Patent Publication Number: US-2005141136-A1

Title: Spindle motor and disk apparatus provided with the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2003-431181, filed Dec. 25, 2003, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to a spindle motor, and more particularly, to a spindle motor of an inner-rotor type having a dynamic pressure fluid bearing and a disk apparatus provided with the spindle motor.  
      2. Description of the Related Art  
      Disk apparatuses, such as magnetic disk apparatuses, optical disk apparatuses, etc., are provided with a spindle motor that supports and drives a disk as a rotating body. The spindle motor, e.g., a fixed-shaft spindle motor, usually comprises a fixed shaft and a rotor that is rotatably supported on the shaft. The rotor is supported on the fixed shaft by radial bearings that support a radial load and a thrust bearing that supports an axial load. A plurality of radial bearings are arranged spaced in the axial direction of the fixed shaft, in order to prevent the rotor from tilting with respect to the shaft, that is, to prevent the rotor from rocking around an axis perpendicular to the shaft. In a spindle motor described in Jpn. Pat. Appln. KOKAI Publication No. 2000-186716, for example, a fine gap is defined between the outer peripheral surface of a fine gap and the inner peripheral surface of a rotor, and two radial bearings are provided in the fine gap. These two radial bearings are arranged in the axial direction of the fixed shaft.  
      In recent years, fluid bearings have been widely used as bearings for spindle motors. A fluid bearing generates a dynamic pressure with use of a fluid, such as air or lubricating oil, filled in fine gaps, thereby supporting a rotating body. Thus, the rotating body can be supported with stability, and the spindle motor can be miniaturized.  
      As described above, the spindle motor is provided with a plurality of radial bearings that are fluid bearings, so that the rotor can be steadily rotated at high speed. With the miniaturization of modern disk apparatuses, spindle motors are expected to be further reduced in size. In the aforementioned configuration in which a plurality of radial bearings are arranged in the axial direction of the fixed shaft, however, the size in the axial direction of the shaft is large and cannot be reduced with ease. Possibly, the axial dimension may be reduced by using only one radial bearing. In this case, however, there is a possibility of the rotor swinging or tilting around the single bearing, so that the rotor cannot be easily supported and rotated with stability.  
     BRIEF SUMMARY OF THE INVENTION  
      According to an aspect of the invention, a spindle motor comprises: a fixed shaft at least one end of which is fixed; a rotating sleeve which is rotatably arranged outside the fixed shaft and which has an inner peripheral surface opposed to an outer peripheral surface of the fixed shaft across a first fine gap, an outer peripheral surface, and a bottom surface extending between the inner and outer peripheral surfaces and; an outer ring member which is provided fixedly and which has an opposite surface opposed to the bottom surface of the rotating sleeve across a second fine gap and an inner peripheral surface opposed to the outer peripheral surface of the rotating sleeve across a third fine gap; a dynamic pressure generating fluid filled in the first, second, and third fine gaps; a hub fixed to the rotating sleeve; a magnetic attraction portion which has a magnet fixed to the hub and a magnetic member fixedly arranged and opposed to the magnet and urges the rotating sleeve in the axial direction of the fixed shaft and in a direction such that the second fine gap narrows, by a force of magnetic attraction between the magnet and the magnetic member; a first radial dynamic pressure generating portion located singly in the first fine gap; a second radial dynamic pressure generating portion located singly in the third fine gap; and a thrust dynamic pressure generating portion located in the second fine gap.  
      According to another aspect of the invention, a spindle motor comprises: a fixed shaft at least one end of which is fixed; a rotating sleeve which is rotatably located outside the fixed shaft and which has an inner peripheral surface opposed to an outer peripheral surface of the fixed shaft across a first fine gap, an outer peripheral surface, a first end face extending between the inner and outer peripheral surfaces, and a second end face spaced in the axial direction of the fixed shaft from the first end face and extending between the inner and outer peripheral surfaces; an outer ring member which is fixedly arranged and which has a first opposite surface opposed to the first end face of the rotating sleeve across a second fine gap, an inner peripheral surface opposed to the outer peripheral surface of the rotating sleeve across a third fine gap, and a second opposite surface opposed to the second end face of the rotating sleeve across a fourth fine gap; a dynamic pressure generating fluid filled in the first, second, third, and fourth fine gaps; a hub fixed to the rotating sleeve; a first radial dynamic pressure generating portion located singly in the first fine gap; a second radial dynamic pressure generating portion located singly in the third fine gap; a first thrust dynamic pressure generating portion located in the second fine gap; and a second thrust dynamic pressure generating portion located in the fourth fine gap.  
      According to still another aspect of the invention, a disk apparatus comprises: a disk-shaped recording medium; and the spindle motor which supports and drives the recording medium. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
      The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
       FIG. 1  is a plan view showing a hard disk drive (hereinafter referred as an HDD) according to a first embodiment of the invention;  
       FIG. 2  is a sectional view showing a spindle motor of the HDD;  
       FIG. 3  is a side view showing a fixed shaft of the spindle motor;  
       FIG. 4  is a side view showing a rotating sleeve of the spindle motor;  
       FIGS. 5A and 5B  are plan views showing thrust dynamic pressure generating grooves on an outer ring member of the spindle motor; and  
       FIG. 6  is a sectional view showing an HDD according to a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      A first embodiment in which a disk apparatus according to this invention is applied to an HDD will now be described in detail with reference to the accompanying drawings.  
      As shown in  FIGS. 1 and 2 , the HDD comprises a case  12  in the form of an open-topped rectangular box and a top cover  14  screwed to the case by screws. The top cover  14  closes a top opening of the case. The case  12  has a flat bottom wall  12   a  in the shape of a flat sheet, which functions as a base. The case  12  is formed of a magnetic material, such as iron.  
      The case  12  contains a magnetic disk  16  for use as a recording medium, a spindle motor  18 , magnetic heads, and a carriage assembly  22 . The motor  18  supports and rotates the disk. The magnetic heads are used to write and read information to and from the disk. The carriage assembly  22  supports the magnetic heads for movement with respect to the magnetic disk  16 . Further, the case  12  houses a voice coil motor (hereinafter referred to as VCM)  24 , a ramp load mechanism  25 , a substrate unit  21 , etc. The VCM  24  rocks and positions the carriage assembly. The ramp load mechanism  25  holds the magnetic heads in a shunt position off the magnetic disk when the magnetic heads are moved to the outermost periphery of the disk. The substrate unit  21  has a read/write amplifier, for use as a processing circuit for recording/reproduction signals, and the like.  
      A printed circuit board (not shown) that controls the operations of the spindle motor  18 , VCM  24 , and magnetic heads through the substrate unit  21  is provided on the outer surface side of the bottom wall  12   a  of the case  12 .  
      The magnetic disk  16  has a magnetic recording layer that is formed on its upper and/or lower surface, and is, for example, about 0.85 inch in diameter. The disk  16  is fitted on a hub of the spindle motor  18 , which will be mentioned later, and is fixedly supported on the hub by means of a clamp spring (not shown). As the motor  18  is driven, the disk  16  is rotated at a given speed, e.g., at 4,200 rpm.  
      The carriage assembly  22  comprises a bearing portion  26  fixed on the bottom wall  12   a  of the case  12 , arms  30  extending from the bearing portion, and a suspension  32  extending from each of the arms. A magnetic head  34  is supported on an extended end of the suspension  32  by means of a gimbals portion (not shown).  
      As shown in  FIG. 1 , the carriage assembly  22  has a support frame  36  that extends in the opposite direction from the bearing portion  26  with respect to the arms  30 . The support frame  36  supports a voice coil  38  that constitutes a part of the VCM  24 . The support frame  36  is molded from synthetic resin and formed integrally on the outer periphery of the voice coil  38 . The coil  38  is situated between a pair of yokes  40  that are fixed on the case  12 . It forms the VCM  24  in conjunction with the yokes and a magnet (not shown) that is fixed to one of the yokes. When the voice coil  38  is energized, the carriage assembly  22  rocks around the bearing portion  26 , whereupon the magnetic head  34  is moved and positioned onto a desired track of the magnetic disk  16 .  
      The ramp load mechanism  25  includes a ramp  42  and a tab  44 . The ramp  42  is provided on the bottom wall of the case  12  and located outside the magnetic disk  16 . The tab  44  extends from the distal end of the suspension  32 . As the carriage assembly  22  rotates to the retreated position outside the disk  16 , the tab  44  engages a ramp surface of the ramp  42 , and is then pulled up by the inclination of the ramp surface. Thereupon, the magnetic head is unloaded.  
      The following is a detailed description of the spindle motor  18 .  
      As shown in  FIG. 2 , the spindle motor  18  comprises a fixed shaft  50 , rotating sleeve  52 , outer ring member  54 , and hub  56 . The sleeve  52  is rotatably supported by the fixed shaft. The outer ring member  54  is fixed to the shaft  50  and the bottom wall  12   a . The hub  56  is fixed to the sleeve  52 . The fixed shaft  50  is set substantially upright on the inner surface of the bottom wall  12   a , and its lower end is fixed to the wall  12   a  with a screw  51   a . The upper end of the shaft  50  is fixed to the top cover  14  with a screw  51   b.    
      The outer ring member  54  integrally has an annular base portion  54   a , a cylindrical portion  54   b  extending from the outer periphery of the base portion, and an annular flange  54   c  on the outer periphery of an extended end of the cylindrical portion. The base portion  54   a  is fitted on the outer periphery of the lower end portion of the fixed shaft  50  and is in contact with the inner surface of the bottom wall  12   a . The upper surface of the base portion  54   a  extends radially with respect to the shaft  50  and forms an annular opposite surface  55 . The cylindrical portion  54   b  is coaxial with the fixed shaft  50  and faces the outside of the shaft across a gap.  
      The rotating sleeve  52  is coaxial with the fixed shaft  50  and is situated between the cylindrical portion  54   b  of the outer ring member  54  and the fixed shaft. The sleeve  52  has an inner peripheral surface  52   a , an outer peripheral surface  52   b , and a bottom surface  52   c . The inner peripheral surface  52   a  faces the outer peripheral surface of the shaft  50  across a first fine gap  57 . The outer peripheral surface  52   b  faces the inner peripheral surface of the cylindrical portion  54   b  across a third fine gap  60 . The bottom surface  52   c  extends between the peripheral surfaces  52   a  and  52   b . The bottom surface  52   c  faces the opposite surface  55  of the base portion  54   a  across a second fine gap  58 . The first and third fine gaps  57  and  60  have their respective open ends that are open to the atmosphere and closed ends that communicate with each other through the second fine gap  58 . The first, second, and third fine gaps  57 ,  58  and  60  are filled with a lubricating oil  62  for use as a dynamic pressure generating fluid. The width of each fine gap ranges from about 2 to 15 μm.  
      The first fine gap  57  is provided with only one first radial dynamic pressure generating portion. The third fine gap  60  is provided with only one second radial dynamic pressure generating portion. Further, the second fine gap  58  is provided with a thrust dynamic pressure generating portion. As shown in  FIGS. 2 and 3 , the first radial dynamic pressure generating portion has a plurality of first radial dynamic pressure generating grooves  64  that are formed of herringbone grooves on the outer peripheral surface of the fixed shaft  50 . The grooves  64  are arranged in the circumferential direction of the shaft  50 , covering its whole circumference. When the rotating sleeve  52  rotates, the grooves  64  cause the lubricating oil  62  in the first fine gap  57  to generate a radial dynamic pressure. The first radial dynamic pressure generating portion has a dynamic pressure generation center a.  
      As shown in  FIGS. 2 and 4 , the second radial dynamic pressure generating portion has a plurality of second radial dynamic pressure generating grooves  66  that are formed of herringbone grooves on the outer peripheral surface  52   b  of the rotating sleeve  52 . The grooves  66  are arranged in the circumferential direction of the sleeve  52 , covering its whole circumference. When the rotating sleeve  52  rotates, the grooves  66  cause the lubricating oil  62  in the third fine gap  60  to generate a radial dynamic pressure. The second radial dynamic pressure generating portion has a dynamic pressure generation center b.  
      The first radial dynamic pressure generating portion in the first fine gap  57  and the second radial dynamic pressure generating portion in the third fine gap  60  are arranged overlapping each other in the radial direction of the fixed shaft  50 . Further, the first radial dynamic pressure generating portion is located so that its dynamic pressure generation center a is kept at a distance h in the axial direction of the shaft  50  from the dynamic pressure generation center b of the second radial dynamic pressure generating portion. The distance h is set to 0.1 mm to 1 mm, for example.  
      As shown in  FIGS. 2 and 5 A, the thrust dynamic pressure generating portion has a plurality of thrust dynamic pressure generating grooves  68  that are formed of spiral grooves on the opposite surface  55  of the outer ring member  54 . The grooves  68  extend spirally around the fixed shaft  50  and are arranged in the circumferential direction of the opposite surface  55 . When the rotating sleeve  52  rotates, the grooves  68  cause the lubricating oil  62  in the second fine gap  58  to generate a thrust-direction dynamic pressure. As shown in  FIG. 5B , the thrust dynamic pressure generating grooves  68  of the thrust dynamic pressure generating portion may alternatively be formed of herringbone grooves that are arranged on the opposite surface  55  of the outer ring member  54  in its circumferential direction.  
      As shown in  FIG. 2 , the hub  56  of the spindle motor  18  is in the form of a ring, which is fixed on the outer periphery of the upper end portion of the rotating sleeve  52 . The hub  56  is coaxial with the fixed shaft  50  and extends outward beyond the cylindrical portion  54   b  of the outer ring member  54  in the radial direction of the shaft. An annular skirt portion  70  that extends toward the bottom wall  12   a  of the case  12  is formed integrally on the outer peripheral portion of the hub  56 . Further, an annular protrusion  72  protrudes from the lower surface of the hub  56  and is situated coaxially with the fixed shaft  50  between the cylindrical portion  54   b  of the rotating sleeve  52  and the skirt portion  70 . An annular stopper sheet  74  is fixed to the lower end of the protrusion  72 . The stopper sheet  74  faces the flange  54   c  of the cylindrical portion  54   b  in the axial direction of the fixed shaft  50 , thereby restraining the rotating sleeve  52  and the hub  56  from slipping off the shaft  50  and the outer ring member  54 . The rotating sleeve  52  and the hub  56  may be molded integrally with each other. The magnetic disk  16  is fixed to the hub  56  with its center hole fitted on the outer peripheral surface of the hub.  
      An annular permanent magnet  76  is fixed on the outer periphery of the lower end portion of the skirt portion  70 , which constitutes a part of the hub  56 , and is situated coaxially with the fixed shaft  50 . The permanent magnet  76  faces the inner surface of the bottom wall  12   a  of the case  12  across a given gap. As mentioned before, the bottom wall  12   a  is formed of a magnetic material and constitutes a magnetic member. The magnet  76 , which functions as a magnetic attraction portion, and the hub  56 , to which the magnet is fixed, are urged toward the bottom wall  12   a  by a force of magnetic attraction between the magnet and the bottom wall. Thus, the hub  56  and the rotating sleeve  52  are urged in the axial direction of the fixed shaft  50  and in a direction such that the second fine gap  58  narrows.  
      On the inner surface of the bottom wall  12   a , a plurality of stator coils  80  are arranged outside the hub  56  and face the permanent magnet  76  across a given gap. When the stator coils  80  are energized, the hub  56  and the rotating sleeve  52  are rotated by interaction between magnetic fields that are formed by the coils and the magnet  76 , individually.  
      According to the HDD with the spindle motor  18  constructed in this manner, a radial dynamic pressure is generated in the first and third fine gaps  57  and  60  when the hub  56  and the rotating sleeve  52  rotate. Under this dynamic pressure, the hub  56  and the rotating sleeve  52  support a radial load. At the same time, the hub  56  and the sleeve  56  support a thrust-direction load under the thrust-direction dynamic pressure generated in the second fine gap  58  and the force of magnetic attraction generated by the magnetic attraction portion. Thus, the hub  56  and the sleeve  52  can smoothly, steadily rotate at high speed without looseness. Likewise, the magnetic disk  16  that is supported by the hub  56  can steadily rotate at high speed. Thus, the magnetic head  34  can perform stable information recording and reproduction.  
      In the spindle motor  18 , the first and second radial dynamic pressure generating portions are arranged so that they are spaced and lapped in the radial direction of the fixed shaft  50  without overlapping in the axial direction of the shaft. Therefore, the dimension of the spindle motor in the axial direction of the shaft  50 , that is, its height, can be reduced to miniaturize the motor. Further, the respective dynamic pressure generation centers a and b of the first and second radial dynamic pressure generating portions are deviated from each other by the distance h in the axial direction of the shaft  50 . Accordingly, the rotating sleeve  52  can be prevented from swinging or tilting around the radial dynamic pressure generating portions, so that the hub  56  and the sleeve  52  can be supported and rotated with stability.  
      Thus, the hub and the magnetic disk as rotating bodies can be supported stably, and the resulting spindle motor can be reduced in size. Further, there may be obtained a small-sized magnetic disk apparatus that ensures stable information recording and reproduction.  
      In the first embodiment described above, the first radial dynamic pressure generating portion is not limited to the outer peripheral surface of the fixed shaft, and may be formed of dynamic pressure generating grooves that are formed on the inner peripheral surface of the rotating sleeve or on both these peripheral surfaces. The second radial dynamic pressure generating portion is not limited to the outer peripheral surface of the rotating sleeve, and may be formed of dynamic pressure generating grooves that are formed on the inner peripheral surface of the outer ring member or on both these peripheral surfaces. Further, the thrust dynamic pressure generating portion is not limited to the opposite surface of the outer ring member, and may be formed of dynamic pressure generating grooves that are formed on the bottom surface of the rotating sleeve or on both these surfaces.  
      The following is a description of an HDD according to a second embodiment of the invention.  
      According to the second embodiment, as shown in  FIG. 6 , a spindle motor  18  of the HDD comprises a fixed shaft  50 , rotating sleeve  52 , outer ring member  54 , and hub  56 . The sleeve  52  is rotatably supported by the fixed shaft. The outer ring member  54  is fixed to the shaft and a bottom wall  12   a  of a case  12 . The hub  56  is fixed to the sleeve  52 .  
      The outer ring member  54  is provided with an annular base portion  54   a , a cylindrical portion  54   b  extending from the outer periphery of the base portion, and an annular stopper sheet  82  extending from an extended end of the cylindrical portion toward the fixed shaft  50  and opposed to the base portion  54   a . The base portion  54   a  is fitted on the outer periphery of the lower end portion of the fixed shaft  50  and is in contact with the inner surface of the bottom wall  12   a . The upper surface of the base portion  54   a  extends radially with respect to the shaft  50  and forms an annular first opposite surface  55   a . The cylindrical portion  54   b  is coaxial with the fixed shaft  50  and faces the outside of the shaft across a gap. The inner surface of the stopper sheet  82  forms an annular second opposite surface  82   a , which faces the first opposite surface  55   a.    
      The rotating sleeve  52  is coaxial with the fixed shaft  50  and is situated between the cylindrical portion  54   b  of the outer ring member  54  and the fixed shaft. The sleeve  52  has an inner peripheral surface  52   a , outer peripheral surface  52   b , first end face  52   c , and second end face  52   d . The inner peripheral surface  52   a  faces the outer peripheral surface of the shaft  50  across a first fine gap  57 . The outer peripheral surface  52   b  faces the inner peripheral surface of the cylindrical portion  54   b  across a third fine gap  60 . The first end face  52   c  extends between the respective lower ends of the inner peripheral surface  52   a  and the outer peripheral surface  52   b . The second end face  52   d  extends from the upper end of the outer peripheral surface  52   b  toward the fixed shaft.  
      The first end face  52   c  faces the first opposite surface  55   a  of the base portion  54   a  across a second fine gap  58 . The second end face  52   d  faces the second opposite surface  82   a  of the stopper sheet  82  across a fourth fine gap  84 . The first and fourth fine gaps  57  and  84  have their respective open ends that are open to the atmosphere. The first and third fine gaps  57  and  60  have their respective closed ends that communicate with each other through the second fine gap  58 . The first, second, third, and fourth fine gaps  57 ,  58 ,  60  and  84  are filled with a lubricating oil  62  for use as a dynamic pressure generating fluid. The width of each fine gap ranges from about 2 to 15 μm. The stopper sheet  82  restrains the rotating sleeve  52  and the hub  56  from slipping off the fixed shaft  50  and the outer ring member  54 .  
      The first fine gap  57  is provided with only one first radial dynamic pressure generating portion. The third fine gap  60  is provided with only one second radial dynamic pressure generating portion. The second fine gap  58  is provided with a first thrust dynamic pressure generating portion, and the fourth fine gap  84  with a second thrust dynamic pressure generating portion. As in the first embodiment, the first radial dynamic pressure generating portion has a plurality of first radial dynamic pressure generating grooves  64  that are formed of herringbone grooves on the outer peripheral surface of the fixed shaft  50 . The grooves  64  are arranged in the circumferential direction of the shaft  50 , covering its whole circumference. When the rotating sleeve  52  rotates, the grooves  64  cause the lubricating oil  62  in the first fine gap  57  to generate a radial dynamic pressure. The first radial dynamic pressure generating portion has a dynamic pressure generation center a.  
      The second radial dynamic pressure generating portion has a plurality of second radial dynamic pressure generating grooves  66  that are formed of herringbone grooves on the outer peripheral surface  52   b  of the rotating sleeve  52 . The grooves  66  are arranged in the circumferential direction of the sleeve  52 , covering its whole circumference. When the rotating sleeve  52  rotates, the grooves  66  cause the lubricating oil  62  in the third fine gap  60  to generate a radial dynamic pressure. The second radial dynamic pressure generating portion has a dynamic pressure generation center b.  
      The first radial dynamic pressure generating portion in the first fine gap  57  and the second radial dynamic pressure generating portion in the third fine gap  60  are arranged overlapping each other in the radial direction of the fixed shaft  50 . Further, the first radial dynamic pressure generating portion is located so that its dynamic pressure generation center a is kept at a distance h in the axial direction of the shaft  50  from the dynamic pressure generation center b of the second radial dynamic pressure generating portion. The distance h is set to 0.1 mm to 1 mm, for example.  
      The first thrust dynamic pressure generating portion has a plurality of thrust dynamic pressure generating grooves  68  that are formed of spiral grooves on the first opposite surface  55   a  of the outer ring member  54 . The grooves  68  extend spirally around the fixed shaft  50  and are arranged in the circumferential direction of the opposite surface  55   a . When the rotating sleeve  52  rotates, the grooves  68  cause the lubricating oil  62  in the second fine gap  58  to generate a thrust-direction dynamic pressure.  
      The second thrust dynamic pressure generating portion has a plurality of thrust dynamic pressure generating grooves  86  that are formed of spiral grooves on the second end face  52   d  of the rotating sleeve  52 . The grooves  86  extend spirally around the fixed shaft  50  and are arranged in the circumferential direction of the second end face  52   d . When the rotating sleeve  52  rotates, the grooves  86  cause the lubricating oil  62  in the fourth fine gap  84  to generate a thrust-direction dynamic pressure. The thrust dynamic pressure generating grooves of the first and second thrust dynamic pressure generating portions may alternatively be formed of herringbone grooves.  
      The hub  56  of the spindle motor  18  is in the form of a ring, which is fixed on the outer periphery of the upper end portion of the rotating sleeve  52 . The hub  56  is coaxial with the fixed shaft  50  and extends outward beyond the cylindrical portion  54   b  of the outer ring member  54  in the radial direction of the shaft. An annular skirt portion  70  that extends toward the bottom wall  12   a  of the case  12  is formed integrally on the outer peripheral portion of the hub  56 . The magnetic disk  16  is fixed to the hub  56  with its center hole fitted on the outer peripheral surface of the hub. The rotating sleeve  52  and the hub  56  may be molded integrally with each other.  
      An annular permanent magnet  76  is fixed on the outer periphery of the lower end portion of the skirt portion  70  and is situated coaxially with the fixed shaft  50 . The permanent magnet  76  faces the inner surface of the bottom wall  12   a  of the case  12  across a given gap. The bottom wall  12   a  is formed of a magnetic material and constitutes a magnetic member. The magnet  76 , which functions as a magnetic attraction portion, and the hub  56 , to which the magnet is fixed, are urged toward the bottom wall  12   a  by a force of magnetic attraction between the magnet and the bottom wall. Thus, the hub  56  and the rotating sleeve  52  are urged in the axial direction of the fixed shaft  50  and in a direction such that the second fine gap  58  narrows.  
      On the inner surface of the bottom wall  12   a , a plurality of stator coils  80  are arranged outside the hub  56  and face the permanent magnet  76  across a given gap. When the stator coils  80  are energized, the hub  56  and the rotating sleeve  52  are rotated by interaction between magnetic fields that are formed by the coils and the magnet  76 , individually.  
      The second embodiment shares the other configurations of the spindle motor  18  and the HDD with the first embodiment. Therefore, like reference numerals are used to designate like portions of the two embodiments, and a detailed description of those portions is omitted.  
      According to the HDD with the spindle motor  18  constructed in this manner, a radial dynamic pressure is generated in the first and third fine gaps  57  and  60  when the hub  56  and the rotating sleeve  52  rotate. Under this dynamic pressure, the hub  56  and the rotating sleeve  52  support a radial load. At the same time, the hub  56  and the sleeve  52  support a thrust-direction load under the thrust-direction dynamic pressures generated in the second and fourth fine gaps  58  and  84  and the force of magnetic attraction generated by the magnetic attraction portion. Thus, the hub  56  and the sleeve  52  can smoothly, steadily rotate at high speed without looseness. Likewise, the magnetic disk  16  that is supported by the hub  56  can steadily rotate at high speed. Thus, the magnetic head  34  can perform stable information recording and reproduction.  
      In the spindle motor  18 , the first and second radial dynamic pressure generating portions are arranged so that they are spaced and lapped in the radial direction of the fixed shaft  50  without overlapping in the axial direction of the shaft. Therefore, the dimension of the spindle motor in the axial direction of the shaft  50 , that is, its height, can be reduced to miniaturize the motor. Further, the respective dynamic pressure generation centers a and b of the first and second radial dynamic pressure generating portions are deviated from each other by the distance h in the axial direction of the shaft  50 . Accordingly, the rotating sleeve  52  can be prevented from swinging or tilting around the radial dynamic pressure generating portions, so that the hub  56  and the sleeve  52  can be supported and rotated with stability.  
      Thus, the hub and the magnetic disk can be supported stably, and the resulting spindle motor can be reduced in size. Further, there may be obtained a small-sized magnetic disk apparatus that ensures stable information recording and reproduction.  
      In the second embodiment described above, the first radial dynamic pressure generating portion is not limited to the outer peripheral surface of the fixed shaft, and may be formed of dynamic pressure generating grooves that are formed on the inner peripheral surface of the rotating sleeve or on both these peripheral surfaces. The second radial dynamic pressure generating portion is not limited to the outer peripheral surface of the rotating sleeve, and may be formed of dynamic pressure generating grooves that are formed on the inner peripheral surface of the outer ring member or on both these peripheral surfaces. The first thrust dynamic pressure generating portion is not limited to the first opposite surface of the outer ring member, and may be formed of dynamic pressure generating grooves that are formed on the first end face of the rotating sleeve or on both these surfaces. Further, the second thrust dynamic pressure generating portion is not limited to the second end face of the rotating sleeve, and may be formed of dynamic pressure generating grooves that are formed on the second opposite surface of the stopper sheet or on both these surfaces.  
      The present invention is not limited directly to the embodiments described above, and various changes or modifications may be effected therein without departing from the scope of the invention. Further, various inventions may be made by suitably combining a plurality of components described in connection with the foregoing embodiments. For example, some of the components according to the foregoing embodiments may be omitted. Furthermore, the components according to the different embodiments may be combined as required.  
      In the embodiments described herein, for example, the magnetic member of the magnetic attraction portion is not limited to the bottom wall of the case, and an alternative magnet member separate from the bottom wall may be opposed to the magnet. In this case, the bottom wall of the case may be formed of a nonmagnetic material, such as aluminum. Further, the present invention is not limited to magnetic disk apparatuses, and may be also applied to any other disk apparatuses, such as optical disk apparatuses.