Patent Publication Number: US-2015086142-A1

Title: Rotating device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a rotating device including a fluid dynamic bearing. 
     2. Description of the Related Art 
     Disk drive devices like hard disk drives that are a kind of rotating devices are becoming compact and increasing the capacity, and are loaded in various electronic devices. In particular, disk drive devices are now loaded in portable electronic devices, such as a laptop computer and a portable music player. The disk drive devices loaded in such portable electronic devices need to improve the shock resistance and the vibration resistance so as to withstand a shock due to falling and a vibration at the time of carriage. 
     For example, JP 2012-191708 A and JP 2013-055791 A disclose a disk drive device employing a fluid dynamic bearing mechanism as a bearing. 
     According to the conventional disk drive devices disclosed in, for example, JP 2012-191708 A and JP 2013-055791 A, dynamic pressure generating grooves are formed in grooved formed portions of a bearing body, and when a rotating device is made compact, the dimension of the groove formed part is reduced, often resulting in a reduction of the rigidity of a fluid dynamic bearing. In addition, according to the conventional disk drive devices, in order to adjust the surface hardness of the groove formed part, a plating layer is formed on the surface of the grooved formed portion after the dynamic pressure generating grooves are formed. The dynamic pressure generating grooves having undergone plating may have a rounded edge in comparison with the grooves prior to the plating, and thus the shape accuracy is deteriorated, resulting in a reduction of dynamic pressure. 
     Still further, according to such disk drive devices, a shaft is held by a shaft encircling portion in a tilted condition when the disk drive devices are still or rotate at a slow speed. In this case, the shaft contacts an external area of the inner circumference of the shaft encircling portion in the direction of a rotation axis, i.e., an area near an inward end. When the disk drive devices repeat activation/deactivation with the shaft being contact, the wear level of the contact portion becomes larger than other portions. Moreover, when a shock or a vibration is applied to the disk drive devices, large stress is applied to the portion of the shaft encircling portion contacting the shaft, and such a portion may be deformed. In the worst case, this causes a breakdown. In particular, when the thickness of the plating layer of the contacting portion of the shaft encircling portion is thin, the durability against the wear at the time of activation/deactivation and the shock resistance may decrease. 
     Such a technical problems are common to, not only the rotating devices loaded in portable electronic devices, but also the rotating devices loaded in other kinds of electronic devices, such as a stationary type electronic device. 
     The present disclosure has been made in view of the aforementioned circumstances, and it is an objective of the present disclosure to provide a rotating device which can improve the wear resistance of dynamic pressure generating grooves, and which can suppress a reduction of a shock resistance when downsized. 
     SUMMARY OF THE INVENTION 
     To accomplish the above objective, a rotating device according to a first aspect of the present disclosure includes: a shaft body including a portion extending in an axial direction; and a bearing body which retains a part of the shaft body, is relatively rotatable to the shaft body, and is provided with a groove formed part having an inner circumference formed with a fluid dynamic pressure generating groove, in which the bearing body comprises, at the groove formed part, a slanted plating layer having a plating thickness gradually becoming thick toward an external side in the axial direction. 
     To accomplish the above objective, a rotating device according to a second aspect of the present disclosure includes: a shaft body including a portion extending in an axial direction; and a bearing body which retains a part of the shaft body, is relatively rotatable to the shaft body, and is provided with a groove formed part having an inner circumference formed with a fluid dynamic pressure generating groove, in which the bearing body comprises, at the groove formed part, a slanted plating layer having a plating thickness gradually becoming thick toward an internal side in the axial direction. 
     To accomplish the above objective, a rotating device according to a third aspect of the present disclosure includes: a shaft body including a portion extending in an axial direction; and a bearing body which retains a part of the shaft body, is relatively rotatable to the shaft body, and is provided with a groove formed part having an inner circumference formed with a fluid dynamic pressure generating groove, in which: the bearing body comprises, at the groove formed part, a slanted plating layer having a plating thickness gradually becoming thick toward a first direction in the axial direction; a difference between a maximum plating thickness of the slanted plating layer and a minimum plating thickness thereof is within a range between equal to or greater than 0.1 μm and equal to or smaller than 0.5 μm; and the fluid dynamic pressure generating groove is engraved in an external surface side of the slanted plating layer. 
     Any arbitrary combination of the aforementioned structural components, and mutual replacement of the subject matter with a method, a device, and a system, etc., are also effective as embodiments of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view illustrating a disk drive device that is a kind of rotating device according to an embodiment of the present disclosure; 
         FIG. 2  is a cross-sectional view mainly illustrating a left part of a cross section taken along a line A-A in  FIG. 1 ; 
         FIG. 3  is an enlarged cross-sectional view exemplarily illustrating a shaft encircling portion of the disk drive device in  FIG. 2 ; 
         FIG. 4  is an enlarged cross-sectional view exemplarily illustrating an example structure of a groove formed part of the shaft encircling portion of the disk drive device; and 
         FIG. 5  is an enlarged cross-sectional view exemplarily illustrating a structure of another example corresponding to  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The same or corresponding structural element and component illustrated in respective figures will be denoted by the same reference numeral below, and the duplicated explanation thereof will be omitted. In addition, the dimension of the component in each figure will be enlarged or scaled-down as needed to facilitate understanding to the present invention. Still further, a part of the component not important to explain the present disclosure will be omitted in respective figures. 
     An embodiment relates to a disk drive device that is a kind of rotating device. The disk drive device of the embodiment is, for example, suitably applied as a hard disk drive on which a magnetic recording disk magnetically recording data is to be mounted, and which rotates and drives the magnetic recording disk. 
       FIG. 1  is a perspective view illustrating a disk drive device  100  according to this embodiment. In  FIG. 1 , a top cover  2  is detached to facilitate understanding to the present disclosure. In  FIG. 1 , for example, an electronic circuit not important to explain the present disclosure is omitted. The disk drive device  100  includes a stationary body, a rotating body rotating relative to the stationary body, a magnetic recording disk  8  to be attached to the rotating body, and a data reader/writer  10 . The stationary body includes a base  4 , a shaft  26  fixed relative to the base  4 , a housing  102  supporting the shaft  26 , the top cover  2 , six screws  20 , and a shaft fastening screw  6 . The rotating body includes a hub  28 , a clamper  36 , and a cover ring  12 . 
     In the following explanation, a side at which the hub  28  is mounted relative to the base  4  will be defined as an upper side. 
     The magnetic recording disk  8  is, for example, a 2.5-inch magnetic recording disk formed of glass and having a diameter of 65 mm. The center hole has a diameter of 20 mm, and the thickness of the magnetic recording disk  8  is 0.65 mm. One magnetic recording disk  8  is to be mounted on the hub  28 . 
     (Base) 
     The base  4  includes a bottom portion  4   a  forming the bottom of the disk drive device  100 , and an outer circumference wall  4   b  formed along the outer circumference of the bottom portion  4   a  so as to encircle an area where the magnetic recording disk  8  is mounted. For example, six screw holes  22  are provided in an upper face  4   c  of the outer circumference wall  4 B. The base  4  is formed and shaped by die-casting of, for example, an aluminum alloy. The base  4  maybe formed by pressing of a steel sheet or an aluminum sheet. A surface coating is applied to the base  4  in order to suppress a peeling from the surface thereof. An example surface coating applied is a resin-material coating like an epoxy resin. As a surface coating, instead of the resin material, a metal material, such as nickel or chrome, may be applied as coating by plating. 
     (Data Reader/Writer) 
     The data reader/writer  10  includes an unillustrated recording/playing head, a swing arm  14 , a voice coil motor  16 , and a pivot assembly  18 . The recoding/playing head is attached to the tip of the swing arm  14 , records data in the magnetic recording disk  8 , or reads the data therefrom. The pivot assembly  18  supports the swing arm  14  in a swingable manner to the base  4  around a head rotating axis S. The voice coil motor  16  allows the swing arm  14  to swing around the head rotating axis S to move the recording/playing head to a desired location over the top face of the magnetic recording disk  8 . The voice coil motor  16  and the pivot assembly  18  are configured by a conventionally well-known technology of controlling the position of a head. 
     (Top Cover) 
     The top cover  2  is fastened to the upper face  4   c  of the outer circumference wall  4   b  of the base  4  using, for example, six screws  20 . The six screws  20  correspond to the six screw holes  22 . In particular, the top cover  2  and the upper face  4   c  of the outer circumference wall  4   b  are fastened together in such a way that no leak to the interior of the disk drive device  100  occurs from the joined portion therebetween. The interior of the disk drive device  100  is, more specifically, a clean space  24  surrounded by the bottom portion  4   a  of the base  4 , the outer circumference wall  4   b  thereof, and the top cover  2 . This clean space  24  is designed so as to be air-tightly sealed, i.e., so as to have no leak-in from the exterior and leak-out to the exterior. The clean space  24  is filled with a clean filler gas having particles eliminated. Example filler gases are various gases including air. In this embodiment, a gas with a smaller molecular weight than nitrogen, such as helium or hydrogen, is filled in the clean space  24 . 
     (Shaft) 
     The shaft  26  has the shaft fastening screw  6  passing all the way through the top cover  2 , and engaged with a shaft-fastening-screw hole  152  provided in an upper end face of the shaft  26 . Hence, the shaft  26  is fixed to the top cover  2 . The shaft  26  is formed by, for example, cutting, machining and grinding of a ferrous material like SUS 420 J2 of stainless-steel, and a heat treatment is applied thereto in order to accomplish a desired hardness. 
       FIG. 2  is a cross-sectional view mainly illustrating the left part of the cross section taken along a line A-A in  FIG. 1 . The cross section illustrated in  FIG. 2  corresponds to a half cross section of a motor component in the disk drive device  100 . 
     The rotating body includes the hub  28 , the clamper  36 , a magnet  32 , and the cover ring  12 . The stationary body includes the base  4 , a stator core  40 , coils  42 , the housing  102 , the shaft  26 , a ring portion  104 , a magnetic plate  34 , and a wire supporting member  72 . A lubricant  92  is continuously present in some gaps between the rotating body and the stationary body. 
     The housing  102 , the shaft  26 , the ring portion  104 , a shaft encircling portion  30  forming the hub  28 , and the lubricant  92  form a fluid dynamic bearing  48 . 
     (Hub) 
     The hub  28  is formed in a predetermined shape like a substantially cup shape around a rotation axis R as viewed from the top. The hub  28  is formed by, for example, cutting and machining or pressing a ferrous material with a soft magnetism like SUS 430 of stainless-steel. In order to suppress a peeling from the surface, a surface layer forming process like electroless nickel plating may be applied to the hub  28 . The hub  28  includes the shaft encircling portion  30  that encircles the shaft  26 , a hub protrusion  28   g  that is provided outwardly in the radial direction relative to the shaft encircling portion  30 , and to be engaged with the center hole of the magnetic recording disk  8 , and, a mount portion  28   h  provided outwardly in the radial direction relative to the hub protrusion  28   g.  An outer circumference  28   d  of the hub protrusion  28   g  includes an electromechanical machined surface having undergone electromechanical machining. 
     (Clamper) 
     The magnetic recording disk  8  is to be mounted on a disk mount face  28   a  that is the upper face of the mount portion  28   h.  The magnetic recording disk  8  is held between the clamper  36  and the mount portion  28   h,  thereby being fastened to the hub  28 . The clamper  36  applies downward force in the axial direction to the upper face of the magnetic recording disk  8  to cause the magnetic recording disk  8  to be in contact with the disk mount face  28   a  in a pressed manner against it. The clamper  36  is engaged with an outer circumference  28   d  of the hub protrusion  28   g.  The clamper  36  and the outer circumference  28   d  of the hub protrusion  28   g  can be joined together by mechanical joining techniques, such as screwing, caulking, and press-fitting. 
     (Magnet) 
     The magnet  32  is formed in a cylindrical shape around the rotation axis R as viewed from the top, and bonded and fastened to a cylindrical inner circumference of the hub  28  corresponding to the internal cylindrical face thereof. The magnet  32  is formed of, for example, a neodymium-based rare-earth magnetic material. The magnet  32  may be formed of other materials like a ferrite magnetic material. In this embodiment, the magnet  32  has, for example,  12  driving polarities in the circumferential direction thereof (a tangent line direction of a vertical circle to the rotation axis R and around it). The magnet  32  faces, for example, nine salient poles of the stator core  40  in the radial direction (i.e., a direction orthogonal to the rotation axis R). 
     (Stator Core) 
     The stator core  40  includes an annular part, and the nine salient poles extending therefrom outwardly in the radial direction, and is fixed on an upper-face- 4   d  side of the base  4 . The stator core  40  is formed by, for example, laminating six thin magnetic steel sheets each having a thickness of 0.2 mm, and caulking and integrating those sheets together. A surface coating, such as electrodeposition coating or powder coating, is applied to the surface of the stator core  40 . The coil  42  is wound around each salient pole of the stator core  40 . When three-phase substantially sinusoidal drive currents are caused to flow through the respective coils, drive magnetic fluxes are generated along the respective salient poles. 
     (Base Protrusion) 
     The base  4  includes an annular base protrusion  4   e  around the rotation axis R of the rotating body. The base protrusion  4   e  protrudes upwardly so as to encircle the housing  102 . When the annular center hole of the annular part of the stator core  40  is engaged with an outer circumference of the base protrusion  4   e,  the stator core  40  is fixed to the base  4 . In particular, the annular part of the stator core  40  is bonded and fixed to the base protrusion  4   e  by press-fitting or loose fitting. 
     (Bearing Hole) 
     The base  4  is provided with a bearing hole  4   k  around the rotation axis R. The bearing hole  4   k  is encircled by the base protrusion  4   e,  and passes all the way through the base  4 . The bearing hole  4   k  may be a non through-hole. A base annular recess  58  concaving downwardly in the axial direction is provided in a middle area between the base protrusion  4   e  and the bearing hole  4   k  in the radial direction. A housing projection  60  to be discussed later enters the base annular recess  58 . An annular projection  62  protruding upwardly in the axial direction is provided at the inner-circumference side of the base annular recess  58 . The projection  62  enters a space in the radial direction between a first base-side encircling portion  112  to be discussed later and the housing projection  60 . A female screw  68  is provided in the lower portion of the inner circumference of the bearing hole  4   k.    
     (Insulation Sheet) 
     An annular insulation sheet  46  is provided on a portion of the upper face  4   d  of the base  4  corresponding to the coils  42 . The insulation sheet  46  is formed of a resin like PET, and is fixed to the base  4  by, for example, bonding. 
     (Magnetic Plate) 
     The magnetic plate  34  in an annular shape as viewed from the top is provided on a portion of the upper face  4   d  of the base  4  corresponding to the magnet  32 . The magnetic plate  34  is formed of a magnetic material, such as a ferrous material like a silicon steel sheet or a rolled steel sheet, or other magnetic materials, and attracts the magnet  32  magnetically. As a result, the rotating body including the magnet  32  is attracted toward the stationary body including the magnetic plate  34 . A distance from the internal portion of the magnetic plate  34  to the rotation axis R is larger than a distance from the inner circumference of the magnet  32  to the rotation axis R. A distance from the external portion of the magnetic plate  34  to the rotation axis R is larger than a distance from the outer circumference of the magnet  32  to the rotation axis R. 
     The stationary body includes a wire hole  70  passing all the way through the base  4  from the upper face  4   d  to the lower face, a wire supporting member  72  fixed to the wire hole  70 , and a drawn electrode  74  supported by the wire supporting member  72 . The wire hole  70  includes multiple annular steps formed circularly as viewed from the top. The wire supporting member  72  is a hollow cylinder, and is formed by, as an example, plastic molding of an insulation material like a resin. External steps corresponding to the annular steps of the wire hole  70  are formed on the outer circumference of the wire supporting member  72 . The wire supporting member  72  has the outer circumference engaged with the wire hole  70 , and is fixed thereto by, for example, bonding. A drawn wire  44  of the coil  42  is led to the lower face of the base  4  through the hollow of the wire supporting member  72 , and is electrically connected with a terminal of a conductor pattern  80  of a wiring member  76  by, for example, soldering. The hollow of the wire supporting member  72  is filled with a sealer  82  so as to suppress a leak-in and leak-out. 
     (Housing) 
     Next, an explanation will be given of the fluid dynamic bearing  48 . 
     The housing  102  forms apart of the external portion of the fluid dynamic bearing  48 . 
     The housing  102  includes a housing bottom  110 , a support protrusion  108 , the first base-side encircling portion  112 , a second base-side encircling portion  122 , the housing projection  60 , a cover portion  124 , and a core contacting portion  126 , all being formed in an annular shape as viewed from the top. 
     The housing bottom  110  is formed in a flat disk shape and spreads in the radial direction. The support protrusion  108  is fixed to the inner circumference thereof and extends like a bar along the rotation axis R. The housing  102  forms, together with the shaft  26 , an annular recess where the lower end of the shaft encircling portion  30  enters. The first base-side encircling portion  112  is a hollow cylindrical portion extending upwardly in the axial direction, and is fixed to the outer circumference of the housing bottom  110 . The second base-side encircling portion  122  is a hollow cylindrical portion extending upwardly in the axial direction, and is provided at a location outwardly in the radial direction relative to the first base-side encircling portion  112 , and above the first bas-side encircling portion  112  in the axial direction. The housing projection  60  extends downwardly in the axial direction, and as an example, fixed to the second base-side encircling portion  122 . The housing projection  60  enters the base annular recess  58  in the axial direction. The cover portion  124  is fixed to the second base-side encircling portion  122  so as to cover the upper end of the base protrusion  4   e.    
     The core contacting portion  126  extends downwardly in the axial direction from the cover portion  124  so as to contact the upper face of the annular part of the stator core  40 . As a result, the annular part of the stator core  40  is held between the base  4  and the core contacting portion  126  in the axial direction, and is fixed therebetween. The upper end portion of the base protrusion  4   e  enters a space between the core contacting portion  126  and the second base-side encircling portion  122  in the axial direction. 
     (Male Screw) 
     A male screw to be engaged with a female screw provided in the base  4  is provided in the outer circumference of the housing  102 . A bond as a sealant is present between the male screw and the female screw. In this embodiment, as an example, a male screw  66  is formed at a portion near the lower end of the outer circumference of the first base-side encircling portion  112 . The male screw  66  is engaged with a female screw  68  provided in the inner circumference of the bearing hole  4   k.  A sealant  94  is present between the male screw  66  and the female screw  68 . The male screw may be provided in the outer circumference of the second base-side encircling portion  122 , and the female screw may be provided in the inner circumference of the base protrusion  4   e.    
     As an example, the housing  102  has the housing bottom  110 , the support protrusion  108 , the first base-side encircling portion  112 , the second base-side encircling portion  122 , the housing projection  60 , the cover portion  124 , and the core contacting portion  126  formed integrally with each other. Any one of the components of the housing  102  may be separately formed, and then joined with the other components. The housing  102  is formed by machining, including cutting and pressing, of a metal like SUS  430  of stainless-steel, or a copper alloy. The housing  102  may include a portion to which a surface process like electroless nickel plating is applied. The housing  102  may include a portion formed by plastic molding of a resin material. 
     (Sealant) 
     The sealant  94  is present in a predetermined area between the fluid dynamic bearing  48  and the base  4 . In this embodiment, the sealant  94  is applied between the outer circumference of the first base-side encircling portion  112 , the outer circumference and a part of lower end face of the second base-side encircling portion  122 , the inner circumference and lower end face of the housing projection  60 , the lower end face of the cover portion  124 , the inner circumference of the core contacting portion  126 , and respective opposing faces. 
     An example sealant  94  is an epoxy-based or acryl-based bond. Different kinds of sealants  94  may be applied to respective sealed faces. In this embodiment, the common sealant  94  is continuously applied to the continuous sealed faces. 
     The shaft-fastening-screw hole  152  that is a non-through hole is formed in an upper end face  108   b  of the support protrusion  108  along the rotation axis R. The shaft fastening screw  6  enters the shaft-fastening-screw hole  152 , and is engaged therewith by screwing. In order to enhance the joining strength, both screwing and bonding may be applied simultaneously. An explanation will now be given of a positional relationship among the shaft  26 , the support protrusion  108 , and the shaft fastening screw  6 . The support protrusion  108  is held between the shaft  26  and the shaft fastening screw  6  in the radial direction, or is present between the shaft  26  and the shaft fastening screw  6 . The shaft fastening screw  6  does not contact the shaft  26 , but is indirectly fixed to the shaft  26 . 
     (Shaft) 
     The shaft  26  extends along the rotation axis R of the hub  28 . In the axial direction, the area where the shaft-fastening-screw hole  152  is formed extends beyond areas where first radial dynamic pressure generating grooves  50  and second radial dynamic pressure generating grooves  52  to be discussed later are formed. The shaft  26  includes a body  26   f  which runs along the rotation axis R and which encircles the support protrusion  108 , and a flange  26   g  extending outwardly in the radial direction from the upper end portion of the body  26   f.  The support protrusion  108  is fixed to a support hole  26   d  by a combination of press-fitting and bonding. 
     (Ring Portion) 
     The ring  104  encircles the flange  26   g,  and is fixed to an outer circumference of the flange  26   g.  The ring  104  is fixed to the flange  26   g  by a combination of press-fitting and bonding. A bond between the ring  104  and the flange  26   g  also serves as a sealant that seals a clearance between the ring  104  and the flange  26   g  to suppress a leak-out of the lubricant  92 . 
     The shaft encircling portion  30  encircles the body  26   f.  The lubricant  92  is applied between the shaft encircling portion  30  and the body  26   f.    
     The shaft encircling portion  30  is held between the flange  26   g,  the ring portion  104  and the housing  102  in the axial direction (i.e., a parallel direction to the rotation axis R). The lubricant  92  is applied between the shaft encircling portion  30  and the ring portion  104 , between the shaft encircling portion  30  and the flange  26   g,  and, between the shaft encircling portion  30  and the housing  102 , respectively. The shaft encircling portion  30  includes a flange opposing face  28   l  which is a surface in a disk shape having a normal line substantially parallel with the rotation axis R. A lower face  28   m  of the shaft encircling portion  30  and an upper face  110   b  of the housing bottom  110  face with each other with a gap in the axial direction, and such a gap is filled with the lubricant  92 . 
     (Tapered Space) 
     A tapered space that gradually becomes widespread between the shaft body and the bearing body toward the area where the gas is present from the area where the lubricant is applied is provided between the stationary body and the rotating body. The tapered space has a gas-liquid interface of the lubricant in the halfway thereof so as to hold the lubricant. The tapered space serves as a capillary seal that suppresses a leak-out of the lubricant by a capillary phenomenon. In particular, in this embodiment, a first tapered space  114  and a second tapered space  118  are provided. The tapered space may be referred to as a tapered seal. 
     (First Tapered Space) 
     The first base-side encircling portion  112  encircles the lower part of the shaft encircling portion  30 . The first tapered space  114  that gradually becomes widespread toward the upper space between the inner circumference of the first base-side encircling portion  112  and the lower outer circumference of the shaft encircling portion  30  is formed between the first base-side encircling portion  112  and the shaft encircling portion  30 . The first tapered space  114  holds the lubricant  92 . The first tapered space  114  contacts, in the halfway thereof, a first gas-liquid interface  116  of the lubricant  92 . 
     (Ring Encircling Portion) 
     The rotating body includes a ring encircling portion  38  that encircles at least a part of the ring portion  104  with a gap. The ring encircling portion  38  is formed in an annular shape around the rotation axis R as viewed from the top. In this embodiment, the ring encircling portion  38  is formed integrally with the hub  28 , but maybe formed separately from the hub  28  and then joined with each other. 
     (Encircling-portion Recess) 
     An annular encircling-portion recess  154  around the rotation axis R is formed in the upper face of the shaft encircling portion  30 . The encircling-portion recess  154  is concaved downwardly. At least a part of the outer circumference of the ring portion  104  enters the encircling-portion recess  154 . The inner circumference of the encircling-portion recess  154  is included in the inner circumference of the ring encircling portion  38 . 
     (Second Tapered Space) 
     A gap between the inner circumference of the ring encircling portion  38  and the outer circumference of the ring portion  104  forms the second tapered space  118  that gradually becomes widespread toward the upper space. The second tapered space  118  includes a second gas-liquid interface  120  of the lubricant  92 , and suppresses a leak-out of the lubricant  92  by a capillary phenomenon. 
     (Radial Dynamic Pressure Generating Groove) 
     Radial dynamic pressure generating grooves that generate, when the shaft body and the bearing body relatively rotate, radial dynamic pressure to the lubricant which is separating force in the radial direction in the gap between the shaft body and the bearing body are formed between the stationary body and the rotating body. In this embodiment, the radial dynamic pressure generating grooves include first radial dynamic pressure generating grooves  50  and second radial dynamic pressure generating grooves  52 . 
     The first and second radial dynamic pressure generating grooves  50 ,  52  that generate dynamic pressure in the radial direction to the lubricant  92  when the hub  28  rotates relative to the shaft  26  are formed in an inner circumference  30   b  of the shaft encircling portion  30  in a manner apart from each other in the axial direction. The first and second radial dynamic pressure generating grooves  50 ,  52  are formed in a herringbone shape or in a spiral shape. 
     At least either one of the first radial dynamic pressure generating grooves  50  and the second radial dynamic pressure generating grooves  52  maybe formed in the outer circumference of the body  26   f  instead of the inner circumference  30   b  of the shaft encircling portion  30 . 
     (Thrust Dynamic Pressure Generating Groove) 
     Thrust dynamic pressure generating grooves that generate, when the shaft body and the bearing body relatively rotate, thrust dynamic pressure to the lubricant which is separating force in the thrust direction in the gap between the shaft body and the bearing body are formed between the stationary body and the rotating body. In this embodiment, the thrust dynamic pressure generating grooves include first thrust dynamic pressure generating grooves  54  and second thrust dynamic pressure generating grooves  56 . 
     (First Thrust Dynamic Pressure Generating Groove) 
     The first thrust dynamic pressure generating grooves  54  that generate dynamic pressure in the axial direction to the lubricant  92  when the hub  28  rotates relative to the shaft  26  are formed in a lower face  28   m  of the shaft encircling portion  30 . The first thrust dynamic pressure generating grooves  54  are formed in a herringbone shape or in a spiral shape. The first thrust dynamic pressure generating grooves  54  may be formed in an upper face  110   b  of the housing bottom  110  instead of the lower face  28   m  of the shaft encircling portion  30 . 
     (Second Thrust Dynamic Pressure Generating Groove) 
     The second thrust dynamic pressure generating grooves  56  that generate dynamic pressure in the axial direction to the lubricant  92  when the hub  28  rotates relative to the shaft  26  are formed in the flange opposing face  28   l  of the shaft encircling portion  30 . The second thrust dynamic pressure generating grooves  56  are formed in a herringbone shape or in a spiral shape. The second thrust dynamic pressure generating grooves  56  may be formed in a lower face  26   i  of the flange  26   g  instead of the flange opposing face  28   l  of the shaft encircling portion  30 . 
     When the rotating body rotates relative to the stationary body, the first and second radial dynamic pressure generating grooves  50 ,  52  and the first and second thrust dynamic pressure generating grooves  54 ,  56  respectively generate dynamic pressure to the lubricant  92 . The rotating body is supported in the radial direction and in the axial direction by such dynamic pressure in a non-contact manner with the stationary body. 
     An explanation will be given of an example method of engraving the radial dynamic pressure generating grooves  50 ,  52  and the thrust dynamic pressure generating grooves  54 ,  56 . The dynamic pressure generating grooves can be formed by, for example, a rolling method of pushing a process tool against a groove formed part  30   c  while applying large force, and of causing the surface of the groove formed part  30   c  to be plastically deformed to engrave the grooves, an etching method like electrochemical machining of causing the groove formed part  30   c  in a predetermined electrolytic solution to face a predetermined electrode, and applying a voltage therebetween to engrave the grooves, or an alternation cutting method of causing the tip of a cutting tool to be alternately driven by alternation drive means like a Piezo element to perform alternation cutting on the surface of the groove formed part  30   c,  thereby engraving the grooves. 
     The radial dynamic pressure generating grooves  50 ,  52  and the thrust dynamic pressure generating grooves  54 ,  56  may be all engraved by the same method, or may be engraved by different methods, respectively. In this embodiment, the first and second radial dynamic pressure generating grooves  50 ,  52  and the first and second thrust dynamic pressure generating grooves  54 ,  56  are engraved by the alternation cutting method. 
     The dynamic pressure generating grooves formed by the rolling method include a plastic deformation portion plastically processed, the dynamic pressure generating grooves formed by the etching method include an etched portion having undergone etching, and the dynamic pressure generating grooves formed by the alternation cutting method include a cut portion. In this embodiment, the radial dynamic pressure generating grooves  50 ,  52  and the thrust dynamic pressure generating grooves  54 ,  56  include respective cut portion having undergone cutting. 
     (Shaft Encircling Portion) 
       FIG. 3  is an enlarged cross-sectional view exemplarily illustrating an area near the inner circumference  30   b  of the shaft encircling portion  30  of the disk drive device  100  in  FIG. 2 , and the slant is emphasized to facilitate understanding to the present disclosure. 
     As to the shaft encircling portion  30 , a base body  30   a  is formed by, for example, cutting a ferrous material like SUS  430  of stainless-steel or a metal like brass. In this embodiment, as an example, the shaft encircling portion  30  has a plating layer  74  formed on an inner circumference  30   d  of the base body  30   a  cut out from brass. The plating layer  74  is formed by, for example, electroless nickel plating. A primer plating layer may be formed by surface treatment like strike plating. When a difference in hardness between the plating layer  74  of the shaft encircling member  30  and the shaft  26  is small, a seizing may occur when those components contact with each other. Hence, the plating layer  74  of the shaft encircling portion  30  is subjected to a thermal process to increase the hardness thereof so as to be higher than that of the shaft  26 . 
     (Plating Layer) 
     Next, an explanation will be given of the plating layer  74  on the inner circumference  30   b  of the shaft encircling portion  30 . The shaft encircling portion  30  has a slanted plating layer  74   b  formed on the inner circumference  30   b  of the groove formed part  30   c  and gradually increasing the plating thickness toward the external side in the direction of the rotation axis R. That is, that shaft encircling portion  30  has the thickness of the plating layer  74  at the possibly contact area thicker than other portions. 
     When the slanted plating layer  74   b  is formed on the inner circumference  30   b,  the parallelism of the inner circumference  30   b  increases, and thus the shaft encircling portion  30  may have the shape accuracy of the radial dynamic pressure generating grooves deteriorated. In this embodiment, the shaft encircling portion  30  has the inner circumference  30   d  of the base body  30   a  without the slanted plating layer  74   b  gradually increasing the diameter toward the external side in the direction of the rotation axis R. As a result, the parallelism of the inner circumference  30   b  formed with the slanted plating layer  74   b  becomes small, and thus it becomes possible to suppress a reduction of the shape accuracy of the radial dynamic pressure generating grooves. 
     Next, an explanation will be given of an example method for forming the slanted plating layer  74   b.    
     As to the plating layer  74 , the pre-processed base body  30   a  of the shaft encircling portion  30  is soaked in an electroless nickel plating solution, and is swung in a predetermined direction. Hence, an oxidization-reduction reaction is caused in the solution, and nickel is precipitated on the surface of the base body  30   a  of the shaft encircling portion  30 . 
     The inventors of the present disclosure keenly studied and found that the slanted plating layer  74   b  gradually becoming thick toward the external side in the direction of the rotation axis R can be formed on the inner circumference  30   b  by combining the following conditions: 
     (1) Swing the base body  30   a  of the shaft encircling portion  30  in the direction of the center axis thereof in the plating solution; 
     (2) When the stroke of the swing of the base body  30   a  of the shaft encircling member  30  is elongated and the cycle of the swing is shortened, the oxidization-reduction reaction becomes remarkable near the ends of the inner circumference  30   b,  and the plating thickness at the external side in the axial direction becomes relatively thick. When, for example, the cycle of the swing is 1.2 seconds, in comparison with a case in which the cycle is 2.5 seconds, a difference between the maximum plating thickness near the ends of the inner circumference  30   b  and the minimum plating thickness near the center becomes remarkably large; 
     (3) When the concentration of a stabilizer that suppresses the oxidization-reduction reaction in the plating solution is increased, the oxidization-reduction reaction near the ends of the inner circumference  30   b  is remarkably suppressed, and thus the plating thickness near the both ends of the inner circumference  30   b  becomes relatively thin; 
     (4) When the inner circumference  30   b  of the base body  30   a  of the shaft encircling portion  30  prior to the plating has a slant gradually increasing the diameter toward the external side in the axial direction, the plating thickness at the external side in the axial direction becomes relatively thick. 
     Based on those results, the condition to form the plating layer with a desired slant can be set through a test with parameters that are the swing direction, the swing stroke, the swing cycle, and the concentration of the stabilizer being as parameters. 
     In this embodiment, the groove formed part  30   c  of the shaft encircling portion  30  has a maximum plating thickness H 2  of the slanted plating layer  74   b  at the external side in the axial direction larger than a minimum plating thickness H 1  at the internal side in the axial direction. When a difference Hd between the maximum plating thickness H 2  and the minimum plating thickness H 1  is set to be equal to or greater than 0.1 μm, the effect of improving the wear resistance can be observed, and it is preferable to set the plating thickness difference Hd to be equal to or greater than 0.2 μm in consideration of a manufacturing error. According to the study by the inventors of the present disclosure, there is no particular practical problem in manufacturing of the shaft encircling portion  30  when the plating thickness difference Hd is equal to or less than 0.5 μm. 
     The above-explained method is merely an example, and the slanted plating layer  74   b  may be formed through other methods. In addition, it is confirmed that when the above-explained method is modified, a reverse-slanted plating layer having a plating thickness gradually becoming thin toward the external side in the direction of the rotation axis R can be formed on the groove formed part  30   c  of the inner circumference  30   b  of the shaft encircling portion  30 . 
     (Dynamic Pressure Generating Groove) 
       FIGS. 4 and 5  are enlarged cross-sectional views exemplarily illustrating an example structure of the groove formed part of the shaft encircling portion of the disk drive device.  FIG. 4  illustrates an example case in which grooves are formed in the surface of the plating layer formed on the base body. In particular, the first radial dynamic pressure generating grooves  50  in  FIG. 4  are engraved in the external surface of the plating layer  74  after the plating layer  74  is formed on the base body  30   a.    FIG. 5  is an enlarged cross-sectional view exemplarily illustrating another example structure corresponding to  FIG. 4 , and illustrates an example case in which the plating layer is formed after the grooves are engraved in the base body. In particular, radial dynamic pressure generating grooves  500  in  FIG. 5  have grooves  500   b  engraved in a base body  300   a  of the shaft encircling portion  300  before a plating layer  740  is formed, and the plating layer  740  is formed thereafter. 
     Although the structure in  FIG. 5  is also practical, this embodiment employs the structure in  FIG. 4 . Edges  50   a  of the first radial dynamic pressure generating grooves  50  in  FIG. 4  are formed so as to be keener than those of edges  500   a  of the radial dynamic pressure generating grooves  500  in  FIG. 5 . 
     In the groove formed part  30   c  in  FIG. 4 , a minimum plating thickness D 1  of a non-grooved part of the plating layer  74  after the first radial dynamic pressure generating grooves  50  are formed is larger than a maximum groove depth D 2  of the first radial dynamic pressure generating grooves  50 . When, for example, the maximum groove depth D 2  of the dynamic pressure generating grooves is set to be 3 to 6 μm, the plating thickness D 1  can be set to be equal to or greater than 7 μm. When the plating thickness D 1  of the plating layer  74  becomes thick, a necessary work time for the plating process may increase, and thus the productivity may decrease. Hence, the plating thickness of the plating layer  74  can be set to be equal to or smaller than 30 μm. In this embodiment, the plating thickness D 1  of the plating layer  74  is set to be within a range between equal to or greater than 8 μm and equal to or smaller than 20 μm. According to such a structure, the shaft encircling portion  30  can be manufactured within a practical work time. 
     The plating layer  74  after the first radial dynamic pressure generating grooves  50  are engraved may have a minimum plating thickness D 3  larger than the maximum groove depth D 2  of the first radial dynamic pressure generating grooves  50 . For example, a plating thickness D can be 5 μm, while the maximum groove depth D 2  can be 4 μm. 
     In this embodiment, the second radial dynamic pressure generating grooves  52  are engraved after the plating layer  74  is formed on the shaft encircling portion  30 , and have the same features as those of the first radial dynamic pressure generating grooves  50 . 
     In addition, the thrust dynamic pressure generating grooves  54 ,  56  are engraved after a plating layer is formed on the lower face  28   m  of the shaft encircling portion  30  and the flange opposing face  28   l  thereof, and have the same features as those of the radial dynamic pressure generating grooves  50 . 
     (Cover Ring) 
     Returning to  FIG. 2 , the cover ring  12  is formed in an annular shape from a metal material like stainless-steel or a resin material, and covers the second tapered space  118 . The cover ring  12  is fixed to, for example, the hub  28  of the rotating body or the flange  26   g  of the stationary body by caulking, press-fitting or a combination thereof. The cover ring  12  may include a lubricant capturer, such as a porous material like a sintered body, or a charcoal filter. 
     (Bypass Communication Hole) 
     The shaft encircling portion  30  is formed with a bypass communication hole  164  passing all the way through the shaft encircling portion  30  in the axial direction. The bypass communication hole  164  suppresses a pressure difference in areas where the lubricant  92  is applied even if, for example, the dynamic pressure becomes unbalanced, thereby maintaining the appropriate levels of the first gas-liquid interface  116  and the second gas-liquid interface  120 . 
     An explanation will be given of an operation of the disk drive device  100  employing the above-explained structure. Three-phase drive currents are applied to the coils  42  through the conductor pattern  80  of the wiring member  76  to rotate the magnetic recording disk  8 . When such drive currents flow through the respective coils  42 , magnetic fluxes are generated along the nine salient poles. Those magnetic fluxes apply torque to the magnet  32 , and thus the rotating body and the magnetic recording disk  8  engaged therewith rotate. While at the same time, when the voice coil motor  16  causes the swing arm  14  to swing, the recording/playing head goes out and comes in the swingable range over the magnetic recording disk  8 . The recording/playing head converts magnetic data recorded in the magnetic recording disk  8  into electrical signals, and transmits the signals to a control board (unillustrated), or writes data transmitted in the form of electrical signals from the control board in the magnetic recording disk  8  as magnetic data. 
     Next, an explanation will be given of advantageous effects of this embodiment in detail. 
     According to the disk drive device  100  of this embodiment, the shaft encircling portion  30  includes the slanted plating layer  74   b  gradually increasing the plating thickness of the groove formed part  30   b  of the shaft encircling portion  30  toward the external side in the direction of the rotation axis. Hence, the plating layer near the ends of the inner circumference  30   b  of the bearing body can be made relatively thick. As a result, the wear resistance against a contact of the portion of the inner circumference  30   b  of the bearing body near the end thereof with the shaft can be improved. 
     According to the disk drive device  100  of this embodiment, the groove formed part  30   c  of the shaft encircling portion  30  has the slanted plating layer  74   b  which has the maximum plating thickness at the external sides in the direction of the rotation axis thicker than the minimum plating thickness at the internal side in the direction of the rotation axis within a range from 0.1 μm to 0.5 μm. Hence, the wear resistance can be improved while suppressing a reduction of the productivity of the shaft encircling portion  30 . 
     According to the disk drive device  100  of this embodiment, the shaft encircling portion  30  has the inner circumference  30   d  of the base body  30   a  without the slanted plating layer  74   b  gradually increasing the diameter toward the external sides in the direction of the rotation axis. Therefore, a reduction of the parallelism of the inner circumference  30   b  can be suppressed. 
     According to the disk drive device  100  of this embodiment, the shaft encircling portion  30  has the dynamic pressure generating grooves engraved in the external surface side of the plating layer. Hence, a reduction of the shape accuracy of the edges of the dynamic pressure generating grooves can be suppressed, and dynamic pressure to be generated can be improved. Even if the plating layer is made thick, a reduction of the shape accuracy of the edges of the dynamic pressure generating grooves can be suppressed. 
     According to the disk drive device  100  of this embodiment, the plating layer  74  having the grooves of the shaft encircling portion  30  engraved therein has the minimum plating thickness D 1  of the non-grooved part larger than the maximum groove depth D 2 . Therefore, even if the plating thickness becomes thin due to a manufacturing error, the surface of the base body  30   a  of the shaft encircling portion  30  can be prevented from being exposed at the bottom of the grooves. 
     According to the disk drive device  100  of this embodiment, the dynamic pressure generating grooves of the shaft encircling portion  30  include at least one of the plastic deformed portion having undergone plastic deformation, a cut portion having undergone cutting, and an etched portion having undergone etching. Therefore, the shape accuracy of the dynamic pressure generating grooves can be improved. 
     According to the disk drive device  100  of this embodiment, the plating layer  74  having the grooves of the shaft encircling portion  30  engraved therein has a higher surface hardness than the portion of the shaft  26  facing the plating layer  74 . Hence, the durability of the plating layer  74  is improved, thereby reducing a possibility of seizing. 
     According to the disk drive device  100  of this embodiment, the plating layer  74  having the grooves of the shaft encircling portion  30  engraved therein has the minimum plating thickness D 3  of the dynamic pressure generating grooves larger than the maximum groove depth D 2  thereof. Therefore, the plating thickness of the plating layer  74  can be made thick entirely. 
     According to the disk drive device  100  of this embodiment, the plating layer  74  having the grooves of the shaft encircling portion  30  engraved therein has the maximum plating thickness D 1  of the non-grooved part that is within a range from 8 μm and 20 μm. Therefore, a reduction of the productivity can be suppressed while improving the durability of the plating layer  74 . 
     According to the disk drive device  100  of this embodiment, the plating layer  74  having the grooves of the shaft encircling portion  30  engraved therein has a primer plating layer formed on the surface of the base body  30   a,  enabling astable formation of an electroless nickel plating layer. 
     The structure of the disk drive device and the operation thereof according to this embodiment were explained above. However, the embodiment is merely an example, and permits various modifications of a combination of structural components, and such modifications should be within the scope of the present disclosure. 
     In the aforementioned embodiment, components explained as being formed by joining equal to or greater than two pieces initially formed separately may be initially formed integrally with each other in accordance with the specification of the product and the demand in manufacturing, and components explained as being formed integrally may be formed by joining equal to or greater than two pieces initially formed separately in accordance with the specification of the product and the demand in manufacturing. According to such a joining, for example, bonding, shrink fitting, welding can be applied in solo or a combination of equal to or greater than two such techniques are also applicable.