Patent Publication Number: US-2011064341-A1

Title: Disk drive device capable of being improved in anti-vibration characteristic

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a disk drive device, in particular, to a disk drive device capable of being improved in anti-vibration characteristic while reducing a drive current. 
     2. Description of the Related Art 
     In recent years, disk drive devices, such as Hard Disk Drives (HDDs), have been developed to be small in size and large in capacity, and been widely used in many electrical appliances. Therefore, disk drive devices have been used in a wide variety of environments. In particular, the disk drive devices are being mounted in portable devices called mobile devices. Mobile devices are frequently used in environments with a lot of vibrations, and therefore the disk drive devices to be mounted in the mobile devices are demanded to have characteristics in which read/write of data can be stably performed even when used in an environment with a lot of vibrations. In order to meet such a demand, there is a disk driver device in which a fluid dynamic bearing capable of stably rotating at high-speed is mounted. For example, Japanese Patent Application Publication No. 2007-198555 discloses an example of the structure of a fluid dynamic bearing unit, in which lubricant is injected into the space between a sleeve of which part of a stator is composed and a shaft of which part of a rotating body is composed. In this fluid dynamic bearing, smooth high-speed rotation of the rotating body can be realized by supporting the rotating body in a non-contact state with dynamic pressure generated in part of the lubricant. 
     Because miniaturization of mobile devices is considered to be important, batteries for mobile devices are often made small with this. As a result, it is often demanded that a drive current should be reduced when a disk drive device is to be mounted in a mobile device. If a drive current in a disk drive device is reduced, the dynamic pressure to be generated by a fluid dynamic bearing unit is decreased accordingly, resulting in decreased bearing stiffness of the fluid dynamic bearing unit. If the bearing stiffness of a fluid dynamic bearing unit is decreased, an axial displacement of a rotating body including a recording disk becomes large when the disk drive device has vibrated after receiving an impact, etc. If a displacement of a recording disk becomes large, the relative distance between the recording disk and a magnetic head becomes unstable, thereby causing the problem that an increase in errors in reading/writing data may be incurred. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of these situations, and a purpose of the invention is to provide a disk drive device in which, even if a drive current is reduced, the bearing stiffness of a fluid dynamic bearing unit is maintained and therefore read/write of data can be stably performed even under an environment with a lot of vibrations. 
     In order to solve the aforementioned problem, a disk drive device according to an embodiment of the present invention comprises: a hub on which a recording disk is to be mounted; a shaft to be the rotational center of the hub; a sleeve configured to house the shaft and to be rotatable relatively with respect to the shaft; a radial space portion formed between the inner circumferential surface of the sleeve and the outer circumferential surface of the shaft; a radial dynamic pressure generating portion configured to generate radial dynamic pressure between at least part of the inner circumferential surface of the sleeve and the outer circumferential surface of the shaft in the radial space portion; and lubricant injected into the radial dynamic pressure generating portion. The axial length of the radial dynamic pressure generating portion is structured to be longer than the diameter of the radial dynamic pressure generating portion such that radial dynamic pressure, which is defined with the axial length of the radial dynamic pressure generating portion and the diameter thereof being parameters, is greater than or equal to a predetermined minimum reference value. 
     According to this embodiment, the relationship between the axial length of the radial dynamic pressure generating portion and the diameter thereof is defined such that the radial dynamic pressure to be generated in the radial dynamic pressure generating portion is greater than or equal to a predetermined design minimum reference value. For example, when the diameter of the radial dynamic pressure generating portion is made small, the diameter of the shaft itself, which is to be housed, is made small accordingly. As a result, the circumferential formation length of the radial dynamic pressure generating portion becomes short. That is, when relative rotation occurs between the shaft and the sleeve, the resistance to the lubricant in the radial space portion is reduced. As a result, a driving force for rotating, in the radial space portion, the hub and the shaft on which a recording disk is mounted, i.e., a drive current can be reduced. Further, the weight of the shaft is reduced by making the diameter thereof small, which can contribute to a reduction in a drive current. On the other hand, the total area of the radial dynamic pressure generating portion is substantially decreased due to the decrease in the circumferential formation length accompanying the reduction in the diameter of the radial dynamic pressure generating portion, and therefore the generation amount of the radial dynamic pressure is decreased. Then, a generation amount of the radial dynamic pressure as a whole is made greater than or equal to the minimum reference value by making the axial length of the radial dynamic pressure generating portion to increase the total area thereof. By making the axial length of the radial dynamic pressure generating portion longer than the diameter thereof as stated above, a generation amount of the radial dynamic pressure can be maintained or improved while reducing a drive current. 
     Another embodiment of the present invention is also a disk drive device. This device comprises: a hub on which a recording disk is to be mounted; a shaft to be the rotational center of the hub; a sleeve configured to house the shaft and to be rotatable relatively with respect to the shaft; a radial space portion formed between the inner circumferential surface of the sleeve and the outer circumferential surface of the shaft; a radial dynamic pressure generating portion configured to generate radial dynamic pressure between at least part of the inner circumferential surface of the sleeve and the outer circumferential surface of the shaft in the radial space portion; and lubricant injected into the radial dynamic pressure generating portion. The radial dynamic pressure generating portion is composed of a plurality of striped groove portions repeatedly arranged along the rotational direction. Each of the striped groove portions is formed by end portions on both side and an intermediate portion sandwiched by the end portions on both sides, and the end portions on both sides and the intermediate portion are arranged such that the lubricant is collected into the intermediate portion by the relative rotation between the shaft and the sleeve, and the width in the rotational direction of the intermediate portion in the striped groove portion is structured to be narrower than that in the rotational direction of each of the end portions on both sides. 
     By making the width in the rotational direction of the intermediate portion in a striped groove portion narrower than that in the rotational direction of each of the end portions on both sides, it becomes possible that an amount of lubricant, which is larger than that acceptable with the intermediate portion, can be scraped up from the end portions to be transferred into the intermediate portion, thereby allowing for radial dynamic pressure to be efficiently increased. 
    
    
     
       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, in which: 
         FIG. 1  is an illustrative view illustrating the internal structure of an HDD, which is an example of a disk drive device according to the present embodiment; 
         FIG. 2  is a schematic cross-sectional view of a brushless motor in the disk drive device according to the present embodiment; 
         FIG. 3  is an illustrative view illustrating the basic shape in the shapes of a striped groove portion in a radial dynamic pressure generating portion in the disk drive device according to the present embodiment; 
         FIG. 4  is a partial cross-sectional view illustrating the relationship between the diameter of the radial dynamic pressure generating portion and the length thereof in the radial dynamic pressure generating portion in the disk drive device according to the present embodiment; 
         FIG. 5  is a cross-sectional view of a hub, illustrating processing procedure of the hub in the disk drive device according to the present embodiment; 
         FIG. 6A  is an illustrative view illustrating a shape of the striped groove portion in the radial dynamic pressure generating portion in the disk drive device according to the present embodiment, the shape being a variation of the basic shape of  FIG. 3 ; 
         FIG. 6B  is an illustrative view illustrating a shape of the striped groove portion in the radial dynamic pressure generating portion in the disk drive device according to the present embodiment, the shape being a variation of the basic shape of  FIG. 3 ; 
         FIG. 6C  is an illustrative view illustrating a shape of the striped groove portion in the radial dynamic pressure generating portion in the disk drive device according to the present embodiment, the shape being a variation of the basic shape of  FIG. 3 ; and 
         FIG. 7  is a schematic cross-sectional view illustrating an example of another structure of the brushless motor in the disk drive device according to the present embodiment. 
     
    
    
     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. 
     Hereinafter, preferred embodiments of the present invention will be described based on the accompanying drawings. The present embodiment can be adopted in a brushless motor, which is mounted in a Hard Disk Drive device (sometimes, simply referred to as an HDD or a disk drive device) to drive a recording disk, or adopted in a disk drive motor to be mounted in an optical disk recording and reproducing device, such as a CD (Compact Disc) device and a DVD (Digital Versatile Disc) device. 
       FIG. 1  is an illustrative view illustrating the internal structure of an HDD  100  (hereinafter, referred to as a disk drive device  100 ), which is an example of a disk drive device according to the present embodiment.  FIG. 1  illustrates the state where a cover is removed to expose the internal structure. 
     A brushless motor  114 , an arm bearing unit  116 , and a voice coil motor  118 , etc., are mounted on the upper surface of a base member  10 . The brushless motor  114  supports, on the rotation axis, a hub  20  on which a recording disk  120  is to be mounted and rotationally drives the recording disk  120  on which data can be recorded, for example, magnetically. The brushless motor  114  can be replaced with, for example, a spindle motor. The brushless motor  114  is driven by a three-phase drive current consisting of a U-phase, a V-phase, and a W-phase. The arm bearing unit  116  supports a swing arm  122  within the movable range AB and in a swing-free manner. The voice coil motor  118  makes the swing arm  122  swing in accordance with external control data. A magnetic head  124  is fixed to the tip of the swing arm  122 . When the disk drive device  100  is in an operating state, the magnetic head  124  moves, with the swing of the swing arm  122 , within the movable range AB and above the surface of the recording disk  120  via a slight gap between the surface of the recording disk  120  and the magnet head  124 , thereby performing read/write of data. It is noted that, in  FIG. 1 , the point A corresponds to the position at the outermost recording track of the recording disk  120  and the point B to the position at the innermost recording track thereof. The swing arm  122  may be transferred to a waiting position provided on the side of the recording disk  120  when the disk drive device  100  is in a stopped state. 
     In the present embodiment, a device including all of the components for reading/writing data, such as the recording disk  120 , the swing arm  122 , the magnetic head  124 , and voice coil motor  118 , etc., is sometimes expressed as a disk drive device, or as an HDD. Alternatively, only the part for rotationally driving the recording disk  120  is sometimes expressed as a disk drive device. 
       FIG. 2  is a schematic cross-sectional view of the brushless motor in the disk drive device  100  according to the present embodiment, the view being taken along the axial direction of a shaft  22  in the disk drive device  100 . The disk drive device  100  comprises a fixed body S and a rotating body R. The fixed body S includes the base member  10 , a stator core  12 , a housing  14 , and a sleeve  16 . The rotating body R includes the hub  20 , the shaft  22 , and a thrust member  26 . The base member  10  includes a cylinder portion  10   a  and the housing  14  includes a groove portion  14   a , a bottom  14   b , a cylinder portion  14   c , and a housing flat portion  14   d . The sleeve  16  includes a cylinder portion inner circumferential surface  16   a , a circumferentially-protruding portion  16   b , and a cylinder portion  16   c , and a coil  18  is wound around the stator core  12 . The hub  20  includes a center hole  20   a , a first cylinder portion  20   b , a second cylinder portion  20   c , a hub outward extending portion  20   d , and a pedestal portion  20   f . The shaft  22  includes a step portion  22   a , a tip portion  22   b , and an outer circumferential surface  22   c ; and the thrust member  26  includes a hanging portion  26   c  and a flange  26   e . It is noted that, in the following descriptions, for convenience, the downward direction illustrated in the drawings is expressed as the bottom and the upward direction illustrated therein as the top, as a whole. 
     The base member  10  has a central hole and the cylinder portion  10   a  provided so as to surround the central hole. The base member  10  holds the housing  14  with the central hole and fixes the stator core  12  to the outer circumference of the cylinder portion  10   a  surrounding the housing  14 . An annular-shaped second area portion  42  is formed between the outer circumference of the housing  14  and the inner circumference of the cylinder portion  10   a . The second area portion  42  has a shape surrounding the central hole of the base member  10 . The base member  10  is formed by cutting an aluminum die casting product or pressing an aluminum plate or a nickel-plated steel plate. 
     The stator core  12  is formed by performing insulation coating made by electro-deposition coating or powder coating, etc., on the surface thereof after a plurality of magnetic plates, such as ferrosilicon plates, are laminated. The stator core  12  is a ring-shaped member having a plurality of salient poles (not illustrated) protruding outwards, around each of which the coil  18  is wound. When the disk drive device  100  is, for example, three-phase driven, the number of the salient poles is made to be nine. The wiring terminal of the coil  18  is soldered on an FPC (Flexible Printed Circuit) arranged on the bottom surface of the base member  10 . The pulled-out wire terminal is fixed with adhesive so as not to unlay. The fixation is performed to prevent disconnection of the wire due to a vibration of large amplitude created by a resonance of the wire during ultrasonic wave cleaning, etc. When a three-phase current having an approximate sine wave shape is applied to the coil  18  through the FPC by a predetermined drive circuit, the coil  18  generates a rotating magnetic field in the salient poles of the stator core  12 . A rotating drive force is generated by the interaction between the driving magnetic poles of the magnet  24  and the rotating magnetic field, which rotates the rotating body R. 
     An attracting plate  44  is fixed to a position on the base member  10  facing the axial lower end surface of the ring-shaped magnet  24  via a gap. The attracting plate  44  is a ring-shaped member and is formed by pressing a soft magnetic material, for example, a cold-rolled steel plate. The attracting plate  44  generates an axial magnetic attracting force between the magnet  24  and itself. That is, the attracting plate  44  generates a force to attract a hub in the direction where the rotating body R is drawn to the base member  10 . The rotating body R is made to rotate in a non-contact state with surrounding members with three forces of a floating force, the force to attract the hub, and the gravity applied to the whole rotating body R, being balanced during the rotation of the rotating body R, the floating force being generated by a bearing including a radial dynamic pressure generating portion RB and a thrust dynamic pressure generating portion SB, which will be described later. 
     The housing  14  is fixed to the inner circumferential surface of the cylinder portion  10   a  by adhesion or press-fitting. The housing  14  is approximately cup-shaped, in which the cylinder portion  14   c  surrounding the sleeve  16 , the housing flat portion  14   d  that is provided at the end portion nearer to the hub  20  and that has the surface facing in the axial direction, and the bottom  14   b  sealing the end portion of the cylinder portion  14   c , the end portions being located on the side opposite to the housing flat portion  14   d , are combined. The housing  14  having such a shape is arranged so as to seal the lower end of the sleeve  16  and to make the upper end thereof protrude. In addition, the bottom  14   b  and the cylinder portion  14   c  may be formed integrally with each other, or both may be fixed together after being formed as different members. The housing  14  may be formed of a copper-based alloy, a sintered alloy by powder metallurgy, stainless steel, or a plastic material, such as polyetherimide, polyimide, polyamide, etc. When the housing  14  is to be formed of a plastic material, it is desirable that the plastic material is structured by containing, for example, carbon fiber, etc., so that the specific resistance of the housing  14  is smaller than or equal to 10 6  Ω·m in order to secure the static eliminating performance of the disk drive device  100 . 
     A groove  14   a  extending in the axial direction is formed on the inner circumferential surface of the housing  14 . The groove  14   a  functions as a communication hole for communicating both end surface sides of the housing  14  when the sleeve  16  is fit into the cylinder portion  14   c . The communication hole becomes a communication channel I by being filled with lubricant  28 . This communication channel I will be described later. The cross-sectional shape of the groove  14   a  may be a concave circular arc shape or a rectangular shape. 
     The sleeve  16  is fixed to the inner circumferential surface of the housing  14  by adhesion or press-fitting and is fixed on the same axis as that of the central hole of the base member  10 . The sleeve  16  has a shape in which the annular cylinder portion  16   c  that supports the shaft  22  by housing the shaft  22  and a circumferentially-protruding portion  16   b  that is extended in the outer diameter direction at the end portion of the cylinder portion  16   c , the end portion being located nearer to the hub  20 , are combined. In addition, the cylinder portion inner circumferential surface  16   a  is formed inside the cylinder portion  16   c  so as to surround the shaft  22 . A radial space portion is formed between the cylinder portion inner circumferential surface  16   a  of the sleeve  16  and the outer circumferential surface  22   c  of the shaft  22 , and a first radial dynamic pressure generating portion RB 1  and a second radial dynamic pressure generating portion RB 2  are arranged in the radial space portion, as individual radial dynamic pressure generating portions for generating radial pressure in the radial space portion. The first radial dynamic pressure generating portion RB 1  and the second radial dynamic pressure generating portion RB 2  as individual radial dynamic pressure generating portions will be described later in detail. The circumferentially-protruding portion  16   b  and the cylinder portion  16   c  may be formed integrally with each other, or both may be fixed together after being formed as different members. An annular first area portion  40  is formed between the circumferentially-protruding portion  16   b  and the cylinder portion  14   c . The sleeve  16  is formed of a copper-based alloy, a sintered alloy by powder metallurgy, stainless steel, etc. Other than that, the sleeve  16  may be formed of a plastic material, such as polyetherimide, polyimide, polyamide, etc. When the sleeve  16  is to be formed of a plastic material, it is desirable that the plastic material is structured by containing, for example, carbon fiber, etc., so that the specific resistance of the housing  14  is smaller than or equal to 10 6  Ω·m in order to secure the static eliminating performance of the disk drive device  100 . 
     The hub  20  is structured to include the center hole  20   a  provided at the center thereof, the first cylinder portion  20   b  provided so as to surround the center hole  20   a , the second cylinder portion  20   c  arranged outside the first cylinder portion  20   b , and the hub outward extending portion  20   d  extending outward in the radial direction at the lower end of the second cylinder portion  20   c . The hub  20  is approximately cup-shaped. The hub  20  has soft magnetism. For example, the hub  20  is formed of a steel material, such as SUS 430F, etc. The hub  20  is formed to have an approximately cup-shaped predetermined shape by pressing or cutting a steel plate. For example, the stainless steel with the product name of DHS1, supplied by Daido Steel Co., Ltd., is preferred as a material for the hub  20  in terms of less outgassing and easy processing. Similarly, the stainless steel with the product name of DHS2 is more preferred as a material for the hub  20  in terms of good corrosion resistance in addition to the foregoing characteristics. 
     The thrust member  26  is fixed to the inner circumferential surface of the first cylinder portion  20   b  of the hub  20 , and the magnet  24  is fixed to the inner circumferential surface of the second cylinder portion  20   c . Herein, the magnet  24  is fixed to the annular portion that is concentric with the shaft  22  so as to face the stator core  12  fixed to the base member  10 . With such a structure, the hub  20  rotates integrally with the shaft  22  to rotate the non-illustrated recording disk  120 . The recording disk  120  is mounted on the hub outward extending portion  20   d  with the center hole of the recording disk  120  being engaged with the outer circumferential surface of the second cylinder portion  20   c.    
     The shaft  22  is fixed to the center hole  20   a  of the hub  20 . Herein, the step portion  22   a  is provided at the upper end portion of the shaft  22  and the shaft  22  is press-fit into the center hole  20   a  when assembled. As a result, the hub  20  is restricted in the axial movement by the step portion  22   a  and is integrated with the shaft  22  at a predetermined right angle. The tip  22   b  of the shaft  22  is housed within the inner circumference of the cylinder portion  16   c . The shaft  22  can be formed of stainless steel. 
     The thrust member  26  has the flange  26   e  surrounding the sleeve  16  and the hanging portion  26   c  surrounding the housing  14 . Herein, the flange  26   e  is fixed to the inner wall of the first cylinder portion  20   b  with adhesive and the hanging portion  26   c  is bound to the outer edge portion of the flange  26   e  and is also fixed to the inner wall of the first cylinder portion  20   b  with adhesive. That is, the outer circumferential surface of the hanging portion  26   c  is fixed to the inner circumferential surface of the first cylinder portion  20   b  by adhesion. Thus, the flange  26   e  surrounds the outer circumference of the cylinder portion  16   c  via a gap and is arranged above the lower surface of the circumferentially-protruding portion  16   b  via a narrow gap. In addition, while the thrust member  26  is rotating integrally with the hub  20 , the flange  26   e  is rotating within the first area portion  40  and the hanging portion  26   c  is rotating within the second area portion  42 . 
     As illustrated in  FIG. 2 , the flange  26   e  has a shape having a thrust upper surface  26   a  and a thrust lower surface  26   b , the shape being thin in the axial direction. The hanging portion  26   c  extends in the axial direction from the lower surface nearer to the outer circumference of the flange  26   e . A first thrust dynamic pressure generating portion SB 1  is composed of the thrust lower surface  26   b  of the flange  26   e  and the housing flat portion  14   d  that is the upper end portion of the housing  14 ; and a second thrust dynamic pressure generating portion SB 2  is composed of the thrust upper surface  26   a  of the flange  26   e  and the lower surface of the circumferentially-protruding portion  16   b . The thrust member  26  is formed by combining the flange  26   e  with the hanging portion  26   c  and has a so-called inverted L-shaped cross section in which the alphabetical capital letter “L” is inverted upside down, as illustrated in  FIG. 2 . Herein, the axial length of the hanging portion  26   c  is larger than the axial length of the flange  26   e . The inner circumferential surface  26   d  of the hanging portion  26   c  is tapered in which the radius thereof is reduced toward the side opposite to the side where the flange  26   e  is formed, thereby composing a capillary seal portion TS, which will be described later. Such a thrust member  26  can be easily and inexpensively formed by, for example, pressing a plate-shaped metal material. Further, the thrust member  26  can be formed with good dimension accuracy by press processing, etc., even if the thrust member  26  becomes small in size and thin. As a result, miniaturization and weight reduction of the disk drive device  100  can be attained by making the thrust member  26  small. 
     The thrust member  26  has the function of preventing the rotating body R from coming off the fixed body S other than composing the thrust dynamic pressure generating portion. If the rotating body R and the fixed body S are relatively transferred due to an impact, the flange  26   e  will be in contact with the lower surface of the circumferentially-protruding portion  16   b . As a result, the thrust member  26  receives stress in the direction where the thrust member  26  will come off the first cylinder portion  20   b . Because the bonding strength between the hanging portion  26   c  and the first cylinder portion  20   b  becomes weak if the bonding distance between the two is short, the possibility that the bonding may be destroyed even by a small impact becomes high. That is, as the bonding distance between the hanging portion  26   c  and the first cylinder portion  20   b  is made longer, the bonding becomes stronger against an impact. 
     On the other hand, when the flange  26   e  becomes thick, the capillary seal portion TS becomes short, thereby causing the capacity of the lubricant  28  that can be held in the capillary seal portion TS to be small. Accordingly, there is the possibility that, when the lubricant  28  is dispersed due to an impact, the lubricant  28  may be immediately lacking. The functions of a fluid dynamic bearing are deteriorated due to such a lack in the lubricant and therefore a malfunction, such as burning, is likely to occur. Accordingly, in the disk drive device  100  according to the present embodiment, the capillary seal portion TS is made long in the up-down direction by thinning the flange  26   e . As a result, an amount of the lubricant  28  that can be held therein becomes large, and the disk drive device  100  is structured such that the lubricant  28  is hardly lacking even if dispersed due to an impact. That is, the axial distance of the thrust member  26  is made to be long with respect to the hanging portion  26   c  and to be short with respect to the flange  26   e.    
     There is a method in which the outer circumferential surface of the hanging portion  26   c  is fixed to the inner circumferential surface of the first cylinder portion  20   b  by press-fitting; however, there is the fear that, when the hanging portion  26   c  receives stress due to the press-fitting, a deformation may occur in the inner circumferential surface of the hanging portion  26   c , thereby possibly impairing the functions of the capillary seal portion TS. Accordingly, in the present embodiment, the outer circumferential surface of the hanging portion  26   c  is made small in diameter than the inner circumferential surface of the first cylinder portion  20   b  and both are fixed together by adhesion. As a result, a deformation in the hanging portion  26   c  is prevented and the functions of the capillary seal portion TS can be sufficiently exhibited. 
     The magnet  24  is fixed to the inner circumference of the second cylinder portion  20   c  and provided so as to face the outer circumference of the stator core  12  via narrow gap. The magnet  24  is formed of an Nd—Fe—B (Neodymium-Ferrum-Boron) material. Electro-deposition coating or spray coating is performed on the surface of the magnet  24 , and the inner circumference thereof is magnetized with twelve poles. 
     Subsequently, a dynamic pressure bearing in the structure of the disk drive device  100  will be described. A radial dynamic pressure bearing includes a radial dynamic pressure generating portion comprising the outer circumferential surface  22   c  of the shaft  22 , the cylinder portion inner circumferential surface  16   a  of the sleeve  16 , and the lubricant  28 , such as oil, etc., which is injected into the gap between the two. The radial dynamic pressure generating portion is composed of a plurality of individual dynamic pressure generating portions. In the present embodiment, as individual radial dynamic pressure generating portions, the first radial dynamic pressure generating portion RB 1  is arranged away from the hub  20  and the second radial dynamic pressure generating portion RB 2  is arranged near to the hub  20  in the state where the two are spaced apart from each other in the axial direction. The first radial dynamic pressure generating portion RB 1  and the second radial dynamic pressure generating portion RB 2  are provided in the gap between the cylinder portion inner circumferential surface  16   a  and the outer circumferential surface  22   c  so as to support the rotating body R by generating radial dynamic pressure. The first radial dynamic pressure generating portion RB 1  and the second dynamic pressure generating portion RB 2  respectively have a first radial dynamic pressure groove and a second radial dynamic pressure groove for generating dynamic pressure on at least one of the outer circumferential surface  22   c  and the cylinder portion inner circumferential surface  16   a , the two surfaces  22   c  and  16   a  facing each other. 
     Subsequently, radial dynamic pressure grooves composing the first radial dynamic pressure generating portion RB 1  and the second radial dynamic pressure generating portion RB 2  will be described. It is noted that, because a radial dynamic pressure groove composing the first radial dynamic pressure generating portion RB 1  and that composing the second radial dynamic pressure generating portion RB 2  can be basically the same as each other, the two will be collectively described as a radial dynamic pressure groove in the radial dynamic pressure generating portion RB.  FIG. 3  is an illustrative view illustrating an example of a state in which the radial groove formed on the cylinder portion inner circumferential surface  16   a  of the sleeve  16 , the cylinder portion inner circumferential surface  16   a  forming the radial space portion including the radial dynamic pressure generating portion RB, is expanded in the circumferential direction. In  FIG. 3 , the hatching areas represent groove portions  502  functioning as radial grooves and other areas represent non-groove portions  504 . As illustrated in  FIG. 3 , the radial dynamic pressure generating portion RB is composed of a plurality of striped groove portions in which the groove portion  502  and the non-groove portion  504  are repeatedly arranged along the rotational direction (the circumferential direction of the sleeve  16 ) Z. In the case of the structure of the present embodiment, each striped groove portion is formed by the end portions E 1  and E 2  on both sides and the intermediate portion P sandwiched by the end portions E 1  and E 2  on both sides, and the end portions E 1  and E 2  on both sides are arranged at positions preceding the intermediate portion P toward the Z direction, which is opposite to the rotational direction of the shaft  22 . That is, the end portions E 1  and E 2  on both sides and the intermediate portion P are arranged such that the lubricant  28  is collected into the intermediate portion P by the relative rotation between the shaft  22  and the sleeve  16 . In the case of the radial dynamic pressure generating portion RB, there are two types of the structures, in one of which the sleeve  16  is fixed and the shaft  22  rotates as illustrated in  FIG. 2 , and in the other of which the shaft  22  is fixed and the sleeve  16  rotates, opposite to the first one. In addition, there are three types with respect to where the radial dynamic pressure is formed, in the first one of which the radial dynamic pressure groove is formed on the sleeve  16  side as illustrated in  FIG. 3 , in the second one of which that is formed on the shaft  22  side, and in the last one of which that are formed on both sides of the sleeve  16  and the shaft  22 . In every case, the end portions E 1  and E 2  on both sides and the intermediate portion P composing the striped groove portion are arranged such that the lubricant  28  is collected into the intermediate portion P by the relative rotation between the shaft  22  and the sleeve  16 . Accordingly, the arrangement relationship between the end portions E 1  and E 2  and the intermediate portion P composing the striped groove portion is determined in accordance with the rotational pattern of the sleeve  16  and the shaft  22  and the formation pattern of the striped groove portion.  FIG. 3  illustrates the case where the radial groove is formed to be herringborn-shaped as an example of the shape of the radial groove. In this case, it can be said that the intermediate portion P represents the periphery of the top of the herringbone-shape. 
     For example, when the diameter of the radial dynamic pressure generating portion RB is 4 mm, the number of the groove portions  502  per circumference can be twelve. Further, the circumferential width Pg of the intermediate portion P and that Eg of each of the end portions E 1  and E 2  in the groove portion  502 , can be approximately 0.52 mm, respectively. Alternatively, when the diameter of the radial dynamic pressure generating portion RB is 3 mm, the number of the groove portions  502  per circumference can be eight, and the circumferential widths Pg and Eg of the intermediate portion P and each of the end portions E 1  and E 2  in the groove portion, can be approximately 0.59 mm, respectively. In addition, the radial depth of the groove portion  502  can be, for example, 5 to 6 μm. The radial gap between the cylinder portion inner circumferential surface  16   a  and the outer circumferential surface  22   c  can be 3 to 4 μm. 
     When the rotating body R is rotating, the radial dynamic pressure groove generates radial dynamic pressure such that the shaft  22  is supported by the radial dynamic pressure via a predetermined radial gap relative to the sleeve  16 . The axial formation width of the first radial dynamic pressure groove in the first radial dynamic pressure generating portion RB 1  is formed to be narrower than that of the second radial dynamic pressure grove in the second radial dynamic pressure generating portion RB 2 . Thereby, a pair of radial dynamic pressure corresponding to a pair of side pressure with different strength in the axial direction of the shaft  22  are generated in the first radial dynamic pressure generating portion RB 1  and the second radial dynamic pressure generating portion RB 2 . The shaft  22  near to a weight member, such as the hub  20 , etc., is stably supported by generating large dynamic pressure in the radial dynamic pressure generating portion RB 2 , as stated above. On the other hand, the shaft  22  is supported by generating radial dynamic pressure in the first radial dynamic pressure generating portion RB 1 , which is smaller than that in the second radial dynamic pressure generating portion RB 2 . As a result, smooth rotation of the shaft  22  is realized, thereby allowing for high shaft stiffness to be obtained. It is noted that the generation of radial dynamic pressure means, in other words, generation of rotational resistance, thereby causing a shaft loss occurring when the shaft  22  is being driven. However, the shaft loss can be reduced by making the radial dynamic pressure generated in the first radial dynamic pressure generating portion RB 1  small. Accordingly, optimal balance between the high shaft stiffness and the low shaft loss can be acquired by adjusting a pair of radial dynamic pressure generated in the first radial dynamic pressure generating portion RB 1  and the second radial dynamic pressure generating portion RB 2 . 
     On the other hand, a thrust dynamic pressure bearing includes the first thrust dynamic pressure generating portion SB 1  and the second thrust dynamic pressure generating portion SB 2 , as illustrated in  FIG. 2 . The first thrust dynamic pressure generating portion SB 1  is formed by the thrust lower surface  26   b  of the flange  26   e , the housing flat portion  14   d , and the lubricant  28  injected into the axial gap between the two. The second thrust dynamic pressure generating portion SB 2  is formed by the thrust upper surface  26   a  of the flange  26   e , the lower surface of the circumferentially-protruding portion  16   b , and the lubricant  28  injected into the axial gap direction between the two. 
     In each of the thrust dynamic pressure generating portion SB 1  and the second thrust dynamic pressure generating portion SB 2 , a thrust dynamic pressure groove (not illustrated) for generating dynamic pressure is formed on at least one of the surfaces in the axial gap. The thrust dynamic pressure groove may be formed, for example, to be spiral-shaped, or to be herringborn-shaped in the same way as the radial dynamic pressure groove. The thrust dynamic pressure generating portion SB generates, as a whole, the dynamic pressure in the pump-in direction, in which the lubricant  28  is transferred from the capillary seal portion TS toward the inside of the bearing unit, with the rotation of the rotating body R, so that an axial force, i.e., a floating force will act on the rotating body by the pressure. The lubricant  28  injected into the gaps in the first radial dynamic pressure generating portion RB 1 , the second radial dynamic pressure generating portion RB 2 , the first thrust dynamic pressure generating portion SB 1 , and the second thrust dynamic pressure generating portion SB 2 , is commonly used with each other and a leak of the lubricant  28  is prevented by being sealed with the capillary seal portion TS. 
     The capillary seal portion TS is composed of the outer circumferential surface  14   e  of the housing  14  and the inner circumferential surface  26   d  of the thrust member  26 . The outer circumferential surface  14   e  is tapered so as to be reduced in diameter going from the upper surface toward the lower surface. On the other hand, the inner circumferential surface  26   d  facing the outer circumferential surface  14   e  is also tapered so as to be reduced in diameter going from the upper surface toward the lower surface, although its tilt angle is smaller than that of the outer circumferential surface  14   e . With such a structure, the outer circumferential surface  14   e  and the inner circumferential surface  26   d  form the capillary seal portion TS in which the gap between the two expands going from the upper surface toward the lower surface. Herein, an injection amount of the lubricant  28  is set such that the boundary surface (gas-liquid interface) between the lubricant  28  and ambient air is located in the middle of the capillary seal portion TS, and therefore the lubricant  28  is sealed by the capillary seal portion TS with capillarity. As a result, a leak of the lubricant  28  is prevented. That is, the lubricant  28  is to be injected into a lubricant holding portion including: the spaces forming the first radial dynamic pressure generating portion RB 1 , the second radial dynamic pressure generating portion RB 2 , the first thrust dynamic pressure generating portion SB 1 , and the second thrust dynamic pressure generating portion SB 2 ; the space between the housing  14  and the thrust member  26 ; and the space between the circumferentially-protruding portion  16   b  and the hub  20 , etc. 
     As stated above, the capillary seal portion TS is designed such that the inner circumferential surface  26   d , which is the outside tilted surface, is reduced in diameter going from the upper surface toward the lower surface. Accordingly, with the rotation of the rotating body R, a centrifugal force acts on the lubricant  28  in the direction where the lubricant  28  is forced to move toward the inside of the space into which the lubricant  28  is injected, and therefore a leak of the lubricant  28  can be more surely prevented. Further, the communication channel I can be secured by the groove  14   a  formed along the axial direction on the inner circumferential surface of the housing  14 . Because both sides of the first radial dynamic pressure generating portion RB 1  and the second radial dynamic pressure generating portion RB 2  are communicated by the communication channel I, the pressure balance in the radial dynamic pressure bearing can be immediately restored even if the pressure balance breaks down, thereby allowing for the total pressure balance to be successfully maintaining. Further, if the dynamic pressure balance in each of the first radial dynamic pressure generating portion RB 1 , the second radial dynamic pressure generating portion RB 2 , and the thrust dynamic pressure generating portion SB, breaks down due to a disturbance, such as application of an external force on the shaft  22  or the rotating body R, the pressure is instantly averaged and the pressure balance can be maintained. As a result, an floating amount of the rotating body R is stabilized relative to the fixed body S, allowing for a disk drive device  100  with high reliability to be obtained. 
     The first radial dynamic pressure generating portion RB 1  and the second radial dynamic pressure generating portion RB 2 , functioning as individual radial dynamic pressure generating portions, will be described in detail with reference to  FIG. 4 .  FIG. 4  is a partial cross-sectional view mainly illustrating the relationship between the diameter of the radial dynamic pressure generating portion RB and the axial length thereof. 
     As stated above, a disk drive device  100  to be mounted in a mobile device is demanded such that a drive current should be reduced. The present inventors have found from experiments that a reduction in a drive current can be realized by making the diameter D of the radial dynamic pressure generating portion RB small in the structures illustrated in  FIGS. 2 and 4 . In the present embodiment, an example in which a radial dynamic pressure groove composing the radial dynamic pressure generating portion RB is formed on the cylinder portion inner circumferential surface  16   a  of the sleeve  16  will be described. 
     The circumferential formation length of the radial dynamic pressure groove becomes short by making the diameter D of the radial dynamic pressure generating portion RB small. That is, the number of the radial dynamic pressure grooves or the capacity of the grooves of the whole radial dynamic pressure groove, the radial dynamic pressure groove(s) being formed on the cylinder portion inner circumferential surface  16   a , is reduced, and thereby the resistance to the lubricant  28  is reduced. As a result, the drive current for rotating the rotating body R can be reduced. Alternatively, if the diameter D of the radial dynamic pressure generating portion RB is made small in the state where the gap size between the outer circumferential surface  22   c  of the shaft  22  and the cylinder portion inner circumferential surface  16   a  of the sleeve  16  is maintained, the diameter of the shaft  22  to be housed in the sleeve  16  also becomes small. That is, the weight as the whole rotating body R can be reduced, thereby contributing to a reduction in a drive current. Similarly, the present inventors have found that a drive current can also be reduced by making the radial depth G of the radial dynamic pressure groove shallow. Also, in this case, the resistance to the lubricant  28  can be reduced by a reduction in the capacity of the grooves of the whole radial dynamic pressure groove. As a result, the drive current for driving the rotating body R can be reduced. Further, the inventors have also found that the sleeve  16  and the radial dynamic pressure groove can be stably processed with the dimension accuracy of the sleeve  16  and the shape accuracy of the radial dynamic pressure groove being maintained, by setting, for example, the diameter D of the radial dynamic pressure generating portion RB to 2.5 mm, the depth G thereof to 3 to 4 μm, and the gap between the cylinder portion inner circumferential surface  16   a  and the outer circumferential surface  22   c  to 2 to 3 μm. In this case, the inventors have confirmed that the sleeve  16  can be processed with high accuracy by cutting. 
     Herein, if the diameter D of the radial dynamic pressure generating portion RB composing the first radial dynamic pressure generating portion RB 1  and the second radial dynamic pressure generating portion RB 2  or the depth G of the radial dynamic pressure groove is made small, generated radial dynamic pressure becomes small in accordance with the reduction amount thereof. That is, the radial bearing stiffness is on the trend of being decreased. As stated above, when an impact is applied to a disk drive device, a displacement of the recording disk  120 , corresponding to the stress occurring due to the acceleration of the impact, becomes large if the radial bearing stiffness is low. And, when an displacement of the recording disk  120  becomes large, the relative distance between the magnetic head and the recording disk  120  becomes unstable, thereby increasing errors in reading/writing data. 
     Because of this, it is needed that radial dynamic pressure should be increased while maintaining the state where the diameter D of the radial dynamic pressure generating portion RB is reduced. The present inventors have found that radial dynamic pressure is approximately defined with the axial length of the radial dynamic pressure generating portion RB and the diameter D thereof being parameters. That is, the inventors have acquired the conclusion that it is successful to determine the axial length of the radial dynamic pressure generating portion RB and the diameter D thereof such that the radial dynamic pressure to be generated is greater than or equal to the minimum reference value determined beforehand in the design step. 
     That is, when the relationship between the sum (L1+L2) of the axial size L1 of the first radial dynamic pressure generating portion RB 1  and that L2 of the second radial dynamic pressure generating portion RB 2 , and the diameter D of the radial dynamic pressure generating portion RB, is made to satisfy (L1+L2)&gt;D, the needed radial dynamic pressure can be secured while reducing a drive current. Herein, when (L1+L2) is increased, the radial dynamic pressure can be increased in accordance with the increase thereof and the impact resistance performance of the disk drive device  100  when in use can be improved, thereby allowing for a stable operation to be realized. In addition, if the diameter D of the shaft  22  can be determined, in the design step, in accordance with the specification of the disk drive device  100 , the diameter D of the radial dynamic pressure generating portion RB can be determined, and further the length of the radial dynamic pressure generating portion RB can be determined, thereby allowing for the radial dynamic pressure generating portion RB to be easily designed. 
     In addition, because the axial length of the radial dynamic pressure generating portion is increased, the capacity of the groove is increased accordingly, thereby increasing the resistance to the lubricant  28 . However, the shaft  22  can be reduced in weight with the shaft  22  becoming narrow in accordance with the reduction in the diameter D of the radial dynamic pressure generating portion RB. Part of the increase in a drive current due to the increase in the axial length of the radial dynamic pressure generating portion can be offset by the reduction in a drive current due to the reduction in weight, a reduction in a drive current can be realized as the whole disk drive device  100 . 
     The present inventors have confirmed from experiments that, when the diameter D of the radial dynamic pressure generating portion RB is 2.4 mm and L1+L2=2.5 mm as a specific example, good balance between a generation amount of the radial dynamic pressure and a reduction in a drive current can be obtained. Further, the inventors have confirmed from experiments that the diameter D of the radial dynamic pressure generating portion RB can be made to be smaller than or equal to 2.4 mm. The inventors have also confirmed that, in this case, there is an advantage because a drive current is further reduced and the period when a battery-powered disk drive device  100  can be used becomes long. Also, in this case, sufficient bearing stiffness can be secured by adjusting the axial size of the radial dynamic pressure generating portion RB in accordance with a decrease in the stiffness. 
     If the axial length of the radial dynamic pressure generating portion RB is made simply long, the axial size of the disk drive device  100  becomes long, contradicting the thinning of a disk drive device  100 , which has been conventionally demanded. The present inventors have learned from experiments that a larger portion of the acceleration due to an impact applied to the recording disk  120  acts on the second radial dynamic pressure generating portion RB 2  located near to the hub  20  than on the first radial dynamic pressure generating portion RB 1  located away from the hub  20 . And, the inventors have found that it is successful to generate larger dynamic pressure in the second radial dynamic pressure generating portion RB 2  by making the axial size L2 of the second radial dynamic pressure generating portion RB 2  located near to the hub  20  longer than that L1 of the first radial dynamic pressure generating portion RB 1  located away from the hub  20 . By forming such generation balance of the radial dynamic pressure, the impact resistance performance of the disk drive device  100  when in use can be improved, and balance distribution of the radial dynamic pressure, corresponding to the weight of the rotating body R, can be concurrently performed as stated above, as stated above, thereby allowing for stable rotation of the rotating body R to be realized. The generation balance of the radial dynamic pressure can be realized by adjusting the axial sizes L1 and L2 of the first radial dynamic pressure generating portion RB 1  and the second radial dynamic pressure generating portion RB 2  or adjusting the depth G of the groove. By performing such adjustment, needed radial dynamic pressure can be secured while reducing a drive current, and the impact resistance performance of a disk drive device  100  when in use can be improved, thereby allowing for an stable operation to be realized. 
     In addition, as one of the demands with respect to a disk drive device  100 , there is the demand that the axial size of a disk drive device  100  should be thinned while improving the impact resistance performance thereof. As illustrated in  FIG. 4 , a liquid reservoir RR for the lubricant  28 , the liquid reservoir RR becoming a non-radial dynamic pressure generating portion, is formed such that the axial length thereof is L3, between the first radial dynamic pressure generating portion RB 1  and the second radial dynamic pressure generating portion RB 2 . In order to stably and efficiently support the shaft  22  with a predetermined length, it is desirable to support the shaft  22  at a plurality of positions spaced apart from each other in the axial direction of the shaft  22 . To realize this, the first radial dynamic pressure generating portion RB 1  and the second radial dynamic pressure generating portion RB 2  are arranged so as to be spaced from each other and the liquid reservoir RR is formed between the two. The liquid reservoir RR also functions as a buffer space of the lubricant  28 , thereby contributing to avoidance of lack in the lubricant  28 . However, the liquid reservoir RR itself does not contribute to the generation of the radial dynamic pressure. Accordingly, the present inventors have made the axial size L3 of the liquid reservoir RR smaller than the sum (L1+L2) of the axial sizes of the radial dynamic pressure generating portion RB, with avoidance of lack in the lubricant  28  being dealt with, for example, the shape of the capillary seal portion TS, etc. As a result, it becomes possible to expand an area where the radial dynamic pressure generating portion RB can be formed, thereby it becomes possible to reduce a drive current and improve the impact resistance performance of the disk drive device  100  while avoiding the expansion of the axial size due to the formation of the radial dynamic pressure generating portion RB, that is, the expansion of the axial size of the disk drive device  100 . Further, the reduction in the axial size L3 of the liquid reservoir RR contributes to the thinning of the axial size of the disk drive device  100 . Alternatively, the axial size L3 of the liquid reservoir RR may be made smaller than the diameter of the liquid reservoir RR in order to further thin the axial size of the disk drive device  100 . This case also leads to the expansion of the area where the radial dynamic pressure generating portion RB can be formed, thereby contributing to suppression of a decrease in the bearing stiffness. 
     The axial size of the hub  20  is sometimes made large in order to realize the stable rotation of the rotating body R. When meeting the demand of thinning the disk drive device  100  in view of this, the axial size of the capillary seal portion TS becomes small. In this case, when an impact is applied to the disk drive device  100 , the lubricant  28  in the capillary seal portion TS is sometimes dispersed, resulting in an decrease in the amount of the lubricant  28 . When the amount of the lubricant  28  is decreased, the lifetime of the disk drive device  100  is sometimes shortened. Then, in the present embodiment, the axial size C of an area where the outer circumferential surface  14   e  of the housing  14  and the inner circumferential surface  26   d  of the hanging portion  26   c  face each other in the radial direction, is made larger than the axial size H of the hub  20  located on the line extended in the axial direction from the outer circumferential surface  14   e  of the housing  14 , in  FIG. 4 . As illustrated in  FIG. 4 , the portion corresponding to the axial size C composes the capillary seal portion TS, and the distance L 4  between the gas-liquid interface  28   a  of the lubricant  28  stored in the capillary seal portion TS and the end surface of the hanging portion  26   c  can be made long by making the axial size C long. With this, the margin space for movement of the lubricant  28 , the movement occurring when the disk drive device  100  receives an impact, can be sufficiently secured, thereby allowing for the performance of preventing dispersion of the lubricant  28  to be improved. As a result, a decrease in the lubricant  28  can be suppressed and a decrease in the lifetime of the disk drive device  100  can be suppressed. As stated above, by improving the performance of preventing a leak of the lubricant  28  in the disk drive device  100 , the load occurring when driving the disk drive device  100  can be reduced and the reliability for maintaining the bearing stiffness can be improved while avoiding excessive consumption of a drive current. 
     In addition, in the present embodiment, application portions  302  and  303  where an oil-repellent agent is applied may be provided, in the circumferential direction, on both the inner circumferential surface  26   d  of the thrust member  26  located near to the open end of the capillary seal portion TS and the outer circumferential surface  14   e  of the housing  14 , as illustrated in  FIG. 4 , in order to suppress a lack in the lubricant  28 . The oil-repellent agent is made by solving, for example, Teflon resin in a solvent, and Teflon resin films are formed by evaporating the solvent in the application portions  302  and  303 . It can be designed that the lubricant  28 , once dispersed from the gas-liquid interface  28   a  of the lubricant  28 , will return to the reservoir area of the lubricant  28  after being repelled by the application portions  302  and  33  due to the Teflon resin films. As a result, a decrease in the lubricant  28  can be easily suppressed. The positions where the application portions  302  and  303  are provided may be appropriately set so long as they are located between the gas-liquid interface  28   a  and the open end. By improving the performance of preventing a leak of the lubricant  28  in the disk drive device  100  as stated above, the load occurring when driving the disk drive device  100  can be reduced and the reliability for maintaining the bearing stiffness can be improved while avoiding excessive consumption of a drive current. 
     In the present embodiment, the diameter of the sleeve  16  between the first radial dynamic pressure generating portion RB 1  and the second radial dynamic pressure generating portion RB 2  is made large as illustrated in  FIG. 4 , in order to form the aforementioned liquid reservoir RR. When receiving an impact by which the lubricant  28  is forced to be displaced upwards in the axial direction, the lubricant  28  held in the liquid reservoir RR makes a force in the direction where the lubricant  28  held in the capillary seal portion TS is extruded. Due to this, if the axial size L3 is large, the amount of the lubricant  28  held in the liquid reservoir RR becomes large and thereby the lubricant  28  held in the capillary seal portion TS is sometimes pushed out and is likely to be dispersed. Accordingly, as illustrated in the present embodiment, the axial size C of an area where the outer circumferential surface  14   e  of the housing  14  and the inner circumferential surface  26   d  of the hanging portion  26   c  of the thrust member  26  face each other in the radial direction, may be larger than the axial size L3 of the liquid reservoir RR, as illustrated in  FIG. 4 . By satisfying L3&lt;C, a dispersion amount of the lubricant  28  can be suppressed by reducing a phenomenon in which the lubricant  28  is pushed out from the capillary seal portion TS when an external force is applied to the lubricant  28 . By improving the performance of preventing a leak of the lubricant  28  in the disk drive device  100  in this way, the load occurring when driving the disk drive device  100  can be reduced and the reliability for maintaining the bearing stiffness can be improved while avoiding excessive consumption of a drive current. 
     Alternatively, the surface roughness of at least one of both areas on the outer circumferential surface  14   e  of the housing  14  and the inner circumferential surface  26   d  of the hanging portion  26   c , both the areas facing each other, may be larger than that of the outer circumferential surface  22   c  of the shaft  22 . The present inventors have confirmed from experiments that, if the surface of the area in this case is formed such that Ry is greater than or equal to 1.6, the resistance force to the movement of the lubricant  28 , occurring due to a surface tension, is increased. As a result, a dispersion amount of the lubricant  28  can be suppressed. By improving the performance of preventing a leak of the lubricant  28  in the disk drive device  100  in this way, the load occurring when driving the disk drive device  100  can be reduced and the reliability for maintaining the bearing stiffness can be improved while avoiding excessive consumption of a drive current. 
     Alternatively, at least one of both areas of the outer circumferential surface  14   e  of the housing  14  and the inner circumferential surface  26   d  of the hanging portion  26   c , both the areas facing each other, may be subjected to a hydrophilic treatment. A hydrophilic treatment is one for making the contact angle with the lubricant  28  small by modifying the surface. By subjecting these areas to a hydrophilic treatment, the resistance force to the movement of the lubricant  28 , occurring due to a surface tension, is increased. As a result, a dispersion amount of the lubricant  28  can be suppressed. As a hydrophilic treatment, various techniques can be employed. For example, hydrophilic treatments by titanium coating, glass coating, silica coating, organic-inorganic composite ceramic coating, and UV irradiation, are preferred for the use in the present embodiment in terms of easy work and a successful hydrophilic effect. Among hydrophilic treatments, a treatment in which the contact angle is made to be smaller than or equal to 10° is sometimes particularly called a superhydrophilic treatment. By subjecting at least one of both areas of the outer circumferential surface  14   e  of the housing  14  and the inner circumferential surface  26   d  of the hanging portion  26   c , both the areas facing each other, to a superhydrophilic treatment, a dispersion amount of the lubricant  28  can be further suppressed. By improving the performance of preventing a leak of the lubricant  28  in the disk drive device  100  in this way, the load occurring when driving the disk drive device  100  can be reduced and the reliability for maintaining the bearing stiffness can be improved while avoiding excessive consumption of a drive current. 
     As another example in which a dispersion amount of the lubricant  28  is suppressed, a structure may be provided in the space between the outer circumferential surface  14   e  of the housing  14  and the inner circumferential surface  26   d  of the hanging portion  26   c , in which the lubricant  28  is forced to move inwards (to the direction of the non-open end) with the rotation of the hub  20 . For example, a lubricant moving portion in which the lubricant  28  is forced to move inwards from the open end of the capillary seal portion TS with the rotation of the hub  20 , may be formed on at least one of the outer circumferential surface  14   e  of the housing  14  and the inner circumferential surface  26   d  of the hanging portion  26   c . The lubricant moving portion can be composed of a spiral groove tilted so as to generate pressure by which the lubricant  28  is guided toward the inside of the capillary seal portion TS when the hanging portion  26   c , together with the hub  20 , is rotating in the rotational direction of the recording disk  120 . By forming such a lubricant moving portion, a stream of the lubricant  28  by which the lubricant  28  is suppressed from moving to the direction where it will be dispersed, can be formed inside the lubricant  28  even when an impact is applied to the disk drive device  100  while the hub  20  is rotating. As a result, the dispersion of the lubricant  28  can be successfully and efficiently suppressed. By improving the performance of preventing a leak of the lubricant  28  in the disk drive device  100  in this way, the load occurring when driving the disk drive device  100  can be reduced and the reliability for maintaining the bearing stiffness can be improved while avoiding excessive consumption of a drive current. 
     In addition, as another example in which a dispersion amount of the lubricant  28  is suppressed, it may be designed that the gas-liquid interface  28   a  of the lubricant  28  at rest is located approximately at the axial center of an area where the outer circumferential surface  14   e  of the housing  14  and the inner circumferential surface  26   d  of the hanging portion  26   c  face each other in the radial direction. By setting the gas-liquid interface  28   a  of the capillary seal portion TS in this way, the distance to the open end of a capillary seal formation area formed by the lower end of the inner circumferential surface  26   d  and the outer circumferential surface  14   e , can be secured. That is, the outward dispersion of the lubricant  28  can be reduced by a buffer space to the open end, even if the level of the gas-liquid interface  28   a  varies when an impact has been applied to the disk drive device  100 . By improving the performance of preventing a leak of the lubricant  28  in the disk drive device  100  in this way, the load occurring when driving the disk drive device  100  can be reduced and the reliability for maintaining the bearing stiffness can be improved while avoiding excessive consumption of a drive current. 
     In the present embodiment, the present inventors have confirmed the following fact: when the diameter D of the radial dynamic pressure generating portion RB is 2.5 mm, it is preferable that the difference between the maximum and the minimum of the axial liquid level in the circumferential direction of the gas-liquid interface  28   a  of the lubricant  28  at rest, is made to be smaller than or equal to 0.2 mm. It is noted that, in this case, the liquid level at the side where the distance to the open end of the capillary seal formation area become smaller is assumed to be the maximum liquid level, and that at the side where the distance thereto becomes larger is assumed to be the minimum liquid level. By setting such an acceptable range of the difference between the liquid levels, the securement of the needed distance to the open end of the capillary seal formation area can be ensured, even if the liquid level of the lubricant  28  varies after an impact has been applied to the disk drive device  100 . By improving the performance of preventing a leak of the lubricant  28  in the disk drive device  100  in this way, the load occurring when driving the disk drive device  100  can be reduced and the reliability for maintaining the bearing stiffness can be improved while avoiding excessive consumption of a drive current. 
     A change in the circumferential liquid level of the gas-liquid interface  28   a  of the lubricant  28  in the capillary seal portion TS is caused due to a circumferential change in the gap between the outer circumferential surface  14   e  of the housing  14  and the inner circumferential surface  26   d  of the hanging portion  26   c . That is, at the portion where the gap is relatively narrow, the gas-liquid interface  28   a  of the lubricant  28  is located downwards in  FIG. 4 , and at the portion where the gap is relative wide, that is located upwards. The circumferential change in the gap becomes small when the coaxial degree of the hanging portion  26   c  relative to the rotational center is high. 
       FIG. 5  is a cross-sectional view illustrating a processed portion of the hub  20  to be used in the present embodiment. As stated above, the hanging portion  26   c  of the thrust member  26  is fixed to the inner circumferential surface of the first cylinder portion  20   b  of the hub  20  with adhesive. Accordingly, when the coaxial degree of the inner circumferential surface  26   d  relative to the outer shape of the hanging portion  26   c  of the thrust member  26  is obtained, it is needed that the inner circumferential surface of the first cylinder portion  20   b  of the hub  20  is formed concentrically with the center hole  20   a  in order to obtain the coaxial degree relative to the rotational center of the hanging portion  26   c . Then, in the present embodiment, the surface represented by the arrow Si and the surface represented by the arrow S 2  in  FIG. 5  are continuously cut. Herein, the continuous cutting means that, for example, the center hole  20   a  and the inner circumferential surface of the first cylinder portion  20   b  of the hub  20  are cut while the hub  20  is being chucked with a lathe. With such cutting, high coaxial degree of the first cylinder portion  20   b  relative to the center hole  20   a  can be obtained, the center hole  20   a  being a processing reference in the hub  20 . As a result, successful coaxial degree can be obtained when fixing the hanging portion  26   c  to the hub  20 . And accordingly, the difference between the maximum and minimum of the axial liquid level in the circumferential direction of the gas-liquid interface  28   a  of the lubricant  28  in the capillary seal portion TS, becomes small. Thus, by improving the performance of preventing a leak of the lubricant  28  in the disk drive device  100 , the load occurring when driving the disk drive device  100  can be reduced and the reliability for maintaining the bearing stiffness can be improved while avoiding excessive consumption of a drive current. 
     Further, in another example, the outer circumferential surface of the second cylinder portion  20   c  of the hub  20  and the inner circumferential surface of the first cylinder portion  20   b  may be continuously processed. As stated above, the hub  20  comprises the second cylinder portion  20   c  to be engaged with the center hole of the recording disk  120 . That is, the surface represented by the arrow S 2  and that represented by the arrow S 3  in  FIG. 5  are continuously cut. Herein, the continuous cutting means that, for example, the outer circumferential surface of the second cylinder portion  20   c  and the inner circumferential surface of the first cylinder portion  20   b  are continuously cut while the hub  20  is being chucked with a lathe. With such cutting, high coaxial degree of the first cylinder portion  20   b  relative to the second cylinder portion  20   c  can be obtained, the second cylinder portion  20   c  being another processing reference in the hub  20 . As a result, successful coaxial degree can be obtained when fixing the hanging portion  26   c  to the hub  20 . And accordingly, the difference between the maximum and the minimum of the axial liquid level in the circumferential direction of the gas-liquid interface  28   a  of the lubricant  28  in the capillary seal portion TS, becomes small. Thus, by improving the performance of preventing a leak of the lubricant  28  in the disk drive device  100 , the load occurring when driving the disk drive device  100  can be reduced and the reliability for maintaining the bearing stiffness can be improved while avoiding excessive consumption of a drive current. 
     In the present embodiment, the radial dynamic pressure generating portion RB is composed of a plurality of the striped groove portions as illustrated in  FIG. 3 , in which the groove portion  502  and the non-groove portion  504  are repeatedly arranged along the Z direction (the circumferential direction of the sleeve  16 ), the Z direction being opposite to the rotational direction of the shaft  22 . The radial dynamic pressure to be generated in the radial dynamic pressure generating portion RB can be adjusted by changing the shape of the striped groove portion.  FIGS. 6A to 6C  are illustrative views illustrating variations of the striped groove portion. Also, in  FIGS. 6A to 6C , the radial grooves formed on the cylinder portion inner circumferential surface  16   a  of the sleeve  16 , the cylinder portion inner circumferential surface  16   a  composing the radial dynamic pressure generating portion RB, are illustrated after being expanded in the circumferential direction. Also, in  FIGS. 6A to 6C , the hatching areas represent the groove portions  502  and other areas represent the non-groove portions  504  in the same way as  FIG. 3 . As illustrated in  FIGS. 6A to 6C , the radial dynamic pressure generating portion RB is composed of a plurality of the striped groove portions in which the groove portion  502  and the non-grove portion  504  are repeatedly arranged along the Z direction. Each striped groove portion is formed by the end portions E 1  and E 2  on both sides and the intermediate portion P sandwiched by the end portions E 1  and E 2  on both sides, and the end portions E 1  and E 2  on both sides are arranged at positions preceding the intermediate portion P toward the Z direction. Also, in  FIGS. 6A to 6C , the case where the radial groove is formed to be herringborn-shaped as an example of the shape is illustrated. Further, each of  FIGS. 6A to 6C  illustrates a type in which the sleeve  16  is fixed, the shaft  22  rotates, and the striped groove portion is formed on the side of the sleeve  16 , in the same way as  FIG. 3 . 
     In the case of  FIG. 6A , the circumferential width Eg of each of the end portions E 1  and E 2  in the groove portion  502  is formed to be larger than the circumferential width Pg of the intermediate portion P in the groove  502 . The lubricant  28  lying in the radial space portion is scraped up in the end portions E 1  and E 2  in the groove portion  502  and guided toward the intermediate portion P by the relative rotation between the cylinder portion inner circumferential surface  16   a  of the sleeve  16  and the outer circumferential surface  22   c  of the shaft  22 . The guided lubricant  28  generates the radial dynamic pressure acting on the shaft  22  by running on a protruding center convex portion Pt in the non-groove portion  504 , the center convex portion Pt corresponding to the intermediate portion P. In this case, by making the circumferential width Pg of the intermediate portion P narrower than that Eg of each of the end portions E 1  and E 2 , a phenomenon can be created in which the lubricant  28  scraped up in the end portions E 1  and E 2  is strongly forced into the intermediate portion P. And accordingly, the radial dynamic pressure generated near the center convex portion Pt can be made large. As a result, the radial stiffness can be increased with the radial dynamic pressure efficiently acting on the shaft  22 . Further, as illustrated in  FIG. 2 , even when a drive current is reduced by making the diameter D of the radial dynamic pressure generating portion RB, a decrease in the radial stiffness can be covered by increasing a generation amount of the radial dynamic pressure with the shape of the groove portion  502 . That is, the radial stiffness can be increased. 
     Alternatively, the circumferential width Pg of the intermediate portion P in the groove portion  502  may be smaller than that Pn of the center convex portion Pt in the non-groove portion  504 , as illustrated in  FIG. 6A . In this case, it is assumed that the circumferential width Eg of each of the end portions E 1  and E 2  in the groove portion  502  is substantially equal to that En of each of protruding end convex portions Et 1  and Et 2  in the non-groove portion  504 . The lubricant  28  lying in the radial space portion is scraped up in the end portions E 1  and E 2  in the groove portion  502  and guided toward the intermediate portion P by the relative rotation between the cylinder portion inner circumferential surface  16   a  of the sleeve  16  and the outer circumferential surface  22   c  of the shaft  22 . The guided lubricant  28  generates the dynamic pressure acting on the shaft  22  by running on the center convex portion Pt in the non-groove portion  504 , near the intermediate portion P. In this case, as the circumferential width Pn of the center convex portion Pt becomes large, the circumferential distance in which the running-on lubricant  28  acts as radial dynamic pressure becomes long. As a result, the radial stiffness can be increased with the radial dynamic pressure efficiently acting on the shaft  22 . 
     For example, when the diameter of the second radial dynamic pressure generating portion RB is 4 mm, the number of the groove portions  502  per circumference can be twelve. Further, the present inventors have confirmed from experiments that the radial dynamic pressure groove can be processed such that: the circumferential width Eg of each of the end portions E 1  and E 2  in the groove portion  502  and the circumferential width En of each of the end convex portions Et 1  and Et 2  are approximately 0.52 mm; the circumferential width Pg of the intermediate portion P in the groove portion  502  is approximately 0.26 mm; and the circumferential width Pn of the center convex portion Pt in the non-groove portion  504  is approximately 0.79 mm. The inventors have also confirmed from experiments that successful radial dynamic pressure can be generated by forming such a radial dynamic pressure groove. 
     Alternatively, when the diameter of the second radial dynamic pressure generating portion RB is, for example, 3 mm, the number of the groove portions  502  per circumference can be eight. Further, the present inventors have confirmed from experiments that the radial dynamic pressure groove can be processed such that: the circumferential width Eg of each the end portions E 1  and E 2  in the groove portion  502  and that En of each of the end convex portions Et 1  and Et 2  are approximately 0.59 mm; the circumferential width Pg of the intermediate portion P in the groove portion  502  is approximately 0.29 mm; and the circumferential width Pn of the center convex portion Pt is approximately 0.89 mm. The inventors have also confirmed from experiments that successful radial dynamic pressure can be generated by forming such a radial dynamic pressure groove. 
     Alternatively, when the diameter of the second radial dynamic pressure generating portion RB is, for example, 2.5 mm, the number of the groove portions  502  per circumference can be eight. Further, the present inventors have confirmed from experiments that the radial dynamic pressure groove can be processed such that: the circumferential width Eg of each of the end portions E 1  and E 2  in the groove portion  502  and that En of each of the end convex portions Et 1  and Et 2  are approximately 0.49 mm; the circumferential width Pg of the intermediate portion P in the groove portion  502  is approximately 0.25 mm; and the circumferential width Pn of the center convex portion Pt is 0.73 mm. The inventors have also confirmed from experiments that successful radial dynamic pressure can be generated by forming such a radial dynamic pressure groove. 
     Alternatively, when the diameter of the second radial dynamic pressure generating portion RB is 2.0 mm, the number of the groove portions  502  per circumference can be six. Further, the present inventors have confirmed from experiments that the radial dynamic pressure groove can be processed such that: the circumferential width Eg of each of the end portions E 1  and E 2  in the groove portion  502  and the circumferential width En of each of the end convex portions Et 1  and Et 2  are approximately 0.52 mm; the circumferential width Pg of the intermediate portion P in the groove portion  502  is approximately 0.26 mm; and the circumferential width Pn of the non-groove portion is approximately 0.79 mm. The inventors have also confirmed from experiments that successful radial dynamic pressure can be generated by forming such a radial dynamic pressure groove. 
     Further, the present inventors have confirmed from experiments that successful radial dynamic pressure effects can be created by making the ratio Pn/Pg of the circumferential width Pn of the non-groove portion  504  to that Pg of the groove portion  502  greater than or equal to 1.2; and the radial dynamic pressure groove can be easily processed by making the ratio Pn/Pg smaller than or equal to 5. 
       FIG. 6B  is an illustrative view illustrating another shape of the radial dynamic pressure groove. In the case of this example, the circumferential width of at least one of the end portions in the groove portion  502  is made larger than that of the non-groove portion  504 .  FIG. 6B  illustrates the case where the circumferential width Eg of each of the end portions E 1  and E 2  in the groove portion  502  is larger than that En of each of the end convex portion Et 1  and Et 2 , and the circumferential width Pg of the intermediate portion P in the groove portion  502  is approximately equal to that Pn of the center convex portion Pt. A larger amount of the lubricant  28  can be scraped up by making the circumferential width Eg of each of the end portions E 1  and E 2  in the groove portion  502  large. With this, a large amount of the lubricant  28  run on the center convex portion Pt in the non-groove portion  504 , near the intermediate portion P, so that the lubricant  28  acts as dynamic pressure. As a result, the radial stiffness can be increased with the dynamic pressure efficiently acting on the shaft  22 . Also, in this example, the present inventors have confirmed from experiments that successful dynamic pressure effects can be created by making the ratio Eg/En of the circumferential width Eg of each of the end portions E 1  and E 2  in the groove portion  502  to that En of each of the end convex portions Et 1  and Et 2  greater than or equal to 1.2, and the radial dynamic pressure groove can be easily processed by making the ratio Eg/En smaller than or equal to 5. 
       FIG. 6C  is an illustrative view illustrating another shape of the radial dynamic pressure groove. In the case of this example, the circumferential width Eg of each of the end portions E 1  and E 2  in the groove portion  502  is made larger than that En of each of the end convex portions Et 1  and Et 2 , and the circumferential width Pn of the center convex portion Pt is made larger than that Pg of the intermediate portion P in the groove portion  502 . In this case, a large amount of the lubricant  28  can be scraped up because the circumferential width Eg of each of the end portions E 1  and E 2  in the groove portion  502  is large. Further, because the circumferential width Pn of the center convex portion Pt is large, the circumferential distance in which the running—on lubricant  28  acts as dynamic pressure becomes long. As a result, the radial stiffness can be further increased with the radial dynamic pressure efficiently acting on the shaft  22 . Also, in this example, the present inventors have confirmed from experiments that successful radial dynamic pressure can be generated by making Pn·Eg/(Pg·En) greater than or equal to 1.4, and the radial dynamic pressure groove can be easily processed by making Pn·Eg/(Pg·En) smaller than or equal to 25. 
     When a radial dynamic pressure groove is formed by ball-rolling, it is difficult to partially make the circumferential width of the groove portion large. Then, the present inventors have found that a desired shape can be obtained by forming a radial dynamic pressure groove with cutting. For example, the main body of the sleeve  16  is at first formed by cutting or resin molding (sleeve main body formation step), and then a groove is formed by contacting the tip of a cutting bite with the inside of the cylinder portion inner circumferential surface  16   a  of the sleeve  16  while rotating the sleeve  16  that is chucked with a lathe (striped pattern formation step). At the time, the tip of the cutting bite is driven in the radial direction with a piezoelectric element. A groove is formed when driving the tip of the cutting bite outwards in the radial direction. A groove is not formed when driving the tip of the cutting bite inwards in the radial direction. A groove with a desired shape can be formed by repeating these drives in accordance with the rotation of the sleeve  16 . A radial dynamic pressure groove as illustrated in  FIGS. 6A to 6C  can be processed with such cutting processing, thereby allowing for an increase in radial dynamic pressure and an increase in the bearing stiffness to be easily realized. 
     Each of the shapes of the radial dynamic pressure grooves illustrated in  FIGS. 3 and 6A  to  6 B represents only one example; and similar effects can be obtained so long as a radial dynamic pressure groove is composed of a plurality of striped groove portions repeatedly arranged along the rotational direction and the groove portion has a shape in which the lubricant  28  scraped up in its end portions is collected into the intermediate portion. Although  FIGS. 3 and 6A  to  6 B illustrate herringborn shapes in which the groove portions are formed linearly; however, a herringborn shape including a curved portion or a herringborn shape in which a straight line and a curved line are combined, can be adopted. 
     Subsequently, a variation of the disk drive device  100  will be described by using the partially enlarged cross-sectional view of the vicinity of the bearing illustrated in  FIG. 7 . In  FIG. 2 , an example in which the thrust member  26  is composed of the flange  26   e  and the hanging portion  26   c  has been described. On the other hand, in the variation illustrated in  FIG. 7 , a thrust member  30  is composed of only a hanging portions  30   b . Although such a hanging portion  30   b  may be referred to as a flange, the portion will be described herein as a hanging portion  30   b . It is noted that, in  FIG. 7 , members common to those in  FIG. 2 , etc., are denoted with the same reference numerals and the descriptions thereof will be omitted. In each of a first radial dynamic pressure generating portion RB 1  and a second radial dynamic pressure generating portion RB 2 , a radial space portion is formed by the outer circumferential surface  22   c  of a shaft  22  and the cylinder portion inner circumferential surface  16   a  of a sleeve  16  in the same way as the structure in  FIG. 2 . For example, a herringborn-shaped radial dynamic pressure groove for generating radial dynamic pressure is formed on at least one of the outer circumferential surface  22   c  and the cylinder portion inner circumferential surface  16   a . On the other hand, a thrust dynamic pressure generating portion SB is formed in the axial gap between the lower surface  20   e  of a hub  20  and the upper surface of a circumferentially-protruding portion  16   b  of the sleeve  16 . That is, for example, a spiral-shaped thrust dynamic pressure groove (not illustrated) for generating thrust dynamic pressure is formed on the lower surface  20   e  and the upper surface of the circumferentially-protruding portion  16   b , the two surfaces facing each other. 
     The thrust member  30  includes an upper end portion  30   a , the hanging portion  30   b , an outer circumferential surface  30   c , and an inner circumferential surface  30   d . That is, the thrust member  30  does not include the flange  26   e  in  FIG. 2  and forms an approximately ring shape only with the hanging portion  30   b  corresponding to the hanging portion  26   c . The upper end portion  30   a  faces the lower surface of the circumferentially-protruding portion  16   b  of the sleeve  16  via a narrow gap and fulfills the function of preventing coming-off. The outer circumferential surface  30   c  of the hanging portion  30   b  is fixed to the inner circumferential surface of a first cylinder portion  20   b  of the hub  20 . When fixed with adhesive, adhesion strength may be improved and protrusion of the adhesive may be prevented by providing a concave portion  20   g , as illustrated in  FIG. 7 , on the inner circumferential surface of the first cylinder portion  20   b  of the hub  20 , the concave portion serving as a reservoir portion for the adhesive. 
     A capillary seal portion TS is composed of the outer circumferential surface of a member composing a fixed body S, such as the sleeve  16  and a housing  14 , etc., (hereinafter, referred to as the “fixed body outer circumferential surface”) and the inner circumferential surfaces  30   d  of the hanging portion  30   b  of the thrust member  30 . The radial size (the size in the horizontal direction of the drawing) of the thrust member  30  is made as short as 0.3 to 0.5 mm such that the space in the radial direction is not uselessly occupied and the sizes of the bearing portion and a stator core portion can be made large. On the other hand, the axial size (the size in the vertical direction of the drawing) of the thrust member is made as long as 1.5 to 3.0 mm such that the capacity of the capillary seal portion TS on the inner circumferential surface is made large. 
     Also, in the structure of the disk drive device  100  illustrated in  FIG. 7 , the embodiments described with reference to  FIGS. 2 to 5  and  6 A to  6 B can be applied, which can provide similar effects. Further, in the examples of the  FIGS. 2 and 7 , the descriptions have been made assuming that the sleeve  16  and the housing  14  are formed as different members; however, similar effects can be obtained when the two are formed integrally with each other. 
     The present invention shall not be limited to the aforementioned embodiments, and various modifications, such as design modifications, can be made with respect to the above embodiments based on the knowledge of those skilled in the art. The structure illustrated in each drawing is intended to exemplify an example, and the structure can be appropriately modified to a structure having a similar function, which can provide similar effects.