Abstract:
Thin-profile spindle motor. Single hydrodynamic-pressure thrust and radial bearings are established about a thrust plate on the end of the motor shaft, encompassed by a cylindrical casing. Hydrodynamic pressure-generating grooves scored on either of the thrust plate end faces, or either of the inner faces of the casing ends opposing the thrust plate end faces, are a component of the single thrust bearing. A micro-gap clearance for the thrust bearing, retaining lubricant continuously with the radial bearing, is established on whichever side of the thrust plate the thrust bearing is formed. Like grooves scored on either the thrust plate circumferential surface, or the casing inner cylindrical surface by which it is opposed at a micro-gap clearance, the clearance itself, and lubricant retained therein form the single radial bearing. Means for magnetically counterbalancing the thrust hydrodynamic-pressure are provided between rotor hub and the stator or casing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a motor, and in particular, to a motor having a hydrodynamic bearing structure which makes use of a lubricating fluid between adjacent surfaces of a rotary member and a stationary member, the motor being adapted to rotate a data storage media such as a hard disk in a hard disk drive. 
     2. Background Information 
     Conventionally, there has been known and used a motor equipped with a hydrodynamic bearing device which uses fluid pressure created between a shaft body and a sleeve structure in order to rotatably support the shaft body and the sleeve structure such that one of them is rotatable relative to the other. An example of such a motor is described in detail below with reference to FIG.  1 . 
     FIG. 1 is a longitudinal sectional view schematically showing structure of a prior art motor  150  equipped with a bearing device using dynamic pressure of a fluid lubricant. As illustrated in FIG. 1, the conventional motor  150  equipped with a bearing configuration using hydrodynamic pressure has a cylindrical shaft housing  151  for rotatably supporting a rotary shaft  154 , and the cylindrical shaft housing  151  has a large diameter base portion  151   a . The outer peripheral surface of a lower portion of the large diameter base portion  151   a  is fixedly fitted in a circular engaging hole  152   a  of a base plate  152  of a recording medium drive device. The base portion  151   a  is integrally formed with an annular ring-shaped plate portion  151   b . The annular ring-shaped plate portion  151   b  is further integrally formed with a small diameter sleeve portion  151   c  that is coaxially aligned with the base portion  151   a  and located above the base portion  151   a . Further, a thrust cover  153  is fixedly engaged with an inner recessed surface of the base portion  151   a  adjacent to the lower end thereof, thereby blocking and sealing a disk shaped internal space defined within the base portion  151   a.    
     In this way, a shaft supporting structure is thus formed with the use of the cylindrical shaft housing  151  and the thrust cover  153 . The rotary shaft  154  is supported in a vertical orientation within the sleeve portion  151   c  of the cylindrical shaft housing  151  by a fluid lubricant  155 , such as lubricating oil, that fills a clearance gap formed between surfaces of the rotary shaft  154  and the sleeve portion  151   c  due to liquid capillary action. The surfaces of the rotary shaft  154  and adjacent surfaces of the sleeve portion  151   c  serve as upper and lower radial bearings  170  and  171  using dynamic pressure of the lubricant  155  to support the rotary shaft  154  within the sleeve portion  151   c  such that the rotary shaft  154  is freely and relatively rotatable within the sleeve portion  151   c.    
     A ring-shaped thrust plate  156  is fixedly fitted to a lower end of the rotary shaft  154  and is positioned in the disk shaped internal space that is defined within the base portion  151   a . A clearance gap defined in the disk shaped internal space between the surfaces of the ring-shaped thrust plate  156  and the inner surface of the base portion  151   a , the inner surfaces of the annular ring-shaped plate portion  151   b  and an upper surface of the thrust cover  153  is filled with lubricant  155  retained therein by capillary action. Upper and lower surfaces of the ring-shaped thrust plate  156  and adjacent surfaces of the base portion  151   a , the plate portion  151   b  and thrust cover  153  serve as upper and lower thrust bearings allowing the annular ring-shaped thrust plate  156  to rotate freely within the cylindrical shaft housing  151  in combination with the dynamic pressure of the lubricant  155 . In this manner, with the use of upper and lower hydrodynamic radial bearings  170  and  171  and upper and lower hydrodynamic thrust bearings, a hydrodynamic fluid bearing structure is formed which makes use of the hydrodynamic pressure of the fluid lubricant  155  during the relative rotation between the rotary shaft  154  (with the thrust plate  156 ) and the cylindrical shaft housing  151 . 
     An annular groove  157  is formed at approximately a middle portion of the rotary shaft  154  separating the upper and lower radial bearings  170  and  171 . The annular groove  157  is surrounded by an adjacent portion of the inner surface of the sleeve portion  151   c  forming an annular air space  159  that communicates with atmosphere outside the motor via a breather hole  158  formed on the sleeve portion  151   c.    
     Herringbone grooves  160   a  and  160   b  are formed on lower and upper surfaces, respectively, of the thrust plate  156 . Herringbone grooves  160   c  and  160   d  are formed on inner surfaces of the sleeve portion  151   b  below and above the annular air space  159 , respectively. In response to rotation of the rotary shaft  154 , radial load supporting pressure and thrust load supporting pressure are generated in the lubricant  155  in and about the herringbone grooves  160   a ,  160   b ,  160   c  and  160   d.    
     A stator  161  formed with coil windings (not shown) around a stator core (not shown) is fixed on an outer surface of the sleeve portion  151   c . A cup-like rotor hub  162  is formed with an outmost enclosure wall  162   a  that encircles the stator  161 . The upper end of the rotary shaft  154  extends into a center hole formed in the cup-like rotor hub  162  such that the rotary shaft  162  is engaged and fixed to the cup-like rotor hub  162 . A rotor magnet  163  is secured on an internal surface of the outmost enclosure wall  162   a  of the rotor hub  162  such that the rotor magnet  163  radially faces the stator  161  with a predetermined clearance space maintained therebetween thereby forming a rotation driving structure. 
     When using the above-described conventional hydrodynamic fluid bearing assembly having upper and lower radial hydrodynamic bearings and upper and lower hydrodynamic thrust bearings, the ring-shaped thrust plate  156  is used in the hydrodynamic thrust bearing structure. In order to ensure a stabilized support for the rotary shaft  154  in the axial direction and thereby minimize possible vibrations in that direction, both upper and lower surfaces of the thrust plate  156  must be used to form upper and lower thrust bearings. However, there is a problem associated with using both upper and lower surfaces of the thrust plate as bearings in that bearing losses due to, for instance, fluid friction, may be large and as a result the electric efficiency of the motor may be low. 
     When a hydrodynamic bearing motor  150  described above is to be installed in a thin hard disk drive (HDD) whose thickness is, for example, less than 5 mm, the sleeve portion  151   c , the rotary shaft  154 , the stator  161  and the rotary magnet  163  somehow have to be made shorter in the vertical direction, as viewed in FIG.  1 . When a hydrodynamic bearing motor  150  is used in such a thin and low-noise hard disk drive with the motor being provided with both upper and lower radial hydrodynamic bearings, upper and lower thrust hydrodynamic bearings provided on upper and lower sides of a thrust plate  156 , the rotary shaft  154  and the thrust plate  156  may be supported stably with minimal axial vibrations. Although the thrust hydrodynamic bearings on both sides of the thrust plate may ensure stability of the rotary shaft  154  in the axial direction, the grooves  160   a  and  160   b  generate large viscous resistance against the flow of the lubricant  155  resulting in bearing loss making the motor electrically inefficient. 
     Also known is a hydrodynamic bearing motor in which no thrust plate  156  is employed. Instead a thrust bearing is formed on an end surface of a rotary shaft. In this case, although bearing loss is small and the motor is relatively electrically efficient, the motor requires some kind of axial movement prevention mechanism to prevent movement of the rotary shaft in the axial direction, since the rotary shaft does not have a projection such as a thrust plate for retaining the rotary shaft within a motor housing. When impact is applied to the motor, large axial movement of the rotary shaft may occur causing undesirable contact between a magnetic head and a data storage medium such as a hard disk on which the magnetic head writes and reads data, thereby adversely affecting the reading and writing functions of the magnetic head and possibly damaging the magnetic head and the recording medium. 
     In view of the above, there exists a need for a motor which overcomes the above mentioned problems in the prior art. The present invention attempts to solve the problems associated with the above described related art, as will become apparent to those skilled in the art from the following description of the present invention. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a thin type motor which requires a small amount of electric energy for rotation of the motor and has a small amount of play of a rotary shaft in the axial direction. 
     Another object of the present invention is to provide a thin type motor which operates stably. 
     Another object of the present invention is to provide a thin motor with a small amount of bearing losses associated with grooves in thrust bearings. 
     In one aspect of the present invention, a motor includes a rotary shaft, a circular or disk-shaped thrust plate fixed to a lower end of the shaft, and a supporting structure for supporting the shaft. The thrust plate is accommodated in a disk shaped space formed in the supporting structure, with a gap or clearance being formed between the outer surfaces of the thrust plate and the inner surfaces of the supporting structure adjacent to the outer surfaces of the thrust plate. One of the axial end surfaces of the thrust plate, an adjacent one of the inner surfaces of the supporting structure and lubricant filling a clearance gap therebetween constitute a thrust bearing of the motor. A radially outer peripheral surface of the thrust plate, an adjacent inner surface of the supporting structure and lubricant oil therebetween constitute a radial bearing of the motor. Grooves are formed on the surfaces of the supporting structure constituting the radial and thrust bearings to create hydrodynamic forces in the lubricant in response to rotation of the thrust plate such that the thrust plate and the rotary shaft are urged in a first axial direction. Magnetic biasing means is provided in the motor to counterbalance the hydrodynamic forces in the thrust bearing urging the rotary shaft and thrust plate in a second axial direction opposite the first axial direction. The magnetic biasing means therefore functions as a second thrust bearing to balance axial forces acting on the rotary shaft and thrust plate in the motor thereby maintaining the rotary shaft and thrust plate in a stable axial position within the support structure of the motor. 
     In one embodiment, a cup-shaped rotor hub is fixed to an upper end of the rotary shaft. The magnetic biasing means includes a pair of permanent magnets provided on adjacent surfaces of the supporting structure and the rotor hub. The pair of magnets are oriented such that their respective magnetic forces repulse one another thereby causing the rotor hub, rotary shaft and thrust plate to be urged upward. The hydrodynamic forces in the lubricant are such that the thrust plate, rotary shaft and rotor hub are urged downward thereby maintaining the rotary shaft in a stable, desirable location with respect to the support structure. 
     Alternatively, the pair of magnets may be oriented such that their respective magnetic forces attract one another causing the rotor hub, rotary shaft and thrust plate to be urged downward. In this alternative arrangement, the grooves formed on the surfaces of the supporting structure constituting the radial and thrust bearings to create hydrodynamic forces that urge the thrust plate, rotary shaft and rotor hub upward to maintain the rotary shaft in a stable, desirable location with respect to the support structure. 
     In another embodiment, a stator coil is fixed to an outer surface of the support structure. A rotor magnet is fixed to an inner surface of a rotor hub that is fixed to an upper end of the rotary shaft. The rotor magnet and the stator each have magnetic centers that are spaced apart from each other in the axial direction such that the magnetic center of the rotor magnet is urged toward the magnetic center of the stator coil by magnetic forces. In this way, the rotor magnet, rotor hub, rotary shaft and thrust plate are urged in an axial direction opposite the hydrodynamic forces created in the lubricant by the grooves formed on the surfaces of the supporting structure constituting the radial and thrust bearings. The magnetic centers of the rotor magnet and the stator coil may be offset such that the rotor magnet is urged downward against upward hydrodynamic forces, or alternatively, the magnetic centers of the rotor magnet and the stator coil may be offset such that the rotor magnet is urged upward against downward hydrodynamic forces. 
     These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal cross sectional side view of a conventional motor that includes upper and lower hydrodynamic thrust bearings on a thrust plate and upper and lower hydrodynamic radial bearings on a rotary shaft; 
     FIG. 2 a schematic perspective view of a motor according to a first embodiment of the present invention; 
     FIG. 3 is a longitudinal cross sectional side view of the motor depicted in FIG. 2 showing internal details of a housing, rotary shaft and a hydrodynamic bearing; 
     FIG. 4 is perspective view of the housing depicted in FIG. 3 looking from an underside thereof with portions of the motor removed to show details of spiral grooves formed on inner surfaces of the housing; 
     FIG. 5 is a bottom view of an annular thrust ring having spiral grooves formed thereon, with the annular thrust ring shown removed from the housing depicted in FIGS. 3 and 4; 
     FIG. 6 is a bottom view of a rotor hub of the motor depicted in FIGS. 2 and 3, shown removed from the motor, showing details of a magnet mounted in a surface of the rotor hub; 
     FIG.  7  and FIG. 8 are bottom views of the rotor hub of the motor, similar to FIG. 6, showing alternate configurations of magnets mounted to the surface of the rotor hub; 
     FIG. 9 is a longitudinal cross sectional side view of a motor showing internal details of a housing, rotary shaft and a hydrodynamic bearing according to a second embodiment of the present invention; 
     FIG. 10 is a cross sectional side view of a sleeve portion of the housing of the motor depicted in FIG. 9, with other portions of the motor removed to show spiral grooves formed on an inner surface of the sleeve portion; 
     FIG. 11 is a top view of a thrust cover of the housing of the motor depicted in FIG. 12, with the thrust cover removed from the motor to show spiral grooves formed on an upper surface of the thrust cover; 
     FIG. 12 is a longitudinal cross sectional side view similar to FIG. 3, showing a third embodiment of the present invention; 
     FIG. 13 is a longitudinal cross sectional side view similar to FIG. 9, showing a fourth embodiment of the present invention; 
     FIG. 14 is a longitudinal cross sectional side view similar to FIG. 9, showing a fifth embodiment of the present invention; and 
     FIG. 15 is a longitudinal cross sectional side view similar to FIG. 3, showing a sixth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A motor having a hydrodynamic bearing in accordance with a first embodiment of the present invention is described below with reference to FIGS. 2-6. However, it should be understood that the present invention is not to be limited to the embodiments described below. 
     Referring to FIG. 2, the motor  1  includes a rotary shaft  2 , a disk-shaped thrust plate  3  having a center bore that is fixedly fitted to an outer periphery of a lower end portion of the rotary shaft  2  and a support housing  4  which supports both the thrust plate  3  and the rotary shaft  2  in a manner described in greater detail below. 
     The support housing  4  includes a cylinder member  6 , an annular thrust ring  7  and a disk-shaped thrust cover  5 . An inner surface of the cylinder member  6  is formed with an annular protrusion  6   p  that divides the inner surface of the cylinder member  6  into upper and lower inner surfaces  6   u  and  61 . The annular thrust ring is fixedly fitted to the cylindrical member  6  such that an outer peripheral surface of the annular thrust ring  7  is in friction engagement with the inner upper surface  61  of the cylinder member  6  and a radially outer portion of a lower surface of the annular thrust ring  7  engages an upper surface of the annular protrusion  6   p . The disk-shaped thrust cover  5  is fixedly fitted to the cylinder member such that an outer peripheral surface of the disk-shaped thrust cover  5  is in friction engagement with the inner lower surface  61  of the support housing  4  and a radially outer portion of an upper surface of the disk-shaped thrust cover  5  engages a lower surface of the annular protrusion  6   p . The annular thrust ring  7  is formed with a center bore though which the rotary shaft  2  extends. 
     A lower portion of the cylinder member  6  of the support housing  4  is fixedly fitted in a circular bore  52   a  of a bracket  52 . The bracket  52  may be a stationary support member within a data storage medium driving device, such as a hard disk drive or other data storage device. A stator  16  is fixedly fitted to the outer periphery of the cylinder member  6  of the support housing  4  above the bracket  52 . An annular stationary magnet  20  is fixedly disposed in a groove formed on the upper surface of the cylinder member  6 , the annular stationary magnet  20  being explained in greater detail below. 
     An upper end of the rotary shaft  2  is fixedly fitted in a central bore formed in a rotor hub  17 . The rotor hub  17  is an annular member having an inverted cup-like shape with a radially outer ring portion that extends downward encircling the stator  16 . A rotor magnet  18  is fixedly fitted to an inner surface of the outer ring portion of the rotor hub  17  encircling and confronting the stator  16  thereby defining a clearance gap between the rotor magnet  18  and the stator  16 . 
     An annular magnet  19  is disposed in a groove formed on a lower surface of the rotor hub  17  immediately above and confronting the annular stationary magnet  20  disposed on the cylinder member  6  with a clearance gap being formed between the annular magnet  19  and the annular stationary magnet  20 . 
     The support housing  4 , the thrust cover  5 , the thrust ring  7 , the bracket  52  and the stator  16  define a rigid stationary support structure. The rotary shaft  2 , the thrust plate  3 , the rotary hub and the rotor magnet  18  define a generally rigid rotatable body that is rotatable with respect to the support structure. 
     The thrust plate  3  includes upper and lower surfaces  3   a  and  3   b , and a radially outer peripheral surface  3   c  defined therebetween. The outer peripheral surface  3   c  of the thrust plate  3  confronts an adjacent inner peripheral surface of the cylinder member  6  with a small gap defined therebetween filled with a lubricant  8  that is preferably a fluid. The bottom surface of the annular thrust ring  7  is adjacent to and confronts the upper surface  3   a  of the thrust plate  3  with a small gap therebetween. The gap is filled with the lubricant  8 . The bottom surface  3   b  of the thrust plate  3  is adjacent to and confronts the upper surface of the thrust cover  5  with a small gap therebetween filled with lubricant  8  only at a radial outer portion thereof, as is explained in greater detail below. The lubricant  8  may be any of a variety of lubricating oils known in the art, filling the above mentioned gaps and retained therein by capillary action between the thrust plate  3  and the annular thrust ring  7 , and between the thrust plate  3  and the thrust cover  5 . It should be understood that the thrust plate  3  is located inside a space defined between the annular thrust ring  7  and the thrust cover  5  and confined radially by the cylinder member  6 . 
     An underside of the cylinder member  6  is depicted in FIG. 4 with the thrust cover  5  removed. As can be seen in FIG. 4, an inner circumferential surface of the cylinder member  6  is formed with spiral grooves  11  and a lower surface of the annular thrust ring  7  is formed with spiral grooves  12 . The spiral grooves  11  and  12  are dynamic pressure generating grooves which induce movement of the lubricant  8  when the rotatable body rotates with respect to the support structure, thus creating fluid dynamic pressure, as is described below in greater detail. 
     The spiral grooves  11  and  12  may have any of a variety of shapes so long as the shape of the grooves induces movement of lubricant in a manner described in greater detail below. For instance, the spiral grooves  12  in the annular thrust ring  7  may be formed as a series of generally straight grooves that are angularly offset from rays extending from a central axis of the annular thrust ring  7 , as is depicted in FIG. 4, or alternatively, the spiral grooves  12  may have an arcuate shape as depicted in FIG.  5 . It should be appreciated that these are only two such examples of the shape of spiral grooves and a variety of alternate shapes and configurations are possible. 
     The spaces in, adjacent to, and around the spiral grooves  11  and  12  retain lubricant  8 . An inner radial portion of the lower surface of the annular thrust ring  7  is formed with a conical surface  7   a  that is inclined upward toward an axial center of the annular thrust ring  7 . The conical surface  7   a  and an adjacent portion of the upper surface  3   a  of the thrust plate  3  define a seal which acts on the lubricant  8  such that surface tension of the lubricant  8  in balance with atmospheric pressure of the surrounding air forms a meniscus which, along with capillary action, retains the lubricant  8  (a fluid) in the gaps between adjacent surfaces, as shown in FIG. 3. A similar seal is formed between a conical surface formed on a radial outward portion of the thrust cover  5  and adjacent portion of the lower surface  3   b.    
     The radial inner surface of the cylinder member  6  around the spiral grooves  11 , the radial outer surface  3   c  of the thrust plate  3  and lubricant  8  therebetween constitutes a radial bearing  9  that bears radial loads of the shaft  2  when the shaft  2  is rotating. 
     The portions of the lower surface of the annular thrust ring  7  radially outward from the conical surface  7   a , the upper surface  3   a  of the thrust plate  3 , the spiral grooves  12 , and the lubricant  8  in the gap therebetween constitute a thrust bearing  10  which bears loads on the shaft  2  which act in an axially upward direction when the shaft  2  is rotating. 
     The radial bearing  9  and the thrust bearing  10  form a hydrodynamic bearing configuration in the support housing  4  which supports the thrust plate  3  fixed to the rotary shaft  2 , such that the thrust plate  3  and rotary shaft  2  are relatively rotatable within the support housing  4  about a rotational axis of the rotary shaft  2 . The thrust plate  3  must be thick enough to: provide strength in the axial direction and to be securely coupled to the rotary shaft  2 ; and insure that the radial bearing  9  generates adequate amounts of radial load support pressure. In view of the above requirements, the thrust plate  3  should ideally be at least 1.0-1.5 mm, or may possibly have a maximum thickness of 2.0 mm if the motor requirements are such that the overall size of the motor may be thicker in the axial direction. 
     When the rotary shaft  2  and thrust plate  3  rotate with respect to the support housing  4 , the spiral grooves  11  in the radial bearing  9  (the spiral grooves  11  being dynamic pressure generating grooves) urge lubricant  8  axially upward toward the annular thrust ring  7  creating a fluid pressure buildup between radially outward portions of the annular thrust ring  7  and the thrust plate  3 . The forces associated with the upward fluid pressure build up is represented as axial force a 1  in FIG.  3 . Further, the spiral grooves  12  on the lower surface of the annular thrust ring  7  of the thrust bearing  10  urge lubricant  8  radially outward toward the cylinder member  6  as the shaft  2  and thrust plate  3  rotate. The radially outward force created in the lubricant  8  when acted upon by the spiral grooves  12  (dynamic pressure generating grooves) is represented in FIG. 3 as radial force b 1 . Specifically, as the thrust plate  3  and rotary shaft  2  rotate, the spiral grooves  11  and  12  generate fluid dynamic pressure by moving the lubricant  8  axially upward and radially outward toward the outer periphery of the upper surface of the thrust plate  3 . The combination of the radial force b 1  and the axial force a 1  is such that the buildup of fluid pressure between the thrust plate  3  and the annular thrust ring  7  causes the thrust plate  7  and the rotary shaft  2  to be urged downward, as is discussed further below. 
     In the first embodiment of the present invention, the spiral grooves  11  are formed on the inner peripheral surface of the cylinder member  6 , whereas the spiral grooves  12  are formed on the bottom surface of the annular thrust ring  7 . By employing the spiral grooves  11  and  12  in the motor of the present invention, it is possible to use thrust plate  3  having a smaller diameter and thickness, compared with a conventional motor in which only herringbone grooves are formed in a conventional hydrodynamic bearing configuration, as is made more clear below. 
     A thrust air interposing space  13  is defined between central portions of the lower surface  3   b  of the thrust plate  3  and the thrust cover  5 , in which air is retained. Another air space  14  is defined above radial inner portions of the upper surface  3   a  of the thrust plate  3  radially inward from the thrust bearing  10 , the air space  14  being in communication with outside air via spaces defined between the rotor hub  17  and stator  16 . The air spaces  13  and  14  are in communication with each other through an air conduit  15  formed in an inner periphery of the thrust plate  3 . The term outside air refers to the air located outside the motor  1  and inside a device to which the motor  1  is installed. The pressure of outside air is not necessarily standard atmospheric pressure, although the pressure may be standard atmospheric pressure. 
     The thrust air interposing space  13  below the lower surface  3   b  of the thrust plate  3  is open to the outside air through the air conduit  15  and the air space  14  formed on the upper surface of the thrust plate  3  such that air pressure in each of the air spaces  13  and  14  are equalized and are the same as the pressure of the outside air. Since the surface tension of the lubricant  8  is affected by the air pressure in the air spaces  13  and  14 , which is the pressure of the outside air, and the air pressure in each of the air spaces  13  and  14  are equal, there is no movement of the lubricant  8  due to pressure differences in either of the air spaces  13  and  14 , thereby ensuring the reliability of the bearings  9  and  10 . Therefore, if the amount of the lubricant  8  decreases due to such factors as evaporation, the volume of air in the air spaces  13  and  14  may expand equally because identical atmospheric pressures act in each space. Further, the lubricant  8  adjacent to but not previously disposed in and about the bearings  9  and  10  is urged into the bearings  9  and  10  to replenish lubricant  8  with an amount equal to an amount of lubricant lost due to, for instance, evaporation. Accordingly, the bearings  9  and  10  are always supplied with a sufficient amount of the lubricant  8 , thereby enabling an improvement of the reliability of the motor  1 . In the first embodiment, as was mentioned above, the bottom surface of the annular thrust ring  7  is formed with the conical surface  7   a  and the upper surface of the thrust cover  5  is formed with a similar conical surface defining seals or boundaries between the lubricant  8  and the air spaces  13  and  14 , where a meniscus forms to assist in retaining the lubricant  8  in the gaps between the thrust plate  3  and the thrust plate  7 , and between the thrust plate  3  and the annular thrust ring  5 . The conical surfaces promote the effects of surface tension on the lubricant  8 , thus defining the meniscus, and also provide a small reservoir of lubricant. The small reservoir of lubricant provides a means for replenishing lost lubricant  8 . 
     In the motor  1 , since the air spaces  13  and  14  are radially inward from the thrust bearing and the air spaces  13  and  14  are connected via the air conduit  15 , the surface tension of the exposed surfaces of the lubricant  8  is acted upon by the same air pressure, whereby the lubricant does not flow into either of the air spaces  13  or  14 , thus insuring reliable operation of the bearings  9  and  10  and even distribution of the lubricant  8 . 
     Since the bearings  9  and  10  include the spiral grooves  11  and  12 , the electrical efficiency is improved, and the structure of the motor  1  is simplified by elimination of cumbersome oil circulating structures present in some prior art configurations, leading to a reduction of production cost of the present invention compared to prior art configurations. 
     Air bubbles may form in the lubricant  8  for a variety of reasons, and in particular as the motor  1  rotates air bubbles may form due to the forces generated by the spiral grooves  11  and  12 . The motor  1  is designed to exhaust such air bubbles to outside the bearings  9  and  10  into the air spaces  13  and  14 , thereby preventing the lubricant  8  from leaking out of the bearings  9  and  10 . Further, the air spaces  13  and  14  also allow for movement of the lubricant  8  due to thermal expansion of the lubricant  8  itself and thermal expansion of the motor  1  components from the heat generated by the motor  1 . 
     The thrust plate  3  limits the upward axial movement of the rotary shaft  2 . An undesirable large amount of axial movement of the rotary shaft  2  is therefore prevented, thereby preventing a data storage medium which is mounted on the rotor, and a magnetic head which is disposed adjacent to the data storage medium to read and write data therefrom and thereto, from contacting each other even when impact is applied thereto, preserving the quality of the data storage medium and the magnetic head. Since the motor  1  has the thrust plate  3  confined within the support housing  4  which limits the axial movement of the rotary shaft  2 , no additional structure is needed to prevent the rotary shaft  2  from detaching from the motor  1 , as has been necessary in prior art motors that do not include an annular projecting portion such as a thrust plate. 
     The thrust air interposing space  13  defined below the lower surface  3   b  of the thrust plate  3  and has a diameter that is greater than the diameter of the air space  14 , the thrust bearing  10  is only formed only on the upper surface  3   a  of the thrust plate  3 . In other words, there is only one hydrodynamic thrust bearing in the motor  1  of the present invention and that thrust bearing  10  is configured such that it only restricts upward movement of the thrust plate  3 . 
     The stationary magnet  20  and the magnet  19  are magnetized in the axial direction with respect to the rotary shaft  3  so have generally identical magnetic poles facing one another. In other words, the stationary magnet  20  is magnetized with, for instance, a north pole facing a north pole of the magnet  19 . Therefore, the stationary magnet  20  and the magnet  19  produce a magnetic repelling force with respect to one another thereby urging the rotor hub  17 , the rotary shaft  3  and the thrust plate  3  upward. However, it should be understood that the stationary magnet  20  and the magnet  19  may also have respective south poles facing one another. The magnetic repelling force produced by the interaction of the stationary magnet  20  and the magnet  19  being close to one another lifts the rotor hub  17  upward by a force represented by the arrow A in FIG.  3 . Since the stationary magnet  20  and the magnet  19  magnetically repel each other, the rotor hub  17  to be urged axially upward, which in turn makes the upper surface of the thrust plate  3  move upward toward the bottom surface of the annular thrust ring  7 , with the thrust bearing  10  therebetween. 
     According to the above described structure, when the stator  16  has electricity applied to it by a power source (not shown), the rotary shaft  2  and the thrust plate  3  start rotating together within the support housing  4 . While the motor  1  is rotating, the spiral grooves  11  and  12  in the radial bearing  9  and the thrust bearing  10 , respectively, urge the lubricant  8  retained in the gaps between the thrust plate  3  and the support housing  4  toward the outer peripheral edge of the upper surface  3   a  of the thrust plate  3 . Specifically the spiral grooves  11  create an upward force al in the lubricant  8  in the vicinity of the radial bearing  9  thereby moving the lubricant  8  upward, and the spiral grooves  12  create a radially outward force b 1  in the lubricant  8  in the vicinity of the thrust bearing  10  thereby moving the lubricant  8  radially outward toward the cylinder member  6 . The combination of the upward force a 1  and the radially outward force b 1  creates fluid pressure between the upper surface  3   a  of the thrust plate  3  and the lower surface of the annular thrust ring  7  urging the thrust plate  3  downward, with respect to FIG.  3 . 
     Simultaneously, the rotor hub  17 , the rotary shaft  2  and the thrust plate  3  are all urged upward by the force A which results from the interaction between the stationary magnet  20  and the magnet  19 . In effect, the interaction between the stationary magnet  20  and the magnet  19  serves as a second thrust bearing. Further, as the fluid pressure between the upper surface  3   a  of the thrust plate  3  and the lower surface of the annular thrust ring  7  increases as rotational speed of the rotary shaft  2  increases, the forces a 1  and b 1  increase correspondingly due to the action of the spiral grooves  11  and  12 , thereby urging the thrust plate  3  downward. As the thrust plate  3  is urged downward in response to increases in the forces a 1  and b 1 , the stationary magnet  20  and the magnet  19  are moved correspondingly closer to one another. Since magnets of like polarity induce exponentially increasingly repellant forces as they get closer to one another, the stationary magnet  20  and the magnet  19  naturally repel one another as they get closer to one another. Therefore, as the forces a 1  and b 1  increase, the force A, due to repelling interaction between the stationary magnet  20  and the magnet  19 , correspondingly increases thereby balancing the overall forces acting on the thrust plate  3 , rotary shaft  2  and rotor hub  17  and maintaining the thrust plate  3  in a desirable location between the thrust cover  5  and annular thrust ring  7 . 
     Therefore, as compared with the conventional motor as shown in FIG. 1, when the hydrodynamic bearing of the present invention is used, for instance, in a rotary shaft type low noise spindle motor having a thrust plate and adapted to be used in a thin hard disk drive (HDD) whose thickness is less than 5 mm, the spiral grooves  11  and  12 , instead of conventional herringbone grooves, are used as the dynamic pressure generating grooves of the radial bearing  9  and the thrust bearing  10 . Since the spiral grooves  11  and  12  have simple structure and can reduce viscous resistance of the lubricant  8 , the motor  1  can have better electric efficiency and a lower production cost than a motor having only herringbone grooves. Also, the thrust plate  3  of the motor  1  can dampen axial vibrations. 
     By virtue of the thrust plate  3  in combination with the magnets  19  and  20 , axial movement of the rotor hub  17 , rotary shaft  2  and thrust plate  3  in the motor can be limited to an amount of movement necessary and sufficient to enable relative rotation of the rotor hub  17 , rotary shaft  2  and thrust plate  3 , with the lubricant  8  disposed in the gaps therebetween, where the gaps have a width of approximately 20 micrometers. Also, a magnetic head and a magnetic disk are protected from damage as a result of undesirable contacts in response to a mechanical shock. The motor  1  is especially suited for a portable personal computer since the motor  1  has a high electrical efficiency. 
     Only one hydrodynamic thrust bearing  10  is formed in the motor  1  on the upper surface  3   a  of the thrust plate  3 , creating downward forces via fluid pressure within the thrust bearing  10  to balance the upward magnetic biasing force A of the stationary magnet  20  and magnet  19  acting on the rotor hub  17 . No thrust bearing is formed on the lower surface  3   b  of the thrust plate  3  and the upward thrust load support pressure which would have been generated by a lower thrust bearing below the lower surface  3   b  of the thrust plate  3  is substituted with the axially upward magnetic biasing force A of the magnet  19  and stationary magnet  20  which serves to balance forces on the rotary shaft  2  and the thrust plate  3 . Since there is no thrust bearing on the bottom surface of the thrust plate  3 , bearing loss due to viscous resistance of the lubricant  8  against the spiral grooves  13  and  14  is reduced, thereby improving the electrical efficiency of the motor  1 . Also since the thrust air interposing space  13  is formed below the bottom surface of the thrust plate  3 , precise tolerances of the thickness of the thrust plate  3  and the widths of the gaps above and below the thrust plate  3  can be relaxed or loosened. For instance, the tolerance in the thickness of the thrust plate  3  can be increased, which makes the press work to produce the thrust plate easy and thereby reduces the production cost. Furthermore, there are no hydrodynamic pressure generating grooves formed on a lower surface of the thrust plate  3 , thereby reducing the manufacturing cost of both elements of the motor  1 . 
     It should be understood that the magnets  19  and  20  may have any of a variety of configurations. For instance, in the embodiment described above, the magnets  10  and  20  may be solid, continuous annular rings as depicted in FIG.  6 . Alternatively, the magnets  19  and  20  may be made up of a plurality of magnet segments, such as magnet segments  19   a  (and  20   a ) depicted in FIG.  7  and magnet segments  19   b  (and  20   b ) depicted in FIG.  8 . 
     As should be understood from the above description, the magnets  19  and  20  functionally act as a thrust bearing urging the rotor hub  17  upward thereby limiting downward axial movement of the thrust plate  3 , rotary shaft  2  and rotor hub  17 . The force A from interaction between the magnets  19  and  20  provide a means for balancing the forces a 1  and b 1  acting to urge the thrust plate  3 , rotary shaft  2  and rotor hub  17  downward. Therefore, the present invention only has one hydrodynamic thrust bearing and has a magnetic thrust bearing. Further, the present invention only has one hydrodynamic radial bearing that is defined on the surface  3   c  of the thrust plate  3  with the adjacent inner surface of the cylinder member  6  proximate the spiral grooves  11 . Having a single hydrodynamic radial bearing and single hydrodynamic thrust bearing formed on the thrust plate  3  balanced by the interaction between the magnets  19  and  20 , simplifies the overall structure of the motor  1  compared to the prior art, making for a smaller and less expensive motor. 
     It should be under stood that the magnets  19  and  20  may be made of any of a variety of magnetic materials. In an alternate embodiment, the magnets  19  and  20  may be made of a stainless steel. It should also be understood, that the rotor hub  17  in the first embodiment should be made of a non-magnetically susceptible material so as not to interfere with the repelling forces present between the magnets  19  and  20 . 
     Second Embodiment 
     A second embodiment of the present invention is depicted in FIG.  9 . 
     In view of the similarities between the second embodiment and the first embodiment, like reference numerals are utilized to refer to like elements where such elements are interchangeable between the first and second embodiments. Moreover, as will be apparent to those skilled in the art from the following description, the various like elements and descriptions thereof with respect to the first embodiment applies to the like elements in the second embodiment. Thus, the motor  1   a  of the second embodiment is not described or illustrated in as great detail as the first embodiment due to the similarities between the two embodiments. Rather, it will be apparent to those skilled in the art from this disclosure that the description of various elements and overall description of the first embodiment apply to the similar or identical elements of the second embodiment. 
     In the first embodiment depicted in FIGS. 3-5, the radial bearing  9  includes the spiral grooves  11  which created upward dynamic pressure in the lubricant  8  retained in the gap between the outer peripheral surface  3   c  of the thrust plate  3  and the inner peripheral surface of the cylinder member  6 . Also in the first embodiment, the thrust bearing  10  includes the spiral grooves  12  which created radially outward dynamic pressure in the lubricant  8  retained in the gap between the upper surface  3   a  of the thrust plate  3  and the bottom surface of the annular thrust ring  7 . In the motor  1   a  depicted in FIG. 9 in the second embodiment, similar spiral grooves are employed in a single thrust bearing and a single radial bearing formed on surfaces of the thrust plate  3  in the second embodiment, but having slightly different effects on the lubricant  8 , as is described in greater detail below. 
     In the second embodiment, the motor  1   a  is supported by a bracket  52 , which may be a support member in, for instance, a hard disc drive. The bracket  52  is formed with a central opening  52   a  into which a lower end of a cylinder member  6   a  is fixedly fitted. The cylinder member  6   a  is formed with spiral grooves  11   a  on a radially inner surface, as is described in greater detail below. 
     An upper portion of the cylinder member  6   a  has a central opening in which an annular plate  7   a  is fixedly fitted. A thrust cover  5   a  is fixedly fitted into an opening formed at the lower end of the cylinder member  6   a.    
     As with the first embodiment, the second embodiment includes a rotary shaft  2  that extends through an opening formed in the annular plate  7   a . A thrust plate  3  is fixedly fitted on a lower end of the rotary shaft  2 , the thrust plate  3  having an upper surface  3   a , a lower surface  3   b  and a radial outer peripheral surface  3   c . A rotor hub  17   a  is fixedly fitted to an upper end of the rotary shaft  2 . A rotor magnet  18   a  is fixedly fitted to an inner radial surface of the rotor hub  17   a  facing, but spaced apart from a stator  16  fixedly fitted to a radially outer surface of the cylinder member  6   a.    
     The stator  16  has a magnetic center  16   c  and the rotor magnet  18   a  also has a magnetic center  18   c . The respective magnetic centers  16   c  and  18   c  are physical locations within the stator  16  and rotor magnet  18   a , respectively, but the magnetic centers are more than just a physical location. The magnetic centers  16   c  and  18   c  represent a focal point of the magnetic forces inherent in a magnet. In other words, the lines of force associated with a magnet are centered about the magnetic center of the magnet. 
     The magnetic center  16   c  of the stator  16  is axially offset from the magnetic center  18   c  of the rotor magnet  18   a  by a distance d, as is shown in FIG. 9, resulting in an axial imbalance with respect to the interaction of the magnetic forces between the rotor magnet  18   a  and stator  16 . Specifically, the rotor magnet  18   a  and stator  16  are configured with opposite magnetic poles facing one another such that the rotor magnet  18   a  and stator  16  are attracted to each other via magnetic forces. Since the magnetic centers  16   c  and  18   c  are axially offset from one another, the stator  16  magnetically urges the rotor magnet  18   a  downward toward the magnetic center  16   c , with respect to the depiction in FIG.  9 . Therefore, due to the above described magnetic forces, the rotor hub  17   a , the shaft  2  and the thrust plate  3  are all urged downward. The magnetic force urging the magnetic center  18   c  toward the magnetic center  16   c  thereby urging the rotary shaft  2  and thrust plate  3  downward is represented by the force B in FIG.  9 . 
     It should be understood that the stator  16  is an electric magnet that may be selectively supplied with electric current in order to cause rotation of the rotor hub  17   a  and shaft  2 , but the stator  16  is also made of a permanently magnetic material and therefore the stator  16  magnetically attracts the rotor magnet  18   a  regardless of whether or not electric current is supplied to the stator  16 . 
     In the motor shown in the FIG. 9, a radial bearing  9   a  is defined by the surface  3   c  and the adjacent inner surface of the cylinder member  6   a . Spiral grooves  11   a  are form on the adjacent inner surface of the cylinder member  6   a , shown more clearly in FIG. 10, also forming part of the radial bearing  9   a . The spiral grooves  11   a  are configured to create downward dynamic pressure as the thrust plate  3  rotates, the dynamic pressure being represented by a force a 2  in FIG.  9 . The force a 2  is created in the lubricant  8  retained in the gap between the outer peripheral surface  3   c  of the thrust plate  3  and the inner peripheral surface of the cylinder member  6   a . A thrust bearing  10   a  is defined in the motor  1   a  by the lower surface  3   b  of the thrust plate  3 , a radial outer portion of an upper surface of the thrust cover  5   a , and spiral grooves  12   a  formed on the thrust cover  5   a.  The spiral grooves  12   a , shown more clearly in FIG. 11, are configured to create radially outward pressure in the lubricant  8  as the thrust plate  3  rotates, the radial outward pressure being represented by force b 2  in FIG.  9 . The force b 2  is dynamic pressure created in the lubricant  8  retained in the gap between the lower surface  3   b  of the thrust plate  3  and the upper surface of the thrust cover  5   a . The forces a 2  and b 2  urge the lubricant  8  toward a radially outer portion of the lower surface  3   b  of the thrust plate  3 , creating an increase in fluid pressure below the thrust plate  3  thereby urging the thrust plate  3 , rotary shaft  2  and rotor hub  17   a  upward. 
     In the motor la shown in FIG. 9, an thrust air interposing space  13   a  is formed between the upper surface  3   a  of the thrust plate  3  and the bottom surface of the annular plate  7   a . An air space  14   a  is formed below radial inner portions of the lower surface  3   b  of the thrust plate  3 , radially inward from the thrust bearing  10   a . The air spaces  13   a  and  14   a  are connected to each other via the air conduit  15  formed on an inner periphery of the thrust plate  3 . As can be seen in FIG. 9, the thrust air interposing space  13   a  has an outer diameter that is much larger than the diameter of the air space  14   a.    
     Since the air space  14   a  is defined on the upper surface  3   a  of the thrust plate  3  and has a relatively large diameter, no thrust bearing per se is formed at the upper side of the thrust plate  3 . There is only one is hydrodymanic thrust bearing present in the motor  1   a  and that is the thrust bearing  10   a  formed with the radially outer portion of the lower surface  3   b  of the thrust plate  3 . 
     As mentioned above, the rotor magnet  18   a  and the stator  16 , are provided with polar orientations whereby they are magnetically attracted to each other. As a result of the magnetic attraction between the magnetic centers  16   c  and  18   c , the rotor magnet  18   a , rotor hub  17   a , rotary shaft  2  and thrust plate  3  are urged downward by the axially downward force B. Although no thrust bearing is formed by the upper surface  3   a  of the thrust plate  3 , force B provides a downward thrust load to balance the upward force created under the thrust plate  3  in the lubricant  8  by action of the spiral grooves  11   a  and  12   a  as the thrust plate  3  rotates. The magnetic biasing force B produced by the rotor magnet  18   a  and the stator  16  in effect functions as a second thrust bearing to counteract and balance the forces a 2  and b 2 . 
     The spiral grooves  11   a  and  12   a  are similar to those described above with respect to the first embodiment and can be in the form of generally straight grooves similar to those depicted in FIGS. 4 and 5 in a manner similar to the first embodiment, or, the spiral grooves  11   a  and  12   a  may have a curved or arcuate form depicted in FIGS. 10 and 11. 
     Although the above described embodiments use spiral grooves for surfaces within both a radial bearing and a thrust bearing, it is possible to include any of a variety of shaped grooves so long as the groove create the desired fluid dynamic pressure in the lubricant between the surfaces of the bearings. For instance, the thrust and radial bearings may each include a respective surface having imbalanced herringbone grooves to create the desired dynamic fluid pressure in the lubricant between the corresponding surfaces of the thrust and radial bearings in order to urge a thrust plate in an axial direction to balance magnetic forces urging the thrust plate in an opposite axial direction. In such motor, the dynamic fluid pressure of the lubricant retained in the spiral grooves of the thrust bearing is also designed to be balanced with the dynamic fluid pressure of the lubricant retained in the unbalanced herringbone grooves of the radial bearing. Even if the balance between the fluid dynamic pressures is lost, the radial bearing may still retain lubricant, which generates pressure to maintain the rotary shaft in a centered position in the housing, thereby continuing to support the rotor securely in the radial direction within the housing. 
     The motor of the present invention having the spiral grooves as dynamic pressure generating grooves in a radial bearing and a thrust bearing has improved electrical efficiency when comparison with a conventional motor having only herringbone grooves because there is, for example, a reduction of bearing losses due to fluid movement in opposing direction within V-shaped herringbone grooves. Furthermore, the structure of the motor is simplified by the elimination of oil circulation structures often used in conventional motors, thereby reducing the production cost. When a motor includes spiral grooves as dynamic pressure generation grooves in the thrust bearing, and imbalanced herringbone grooves as dynamic pressure generation grooves in the radial bearing, the radial bearing is capable of retaining an amount of circulating lubricant necessary and sufficient to generate pressure to center of the rotor in the motor housing and to axially support the rotor in the radial bearing. Also, since the thrust plate limits the axial vibrations of the rotor along with a magnetic biasing in an opposing direction, the amount of axial movement of the rotor can be kept minimal, thereby protecting the data storage medium and the magnetic head which is disposed adjacent to the data storage medium to read and write data therefrom and thereto, maintaining the reading and writing functions of the data storage device. 
     The present invention provides a motor having a hydrodynamic bearing configuration and adapted to be used in a thin hard disk drive device with an improved electrical efficiency and a reduction in production costs. 
     It should be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. 
     For instance, in the first embodiment depicted in FIG. 3, the spiral grooves  11  are formed on the surface of protrusion  6   p  of the cylinder member  6 . The spiral grooves  11  could alternatively be formed on the outer peripheral surface  3   c  of a thrust plate  3 ′, as depicted in FIG. 12 in a third embodiment. Similarly the spiral grooves  12  formed on the lower surface of the thrust ring  7  in FIG. 3 could alternatively be formed on the upper surface  3   a  of the thrust plate  3 ′, as is also shown in FIG.  12 . It should be understood that all of the various features depicted in FIG.  12  and identified by common reference numbers are the same as those depicted in FIG. 3 with the exception that the spiral grooves  11  and  12  are now formed on the thrust plate  3 ′ in the third embodiment in FIG.  12 . 
     Further, in the second embodiment depicted in FIG. 9, the spiral grooves  11   a  could alternatively be formed on the outer peripheral surface  3   c  of the thrust plate  3 ″, as is shown in FIG. 13 in a fourth embodiment. Similarly the spiral grooves  12   a  formed on the upper surface of the thrust cover  5   a  in FIG. 9 could alternatively be formed on the lower surface  3   b  of the thrust plate  3 ″, as is shown in FIG.  13 . It should be understood that all of the various features depicted in FIG.  13  and identified by common reference numbers are the same as those depicted in FIG. 9 with the exception that the spiral grooves  11   a  and  12   a  are now formed on the thrust plate  3 ″ in the fourth embodiment in FIG.  12 . 
     Other alternative configurations of the present invention are also considered by the inventors. For example, the magnets  19  and  20  of the first embodiment depicted in FIG. 3 may be added to the motor la depicted in FIG. 9 with the magnets  19  and  20  being oriented to be magnetically attracted to one other to counterbalance the fluid dynamic pressure created between the thrust cover  5   a  and the thrust plate  3 , as is shown in FIG. 14 in a fifth embodiment. In such a configuration, the offset d between the magnetic centers  18   c  and  16   c  of the rotor magnet  18   a  and the stator  16 , respectively, depicted in FIG. 9, are now unnecessary. In such a configuration, the fluid dynamic pressure generated in the bearings  9   a  and  10   a  urge the thrust plate  3  upward and the magnetic attraction between the magnets  19  and  20  urge the rotor hub  17   a , shaft  2  and thrust plate  3  downward. It should be understood that all of the various features depicted in FIG.  14  and identified by common reference numbers are the same as those depicted in FIG. 9 with the exception that the magnets  19  and  20  are employed to urge the rotor hub  17   a , rotary shaft  2  and thrust plate  3  downward in the fifth embodiment depicted in FIG.  14 . 
     In a sixth embodiment of the present invention, the motor depicted in FIG. 3 may be manufactured without the magnets  19  and  20  and the magnetic center of the rotor magnet  18  may be offset from the magnetic center of the stator  16  by a distance d′, as depicted in FIG.  15 . The offset between the magnetic center of the rotor magnet  18  and the magnetic center of the stator  16  is such that the magnetic attraction therebetween urges the rotor hub  17 , the shaft  2  and the thrust plate  3  upward against the downward urging of the hydrodynamic forces created in the lubricant of the bearings  9  and  10 . It should be understood that all of the various features depicted in the sixth embodiment in FIG.  15  and identified by common reference numbers are the same as those depicted in FIG. 3 with the exception that the magnets  19  and  20  are now unnecessary due to offset d′ between the rotor magnet  18  and the stator  16 . 
     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.