Abstract:
In a thrust dynamic-pressure bearing that through pump-in type spiral grooves generates dynamic pressure, a simple configuration to eliminate at the bearing exterior air bubbles liable to build up nearby the central portion of the bearing, yielding stabilized axial bearing force. The configuration lends superior endurance and reliability to the thrust bearing, and to a spindle motor furnished with the bearing. The configuration makes the circularly symmetrical dynamic pressure distribution, created by spiral grooves formed circularly symmetrical with respect to the bearing axial center, asymmetrical by the addition of an asymmetrical auxiliary groove(s). Air bubbles building up near the center of the spiral grooves are thus shifted to the area where the spiral grooves are formed. In the area where the spiral grooves are formed, the more inward the oil the higher the dynamic pressure. Therefore, because air bubbles will tend to shift from a region of high to a region of low oil pressure, the air bubbles get pushed out to the outer circumferential margin of the spiral grooves.

Description:
BACKGROUND OF INVENTION 
     The present invention relates to a thrust hydrodynamic bearing, to a spindle motor equipped with the thrust hydrodynamic bearing, and to a disk drive utilizing the spindle motor, that are capable of eliminating air bubbles that can build up nearby the bearing center. 
     Spindle motors for driving recording disks such as hard disks include bearing means for supporting axial loads that act on the rotor. Thrust hydrodynamic bearings are conventionally employed as the axial load bearing means. In a fluid such as oil, retained between two axially opposing planar surfaces, thrust hydrodynamic bearings generate dynamic pressure when the rotor rotates. The hydrodynamically generated thrust pressure serves as the axial load bearing pressure. 
     FIG. 7 depicts a conventional thrust hydrodynamic bearing  1 . The dynamic-pressure generating grooves formed in this conventional thrust bearing  1  for generating hydrodynamic pressure are so-called pump-in type spiral grooves  2 , which induce radially inward acting dynamic pressure in the oil. By virtue of the spiral grooves, nearby the bearing center a pressure peak where the dynamic pressure becomes quite large appears, while heading radially outward the pressure declines. This pressure peak area in the dynamic bearing supports loads acting on the rotor. 
     The spiral grooves  2  develop dynamic pressure with comparatively better energy efficiency than herringbone grooves formed by two sets of spiral grooves in combination. In addition, the spiral grooves  2  make diametric reduction of the thrust hydrodynamic bearing  1  possible, which lets the motor be run at low peripheral speeds, and which is a way to decrease bearing losses. 
     Nevertheless, air bubbles will eventually become present in the area where the spiral grooves  2  are formed. The air bubbles will shift from the high end to the low end along the pressure gradient at which the thrust bearing  1  generates dynamic pressure. Accordingly, the air bubbles shift radially outward, toward where the pressure is lower. Herein, one way to exhaust the air bubbles to the bearing exterior is to arrange a communicating hole in the radially outward area of the bearing, as a communication to the bearing exterior. Because the bearing pressure distribution develops with axial symmetry, however, in the bearing center vicinity there will only be a slight pressure gradient. Despite the communicating hole, therefore, the air bubbles that eventually will be present nearby the center are less likely to be eliminated. 
     When air bubbles build up like this nearby the center of the bearing under a higher-temperature environment, they increase in volume because the coefficient of thermal expansion of the air bubbles is greater than that of the oil. The expanding air bubbles cause the oil to effuse to the bearing exterior. A similar phenomenon occurs even under a lower-temperature environment. Effusion of oil decreases the amount of oil retained in the bearing. Consequently the rigidity of the bearing declines and moreover the oil reserve depletes prematurely; and other problems, such as deterioration in endurance and degradation in reliability of the bearing, arise. 
     SUMMARY OF INVENTION 
     An object of the present invention is to eliminate air bubbles liable to build up nearby the bearing center, and thereby to yield stabilized axial bearing force, in a thrust hydrodynamic bearing in which dynamic pressure is generated by pump-in type spiral grooves. 
     Yet another object of the present invention is with a simple structure to eliminate to the bearing exterior air bubbles liable to build up nearby the center of a pump-in type thrust hydrodynamic bearing. 
     A still further object of the invention is to structure a pump-in type thrust hydrodynamic bearing for superior endurance and reliability of the bearing, and of a spindle motor furnished with the bearing. 
     An additional object is to provide a disk drive that operates stably over the long term. 
     In order to achieve the foregoing objects, in a pump-in type thrust hydrodynamic bearing construction according to the present invention an asymnmetrical auxiliary groove is added to spiral grooves formed circularly symmetrical with respect to the axial center. The addition of the asymmetrical auxiliary groove makes the circularly symmetrical pressure distribution created by the spiral grooves asymmetrical. Air bubbles building up nearby the center of the spiral grooves are thereby shifted to the area where the spiral grooves are formed. Around the inside of the area where the spiral grooves are formed, the dynamic pressure of the oil will be high. Air bubbles in a hydraulic fluid tend to shift from a region where the fluid pressure is high to a region where the fluid pressure is low. Owing to this tendency, air bubbles inside the area where the spiral grooves are formed get pushed out beyond the periphery. 
     This accordingly is a way surely and readily to eliminate air bubbles intermixing with the oil. Thus eliminating air bubbles lets movement of oil in the direction of the bearing center by virtue of the pump-in type spiral grooves occur smoothly, which yields stabilized axial bearing force. In addition, oil leakage from the bearings occurring due to air bubbles is effectively checked, which improves the bearing reliability and endurance. 
     As far as manufacturing is concerned the spiral grooves and/or the auxiliary groove can by formed by electrochemical machining, cutting, or pressworking processes. Further, the auxiliary groove should be of a shape or configuration that makes the pressure distribution that the spiral grooves develop asymmetric with respect to the axial center. For example, the auxiliary groove may be straight or circularly arcuate, and positioned radially inward of the spiral grooves, asymmetric with respect to the axial center and having a radially outward end from which the auxiliary groove extends towards the bearing center. Alternatively, the auxiliary groove may be formed by extending radially inward a part of the inner edge of the spiral grooves. Furthermore, furnishing in the outer margin of the thrust plate a communication hole that communicates with the bearing exterior is a way to exhaust air bubbles pushed out beyond the periphery of the spiral grooves. 
     In addition, the present invention can be realized as a spindle motor having a thrust bearing as described above. Spindle motors thus embodied may be employed in spindle motor applications for driving magnetic disks such as hard disks, magneto-optical disks, optical disks such as CD-ROMs and DVDs, and like recording disks. These spindle motors could be used under various environments, particularly in high-temperature, low-pressure environments. Air assimilated into oil under a high-temperature, low-pressure environment is liable to turn into bubbles. Air bubbles generated within the oil are eliminated surely and readily by establishing an auxiliary groove like the foregoing, associated with the thrust bearing spiral grooves. Consequently, problems originating in air bubbles, such as oil effusion and degradation in bearing rigidity, are effectively averted. 
     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 an elevational section view illustrating the configurational outline of a spindle motor in the present invention; 
     FIG. 2 is a partial section view of the bearing portion of the spindle motor, enlarged from FIG. 1; 
     FIG. 3 is a section view through the spindle motor sleeve, taken along A—A in FIG.  2  and seen in the direction of the arrows; 
     FIG. 4 is a plan view of an upper thrust bearing from the spindle motor shown in FIG. 1; 
     FIG. 5 is a plan view of a lower thrust bearing from the spindle motor shown in FIG. 1; 
     FIG. 6 is a plan view illustrating an example variation of the lower thrust bearing shown in FIG. 5; 
     FIG. 7 is a plan view illustrating dynamic pressure generating grooves formed in a conventional thrust hydrodynamic bearing for generating dynamic pressure; and 
     FIG. 8 is a diagram schematically representing a general disk drive in section. 
    
    
     DETAILED DESCRIPTION 
     Reference is made initially to FIG. 8, schematically diagramming the internal configuration of a general disk drive. The interior of its housing  71  is formed as a clean space in which dirt/dust is extremely slight. A spindle motor  72  onto which information-recording disks are mounted is installed within the clean space. In addition, a head-shifting mechanism  77  for reading/writing information from/onto the disks  73  is installed within the housing. The head-shifting mechanism  77  is composed of: heads  76  for reading/writing disk information; arms  75  for supporting the heads  76 ; and an actuator  74  for shifting the heads  76  on the arms  75  into requisite disk positions. 
     With reference then to FIGS. 1 through 6, the following describes embodiments of the present invention. For the following description, the illustrated spindle motor and hydrodynamic bearing depict an example in which the present invention is applied to a disk-drive spindle motor, which may be the motor  72  just mentioned. 
     FIG. 1 is a section through a spindle motor in which the present invention is adapted, giving an overview of the motor configuration. FIG. 2 then illustrates the bearing portion of the spindle motor, removed from the FIG. 1 section and enlarged for detail. Shown likewise in FIGS. 1 and 2, a shaft  12   a  and an annular thrust plate  12   b  located at the bottom end of the shaft  12   a  integrally form a rotary shaft part  12 . An annular recess  12   a   1  circularly arcuate in cross-section is formed in the outer circumferential surface of the shaft  12   a  at an axially intermediate portion. The outer diameter of the shaft  12   a  heading upward gradually contracts; i.e., the shaft  12   a  is slightly tapered along the upper-end outer circumferential surface. A smaller outer diameter portion  12   a   2  is formed continuous with the tapered portion of the shaft  12   a . The smaller outer diameter portion  12   a   2  is fixed to the inner circumferential surface of an installation hole  38   a  in a cap-shaped rotor hub  38 . A rotor magnet  42  is mounted on the inner circumferential surface of an outer circumferential wall portion  38   b  of the rotor hub  38 . 
     An upward-opening, male-threaded hole  13  oriented axially is furnished in the shaft  12   a . A clamp element (not illustrated) for retaining hard disks (also not illustrated), loaded around the outer surface of the rotor hub  38  circumferential wall portion  38   b , is screwed fast by a screw (again, not illustrated) fitted into the hole  13 . 
     The rotary shaft part  12  is accommodated within a stationary sleeve  14 . The sleeve  14  is formed axially penetrated by a central hole  14   a  made up of upper/lower smaller inner-diameter portions  14   a   1  , a medium inner diameter portion  14   a   2 , a larger inner-diameter portion  14   a   3 , as well as an annular channel  14   a   4 . Portions  14   a   1  oppose the outer circumferential surface of the shaft  12   a  via micro-gaps. Portion  14   a   2  is where the hole  14   a  radially enlarges to oppose the thrust plate  12   b  along its outer circumferential surface. Portion  14   a   3  is where the hole  14   a  radially enlarges further from portion  14   a   2 . The annular channel  14   a   4 , whose bottom is flat in cross-section, is in a position along the hole  14   a  that divides the portions  14   a   1  into upper and lower. The lower end opening of the hole  14   a  is closed off by a counter-plate  14   b  that mates exteriorly with the inner circumferential surface of the large inner diameter portion  14   a   2 , wherein the counter-plate  14   b  is fixedly fitted. The upper end opening of the hole  14   a  is open to the external atmosphere in a tapered area  24  defined between the tapered portion of the shaft  12   a  , and the inner circumferential surface of the radially opposing sleeve portion  14   a   1  . Finally, the sleeve  14  along its outer periphery is fitted into the inner circumferential surface of an annular cylindrical wall  36   a  furnished on a bracket  36 . 
     A stator  40  is installed on the outer circumferential periphery of the annular wall  36   a , in radial opposition to the rotor magnets  42 . 
     A radial gap enlargement  18  is defined between the annular recess  12   a   1  and the annular channel  14   a   4 . An intermediate vent hole  19  that, opening on the annular channel  14   a   4  and the outer circumferential surface of the sleeve  14 , opens the radial gap enlargement  18  to the bearing exterior, is provided on the sleeve  14 . Air taken in through the intermediate vent hole  19  is retained in the radial gap enlargement  18 . 
     Oil Vis retained axially above and below the radial gap enlargement  18 , between the outer circumferential surface of the shaft  12   a  and the inner circumferential surface of the smaller inner-diameter portions  14   a   1  of the central hole  14   a . In these respective areas, an upper radial bearing  20  and a lower radial bearing  22  are constituted. 
     The tapered area  24 , which as described earlier is defined between the tapered portion of the shaft  12   a  and the inner circumferential surface of the sleeve portion  14   a   1  , is located at the upper end of the upper radial bearing  20 . In the tapered area  24 , the radial dimension of the gap between the inner circumferential surface of the sleeve portion  14   a   1  and the outer circumferential surface of the shaft  12   a  gradually enlarges heading axially upward. The earlier described annular recess  12   a   1  in the outer circumferential surface of the shaft  12   a  is located at the lower end of the upper radial bearing  20 . The radial gap enlargement  18 , defined, as noted earlier, between the annular recess  12   a   1  and the annular channel  14   a   4  gradually contracts in dimension going axially upward/downward from where the intermediate vent hole  19  in the sleeve portion  14   a   1  opens. The interfaces between the oil V retained in the upper radial bearing  20  and the external air form in the positions where external-air surface tension pressures acting respectively on the oil V in the tapered area  24 , and in the radial gap enlargement  18 , balance. 
     When due to long-term use the oil V retained in the upper radial bearing  20  has decreased, oil V retained in the tapered area  24  and the radial gap enlargement  18  will replenish the upper radial bearing  20 . 
     The radial gap enlargement  18  meanwhile is located at the upper end of the lower radial bearing  22 . The upper end interface on the oil V retained in the lower radial bearing  22  is located beneath where the intermediate vent hole  19  opens on the radial gap enlargement  18 . 
     FIG. 3 depicts the sleeve  14  with the rotary shaft part  12  and counter-plate  14   b  removed, in a section taken along A—A through the spindle motor bearing portion shown in FIG.  2 . Herringbone grooves  20   a  are furnished on the inner circumferential surface of the smaller inner-diameter portion  14   a   1  in the upper end of the sleeve  14 . The herringbone grooves  20   a  are axially symmetrical, formed so that the part where the grooves fold over corresponds to the axially mid position in the upper radial bearing  20 . Rotation of the rotary shaft part  12  then generates oil dynamic pressure that acts heading from either marginal end (axially upper/lower ends) of the herringbone grooves  20   a  to where the grooves fold over. That is, the upper radial bearing  20  configuration generates a pressure peak in the axially mid position, and leaves the pressure lowest at either end. 
     On the other hand, herringbone grooves  22   a  are furnished on the inner circumferential surface of the smaller inner-diameter portion  14   a   1  in the lower end of the sleeve  14 . The herringbone grooves  22   a  are axially asymmetrical, formed so that the part where the grooves fold over is biased downward in the lower radial bearing  22 . Rotation of the rotary shaft part  12  then generates oil dynamic pressure that acts heading from either marginal end (axially upper/lower ends) of the herringbone grooves  22   a  to where the grooves fold over. That is, the lower radial bearing  22  configuration generates oil dynamic pressure having a pressure peak nearby the axially lower end, and leaves the pressure lowest at the upper end. 
     Pump-in type spiral grooves  26   a  as shown in FIG. 4 are formed as dynamic-pressure generating grooves along the inner circumferential margin of the sleeve  14  lower surface axially opposing the thrust plate  12   b  upper surface This composes an upper thrust bearing  26 . Meanwhile, pump-in type spiral grooves  28   a  as shown in FIG. 5 are formed as dynamic-pressure generating grooves in a ring-shaped area in the counter-plate  14   b  upper surface axially opposing the thrust plate  12   b  lower surface Likewise as with the upper thrust bearing  26 , this composes a lower thrust bearing  28 . When the rotary shaft part  12  rotates, the spiral grooves  26   a  and  28   a  generate dynamic pressure that acts on the oil V retained in the upper and lower thrust bearings  26  and  28 . The dynamic-pressure generating action of the grooves  26   a  and  28   a  is such that the pressure grows higher heading radially inward. 
     The clearance between the thrust plate  12   b  upper surface and the axially opposing sleeve  14  lower surface is narrower in the ring-shaped locus corresponding to the upper thrust bearing  26 —where the two surfaces confront in parallel—than radially outward of the locus. Proceeding radially outward beyond the locus, the clearance at which the two surfaces confront is structured to widen axially, then remain thus widened. 
     Likewise, the clearance between the thrust plate  12   b  lower surface and the axially opposing counter-plate  14   b  upper surface is narrower in the ring-shaped locus corresponding to the lower thrust bearing  28 —where the two surfaces confront in parallel—than radially outward of the locus. Proceeding radially outward beyond the locus, the clearance at which the two surfaces confront is structured to widen axially, then remain thus widened. 
     Then, oil V continuously fills the gaps: from that in the lower radial bearing  22  to begin with, to that in the upper thrust bearing  26  comprising the thrust plate  12   b  top and outer circumferential surfaces, and to that in the lower thrust bearing  28  comprising the under surface of the thrust plate  12   b.    
     When the rotary shaft part  12  rotates, the spiral grooves  26   a  generate radially inward acting dynamic pressure in the oil V retained in the upper thrust bearing  26 . The shaft  12   a , because it is located on the rotational center of the thrust plate  12   b , however, obstructs action of the spiral grooves  26   a  radially inwardly on the oil V. On the other hand, the herringbone grooves  22   a , axially unbalanced as described earlier, are formed to generate a pressure peak near the axial lower end of the lower radial bearing  22 —that is, neighboring the upper thrust bearing  26 . Meanwhile oil Vis retained continuously in between the lower radial bearing  22  and the upper thrust bearing  26 . Consequently, the mutual action of these two bearings generates an oil-pressure peak region nearby the boundary between the lower radial bearing  22  and the upper thrust bearing  26 . Accordingly, the lower radial bearing  22  and the upper thrust bearing  26  collaborate to generate the dynamic pressure needed to support the rotary shaft part  12 . 
     In contrast, in the lower thrust bearing  28 , an oil-compression action radially inward, arising from the spiral grooves  28   a , generates an oil pressure peak region nearby the rotational center of the shaft  12   a . The form that the oil pressure distribution arising from the spiral grooves  28   a  alone assumes is roughly symmetrical with respect to the axial center. 
     A circularly arcuate auxiliary groove  28   b  is furnished in the bearing center of the lower thrust bearing  28  (an area that matches the rotational center of the rotary shaft part  12 ). As shown in FIG. 5, the auxiliary groove  28   b  is disposed radially inward of the spiral grooves  28   a , extending radially outward from the axial center. 
     When the rotary shaft part  12  rotates, the auxiliary groove  28   b  induces dynamic pressure that, as the spiral grooves  28   a  do, compresses the oil V toward the bearing center. That is, from where the outer end of the auxiliary groove  28   b  is positioned, the distribution form of the dynamic pressure in the lower thrust bearing  28  becomes asymmetrical with respect to the axial center. This asymmetrical pressure distribution is due to the dynamic pressure generated by the auxiliary groove  28   b  in addition the dynamic pressure generated by the spiral grooves  28   a . Therefore, though air bubbles develop around the central portion of the thrust plate  12   b  lower surface, they get sent conversely to the spiral grooves along the pressure gradient at which the auxiliary groove  28   b  generates dynamic pressure in the oil. In the spiral grooves is a pressure distribution according to which the oil dynamic pressure gradually reduces from the inner margin to the outer margin. Since air bubbles by nature will shift from a region of high to a region of low oil pressure, ultimately the air bubbles are exhausted to the outer circumferential region of the lower thrust bearing  28 . The air bubbles are then sent in turn to the low-pressure outer circumferential area of the thrust plate  12   b  i.e., where the axial clearance between the thrust plate  12   b  lower surface and counter-plate  14   b  upper surface widens, as described earlier. 
     An axially extending breathing hole  34 , as FIG. 2 illustrates, is furnished in the sleeve  14 . One end of the breathing hole  34  is located at the circumferential margin of the upper thrust bearing  26 , and the other end is open to the external atmosphere. Air bubbles sent toward the outer peripheral area of the upper thrust plate  12   b  exhaust to the external atmosphere through the breathing hole  34 . 
     With regard to the configuration of the auxiliary groove, as shown in FIG. 6, an arcuate groove  28   c  of greater curvature than that of the auxiliary groove  28   b  is feasible. The auxiliary groove  28   b  is extended toward the inner margin of part of the spiral grooves  128   a  to be effective in sending air bubbles there. Since the crucial point is, with a dynamic-pressure groove asymmetrical with respect to the axial center, to make the generated dynamic pressure distribution asymmetrical with respect to the axial center, various configurations are feasible. 
     Furthermore, in the spindle motor embodiment illustrated in FIGS. 1 and 2, as already explained, an oil-pressure peak region is generated near the boundary between the lower radial bearing  22  and the upper thrust bearing  26 . Accordingly, should air bubbles develop in the oil present therein, the bubbles will shift to a region where the oil pressure is lower. Consequently, air bubbles arising on the lower radial-bearing end will be sent from the oil-pressure peak region just noted, toward the radial gap enlargement  18 , and exhausted through the intermediate vent hole  19  to the external atmosphere. Meanwhile, air bubbles arising on the upper thrust-bearing end will shift from the aforementioned oil-pressure peak region across the upper surface of the thrust plate  12   b , and reach the thrust plate outer circumferential gap. From there, the air bubbles will exhaust through the breathing hole  34  to the external atmosphere. 
     A bearing configured in the foregoing manner functions to exhaust air bubbles; moreover, the present invention is the realization of spiral grooves formed on the lower surface of the thrust plate  12   b  functioning to exhaust air bubbles more completely from the thrust bearing. 
     While embodiments of the invention in a thrust dynamic-pressure bearing, and spindle motor furnished therewith, have been explained above, the present invention is not limited to these embodiments. Various changes and modifications can be made to the embodiments herein set forth, without departing from the scope of the invention. 
     For example, in the foregoing embodiments, the configuration is furnished with only one auxiliary groove  28   b  or  28   c . A plurality of auxiliary grooves  28   b / 28   c  may be furnished, however, or the auxiliary groove(s) may be provided unitarily with the spiral grooves  28   a . Such alternative configurations are possible, as long as with the dynamic-pressure groove(s) asymmetrical with respect to the axial center, the generated dynamic pressure distribution is made asymmetrical with respect to the axial center. 
     In the foregoing embodiments, the spiral grooves are formed on the lower end surface of the sleeve and on the counter-plate. Given that, an auxiliary groove(s) can be formed on the surface opposing the counter-plate/spiral grooves, i.e. the thrust plate lower surface, as another example of a variation.