Patent Publication Number: US-8995084-B2

Title: Disk drive device with hub and wiring member with increased thinness

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Divisional Application based on U.S. Ser. No. 14/011,301 filed on Aug. 27, 2013, which is based on U.S. Ser. No. 13/760,260, filed on Feb. 6, 2013, which is based on U.S. Ser. No. 13/530,433 filed on Jun. 22, 2012, U.S. Pat. No. 8,395,826, issued Mar. 12, 2013, which is a Divisional of U.S. Ser. No. 12/545,751, filed Aug. 21, 2009, U.S. Pat. No. 8,213,114, issued Jul. 3, 2012, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a disk drive device with a reduced height. 
     2. Description of the Related Art 
     Recently, a disk drive device such as an HDD has been improved in bearing stiffness by incorporating a dynamic pressure fluid bearing unit. There is a case that such a disk drive device having the dynamic pressure fluid bearing unit is mounted on a small portable apparatus. A portable apparatus is desired to be further thinned and lightened. Therefore, the disk drive device which is mounted on the portable apparatus is desired to be further thinned and lightened. 
     For example, patent document 1 has disclosed a disk drive device having a dynamic pressure fluid bearing unit with a first radial dynamic pressure groove of which the formed width in the axial direction is narrower than that of a second radial dynamic pressure groove.
     Patent document 1: Japanese Patent Application Laid-Open No. 2007-198555   

     In order to thin a disk drive device, it is necessary to thin a spindle drive unit. 
     SUMMARY OF THE INVENTION 
     The present invention is devised in view of the abovementioned situation, and a purpose thereof is to provide a disk drive device with a reduced height. 
     In view of the above mentioned, a disk drive device according to an aspect of the present invention comprises: a base member; a hub; a bearing unit which is arranged on the base member and which rotatably supports the hub; and a spindle drive unit, which drives the hub to rotate, wherein the spindle drive unit includes a stator core having a salient pole, a coil wound around the salient pole, and a magnet that is opposite to the salient pole, wherein the hub is formed of a magnetic material, wherein the base member includes a wire hole through which a wire for forming the coil passes, and a concavity that is formed at a bottom surface of the base member at a position that is arranged radially outside of the outer diameter of the magnet, wherein the wire passes through the wire hole and is arranged along the bottom face of the base member to a wiring member to which the wire is connected at the concavity at the bottom surface of the base member, and wherein the hub is provided with a fitting portion located radially outside of a space in the bearing unit for storing lubricant and configured to be fitted to the end of the clamper that defines the central hole. 
     According to this aspect, the diameter of the magnet can be larger than the inner circumference of the recording disk by setting, a slim disk drive device can be produced even in the presence of both the wiring member and the wire, by connecting the wire to the wiring member at the lowered portion, so that the thickness of the disk drive device can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures. 
         FIG. 1A  is a view illustrating a disk drive device according to an embodiment. 
         FIG. 1B  is a view illustrating the disk drive device according to the present embodiment. 
         FIG. 2  is a sectional view of a part of the disk drive device according to the present embodiment. 
         FIG. 3  is a sectional view of a hub according to the present embodiment. 
         FIG. 4  is a sectional view of a part of a disk drive device according to the related art. 
         FIG. 5A  is a view illustrating a method of forming a coil according to the present embodiment. 
         FIG. 5B  is a view illustrating the method of forming a coil according to the present embodiment. 
         FIG. 5C  is a view illustrating the method of forming a coil according to the present embodiment. 
         FIG. 6  is a schematic view schematically illustrating behavior of momentary vibration of a recording disk surface. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention. 
     In the following, the same numeral is given to the same or a similar structural element or member, which is illustrated in each of the drawings, and redundant description will not be repeated. For ease of understanding, members of each of the drawings are appropriately enlarged or reduced in scale Here, in the following description, the terms “lower” and the “upper” in regard to the drawings are respectively expressed as the “bottom” and the “top”, for convenience. 
       FIGS. 1A and 1B  illustrate a disk drive device  100  according to an embodiment.  FIG. 1A  is a top view of the disk drive device  100  and  FIG. 1B  is a side view of the disk drive device  100 . Here,  FIG. 1A  illustrates a state where a top cover  2  is removed. Further.  FIG. 2  is a sectional view of a part of the disk drive device  100  according to the present embodiment. Furthermore,  FIG. 3  is a sectional view of a hub  4  according to the present embodiment. Each of  FIG. 2  and  FIG. 3  is the view sectioned at line A-B in  FIG. 1A . 
     The disk drive device  100  includes a chassis  10 , which has a base member  3  and a looped circumference wall portion  15 , the hub  4  to which a ring-shaped recording disk  1  is mounted, a bearing unit  5 , which is arranged on the base member  3  so as to rotatably support the hub  4 , a spindle drive unit  6 , which drives the hub  4  to rotate, a head drive unit  17 , the top cover  2 , and a screw  9 . Further, the disk drive device  100  includes a fixed body portion  7  configured with members that do not rotate and a rotating body portion  8  configured with members that rotate. The fixed body portion  7  and the rotating body portion  8  include the bearing unit  5 , which supports the hub  4  in order to be relatively rotatable, and the spindle drive unit  6 , which drives the hub  4  to rotate. 
     The chassis  10  includes the base member  3 , which is a plane area of a hollowed portion, and the looped circumference wall portion  15 , which is formed in a wall shape at the outer circumference of the base member  3 . The base member  3  has a bearing hole  3 A into which a housing  13 , a sleeve  14 , and a shaft  16  are inserted. The outer circumference surface of the looped circumference wall portion  15  is rectangularly formed. The inner circumference surface of the looped circumference wall portion  15  is formed by connecting a circular portion  15 A for surrounding the recording disk  1  and a rectangular portion  15 B for surrounding an area to which the head drive unit  17  is mounted. The looped circumference wall portion  15  functions as a support member of the disk drive device  100  for supporting in the rotation axial direction of the shaft  16 . On the other hand, the base member  3  functions as a support member of the disk drive device  100  for supporting in the direction perpendicular to the rotation axial direction of the shaft  16 . 
     The top cover  2 , which is illustrated in  FIG. 1B , is arranged and fixed at the upper end of the looped circumference wall portion  15  by screwing the screw  9  into a screw hole  15 C, which is formed at the upper end surface side of the looped circumference wall portion  15 . A clean air chamber is formed by being enclosed with the chassis  10  and the top cover  2  for covering the hollow portion of the chassis  10 . The clean air chamber is filled with clean air from which particles are removed. The recording disk  1 , which is a magnetic recording medium, the rotating body portion  8 , and the head drive unit  17  are arranged in the clean air chamber. 
     As shown in  FIG. 2 , the bearing unit  5  is arranged on the base member  3  and includes the shaft  16 , the sleeve  14 , the housing  13 , an overhanging member  19 , and a descent portion  20 . Further, the bearing unit  5  includes a radial dynamic pressure groove  22 , a thrust dynamic pressure groove  23 , and a capillary seal portion  24 . 
     The shaft  16  functions as the rotation axis. The upper end of the shaft  16  is fixed to a shaft hole  4 M, which is formed at the center of the hub  4 . The shaft  16  is inserted into the sleeve  14 . The sleeve  14 , which is approximately cylindrical, is inserted into the housing  13 . A part of the surface of the outer circumference of the sleeve  14  is fixed to the surface of the inner circumference of the housing  13  by bonding and the like. The overhanging member  19 , which overhangs outward in the radial direction, is fixed to an opening end surface  14 A at the upper side of the sleeve  14 . The overhanging member  19  restricts the movement of the hub  4  in the axial direction in cooperation with the descent portion  20 . Further, the overhanging member  19  and the descent portion  20  prevent the rotating body portion  8  from coming out of place. 
     The housing  13  is in the form of a cup, with a bottom, such that a cylinder portion and a bottom portion are integrally formed. A part of the surface of the outer circumference of the housing  13  is fixed to a bearing hole  3 A, which positions is positioned approximately at the center of the base member  3 . The bottom portion of the housing  13  is formed at the lower end of the housing  13  for sealing so that lubricant does not leak to the outside of the housing  13 . 
     The radial dynamic pressure groove  22  and the thrust dynamic pressure groove  23  function as the bearing to rotatably support the hub  4 . Two herringbone-shaped radial dynamic pressure grooves  22  are formed to be vertically apart, at least at either of the inner circumferential surface of the sleeve  14  or the outer circumferential surface of the shaft  16 . Further, the thrust dynamic pressure grooves  23 , which are herringbone-shaped or spiral-shaped, are formed at both a surface of the descent portion  20  opposed to the surface of the opening end of the housing  13  and the upper surface of the descent portion  20  opposed to a lower surface of the overhanging member  19 . Here, the thrust dynamic pressure grooves  23  may be formed at least at either the opening end surface  14 A of the sleeve  14  or the lower end surface  4 F of the hub  4 , which is opposed to the opening end surface  14 A. 
     When the shaft  16  is rotated, a radial dynamic pressure is generated at the lubricant by the radial dynamic pressure groove  22 , so that the rotating body portion  8  is supported in the radial direction. Further, when the descent portion  20  is rotated, a thrust dynamic pressure is generated at the lubricant by the thrust dynamic pressure groove  23 , so that the rotating body portion  8  is supported in the axial thrust direction. 
     The capillary seal portion  24  is formed with the inner circumference surface of a cylinder portion of the descent portion  20  and the outer circumference surface of the housing  13  so that the gap between the inner circumference surface of the descent portion  20  and the outer circumference surface of the housing  13  gradually becomes larger toward the opening end at the lower side of the descent portion. The lubricant such as oil is infused to a space defined by the radial dynamic pressure groove  22  and the opposing surface thereto, the thrust dynamic pressure groove  23  and the opposing surface thereto, and the capillary seal portion  24 . The boundary liquid level at which the lubricant contacts outside air is set at some mid-part of the capillary seal portion  24 . The capillary seal portion  24  prevents leaking of the lubricant with capillarity. 
     The spindle drive unit  6  includes a stator core  11  that is fixed to the base member  3 , a three-phase coil  12 , which is wound around a salient pole of the stator core  11 , and an approximately cylindrical magnet  21 , which is fixed at an interior cylinder portion  4 D of the hub  4 , as seen in  FIG. 4 . 
     The stator core  11  includes a circular portion and nine salient poles that are extended in the radial direction therefrom. The stator core  11  is formed by performing insulation coating such as electro-deposition coating and powder coating on the surface thereof after a plurality of magnetic plates such as feffosilicon plates are laminated. The magnet  21  is formed of rare-earth material such as Nd—Fe—B (neodymium-ferrum-boron), for example. Rustproofing such as electro-deposition coating and spray coating is performed on the surface of the magnet  21 . Further, for example, the magnet  21  has driving magnetic poles of twelve poles along the circumferential direction of a portion of the inner circumference of the magnet  21 . The magnet  21  opposes to distal ends  11 A of the salient poles of the stator core. 
     For the coil  12 , a wire  25  is wound a predetermined number of times around the salient pole of the stator core  11  from the lower side of the salient pole, and then, is wound around the adjacent salient pole of the stator core  11  from the upper side of the salient pole. After the wire  25  is continuously wound a predetermined number of times around the salient poles of the stator core  11  in this manner, the wound end of the wire  25  is drawn to the lower side of the salient pole of the stator core  11 . Subsequently, the wound end of the wire  25  is drawn to the opposite side of the base member  3  through a wire hole  3 B, which is disposed at the base member  3 , The wire  25  is disposed through the wire hole  3 B, with a first opening that is on an upper surface  3 E of the base member  3  and a second opening that is located at the lower surface  3 C of the base member  3 . “First opening” and “second opening” may refer to the wire hole and the surface base member  3  surrounding the wire hole. From the second opening of the wire hole  3 B, the wire is electrically connected to a wiring member  26 , which is arranged at the lower surface  3 C of the base member  3  in a lower sloped surface  26 C of a concavity  26 B. The lower sloped surface  26 C is disposed on the base member so as to axially overlap with an upper sloped surface  26 D on an upper surface  3 E of the base member  3 . The wound end of the drawn wire  25  is fixed with a bond so as not to be released. Such fixing prevents the wire  25  from disconnection due to large-amplitude vibration caused by resonance during ultra-sonic cleaning. When the coil  12  is powered with a three-phase current of an approximate sine-wave by a predetermined drive circuit via the wiring member  26 , the coil  12  generates a magnetic field for rotation at the salient poles of the stator core  11 . A rotational driving force is generated by the interaction between the driving poles of the magnet  21  and the magnetic field for rotation so that the rotating body portion  8  is rotated. Namely, the spindle drive unit  6  drives the rotating body portion  8  to rotate. 
     The fixed body portion  7  is configured to include the chassis  10  of which section is an approximate hollow shape, the stator core  11 , the coil  12 , the housing  13 , and the sleeve  14 . Further, the rotating body portion  8  is configured to include the approximately pan-shaped hub  4  to which the recording disk  1  is mounted, the shaft  16  and the magnet  21 . 
     In the following, the hub  4  is specifically described with reference to  FIG. 3 . The hub  4  is formed of magnetic material such as SUS430F which has soft magnetism. It is preferable to form the whole hub  4  with magnetic material in view of generating an effect of magnetic shield. The hub  4  is formed by machining, such as by pressing and cutting so as to create the predetermined shape of being approximately pan-shaped. For example, stainless steel DHS1 manufactured by Daido Steel Co. Ltd. is preferable in view of resources expended. In addition, stainless steel DHS2 is further preferable in view of its excellent corrosion resistance. 
     The shaft hole  4 M is formed at the center of the hub  4  and a circular center portion  4 I is formed around the shaft hole  4 M. The shaft hole  4 M is formed so that the dimension thereof in the axial direction is larger than the dimension in the axial direction of a part of the center portion  4 I opposing to the upper end surface of the sleeve  14 . A part of the outer circumference of the shaft hole  4 M is projected downward. Accordingly, connecting surface between the hub  4  which is thinned and the shaft  16  is ensured. 
     The upper end surface  4 A of the hub  4  includes at least two leveled surface. The center portion  4 I is located at a highest leveled surface of the upper end surface  4 A. A recess portion  4 J, which is at a lower leveled surface of the upper end surface  4 A, is formed to be ring-shaped. A plurality of threaded holes  4 K are disposed at the upper surface of the recess portion  4 J at the same circular intervals. A clamper  29  is disposed on the recess portion  4 J. Then, a circular step between the leveled surface of the center portion  4 I and the leveled surface of the recess portion  4 J is fitted to a center hole of the clamper  29 . The clamper  29  is fixed by screwing screws  30  to the threaded holes  4 K. 
     A circular exterior cylinder portion  4 B is formed as a stepped portion lowered from the periphery of the recess portion  4 J. A annular extension portion  4 C is formed to extend outward in the radial direction from the lower end of the periphery of the exterior cylinder portion  4 B. The inner circumference of the center hole of the recording disk  1  is engaged with the exterior cylinder portion  4 B of the hub  4  so that the recording disk  1  is mounted on the upper surface of the annular extension portion  4 C. The annular extension portion  4 C sags to the base member  3  side. The outer circumference of the magnet  21  is fixed to the interior cylinder portion  4 D. The annular extension portion  4 C, which is located in an area outside the outer circumference of the magnet  21  in the radial direction, functions as a back yoke for the magnet  21 . 
     A circular projecting portion  4 E, which projects in the direction toward the base member  3  between the housing  13  and the stator core  11 , is formed at the lower surface of the hub  4 . The circular descent portion  20  is fixed to the inner circumference surface of the circular projecting portion  4 E of the hub  4  through bonding. 
     A lower end surface  4 F of the hub  4  opposing an opening end surface  14 A of the sleeve  14  is located at the back surface of the center portion  4 I. A portion  4 H of the hub  4  opposing the coil  12  is located at the back surface of the recess portion  4 J. 
     Here, a problem with respect to the related art, which is recognized by the present inventor, is described based on the structure according to  FIG. 4 .  FIG. 4  is a sectional view of a part of a disk drive device  200  of the related art. When the disk drive device  200  of the related art is thinned, a spindle unit  52  such as a stator core  50  becomes thin accordingly. When the spindle unit  52  becomes thin, the rotation becomes unstable due to torque decrease. When the rotation becomes unstable, the unstableness may impair normal read/write operation of magnetic data, at worst. 
     There may be a solution for recapturing a decrease in torque by using a magnet  54  with a larger diameter. However, with the configuration that the recording disk  1  is located at an area on the extension of the outer circumference of the magnet  54 , a back yoke portion  58  of the hub  56  sandwiched by the inner circumference of the recording disk  1 , and the outer circumference of the magnet  54  becomes thin in accordance with increase of the diameter of the magnet  54 . The back yoke portion  58  constitutes a part of a magnetic circuit through which magnetic flux departing from the outer circumference of the magnet  54 . Thus, when the back yoke portion  58  becomes thin, magnetic saturation occurs. When the magnetic saturation occurs, the magnetic flux is hardly increased even though the magnetic field is strengthened. Accordingly, the torque cannot be increased since the increase of the magnetic flux contributing to the torque is slight. On the other hand, the magnetic flux leaking to the recording disk  1  side is extremely increased. Therefore, with the configuration that the recording disk  1  is located at the area on the extension of the outer circumference of the magnet  54 , there is a possibility that normal read/write operation of magnetic data is impaired by the leaked magnetic flux. This has been an inhibitor of thinning the disk drive device  200 . Further, even in the case that the back yoke portion  58  is thickened in the related art, increase of the torque is not expected since the magnet  54  has to be decreased in size accordingly. 
     In view of the abovementioned problem, the recording disk  1  according to the present embodiment is arranged at a position to be apart upward from the magnet  21  in the axial direction being away from the area on the extension of the outer circumference of the magnet  21  in the radial direction, as illustrated in  FIG. 2 . With this configuration, the magnetic flux leaking to the recording disk  1  side can be decreased. Then, the interior cylinder portion  4 D of the hub  4  is configured so that the diameter  90  thereof is larger than the diameter  92  of the exterior cylinder portion  4 B of the hub  4 , as seen in  FIG. 3 . As a result, it becomes possible to enlarge the outer circumference of the magnet  21  so that the torque is increased due to increase of the magnetic flux amount of the magnetic poles for driving. Accordingly, the configuration is preferable for the thinned disk drive device  100 . Further, since the recording disk  1  is arranged at the upper position in the axial direction away from the area on the extension of the outer circumference of the magnet  21  in the radial direction, the back yoke of the hub  4  can be configured to be sufficiently thick in the radial direction. As a result, even in the case that the magnet  21  of which the energy product is larger is used, the leak in magnetic flux can be suppressed and the torque can be increased. 
     The diameter  94  of a circle connecting the ends  11 A of the salient poles of the stator core  11  may be set to be 80% or more of the diameter  92  of the exterior cylinder portion  4 B of the hub  4 . By configuring the stator core  11  to be large, as mentioned above, more winding can be performed for the coil  12  no that torque increase is expected. Here, when the diameter  94  of the circle connecting the salient ends  11 A of the salient poles of the stator core  11  exceeds 100% of the diameter  92  of the exterior cylinder portion  4 B, the leaked magnetic flux of the magnet  21  may affect the recording disk  1  and may impair the normal read/write operation of magnetic data. Therefore, the diameter of the circle connecting the salient ends  11 A of the salient poles of the stator core  11  is within a range of 80% to 100% of the diameter  92  of the exterior cylinder portion  4 B. 
     The base member  3  has a wire hole  3 B through which the wire  25  for forming the coil  12  is inserted. A drawing line of the wire  25  which forms the coil  12  is introduced to exit through the wire hole  3 B to the back surface  3 C of the upper surface of the base member  3  on which the bearing unit  5  is arranged. In the related art of  FIG. 4 , the drawing line of the wire is connected to a wiring member  66  by soldering at a position strictly below the position where the drawing line is drawn from the coil  12 . The thickness of the wiring member  66  and the height of the connection portion  68  at the position strictly below the coil  62  are to be a barrier of thinning the spindle drive unit  5 . 
     Next, in the disk drive device  200  of the related art which is illustrated in  FIG. 4 , a cylinder portion  76  and a bottom portion  78  of the housing is fixed by bonding as separate members. In this case, when the disk drive device is thinned, the bonding part also becomes thin. When the bonding part becomes thin, the connection strength is decreased. Accordingly, there is a possibility of disconnection due to impact. 
     Here, there is a case that the base member  3  is formed of metal such as aluminum. In this case, there is a possibility that the wire  25 , which is drawn to the lower surface  3 C of the base member  3 , may be electrically short-circuited by directly contacting to the base member  3 . In order to cope with this problem, a channel portion  3 D, which introduces the wire to exit through the wire hole  3 B for connecting to the wiring member  26 , is disposed at the back surface  3 C of the surface of the base member  3  to which the bearing unit  5  is disposed. The channel portion  3 D is insulation treated. As a result, the problem that the wire  25  is electrical short-circuited with the base member  3  is relieved. Further, by combining the positioning of the abovementioned connection portion  26 A with the configuration of positioning outside the magnet  21  in the radial direction, in concavity  26 B, the spindle drive unit  6  can be thinned by the amount based on the thickness of the wiring member  66  and the height of the connection portion  68  of the related art which is illustrated in  FIG. 4 . Here, for example, cationic electro-deposition coating (hereinafter, called ED coating) onto the base member  3 , which is molded with aluminum die-casting, is preferable as the isolation process in view of less pin holes. 
     Here, when the disk drive device  100  is configured to be further thinned, the coil  12 , which is wound around the salient poles and the lower surface of the hub  4 , becomes extremely close. In this case, the possibility that the coil  12  contacts the rotating hub  4  is increased. When the coil  12  contacts the hub  4 , an electrical short-circuit may occur. In order to cope with this problem, the coil  12  is leveled so that the surface opposing to the hub  4  and the surface opposing to the base member  3  are to be level. 
       FIGS. 5A to 5C  illustrate a forming method of the coil  12  according to the present embodiment.  FIG. 5A  illustrates the coil  12  before forming,  FIG. 5B  illustrates the coil  12  during pressing, and  FIG. 5C  illustrates the coil  12  after pressing. As illustrated in the drawings, the coil  12  is formed by being pressed between a first pressing die  40  and a second pressing die  42  after the wire  25  is wound around the salient poles of the stator core  11 . The pressing surfaces of the first pressing die  40  and the second pressing die  42  are flat. By forming the coil  12  to be flat with pressing, the dimension of the coil  12  in the axial direction is stabilized so that the possibility that the coil  12  comes in contact with the rotating hub  4  can be decreased. Accordingly, the dimension of the coil  12  in the axial direction can be thinned. 
     Further, in order to cope with the contacting problem of the coil  12  to the rotating hub  4 , the flattening ratio of the wire  25 , which forms the leveled coil  12 , may be 90% or less. The flattening ratio of the wire  25  is expressed by a percentage of the dimension “b” of the section of the single wire  25  in the axial direction against the dimension “a” in the radial direction. Here, the flattening ratio of the wire  25  of the coil  12  is defined at a part of which the flattening ratio is the lowest. The equation thereof is as follows:
 
The flattening ratio of the wire 25=( b/a )×100
 
     When the coil  12  is formed with pressing so as to limit the dimension of the coil  12  in the axial direction, the part of the wire  25  of which the flattening ratio becomes lowest is the part that is thickest in the axial direction. As a result, the possibility of contact of the coil  12  to the rotating hub  4  is further decreased. 
     Furthermore, in order to cope with the contacting problem of the coil  12  to the rotating hub  4 , it is also possible to perform the insulation treatment on the surface of the hub  4  opposing to the coil  12 . As a result, the possibility of the electrical short-circuiting, which causes a malfunction, is decreased. For example, a circular film  27  that is made of Polyethylene terephthalate (PET) may be stuck with double-faced tape to the surface  96  of the hub  4  opposed to the surface  12 A of the coil  12 . This method is preferable in view of easy operation. 
     Further, there may be a problem that an electrical short-circuit occurs due to the coil  12  coming in contact with the base member  3 . In order to cope with this problem, it is also possible to perform an insulation treatment on the surface  98  of the base member  3  opposed to the surface  12 B of the coil  12 . As a result, the possibility that the coil  12  comes in contact with the base member  3  to cause an electrical short-circuit is decreased. For example, it is also possible to perform the ED coating on the base member  3  which is molded with aluminum die-casting as the insulation treatment. This is preferable in view of less pin holes. Further, a circular film  28 , which is made of PET, may be stuck with double-faced tape to the surface of the base member  3  opposing to the coil  12 . This method is preferable in view of easy operation. 
     By the way, in the case that the disk drive device  100  is thinned, stiffness is decreased and rocking-mode resonance frequency is decreased when the portion  4 H of the hub  4  opposed to the coil  12  in the axial direction is shortened. Here, the rocking-mode resonance is described with reference to  FIG. 6 .  FIG. 6  is a schematic view, which schematically illustrates behavior of momentary vibration of a recording disk  1  surface. In  FIG. 6 , dashed lines illustrate a nodal diameter  36  and a nodal circle  38  at the vicinity of toque-ripple frequency. The area with hatching indicates that the vibration phase thereof at the vicinity of the torque-ripple frequency is reverse to that of the area without hatching. Solid lines are contour lines of vibration displacement at the vicinity of the torque-ripple frequency. 
     The resonance of the disk drive device  100  during non-rotating has been examined in the state that the recording disk  1  is mounted in the disk drive device  100 . As a result, the rocking-mode resonance with the single nodal diameter  36  and the nodal circle  38  as an intermediate portion was observed in the recording disk  1  at the vicinity of the torque-ripple frequency. Through the study of the present inventor, the main factors that determine the frequency of the rocking-mode resonance are discovered to be the stiffness of the bearing, the stiffness of the connecting portion between the hub  4  and the shaft  16 , the stiffness of the connecting portion between the recording disk  1  and the hub  4 , the stiffness of the recording disk  1  itself, the lateral moment of inertia of the recording disk  1 , and the lateral moment of inertia of the hub  4 . 
     When the frequency of the rocking-mode resonance becomes low, there may be a case that large vibration occurs due to resonance with the variation of the drive torque. There may be a problem that such vibration causes a malfunction of normal read/write operation of magnetic data, at worst. In order to cope with this problem, the width  102  in the axial direction of the hub  4  opposed to the coil  12  may be larger than the width  104  in the axial direction of the base member  3  opposed to the coil  12 . This is for the relative relation of dimensions of the base member  3  and the hub  4  in the axial direction in the case that the disk drive device  100  is thinned. As a result, the problem caused by decrease of the frequency of the rocking-mode resonance is relieved. 
     Next, in the disk drive device  200  of the related art, which is illustrated in  FIG. 4 , a center part of a clamper  70  is fixed at the center of the shaft  74  with a screw  72 . Therefore, the center part of the hub  56  is to be thinned in the axial direction by the amount of the clamper  70  and the screw  72 . When the center part of the hub  56  is thinned in the axial direction, the frequency of rocking-mode resonance becomes low. Accordingly, there may be a case that large vibration occurs due to the resonance with the variation of the drive torque. 
     In order to cope with this problem, in the disk drive device  100  according to the present embodiment, the hub  4  includes the recess portion  4 J, which is formed on the surface of the hub  4  at the side to which the recording disk  1  is mounted, and the threaded hole  4 K, which is formed at the recess portion  4 J, as illustrated in  FIG. 2  and  FIG. 3 . The clamper  29  is fixed to the threaded hole  4 K with the screw  30 . As a result, the problem caused by thinning the center portion  4 I of the hub  4  in the axial direction is relieved. In addition, since the shaft  16  can be configured to be long, it is preferable because decrease of the bearing stiffness can be prevented. 
     Further, there may be a case that the dimension in the axial direction of the thread portion of the threaded hole  4 K, which is formed at the hub  4 , is insufficient. In order to cope with this problem, the threaded portion  4 K is formed to penetrate in the axial direction. Further, a cover member  31  is disposed at the surface  96  of the hub  4  to which the threaded portion  4 K is formed and which is opposed to the surface  12 A of the coil  12 . As a result, the problem that the dimension in the axial direction of the thread portion of the threaded hole  4 K is insufficient is relieved. A variety of materials can be used for the cover member  31 . For example, a PET film may be stuck with double-faced tape to the surface  98  of the base member  3  opposed to the surface  12 B of the coil  12 . This method is preferable in view of easy operation as well as functioning as the insulation treatment against the coil  12 . 
     Next, in the disk drive device  200  of the related art, which is illustrated in  FIG. 4 , a cylinder portion  76  and a bottom portion  78  of the housing is fixed by bonding as separate members. In this case, when the disk drive device is thinned, the bonding part also becomes thin. When the bonding part becomes thin, the connection strength is decreased. Accordingly, there is a possibility of disconnection due to impact. 
     In order to cope with this problem, in the disk drive device  100  of the present embodiment, the housing  13  is in the form of a cup with a bottom such that a cylinder portion and a bottom portion are integrally formed, as illustrated in  FIG. 2 . As a result, the problem of disconnection between the cylinder portion and the bottom portion of the housing  13  is relieved even when the disk drive device  100  becomes thin. 
     Next, in the disk drive device  200  of the related art which is illustrated in  FIG. 4 , a circular member  80  is fixed by bonding to the inner circumference of a circular projecting portion of the hub  56 . The doughnut-shaped circular member  80  is formed so that the dimension thereof in the axial direction is 1.2 mm or more for ensuring connection strength with the hub  56 . When the disk drive device  200  becomes thin, the center portion of the hub  56  in the axial direction is to be thin by the amount of the circular member  80 . When the center portion of the hub  56  in the axial direction becomes thin, the frequency of the rocking-mode resonance becomes low. Accordingly, there may be a case that large vibration occurs due to resonance with the variation of the drive torque. 
     In order to cope with this problem, the bearing unit  5  of the present embodiment includes the descent portion  20 , which is rotated integrally with the hub  4 , and the overhanging member  19 , which is arranged so as to be nonrotatable at a position opposed to the descent portion  20  in the axial direction, as illustrated in  FIG. 2 . Further, it is also possible that the descent portion  20  restricts movement of the hub  4  in cooperation with the overhanging member  19  and the width in the axial direction of the descent portion  20  opposed to the overhanging member  19  is set to be 0.6 mm or less. Namely, the thickness of a disk portion  20 A of the descent portion  20  is set to be 0.6 mm or less. As a result, in the case that the disk drive device  100  is thinned, the dimension of the center portion  4 I of the hub  4  in the axial direction can be ensured. In addition, it is preferable that the dimension of the descent portion  20  is set to be 0.4 mm or less because the center portion  4 I of the hub  4  can be further thickened in the axial direction. 
     Further, when the circular member  80  of  FIG. 4  is thinned in the axial direction, there may be a problem of disconnection due to impact since the connection strength of the circular member  80  with the hub  56  is decreased. In order to cope with this problem, the descent portion  20  is formed by integrating the disk portion  20 A, which is opposed to the overhanging member  19  in the axial direction, and a cylinder portion  20 B, which is connected to the periphery portion of the disk portion  20 A. With this configuration, the dimension in the axial direction for connecting to the circular projecting portion  4 E can be sufficiently ensured. As a result, the problem of disconnection between the descent portion  20  and the hub  4  is relieved. For example, by setting the dimension of the cylinder portion  20 B in the axial direction to be 2.0 mm or more, the connection strength with the hub  4  is sufficiently ensured. In addition, by setting the dimension of the disk portion  20 A in the axial direction to be 0.4 mm or less thereafter, the dimension of the center portion  4 I of the hub  4  can be configured to be thick in the axial direction. 
     There may be a problem that the machining of the descent portion  20  requires much expense in time. In order to cope with this problem, the descent portion  20  may be formed by the pressing of metal material. As a result, the problem of machining expense of the descent portion  20  in time can be relieved. 
     There may be a problem that the machining of the descent portion  20  requires much expense in time. In order to cope with this problem, the descent portion  20  may be formed by pressing of metal material. As a result, the problem of machining expense of the descent portion  20  in time can be relieved. 
     Further, the thrust dynamic pressure groove  23  may be formed at least at any surface of the disk portion  20 A of the descent portion  20 . Specifically, the thrust dynamic pressure groove  23  is formed at least either at the surface of the disk portion  20 A opposed to the opening upper end surface of the housing  13  or the surface of the disk portion  20 A opposed to the overhanging member  19 . As a result, machining of the thrust dynamic pressure groove  23  becomes easy. 
     Here, in accordance with thinning of the disk drive device  100 , the stator core  11  is configured to be thin. When the stator core  11  becomes thin, there is a possibility that the stator core  11  is attached to be inclined when the circular portion thereof is fitted to the base member  3 . In order to cope with this problem, a stator core supporting member  32  is disposed between the salient pole of the stator core  11  and the base member  3 . The stator core supporting member  32  is arranged to circularly project from the base member  3  toward the salient pole of the stator core  11  at which the coil  12  is not arranged. As a result, the stator core  11  is supported at the inner circumference and the outer circumference. Accordingly, the problem of the stator core  11  inclining by the thinning is relieved. 
     A part of the hub  4  covering the outer circumference of the magnet  21  performs a function of so-called back yoke. When the back yoke becomes thin, the magnetic resistance is increased. When the magnetic resistance is increased, the magnetic flux which is generated by the magnet  21  is decreased. When the magnetic flux is decreased, the torque is decreased. Accordingly, there may be a problem that a malfunction such as unstable rotation occurs. In order to cope with this problem, the hub  4  has the annular extension portion  4 C, which extends outward, and the diameter  106  of the outer circumference end of the annular extension portion  4 C is set to be larger than the diameter  90  of the interior cylinder portion  4 D of the hub  4  by 4 mm or more. As a result, the thickness of the back yoke is sufficiently ensured and the problem caused by the torque decrease is relieved. 
     Here, when strong magnetic material is used for increasing the torque, there is a case that leaked magnetic flux is increased due to magnetic saturation in the back yoke. When the leaked magnetic flux is increased, a noise signal may be generated at a magnetic head for reading/writing data. When the noise signal is large, there is a possibility that normal operation of reading/writing of magnetic data is impaired. In order to cope with this problem, the saturation magnetic flux density of the annular extension portion  4 C, which functions as the back yoke, is set to be 1 T (tesla) or more. With this configuration, the saturation magnetic flux density can be sufficiently ensured at the back yoke and the problem of increasing of the leaked magnetic flux is relieved. Here, when the saturation magnetic flux density of the hub  4  is set to be 1.2 T or more, stronger magnetic material can be used. 
     Here, there may be a demand to increase the torque so as to stabilize the rotation. In order to cope with this demand, the opposing clearance between the distal end  11 A of the salient pole and the magnet  21  is set to be 0.4 mm or less. Namely, a gap where the distal end  11 A of the salient pole and the magnet  21  face each other is set to be 0.4 mm or less. As a result, an air gap of the magnetic circuit becomes small and the magnetic flux amount of the magnet  21  is increased so that the torque is increased. The opposing clearance between the salient pole and the magnet  21  is preferable to be 0.4 mm or less in view of ensuring effect to increase the torque and to be 0.2 mm or more in view of preventing contact between the salient pole and the magnet  21 . 
     Further, the maximum energy product of the magnet  21  according to the present embodiment may be set to be 10 megagauss-oersted (MGOe) or more. Accordingly, the magnetic flux amount of the magnet  21  is increased so that the torque is increased. The maximum energy product of the magnet  21  is preferably to be 10 MGOe or more in view of ensuring the torque increasing effect and to be 16 MGOe or less in view of easiness of magnetizing. Here, by combining the magnet  21  of the abovementioned maximum energy product with the back yoke of which saturation magnetic density is 1 T or more, the leaking of the magnetic flux from the back yoke can be suppressed even in the thinned disk drive device  100 . 
     There is a demand of further thinning and lightening for the disk drive device  100 , which is mounted on a portable apparatus. In order to cope with this demand, the inner diameter of the recording disk  1  is set to be 20 mm, and the thickness of the disk drive device in the axial direction is set to be 7.5 mm or less. As a result, the portable apparatus can be configured to be thin and light and whose construction may further be resource-saving. 
     As described above, the disk drive device  100  according to the present embodiment can stabilize the rotation of the recording disk  1  while achieving further thinning so as to be a preferable shape for a portable apparatus and the like. 
     Not limited to the abovementioned embodiments, the present invention is possible to be modified by various design changes based on knowledge of skilled persons. The configuration illustrated in each of the drawings is simply for describing an example and can be appropriately modified so that the similar effects are obtained as long as the similar functions can be achieved.