Patent Publication Number: US-9903385-B2

Title: Impeller, rotary machine including the same, and method for manufacturing impeller

Description:
TECHNICAL FIELD 
     The present invention relates to an impeller in rotary machines, such as a centrifugal compressor, a rotary machine including the impeller, and a method for manufacturing the impeller. Priority is claimed on Japanese Patent Application No. 2011-185838, filed Aug. 29, 2011, the content of which is incorporated herein by reference. 
     BACKGROUND ART 
     As shown in  FIGS. 12 and 14 , generally, an impeller  101  used for rotary machines, such as a centrifugal compressor, has a fixed hub portion  112  of a rotating shaft S, a disc-shaped disc  107  that is provided integrally with the hub portion  112 , a shroud  108  that is arranged so as to be spaced apart from the disc  107  in the axial direction of the central axis L, and a plurality of blades  106  that are provided in a circumferential direction and connect the disc  107  to the shroud  108 . In this type of impeller, a portion surrounded by the side surfaces of the blades  106  and two mutually facing surfaces of the disc  107  and the shroud  108  formed a flow passage  103  for compressing air. Additionally, by shrink-fitting the hub portion  112  to the rotating shaft S of the rotary machine, the impeller  101  is fixed to the rotating shaft S. 
     The flow passage  103  opens toward a first end side of the central axis L on the inner peripheral side, curves gradually so as to be directed to a radial outer peripheral side, and opens toward the radial direction on outer peripheral side. That is, the flow passage  103  is formed in a curved shape as viewed from the circumferential direction in order to direct a fluid, which is introduced from the first end side along a second end side, to the radial outer peripheral side (particularly, refer to  FIG. 14 ). Moreover, as shown in  FIG. 13 , as the blades  106  are obliquely connected to the disc  107 , the compression performance of the impeller  101  is improved, and thereby, the flow passage  103  assumes a complicated three-dimensional shape. 
     As a method for manufacturing the impeller  101 , a method in which the blades  106  and one of the disc  107  and the shroud  108  are integrally formed, the other of the disc and shroud is separately manufactured, and these disc and shroud are integrated by welding or brazing, is known. Additionally, since the impeller requires high rigidity, a one-piece impeller with high strength reliability is manufactured by shaving out the disc  107 , the shroud  108 , and the blade  106  from a single base material (for example, refer to PTL  1 ). 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Patent Application, First Publication No. 2010-285919 
     SUMMARY OF INVENTION 
     Problem to Be Solved by the Invention 
     Incidentally, as described above, the flow passage  103  of the impeller  101  has a complicated shape having a curved portion, and the inside of the flow passage is narrow. Therefore, during the manufacture of the one-piece impeller  101 , it is necessary to perform complicated cutting while inserting, for example, machining members, such as an electrode for machining, from positions to be used as an inlet and an outlet of the flow passage  103 . Additionally, in the manufacturing method as described in PTL 1, it is necessary to form the flow passage using a special machining member and substantial manufacturing costs are incurred. 
     The present invention has been made in consideration of such a situation, and an object thereof is to provide an impeller, a rotary machine including the impeller, and a method for manufacturing the impeller that maintains the performance of the related art while being capable of being manufactured at low cost. 
     Means for Solving the Problem 
     In order to achieve the above object, an impeller related to a first aspect of the present invention is an impeller including a plurality of blades disposed in a circumferential direction of the impeller, in which each of the blades directs outward from inward in a radial direction of the impeller; a shroud located close to a first end side of an axis of the impeller with respect to the blades, and to which the blades are attached; and a disk located close to a second end side of an axis of the impeller with respect to the blades, to which the blades are attached, and configured to be attached to a rotating shaft. A plurality of flow passages are formed by the blades, the disc, and the shroud. The blades, the shroud and a first portion of the disk close to the second end side of the axis of the impeller are integrated so as to form a first member. A second portion of the disk close to the first end side of the axis of the impeller forms a second member. 
     According to the first aspect of the present invention, when a flow passage portion of the first member is formed by splitting the impeller into the first member and the second member and making the second member into a part that constitutes the portion of the disc on the first end side, the accessibility of the machining tool improves. That is, when the machining tool is inserted from a position to be used as the outlet of the flow passage, the machining when forming the flow passage becomes easy by making a part that becomes an interference object, on the inner peripheral side, into a separate second member. Additionally, when the machining tool is inserted from a position to be used as the introduction port that is the inlet of the flow passage, the machining of the introduction port becomes easy by making a part that becomes an interference object into a separate second member. Thereby, the manufacturing time can be shortened, and the manufacturing costs can be kept down. 
     In a second aspect of the present invention, in the above impeller, at least one of mutually facing surfaces of the disc and the shroud in the first member is formed into a flat surface. 
     According to the second aspect of the present invention, the shape of the flow passage defined by the disc, the shroud, and the blades are further simplified. Therefore, the accessibility of the machining tool can be improved and the man-hours of machining when forming the flow passage can be further reduced. 
     In a third aspect of the present invention, in the above impeller, both of the mutually facing surfaces of the disc and the shroud in the first member are formed into flat surfaces. 
     According to the third aspect of the present invention, a curved portion is eliminated in a cross-sectional shape when viewed from the circumferential direction in the shape of the flow passage defined by the disc, the shroud, and the blades. Therefore, the man-hours of machining when forming the flow passage can be further reduced. 
     In a fourth aspect of the present invention, in the impeller of any one aspect of the second and third aspects, the blades are provided within a range of the flat surface of the disc or the shroud as viewed from the axial direction. 
     According to the fourth aspect of the present invention, the shape of the flow passage defined by the disc, the shroud, and the blades are further simplified. Therefore, the accessibility of the machining tool can be improved and the man-hours of machining when forming the flow passage can be further reduced. 
     In a fifth aspect of the present invention, in the impeller of any one aspect of the first to four aspects, the surface of the second member that faces the first end side is formed in a curved shape so as to be directed to the radial outer peripheral side as it goes from the first end side to the second end side. 
     According to the fifth aspect of the present invention, a fluid introduced into the impeller can be guided to the flow passage without any delay by the surface of the second member that faces the first end side. This can maintain the compression performance of the impeller. 
     In a sixth aspect of the present invention, in the impeller of any one aspect of the first to fifth aspects, the first member has a fixed portion that is fixed to the rotating shaft. 
     According to the sixth aspect of the present invention, the impeller can be more firmly fixed to the rotating shaft compared to a case where the second member equivalent to a hub portion of a disc of the related art is fixed to the rotating shaft after the first member and the second member are integrated. That is, the impeller can be more firmly fixed to the rotating shaft by directly fixing the first member, which has a weight more than the second member, to the rotating shaft. 
     In a seventh aspect of the present invention, in the impeller of any one aspect of the first to sixth aspects, the surface of the blade that forms the flow passage is formed so as to be orthogonal to the surface of the disc that faces the shroud. 
     In the seventh aspect of the present invention, the shape of the blade is further simplified as compared to a shape where the surface of the blade that forms the flow passage inclines with respect to the disc. Therefore, the man-hours of machining when forming the flow passage can be further reduced. 
     Additionally, an eighth aspect of the present invention provides a rotary machine including the impeller related to any one aspect of the first to seventh aspects. 
     By adopting the above impeller, the rotary machine can be provided at low costs. 
     Additionally, a method for manufacturing an impeller related to a ninth aspect of the present invention is a method for manufacturing an impeller including a plurality of blades disposed in a circumferential direction of the impeller, in which each of the blades directs outward from inward in a radial direction of the impeller; a shroud located close to a first end side of an axis of the impeller with respect to the blades, and to which the blades are attached; and a disk located close to a second end side of an axis of the impeller with respect to the blades, to which the blades are attached, a plurality of flow passages being formed by the blades, the disc, and the shroud. The method for manufacturing an impeller includes a first member forming step of performing cutting on a base material for forming the second end side of the axis of the impeller to form the flow passages, and forming a first member in which the blades, the shroud, and a first portion of the disc close to the second end side of the impeller are integrally formed; and a second member forming step of forming a second member that constitutes a second portion of the disc close to the first end side of the impeller. 
     According to the ninth aspect of the present invention, after the impeller is split into the first member and the second member, these members are formed in separate steps, and the second member formed in the second member forming step is made into a part that constitutes the portion of the disc on the first end side. This improves the accessibility of a machining tool when a flow passage portion of the first member is formed. That is, when the machining tool is inserted from a position to be used as the outlet of the flow passage, the machining when forming the flow passage becomes easy by making a part that becomes an interference object on the inner peripheral side into a separate second member. Additionally, when the machining tool is inserted from a position to be used as the introduction port that is the inlet of the flow passage, the machining of the introduction port becomes easy by making a part that becomes an interference object into a separate second member. Thereby, the manufacturing time can be shortened, and the manufacturing costs can be kept down. 
     In a tenth aspect of the present invention, in the above method for manufacturing an impeller, in the second member forming step, the surface of the second member that faces the first end side is formed with a curved portion that is directed to the radial outer peripheral side as it goes from the first end side to the second end side. 
     According to the tenth aspect of the present invention, the air introduced into the impeller including the second member formed by the second member forming step can be guided to the flow passage without any delay by the surface of the second member that faces the first end side. Therefore, the performance of the impeller does not degrade. 
     Advantageous Effects of Invention 
     According to the present invention, the accessibility of a machining tool improves when a flow passage portion of the impeller is formed. Therefore, the manufacturing time can be shortened and the manufacturing costs can be kept down. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view showing a centrifugal compressor to which impellers of an embodiment of the present invention is applied. 
         FIG. 2  is a perspective view showing an impeller of the embodiment of the present invention. 
         FIG. 3  is an enlarged view of a part A of  FIG. 2 . 
         FIG. 4  is a cross-sectional view of the impeller of the embodiment of the present invention. 
         FIG. 5  is an exploded cross-sectional view of the impeller of the embodiment of the present invention. 
         FIG. 6  is a view showing a manufacturing step of the impeller of the embodiment of the present invention. 
         FIG. 7  is a view showing a manufacturing step of the impeller of the embodiment of the present invention. 
         FIG. 8  is a view showing a manufacturing step of the impeller of the embodiment of the present invention. 
         FIG. 9  is a view showing a manufacturing step of the impeller of the embodiment of the present invention. 
         FIG. 10  is a cross-sectional view showing another form of the impeller of the embodiment of the present invention. 
         FIG. 11  is a cross-sectional view showing still another form of the impeller of the embodiment of the present invention. 
         FIG. 12  is a perspective view showing an impeller of the related art. 
         FIG. 13  is an enlarged view of a part B of  FIG. 12 . 
         FIG. 14  is a cross-sectional view of the impeller of the related art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be described in detail referring to the drawings. 
     A centrifugal compressor  50  is shown as an example of a rotary machine of the present embodiment in  FIG. 1 . The centrifugal compressor  50  is mainly constituted by a rotating shaft S that is rotated around an axis P, impellers  1  that are attached to the shaft S and compress a fluid utilizing centrifugal force, and a casing  53  that rotatably supports the rotating shaft S and is formed with flow passages  52  that allow the fluid to flow therethrough from the upstream to the downstream. 
     The casing  53  is formed so as to form a substantially columnar outline, and the rotating shaft S is arranged so as to pass through the center of the casing. Journal bearings  54  are provided at both axial ends of the rotating shaft S of the casing  53 , and a thrust bearing  55  is provided at one end of the rotating shaft. The journal bearings  54  and the thrust bearing  55  rotatably support the rotating shaft S. That is, the rotating shaft S is supported by the casing  53  via the journal bearings  54  and the thrust bearing  55 . 
     Additionally, a suction port  56  into which a fluid is made to flow from the outside is provided on one end side of the casing  53  in the axial direction, and a discharge port  57  through which the fluid flows out to the outside is provided on the other end side. An internal space, which communicates with the suction port  56  and the discharge port  57 , respectively, and repeats diameter reduction and diameter increase, is provided within the casing  53 . This internal space functions as a space that accommodates the impeller  1 , and also functions as the above flow passages  52 . That is, the suction port  56  and the discharge port  57  communicate with each other via the impellers  1  and the flow passages  52 . 
     A plurality of impellers  1  is arranged at intervals in the axial direction of the rotating shaft S. In addition, although six impellers  1  are provided in the illustrated example, at least one or more impellers may be provided. 
     As shown in  FIGS. 2 and 4 , the impeller  1  has a substantially disc shape, and is configured so that a fluid suctioned from an introduction port  2  that opens to a first side in the direction (hereinafter referred to as an axial direction) of a central axis L is discharged toward the radial outer peripheral side via flow passages  3  formed inside the impeller  1 . 
     In addition, in the following, the outer peripheral side of the impeller  1  in the radial direction is simply referred to as outer peripheral side. Additionally, the inner peripheral side of the impeller  1  in the radial direction is simply referred to as inner peripheral side. Additionally, the upper side of  FIGS. 2 and 4  that becomes the upstream side of the fluid is referred to as a first end side, and the lower side of  FIGS. 2 and 4  that becomes the downstream side of the fluid is referred to as a second end side. 
     The impeller  1  of the present embodiment is equipped with a substantially disc-shaped first member  4  that forms the second end side, and a substantially cylindrical second member  5  that forms the first end side and that has an outer peripheral surface that is gradually increased in diameter toward the second end side. A disc  7  that is fixed to the rotating shaft S in the impeller  1 , a plurality of blades  6  that are provided in the circumferential direction on the first end side of the disc  7  so as to be directed from the inner peripheral side to the outer peripheral side, and a shroud  8  that is provided to face the disc  7  on the first end side and is attached to the blades  6  are constituted by the first member  4  and the second member  5 . The first member  4  and the second member  5  are not fixed to each other in the present embodiment but are fixed to the rotating shaft S, respectively, whereby the introduction port  2  is defined between the first member  4  and the second member  5 , and the first member  4  and the second member  5  further defines a suction portion  9  that connects the introduction port  2  and the flow passages  3 . 
     The first member  4  is arranged from the radial inner peripheral side toward the radial outer peripheral side. The first member  4  is equipped with a plurality of blades  6  disposed in a circumferential direction of the impeller, a first portion  7   a  that is provided on the second end side of the blades  6  and constitutes the second end side of the disc  7  to which the blades  6  are attached, and the shroud  8  that is provided on the first end side of the blades  6  and has the blades  6  attached thereto. That is, the shroud  8  is arranged so as to be spaced apart from the first portion  7   a  of the disc  7  by a predetermined distance. The first member  4  is formed from, for example, precipitation-hardened stainless steel. 
     The first portion  7   a  of the disc  7  includes a fixed portion  12  that is fixed to the rotating shaft S, and a disc body portion  11  that is formed integrally with the fixed portion  12  and has a substantially disc shape. 
     The fixed portion  12  is formed in a cylindrical shape that has a fitting hole  13 , which penetrates in the axial direction, at a central portion thereof. The fitting hole  13  is a hole that is inserted and fitted to the rotating shaft S when the impeller  1  is fixed to the rotating shaft S. The disc body portion  11  has a substantially circular shape as viewed from the axial direction and is formed at one axial end of the fixed portion  12 . Additionally, one surface  11   a  of the disc body portion  11  on the first end side is formed into a substantially flat surface. 
     In other words, the fixed portion  12  is a columnar part that protrudes to the second end side in the central portion of the disc body portion  11 . The fixed portion  12  protrudes to the second end side by a predetermined amount. This protruding amount is appropriately set according to a fastening force required in order to shrink-fit and fix the impeller  1  to the rotating shaft S. 
     The plurality of blades  6  are provided in the one surface  11   a  of the disc body portion  11 . The plurality of blades  6  have a constant plate thickness (blade thickness), respectively, and are provided at regular intervals in the circumferential direction in a substantially radial shape from the radial inner peripheral side toward the radial outer peripheral side. Additionally, the blades  6  extend so as to curve toward one direction in the circumferential direction as they go from the radial inner peripheral side of the disc  7  to the radial outer peripheral side, respectively. 
     Additionally, an inner end portion  6   a  of the blade  6  on the radial inner peripheral side is spaced apart from an inner peripheral surface  13   a  of the fitting hole  13  by a predetermined distance G This distance G is appropriately set according to the shapes or the like of the suction portion  9  and the flow passages  3  that communicate with the suction portion  9 , and is set so as to be located closer to the outer peripheral side than an outer peripheral end of the second member  5 . 
     The shroud  8  is a substantially disc-shaped member that is provided integrally with the blades  6  so as to cover the plurality of blades  6  from the first end side. The shroud  8  is formed in the shape of a disc centered on the central axis L. Specifically, the shroud  8  is formed in the shape of an umbrella that is gradually reduced in diameter as it goes to the first end side. Additionally, the radial inner peripheral side of the shroud  8  constitutes a cylindrical portion  14  that rises to the first end side. The cylindrical portion  14  defines the introduction port  2  together with a smaller-diameter surface  17  (refer to  FIG. 5 ) of the second member  5  by combining the first member  4  and the second member  5 . 
     Additionally, a range where the blades  6  are formed, in the other surface  8   a  of the shroud  8  on the second end side, that is, the surface of the shroud  8  that faces the one surface  11   a  of the disc  7 , is formed into a substantially flat surface. That is, as for the other surface  8   a  of the shroud  8 , a cross-section perpendicular to the axial direction in the range where the blades  6  are formed can be drawn in a straight line. 
     As shown in  FIG. 3 , the flow passage  3  is formed between the blades  6 , the first portion  7   a  of the disc  7 , and the shroud  8 . In other words, the flow passage  3  is configured by a space surrounded by the one surface  11   a  of the disc  7 , the other surface  8   a  that is the surface of the shroud  8  on the second end side, and a surface  6   b  of one blade  6  on the other circumferential side, and a surface  6   c  of the other blade  6  on one circumferential side, in the blades  6  that are adjacent to each other. 
     Additionally, in the present embodiment, the blade  6  is provided so as to become substantially perpendicular to the one surface  11   a  of the disc  7 . In other words, the cross-sectional shape of the flow passage  3  defined by the blades  6 , the disc  7 , and the shroud  8  becomes rectangular. That is, the surfaces of the blades  6  that form the flow passage  3  are formed so as to be substantially orthogonal to the one surface  11   a  of the disc  7 . 
     The second member  5  has the second portion  7   b  of the disc  7 , and is a substantially cylindrical member centered on the central axis L. The second member  5  has the outer peripheral surface  16  that is gradually increased in diameter toward the second end side. A radial central portion of the second member  5  is formed with a second fitting hole  15  that has almost the same internal diameter as the fitting hole  13 . Additionally, the other end surface  5   a  of the second member  5  is formed into a flat surface. 
     The outer peripheral surface  16  of the second member  5  includes the smaller-diameter surface  17  and an increased diameter surface  18 . The smaller-diameter surface  17  including an end portion of the second member  5  on the first end side is formed so as to have the same diameter along the axial direction. 
     The increased diameter surface  18  including an end portion of the second member  5  on the second end side is formed into a curved surface that is gradually increased in diameter toward the other end surface  5   a.  The smaller-diameter surface  17  and the increased diameter surface  18  are gently connected. Additionally, the increased diameter surface  18  is formed so that the normal line of the increased diameter surface  18  substantially faces the axial direction on the other end portion. 
     That is, when the first member  4  and the second member  5  are combined, these members are formed so that the increased diameter surface  18  and the one surface  11   a  of the disc  7  are gently connected. 
     Additionally, the diameter of the other end surface  5   a  of the second member  5  is formed so as to become smaller than the internal diameter of the cylindrical portion  14  of the shroud  8 . 
     In addition, the shape of the increased diameter surface  18  may be an oblique surface with a constant angle, without being limited to the curved surface as described above. Additionally, and in particular the smaller-diameter surface  17  does not need to be provided, and the outer peripheral surface  16  may be constituted only by the increased diameter surface  18 . 
     Next, a method for assembling the impeller  1  of the present embodiment to the rotating shaft S will be described. First, as shown in  FIG. 5 , the inner peripheral surface of the fixed portion  12  of the first member  4  is fixed to the rotating shaft S by shrink-fitting. Specifically, the inner peripheral surface of the fitting hole  13  of the first member  4  is heated whereby the fitting hole  13  is increased in diameter, and in this state, the fitting hole  13  is inserted through the rotating shaft S. Then, the first member  4  and the rotating shaft S are integrally anchored by cooling the periphery of the fitting hole  13  to reduce the diameter thereof, and bringing the fitting hole  13  into contact with the outer peripheral surface of the rotating shaft S. 
     Next, similarly to the first member  4 , the second member  5  is fixed to the rotating shaft S by shrink-fitting. In this case, the shrink-fitting is performed after the other end surface  5   a  of the second member  5  is made to abut against the one surface  11   a  of the disc  7  of the first member. 
     In addition, the order of being fixed to the rotating shaft S is not limited to the above-described order, and the first member  4  may be fixed to the rotating shaft S after the second member  5  is fixed to the rotating shaft S. 
     As described above, the impeller  1  is formed by the first member  4  and the second member  5  that are assembled to the rotating shaft S. As the other end surface  5   a  of the second member  5  and the one surface  11   a  of the first member  4  abut against each other, the relative positions of the first member  4  and the second member  5  are determined, and thereby, the introduction port  2  and the suction portion  9  are defined. 
     In addition, the assembling method is not limited to the above-described method, for example, a method for fixing the first member  4  and the second member  5  to the rotating shaft S after the second member  5  is joined to the first member  4  by methods, such as welding, may be used. 
     In the impeller  1  shown above, a fluid that has flowed in from the introduction port  2  is directed to the outer peripheral side from the inner peripheral side by the increased diameter surface  18  of the second member  5  in the suction portion  9 . Next, the fluid that has flowed into the flow passages  3  from the suction portion  9  is accelerated by a centrifugal force generated by the rotation of the rotating shaft S by a driving source that is not shown, and is discharged from the outer peripheral ends of the flow passages  3 . 
     A method for manufacturing the above-described impeller  1  of the present embodiment will be described. The method for manufacturing the impeller  1  related to the present embodiment has a first member forming step of forming the first member  4 , and a second member forming step of forming the second member  5 . 
     The first member forming step has a first base material forming step and a cutting step. First, as shown in  FIG. 6 , as the first base material forming step, a substantially cylindrical base material  30 , which is formed with the fitting hole  13  through which the rotating shaft S is inserted and the fixed portion  12 , is forged. Then, as shown in  FIG. 7 , an inclined surface  8   b  that is the surface of the shroud  8  on the first end side is formed by, for example, lathing or the like to form a disc body  32 . 
     In addition, here, although the base material  30  is subjected to lathing or the like so as to form the disc body  32 , the disc body  32  may be formed only by forging. Additionally, here, although the cylindrical base material  30  which is formed with the fitting hole  13  and the fixed portion  12  by forging, is adopted, the fitting hole  13  and the fixed portion  12  are subjected to lathing or the like, for example, using a disc-shaped base material. 
     Next, as shown in  FIG. 8 , as the cutting step, the flow passage  3  is formed from the outer peripheral side of the disc body  32 . Specifically, the flow passage  3  is formed by inserting an electrode  33  corresponding to the shape of the flow passage  3  from a position to be used as an outlet of the flow passage  3 , by a spark erosion method. 
     Here, the electrode  33  is a rectangular elongated member as viewed from the cross-section thereof. Additionally, the electrode  33  has a shape having a height smaller than the height of the flow passage  3 , and has a curved shape and a width dimension corresponding to a shape viewed from the axial direction of the flow passage  3 . Additionally, the electrode  33  is formed from, for example, graphite, copper, or the like, and is attached to an electrical discharge machine that is not shown. 
     As for spark erosion, first, the disc body  32  is dipped in, for example, spark erosion oil that is not shown. Next, as shown in  FIG. 8 , the disc body  32  and the flow passage  3  are relatively moved in the radial direction and the circumferential direction, respectively, while a portion that becomes the flow passage  3  are inserted using the electrode  33 . Additionally, the disc body is also moved in the axial direction if necessary, and spark erosion is performed. In addition, in this case, the machining conditions (a current, a voltage, a pulse, and a feed rate) of the spark erosion by the electrode  33  may be appropriately changed. 
     A plurality of the flow passages  3  are formed by repeatedly carrying out the steps shown above, regarding each flow passage  3  to be formed in the impeller  1 . 
     Next, as shown in  FIG. 9 , the electrode  33  is inserted from the first end side, and the inner peripheral surface of the shroud  8  is machined. 
     In addition, in the present embodiment, spark erosion is performed by one type of electrode  33 . However, the electrode is not limited to this. For example, roughing, intermediate machining, and finishing may be performed using two or more types of electrodes with different sizes or materials. 
     Next, in the second member forming step, the second member  5  (refer to  FIG. 5 ) is formed by performing lathing of the cylindrical base material. In the second member forming step, the outer peripheral surface  16  that has the curved increased diameter surface  18  that goes to the radial outer peripheral side is formed as it goes from the second direction to the first direction in the axial direction in the second member  5 . 
     In addition, not only the second member  5  may be obtained by performing lathing of the base material but the second member  5  may be formed only by forging. 
     According to the above embodiment, the shape of the flow passage  3  formed by the blades  6 , the disc  7 , and the shroud  8  of the first member  4  forms a substantially straight shape as viewed from the circumferential direction. Therefore, the spark erosion using the straight electrode  33  becomes easier. Since the second member  5  equivalent to a hub portion of related art for directing the air introduced in the axial direction to the radial direction is a separate member, machining of the introduction port  2  in the vicinity of the impeller  1  becomes easier. 
     In other words, when the flow passage  3  of the first member  4  is formed by splitting the impeller  1  into the first member  4  and the second member  5  and making the second member  5  into a part that constitutes the portion of the disc  7  on the first end side, the accessibility of the electrode  33  improves. That is, when the electrode  33  is inserted from a position to be used as the outlet of the flow passage  3 , the machining when forming the flow passage  3  becomes easy by making a part that becomes an interference object on the inner peripheral side into a separate second member  5 . Additionally, when the electrode  33  is inserted from a position to be used as the introduction port  2  that is the inlet of the flow passage  3 , the machining of the introduction port  2  becomes easy by making a part that becomes an interference object on the second end side into a separate second member  5 . Thereby, the manufacturing time can be shortened, and the manufacturing costs can be kept down. 
     Additionally, since the fluid introduced into the impeller  1  can be guided to the flow passage  3  without any delay by the increased diameter surface  18  in the second member  5 , the compression performance of the impeller  1  can be maintained. 
     Additionally, since the first member  4  and the second member  5  are separately shrink-fitted and fixed to the rotating shaft S, respectively, the impeller  1  can be more firmly fixed to the rotating shaft S as compared to a case where any member is fixed to the rotating shaft S after the first member  4  and the second member  5  are integrated. 
     Additionally, the surface of the blades  6  that forms the flow passage  3  are formed so as to be orthogonal to the disc  7  whereby the shape of the blades  6  are further simplified as compared to a shape where the surfaces of the blades  6  that form the flow passage  3  inclines with respect to the disc  7 . Therefore, the man-hours of machining when the flow passage  3  is formed can be further reduced. 
     In addition, the technical scope of the present invention is not limited to the above embodiment, and various changes can be made without departing from the scope of the present invention. 
     In the above embodiment, the other end surface  5   a  of the second member  5  and the one surface  11   a  of the first member  4  are made into substantially flat surfaces, respectively. In contrast, as shown in  FIG. 10 , a configuration in which the other end surface  5 Ba of a second member  5 B that constitutes an impeller  1 B is provided with a convex portion  20  of a shape that extends the other end surface  5 Ba to the second end side, and a concave portion  21  corresponding to the convex portion  20  is provided in one surface  11 Ba of a first member  4 B may be adopted. 
     Here, although the second member  5 B and the first member  4 B are fixed by shrink-fitting or the like in the other end surface  5 Ba of the second member  5 B, it is not necessary to fix the second member  5 B and the rotating shaft S. 
     Since the outer peripheral side of the other end surface  5 Ba of the second member  5 B does not have a thin-walled shape in such a configuration, machining of the second member  5 B becomes easy. 
     Additionally, as shown in  FIG. 11 , the first member  4 C that constitutes the impeller  1 C is not necessarily provided with a fixed portion that extends to the second side of the disc  7 C. In the case of this form, it is preferable that the dimension d from the end portion of the second member  5 C on the second end side be as great as possible within a range where the fixation strength between the first member  4 C and the rotating shaft S can be sufficiently secured. As the dimension d is enlarged, the accessibility of a machining member during machining of the flow passage  3 C and the introduction port  2 C is increased, which is preferable. 
     Moreover, in  FIG. 10 , the fixed portion is not necessarily provided even in a case where the second member  5 B is, for example, shrink-fitted to the rotating shaft S and the first member  4 B. 
     Additionally, the method for machining the flow passage or the like may not be limited to the spark erosion, and a flow passage or the like may be worked by machining. 
     INDUSTRIAL APPLICABILITY 
     According to the impeller of the present invention, the accessibility of a machining tool improves when a flow passage portion of the impeller is formed. Therefore, manufacturing time can be shortened. Additionally, in the impeller of the present invention, manufacturing costs can be kept down. 
     REFERENCE SIGNS LIST 
     S: ROTATING SHAFT 
       1 : IMPELLER 
       3 : FLOW PASSAGE 
       4 : FIRST MEMBER 
       5 : SECOND MEMBER 
       6 : BLADE 
       7 : DISC 
       8 : SHROUD 
       8 A: OTHER SURFACE 
       11 : DISC BODY PORTION 
       11 A: ONE SURFACE 
       12 : FIXED PORTION 
       18 : INCREASED DIAMETER SURFACE 
       50 : CENTRIFUGAL COMPRESSOR (ROTARY MACHINE)