Patent Publication Number: US-2016233750-A1

Title: Rotor core heating device and rotor core shrink-fitting method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rotor core heating device and a rotor core shrink-fitting method. 
     2. Description of Related Art 
     A rotor core is a component of a motor. The motor is constituted by a shaft rotatably supported in a sealed case and having a rotor formed integrally at one end portion, a rotor core externally fitted on the shaft, and a stator fixed to the sealed case side to face the outer peripheral surface of the rotor with a predetermined gap therebetween. 
     In order to manufacture the motor, it is necessary to externally fit the rotor core onto the shaft. A shrink-fitting method is known as a method of externally fitting the rotor core. In shrink-fitting the rotor core onto the shaft, the rotor core is heated by a rotor core heating device, and the heated rotor core is cooled after being fitted onto the shaft. 
     For example, Japanese Patent Application Publication No. 07-022168 (JP 07-022168 A) and Japanese Patent Application Publication No. 2013-102622 (JP 2013-102622 A) disclose a rotor core heating device including a first heater that heats the inner peripheral side surface of a hollow cylindrical rotor core with a coil through induction heating, and a second heater that heats the outer peripheral side surface of the hollow cylindrical rotor core with a coil through induction heating. 
     The configuration of a rotor core heating device  500  according to the related art represented by JP 07-022168 A will be described with reference to  FIG. 10A  and  FIG. 10B . In  FIG. 10A  and  FIG. 10B , the configuration of the rotor core heating device  500  according to the related art is schematically illustrated as viewed in a cross section. In the following, description is made with reference to the axial direction indicated in  FIG. 10A  and  FIG. 10B . 
     The rotor core heating device  500  is a device that heats a rotor core  550  through induction heating to shrink-fit the rotor core  550  onto a shaft (not illustrated). The rotor core heating device  500  includes an inner coil  510 , an outer coil  520 , and an induction heater (not illustrated). 
     The rotor core  550  is formed to have a cylindrical shape, and includes a hollow portion  560  formed to extend in the axial direction (see  FIG. 10A ). The rotor core  550  is constituted by stacking a plurality of steel plates. 
     The inner coil  510  is formed to have a spiral shape, and disposed on the inner peripheral side of the rotor core  550  (in the hollow portion  560 ). The inner coil  510  is disposed in the hollow portion  560  so as to extend spirally in the axial direction. 
     The outer coil  520  is formed to have a spiral shape, and disposed on the outer peripheral side of the rotor core  550 . The outer coil  520  is disposed around the outer periphery of the rotor core  550  so as to extend spirally in the axial direction. 
     The induction heater applies an alternating current to the inner coil  510  and the outer coil  520  to generate magnetic force lines around the inner coil  510  and the outer coil  520 . 
     In  FIG. 10A , the length of the rotor core  550  in the axial direction is generally the same as the length of the inner coil  510  and the outer coil  520  in the axial direction. In  FIG. 10B , meanwhile, the length of a rotor core  580  in the axial direction is shorter than the length of the inner coil  510  and the outer coil  520  in the axial direction. 
     The function of the rotor core heating device  500  according to the related art will be described with reference to  FIG. 11 . In  FIG. 11 , the function of the rotor core heating device  500  according to the related art is schematically illustrated as viewed in the cross-section. In  FIG. 11 , the length of the rotor core  580  in the axial direction is shorter than the length of the inner coil  510  and the outer coil  520  in the axial direction. 
     When magnetic force lines are generated around the inner coil  510  and the outer coil  520 , the rotor core  580  disposed in the vicinity is affected by the magnetic force lines so that an eddy current flows in the rotor core  580 . When a current flows in the rotor core  580 , Joule heat is generated because of the electrical resistance of the rotor core  580  so that the rotor core  580  is self-heated. 
     In  FIG. 11 , as described above, the length of the rotor core  580  in the axial direction is shorter than the length of the inner coil  510  and the outer coil  520  in the axial direction. When the rotor core  580  is affected by the magnetic force lines, magnetic flux concentrates on the upper end surface of the rotor core  580  in the axial direction (location C in  FIG. 11 ), which may cause a curl of a steel plate positioned at the upper end portion of the rotor core  580  due to abnormal heat generation. 
     For example, in the case where a steel plate is curled, the curled steel plate is thermally insulated from the other steel plates. Thus, the steel plate is further curled to reach a plastic region, which may deform the rotor core  580 . 
     Therefore, in the related art, it is necessary to prepare dedicated rotor core heating devices corresponding to various lengths of a rotor core in the axial direction, which may increase the equipment cost. Thus, there is desired a general-purpose rotor core heating device capable of accommodating differences in length of a rotor core in the axial direction. 
     SUMMARY OF THE INVENTION 
     The present invention provides a rotor core heating device and a rotor core shrink-fitting method capable of accommodating differences in length of a rotor core in the axial direction. 
     A rotor core heating device according to a first aspect of the present invention is configured to heat an inner peripheral side surface and an outer peripheral side surface of a rotor core through induction heating. The rotor core has a hollow cylindrical shape. The rotor core heating device includes a first coil, a second coil and a magnetic flux shielding jig. The first coil is disposed inside the rotor core and is configured to heat the inner peripheral side surface of the rotor core through induction heating. The second coil is disposed outside the rotor core and is configured to heat the outer peripheral side surface of the rotor core through induction heating. The magnetic flux shielding jig has a hollow cylindrical shape and is disposed opposite a first end surface of the rotor core with a gap provided between the first end surface and the magnetic flux shielding jig in an axial direction of the rotor core. 
     In the rotor core heating device according to the first aspect of the present invention, the magnetic flux shielding jig may include a first magnetic flux shielding jig that is opposite to the first end surface, and a second magnetic flux shielding jig that is opposite to a second end surface of the rotor core. The first magnetic flux shielding jig is disposed with the gap provided between the first end surface and the first magnetic flux shielding jig in the axial direction. The second magnetic flux shielding jig is disposed with a gap provided between the second end surface and the second magnetic flux shielding jig in the axial direction. Furthermore, both ends of the first coil in the axial direction may project from the rotor core. 
     With the rotor core heating device described above, differences in length of the rotor core in the axial direction can be accommodated. 
     In the rotor core heating device according to the first aspect of the present invention, a through portion that penetrates in the axial direction may be formed in the magnetic flux shielding jig. 
     With the rotor core heating device described above, the inside of the rotor core can be reliably heated. 
     In the rotor core heating device according to the first aspect of the present invention, the magnetic flux shielding jig may be made of copper. 
     A rotor core shrink-fitting method according to a second aspect of the present invention includes: heating a rotor core with the rotor core heating device according to the first aspect of the present invention to increase an inside diameter of the rotor core; and shrink-fitting the rotor core, an inside diameter of which has been increased, onto a shaft to fasten the rotor core to the shaft. 
     With the rotor core shrink-fitting method described above, differences in length of the rotor core in the axial direction can be accommodated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a schematic view illustrating the configuration of a rotor core heating device according to a first embodiment of the present invention; 
         FIG. 2  is a schematic view illustrating the function of the rotor core heating device according to the first embodiment of the present invention; 
         FIG. 3  is a schematic view illustrating the function of the rotor core heating device according to the first embodiment of the present invention; 
         FIG. 4  is a schematic view illustrating the configuration of a rotor core heating device according to a second embodiment of the present invention; 
         FIG. 5  is a schematic view illustrating the function of the rotor core heating device according to the second embodiment of the present invention; 
         FIG. 6A  is a schematic view illustrating the configuration of a magnetic flux shielding jig according to a third embodiment of the present invention; 
         FIG. 6B  is a schematic view illustrating the configuration of a rotor core according to the third embodiment of the present invention; 
         FIG. 7  is a schematic view illustrating the configuration of a rotor core heating device according to the third embodiment of the present invention; 
         FIG. 8  is a schematic view illustrating the function of the rotor core heating device according to the third embodiment of the present invention; 
         FIG. 9A  is a schematic view illustrating the configuration of another magnetic flux shielding jig according to a fourth embodiment of the present invention; 
         FIG. 9B  is a schematic view illustrating the configuration of a rotor core according to the fourth embodiment of the present invention; 
         FIG. 10A  is a schematic view illustrating the configuration of a rotor core heating device according to the related art; 
         FIG. 10B  is a schematic view illustrating the configuration of a rotor core heating device according to the related art; and 
         FIG. 11  is a schematic view illustrating the function of the rotor core heating device according to the related art. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     The configuration of a rotor core heating device  100  will be described with reference to  FIG. 1 . In  FIG. 1 , the configuration of the rotor core heating device  100  is schematically illustrated as viewed in the cross-section. In the following, description is made with reference to the axial direction indicated in  FIG. 1 . 
     The rotor core heating device  100  is a rotor core heating device according to a first embodiment of the present invention. The rotor core heating device  100  is a device that heats a rotor core  150  through induction heating to shrink-fit the rotor core  150  onto a shaft (not illustrated). 
     The rotor core  150  is a component of a motor (not illustrated). The motor is constituted by a shaft (not illustrated), the rotor core  150  externally fitted on the shaft, and a stator (not illustrated). The shaft is rotatably supported in a sealed case (not illustrated) and has a rotor formed integrally at one end portion. The stator is fixed to the sealed case side to face the outer peripheral surface of the rotor with a predetermined gap therebetween. 
     In order to manufacture the motor, it is necessary to externally fit the rotor core  150  onto the shaft. A shrink-fitting method is known as a method of externally fitting the rotor core  150 . In shrink-fitting the rotor core  150  onto the shaft, the rotor core  150  is heated by the rotor core heating device  100 , and the heated rotor core  150  is cooled after being fitted onto the shaft. 
     The rotor core heating device  100  includes an inner coil  110 , an outer coil  120 , an induction heater (not illustrated), and a magnetic flux shielding jig  170 . The rotor core  150  is formed to have a cylindrical shape, and includes a hollow portion  160  formed to extend in the axial direction. The rotor core  150  is constituted by stacking a plurality of steel plates. 
     The inner coil  110  is formed to have a spiral shape, and disposed on the inner peripheral side of the rotor core  150  (in the hollow portion  160 ). The inner coil  110  is disposed in the hollow portion  160  so as to extend spirally in the axial direction. 
     The outer coil  120  is formed to have a spiral shape, and disposed on the outer peripheral side of the rotor core  150 . The outer coil  120  is disposed around the outer periphery of the rotor core  150  so as to extend spirally in the axial direction. 
     The induction heater applies an alternating current to the inner coil  110  and the outer coil  120  to generate magnetic force lines around the inner coil  110  and the outer coil  120 . 
     The magnetic flux shielding jig  170  is formed to have a cylindrical shape, and includes a hollow portion  180  formed to extend in the axial direction. The magnetic flux shielding jig  170  is made of copper. The cross-sectional shape of the magnetic flux shielding jig  170  as viewed in the axial direction is generally the same as the cross-sectional shape of the rotor core  150 . 
     The magnetic flux shielding jig  170  is disposed above the rotor core  150  in the axial direction when the rotor core  150  is heated by the rotor core heating device  100 . The magnetic flux shielding jig  170  is disposed with a gap provided between the rotor core  150  and the magnetic flux shielding jig  170  so as not to contact the rotor core  150 . In the present embodiment, the sum of the length of the magnetic flux shielding jig  170  in the axial direction and the length of the rotor core  150  in the axial direction is generally the same as the length of the inner coil  110  and the outer coil  120  in the axial direction. 
     Preferably, the length of the inner coil  110  and the outer coil  120  in the axial direction is generally the same as the length of the longest rotor core, among rotor cores assumed to be heated, in the axial direction. 
     On the inner peripheral side of the axial end surface of the rotor core  150 , magnetic flux tends to concentrate to generate abnormal heat. On the outer peripheral side of the axial end surface of the rotor core  150 , on the other hand, magnetic flux is less likely to concentrate to generate abnormal heat than on the inner peripheral side. Therefore, although the outer shape of the magnetic flux shielding jig  170  is generally the same as the outer shape of the rotor core  150 , the outside diameter of the magnetic flux shielding jig  170  may be larger than the outside diameter of the rotor core  150 . 
     The function of the rotor core heating device  100  will be described with reference to  FIG. 2  and  FIG. 3 . In  FIG. 2  and  FIG. 3 , the function of the rotor core heating device  100  is schematically illustrated as viewed in the cross-section. In  FIG. 2 , magnetic flux lines are indicated by dash-double-dot lines. 
     When magnetic force lines are generated around the inner coil  110  and the outer coil  120 , the rotor core  150  disposed in the vicinity is affected by the magnetic force lines so that an eddy current flows in the rotor core  150 . When a current flows in the rotor core  150 , Joule heat is generated because of the electrical resistance of the rotor core  150  so that the rotor core  150  is self-heated. 
     At this time, the magnetic flux shielding jig  170  is disposed above the rotor core  150  in the axial direction, and therefore concentration of magnetic flux on the upper end surface of the rotor core  150  in the axial direction is prevented. Magnetic flux is distributed as if the length of the rotor core  150  in the axial direction were generally the same as the length of the inner coil  110  and the outer coil  120  in the axial direction. 
     Therefore, magnetic flux does not concentrate on the upper end surface of the rotor core  150  in the axial direction (location A in  FIG. 3 ), which prevents a curl of a steel plate from occurring because of abnormal heat generation. 
     The effect of the rotor core heating device  100  will be described. According to the rotor core heating device  100 , differences in length of the rotor core  150  in the axial direction can be accommodated by preparing a plurality of types of the magnetic flux shielding jig  170  corresponding to various lengths of the rotor core  150  in the axial direction based on differences in lengths of the rotor core  150  in the axial direction. 
     That is, differences in length of the rotor core  150  in the axial direction can be accommodated by preparing a plurality of types of the magnetic flux shielding jig  170  such that the sum of the length of a magnetic flux shielding jig  170  in the axial direction and the length of the rotor core  150  in the axial direction is generally the same as the length of the inner coil  110  and the outer coil  120  in the axial direction for each set of the inner coil  110  and the outer coil  120 . 
     In the present embodiment, the magnetic flux shielding jig  170  is made of cupper. However, the present invention is not limited thereto. For example, if the magnetic flux shielding jig  170  is made of any magnetic material such as iron, the same function and effect as those of the first embodiment can be obtained. 
     In the present embodiment, the sum of the length of the magnetic flux shielding jig  170  in the axial direction and the length of the rotor core  150  in the axial direction is generally the same as the length of the inner coil  110  and the outer coil  120  in the axial direction. However, the present invention is not limited thereto. 
     The sum of the length of the magnetic flux shielding jig  170  in the axial direction and the length of the rotor core  150  in the axial direction may be longer than the length of the inner coil  110  and the outer coil  120  in the axial direction. Alternatively, the sum of the length of the magnetic flux shielding jig  170  in the axial direction and the length of the rotor core  150  in the axial direction may be shorter than the length of the inner coil  110  and the outer coil  120  in the axial direction. In either case, the same function and effect as those of the first embodiment can be obtained. 
     Second Embodiment 
     The configuration of a rotor core heating device  200  will be described with reference to  FIG. 4 . In  FIG. 4 , the configuration of the rotor core heating device  200  is schematically illustrated as viewed in the cross-section. In the following, description is made with reference to the axial direction indicated in  FIG. 4 . 
     The rotor core heating device  200  is a rotor core heating device according to a second embodiment of the present invention. The rotor core heating device  200  is a device that heats a rotor core  250  through induction heating to shrink-fit the rotor core  250  onto a shaft (not illustrated). 
     The rotor core  250  is a component of a motor (not illustrated). The motor is constituted by a shaft (not illustrated), the rotor core  250  externally fitted on the shaft, and a stator (not illustrated). The shaft is rotatably supported in a sealed case (not illustrated) and has a rotor formed integrally at one end portion. The stator is fixed to the sealed case side to face the outer peripheral surface of the rotor with a predetermined gap therebetween 
     In order to manufacture the motor, it is necessary to externally fit the rotor core  250  onto the shaft. A shrink-fitting method is known as a method of externally fitting the rotor core  250 . In shrink-fitting the rotor core  250  onto the shaft, the rotor core  250  is heated by the rotor core heating device  200 , and the heated rotor core  250  is cooled after being fitted onto the shaft. 
     The rotor core heating device  200  includes an inner coil  210 , an outer coil  220 , an induction heater (not illustrated), and magnetic flux shielding jigs  270 . The rotor core  250  is formed to have a cylindrical shape, and includes a hollow portion  260  formed to extend in the axial direction. The rotor core  250  is constituted by stacking a plurality of steel plates. 
     The inner coil  210  is formed to have a spiral shape, and disposed on the inner peripheral side of the rotor core  250  (in the hollow portion  260 ). The inner coil  210  is disposed in the hollow portion  260  so as to extend spirally in the axial direction. The length of the inner coil  210  in the axial direction is longer than the length of the rotor core  250  in the axial direction. 
     The inner coil  210  is disposed with respect to the rotor core  250  such that both the upper and lower ends of the inner coil  210  in the axial direction project from the rotor core  250 . More particularly, the inner coil  210  is preferably disposed at a position at which the middle portion of the inner coil  210  and the middle portion of the rotor core  250 , generally coincide with each other in the axial direction. 
     The outer coil  220  is formed to have a spiral shape, and disposed on the outer peripheral side of the rotor core  250 . The outer coil  220  is disposed around the outer periphery of the rotor core  250  so as to extend spirally in the axial direction. 
     The induction heater applies an alternating current to the inner coil  210  and the outer coil  220  to generate magnetic force lines around the inner coil  210  and the outer coil  220 . 
     The magnetic flux shielding jigs  270  are formed to have a cylindrical shape, and include a hollow portion  280  formed to extend in the axial direction. The magnetic flux shielding jigs  270  are made of copper. The cross-sectional shape of the magnetic flux shielding jigs  270  as viewed in the axial direction is generally the same as the cross-sectional shape of the rotor core  250 . 
     The magnetic flux shielding jigs  270  are disposed above and below the rotor core  250  in the axial direction when the rotor core  250  is heated by the rotor core heating device  200 . The magnetic flux shielding jigs  270  are disposed with a gap provided between the rotor core  250  and each of the magnetic flux shielding jigs  270  so as not to contact the rotor core  250 . 
     On the inner peripheral side of the axial end surface of the rotor core  250 , magnetic flux tends to concentrate to generate abnormal heat. On the outer peripheral side of the axial end surface of the rotor core  250 , on the other hand, magnetic flux is less likely to concentrate to generate abnormal heat than on the inner peripheral side. Therefore, although the outer shape of the magnetic flux shielding jigs  270  is generally the same as the outer shape of the rotor core  250 , the outside diameter of the magnetic flux shielding jigs  270  may be larger than the outside diameter of the rotor core  250 . 
     The function of the rotor core heating device  200  will be described with reference to  FIG. 5 . In  FIG. 5 , the function of the rotor core heating device  200  is schematically illustrated as viewed in the cross-section. 
     When magnetic force lines are generated around the inner coil  210  and the outer coil  220 , the rotor core  250  disposed in the vicinity is affected by the magnetic force lines so that an eddy current flows in the rotor core  250 . When a current flows in the rotor core  250 , Joule heat is generated because of the electrical resistance of the rotor core  250  so that the rotor core  250  is self-heated. 
     At this time, the magnetic flux shielding jigs  270  are disposed above and below the rotor core  250  in the axial direction, and therefore concentration of magnetic flux on the upper end surface and the lower end surface of the rotor core  250  in the axial direction is prevented. Magnetic flux is distributed as if the length of the rotor core  250  in the axial direction were generally the same as the sum of the respective lengths, in the axial direction, of the magnetic flux shielding jig  270  disposed on the upper side and the magnetic flux shielding jig  270  disposed on the lower side. 
     Therefore, magnetic flux does not concentrate on the upper end surface or the lower end surface of the rotor core  250  in the axial direction (location B in  FIG. 5 ), which prevents a curl of a steel plate from occurring because of abnormal heat generation. In addition, the magnetic flux shielding jigs  270  are disposed above and below the rotor core  250  in the axial direction, and thus the rotor core  250  generates a magnetic field that is uniform in the axial direction. Consequently, the rotor core  250  is heated uniformly in the axial direction so that the inside diameter of the rotor core  250  is increased uniformly. 
     The effect of the rotor core heating device  200  will be described. According to the rotor core heating device  200 , differences in length of the rotor core  250  in the axial direction can be accommodated. That is, differences in length of the rotor core  250  in the axial direction can be accommodated by disposing the magnetic flux shielding jigs  270  above and below the rotor core  250  if the rotor core  250  has a length, in the axial direction, that is shorter than the length of the inner coil  210  in the axial direction, for each set of the inner coil  210  and the outer coil  220 . 
     In the rotor core heating device  200  in which the magnetic flux shielding jigs  270  are disposed above and below the rotor core  250  in the axial direction, in addition, a magnetic field that is uniform in the axial direction of the rotor core  250  is generated in contrast to the rotor core heating device  100  according to the first embodiment. Consequently, the rotor core  250  can be heated uniformly in the axial direction so that the inside diameter of the rotor core  250  can be increased uniformly. 
     In the present embodiment, the magnetic flux shielding jigs  270  are made of cupper. However, the present invention is not limited thereto. For example, if the magnetic flux shielding jigs are made of any magnetic material such as iron, the same function and effect as those of the second embodiment can be obtained. 
     A rotor core shrink-fitting method according to an embodiment of the present invention will be described. The rotor core shrink-fitting method according to the embodiment includes: heating the rotor core  150  or the rotor core  250  with the rotor core heating device  100  or the rotor core heating device  200  to increase the inside diameter of the rotor core  150  or the rotor core  250 ; and shrink-fitting the rotor core  150  or the rotor core  250 , the inside diameter of which has been increased, onto a shaft to fasten the rotor core  150  or the rotor core  250  to the shaft. 
     Third Embodiment 
     If the magnetic flux shielding jig  170  is disposed above the rotor core  150  in the axial direction in the rotor core heating device  100 ′ according to the first embodiment, magnetic flux that passes through the inside of the rotor core  150  may be blocked so that the inside of the rotor core  150  may be heated to a reduced degree. 
     That is, the rotor core heating device  100  according to the first embodiment has room for improvement of the working efficiency in reliably heating the inside of the rotor core  150  and shortening the heating time. 
     The configuration of a rotor core  50  and a magnetic flux shielding jig  350  according to a third embodiment of the present invention will be described with reference to  FIG. 6A  and  FIG. 6B .  FIG. 6A  is a perspective view schematically illustrating the configuration of the magnetic flux shielding jig  350 .  FIG. 6B  is a perspective view schematically illustrating the configuration of the rotor core  50 . In the following, description is made with reference to the axial direction and the circumferential direction indicated in  FIG. 6A  and  FIG. 6B . 
     The rotor core  50  is a rotor core according to the third embodiment of the present invention. The rotor core  50  is to be heated by a rotor core heating device  300  to be discussed later. 
     The rotor core  50  is a component of a motor (not illustrated). The motor is constituted by a shaft (not illustrated), the rotor core  50  externally fitted on the shaft, and a stator (not illustrated). The shaft is rotatably supported in a sealed case (not illustrated) and has a rotor formed integrally at one end portion. The stator is fixed to the sealed case side to face the outer peripheral surface of the rotor with a predetermined gap therebetween. 
     In order to manufacture the motor, it is necessary to externally fit the rotor core  50  onto the shaft. A shrink-fitting method is known as a method of externally fitting the rotor core  50 . In shrink-fitting the rotor core  50  onto the shaft, the rotor core  50  is heated by the rotor core heating device  300 , and the heated rotor core  50  is cooled after being fitted onto the shaft. 
     The rotor core  50  is constituted by stacking a plurality of steel plates, and formed to have a hollow cylindrical shape. The rotor core  50  has a hollow portion  60  formed to penetrate in the axial direction. 
     The hollow portion  60  is a hole into which a shaft is inserted when the rotor core  50  is assembled into the motor. The hollow portion  60  is formed in the center portion of the rotor core  50  to have a circular shape as viewed in a plan. 
     The magnetic flux shielding jig  350  is formed to have a hollow cylindrical shape, and disposed above the rotor core  50  in the axial direction when the rotor core  50  is heated by the rotor core heating device  300 . The magnetic flux shielding jig  350  is constituted to have a generally cylindrical shape. The magnetic flux shielding jig  350  has a hollow portion  360  that penetrate in the axial direction, and a plurality of through holes  370  that serve as a through portion. 
     The hollow portion  360  is formed in the center portion of the magnetic flux shielding jig  350  to have a circular shape as viewed in the plan. The hollow portion  360  is formed to have generally the same diameter as the hollow portion  60  of the rotor core  50 , and formed generally at the same position as the hollow portion  60  of the rotor core  50  as viewed in the plan when the magnetic flux shielding jig  350  is disposed above the rotor core  50  in the axial direction and generally coaxially with the rotor core  50 . 
     The plurality of through holes  370  are disposed at equal intervals in the circumferential direction generally at the edge portion of the magnetic flux shielding jig  350  on the outer peripheral side as viewed in the plan. 
     The configuration of a rotor core heating device  300  will be described with reference to  FIG. 7 . In  FIG. 7 , the configuration of the rotor core heating device  300  is schematically illustrated as viewed in the cross-section. In the following, description is made with reference to the axial direction indicated in  FIG. 7 . 
     The rotor core heating device  300  is a rotor core heating device according to an embodiment of the present invention. The rotor core heating device  300  is a device that heats a rotor core  50  through induction heating to shrink-fit the rotor core  50  onto a shaft (not illustrated). 
     The rotor core heating device  300  includes an inner coil  310 , an outer coil  320 , an induction heater (not illustrated), and the magnetic flux shielding jig  350  discussed above. 
     The inner coil  310  is formed to have a spiral shape, and disposed on the inner peripheral side of the rotor core  50  (in the hollow portion  60 ). The inner coil  310  is disposed in the hollow portion  60  so as to extend spirally in the axial direction. 
     The outer coil  320  is formed to have a spiral shape, and disposed on the outer peripheral side of the rotor core  50 . The outer coil  320  is disposed around the outer periphery of the rotor core  50  so as to extend spirally in the axial direction. 
     The induction heater applies an alternating current to the inner coil  310  and the outer coil  320  to generate magnetic force lines around the inner coil  310  and the outer coil  320 . 
     The magnetic flux shielding jig  350  is disposed above the rotor core  50  in the axial direction when the rotor core  50  is heated by the rotor core heating device  300 . 
     The magnetic flux shielding jig  350  is disposed with a gap provided between the rotor core  50  and the magnetic flux shielding jig  350  so as not to contact the rotor core  50 . In the present embodiment, the sum of the length of the magnetic flux shielding jig  350  in the axial direction and the length of the rotor core  50  in the axial direction is generally the same as the length of the inner coil  310  and the outer coil  320  in the axial direction. 
     In the present embodiment, the magnetic flux shielding jig  350  is disposed above the rotor core  50  in the axial direction. However, the present invention is not limited thereto. For example, the magnetic flux shielding jig  350  may be disposed below the rotor core  50  in the axial direction. 
     The function of the rotor core heating device  300  will be described with reference to  FIG. 8 . In  FIG. 8 , the function of the rotor core heating device  300  is schematically illustrated as viewed in the cross-section. In  FIG. 8 , in addition, magnetic flux lines are indicated by dash-double-dot lines. 
     When magnetic flux is generated around the inner coil  310  and the outer coil  320 , the rotor core  50  disposed in the vicinity is affected by the magnetic flux so that an eddy current flows in the rotor core  50 . When a current flows in the rotor core  50 , Joule heat is generated because of the electrical resistance of the rotor core  50  so that the rotor core  50  is self-heated. 
     It is assumed that magnetic flux is generated from at least one of the inner coil  310  and the outer coil  320 . 
     In the rotor core heating device  300 , the plurality of through holes  370  are formed in the magnetic flux shielding jig  350  as viewed in the plan. Therefore, magnetic flux is not blocked by the magnetic flux shielding jig  350 , but passes through the through holes  370  of the magnetic flux shielding jig  350 . Therefore, the inside of the rotor core  50  is sufficiently heated. 
     The effect of the rotor core heating device  300  will be described. According to the rotor core heating device  300 , the inside of the rotor core  50  can be reliably heated. That is, the inside of the rotor core  50  is sufficiently heated by forming the through holes  370  in the magnetic flux shielding jig  350  and allowing magnetic flux to pass through the through holes  370 . 
     Fourth Embodiment 
     The configuration of a rotor core  50  and a magnetic flux shielding jig  450  according to a fourth embodiment of the present invention will be described with reference to  FIG. 9A  and  FIG. 9B .  FIG. 9A  is a perspective view schematically illustrating the configuration of the magnetic flux shielding jig  450 .  FIG. 9B  is a perspective view schematically illustrating the configuration of the rotor core  50 . 
     The rotor core  50  has the configuration discussed above, and will not be described in detail. 
     The magnetic flux shielding jig  450  is constituted by an inner peripheral portion  451  and an outer peripheral portion  452 . The inner peripheral portion  451  is formed to have a hollow cylindrical shape. The outer peripheral portion  452  is also formed to have a hollow cylindrical shape. The inner peripheral portion  451  is disposed inside the outer peripheral portion  452 . The inner peripheral portion  451  and the outer peripheral portion  452  are disposed with a predetermined gap D, which serves as a through portion, provided therebetween. 
     A rotor core heating device, having the magnetic flux shielding jig  450  configured in this way achieves the same function and effect as those of the rotor core heating device  300 . 
     The technical features of the first to fourth embodiments described above may be used in appropriate combination.