Patent Publication Number: US-2013234557-A1

Title: Rotor for electric motor including rotational shaft and yoke securely fitted on the rotational shaft

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rotor for an electric motor including a rotational shaft and a yoke fitted on the rotational shaft. 
     2. Description of the Related Art 
     In a rotor for an electric motor, fitting between a yoke and a shaft is generally carried out by means of shrinkage fit. However, in a rotor for a small-sized electric motor with a thinner yoke, fitting allowance has to be strictly controlled so that the yoke undergoes elastic deformation during a shrinkage fit process. Thus, it is necessary to form an inner diameter of the yoke and an outer diameter of the shaft with high precision. Such forming with high precision tends to increase the cost. In addition, it is difficult to increase precision of a punching process, which is generally carried out in order to form the yoke as a stacked structure of steel plates. Without a sufficient degree of precision, the yoke undergoes plastic deformation, which could result in reduced fastening force, and therefore misalignment of the yoke. 
     It is known to fit a yoke and a shaft on each other by means of an adhesive. However, the process is rather complicated because of a need to control the amount of adhesive, clean the yoke and the shaft, and remove excessive adhesives, and therefore, quality control may be challenging. It is also known to fasten a yoke and a shaft together by fitting a key into a keyway. However, a key structure for fastening the yoke and the shaft provides relatively small fastening force in an axial direction, while providing large fastening force in a rotational direction. Therefore, it is necessary to provide separate fixing means in an axial direction, complicating the structure. JP-A-61-266041 and JP-A-2002-295500, which disclose the related art, should also be referred to. 
     Therefore, there is a need for a motor for an electric motor including a rotational shaft and a yoke fitting on the rotational shaft, in which separate fixing means and adhesives are not required. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, a rotor for an electric motor comprises: a rotational shaft having a cylindrical contour and capable of rotating around an axis; and a yoke fitted on an outer circumferential surface of the rotational shaft, wherein the rotational shaft has on the outer circumferential surface at least one concave portion or convex portion extending in parallel to the axis, wherein the yoke has on an inner circumferential surface a convex portion or a concave portion extending in parallel to the axis and adapted to be fitted on the at least one concave portion or convex portion of the rotational shaft, and wherein fitting between the outer circumferential surface of the rotational shaft and the inner circumferential surface of the yoke is interference fit, and fitting between the concave portion or the convex portion of the rotational shaft and the convex portion or the concave portion of the yoke is interference fit. 
     According to a second aspect, in the rotor for an electric motor according to the first aspect, the concave portion of the rotational shaft or the yoke has an enlarged portion having a width in a direction perpendicular to the axis, the width gradually increasing toward at least one end of the concave portion. 
     According to a third aspect, in the rotor for an electric motor according to the second aspect, the concave portion of the rotational shaft or the yoke has a widened portion extending from a tip end of the enlarged portion and having a constant width in a direction perpendicular to the axis. 
     According to a fourth aspect, in the rotor for an electric motor according to any one of the first to third aspects, the rotational shaft has at at least one end of the rotational shaft a smaller diameter portion having an outer diameter smaller than an inner diameter of the yoke. 
     According to a fifth aspect, in the rotor for an electric motor according to the second or third aspect, the rotational shaft has at at least one end of the rotational shaft a smaller diameter portion having an outer diameter smaller than an inner diameter of the yoke, the smaller diameter portion extending from the at least one end of the rotational shaft to an end of the enlarged portion situated distant from the at least one end of the rotational shaft. 
     According to a sixth aspect, in the rotor for an electric motor according to the fourth or fifth aspect, the smaller diameter portion has a tapered shape which gradually decreases in an outer diameter toward the at least one end of the rotational shaft where the smaller diameter portion is situated. 
     According to a seventh aspect, in the rotor for an electric motor according to any one of the first to sixth aspects, a plurality of the concave portions or the convex portions are situated on the outer circumferential surface of the rotational shaft or the inner circumferential surface of the yoke at an equal distance from each other. 
     According to an eighth aspect, in the rotor for an electric motor according to any one of the first to seventh aspects, the yoke has a stacked structure of steel plates. 
     These and other objects, features and advantages of the present invention will become more apparent in light of the detailed description of exemplary embodiments thereof as illustrated by the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating a rotor including a rotational shaft and a yoke according to one embodiment of the present invention. 
         FIG. 2  is a sectional view illustrating the yoke in the embodiment of  FIG. 1 . 
         FIG. 3  is a sectional view illustrating the rotational shaft in the embodiment of  FIG. 1 . 
         FIG. 4  is a sectional view illustrating a yoke according to another embodiment of the present invention. 
         FIG. 5  is a sectional view illustrating a rotational shaft used together with the yoke shown in  FIG. 4 . 
         FIG. 6  is an exploded perspective view illustrating a yoke and a rotational shaft of a rotor according to a variant of the present invention. 
         FIG. 7  is an exploded perspective view illustrating a yoke and a rotational shaft of a rotor according to another variant of the present invention. 
         FIG. 8  is an exploded perspective view illustrating a yoke and a rotational shaft of a rotor according to yet another variant of the present invention. 
         FIG. 9  is a sectional view illustrating the yoke and the rotational shaft shown in  FIG. 8 . 
         FIG. 10  is an exploded perspective view illustrating a yoke and a rotational shaft of a rotor according to yet another variant of the present invention. 
         FIG. 11  is a sectional view illustrating the yoke and the rotational shaft shown in  FIG. 10 . 
         FIG. 12  is a sectional view illustrating a yoke of a rotor according to yet another variant of the present invention. 
         FIG. 13  is a sectional view illustrating a yoke of a rotor according to yet another variant of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be described below with reference to the accompanying drawings. In the illustrated embodiments, each constituent element may be modified in size in relation to another from the practical application for better understanding.  FIG. 1  is a perspective view illustrating a rotor  14  including a rotational shaft  10  and a yoke  12  according to one embodiment of the present invention. 
       FIG. 2  is a sectional view illustrating the yoke  12  in the embodiment of  FIG. 1 .  FIG. 3  is a sectional view illustrating the rotational shaft  10  in the embodiment of  FIG. 1 . 
     The rotor  14  is a rotor used for an electric motor (not shown). As shown in  FIG. 1 , the rotor  14  includes a rotational shaft  10  having a cylindrical contour and capable of rotating around an axis X, and a yoke  12  fitted on an outer circumferential surface  10   a  of the rotational shaft  10 . The rotational shaft  10  has a convex portion  10   b  extending in parallel to the axis X on the outer circumferential surface  10   a . The yoke  12  has a concave portion  12   b  extending in parallel to the axis X on an inner circumferential surface  12   a , as can be clearly seen in  FIG. 2 . The convex portion  10   b  of the rotational shaft  10  is adapted to be fitted on the concave portion  12   b  of the yoke  12 . 
     Referring to  FIG. 2 , the yoke  12  is a tubular member having a constant inner diameter D 1  except for portions where the concave portion  12   b  is provided. The yoke  12  is formed from a magnetic material and serves as a magnetic path during operation of an electric motor. For example, the yoke  12  may have a stacked structure of steel plates. The yoke formed from stacked steel plates is effective to prevent an eddy current from generating. As a result, iron loss is decreased, and efficiency of the electric motor can be improved. In addition, each steel plate may be formed by means of punching, which is inexpensive, so that total manufacturing cost can be reduced. The concave portion  12   b  extends like a groove on the inner circumferential surface  12   a  of the yoke  12  and has substantially the same cross-section over the entire length of the yoke  12  in a direction in parallel to the axis X. The concave portion  12   b  has a width W 1  defined in a direction perpendicular to the axis X. 
     Referring to  FIG. 3 , the rotational shaft  10  substantially has a circular cross-section having a constant outer diameter D 2 , except for portions where the convex portion  10   b  is provided. The rotational shaft  10  is a shaft of a rotor for an electric motor, for example. The rotor produces power by rotating around the axis X due to magnetic action in cooperation with a stator of an electric motor, which is not shown. The convex portion  10   b  protrudes radially outwardly from the outer circumferential surface  10   a  of the rotational shaft  10 . The convex portion  10   b  has substantially the same cross-section over the entire length of the rotational shaft  10  in a direction in parallel to the axis X. The convex portion  10   b  has a width W 2  defined in a direction perpendicular to the axis X. 
     In the present embodiment, D 1 &lt;D 2  and W 1 &lt;W 2  are satisfied. In other words, the rotational shaft  10  and the yoke  12  are sized in relation to each other so that fitting between the outer circumferential surface  10   a  of the rotational shaft  10  and the inner circumferential surface  12   b  of the yoke  12  is interference fit. Likewise, fitting between the convex portion  10   b  of the rotational shaft  10  and the concave portion  12   b  of the yoke  12  is interference fit. The respective fitting allowances may be determined accordingly by taking materials of the rotational shaft  10  and the yoke  12  and the size thereof, in particular the thickness of the yoke  12  into consideration. 
     As described above, according to the present embodiment, both of fitting can be realized by means of interference fit. The interference fit may be shrinkage fit, expansion fit or press-fit, for example. Therefore, according to the present invention, the rotor with reliable quality can be provided, as compared to a rotor in which a yoke is attached to a rotational shaft by means of an adhesive. In addition, a mounting process in the rotor according to the present embodiment is relatively simple, as compared to the case where an adhesive is used, which requires additional processes such as removing excessive adhesives. Thus, a manufacturing process can be easily automated. 
     With the configuration of the present embodiment, fitting between the outer circumferential surface  10   a  of the rotational shaft  10  and the inner circumferential surface  12   a  of the yoke  12  and fitting between the convex portion  10   b  and the concave portion  12   b  function to compensate each other, and as a result, a reliable fastening effect can be achieved. For example, if the yoke  12  plastically deforms, fastening force acting between the inner circumferential surface  12   a  of the yoke  12  and the outer circumferential surface  10   a  of the rotational shaft  10  may be decreased. Even if this is the case, due to fitting between the convex portion  10   b  and the concave portion  12   b , fastening force acting in the rotor  14  in a rotational direction can be substantially maintained. On the other hand, only with fitting between the convex portion  10   b  and the concave portion  12   b , fastening force acting in a direction of the axis X of the rotor  14  is generally insufficiently small. However, according to the present embodiment, sufficiently great fastening force acts in the direction of the axis X as well, due to the interference fit between the outer circumferential surface  10   a  of the rotational shaft  10  and the inner circumferential surface  12   a  of the yoke  12 . Therefore, there is no need for additional supporting means for providing support in the axial direction. As a result, the number of parts can be decreased, the structure can be simplified. 
     It should be noted that smaller fastening force is required in the direction of the axis X, as compared to the rotational direction, in a rotor for an electric motor. Therefore, in the present embodiment, even in the case where the yoke  12  undergoes plastic deformation, fastening force can be sufficiently maintained both in the rotational direction and the direction of the axis X. In addition, since the convex portion  10   b  and the concave portion  12   b  are fitted together by means of interference fit, the rotational shaft  10  and the yoke  12  can be prevented from being misaligned relative to each other in the rotational direction. Therefore, fretting can be effectively prevented from occurring, and a more durable rotor can be provided. 
     Other embodiments and variants of the present invention will be described below. The matters that have been already described in relation to the above embodiment will be omitted in the following explanation. The above matters may be applied to the following embodiments and variants in the same way, unless expressly stated otherwise. 
       FIG. 4  is a sectional view illustrating a yoke  20  according to another embodiment of the present invention. 
       FIG. 5  is a sectional view illustrating a rotational shaft  22  used together with the yoke  20 . In the present embodiment, the yoke  20  has on an inner circumferential surface  20   a  a convex portion  20   b  extending radially inwardly and substantially having a constant width W 3  over the entire length of the yoke  20 . Corresponding thereto, the rotational shaft  22  has on an outer circumferential surface  22   a  a concave portion  22   b  adapted to be fitted on the convex portion  20   b  of the yoke  20 . In the present embodiment, D 2 &lt;D 4  is satisfied, where D 3  is an inner diameter of the yoke  20  and D 4  is an outer diameter of the rotational shaft  22 . Further, W 4 &lt;W 3  is satisfied, where W 3  is a width of the convex portion  20   b  of the yoke  20 , and W 4  is a width of the concave portion  22   b  of the rotational shaft  22 . 
     Accordingly, according to the present embodiment, fitting between the inner circumferential surface  20   a  of the yoke  20  and the outer circumferential surface  22   a  of the rotational shaft  22  is interference fit, and fitting between the convex portion  20   b  of the yoke  20  and the concave portion  22   b  of the rotational shaft  22  is also interference fit. Due to fastening force provided by the interference fit, the yoke  20  and the rotational shaft  22  are securely fastened together both in a rotational direction and an axial direction, similarly to the above embodiment. 
       FIG. 6  is an exploded perspective view illustrating a yoke  30  and a rotational shaft  32  of a rotor according to a variant of the present invention. In this variant, similarly to the above embodiment shown in  FIGS. 4 and 5 , the yoke  30  has on an inner circumferential surface  30   a  a convex portion  34 , and the rotational shaft  32  has on an outer circumferential surface  32   a  a concave portion  36 . More specifically, the yoke  30  has a pair of the convex portions  34  and  34  situated substantially opposite to each other, as illustrated. Similarly, the rotational shaft  32  has a pair of the concave portions  36  and  36  situated substantially opposite to each other, as illustrated. As can be seen in  FIG. 6 , the concave portion  36  further has a fitting portion  38  sized so as to be fitted on the convex portion  34  by interference fit, and an enlarged portion  40  extending from a tip end of the fitting portion  38 . The enlarged portion  40  has a width defined in a direction perpendicular to an axis of the rotational shaft  32 , and the width gradually increases toward an end  36   a  of the concave portion  36 . The width of the enlarged portion  40  gradually increases at least to the extent that it becomes larger than a width of the convex portion  34  of the yoke  30 . The enlarged portion  40  serves as guiding means, when the yoke  30  is mounted on the rotational shaft  32 , and with the aid thereof, the convex portion  34  of the yoke  30  can be smoothly introduced to the concave portion  36  of the rotational shaft  32 . Therefore, a mounting process is facilitated and is more efficient, and as a result, manufacturing cost can be reduced. 
       FIG. 7  is an exploded perspective view illustrating a yoke  30  and a rotational shaft  32  of a rotor according to another variant of the present invention. In  FIG. 7 , constituent elements which are the same as or correspond to those in  FIG. 6  are designated with the same referential numerals, and explanation directed thereto will be omitted to avoid redundancy. In this variant, the concave portion  36  of the rotational shaft  32  has, in addition to an enlarged portion  40  whose width in a direction perpendicular to the axis of the rotational shaft  32  gradually increases toward an end  36   a  of the concave portion  36 , a widened portion  42  extending from a tip end of the enlarged portion  40 . The widened portion  42  has a constant width in a direction perpendicular to the axis of the rotational shaft  32 , and the width is larger than a width of the convex portion  34  of the yoke  30  and than a width of the fitting portion  38  of the rotational shaft  32 . In this variant, similarly to the above variant shown in  FIG. 6 , the enlarged portion  40  and the widened portion  42  cooperate with each other to function as guiding means when the yoke  30  is mounted on the rotational shaft  32 . Therefore, a mounting process is facilitated and is more efficient, and as a result, manufacturing cost can be reduced. 
     Next, another variant of the present invention will be described with reference to  FIGS. 8 and 9 .  FIG. 8  is an exploded perspective view illustrating a yoke  30  and a rotational shaft  72  of a rotor  70  according to the variant.  FIG. 9  is a sectional view illustrating the yoke  30  and the rotational shaft  72  shown in  FIG. 8 . In this variant, similarly to the above variants shown in  FIGS. 6 and 7 , the yoke  30  has a convex portion  34  on an inner circumferential surface  30   a  and the rotational shaft  72  has a concave portion  74  on an outer circumferential surface  72   a . Since configuration of the yoke  30  is the same as that in the above variant described in relation to  FIGS. 6 and 7 , the detailed explanation thereon will be omitted. The concave portion  74  of the rotational shaft  72  has a fitting portion  74   a  and an enlarged portion  74   b , similarly to the concave portion  36  of the rotational shaft  32  in the variant of  FIG. 6 . Specifically, the fitting portion  74   a  is sized so as to have a width smaller than a width of the convex portion  34  such that the fitting portion  74   a  is fitted on the convex portion  34  of the yoke  30  by interference fit. The enlarged portion  74   b  has a width defined in a direction perpendicular to the axis X of the rotational shaft  72  and the width gradually increases toward an end  74   c  of the concave portion  74 . 
     As can be more clearly seen in  FIG. 9 , the rotational shaft  72  has a smaller diameter portion  76  at an end  72   b  directed to the side on which the yoke  30  is introduced. The smaller diameter portion  76  has an outer diameter D 6  smaller than an inner diameter D 5  of the yoke  30 . Therefore, D 6 &lt;D 5 &lt;D 7  is satisfied, where D 5  is the inner diameter of the yoke  30 , D 6  is the outer diameter of the smaller diameter portion  76  and D 7  is an outer diameter of the rotational shaft  72 . With the rotor  70  having such configuration, the smaller diameter portion  76  serves as guiding means when the yoke  30  is mounted on the rotational shaft  72 . Therefore, a mounting process is facilitated and is more efficient, and as a result, manufacturing cost can be reduced. The smaller diameter portion  76  preferably extends at least from the end  72   b  of the rotational shaft  72  to an end  74   d  of the enlarged portion  74   b  situated distant from the end  72   b . With such configuration, the enlarged portion  74   b  and the smaller diameter portion  76  cooperate with each other over the area of the enlarged portion  74   b , and therefore, a mounting process of the yoke  30  can be smoothly carried out. The smaller diameter portion  76  may also extend beyond the end  74   d  of the enlarged portion  74   b . In this case, after the convex portion  34  of the yoke  30  and the concave portion  74  of the rotational shaft  72  are fitted together, the inner circumferential surface  30   a  of the yoke  30  and the outer circumferential surface  72   a  of the rotational shaft  72  are fitted together. Therefore, the yoke  30  can be prevented from being misaligned and unintentional great force can be prevented from acting on the yoke  30  or the rotational shaft  72 . 
       FIG. 10  is an exploded perspective view illustrating a yoke  30  and a rotational shaft  82  of a rotor  80  according to yet another variant of the present invention.  FIG. 11  is a sectional view illustrating the yoke  30  and the rotational shaft  82  shown in  FIG. 10 . In this variant, similarly to the above variants shown in  FIGS. 6 and 7 , the yoke  30  has a convex portion  34  on an inner circumferential surface  30   a  and the rotational shaft  82  has a concave portion  84  on an outer circumferential surface  82   a . Detailed explanation on the yoke  30  will be omitted. The concave portion  84  of the rotational shaft  82  has a fitting portion  84   a , an enlarged portion  84   b  and a widened portion  84   c , similarly to the concave portion  36  of the rotational shaft  32  according to the variant shown in  FIG. 7 . Specifically, the fitting portion  84   a  is sized so as to have a width smaller than a width of the convex portion  34  such that the fitting portion  84   a  is fitted on the concave portion  34  of the yoke  30  by interference fit. The enlarged portion  84   b  has a width defined in a direction perpendicular to the axis X of the rotational shaft  82 , and the width gradually increases toward an end  84   d  of the concave portion  84 . The widened portion  84   c  has a constant width in a direction perpendicular to the axis X of the rotational shaft  82 . The width of the widened portion  84   c  is larger than a width of the convex portion  34  of the yoke  30  and equal to or larger than a width of the enlarged portion  84   b  of the rotational axis  82 . 
     As can be more clearly seen in  FIG. 11 , the rotational shaft  82  has a smaller diameter portion  86  at an end  82   b  directed to the side on which the yoke  30  is introduced. The smaller diameter portion  86  has an outer diameter D 8  smaller than an inner diameter D 5  of the yoke  30 . The smaller diameter portion  86  has a tapered shape so that the outer diameter D 8  gradually decreases toward the end  82   b  of the rotational shaft  82 . Further, the smaller diameter portion  86  has at a tip end of the smaller diameter portion  86 , or the end  82   b  of the rotational shaft  82 , an outer diameter D 81  smaller than the inner diameter D 5  of the yoke  30 . Therefore, D 81 &lt;D 5 &lt;D 9  is satisfied, where D 5  is the inner diameter of the yoke  30 , D. is the outer diameter of the smaller diameter portion  86  at the tip end thereof and D 9  is an outer diameter of the rotational shaft  82 . With the rotor  80  having such configuration, the smaller diameter portion  86  serves as guiding means when the yoke  30  is mounted on the rotational shaft  82 . Therefore, a mounting process is facilitated and is more efficient, and as a result, manufacturing cost can be reduced. The smaller diameter portion  86  preferably extends from the end  82   b  of the rotational shaft  82  to an end  84   e  of the enlarged portion  84   b  situated distant from the end  82   b . With such configuration, the enlarged portion  84   b  and the smaller diameter portion  86  cooperate with each other over the area of the enlarged portion  84   b , and therefore, a mounting process of the yoke  30  can be smoothly carried out. In this variant, the smaller diameter portion  86  may also extend beyond the end  84   e  of the enlarged portion  84   b.    
     Although the exemplary variants in which the concave portion is formed on the rotational shaft have been explained with reference to  FIGS. 6 to 11 , the same matters will be applied to embodiments in which the concave portion is formed on the yoke, rather than on the rotational shaft. If this is the case, it is evident that the enlarged portion  40 ,  74   b  or  84   b , the widened portion  42  or  84   c , or the smaller diameter portion  76  or  86  of the concave portion also serve as guiding means in the same way as explained in relation to the illustrated variants. Further, the enlarged portion  40 ,  74   b  or  84   b , the widened portion  42  or  84   c  and the smaller diameter portion  76  or  86  may also be provided at the other end, which is not shown. With the enlarged portion  40 ,  74   b  or  84   b , the widened portion  42  or  84   c  and the smaller diameter portion  76  or  86  provided at both ends, the yoke  30  can be easily mounted on the rotational shaft  32 ,  72  or  82  from either side thereof. This advantageously increases freedom of how the mounting process is carried out. 
       FIGS. 12 and 13  are sectional views illustrating yokes  50  and  60  of a rotor according to yet another variant, respectively. In these variants, the above-described fitting between the concave portion and the convex portion is evenly provided at a plurality of positions. Specifically, the concave portions and the convex portions are provided so as to be equally distant from each other on the inner circumferential surface of the yoke or the outer circumferential of the rotational shaft. 
     The yoke  50  shown in  FIG. 12  has two convex portions  54  and  54  on an inner circumferential surface  52  of the yoke  50 . The convex portions  54  and  54  are situated so as to be opposite to each other. Although not illustrated, the rotational shaft has two concave portions situated so as to be opposite to each other, corresponding to the convex portions  54  and  54 . The yoke  60  shown in  FIG. 13  has on an inner circumferential surface  62  three convex portions  64  at an equal distance from each other in a circumferential direction of the yoke  60 , or in other words, distant from each other by 120 degrees. It is evident that the yoke may also have a plurality of concave portions at an equal distance from each other in a circumferential direction. In this case, the rotational shaft has a plurality of convex portions at an equal distance from each other in a circumferential direction, corresponding to the concave portions of the yoke. 
     As illustrated in  FIGS. 12 and 13  by way of example, balance of the rotor during rotational movement can be maintained by providing the convex portions and the concave portions fitted thereon at an equal distance from each other in a circumferential direction, respectively. In the illustrated variants, two or three convex portions and concave portions are provided, the present invention is not limited to such particular configuration, but may also include configuration in which four or more convex portions and concave portions are provided at an equal distance from each other. The present invention is not limited to any particular embodiment expressly described in the present specification in relation to other embodiments, either. For example, it is evident to a person skilled in the art that it is possible to combine the embodiments and variants thereof described in the present specification in any way in order to implement the present invention. 
     Effect of the Invention 
     According to the first aspect, fitting between the outer circumferential surface of the rotational shaft and the inner circumferential surface of the yoke is interference fit, and fitting between the concave portion and the convex portion of the rotational shaft and the yoke is interference fit. Therefore, a rotor in which the yoke and the rotational shaft are securely fitted on each other both in an axial direction and a rotational direction of the rotor can be provided without adhesive or separate fixing means. With such configuration, even in the case where the yoke undergoes plastic deformation when fitted on the shaft, the yoke and the shaft are securely fitted on each other, due to the interference fit between the concave portion and the convex portion. In contrast, in the related art, fitting between the concave portion and the convex portion is generally carried out by transition fit in the case where the yoke is most closely fitted on the shaft. In such a case, a gap may be formed between the concave portion and the convex portion. As a result, fastening force may be decreased upon plastic deformation of the yoke. Further, fretting may occur due to the gap between the concave portion and the convex portion. 
     According to the second aspect, the enlarged portion serves as guiding means when the concave portion and the convex portion are fitted on each other. This allows a mounting process for mounting the yoke on the rotational shaft to be facilitated. 
     According to the third aspect, the widened portion serves as guiding means, in addition to the enlarged portion. This allows a mounting process for mounting the yoke on the rotational shaft to be facilitated. Further, the widened portion can be formed relatively easily because it has a constant width. 
     According to the fourth aspect, the smaller diameter portion of the rotational shaft serves as guiding means when the yoke is fitted on the rotational shaft. Accordingly, the yoke can be smoothly introduced without slanting the yoke. 
     According to the fifth aspect, the smaller diameter portion extends at least over the area of the enlarged portion of the concave portion. The smaller diameter portion and the enlarged portion cooperate with each other to serve as guiding means, when the concave portion and the convex portion are fitted on each other. Therefore, a mounting process for mounting the yoke on the rotational shaft can be facilitated. 
     According to the sixth aspect, the smaller diameter portion has a tapered shape. Therefore, the yoke can be introduced smoothly. 
     According to the seventh aspect, the concave portions and the convex portions are provided on the rotor so as to be evenly distributed. This allows balance of the rotor during rotational movement to be maintained. 
     According to the eighth aspect, a manufacturing process can become relatively easier since the yoke is formed from steel plates stacked one on top of another and the steel plates are formed by means of punching. With the yoke having a stacked structure, an eddy current can be prevented from generating, and as a result, iron loss can be reduced. 
     Although the invention has been shown and described with exemplary embodiments thereof, it should be understood by a person skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto without departing from the spirit and scope of the invention.