exhaust gas and to achieve a higher-pressure ratio of the turbo charger  11 . In other words, the compressor wheel  16  must be rotated at a higher rotating speed. The prior art technology is not yet sufficient and a compressor wheel  16  having higher durability has been desired.  
       SUMMARY OF THE INVENTION  
       [0015]     In view of the problems described above, the invention aims at providing a compressor of a turbo machine exhibiting less breaking even when rotated at a high rotating speed and its compressor wheel.  
         [0016]     To accomplish this object, a compressor of a turbo machine according to the invention includes a male screw portion integrally formed on a main body portion of a compressor wheel and a male screw portion disposed at a distal end of a shaft for driving the compressor wheel that are coupled with each other through a sleeve equipped at one of the ends thereof with a female screw portion capable of meshing with the male screw portion of the compressor wheel and at the other end with a female screw portion capable of meshing with the male screw portion of the shaft.  
         [0017]     In the invention, a diameter of the male screw portion of the compressor wheel may be greater than a diameter of the male screw portion of the shaft.  
         [0018]     In the invention, at least either one of centering between the compressor wheel and the sleeve and centering between the sleeve and the shaft may be made in a spigot joint arrangement.  
         [0019]     In the invention, a seal groove may be formed around an outer circumferential portion of the sleeve and a seal ring may be fitted into the seal groove so as to prevent leakage of air and oil between a back surface chamber of the compressor wheel and a bearing chamber.  
         [0020]     The invention may also have a construction which includes a thrust bearing fixed to a non-rotary member not executing revolution in synchronism with the shaft and a disk-like thrust collar fixed to the shaft, and wherein the thrust collar and the sleeve sandwich the thrust bearing between them.  
         [0021]     In the invention, a distal end of a cylindrical portion of a back surface-side disk portion of the compressor wheel is a male screw processed portion.  
         [0022]     The following can be listed up as the effects of the invention.  
         [0023]     The fitting hole or aperture for coupling with the shaft need not be disposed in the compressor wheel main body portion. As a result, the stress acting on the compressor wheel becomes small and breaking becomes less even when the compressor wheel is rotated at a high number of revolutions.  
         [0024]     The compressor wheel is formed in many cases of a material having a lower intensity than the shaft to reduce the weight. Therefore, when the male screw portion of the compressor wheel is rendered thick, the problem that the male screw portion of the compressor wheel is particularly likely to be broken becomes small and overall durability can be improved.  
         [0025]     When the rotation balance of the compressor wheel and the turbine wheel is individually adjusted and these wheels are assembled, a centering error after assembly becomes small. Therefore, the frequency of re-adjustment of the rotation balance of the compressor of the turbo machine becomes small.  
         [0026]     Members for sealing air and oil need not be disposed separately, and air and oil can be sealed with a compact construction.  
         [0027]     It is also possible to support the thrust bearing with a simple construction and to effectively bear the force in the thrust direction that acts on the shaft.  
         [0028]     Because breaking of the compressor wheel becomes more difficult to occur and durability can be improved, the pressure ratio of the turbo charger using this compressor wheel can be increased. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]      FIG. 1  is a side view of a compressor wheel according to the invention;  
         [0030]      FIG. 2  is a sectional view of  FIG. 1 ;  
         [0031]      FIG. 3  is a sectional view of a turbo charger according to the invention;  
         [0032]      FIG. 4  is a detailed view of a portion P in  FIG. 3 ;  
         [0033]      FIG. 5  is a flowchart showing a procedure for assembling the compressor wheel;  
         [0034]      FIG. 6  is a graph showing the relation between an inner diameter of a known fitting hole and a magnitude of a stress;  
         [0035]      FIG. 7  is a sectional side view of a turbo charger according to a prior art;  
         [0036]      FIG. 8  is a sectional side view of a compressor wheel shown in  FIG. 7 ; and  
         [0037]      FIG. 9  is a sectional view of another compressor wheel according to the prior art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0038]     A preferred embodiment of the invention will be hereinafter explained in detail with reference to the accompanying drawings.  
         [0039]     Referring to  FIG. 3 , an exhaust-side unit  12  includes an exhaust-side housing  15  and a turbine wheel  14  having a plurality of vanes and supported by a shaft  23 .  
         [0040]     The exhaust-side housing  15  has an exhaust inflow passage  19  for supplying an exhaust gas to the turbine wheel  14 . The exhaust inflow passage  19  is formed into an annular shape in such a manner as to encompass the outer periphery of the turbine wheel  14  and is connected to an engine exhaust flow passage through which the exhaust gas emitted from an engine, not shown, flows. The exhaust-side housing  15  has also an exhaust outflow port  21  for emitting the exhaust gas after imparting energy to the turbine wheel  14 . The exhaust outflow port  21  is formed substantially into a cylindrical shape that is concentric with the center of revolution of the turbine wheel  14 . An opening on the opposite side to the exhaust outflow port  21  is closed by an exhaust-side inner plate  22 .  
         [0041]     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  and the shaft  23  are ordinarily formed of a nickel base super-alloy, carbon steel or alloy steel.  
         [0042]     A compressor wheel  16  is accommodated inside an intake-side housing  17 . The intake-side housing  17  has an intake inflow port  27  for sucking air into the compressor wheel  16 . The intake inflow port  27  is formed substantially into a cylindrical shape that is concentric with the center of revolutions of the compressor wheel  16 . An opening on the opposite side to the intake inflow port  27  is closed by an intake-side inner plate  55 .  
         [0043]     Air that is compressed by the compressor wheel  16  is centrifugally discharged and is supplied to a feed port of the engine, not shown, while passing through an intake exhaust passage  28  annularly formed in such a manner as to encompass the outer peripheral portion of the compressor wheel  16 .  
         [0044]     The vanes  18  include full vanes  18 A having a large width in an axial direction of the vanes and intermediate vanes  18 B in which a vane inlet starts from an intermediate part in the axial direction with respect to the full vanes  18 A. These full vanes  18 A and intermediate vanes  18 B are alternately arranged.  
         [0045]     A group of rotary members including the turbine wheel  14 , the compressor wheel  16  and the shaft  23  will be hereinafter called the “rotary members”. A group of stationary members including the intake-side housing  17 , the exhaust-side housing  15  and a bearing housing  45  will be hereinafter called the “non-rotary members”. A penetrating direction of the fitting hole  25  will be called the “axial direction”.  
         [0046]     As shown in  FIGS. 1 and 2 , a main body portion  29  of the compressor wheel  16  according to the invention is solid and does not have any fitting holes or apertures.  
         [0047]     A portion for sucking air into the compressor wheel  16  will be called a“compressor wheel inlet portion 35” and a portion for discharging air in a radial direction will be called a “compressor wheel outlet portion 33”. A curve surface of an intermediate portion between the compressor wheel inlet portion  35  and the compressor wheel outlet portion  33  will be called a “disk center portion 34”.  
         [0048]     A portion in the axial direction at which the outer peripheral portion of the compressor wheel  16  becomes maximal will be called the “maximum outer peripheral portion 30”. The main body portion  29  of the compressor wheel  16  has an inlet-side disk portion  29 A and a rear surface-side disk portion  29 B. A cylindrical portion  43  is integrally arranged on the rearmost portion of the rear surface-side disk portion  29 B while its axis is in alignment with the main body portion  29 . A male screw  44  having a smaller diameter than that of the cylindrical portion  43  is integrally formed at the lower end of the cylindrical portion  43 . The male screw  44  will be called a “compressor wheel male screw portion 44”.  
         [0049]     Processing for securing a width across flats or nut-like processing, for example, is applied to the outer peripheral portion of the compressor wheel inlet portion  35  of the compressor wheel  16  and this portion can be clamped by use of a wrench, or the like.  
         [0050]     Referring to  FIGS. 3 and 4 , the distal end portion  60  of the shaft  23  fixed to the turbine wheel  14  is precisely machined into a cylindrical shape that is concentric with the shaft  23 . This cylindrical portion will be called a “shaft cylindrical portion 60”. A male screw  46  is formed at a further distal end of the shaft cylindrical portion  60 . This male screw  46  will be called a “shaft male screw portion 46”. The outer diameter of the shaft male screw portion  46  is smaller than the outer diameter of the compressor wheel male screw portion  44 . The shaft male screw portion  46  and the compressor wheel screw portion  44  are connected to each other through a sleeve  49  having female screws at both of its ends.  
         [0051]     As shown in  FIG. 4 , spigot joint processing is applied to the inner peripheral portion  58  at the end of the sleeve  49  on the side of the shaft  23  with respect to the shaft cylindrical portion  60 . A female screw  53  (hereinafter called a “shaft-side female screw portion 53”) meshing with the shaft male screw portion  46  is formed at the depth of the inner peripheral portion  58  (on the side of the compressor wheel  16 ).  
         [0052]     Spigot joint processing is applied to the inner peripheral portion  57  at the end of the sleeve  49  on the side of the compressor wheel  16  with respect to the cylindrical portion  43  formed on the rear surface of the compressor wheel  16 . A female screw  52  (hereinafter called a “compressor wheel-side female screw portion 52”) meshing with the compressor wheel male screw portion  44  is formed at the depth of the inner peripheral portion  57  (on the side of the shaft  23 ).  
         [0053]     Incidentally, the shaft-side female screw portion  53  and the compressor wheel-side female screw portion  52  in the sleeve  49  are shown penetrated but they need not always be penetrated. Processing for securing a width across flats or nut-like processing, for example, is applied to the outer peripheral portion  61  of the of the sleeve  49  on the side of the compressor wheel  16  and this portion can be clamped by use of a wrench, or the like. A seal groove  50  is formed in the entire outer peripheral portion at an intermediate part of the sleeve  49  in the axial direction and a seal ring  51  formed of an FC material, etc, is fitted into the seal groove  50 . The seal ring  51  is formed in such a manner that when force for reducing the diameter is applied, the outer peripheral portion of the sealing ring  51  tightly fits into the inner peripheral portion of the intake-side inner plate  55 .  
         [0054]     The bearing  24  is accommodated in a bearing box  63  of the bearing housing  45  that connects the intake-side housing  17  and the exhaust-side housing  15 . An oil-feed port  59  is formed in the bearing housing  45  to supply a lubricant to the bearing  24  and the thrust bearing  48 .  
         [0055]      FIG. 5  is a flowchart showing the procedure for assembling the compressor wheel  16  into the shaft  23 . First, a disk-like thrust collar  47  having a round hole at its center is fitted to the shaft  23  supported by the bearing  24  (Step S 11 ).  
         [0056]     Next, the thrust bearing  48  is fitted to the bearing housing  45  (Step S 12 ). An oil passage  56  through which lubricant oil flows is disposed in the thrust bearing  48 .  
         [0057]     The sleeve  49  is screwed into the shaft  23  (Step S 13 ). In this instance, the sleeve  49  is screwed into the shaft male screw portion  46  while the outer peripheral portion  61  of the sleeve  49  processed into the nut shape is clamped by the wrench, or the like. In consequence, the sleeve  49  and the thrust collar  47  rotate integrally with the shaft  23 .  
         [0058]     Next, the intake side inner plate  55  is fixed to the bearing housing  45  (Step S 14 ). Consequently, the thrust bearing  48  is fixed to the non-rotary members while being sandwiched between the bearing housing  45  and the intake-side inner plate  55 .  
         [0059]     As a result, the thrust bearing  48  fixed to the non-rotary members in Step S 13  is sandwiched between the thrust collar  47  and the sleeve  49  as the rotary members rotating integrally with the shaft  23 . Therefore, the force imparted in the thrust direction of the shaft  23  during revolution is received by the thrust bearing  48  and the position in the axial direction is limited. When the sleeve  49  is screwed in Step S 14 , the outer peripheral portion of the seal ring  51  comes into adhesion with the inner peripheral portion of the intake-side inner plate  55 . Consequently, the oil for lubricating the bearing  24  and the thrust bearing  48  is prevented from flowing out to the space (called a “back surface chamber 62”) of the back surface of the compressor wheel  16 .  
         [0060]     Next, the compressor wheel  16  is screwed into the sleeve  49  (Step S 15 ). In this instance, the nut-like processed portion of the compressor wheel inlet portion  35  of the compressor wheel  16  and the nut-like processed portion of the outlet portion of the turbine wheel  14  are screwed to each other while being clamped by the wrench, or the like. The compressor wheel  16  and the shaft  23  are thus coupled with each other.  
         [0061]     As explained above, in the invention, the compressor wheel male screw portion  44  is arranged round the outer periphery of the cylindrical portion  43  at the rearmost surface portion of the back surface-side disk portion  29 B of the compressor wheel  16 . The impeller male screw portion  44  and the shaft male screw portion  46  disposed at the distal end of the shaft  23  are connected to each other through the sleeve  49  having the female screw portions  52  and  53  at both of its ends.  
         [0062]     Therefore, even when the compressor wheel  16  is solid, the compressor wheel  16  and the shaft  23  can be connected to each other. For this reason, the stress acting on the compressor wheel  16  becomes small and breakage does not occur even at a high rotating speed.  
         [0063]      FIG. 6  is a graph showing the relation between the inner diameter Φ of the fitting hole  25  of the compressor wheel  16  and the magnitude of the stress T acting on the compressor wheel  16  at the maximum outer circumference portion  30  at which the outer circumferential portion of the compressor wheel  16  becomes maximal in the axial direction of the rotary shaft of the compressor wheel  16  in the prior art technology. In the graph, the stress T is small when the inner diameter of the fitting hole  25  is 0 and becomes extremely great when the inner diameter is excessively small. At a certain inner diameter D or above, the stress T becomes greater with the increase of the inner diameter of the fitting hole  25 . Therefore, it can be understood that when the fitting hole  25  does not exist and the compressor wheel  16  is solid as in the present invention, the stress becomes small.  
         [0064]     According to the invention, the diameter of the compressor wheel male screw portion  44  formed integrally with the compressor wheel  16  is greater than the diameter of the shaft male screw portion  46  formed at the distal end of the shaft  23 . The compressor wheel  16  and the compressor wheel male screw portion  44  are formed of a casting of an aluminum alloy, for example. On the other hand, the shaft  23  and the shaft male screw portion  46  are formed of a hard material such as iron or its alloy. Therefore, when the thickness of the casting of the aluminum alloy having a lower strength is increased, it is possible to prevent the problem that one of them is particularly likely to be broken.  
         [0065]     Furthermore, the shaft male screw portion  46  is formed at the distal end portion of the shaft  23  and the sleeve  49  having the female screw portion  53  is screwed to the shaft male screw portion  46 . This configuration makes it possible to reduce the outer diameter of the portion of the shaft  23 , which is supported by the shaft  24 , when compared to the configuration in which, for example, a female screw is disposed in the shaft  23 . Therefore, because the speed of the outer peripheral portion of the shaft  23  becomes lower, the rotation frictional loss with the bearing  24  becomes smaller and breaking of the shaft  23  and the bearing  24  does not easily occur.  
         [0066]     The seal groove  50  is disposed around the outer circumferential portion of the sleeve  49  and the oil can be sealed by a compact construction. Because the sleeve  49  and the compressor wheel  16  are centered with each other in the spigot joint, unbalance during revolution can be reduced.  
         [0067]     Incidentally, the outer circumferential portion of the compressor wheel inlet portion  35  of the compressor wheel  16  is sufficient so long as the compressor wheel  16  can be fixed when screwed to the sleeve  49  and may have a bolt shape having a hexagonal boss, for example.  
         [0068]     The invention has been explained about only its application example to the turbo charger but can be similarly applied to other turbo machines and mechanical driving centrifugal compressors such as a micro-gas turbine.