Abstract:
A brushless motor includes a plate-shaped armature and a magnet facing either one of a top surface and a bottom surface of the armature. An inner peripheral surface of a base member and an outer peripheral surface of a bearing mechanism contact with each other or face each other in a diametrical direction with an adhesive therebetween in a first fixing part. Further, a rotor yoke and a hub protrusion contact with each other or face each other in a diametrical direction with an adhesive therebetween in a second fixing part. Then, a press-fitting or adhesion is carried out in the other of the first and the second fixing part.

Description:
FIELD 
       [0001]    The present invention relates to a brushless motor, a disk drive apparatus and a brushless motor manufacturing method. 
       BACKGROUND 
       [0002]    A hard disk apparatus is equipped with a brushless motor for rotating a disk. A conventional brushless motor is disclosed in, e.g., SG185981. A permanent-magnet synchronous motor of SG185981 includes a permanent magnet mounted to at least one of a top yoke and a bottom yoke and an armature arranged within an air gap between the permanent magnet and the other yoke. 
         [0003]    When assembling a brushless motor, an inner member is inserted or fitted into an outer member. If necessary, the inner member and the outer member are fixed to each other through the use of an adhesive agent. In particular, an armature is axially opposed to a magnet. In a so-called axial-gap-type brushless motor, a plurality of members is intricately arranged in an axially overlapping relationship. For that reason, in a process of manufacturing the axial-gap-type brushless motor, it is sometimes the case that bonding or press-fitting is performed with respect to a plurality of fixing portions during one insertion work in which two units are combined together. A demand has existed for a technology capable of accurately fixing individual members in the fixing portions. 
       SUMMARY 
       [0004]    A brushless motor according to a first illustrative invention of the subject application includes a stationary unit and a rotary unit rotatably supported with respect to the stationary unit through a bearing mechanism. The stationary unit includes a flat armature and a base member. The flat armature arranged radially outward of the bearing mechanism to extend in a direction orthogonal to a center axis extending in an up-down direction. The base member includes a base through-hole extending in the up-down direction. The base member is arranged to support the armature. The rotary unit includes a hub, a rotor yoke made of a magnetic material, and a magnet. The hub includes a top plate portion made of a magnetic material and a hub protrusion portion made of a magnetic material and arranged to extend downward from the top plate portion. The rotor yoke is fixed to the hub protrusion portion and is positioned below the armature. The magnet is fixed to the hub or the rotor yoke and is opposed to one of upper and lower surfaces of the armature. 
         [0005]    The base member includes an inner circumferential surface defining the base through-hole. The bearing mechanism includes an outer circumferential surface. The inner circumferential surface of the base member is arranged to make contact with the outer circumferential surface of the bearing mechanism or to radially oppose the outer circumferential surface of the bearing mechanism across an adhesive agent in a first fixing portion. The rotor yoke is arranged to make contact with the hub protrusion portion or to radially oppose the hub protrusion portion across an adhesive agent in a second fixing portion. 
         [0006]    An axial length between a lower end of a constant diameter surface of the bearing mechanism extending along the first fixing portion or further extending downward from the first fixing portion and an upper end of a constant diameter surface of the base member extending along the first fixing portion or further extending upward from the first fixing portion is assumed to be d1. An axial length between a lower end of a constant diameter surface of the hub protrusion portion extending along the second fixing portion or further extending downward from the second fixing portion and an upper end of a constant diameter surface of the rotor yoke extending along the second fixing portion or further extending upward from the second fixing portion is assumed to be d2. The d1 and the d2 are set to satisfy a relationship of d1&gt;d2. 
         [0007]    With the first illustrative invention of the subject application, the respective members are substantially coaxially located in position on the basis of the first fixing portion. As a result, the rotor yoke and the hub protrusion portion are accurately fixed to each other in the second fixing portion. 
         [0008]    A brushless motor according to a second illustrative invention of the subject application includes a stationary unit and a rotary unit rotatably supported with respect to the stationary unit through a bearing mechanism. The stationary unit includes a flat armature and a base member. The flat armature is arranged radially outward of the bearing mechanism to extend in a direction orthogonal to a center axis extending in an up-down direction. The base member includes a base through-hole extending in the up-down direction. The base member is arranged to support the armature. The rotary unit includes a hub, a rotor yoke made of a magnetic material, and a magnet. The hub includes a top plate portion made of a magnetic material and a hub protrusion portion made of a magnetic material and arranged to extend downward from the top plate portion. The rotor yoke is fixed to the hub protrusion portion and is positioned below the armature. The magnet is fixed to the hub or the rotor yoke and is opposed to one of upper and lower surfaces of the armature. 
         [0009]    The base member includes an inner circumferential surface defining the base through-hole. The bearing mechanism includes an outer circumferential surface. The inner circumferential surface of the base member is arranged to make contact with the outer circumferential surface of the bearing mechanism or to radially oppose the outer circumferential surface of the bearing mechanism across an adhesive agent in a first fixing portion. The rotor yoke is arranged to make contact with the hub protrusion portion or to radially oppose the hub protrusion portion across an adhesive agent in a second fixing portion. 
         [0010]    An axial length between a lower end of a constant diameter surface of the bearing mechanism extending along the first fixing portion or further extending downward from the first fixing portion and an upper end of a constant diameter surface of the base member extending along the first fixing portion or further extending upward from the first fixing portion is assumed to be d1. An axial length between a lower end of a constant diameter surface of the hub protrusion portion extending along the second fixing portion or further extending downward from the second fixing portion and an upper end of a constant diameter surface of the rotor yoke extending along the second fixing portion or further extending upward from the second fixing portion is assumed to be d2. The d1 and the d2 are set to satisfy a relationship of d1&lt;d2. 
         [0011]    With the second illustrative invention of the subject application, the respective members are substantially coaxially located in position on the basis of the second fixing portion. As a result, the base member and the bearing mechanism are accurately fixed to each other in the first fixing portion. 
         [0012]    A method for manufacturing a brushless motor according to a third illustrative invention of the subject application is provided. The brushless motor includes a bearing mechanism, a flat armature, a base member arranged to support the armature, a hub, a rotor yoke made of a magnetic material, and a magnet. The armature is arranged radially outward of the bearing mechanism to extend in a direction orthogonal to a center axis extending in an up-down direction. The hub is arranged to rotate with respect to the base member. The rotor yoke is fixed to the hub and is positioned below the armature. The magnet is fixed to the hub or the rotor yoke and is opposed to one of upper and lower surfaces of the armature. The method according to the third illustrative invention includes steps a), b) and c). In the step a), a first unit including the base member, the armature and the rotor yoke is prepared. In the step b), a second unit including the bearing mechanism, the hub and the magnet is prepared. In the step c), the first unit and the second unit are combined together. The step c) includes steps c 1 ) and c 2 ). In the step c 1 ), an inner circumferential surface of the base member is caused to make contact with an outer circumferential surface of the bearing mechanism or to radially oppose the outer circumferential surface of the bearing mechanism across an adhesive agent. In the step c 2 ), after the step c 1 ), the hub is caused to make contact with the rotor yoke or to radially oppose the rotor yoke across an adhesive agent. 
         [0013]    With the third illustrative invention of the subject application, the hub can be accurately fixed with respect to the rotor yoke. 
         [0014]    A method for manufacturing a brushless motor according to a fourth illustrative invention of the subject application is provided. The brushless motor includes a bearing mechanism, a flat armature, a base member arranged to support the armature, a hub, a rotor yoke made of a magnetic material, and a magnet. The armature is arranged radially outward of the bearing mechanism to extend in a direction orthogonal to a center axis extending in an up-down direction. The hub is arranged to rotate with respect to the base member. The rotor yoke is fixed to the hub and is positioned below the armature. The magnet is fixed to the hub or the rotor yoke and is opposed to one of upper and lower surfaces of the armature. The method according to the fourth illustrative invention includes steps a), b) and c). In the step a), a first unit including the base member, the armature and the rotor yoke is prepared. In the step b), a second unit including the bearing mechanism, the hub and the magnet is prepared. In the step c), the first unit and the second unit are combined together. The step c) includes steps c 1 ) and c 2 ). In the step c 1 ), the hub is caused to make contact with the rotor yoke or to radially oppose the rotor yoke across an adhesive agent. In the step c 2 ), after the step c 1 ), an inner circumferential surface of the base member is caused to make contact with an outer circumferential surface of the bearing mechanism or to radially oppose the outer circumferential surface of the bearing mechanism across an adhesive agent. 
         [0015]    With the fourth illustrative invention of the subject application, the bearing mechanism can be accurately fixed with respect to the base member. 
         [0016]    The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  is a vertical section view showing a brushless motor according to a first preferred embodiment. 
           [0018]      FIG. 2  is a vertical section view showing a disk drive apparatus according to a second preferred embodiment. 
           [0019]      FIG. 3  is a vertical section view showing a brushless motor according to the second preferred embodiment. 
           [0020]      FIG. 4  a partial vertical section view of the brushless motor according to the second preferred embodiment. 
           [0021]      FIG. 5  is a flowchart illustrating some steps for manufacturing the brushless motor according to the second preferred embodiment. 
           [0022]      FIG. 6  is a vertical section view showing the brushless motor according to the second preferred embodiment, which is under a manufacturing process. 
           [0023]      FIG. 7  is a vertical section view showing the brushless motor according to the second preferred embodiment, which is under a manufacturing process. 
           [0024]      FIG. 8  is a vertical section view showing the brushless motor according to the second preferred embodiment, which is under a manufacturing process. 
           [0025]      FIG. 9  is a partial vertical section view showing a brushless motor according to a third preferred embodiment. 
           [0026]      FIG. 10  is a flowchart illustrating some steps for manufacturing the brushless motor according to the third preferred embodiment. 
           [0027]      FIG. 11  is a partial vertical section view showing the brushless motor according to the third preferred embodiment, which is under a manufacturing process. 
           [0028]      FIG. 12  is a partial vertical section view of a brushless motor according to one modified example. 
           [0029]      FIG. 13  is a partial vertical section view of a brushless motor according to another modified example. 
           [0030]      FIG. 14  is a partial vertical section view of a brushless motor according to a further modified example. 
           [0031]      FIG. 15  is a partial vertical section view of a brushless motor according to a still further modified example. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0032]    Hereinafter, illustrative embodiments of the present invention will now be described with reference to the accompanying drawings. In the subject application, the direction parallel to the center axis of a brushless motor will be referred to as “axial”. The direction orthogonal to the center axis of the brushless motor will be referred to as “radial”. The direction extending along an arc about the center axis of the brushless motor will be referred to as “circumferential”. In the subject application, the shape and positional relationship of the respective portions will be described under the assumption that the axial direction extends in an up-down direction and that the side of a hub with respect to a base member is upper. However, the up-down direction is defined merely for the sake of convenience in description and is not intended to limit the direction in the manufacture and use of the brushless motor and the disk drive apparatus according to the present invention. 
         [0033]    In the subject application, the term “parallel” is intended to include a substantially parallel direction. Likewise, the term “orthogonal” is intended to include a substantially orthogonal direction. 
         [0034]      FIG. 1  is a vertical section view showing a brushless motor  11 A according to a first preferred embodiment. As shown in  FIG. 1 , the brushless motor  11 A includes a stationary unit  2 A and a rotary unit  3 A. The rotary unit  3 A is rotatably supported with respect to the stationary unit  2 A through a bearing mechanism  4 A. 
         [0035]    The stationary unit  2 A includes a base member  21 A and an armature  22 A. The armature  22 A extends in a flat shape in the direction orthogonal to a center axis  9 A. The armature  22 A is supported on the base member  21 A. The base member  21 A includes a base through-hole  53 A extending in an up-down direction. The inner circumferential surface of the base member  21 A defining the base through-hole  53 A makes contact with the outer circumferential surface of the bearing mechanism  4 A or remains radially opposed to the outer circumferential surface of the bearing mechanism  4 A across an adhesive agent in a first fixing portion  81 A. In other words, the inner circumferential surface of the base member  21 A may be opposed to the outer circumferential surface of the bearing mechanism  4 A in a contact state or may be opposed to the outer circumferential surface of the bearing mechanism  4 A in a non-contact state with a radial gap left therebetween. 
         [0036]    The rotary unit  3 A includes a hub  32 A, a magnet  34 A and a rotor yoke  35 A. The hub  32 A includes a top plate portion  61 A made of a magnetic material and a hub protrusion portion  62 A made of a magnetic material. The hub protrusion portion  62 A extends downward from the top plate portion  61 A. The magnet  34 A is fixed to the hub  32 A and is opposed to the upper surface of the armature  22 A. Alternatively, the magnet  34 A may be fixed to the rotor yoke  35 A and may be opposed to the lower surface of the armature  22 A. The rotor yoke  35 A is a magnetic body fixed to the hub protrusion portion  62 A. The rotor yoke  35 A is positioned below the armature  22 A. The rotor yoke  35 A makes contact with the hub protrusion portion  62 A or remains radially opposed to the hub protrusion portion  62 A across an adhesive agent in a second fixing portion  82 A. In other words, the rotor yoke  35 A may be opposed to the hub protrusion portion  62 A in a contact state or may be opposed to the hub protrusion portion  62 A in a non-contact state with a radial gap left therebetween. 
         [0037]    In this regard, an axial length between a lower end of a constant diameter surface of the bearing mechanism  4 A extending along the first fixing portion  81 A or further extending downward from the first fixing portion  81 A and an upper end of a constant diameter surface of the base member  21 A extending along the first fixing portion  81 A or further extending upward from the first fixing portion  81 A is assumed to be d1. An axial length between a lower end of a constant diameter surface of the hub protrusion portion  62 A extending along the second fixing portion  82 A or further extending downward from the second fixing portion  82 A and an upper end of a constant diameter surface of the rotor yoke  35 A extending along the second fixing portion  82 A or further extending upward from the second fixing portion  82 A is assumed to be d2. In the brushless motor  11 A, as shown in  FIG. 1 , d1 and d2 are set to satisfy a relationship of d1&gt;d2. The term “constant diameter surface” includes a substantially constant diameter surface. 
         [0038]    When manufacturing the brushless motor  11 A, a first unit including the base member  21 A, the armature  22 A and the rotor yoke  35 A is prepared first. Furthermore a second unit including the bearing mechanism  4 A, the hub  32 A and the magnet  34 A is prepared. Then, the first unit and the second unit are combined together. 
         [0039]    In order to combine the first unit and the second unit together, the bearing mechanism  4 A is first inserted into the base member  21 A. In other words, the inner circumferential surface of the base member  21 A is brought into contact with the outer circumferential surface of the bearing mechanism  4 A or is caused to radially oppose the outer circumferential surface of the bearing mechanism  4 A across an adhesive agent. As a consequence, the respective members are substantially coaxially located in position on the basis of the first fixing portion  81 A. Thereafter, the hub protrusion portion  62 A is brought into contact with the rotor yoke  35 A or is caused to radially oppose the rotor yoke  35 A across an adhesive agent. As a result, the rotor yoke  35 A and the hub protrusion portion  62 A are accurately fixed to each other in the second fixing portion  82 A. 
         [0040]    The brushless motor  11 A may be modified to satisfy a relationship of d1&lt;d2. in this case, when combining the first unit and the second unit together, the hub protrusion portion is first inserted into the rotor yoke. In other words, the rotor yoke is brought into contact with the hub protrusion portion or is caused to radially oppose the hub protrusion portion across an adhesive agent. As a consequence, the respective members are substantially coaxially located in position on the basis of the second fixing portion. Thereafter, the inner circumferential surface of the base member is brought into contact with the outer circumferential surface of the bearing mechanism or is caused to radially oppose the outer circumferential surface of the bearing mechanism across an adhesive agent. As a result, the base member and the bearing mechanism are accurately fixed to each other in the first fixing portion. 
         [0041]      FIG. 2  is a vertical section view showing a disk drive apparatus  1  according to a second preferred embodiment. The disk drive apparatus  1  is an apparatus for performing information reading and writing tasks with respect to a magnetic disk  12  while rotating the magnetic disk  12 . As shown in  FIG. 2 , the disk drive apparatus  1  includes a brushless motor  11 , a magnetic disk  12 , an access unit  13  and a cover  14 . 
         [0042]    The brushless motor  11  supports the magnetic disk  12  and rotates the magnetic disk  12  about the center axis  9 . The brushless motor  11  includes a base member  21  extending radially at the lower side of the magnetic disk  12 . The rotary unit  3  of the brushless motor  11 , the magnetic disk  12  and the access unit  13  are accommodated within a housing made up of the base member  21  and the cover  14 . The access unit  13  displaces a head  131  along the recording surface of the magnetic disk  12  and performs information reading and writing tasks with respect to the magnetic disk  12 . The base member  21  may be formed of a single member or an assembly including a plurality of members. 
         [0043]    The disk drive apparatus may include two or more magnetic disks  12 . The access unit  13  may perform only one of information reading and writing tasks with respect to the magnetic disk  12 . 
         [0044]    Next, description will be made on the configuration of the brushless motor  11 .  FIG. 3  is a vertical section view of the brushless motor  11 . As shown in  FIG. 3 , the brushless motor  11  includes a stationary unit  2  and a rotary unit  3 . The stationary unit  2  is kept stopped with respect to the base member  21  and the cover  14 . The rotary unit  3  is rotatably supported with respect to the stationary unit  2  through a bearing mechanism  4 . 
         [0045]    The stationary unit  2  of the present embodiment includes a base member  21 , an armature  22 , a sleeve  23  and a cap  24 . 
         [0046]    The base member  21  supports the armature  22  and the sleeve  23 . The base member  21  can be formed by casting metal, e.g., aluminum. As shown in  FIG. 3 , the base member includes a bottom plate portion  51 , a base protrusion portion  52 , a base through-hole  53  and an armature holding portion  54 . 
         [0047]    The bottom plate portion  51  lies below the armature  22  and extends in the direction orthogonal to the center axis  9 . The base protrusion portion  52  extends upward from the inner periphery of the bottom plate portion  51  in a substantially cylindrical shape. The base protrusion portion  52  is positioned radially inward of a hub protrusion portion  62  to be described later. The base through-hole  53  lies radially inward of the base protrusion portion  52  and extends in the up-down direction. 
         [0048]    The armature holding portion  54  lies radially outward of a rotor yoke  35  to be described later and protrudes upward from the bottom plate portion  51 . The armature holding portion  54  includes a wall portion  541  and an annular surface  542 . The wall portion  541  is radially opposed to the outer edge portion of the armature  22 . The annular surface  542  extends radially inward from the lower end of the wall portion  541 . The armature  22  is arranged radially inward of the wall portion  541  and is supported on the annular surface  542 . The outer edge portion may oppose the wall portion  541  in a contact state or may oppose the wall portion  541  with a radial gap left therebetween. 
         [0049]    The armature  22  is a flat circuit board extending in the direction orthogonal to the center axis  9 . The armature  22  is arranged radially outward of the bearing mechanism  4 , below the magnet  34  to be described later and above the disc portion  71  of the rotor yoke  35  to be described later. The radial outer edge portion of the armature  22  is fixed to the armature holding portion  54  by, e.g., an adhesive agent. The armature  22  includes a plurality of electrically conductive coil patterns arranged along the circumferential direction. Preferably, each of the coil patterns is spirally arranged about a coil axis extending in the axial direction. 
         [0050]    The sleeve  23  extends axially in a substantially cylindrical shape around a shaft  31  to be described later. The lower portion of the sleeve  23  is inserted into the base through-hole  53  and is fixed to the inner circumferential surface of the base protrusion portion  52 . The inner circumferential surface of the sleeve  23  is radially opposed to the outer circumferential surface of the shaft  31 . The lower opening of the sleeve  23  is closed by the cap  24 . 
         [0051]    The rotary unit  3  of the present embodiment includes a shaft  31 , a hub  32 , a ring-shaped member  33 , a plurality of magnets  34  and a rotor yoke  35 . 
         [0052]    The shaft  31  is a member extending in the axial direction. The shaft  31  is made of metal, e.g., stainless steel. The shaft  31  is supported on the sleeve  23  and the cap  24  through a lubricant  41  and is rotated about the center axis  9 . The upper end portion of the shaft  31  protrudes upward beyond the upper surface of the sleeve  23 . 
         [0053]    The hub  32  includes a top plate portion  61 , a hub protrusion portion  62  and a disk support portion  63 . In the present embodiment, the hub  32  as a whole is made of a magnetic material. The top plate portion  61  lies above the armature  22  and extends radially and circumferentially. The radial inner edge portion of the top plate portion  61  is fixed to the upper end portion of the shaft  31 . The hub protrusion portion  62  extends downward from the top plate portion  61  in a substantially cylindrical shape. The shaft  31  and the hub  32  may be a single member. The hub  32  may be formed of a single member or an assembly including a plurality of members. 
         [0054]    The disk support portion  63  is arranged radially outward of the top plate portion  61  to support the magnetic disk  12 . The disk support portion  63  includes a substantially cylindrical first support surface  631  and a second support surface  632  extending radially outward from the lower end of the first support surface  631 . The inner circumferential portion of the magnetic disk  12  makes contact with the first support surface  631 . Thus the magnetic disk  12  is radially located in position. The lower surface of the magnetic disk  12  makes contact with the second support surface  632 . Accordingly, the magnetic disk  12  is axially located in position. 
         [0055]    The ring-shaped member  33  is a circular ring-shaped member positioned radially inward of the hub protrusion portion  62 . The upper surface of the ring-shaped member  33  makes contact with the lower surface of the top plate portion  61 . The radial outer surface of the ring-shaped member  33  is fixed to the inner circumferential surface of the hub protrusion portion  62  by press fit and/or by a fixing means such as an adhesive agent or the like. 
         [0056]    A lubricant  41  exists between the combination of the sleeve  23  and the cap  24  and the combination of the shaft  31 , the hub  32  and the ring-shaped member  33 . The liquid level of the lubricant  41  is positioned between the outer circumferential surface of the sleeve  23  and the inner circumferential surface of the ring-shaped member  33 . The shaft  31 , the hub  32  and the ring-shaped member  33  are rotatably supported with respect to the sleeve  23  and the cap  24  through the lubricant  41 . In the present embodiment, the bearing mechanism  4  is made up of: the sleeve  23  and the cap  24  which belong to the stationary unit  2 ; the shaft  31 , the hub  32  and the ring-shaped member  33  which belong to the rotary unit  3 ; and the lubricant  41  existing therebetween. For example, polyol ester-based oil or diester-based lubricating liquid is used as the lubricant  41 . 
         [0057]    A radial dynamic pressure groove array is provided on at least one of the inner circumferential surface of the sleeve  23  and the outer circumferential surface of the shaft  31 . The radial dynamic pressure groove array is formed into, e.g., a herringbone shape. Upon driving the brushless motor  11 , the radial dynamic pressure groove array induces a radial dynamic pressure in the lubricant  41  existing between the sleeve  23  and the shaft  31 . The shaft  31  is radially supported with respect to the sleeve  23  by the dynamic pressure thus induced. 
         [0058]    A thrust dynamic pressure groove array is provided on at least one of the upper surface of the sleeve  23  and the lower surface of the top plate portion  61 . The thrust dynamic pressure groove array is formed into, e.g., a herringbone shape. Upon driving the brushless motor  11 , the thrust dynamic pressure groove array induces an axial dynamic pressure in the lubricant  41  existing between the sleeve  23  and the hub  32 . The hub  32  is axially supported with respect to the sleeve  23  by the axial dynamic pressure thus induced. The thrust dynamic pressure groove array may be, e.g., a spiral groove array for increasing a pressure at the radial inner side. 
         [0059]    The magnets  34  are fixed to the lower surface of the top plate portion  61  of the hub  32  by means of, e.g., an adhesive agent. The magnets  34  are positioned above the armature  22 . The lower surface of each of the magnets  34  is a magnetic pole surface axially opposed to each of the coil patterns provided on the upper surface of the armature  22 . The magnets  34  are arranged along the circumferential direction in such a way that the magnetic pole surfaces of N-pole and the magnetic pole surfaces of S-pole can be alternately arranged side by side. A single annular magnet alternately magnetized with N-poles and S-poles along the circumferential direction may be used in place of the magnets  34 . 
         [0060]    The rotor yoke  35  is a magnetic body rotating together with the hub  32  and the magnets  34 . The rotor yoke  35  of the present embodiment includes a disc portion  71  and a yoke protrusion portion  72 . The disc portion  71  is positioned below the armature  22  and above the bottom plate portion  51  of the base member  21  so as to extend in a circular ring shape. The yoke protrusion portion  72  lies radially inward of the armature  22  and extends upward from the disc portion  71  in a substantially cylindrical shape. The yoke protrusion portion  72  is fixed to the hub protrusion portion  62 . 
         [0061]    In the brushless motor  11  described above, magnetic fluxes axially penetrating the armature  22  are generated if a drive current is supplied to the coil patterns of the armature  22 . Also formed is a magnetic circuit extending through the armature  22 , the magnets  34 , the hub  32  and the rotor yoke  35 . Circumferential torque is generated under the action of the magnetic fluxes. As a result, the rotary unit  3  is rotated about the center axis  9  with respect to the stationary unit  2  The magnetic disk  12  supported on the hub  32  is rotated about the center axis  9  together with the rotary unit  3 . 
         [0062]    Next, description will be made on a structure for fixing the base member  21  and the bearing mechanism  4  and a structure for fixing the hub  32  and the rotor yoke  35 . 
         [0063]      FIG. 4  is a partial vertical section view of the brushless motor  11 . As shown in  FIG. 4 , the lower portion of the sleeve  23  is arranged radially inward of the base protrusion portion  52 . The inner circumferential surface of the base protrusion portion  52  and the outer circumferential surface of the sleeve  23  are fixed to each other by an adhesive agent  80 . In the following description, the portion where the inner circumferential surface of the base member  21  and the outer circumferential surface of the bearing mechanism  4  are fixed to each other will be referred to as first fixing portion  81 . In the first fixing portion  81 , the inner circumferential surface of the base protrusion portion  52  defining the base through-hole  53  is radially opposed to the outer circumferential surface of the sleeve  23 , i.e., the outer circumferential surface of the bearing mechanism  4 , with a gap left therebetween. An adhesive agent exists in the gap. 
         [0064]    In the brushless motor  11  of the present embodiment, the yoke protrusion portion  72  is positioned radially outward of the hub protrusion portion  62 . The hub protrusion portion  62  is press-fitted to the yoke protrusion portion  72 . In the following description, the portion where the hub protrusion portion  62  and the rotor yoke  35  are fixed to each other will be referred to as second fixing portion  82 . In the second fixing portion  82 , the outer circumferential surface of the hub protrusion portion  62  makes contact with the inner circumferential surface of the yoke protrusion portion  72 . 
         [0065]    In the present embodiment, the outer circumferential surface of the hub protrusion portion  62  includes a hub-side contact surface  621  and a hub-side opposing surface  622  positioned below the hub-side contact surface  621 . The hub-side contact surface  621  protrudes radially outward of the hub-side opposing surface  622 . In the present embodiment, therefore, only the hub-side contact surface  621  makes contact with the inner circumferential surface of the yoke protrusion portion  72 . The hub-side opposing surface  622  is radially opposed to the inner circumferential surface of the yoke protrusion portion  72  with a gap left therebetween. 
         [0066]    In this regard, an axial length between a lower end of a constant diameter surface of the sleeve  23  extending along the first fixing portion  81  or further extending downward from the first fixing portion  81  and an upper end of a constant diameter surface of the base protrusion portion  52  extending along the first fixing portion  81  or further extending upward from the first fixing portion  81  is assumed to be d1. In the example shown in  FIG. 4 , the constant diameter surface of the sleeve  23  does not extend below the first fixing portion  81 . Moreover, the constant diameter surface of the base protrusion portion  52  does not extend above the first fixing portion  81 . Consequently, the axial length of the first fixing portion  81  is equal to d1. 
         [0067]    An axial length between a lower end of a constant diameter surface of the hub protrusion portion  62  extending along the second fixing portion  82  or further extending downward from the second fixing portion  82  and an upper end of a constant diameter surface of the yoke protrusion portion  72  extending along the second fixing portion  82  or further extending upward from the second fixing portion  82  is assumed to be d2. In the example shown in  FIG. 4 , the constant diameter surface of hub protrusion portion  62  does not extend below the second fixing portion  82 . Consequently, the axial length between the lower end of the second fixing portion  82  and the upper end of a constant diameter surface of the yoke protrusion portion  72  extending along the second fixing portion  82  is equal to d2. 
         [0068]    In the brushless motor  11 , as shown in  FIG. 4 , d1 and d2 are set to satisfy a relationship of d1&gt;d2. For that reason, when manufacturing the brushless motor  11 , the bonding of the sleeve  23  and the base protrusion portion  52  in the first fixing portion  81  is started prior to the hub protrusion portion  62  coming into contact with the yoke protrusion portion  72  in the second fixing portion  82 . Thus, the respective members are substantially coaxially located in position on the basis of the first fixing portion  81 . As a result, the rotor yoke  35  and the hub  32  are accurately fixed to each other in the second fixing portion  82 . The term “constant diameter surface” includes a substantially constant diameter surface. 
         [0069]    Particularly, in the present embodiment, the rotor yoke  35  is provided with an axially-extending cylindrical yoke protrusion portion  72 . The hub protrusion portion  62  is press-fitted to the yoke protrusion portion  72 . This makes it possible to lengthen the second fixing portion  82  in the axial direction. Accordingly, it is possible to strongly fix the hub  32  and the rotor yoke  35 . 
         [0070]    In the present embodiment, the yoke protrusion portion  72  is positioned radially outward of the hub protrusion portion  62 . In other words, the hub protrusion portion  62  is arranged between the yoke protrusion portion  72  and the base protrusion portion  52 . Thus, the yoke protrusion portion  72  belonging to the rotary unit  3  is restrained from making contact with the base protrusion portion  52  belonging to the stationary unit  2 . 
         [0071]    In the present embodiment, the hub protrusion portion  62  includes a hub-side opposing surface  622  positioned below a hub-side contact surface  621 . The hub-side opposing surface  622  is a little smaller in diameter than the hub-side contact surface  621 . For that reason, in the manufacturing process to be described later, the hub-side contact surface  621  can be press-fitted to the inner circumferential surface of the yoke protrusion portion  72  after the hub-side opposing surface  622  is inserted into the radial inner side of the yoke protrusion portion  72 . This makes it possible to restrain the hub  32  from being inclined with respect to the rotor yoke  35  in the press-fitting process. Accordingly, it becomes possible to accurately fix the hub  32  with respect to the rotor yoke  35 . 
         [0072]    The outer circumferential surface of the ring-shaped member  33  makes contact with the inner circumferential surface of the hub protrusion portion  62  or stays radially opposed to the inner circumferential surface of the hub protrusion portion  62  across an adhesive agent. In the present embodiment, the lower end of a third fixing portion  83  where the ring-shaped member  33  and the hub protrusion portion  62  are fixed to each other is positioned above the upper end of the second fixing portion  82 . In other words, the second fixing portion  82  and the third fixing portion  83  do not radially overlap with each other. For that reason, the press-fitting load applied to the second fixing portion  82  is hard to be transferred to the third fixing portion  83 . This helps restrain the ring-shaped member  33  from being displaced radially inward. As a result, it is possible to accurately set the radial gap between the sleeve  23  and the ring-shaped member  33  in a position near the liquid level of the lubricant  41 . 
         [0073]      FIG. 5  is a flowchart illustrating some steps for manufacturing the brushless motor  11  according to the second preferred embodiment.  FIGS. 6 through 8  are vertical section views showing the brushless motor  11  which is under a manufacturing process. A manufacturing sequence of the brushless motor  11  will now be described with reference to  FIGS. 5 through 8 . 
         [0074]    When manufacturing the brushless motor  11 , a first unit  111  including the base member  21 , the armature  22  and the rotor yoke  35  is prepared first (step S 1 ). As shown in  FIG. 6 , the base member  21  includes a tubular surface  55  and a slant surface  56 . The tubular surface  55  is radially opposed to the outer circumferential portion of the rotor yoke  35 . The slant surface  56  obliquely extends such that the diameter of the slant surface  56  grows smaller as the slant surface  56  extends downward from the lower end portion of the tubular surface  55 . In the first unit  111 , the rotor yoke  35  is arranged inside the tubular surface  55 . The lower end portion of the outer circumferential surface of the rotor yoke  35  is arranged on the slant surface  56 . As a consequence, the rotor yoke  35  is arranged in a substantially coaxial relationship with the center axis  9 . 
         [0075]    Next, a second unit  112  including the sleeve  23 , the shaft  31 , the hub  32 , the ring-shaped member  33  and the magnets  34  is prepared (step S 2 ). Step S 2  may be performed prior to step S 1  or may be performed simultaneously with step S 1 . Thereafter, the first unit  111  and the second unit  112  are combined together (steps S 3  and S 4 ). 
         [0076]    In order to combine the first unit  111  and the second unit  112  together, an adhesive agent  80  is first applied on the upper end portion of the inner circumferential surface of the base protrusion portion  52  or on the lower end portion of the outer circumferential surface of the sleeve  23 . Then, as shown in  FIG. 7 , the sleeve  23  is inserted into the base through-hole  53  (step S 3 ). The inner circumferential surface of the base protrusion portion  52  is radially opposed to the outer circumferential surface of the sleeve  23  across the adhesive agent  80 . Thus, the respective members are substantially coaxially located in position on the basis of the inner circumferential surface of the base protrusion portion  52  and the outer circumferential surface of the sleeve  23 . 
         [0077]    At the time point shown in  FIG. 7 , the lower end portion of the hub protrusion portion  62  stays inserted into the radial inner side of the yoke protrusion portion  72 . Since the diameter of the hub-side opposing surface  622  is smaller than the inner diameter of the yoke protrusion portion  72 , the hub protrusion portion  62  does not make contact with the yoke protrusion portion  72 . 
         [0078]    Thereafter, the second unit  112  is further moved down with respect to the first unit  111 . Consequently, as shown in  FIG. 8 , the hub protrusion portion  62  is press-fitted to the yoke protrusion portion  72  (step S 4 ). In other words, the outer circumferential surface of the yoke protrusion portion  72  comes into contact with the hub-side contact surface  621  of the hub protrusion portion  62 . In the present embodiment, as stated above, the sleeve  23  is first inserted into the base protrusion portion  52 . Thereafter, the yoke protrusion portion  72  and the hub protrusion portion  62  are brought into contact with each other. Accordingly, the hub  32  is accurately fixed with respect to the rotor yoke  35 . 
         [0079]      FIG. 9  is a partial vertical section view showing a brushless motor  11 B according to a third preferred embodiment. Description will now be made on the brushless motor  11 B according to the third preferred embodiment, with an emphasis placed on the points differing from the second embodiment. 
         [0080]    In the brushless motor  11 B shown in  FIG. 9 , the hub-side contact surface  621 B of the hub protrusion portion  62 B is positioned below in the second embodiment. The hub-side opposing surface  622 B is positioned above the hub-side contact surface  621 B. If d1 and d2 are defined as in the second embodiment, a relationship of d1&lt;d2 is satisfied in the brushless motor  11 B. 
         [0081]    For that reason, when manufacturing the brushless motor  11 B, the contact of the hub protrusion portion  62 B with the yoke protrusion portion  72 B in the second fixing portion  82 B is started prior to the sleeve  23 B being bonded to the base protrusion portion  52 B in the first fixing portion  81 B. Thus, the respective members are substantially coaxially located in position on the basis of the second fixing portion  82 B. As a result, the base member  21 B and the sleeve  23 B are accurately fixed to each other in the first fixing portion  81 B. 
         [0082]      FIG. 10  is a flowchart illustrating some steps for manufacturing the brushless motor  11 B.  FIG. 11  is a partial vertical section view showing the brushless motor  11 B which is under a manufacturing process. A manufacturing sequence of the brushless motor  11 B will now be described with reference to  FIGS. 10 and 11 . 
         [0083]    When manufacturing the brushless motor  11 B, just like the second preferred embodiment, a first unit  111 B including the base member  21 B, the armature  22 B and the rotor yoke  35 B and a second unit  112 B including the sleeve  23 B, the shaft  31 B, the hub  32 B, the ring-shaped member  33 B and the magnets  34 B are prepared (steps S 1 B and S 2 B). Thereafter, the first unit  111 B and the second unit  112 B are combined together (steps S 3 B and S 4 B). 
         [0084]    In order to combine the first unit  111 B and the second unit  112 B together, an adhesive agent SOB is first applied on the upper end portion of the inner circumferential surface of the base protrusion portion  52 B or on the lower end portion of the outer circumferential surface of the sleeve  23 B. Then, as shown in  FIG. 11 , the hub protrusion portion  62 B is press-fitted to the yoke protrusion portion  72 B (step S 3 B). In other words, the outer circumferential surface of the yoke protrusion portion  72 B is brought into contact with the hub-side contact surface  621 B of the hub protrusion portion  62 B. As a consequence, the respective members are substantially coaxially located in position on the basis of the hub-side contact surface  621 B and the inner circumferential surface of the yoke protrusion portion  72 B. 
         [0085]    Thereafter, the second unit  112 B is further moved down with respect to the first unit  111 B. As a result, the sleeve  23  is inserted into the base through-hole  53 B (step S 4 B). Thus, the inner circumferential surface of the base protrusion portion  52 B is radially opposed to the outer circumferential surface of the sleeve  23 B across the adhesive agent  80 B. In the present embodiment, as stated above, the hub protrusion portion  62 B is first press-fitted to the yoke protrusion portion  72 B. Thereafter, the sleeve  23  B is inserted into the base protrusion portion  52  B. Accordingly, the sleeve  23 B is accurately fixed with respect to the base member  21 B. 
         [0086]    Features of the above-described preferred embodiments and the modifications thereof may be combined appropriately as long as no conflict arises. 
         [0087]    While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims. 
         [0088]      FIG. 12  is a partial vertical section view of a brushless motor  11 C according to one modified example. In the example shown in  FIG. 12 , the yoke protrusion portion  72 C is positioned radially inward of the hub protrusion portion  62 C. The yoke protrusion portion  72 C is press-fitted to the hub protrusion portion  62 C. In the second fixing portion  82 C, the outer circumferential surface of the yoke protrusion portion  72 C makes contact with the inner circumferential surface of the hub protrusion portion  62 C. This makes it possible to prevent the hub protrusion portion  62 C from being displaced radially inward by the press-fitting load applied to the second fixing portion  82 C. Accordingly, it is possible to prevent the ring-shaped member  33 C from being displaced radially inward. 
         [0089]      FIG. 13  is a partial vertical section view of a brushless motor  11 D according to another modified example. In the example shown in  FIG. 13 , the rotor yoke  35 D does not include any yoke protrusion portion. In other words, the rotor yoke  35 D is composed of only a disc portion  71 D. In the second fixing portion  83  D, the inner circumferential surface of the disc portion  71 D makes contact with the inner circumferential surface of the hub protrusion portion  62 D. This makes it easy to manufacture the rotor yoke  35 D. Moreover, the axial position of the second fixing portion  82 D can be spaced apart downward from the axial position of the third fixing portion  82 D. Accordingly, it is possible to restrain the ring-shaped member  33 D from being displaced radially inward. 
         [0090]      FIG. 14  is a partial vertical section view of a brushless motor  11 E according to a further modified example. 
         [0091]    In the example shown in  FIG. 14 , the inner circumferential surface of the yoke protrusion portion  72 E includes a yoke-side contact surface  721 E and a yoke-side opposing surface  722 E positioned above the yoke-side contact surface  721 E. The yoke-side contact surface  721 E protrudes radially inward of the yoke-side opposing surface  722 E. For that reason, in the example shown in  FIG. 14 , only the yoke-side contact surface  721 E makes contact with the outer circumferential surface of the hub protrusion portion  62 E. The yoke-side opposing surface  722 E is radially opposed to the outer circumferential surface of the hub protrusion portion  62 E across a gap. 
         [0092]    When manufacturing the brushless motor  11 E, the hub protrusion portion  62 E is inserted into the radial inner side of the yoke-side opposing surface  722 E. Thereafter, the hub protrusion portion  62 E is press-fitted to the yoke-side contact surface  721 E. This makes it possible to restrain the hub  32 E from being inclined with respect to the rotor yoke  35 E in the press-fitting process. Accordingly, it becomes possible to accurately fix the hub  32 E with respect to the rotor yoke  35 E. 
         [0093]      FIG. 15  is a partial vertical section view of a brushless motor  11 F according to a still further modified example. In the example shown in  FIG. 15 , the outer circumferential surface of the hub protrusion portion  62 F includes a pair of hub-side contact surfaces  621 F and  623 F arranged in an axially spaced-apart relationship and a hub-side opposing surface  622 F positioned between the hub-side contact surfaces  621 F and  623 F. The hub-side contact surfaces  621 F and  623 F protrude radially outward of the hub-side opposing surface  622 F. For that reason, in the example shown in  FIG. 15 , only the hub-side contact surfaces  621 F and  623 F make contact with the inner circumferential surface of the yoke protrusion portion  72 F. The hub-side opposing surface  622 F is radially opposed to the inner circumferential surface of the yoke protrusion portion  72 F across a gap. 
         [0094]    In the example shown in  FIG. 15 , the hub protrusion portion  62 F makes contact with the yoke protrusion portion  72 F at two upper and lower points. Thus, the hub  32 F is restrained from being inclined with respect to the rotor yoke  35 F. In addition, the fixing strength of the rotor yoke  35 F and the hub  32 F becomes higher. 
         [0095]    The bearing mechanism may be a fluidic dynamic-pressure bearing mechanism as employed in the second and third preferred embodiments or may be other kinds of bearings such as an oil-containing sintered bearing, a ball bearing and a slide bearing. The member defining the outer circumferential surface of the bearing mechanism may be a cup-shaped housing for holding the sleeve or an outer race of a ball bearing. 
         [0096]    The brushless motor of the present invention may be a so-called fixed-shaft-type motor in which a shaft belongs to a stationary unit and a sleeve belongs to a rotary unit. In this case, the outer circumferential surface of the shaft or the outer circumferential surface of the ring-shaped member fixed to the shaft makes up the outer circumferential surface of the bearing mechanism. The outer circumferential surface of the shaft or the outer circumferential surface of the ring-shaped member is fixed to the inner circumferential surface of the base member. 
         [0097]    As another modification, the outer circumferential surface of the bearing mechanism may be press-fitted to the inner circumferential surface of the base member. In other words, the inner circumferential surface of the base member may make contact with the outer circumferential surface of the bearing mechanism in the first fixing portion. In addition, an adhesive agent may be used in combination when the outer circumferential surface of the bearing mechanism is press-fitted to the base member. 
         [0098]    As a further modification, the hub protrusion portion may be fixed to the rotor yoke by an adhesive agent. In the second fixing portion, the inner circumferential surface of the rotor yoke may be radially opposed to the outer circumferential surface of the hub protrusion portion across a gap. An adhesive agent exists in the gap. In addition, an adhesive agent may be used in combination when the hub protrusion portion is press-fitted to the rotor yoke. 
         [0099]    As a still further modification, the magnets may be arranged below the armature. For example, the magnets may be fixed to the upper surface of the rotor yoke. In other words, the magnets may be fixed to the hub or the rotor yoke and may be opposed to one of the upper and lower surfaces of the armature. The hub and the rotor yoke may be formed of a single member. 
         [0100]    As a yet still further modification, the hub may be made of a non-magnetic material such as aluminum or the like. In this case, for example, a magnetic body making up the top plate portion and the hub protrusion portion may be fixed to the lower surface of the hub made of a non-magnetic material. 
         [0101]    The brushless motor and the disk drive apparatus of the present invention may be used to rotate a disk other than the magnetic disk, e.g., an optical disk. 
         [0102]    The specific shapes of the respective members may differ from those shown in the respective figures of the subject application. 
         [0103]    The respective elements appearing in the preferred embodiments and the modified examples described above may be combined appropriately as long as no conflict arises.