Abstract:
Reduced axial height, compact disk-drive motor rotating on a hydrodynamic radial bearing and a magnetically counterbalanced single hydrodynamic thrust bearing. The single hydrodynamic thrust bearing is configured between the underside of the rotor hub and the adjacent end face of a support cylinder in which the motor shaft rotates. To make the motor rotationally operable, instead of another hydrodynamic thrust bearing, the reduced axial-height configuration employs magnetic counterbalancing means associated with the cylindrical wall of the rotor hub. The magnetic counterbalancing means counterbalances thrust hydrodynamic lifting pressure generated in the single thrust-hydrodynamic pressure bearing and acting on the rotor hub when it spins.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of Application Ser. No. 09/369,156, filed Aug. 6, 1999 now abandoned. 
    
    
     BACKGROUND OF INVENTION 
     1. Technical Field 
     The present invention relates to spindle motors; in particular to disk-drive spindle motors employing hydrodynamic bearings that generate rotational dynamic pressure in shaft-sleeve interposed lubricant for rotationally bearing the disk-supporting shaft/sleeve. 
     2. Description of Related Art 
     Disk drive devices, such as computer hard disk drives, in which spindle motors are employed to drive data-storing disks, are well known. Spindle motors of this type may include hydrodynamic bearing configurations that generate bearing pressure in the lubricating fluid dynamically when the motor spins, thereby stabilizing, for example, a disk-supporting sleeve rotationally against a stationary spindle shaft. 
     One such motor is disclosed in U.S. Pat. No. 5,504,637. The disclosed motor includes a stationary shaft, a thrust plate fixed endwise to the stationary shaft, and a rotary hub having an annular central recess encompassing the thrust plate and integral with a sleeve surrounding the shaft. The motor further includes a thrust washer fixed over the central recess in the hub, confining the thrust plate in the rotary hub recess. Lubricant is retained in a clearance between the base of the recess and the axially adjacent surface of the thrust plate; the lubricant and the clearance-defining surfaces of the recess and thrust plate form a first hydrodynamic thrust bearing. Axially adjacent, clearance-defining surfaces of the thrust plate and thrust washer, together with lubricant retained in the clearance form a second hydrodynamic thrust bearing. 
     Herringbone grooves for generating hydrodynamic pressure are formed in a first portion of the cylindrical surface of the shaft. The first portion of the shaft cylindrical surface is surrounded by the radially adjacent inner circumferential surface of the sleeve at an annular micro-gap filled with lubricant. The grooved first portion of the shaft cylindrical surface, the adjacent inner surface of the sleeve, and the lubricant in the micro-gap establish a first radial bearing. Hydrodynamic-pressure-generating herringbone grooves are also formed in a second portion of the shaft cylindrical surface, radially adjacent the inner surface of the sleeve at another annular micro-gap filled with lubricant. The grooved second portion of the shaft cylindrical surface, the adjacent inner surface of the sleeve, and the lubricant in the micro-gap establish a second radial bearing. The hydrodynamic-pressure-generating grooves in the first and second radial bearings generate hydrodynamic pressure when the sleeve rotates relative to the shaft. 
     The hydrodynamic pressure thus generated in the radial bearings imparts high rigidity to the radial bearings to stabilize sleeve rotation. To stabilize sleeve rotation further, the first and second radial bearings are spaced apart at a predetermined distance, supporting the sleeve to eliminate wobble as it rotates about the shaft. 
     In a conventional motor of the foregoing type, the first and second radial bearings provide radial stability to the sleeve, maintaining the rotary hub in a vertical orientation with respect to the stationary shaft. Further, the first and second radial bearings maintain the sleeve in a concentric relationship with respect to the stationary shaft during rotation of the rotary hub. The effectiveness of the radial bearings in maintaining the rotary hub in a constant concentric relationship with respect to the shaft depends on the rigidity of each of the radial bearings and the axial distance between their respective centers. The farther apart the first and second radial bearings are, the more stable the rotary hub rotation will be against radial movement with respect to the shaft, since the thrust bearings primarily only restrain axial movement of the rotary hub. 
     Personal computers, in which disk-drive storage devices driven by conventional motors such as described above are utilized, are continually becoming smaller and thinner. The motors for spinning the hard disk in these disk-drive storage devices are expected to become smaller and thinner as well. Because the radial bearings are essential to the radial support of the rotary hub, however, and because making the distance between the radial bearings as large as possible is advantageous for imparting greater rotational stability to the rotary hub, reducing the axial height of motors employing first and second radial bearings presents difficulties. 
     Moreover, simply making motor structural components, such as the rotary hub and the thrust plate, thinner in order to reduce the motor axial height makes secure, precision assembly of the motor components to one another difficult. In particular, if the shaft and the thrust plate are not securely fixed to one another, the rotational precision of the motor is negatively affected. 
     Japanese laid patent application 08331796 (1996) discloses a different type of motor that includes a stationary sleeve encompassing a rotational shaft. In this case as well two axially separated sets of hydrodynamic-pressure-generating grooves are formed in the cylindrical surface of the shaft. The grooved sections of the shaft cylindrical surface and radially adjacent sections of the inner circumferential surface of the stationary sleeve define annular micro-gaps in which lubricant is retained, and together with the lubricant form upper and lower radial hydrodynamic bearings for supporting the rotational shaft. 
     However, the motor configuration disclosed in this Japanese publication does not utilize the radially extensive surface(s) of a thrust plate to establish hydrodynamic thrust bearing(s) as, in contrast, does the first motor configuration discussed above. Rather, hydrodynamic pressure generation grooves formed on the base-end surface of the shaft and/or the adjacent surface of a plate fixed to the sleeve, and lubricant in the clearance defined between the two surfaces, form a single hydrodynamic thrust bearing. Since the shaft is of diameter that is proportionately smaller than the thrust plate in the first motor discussed above, the grooves formed on the base-end surface of the shaft and/or the adjacent surface of the plate may not be able to generate sufficient thrust hydrodynamic pressure to support adequately the thrust load generated by rotation of the motor. Increasing the diameter of the shaft in order to obtain increased hydrodynamic pressure in the lubricant in the thrust bearing is not a practical consideration for this motor, because an increased shaft diameter in such a motor would result in greater energy loss that would decrease the electrical efficiency of the motor. 
     U.S. Pat. No. 5,659,445 to Yoshida et al. is directed to improving the lubricating configuration in a recording disk-drive motor. As set forth in the Summary section, Yoshida et al. accomplish this i) by employing tapered lubricant clearances in the thrust and radial dynamic-pressure bearing sections to increase dynamic lubricant pressure, and ii) by containing the dynamic pressure lubricant with a magnetic fluid seal device. Yoshida et al. thus seek to improve bearing and lubricating performance by increasing the dynamic pressure generated in, and at the same time keeping air out of, the radial and thrust bearing sections. The magnetic fluid seal device taught by Yoshida et al. is to prevent air, which has a larger coefficient of thermal expansion/contraction than the lubricant, from entering the bearing sections and destabilizing their performance. 
     Yoshida et al. thus teach improving motor bearing performance by employing tapered clearances to increase rotational dynamic pressure in the lubricant, which at the same time necessitates containing the lubricant with magnetic fluid seal devices. In turn, the magnetic fluid seal devices taught in every pertinent Yoshida et al. embodiment require a separate thrust plate/member for at least the thrust bearing on the rotor-hub adjacent end. 
     For example, Yoshida et al. discloses, as shown in FIG. 18, a recording-disk rotating device that includes a radial bearing portion  117  and thrust bearing portions  118  and  119 . The radial bearing portion  117  is constituted by a combination of the inner circumferential surface of the circular hole in the bush  107  and helical or herringbone grooves  131  on the cylindrical surface of the shaft  105 . The thrust bearing portions  118  and  119  are constituted by a combination of both the flat axial-end surfaces of the bush  107  and the respectively adjacent thrust plate  122  and thrust member  121 , as well as tapered grooves  130  or spiral grooves  132 . 
     Accordingly, the Yoshida et al. configuration employs dual thrust bearing portions  118  and  119 . Consequently, bearing losses due, for instance, to fluid friction in the lubricant filling the clearances formed in both the thrust bearing portions  118  and  119  may be large, which lowers the electrical efficiency of the motor. 
     Furthermore, the necessity of the thrust plate/member in the recording disk rotating device as taught by Yoshida et al. significantly limits the amount by which the axial height can be reduced. For example, in the Yoshida et al. embodiment described above, the configuration of the thrust bearings  118  and  119  requires the thrust member  121  on the rotor hub  104  end of the shaft  105 , and employs a thrust plate  122  as well. The magnetically conductive thrust member  121  is required to form a magnetic fluid seal device together with the magnet assembly  125 . 
     Yoshida et al. employ tapered sections within the dynamic-pressure generating portions of the lubricant clearances to increase hydrodynamic pressure; thus instead of using taper seals along the lubricant boundaries to prevent leakage, special magnetic seals requiring at least one extra axially disposed part a thrust plate/member are used to magnetically seal a magnetic fluid as the dynamic-pressure generating lubricant in the thrust bearings. 
     The hydrodynamic bearings in the motor taught by Yoshida et al. are constituted by four components, namely: the shaft  105 , the bush  107 , the thrust member  121 , and the thrust plate  122 . Further, the hydrodynamic bearings include bearing gaps defined by six surfaces, namely: the outer circumferential surface of the shaft  105 ; the inner circumferential surface, and the axial upper and lower surfaces, of the bush  107 ; the axial lower surface of the thrust member  121 ; and the axial upper surface of the thrust plate  122 . 
     In general, bearing surfaces that define hydrodynamic bearing gaps must be precisely machined to close tolerances, and the hydrodynamic bearing components must be precision-assembled, which makes hydrodynamic bearing manufacturing costs high. Accordingly, the number of bearing surfaces and parts desirably should be reduced to cut down motor manufacturing costs. 
     In view of the foregoing there exists a need for a spindle motor that overcomes the prior art problems mentioned above. 
     SUMMARY OF INVENTION 
     An object of the present invention is to reduce the axial height of and otherwise make smaller a spindle motor. 
     Another object is to facilitate the assembly of such a smaller and thinner spindle motor in order to minimize production costs. 
     Still another object is to configure a bearing structure for such a smaller and thinner spindle motor to provide the motor with a high level of rigidity. 
     Yet another object is to reduce the axial height of and make smaller a spindle motor by eliminating hydrodynamic bearing thrust plates entirely from the motor configuration, and yet to maintain thrust-load support in such a smaller and thinner spindle motor and make its manufacture and assembly simpler and less costly. 
     Another object of the present invention is to reduce the axial height of and otherwise make smaller a disk-drive device. 
     A further object is to facilitate and make less costly the assembly of such a smaller and thinner disk-drive device. 
     Still another object is to configure a bearing structure for such a smaller and thinner disk-drive device to provide the device with a high level of rigidity. 
     A yet further object is to reduce the axial height of and make smaller a disk-drive device by eliminating hydrodynamic bearing thrust plates entirely from the hydrodynamic thrust bearing configuration, to make manufacture and assembly of the disk-drive device simpler and less costly. 
     In accordance with one aspect of the present invention, a disk-drive motor is configured to rotate on a hydrodynamic radial bearing and a magnetically counterbalanced single hydrodynamic thrust bearing. The motor includes a support cylinder defining a central bore and a shaft coaxially inserted and extending at least partially into the support cylinder bore. An axially extending micro-gap is defined radially between the shaft circumferentially and the bore. A rotor hub is fixed axially endwise to the shaft, and the rotor hub itself constitutes a circular inner face opposing the support cylinder endwise. A radially extending micro-gap is defined axially between the circular inner face of the rotor hub and the end of the support cylinder. From the outer circumference of the rotor hub a cylindrical wall extends coaxially with the shaft, encompassing the stator, and a rotor magnet is fixed to the inner margin of the cylindrical wall, opposing the stator. Lubricant fills the axially and radially extending micro-gaps. 
     A radial-hydrodynamic pressure bearing including the lubricant-filled axially extending micro-gap, and hydrodynamic pressure-generating grooves formed in either the circumferential surface of the shaft or the bore, is thus established in this configuration. One single thrust-hydrodynamic pressure bearing including the lubricant-filled radially extending micro-gap, and hydrodynamic pressure-generating grooves formed in either the circular inner face of the rotor hub or the end of the shaft-support ring is also thus established in this configuration. 
     Further, magnetic counterbalancing means associated with the cylindrical wall of the rotor hub are provided for generating magnetically attractive force attracting the rotor hub axially toward the shaft-support ring. The magnetic counterbalancing means make the motor rotationally operable by counterbalancing thrust hydrodynamic lifting pressure acting on the rotor hub and generated in the single thrust-hydrodynamic pressure bearing when the rotor hub rotates. 
     Thus a spindle motor, as well as a disk-drive device employing the spindle motor, according to the present invention employs only one thrust bearing, defined between the lower surface of an upper wall section of the rotor hub, and the upper surface of the support cylinder. Therefore, because hydrodynamic thrust-bearing surface area is reduced over that in conventional motors, fluid friction due to the lubricant is reduced, improving the electrical efficiency of the motor. Moreover, since the only one hydrodynamic thrust bearing between the lower surface of the rotor hub and the upper wall portion of the support cylinder is formed without using a thrust plate, the axial height of the motor thus is reduced. 
     Moreover, in a disk-drive motor rotating on a hydrodynamic radial bearing and a magnetically counterbalanced single hydrodynamic thrust bearing according to the present invention, the underside surface of the rotor hub is employed in lieu of a thrust plate/member in configuring the lone hydrodynamic thrust bearing. Accordingly, in contrast to conventional disk-drive motor configurations, the end surface of the rotor hub itself is employed as a component of the hydrodynamic thrust bearing. 
     Three parts constitute the hydrodynamic bearings in a motor according to the present invention, namely: the rotor hub, the shaft, and the support cylinder. The hydrodynamic bearings include bearing gaps defined by four surfaces, namely: the lower surface of the rotor hub upper wall portion; the outer circumferential surface of the shaft; and the upper and the inner circumferential surfaces of the support cylinder. Therefore, motor configurations according to the present invention enable motor manufacturing costs to be reduced. 
     In sum, a disk-drive motor according to the present invention comprises one single hydrodynamic thrust bearing, configured between the underside of the rotor hub and the adjacent end face of the support cylinder. Configuring the motor to have unilaterally a lone thrust-hydrodynamic pressure bearing supporting the shaft necessitates counterbalancing means to achieve the axial balance essential to make the motor rotationally operable. Rather than another thrust-hydrodynamic pressure bearing, the reduced axial-height configuration according to the present invention employs the magnetic counterbalancing means. 
     Thus, no hydrodynamic thrust plates are employed in a spindle motor/disk-drive device embodied according to the present invention, and the motor/disk-drive device can be made smaller and reduced in axial height. Moreover, since hydrodynamic thrust plates must be manufactured within close tolerances and precise assembly techniques, the spindle motor of the present invention that lacks a hydrodynamic thrust plate entirely is easier to manufacture and assemble, thereby making the manufacturing process less costly. In the motor of the present invention, thrust loads are balanced by the single thrust bearing in combination with magnetic attraction whereby the single thrust bearing supports the rotor hub in a first axial direction and the magnetic attraction supports the rotor hub in a second axial direction. 
     A hard disk driving device according to the present invention utilizing the motor of the present invention described above, can be made thinner, smaller, less costly to manufacture, and capable of rotating a hard disk with precision. 
     From the following detailed description in conjunction with the accompanying drawings, the foregoing and other objects, features, aspects and advantages of the present invention will become readily apparent to those skilled in the art. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a fragmentary side view of a hard disk drive having a motor according to the present invention; 
     FIG. 2A is a fragmentary, cross-sectional side view of the hard disk drive depicted in FIG. 1, on an enlarged scale, showing details of a motor having a rotor hub fixed to a rotational shaft and a motor housing having a stationary sleeve according to a first embodiment of the present invention; 
     FIG. 2B is a bottom view of the rotor hub depicted in FIG. 2A showing hydrodynamic grooves formed on the rotor hub which in part form a thrust bearing of the motor; 
     FIG. 2C is a cross-sectional side view of the stationary sleeve shown removed from the motor depicted in FIG. 2A, showing details of hydrodynamic grooves formed on an inner circumferentially extending surface of the sleeve; 
     FIG. 2D is a fragmentary, cross-sectional side view similar to FIG. 2A, showing details of a motor according to a second embodiment of the present invention; 
     FIG. 3A is a fragmentary, cross-sectional side view of the hard disk drive depicted in FIG. 1, on an enlarged scale, showing details of a motor having a rotor hub fixed to a rotational shaft and a motor housing having a stationary sleeve according to a third embodiment of the present invention; 
     FIG. 3B is a bottom view of the rotor hub depicted in FIG. 3A showing hydrodynamic grooves formed on the rotor hub which in part form a thrust bearing of the motor; 
     FIG. 3C is a cross-sectional side view of the stationary sleeve shown removed from the motor depicted in FIG. 3A, showing details of hydrodynamic grooves formed on an inner circumferentially extending surface of the sleeve; 
     FIG. 3D is a fragmentary, cross-sectional side view similar to FIG. 3A, showing details of a motor according to a fourth embodiment of the present invention; 
     FIG. 4A is a fragmentary, cross-sectional side view of the hard disk drive depicted in FIG. 1, on an enlarged scale, showing details of a motor having a rotor hub fixed to a rotational shaft and a motor housing having a stationary sleeve according to a fifth embodiment of the present invention; 
     FIG. 4B is a bottom view of the rotor hub depicted in FIG. 4A showing hydrodynamic grooves formed on the rotor hub which in part form a thrust bearing of the motor; 
     FIG. 4C is a cross-sectional side view of the stationary sleeve shown removed from the motor depicted in FIG. 4A, showing details of hydrodynamic grooves formed on an inner circumferentially extending surface of the sleeve; 
     FIG. 4D is a fragmentary, cross-sectional side view similar to FIG. 4A, showing details of a motor according to a sixth embodiment of the present invention; 
     FIG. 4E is a bottom view of the rotor hub depicted in FIG. 4A showing an alternative configuration of hydrodynamic grooves formed on the rotor hub; 
     FIG. 5A is a fragmentary, cross-sectional side view of the hard disk drive depicted in FIG. 1, on an enlarged scale, showing details of a motor having a rotor hub fixed to a rotational shaft and a motor housing having a stationary sleeve according to a seventh embodiment of the present invention; 
     FIG. 5B is a cross-sectional side view of the stationary sleeve shown removed from the motor depicted in FIG. 5A, showing details of hydrodynamic grooves formed on an inner circumferentially extending surface of the sleeve; 
     FIG. 5C is a bottom view of the rotor hub depicted in FIG. 5A showing hydrodynamic grooves formed on the rotor hub which in part form a thrust bearing of the motor; and 
     FIG. 5D is a fragmentary, cross-sectional side view similar to FIG. 5A, showing details of a motor according to an eighth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     A motor for a hard disk drive device in accordance with several embodiments of the present invention are described in detail below with reference to the accompanying drawings. Specifically, FIG. 1 shows a hard disk drive for use in, for example, a computer system. The hard disk drive includes a housing  200 , a hard disk  208 , a motor M (described in greater detail below) and a clamp member  212 . The housing  200  includes a base member  202  and an upper lid  206 . The motor M is supported on the base member  202  and the hard disk  208  is supported by a rotor hub (described below) of the motor. The upper lid  206  along with the base member  202  covers and protects the motor and hard disk  208  by forming a sealed, dust free space  204 . The clamp member  212  is fixed to the rotor hub of the motor M by a screw  210  for supporting the hard disk  208 . 
     The motor M of the hard disk drive shown in FIG. 1, in accordance with the present invention, includes several embodiments that are described below. In the following description of the various embodiments of the motor M, reference is made to upper and lower portions of the various members. It should be understood that the directions upper and lower are relative to the orientation of the motor in, for instance, FIGS. 2A,  2 D,  3 A,  3 D,  4 A,  4 D,  5 A and  5 D, and not to any specific orientation of the motor M since the motor of the present invention may be oriented in any of a variety of ways. 
     First Embodiment 
     FIGS. 2A,  2 B,  2 C and  2 D show a first embodiment of the motor M. The first embodiment of the motor M is identified in FIG. 2A as a motor  10  that includes a rotor hub  12 , a shaft  14 , an annular support body  16 , a disk-shaped cover  18 , a supporting cylinder  20 , a bracket  22 , lubricant  24  such as lubricating oil, a radial bearing  28 , a thrust bearing  32 , a stator  34 , and a rotor magnet  36 , all described in greater detail below. 
     An overview of the structure of the motor  10  is described below. The bracket  22  of the motor  10  is fixedly attached to the base member  202  of the hard disk drive shown in FIG.  1 . The supporting cylinder  20  partially extends into a central opening formed in the bracket  22  and the supporting cylinder  20  is fixed to bracket  22  by, for instance, adhesive and/or may be press fitted into the opening of the bracket  22 . The annular support body  16  is fixed to the supporting cylinder  20  by, for instance, adhesive. The annular support body  16  is a hollow cylindrical member adapted to rotatably support the shaft  14  in a manner described in greater detail below. The cover  18  is fixed to the inner periphery of a lower end portion of the annular support body  16  by, for instance, adhesive and/or press fitting. The shaft  14  extends into a central opening formed in the annular support body  16 . The rotor hub  12  is fixedly fitted to an upper portion of the shaft  14 . 
     The rotor hub  12  includes a substantially disk-shaped upper wall portion  12   a , a cylindrical side wall portion  12   b , and a flange portion  12   c . The cylindrical sidewall portion  12   b  extends in an axial direction from the outer periphery of the upper wall portion  12   a  toward the bracket  22 . The flange portion  12   c  extends in a radially outward direction from a lower portion of the outer peripheral surface of the side wall portion  12   b , thereby forming an annular lip that supports the hard disk  208  (depicted by phantom lines in FIG.  2 ). 
     An upper end of the shaft  14  is fixedly fitted into a central bore of the upper wall portion  12   a  of the rotor hub  12 , whereby the shaft  14  rotates together with the rotor hub  12 . The shaft  14  is fixed to the rotor hub  12  with an adhesive and/or by press fitting. 
     As is explained in greater detail below, lubricant  24  is retained by a capillary action in a small gap that extends between a lower surface of the upper wall portion  12   a  of the rotor hub  12  and an upper end surface of the annular support body  16 . The small gap also extends between an inner peripheral surface of the annular support body  16  and an outer peripheral surface of the shaft  14 , and further extends between a lower surface of the shaft  14  and an upper surface of the cover  18 . The inner peripheral surface of the annular support body  16  is formed with herringbone grooves  26  (see FIG.  2 C). 
     The herringbone grooves  26 , a surrounding portion of the inner peripheral surface of the annular support body  16 , and an adjacent portion of the outer peripheral surface of the shaft  14  form the radial bearing  28 . In response to rotation of the shaft  14 , the herringbone grooves  26  generates hydrodynamic pressure in the lubricant  24  in the radial bearing  28  to support loads in the radial direction, as is described further below. 
     The lower surface of the upper wall portion  12   a  is formed with herringbone grooves  30 , shown in FIG. 2B, that together with the surrounding portions of the lower surface of the upper wall portion  12   a  of the rotor hub  12  and the adjacent portions of the upper end surface of the annular support body  16  form the thrust bearing  32 . Upon rotation of the shaft  14  and rotor hub  12 , the herringbone grooves  30  generate hydrodynamic pressure in the lubricant  24  in the thrust bearing  32  to support axial loads acting on the motor, as is described further below. 
     The stator  34  is fixed to an outer periphery of the supporting cylinder  20 . The rotor magnet  36  is fixed to an inner peripheral surface of the side wall portion  12   b  of the rotor hub  12 , such that the rotor magnet  36  cooperates with the stator  34  to cause rotation of the rotor hub  12  and the shaft  14  within the annular support body  16  and the cover  18 . The stator  34  has a magnetic center  34   c  which corresponds to the center of magnetic forces of attraction of the stator  34 . The rotor magnet  36  also has a magnetic center  36   c  which corresponds to the center of magnetic forces of attraction of the rotor magnet  36 . 
     As is shown in FIG. 2A, the magnetic centers  34   c  and  36   c  are provided with an axial offset  35  in an axial direction with respect to the shaft  14  of the motor  10 . The axial offset  35  is such that the rotor magnet  36  is urged axially downward toward the bracket  22  by the magnetic attraction between the rotor magnet  36  and stator  34 . As a result of the magnetic attraction between the rotor magnet  36  and stator  34 , the rotor hub  12  is acted upon by a downward force that acts against upward thrust forces. Consequently, the magnetic attraction between the rotor magnet  36  and stator  34  acts as a thrust bearing in a first axial direction. As is explained in greater detail below, hydrodynamic forces created by the herringbone grooves  30  of the thrust bearing  32  act in a second axial direction opposite the magnetic attraction forces between the stator  34  and the rotor magnet  36 , thereby balancing axial or thrust forces in the motor  10 . 
     The herringbone grooves  30  are shown more clearly in FIG.  2 B. The herringbone grooves  30  include two sections, spiral grooves  30   a  and  30   b  that extend in opposite rotational directions. The spiral grooves  30   a  and  30   b  intersect at corner portions  30   c  that connect the spiral grooves  30   a  and  30   b , whereby each of the herringbone grooves  30  has a shape of a letter V. In the herringbone grooves  30 , the radially outer spiral grooves  30   b  are slightly longer than the radially inner spiral grooves  30   a . In other words, the corner portions  30   c  are formed at radially inward positions with respect to a radial center of the thrust bearing  32 . Therefore, as the rotor hub  12  and the shaft  14  rotate, the lubricant  24  is urged in a direction shown by an arrow A in FIG. 2 radially inward, thereby generating a dynamic pressure within the lubricant  24 . Further, as indicated in FIG. 2B in the graph to the right of the rotor hub  12 , the hydrodynamic pressure is greatest in the vicinity of the corner portion  30   c  due to the action of the spiral grooves  30   a  and  30   b . It should be understood that although the spiral grooves  30   a  and  30   b  are depicted as having a generally straight configuration, the spiral grooves  30   a  and  30   b  also may have a curved or arcuate configuration. The maximum pressure generated by the grooves  30  in the vicinity of the corner portions  30   c  causes an upward force to act against the underside of rotor hub  12 . This upward force is balanced by the downward oriented magnetic attraction between the stator  34  and the rotor magnet  36 . These two forces together maintain the rotor hub  12  and shaft  14  axially positioned within the motor housing. 
     The herringbone grooves  26  of the radial bearing  28  include several portions such as spiral grooves  26   a  and  26   b  that extend in opposite rotational directions, and axial grooves  26   c  that connect the spiral grooves  26   a  to the spiral grooves  26   b , as is more clearly shown in FIG.  2 C. As the rotor hub  12  and the shaft  14  rotate, the lubricant  24  in the gap between the surfaces of the radial bearing  28  is urged toward the center of the axial grooves  26   c  from the spiral grooves  26   a  and  26   b , thereby generating hydrodynamic pressure between the outer surface of the shaft  14  and the inner surface of the annular support body  16 . As is indicated in the graph to the right of the support body  16  in FIG. 2C, the hydrodynamic pressure in the radial bearing is greatest along the length of the axial grooves  26   c . The axial length of the maximum pressure area in the radial bearing  28  acts to support the shaft  14  to prevent wobble and other undesirable movement in radial directions. 
     The annular support body  16  is made of a porous material that absorbs and retains oil. For instance, the annular support body  16  is made of a sintered metal that is typically formed with pores and voids which can retain and hold liquid lubricant. The sintered metal may be formed by any of a variety of procedures and appropriate materials by, for instance, compression-molding powders of flake graphite cast iron, sintering the molded material, and thereafter impregnating the sintered material with lubricant. 
     It should be appreciated that the radial bearing  28  is exposed to the sealed, dust-free air within the housing  200  via the pores and voids in the oil-bearing metal that forms the annular support body  16 . As a result, bubbles or vapor pressure that may develop in the lubricant  24  while the motor rotates can be vented to outside of the motor  10  through the pores and voids in the annular support body  16 . As a result, the lubricant  24  generally does not leak out of the motor or bearings in the motor  10  because the porous nature of the annular support body  16  eliminates the possibility of vapor pressure building up due to an increase of temperature of the motor. 
     The upper surface of the annular support body  16  facing the herringbone grooves  30  and inner peripheral surface of the annular support body  16  surrounding the herringbone grooves  26  are treated with a sealant so that the pores and voids are filled up for preventing the lubricant  24  from penetrating into the bores of the annular support body  16 . Specifically, hydrodynamic pressure created by the grooves  26  and  30  as the shaft  14  and rotor hub  16  rotate is only generated if the lubricant  24  has a sealed surface to act against. Therefore, the pores and voids formed in the vicinity of the herringbone grooves  26  and  30  must be filled in or sealed in order for the radial bearing  28  and thrust bearing  32  to operate properly. 
     The pores and voids on the upper surface of the annular support body  16  facing the herringbone grooves  30  and inner peripheral surface of the annular support body  16  surrounding the herringbone grooves  26  are sealed with any of a variety of sealing materials that withstand both the heat and temperature requirements of the motor  10 . As a result of the inclusion of the sealing material on the upper surface of the annular support body  16  and inner peripheral surface of the annular support body  16  surrounding the herringbone grooves  26 , lubricant  24  is retained in the annular support body  16 , hydrodynamic pressure generated in the radial bearing  28  and the thrust bearing  32 , and the hydrodynamic pressure can be used to support the shaft  14  and rotor hub  16 . Alternatively, the pores and voids of the upper surface of the annular support body  16  and inner peripheral su race of the annular support body  16  can be filled in and covered by coating or plating. 
     An annular projection  12   d  is formed on the lower surface of the upper wall portion  12   a  of the rotor hub  12 . The annular projection  12   d  extends downward with respect to FIG. 2A part way down an outer peripheral surface of the annular support body  16  for forming a gap therebetween. An inner peripheral surface of the annular projection  12   d  is therefore adjacent to a portion of the outer peripheral surface of the annular support body  16  thereby forming a tapered sealing portion  38  that retains the lubricant  24  on an outer peripheral side of the thrust bearing  32 . An upper edge of the outer peripheral surface of the annular support body  16  adjacent to the annular projection  12   d  is radially outwardly inclined such that the gap between the annular projection  12   d  and the annular support body  16  becomes larger as the annular projection  12   d  extends downward. Surface tension on the lubricant  24  is in balance with atmospheric pressure of the surrounding air, forming a meniscus, which along with capillary action in the gap, retains the lubricant  24  in the thrust bearing  32 . 
     Alternatively, the tapered sealing portion  38  can also have a structure in which radially outward edge of the upper surface of the annular support body  16  is inclined downward such that a meniscus if formed by the inclined surface adjacent to a radially outward edge of the thrust bearing  32 . 
     To further prevent the lubricant  24  from leaking out of the thrust bearing through the inner peripheral surface of the annular projection  12   d  or the outer peripheral surface of the annular support body  16 , which is generally known as an oil migration phenomenon, it is preferable that an oil repellent material such as a fluorine material is applied to the inner peripheral surface of the annular projection  12   d  and the outer peripheral surface of the annular support body  16  in the region of the tapered sealing portion  38 . 
     An annular recess  14   a  is formed on an axial lower portion of the shaft  14 . A ring member  40  is fixed to the annular recess  14   a  such that the ring member  40  projects radially outward from the outer peripheral surface of the shaft  14 . An annular recess  16   a  is formed on the inner peripheral surface of a lower end of the annular support body  16  such that the ring member  40  extends into the annular recess  16   a  without contacting the annular support body  16 . As is shown in FIG. 2A, a gap is formed between the adjacent surfaces of the annular recess  16   a  and the ring member  40 , and the gap further continues between a portion of the upper surface of the cover and the lower surface the ring member  40 . The gap between the cover  18 , the annular recess  16   a  and the ring member  40  does not generally act as a bearing since no herringbone grooves are present to create hydrodynamic pressure. Rather, the ring member  40  and the annular recess  16   a  prevent the shaft  14  from slipping out of the rotor hub  12  when the motor is at rest (not rotating). By forming a structure to keep the shaft  14  from slipping out of the annular support body  16 , axial vibrations of the rotor hub  12  can be reduced. Even when the motor receives a shock or impact, the movements of the rotor hub  12  are limited by the ring member  40  and the annular recess  16   a  such that the hard disk  208  that is mounted on the rotor hub  12  and a read/write head (not shown) adjacent the hard disk  208  for reading and writing data to and from the hard disk, do not collide. 
     The ring member  40  extends slightly below the lower end of the shaft  14  such that a relatively large disk-shaped chamber is formed between the lower end surface of the shaft  14  and the cover  18  that is larger than the gap between the lower surface of the ring member  40  and the cover  18 . Therefore, the disk-shaped chamber formed between the lower end surface of the shaft  14  and the cover  18  functions as a reservoir for lubricant  24  thereby supplying the lubricant  24  in the event that insufficient amounts of the lubricant  24  are retained the bearings  28  and  32 , thereby enabling the bearings to function for a long period of time. 
     An annual groove  12   e  is formed on the upper wall portion  12   a , between the annular projection  12   d  and the side wall portion  12   b . The portion of the upper wall portion  12   a  where the annular groove  12   e  is formed is thinner than the rest of the upper wall portion  12   a . This thinner portion where the annular groove  12   e  is formed allows the upper wall portion  12   a  to deflect in order to absorb stresses that occur when the hard disk  208  is clamped by the clamping member  212  shown in FIG.  1 . As a result, the portion of the upper wall portion  12   a  forming the thrust bearing  32  is not easily deformed thereby avoiding problems associated with an unbalanced condition. 
     In the above-described structure, once the stator  34  is electrically activated in a known manner, the rotor hub  12  and the shaft  14  start rotating within the annular support body  16  and the cover  18 . As the rotor hub  12  rotates, the herringbone grooves  30  in the thrust bearing  32  act on the lubricant  24  retained between the lower surface of the upper wall portion  12   a  of the rotor hub  12  and upper surface of the annular support body  16  thereby generating hydrodynamic pressure urging the rotor hub  12  upward relative to FIG. 2A, thereby support thrust loads in one axial direction of the motor  10 . Simultaneously, the herringbone grooves  26  in the radial bearing  28  acts on the lubricant  24  retained in the small gap defined between the outer surface of the shaft  14  and inner surface of the annular support body  16  thereby generating hydrodynamic pressure to support radial loads in the motor  10 . Also simultaneously, the rotor hub  12  and the shaft  14  are magnetically biased toward the bracket  22  (in a downward direction) due to the offset  35  between the magnetic centers  34   c  and  36   c  of the stator  34  and rotor magnet  36  thereby counterbalancing the hydrodynamic pressure generated in the lubricant  24  in the thrust bearing  32 . 
     Since the thrust bearing  32  is formed between the rotor hub  12  and the annular support body  16  without use of a thrust plate such as that in the prior art, the motor of the present invention can be manufactured and assembled with a high degree of accuracy utilizing a simple bearing structure. Therefore, motors according to the present invention can be produced more efficiently thereby increasing productivity. Also, since a surface of the rotor hub  12  is utilized to form the thrust bearing  32 , the thrust bearing  32  assists greatly in keeping the rotor hub  12  vertical and concentric within the annular support body  16  during rotation of the rotor hub  12  thereby helping to eliminate wobble and tilting of the rotor hub  12  and shaft  14 . Also since the surface of the rotor hub  12  used to form the thrust bearing  32  has a proportionately greater surface area than the conventional thrust plate, there is greater flexibility in the size, configuration and orientation of herringbone grooves of the thrust bearing. Since a thrust plate is not utilized in the thrust bearing of the present invention, rotation of the rotor hub  12  is not affected by the preciseness or tightness of tolerances between the shaft and a thrust plate. The structure of a radial bearing can also be simplified in a motor that does not have a thrust plate. As a result, the motor  10  can be made smaller, thinner, and less costly to manufacture. 
     Since a thrust plate is not utilized in the thrust bearing of the present invention, rotation of the rotor hub  12  is not affected by the preciseness or tightness of tolerances between the shaft and a thrust plate. The structure of a radial bearing can also be simplified in a motor that does not have a thrust plate. As a result, the motor  10  can be made smaller, thinner, and less costly to manufacture. 
     In the structure of the motor  10  described above, the pressure generated by the herringbone grooves  30  in the thrust bearing  32  imparts a lifting force on the rotor hub  12  and the shaft  14  that is balanced by the magnetic attraction acting between the stator  34  and the rotor magnet  36 . In other words, the motor  10  does net include a conventional thrust plate and conventional thrust bearings formed on upper and lower surfaces of a thrust plate as in a conventional motor. Since members for thrust bearings formed on a thrust plate require precise manufacturing, the motor  10  of the present invention is easy to manufacture and assemble, thereby reducing production costs of a hard disk drive. As well, the configuration of the motor  10  of the present invention reduces viscous resistance of the lubricant  24  during rotation of the motor in comparison with a conventional motor where thrust bearings are formed on upper and lower surfaces of a thrust plate, thereby improving the electrical efficiency of the motor  10 . 
     Additionally, since the annular support body  16  is made of a porous oil-bearing metal, the radial bearing  28  is provided with a vent to the air within the housing  200  through the pores and voids in the oil-bearing sintered metal material. Therefore, no air vent needs to be formed to expose the radial bearing to the outside air, which further simplifies the structure of the motor and reduces the production cost of the motor. 
     It should be understood that in the above described embodiment of the present invention, the herringbone grooves  26  of the radial bearing  28  are not limited, with respect to location, to the inner surface of the annular support body  16 . In other words, the herringbone grooves  26  may alternatively be located on adjacent the portion of the outer surface of the shaft  14 . Further, the herringbone grooves  30  in the thrust bearing  32  may alternatively be formed on the upper surface of the annular support body  16  facing the upper wall portion  12   a  of the rotor hub  12 . 
     Second Embodiment 
     The motor  10  described above with respect to FIGS. 2A,  2 B and  2 C may be modified in a variety of different ways and still remain within the scope of the invention. For example, a motor  10 ′ depicted in FIG. 2D may include generally all of the features described above with respect to FIGS. 2A,  2 B and  2 C except that a stator  34 ′ and a rotor magnet  36 ′ may alternatively be employed. All other features of the motor  10  are generally present in the motor  10 ′ and therefore, repetitious description will not be repeated. Rather, only those features of the motor  10 ′ that differ from the motor  10  will be described. 
     As mentioned above, the motor  10 ′ shown in FIG. 2D includes the stator  34 ′ and the rotor magnet  36 ′ having magnetic centers axially aligned with one another. Therefore, there is no magnetic attraction acting therebetween so as to urge the rotor hub  12  in an axial direction with respect to the annular support body  16 . An axial or thrust force is provided in an alternative way. 
     The bracket  22  is formed with an annular recess that retains a first magnet  46 . The flange portion  12   c  is formed with a recess that retains a second magnet  44 . The first and second magnets  46  and  44  are oriented such that there is a magnetic attraction force acting therebetween so as to urge the rotor hub  12  toward the bracket  22  balancing the upward lifting force that results from hydrodynamic pressure generated in the lubricant  24  in the thrust bearing  32 . 
     Third Embodiment 
     The motor M in FIG. 1 may also correspond to a motor  10 ″ as shown in FIGS. 3A,  3 B and  3 C. As shown in FIG. 3, the motor  10 ″ includes many of the features of the above described first embodiment. For instance, the motor  10 ″ includes the shaft  14 , the disk-shaped cover  18 , the supporting cylinder  20 , the bracket  22 , lubricant  24 , the stator  34 , and the rotor magnet  36 . As can be seen in FIG. 3A, the stator  34  and rotor magnet  36  further have magnetic centers  34   c  and  36   c , respectively, which are axially offset thereby acting as a thrust bearing in a manner described above with respect to the first embodiment. 
     The motor  10 ″ further includes a rotor hub  12 ′ that is generally the same as the rotor hub  12  described above with respect to the first embodiment, but is formed with herringbone grooves  30 ′ that differ from those in the first embodiment, as described further below. The motor  10 ″ also includes an annular support body  16 ′ that is generally the same as the annular support body  16  described above with respect to the first embodiment, but has been modified slightly, as is described below. 
     The motor  10 ″ further includes a radial bearing  28 ′ and a thrust bearing  32 ′ that are located in the same relative positions as described above with respect to the first embodiment, however, the herringbone grooves in the respective bearings differ from those described in the first embodiment, as is described in greater detail below. 
     In the radial bearing  28 ′, herringbone grooves  26 ′ (FIG. 3A) are formed with spiral grooves  26   a   1  and  26   b   1  and corner portions  26   c   1 , as shown in FIG.  3 C. The spiral grooves  26   a   1  and  26   b   1  have a substantially identical axial length, and extend in opposite rotational directions. The corner portions  26   c   1  connect the spiral grooves  26   a   1  and  26   b   1 . As the rotor hub  12  and the shaft  14  rotate, the lubricant  24  is urged along the spiral grooves  26   a   1  and  26   b   1  toward the corner portions  26   c   1 , thereby generating hydrodynamic pressure in the lubricant  24  between the outer surface of the shaft  14  and radially inward surface of the annular support body  16 ′. A graph to the right of the annular support body  16 ′ in FIG. 3C shows the pressure distribution in an axial direction along the length of the shaft  14  indicating that a high pressure area is generated in the vicinity of the corner portion  26   c   1 . 
     The herringbone grooves  30 ′ formed in the thrust bearing  32 ′ have spiral grooves  30   a   1  and  30   b   1 , and corner portions  30   c   1 . The spiral grooves  30   a   1  and  30   b   1  have substantially identical radial lengths, and extend in opposite rotational directions. The corner portions  30   c   1  connect the spiral grooves  30   a   1  and  30   b   1 . The lubricant  24  is urged along the spiral grooves  30   a   1  and  30   b   1  toward the corner portions  30   c   1  while the rotor hub  12  and the shaft  14  rotate, thereby generating hydrodynamic pressure in the lubricant  24  between the upper surface of the annular support body  16 ′ and the adjacent portion of the lower surface of the rotor hub  12 ′. The graph to the right of the rotor hub  12 ′ in FIG. 3B show the fluid pressure distribution along a radial direction of the thrust bearing  32 ′ indicating that a high fluid pressure area is generated in the vicinity of the corner portion  30   c   1 . 
     The annular support body  16 ′ and supporting cylinder  20  of the motor  10 ″ shown in FIG. 3A are formed with an air conduit  42  that facilitates venting of bubbles and vapor pressure present in an area near the axial lower end of the annular support body  16 ′ where the lubricant  24  is under a lower pressure than in the radial bearing  28 ′. The air conduit  42  includes a first air conduit  42   a  and a second air conduit  42   b . The first air conduit  42   a  extends in the axial direction along an inner surface of the supporting cylinder  20  and is open at the upper end surface of the supporting cylinder  20 . The second air conduit  42   b  extends in the radial direction on a lower end surface of the annular support body  16 ′ and is open to an axial lower end portion of the small gap defined between the shaft  14  and the annular support body  16  adjacent to the ring member  40 . The second air conduit  42   b  also connects to the first air conduit  42   a . Although FIG. 3A shows only one air conduit  42 , the motor  10 ″ can be provided with a plurality of circumferentially spaced apart air conduits  42 . 
     The annular support body  16 ′ may be made of a sintered metal material in order to provide the oil retaining properties described above with respect to the first embodiment. However, the annular support body  16 ′ may alternatively be made of a solid metal material such as stainless steel or brass. 
     It should be understood that in the above described embodiment of the present invention, the grooves  26 ′ of the radial bearing  28 ′ are not limited, with respect to location, to the inner surface of the annular support body  16 ′. In other words, the grooves  26 ′ may alternatively be located on adjacent the portion of the outer surface of the shaft  14 . Further, the grooves  30 ′ in the thrust bearing  32 ′ may alternatively be formed on the upper surface of the annular support body  16 ′ facing the upper wall portion  12   a  of the rotor hub  12 ′. 
     Fourth Embodiment 
     The motor  10 ″ described above with respect to FIGS. 3A,  3 B and  3 C may be modified in a variety of different ways and still remain within the scope of the invention. For example, a motor  10 ′″ depicted in FIG. 3D may include generally all of the features described above with respect to FIGS. 3A,  3 B and  3 C except that a stator  34 ′ and a rotor magnet  36 ′ may alternatively be employed in a manner previously described with respect to the second embodiment depicted in FIG.  2 D. All other features of the motor  10 ″ of the third embodiment are generally present in the motor  10 ′″ depicted in FIG.  3 D and therefore, repetitious description will not be repeated. Rather, only those features of the motor  10 ′″ that differ from the motor  10 ″ will be described. 
     As mentioned above, the motor  10 ′″ shown in FIG. 3D includes the stator  34 ′ and the rotor magnet  36 ′ having magnetic centers axially aligned with one another. Therefore, there is no magnetic attraction acting therebetween so as to urge the rotor hub  12 ′ in an axial direction with respect to the annular support body  16 ′. An axial or thrust force is provided in an alternative way. 
     The bracket  22  is formed with an annular recess that retains a first magnet  46 . The flange portion  12   c  is formed with a recess that retains a second magnet  44 . The first and second magnets  46  and  44  are oriented such that there is a magnetic attraction force acting therebetween so as to urge the rotor hub  12  toward the bracket  22  balancing the upward lifting force that results from hydrodynamic pressure generated in the lubricant  24  in the thrust bearing  32 . 
     Fifth Embodiment 
     A fifth embodiment of the present invention is described below with reference to FIGS. 4A,  4 B and  4 C which show details of a disk drive motor  50 . The motor  50  includes a rotor hub  52 , a shaft  54 , an annular support body  56 , a disk-shaped cover  58 , a supporting cylinder  60 , a bracket  62 , lubricant  64 , radial bearings  70  and  72 , a thrust bearing  76 , a stator  78 , and a rotor magnet  80 , all of which have aspects similar to the motor described above with respect to the first embodiment, as described below. 
     The rotor hub  52  is formed with a substantially disk-shaped upper wall portion  52   a , a cylindrical side wall portion  52   b , and a flange portion  52   c . The cylindrical side wall portion  52   b  extends downward from the outer periphery of the upper wall portion  52   a . The flange portion  52   c  projects in a radially outward direction from the bottom of the outer peripheral surface of the side wall portion  52   b , thereby providing a surface for supporting the hard disk  208 , depicted in phantom lines in FIG.  4 A. The upper end of the shaft  54  is fixedly fitted in a central bore formed in the upper wall portion  52   a  of the rotor hub  52 , whereby the shaft  54  rotates together with the rotor hub  52 . 
     The annular support body  56  is a hollow cylindrical member adapted to rotatably support the shaft  54 , as described in greater detail below. The cover  58  is coupled to the inner periphery of a lower end of the annular support body  56 . The supporting cylinder  60  supports the annular support body  56 . The supporting cylinder  60  is fixed to the bracket  62 . 
     The lubricant  64  is retained by capillary action in a small gap defined between a portion of the lower surface of the upper wall portion  52   a  of the rotor hub  52  and an upper surface of the annular support body  56 . The lubricant  64  is further retained in the small gap as it continues in an axial direction between inner peripheral surfaces the annular support body  56  and portions of an outer surface of the shaft  54 . Further, the lubricant  64  is retained in a reservoir formed at the bottom of the motor  50  between a lower end surface of the shaft  54  and an upper surface of the cover  58 . 
     A portion of the lower surface of the upper wall portion  52   a  of the rotor hub  52  is formed with spiral grooves  74 . The spiral grooves  74 , the surrounding portions of the lower surface of the upper wall portion  52   a  of the rotor hub  52 , the upper surfaces of the annular support body  56 , and the lubricant  64  retained therebetween together define the thrust bearing  76 . 
     Spiral grooves  68  as depicted in FIG. 4C are formed in the upper portion of the inner peripheral surface of the annular support body  56 . The spiral grooves  68 , the surrounding inner peripheral surface of the annular support body  56 , the adjacent portion of an outer surface of the shaft  54 , and the lubricant  64  retained therebetween define the upper radial bearing  72 . 
     A lower portion of the inner peripheral surface of the annular support body  56  is formed with herringbone grooves  66 , also depicted in FIG.  4 C. The herringbone grooves  66 , the surrounding inner peripheral surface of the annular support body  56 , the adjacent portion of an outer surface of the shaft  54 , and the lubricant  64  retained therebetween define the lower radial bearing  70 . 
     The lower and upper radial bearings  70  and  72  are configured to generate hydrodynamic fluid pressure to support loads in the radial direction by virtue of the grooves  66  and  68 , as is described in greater detail below. The thrust bearing  76  are also configured to generate hydrodynamic fluid pressure to support loads in the axial direction by virtue of grooves  74 , as is described in greater detail below. 
     The stator  78  is fixed to an outer periphery of the supporting cylinder  60 . The rotor magnet  80  is fixedly attached to an inner peripheral surface of the side wall portion  52   b  of the rotor hub  52 , such that the rotor magnet  80  cooperates with the stator  78  so as to rotate the rotor hub  52  and the shaft  54  within the annular support body  56  and the cover  58 . Although not shown, the bracket  62  is fixedly attached to the base member  202  of the hard disk drive shown in FIG.  1 . 
     The herringbone grooves  66  in the lower radial bearing  70  are formed with spiral grooves  66   a  and  66   b  that extend in opposite rotational directions and corner portions  66   c  that connect the spiral grooves  66   a  and  66   b , as shown in FIG.  4 C. As the rotor hub  52  and the shaft  54  rotate, the lubricant  64  is pumped along the spiral grooves  66   a  and  66   b  toward the corner portions  66   c , thereby generating hydrodynamic fluid pressure in the lubricant  64  to support radial loads acting on the motor  50 . 
     The spiral grooves  68  in the upper radial bearing  72  are oriented to pump lubricant  64  in an upward direction as indicated in FIG. 4A by the arrow B. The spiral grooves  74  in the thrust bearing  76  depicted in FIG. 4B is oriented to pump lubricant  64  in a radially inward direction as indicated by arrows C in FIG.  4 A. The combined hydrodynamic fluid pressure generated by the spiral grooves  68  and  74  supports both radial and thrust loads in the motor  50 . Specifically, the hydrodynamic pressure generated by the spiral grooves  74  in the thrust bearing  76  is sufficient to counter loads in one axial direction (thrust loads) and is sufficient to counter the hydrodynamic pressure generated by the spiral grooves  68  in the upper radial bearings  72 . In other words, since the hydrodynamic pressure from each of the grooves  68  and  74  are directed toward an intersection between the upper radial bearing  72  and the thrust bearing  76 , the fluid pressure generated by the grooves  68  acts to maintain the fluid pressure generated by the grooves  74  and vice verse. 
     The annular support body  56  is made of any of a variety of metal materials, such as copper, copper alloy, brass or stainless steel. The annular support body  56  is formed with a first bore  81  and a first air conduit  82   a  for venting the radial bearings  70  and  72  to the outside air within the housing  200  (FIG.  1 ). The first bore  81  is open to an annular recess  84  formed on the inner surface of the annular support body  56  between the upper and lower radial bearings  72  and  70 . The first bore  81  is also open to a portion of the outer peripheral surface of the annular support body  56  that is exposed to the outside air within the housing  200 . The annular recess  84  defines an annular air separation space  86  between the upper and lower radial bearings  72  and  70 . 
     The first air conduit  82   a  extends along the bottom of the annular support body  56  and is open to the space defined between the lower end of the shaft  54  and cover  58 . The first air conduit  82   a  is also connected to a second air conduit  82   b  formed in the supporting cylinder  60 . The second air conduit  82   b  extends from the first air conduit  82   a  in an upward axial direction and is open to an upper surface of the supporting cylinder  60  thereby forming a second air conduit  82 . 
     The radial bearings  70  and  72  are exposed to the outside air through the first air conduit  81  and the second air conduit  82 . Bubbles that form within the lubricant  64  as the lubricant  64  is filled or injected into the motor  50  or vapor pressure and that may form while the motor rotates are vented or exhausted out of the bearings through the first air conduit  81  and the second air conduit  82 . In this way, leakage of lubricant  64  out of the bearings due to vapor pressure that is produced as a result of heightened temperatures during operation of the motor is minimized. 
     Since the radial bearings  70  and  72  are exposed the outside air through the first and the second air conduits  81  and  82 , the bubbles and vapor pressure in the lubricant  64  can be vented out of the bearings easily. 
     The space between the bottom surface of the shaft  54  and the upper surface of the cover  58  serves as a reservoir for lubricant. As the lubricant  64  either vaporizes, or otherwise is lost from the bearings, the lubricant contained in the reservoir replenishes the lost lubricant, whereby the bearings can function for an extended period of time. 
     The motor  50  includes the upper radial bearing  72  having spiral grooves  68  to generate hydrodynamic pressure in the axial upper portion of the motor, and the lower radial bearing  70  having herringbone grooves  66  to generate hydrodynamic pressure in the axial lower portion of the motor. Therefore, the radial bearings  70  and  72  can be positioned through enough axial distance to maintain necessary rigidity of the bearings, even though the axial length of the shaft  54  of the motor  50  is shorter than in prior art configurations, thereby making it possible to make a motor that is axially thinner than conventional motors. 
     An annular projection  52   d  is formed on a lower surface of the upper wall portion  52   a  of the rotor hub  52 . A tapered seal  88  is formed at an outer peripheral portion with respect to the thrust bearing  76 . The configuration of the tapered seal  88  is generally the same as the tapered seal  38  described above with respect to the first embodiment shown in FIG.  2 A. 
     An annular recess  54   a  is formed in an axial lower portion of the shaft  54 . A ring member  90  is attached to the annular recess  54   a  such that the ring member  90  extends radially outward from the outer peripheral surface of the shaft  54 . An annular recess  56   a  is formed on a lower portion of an inner peripheral surface of the annular support body  56  that radially faces the ring member  90 , such that the ring member  90  extends into to the annular recess  56   a . However, the surfaces of the annular recess  56   a  are spaced apart from the adjacent surfaces of the ring member  90 . 
     The ring member  90  has a larger diameter than the inner diameter of the annular support body  56  above the recess  56   a . Therefore, the ring member  90  serves as a retainer that prevents the shaft  54  from slipping out of the rotor hub  52 . However, the surfaces of the ring member  90  are spaced apart from the adjacent surfaces of the recess  56   a  by a distance sufficient to prevent the ring member  90  from serving as a bearing. 
     An annular groove  52   e  is defined under the upper wall portion  52   a  between the outer peripheral surface of the annular projection  52   d  and the side wall portion  52   b , such that the portion of the upper wall portion  52   a  defining the annular groove  52   e  is thinner than the remainder of the upper wall portion  52   a . The tapered sealing  88 , the ring member  90  and recess  56   a , the space at the lower end of the shaft  54 , and the annular groove  52   e  all have generally the same functions as similar structure described above with respect to the first embodiment depicted in FIG.  2 A. 
     In the above-described structure, once the stator  78  is electrically excited, the rotor hub  52  and the shaft  54  rotate with respect to the annular support body  56  and the cover  58 . As the rotor hub  52  rotates, the grooves  74  of the thrust bearing  76  generate hydrodynamic pressure in the lubricant  64  imparting an upward force on the lower surface of the upper wall portion  52   a  of the rotor hub  52  thereby supporting the motor against axial or thrust loads. Simultaneously, the grooves  68  in the upper radial bearing  72  generate upwardly directed hydrodynamic pressure in the lubricant  64  between the surfaces of the radial bearing  72  to radially support an upper portion the shaft  54  as the shaft  54  and rotor hub  52  rotate. Further simultaneously, the grooves  66  in the lower radial bearing  70  generate hydrodynamic forces which support a lower portion of the shaft  54  as the shaft  54  rotor hub  52  rotate. 
     The rotor hub  52  and the shaft  54  are magnetically biased toward the bracket  62  in a downward direction to balance the upward acting hydrodynamic pressure in the thrust bearing  76 . The magnetic biasing force provided by the configuration of the stator  78  and the rotor magnet  80 . Specifically a magnetic center  78   c  of the stator  78  is axially offset from a magnetic center  80   c  of the rotor magnet  80 . As a result, magnetic attraction acting between the stator  78  and the rotor magnet  80  urge the rotor hub  52  downward toward the bracket  62  against the upward force in the thrust bearing  76 . 
     Since the thrust bearing  76  is formed between the rotor hub  52  and the annular support body  56  without use of a thrust plate, the structure of the bearings, which often requires precise manufacturing and assembly, is simplified. Therefore, motors according to the present invention can be produced more efficiently than more complex prior art configurations. Since the thrust bearing is formed by a portion of the lower surface of the rotor hub and a portion of the upper surface of the annular support body, it is possible to have a thrust bearing on a surface having a greater surface area than the surface area of a thrust plate such as in the prior art. Therefore, the thrust bearing of the present invention can serve to support the shaft against tilting and wobbling during rotation. The thrust bearing further maintains the shaft in an axially aligned orientation within the annular support body. As further result, the motor of the present invention can be made smaller and thinner in the axial direction than conventional motors. 
     Moreover, the motor  50  having the thrust bearing  76  formed between the upper wall portion  52   a  of the rotor hub and the upper surface of the annular support body  56 , that generates hydrodynamic pressure for urging the rotor hub  52  and the shaft  54 , is balanced by the magnetic attraction acting between the stator  78  and the rotor magnet  80  so as to urge the rotor hub  52  and shaft  54  downward. In other words, the motor  50  does not have a thrust plate and therefore does not have thrust bearings formed on upper and lower surfaces of the thrust plate as in a conventional motor. Rather, the magnetic attraction defines one thrust bearing. Since members for thrust bearings formed on a thrust plate require precise manufacturing techniques, the motor  50  of the present invention, which does not have such thrust plate, is easier to manufacture thereby reducing the cost of a hard disk drive. 
     Further, since a thrust plate is not utilized in a thrust bearing, rotation of the rotor hub  52  is not affected by the preciseness and tightness of adherence between the shaft and the thrust plate. The structure of a radial bearing can also be simplified in a motor that does not have a thrust plate. As a result, the motor  50  can be made smaller, thinner, and less costly to manufacture. 
     Additionally, if spiral grooves  74  are utilized in the thrust bearing  76  as dynamic pressure generation grooves, viscous resistance of the lubricant  64  during rotation of the motor  50  can be reduced, making the motor  50  even more electrically efficient. 
     It should be understood that in the above described embodiment of the present invention, the grooves  66  and  68  of the radial bearings  70  and  71  are not limited, with respect to location, to the inner surface of the annular support body  56 . In other words, the grooves  66  and  68  may alternatively be located on adjacent the portion of the outer surface of the shaft  54 . Further, the grooves  74  in the thrust bearing  76  may alternatively be formed on the upper surface of the annular support body  56  facing the upper wall portion  52   a  of the rotor hub  52 . 
     Sixth Embodiment 
     A sixth embodiment of the present invention is depicted in FIG.  4 D. The motor  50 ′ in FIG.  4 D is generally identical to the motor  50  depicted in FIG. 4A having all the same structural features elements except that the motor  50 ′ is provided with an alternative form of magnetic attraction to provide thrust bearing support. 
     Specifically, the motor  50 ′ in FIG. 4D has a stator  78 ′ and a rotor magnet  80  having centers that are not axially offset from one another as in the first embodiment and fifth embodiment. Rather, the rotor hub  52  is formed with a recess at a radial lower and outward portion having a first magnet  144  fixed therein. Further, the bracket  62  is formed with a recess having a second magnet  146  fixed therein. Magnetic attraction acting between the first and second magnets  144  and  146  urges the rotor hub  52  downward toward the bracket  62  to balance upward forces generated by the grooves  74  in the thrust bearing  76 . 
     It should be appreciated that in either the fifth or sixth embodiments of the present invention, the grooves  74  may have the generally curved shape as shown in FIG. 4B or the generally linear shape depicted in  4 E where the grooves  74 ′ are depicted. 
     Seventh Embodiment 
     A seventh embodiment of the present invention is shown in FIGS. 5A,  5 B and  5 C, where a motor  50 ″ is depicted. The motor  50 ″ has many of the features described above with respect to the sixth embodiment shown in FIG.  4 A. For instance, the motor  50 ″ in FIG. 5A includes the bracket  62 , the supporting cylinder  60  having first and second air conduits  82   a  and  82   b , the rotational shaft  54  and a stator  78 . 
     The motor  50 ″ also includes an annular support body  56 ′ that is formed with herringbone grooves  92 , described in greater detail below. The motor  50 ″ also includes a rotor hub  52 ′ having many of the same features as the rotor hub  52  of the sixth embodiment in FIG. 4A such as the rotor magnet  80  that is axially offset from the stator  78  to provide magnetic attraction that urges the rotor hub  52 ′ downward toward the bracket  62  thereby acting as a thrust bearing to balance upward forces acting on the rotor hub  52 ′ generated in the thrust bearing  76 ′ (described in greater detail below). 
     However, unlike the sixth embodiment, the motor  50 ″ only includes one radial bearing  94  that is formed by an inner surface of the annular support body  56 ′, the herringbone grooves  92  formed on the inner surface of the annular support body  56 ′, the outer surface of the shaft  54  adjacent to the herringbone grooves  92  and the lubricant  64  retained therebetween. 
     The herringbone grooves  92 , as shown more clearly in FIG. 5B, include spiral grooves  92   a  and  92   b  and corner portions  92   c.  The spiral grooves  92   a  and  92   b  extend in opposite rotational directions and are connected to one another by the corner bent portions  92   c . When the shaft  54  rotates within the annular support body  56 ′, the herringbone grooves  92  generate hydrodynamic fluid pressure within the lubricant  64  in the radial bearing  94 . The hydrodynamic fluid pressure provides support against radial loads acting on the shaft  54 . 
     As shown in FIG. 5B, the axial length of the spiral grooves  92   a  is less than half the axial length of the spiral grooves  92   b . Therefore, the spiral grooves  92   b  are able to generate a greater amount of hydrodynamic pressure in the lubricant than the spiral grooves  92   a . In this way, the radial bearing  94  generates a radial load support pressure in the axial direction represented by the arrow D in FIG.  5 A. 
     The motor  50 ″ also includes a thrust bearing  76 ′ that is defined by a lower surface of the portion  52   a  of the rotor hub  52 ′, spiral grooves  74 ′ formed on the lower surface of the portion  52   a  of the rotor hub  52 ′, an adjacent surface of the annular support body  56 ′ and the lubricant  64  retained therebetween. As the rotor hub  52 ′ and the shaft  54  rotate with respect to the annular support body  56 ′, the spiral grooves  74 ′ shown in FIG. 5C generate hydrodynamic fluid pressure in the lubricant  64  that is directed radially inward as represented by the arrow C in FIG.  5 A. 
     A small annular space  104  is defined adjacent to an upper portion of the shaft  54  at the intersection of the thrust bearing  76 ′ and the radial bearing  94 . Since the hydrodynamic pressure generated by the grooves  92  in the radial bearing  94  is directed upward toward the annular space  104 , and the hydrodynamic pressure generated by the grooves  74 ′ in the thrust bearing  76 ′ is directed radially inward toward the annular space  104 , the fluid pressure of the lubricant  64  in the annular space  104  is high during rotation of the motor  50 ″. 
     As the motor  50 ″ rotates, bubbles and vapor pressure are often formed in the lubricant  64  retained in the thrust bearing  76 ′ and the radial bearing  94 . Bubbles may form as a result of air being released from the lubricant. Vapor pressure may be created as lubricant vaporizes. As the bubbles and vapor pressure are generated in the lubricant  64 , they tend to migrate toward portions of the lubricant  64  under low pressure. Therefore, since the annular space  104  is generally a high fluid pressure area during operation of the motor  50 ″, bubbles and vapor pressure will tend to migrate away from the annular space  104  out of the radial bearing  94  and out of the thrust bearing  76 ′. Due to the hydrodynamic pressure generated by the grooves in the thrust bearing  76 ′ and the radial bearing  94 , the motor  50 ″ shown in FIG. 5A does not need venting such as the venting provided by the annular recess  84  and vent  81  in the embodiment depicted in FIG.  4 A. Further, such venting is not necessary in the motor  50 ″ in FIG. 5A because the motor  50 ″ only has one radial bearing  94 , and also because herringbone grooves  92  and spiral grooves  74 ′ are designed to prevent bubbles from staying in the annular space  104  due to the fluid pressure therein. 
     Due to the hydrodynamic pressure generated by the grooves in the thrust bearing  76 ′ and the radial bearing  94 , the motor  50 ″ shown in FIG. 5A does not need venting such as the venting provided by the annular recess  84  and vent  81  in the embodiment depicted in FIG.  4 A. Further, such venting is not necessary in the motor  50 ″ in FIG. 5A because the motor  50 ″ only has one radial bearing  94 , and also because herringbone grooves  92  and spiral grooves  74 ′ are designed to prevent bubbles from staying in the annular space  104  due to the fluid pressure therein. 
     It should be understood that in the above described embodiment of the present invention, the herringbone grooves  92  of the radial bearing  94  are not limited, with respect to location, to the inner surface of the annular support body  56 ′. In other words, the herringbone grooves  92  may alternatively be located on adjacent the portion of the outer surface of the shaft  54 . Further, the spiral grooves  74 ′ in the thrust bearing  76  may alternatively be formed on the upper surface of the annular support body  56 ′ facing the upper wall portion  52   a  of the rotor hub  52 . 
     Eighth Embodiment 
     An eighth embodiment of the present invention is depicted in FIG. 5D. A motor  50 ′″ depicted in FIG. 5D is almost identical to the motor  50 ″ depicted in FIG.  5 A and includes all the features described with respect to the seventh embodiment except that the motor  50 ′″ includes a stator  78 ′ and a rotor magnet  80 ′ that are axially aligned such that there is no magnetic attraction acting in an axial direction acting on the rotor hub  52 ′. 
     Instead, the motor  50 ′″ in FIG. 5D includes a first annular magnet  144  mounted in a recess in the annular lip  52   c  of the rotor hub  52 ′. A second annular magnet  146  is mounted in a recess formed in the bracket  62  adjacent to the first annular magnet  144 . Magnetic attraction acting between the first and second annular magnets  144  and  146  urges the rotor hub  52 ′ downward against the upward force acting on the rotor hub  52 ′ that results from the hydrodynamic fluid pressure generated by the spiral grooves  74 ′ in the thrust bearing  76 ′. Therefore, the magnetic attraction acting between the first and second annular magnets  144  and  146  functions as a second thrust bearing. 
     Although the various embodiments described above are directed to motors for rotating a hard disk in a hard disk drive in which the brackets  22  and  62  of the motor are installed on the base member  202  of the hard disk drive housing, the present invention may also be applied to a hard disk drive in which the base member  202  serves the same function and has the same structure as the brackets  22  and  62  described above. Specifically, the base member  202  may be formed with an opening that supports a supporting cylinder such as the supporting cylinders  20  and  60  described above in the various embodiments of the present invention. 
     The foregoing description of the embodiments according to the present invention is provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.