Abstract:
A method and apparatus for rotatably interconnecting computer storage disks to a stationary base is provided. The apparatus provides a hybrid bearing assembly having an oil-filled thrust bearing and an air-filled radial bearing. In particular, the apparatus provides a hybrid bearing having an oil-filled thrust bearing that is highly resistant to leakage. In addition, the apparatus provides a hybrid bearing having a radial air bearing with a very large diameter to provide great stiffness in a radial direction using common manufacturing tolerances. The method of the present invention includes rotatably interconnecting a hub for suspending computer storage media to a base or enclosure using an oil-filled thrust bearing and an air-filled radial bearing. The method further includes balancing the pressures in both directions along the axis of the thrust bearing to prevent leakage of the oil from the bearing. The method also includes providing an air bearing having a very large diameter.

Description:
FIELD OF THE INVENTION 
     The present invention relates to hybrid bearings, and in particular to hybrid bearings used in conjunction with hard disk drive spindle motors. The invention further relates to hybrid bearings used in disk drive spindle motors having an air radial bearing and a fluid thrust bearing. 
     BACKGROUND OF THE INVENTION 
     Disk drive memory systems store digital information on magnetic disks. The information is stored on the disks in concentric tracks divided into sectors. The disks themselves are mounted on a hub, which rotates relative to the disk drive enclosure. Information is accessed by means of read/write heads mounted on pivoting arms able to move radially over the surface of the disks. This radial movement of the transducer heads allows different tracks to be accessed. Rotation of the disks allows the read/write head to access different sectors on the disks. 
     In operation, the disk or disks comprising the magnetic media are rotated at very high speeds by means of an electric motor generally, but not necessarily, located inside the hub that supports the individual disks. Bearings mounted inside the hub allow the hub to rotate about a fixed shaft. Alternatively, the hub is fixed to a rotating shaft carried by bearings mounted to the base or enclosure of the disk drive. In either configuration, the bearings are typically ball-bearings or fluid bearings. Bearings having a fluid lubricant are desirable for disk drive applications because of their inherently low nonrepeatable runout and low acoustical noise. However, these bearings suffer from several shortcomings. For instance, oil filled bearing designs have been difficult to seal. In particular, where oil filled bearings are used to support a rotatable hub in radial and axial directions, it is extremely difficult to balance the pressure exerted on the oil between all of the surfaces of the bearing. As a result, oil can be forced from between the bearing surfaces, contaminating the interior of the disk drive. Such contamination may cause a failure or a decreased performance of the drive. Bearing systems incorporating an oil lubricant also have a limited maximum rotational speed, due to large power consumption at high speeds. 
     Alternative bearing designs have utilized air as the lubricant with bearings having grooved bearing surfaces to generate areas of increased pressure when the surfaces of the bearing move in opposition to each other. However, such designs typically only have a unidirectional thrust mechanism, and therefore the disk drive can only be operated when the device is in certain orientations (e.g. upright) or the device cannot withstand shock in certain directions (e.g. the axial direction). Furthermore, air bearing designs have typically featured a relatively small diameter radial bearing surface, resulting in bearings that have inadequate stiffness. Adequate stiffness is difficult to achieve in an air bearing because air has a viscosity that is much lower than the viscosity of oil or other conventional liquid lubricants. Therefore, conventional air bearing designs result in a bearing that cannot maintain the rotating components in a precise relationship to the stationary components when bearings constructed in accordance with those designs are subjected to external forces. Stated differently, a bearing that lacks stiffness will allow the rotating disks to deviate from the desired alignment when the drive is subjected to external forces. 
     Air is desirable as a bearing lubricant because its use removes concerns about leakage and outgassing resulting from the presence of oil. In addition, the viscosity of air varies less with changes in temperature compared to the viscosity of oil or other lubricants. Furthermore, air bearings generate less acoustical noise and less nonrepeatable runout than ball-bearing designs, and consume less power in comparable situations due to decreased friction compared to oil filled bearings. However, existing air bearing designs use extremely high rotational speeds and/or extremely tight internal clearances to increase the stiffness of the bearing in order to achieve stiffness levels that are comparable to the stiffness of oil-filled bearings. This is due in large part to the fact that the viscosity of air is approximately {fraction (1/700)}th the viscosity of oil. However, increased rotational speeds generally reduce the storage capacity of the disk drive because of limitations in read/write channel data rates. Also, the tight internal clearances typically employed by known air bearing designs increase manufacturing costs. 
     It would be desirable to provide a bearing system for a disk drive motor assembly that presents a low risk of contamination to the storage media, and one that is stiff enough to support a heavy disk pack and/or withstand external shocks. In addition, it would be desirable to provide such a device with reduced friction, reduced power consumption and wear and tear, and a longer life than conventional bearing systems. Furthermore, it would be desirable that such a device be easy to manufacture in large volumes and at low cost. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an apparatus and method for supporting a rotating component is disclosed. In particular, the invention provides a computer disk drive spindle bearing having an air-filled bearing for support in a radial direction and an oil-filled bearing for support in an axial direction. In a preferred embodiment, the air bearing has a large diameter with respect to the spindle. In addition, the present invention provides an oil-filled bearing that is resistant to leakage for supporting the rotating hub in an axial direction. 
     The device generally includes a stationary shaft fixed or interconnected to a base. Affixed to the stationary shaft is a thrust plate for use as a part of an oil filled bearing. The thrust plate cooperates with upper and lower bearing plates interconnected to a hub to support the hub in an axial direction. The space between the thrust plate and the upper and lower bearing plates is filled with a viscous oil. The viscous oil insures that the bearing has high stiffness and high load capacity. 
     Also interconnected to the stationary shaft is an air bearing element having a large diameter. A large diameter is desirable because the stiffness of such a bearing increases with the cube of the diameter. In a preferred embodiment, the air bearing element substantially fills the volume defined by the interior surface of the rotating hub. In a more preferred embodiment, the diameter of the air bearing is greater than or equal to the diameter of the thrust plate of the oil filled bearing. In another preferred embodiment, the volume of the air bearing element is at least about 80% of the volume defined by the interior surface of the rotating hub. 
     The air bearing is also preferably relatively tall. As with the relatively large diameter, a relatively tall bearing is desirable because it provides greater stiffness and greater load capacity. Furthermore, increases in stiffness and load capacity gained by making the bearing larger can allow the bearing to be manufactured to tolerances that are no more stringent than those typically maintained for oil-filled bearing disk drive designs. In a preferred embodiment, the length of the air bearing is at least about 50% of the height of the hub surface about which storage media disks are stacked. In a more preferred embodiment, the air bearing extends from the center line of the rotor of the electric motor used to rotate the hub about the spindle, to a level at least about halfway along the hub between the hub flange and the disk clamp. 
     As discussed above, the relatively large size of the air bearing element increases the stiffness of the air bearing. In addition, the air bearing element may be provided with pressure generating ridges or grooves. These ridges or grooves direct air away from the edges of the bearing to increase the air pressure along the surface of the bearing, improving the stiffness of the bearing. Increased stiffness allows the bearing to support a heavier disk pack, and to avoid damage that might otherwise be sustained due to external forces or shocks. 
     The bearing of the present invention also provides a disk drive spindle and housing that are themselves very stiff. This is due to the use of a stationary shaft, which can be affixed to the enclosure at both ends to ensure a very stiff disk drive assembly. The increased stiffness of the design improves the accuracy of the read/write head in relation to the storage disks, and lessens low frequency vibrations. 
     The unique combination of an oil-filled bearing for supporting the rotatable hub in an axial direction and an air-filled bearing for supporting the rotatable hub in a radial direction provides a bearing apparatus having lower friction than an all oil-filled bearing design. Therefore, the bearing of the present invention consumes less power in operation and permits the disks to rotate at higher speeds than an all oil-filled bearing design having similar load bearing capabilities. In addition, the bearing of the present invention has greater load capacity than an air bearing of similar dimensions, because an air bearing must be extremely large to provide comparable support in an axial direction. Therefore, the present design offers a bearing with a higher load capacity and/or smaller size than an all air bearing design. Yet another advantage of the bearing of the present invention is the low nonrepeatable runout obtained by using fluid filled bearings exclusively. Therefore, runout problems associated with ball bearing or contact bearing designs are avoided. 
     Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cutaway view of a disk storage drive having an oil-filled thrust bearing and an air-filled radial bearing in accordance with an embodiment of the present invention; 
     FIG. 2 is a top view of a thrust plate of an oil-filled bearing according to one embodiment of the present invention; 
     FIG. 3 is a side view of an air bearing element having a grooved surface in accordance with one embodiment of the present invention; and 
     FIG. 4 is a cutaway view of a disk storage drive having an oil-filled thrust bearing and an air-filled radial bearing in accordance with yet another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, a hybrid disk drive bearing having an oil-filled thrust bearing and an air-filled radial bearing is provided. 
     With reference to FIG. 1, a hybrid bearing having oil-filled and air-filled bearing components constructed in accordance with one embodiment of the present invention is generally identified as hybrid bearing  104 . The hybrid bearing  104  generally comprises a rotatable hub assembly  108 , a stationary base assembly  112 , rotating oil-filled bearing  116 , and a rotating air-filled bearing  120 . The rotatable hub assembly  108  generally comprises a hub  124 . The hub  124  includes a retaining clamp groove  128 , and a flange  132  to support storage disks (not shown) that are stacked on the hub  124  when the device is used in a computer storage drive. The hub  124  typically features a constant diameter outer mounting surface  136  to securely locate the storage disks (not shown) in a radial direction. The retaining clamp groove  128  is adapted to receive a retainer (not shown) to securely hold the disk stack (not shown) against the flange  132 . 
     The rotating oil-filled bearing  116  generally comprises an upper bearing plate  140 , a lower bearing plate  144 , and a spacer  148 . The upper  140  and lower  144  bearing plates are generally annular in shape and are fixed to the hub  124 . Annular spacer  148  is interposed between the upper  140  and lower  144  bearing plates to maintain space  152  between the bearing plates  140  and  144 . 
     A number of bearing components are fixed or interconnected to the base plate  156 . These include a spindle  160 , an air bearing element  164 , an oil-filled bearing thrust plate  168 , and a stator assembly  172 . The spindle  160  is affixed to the base  156  at a first end and may also be secured to the top of the disk drive enclosure (not shown) at a second end using a fastener (not shown) inserted into a threaded hole  176  or by other means of attachment known in the art. Support of the shaft  160  on both ends is desirable because it increases the overall stiffness of the disk drive assembly. 
     The stationary air bearing element  164  is concentric to the spindle  160 , and may be formed from a separate piece of material, or may be integral to the spindle  160 . The air bearing element  164  has a very large diameter to increase the stiffness of the air bearing. The air bearing sleeve  180 , which is part of the rotating hub assembly  108 , generally encloses the diameter of the air bearing element  164 . The air bearing sleeve  180  has an inside diameter that is slightly larger than the outside diameter of the air bearing element  164 , so that a gap  184  is formed between the air bearing element  164  and the air bearing sleeve  180 . The size of the gap  184  influences the stiffness of the bearing. As the gap  184  size decreases, the stiffness of the air bearing  120  increases. However, due to the large air bearing element  164  provided by the present invention, manufacturing tolerances of the air bearing element  164  and the air bearing sleeve  180  need not be any more stringent than the tolerances adhered to in conventional disk drive spindle bearing designs. In a preferred embodiment, the diameter of the air bearing element  164  is at least about 60% of the outside diameter of the hub outer mounting surface  136 . In a more preferred embodiment, the diameter of the air bearing element  164  is at least about 75% of the outside diameter of the hub outer mounting surface  136 . In yet another preferred embodiment, the diameter of the air bearing element  164  is at least as large as the thrust plate  168  of the oil-filled bearing  116 . 
     The length or height L 1  of the air bearing  120  also influences the stiffness of the air bearing  120  in a radial direction. According to a preferred embodiment of the present invention, the length L 1  of the air bearing element  164  is at least about 50% of the length L 2  of the constant diameter outer mounting surface  136  of the hub  124 . In a more preferred embodiment, the length of the air bearing element  164  is at least about 50% of the vertical distance D 1  between the horizontal centerline of the stator assembly  172  and the upper most extent of the constant diameter outer mounting surface  136 . 
     The thrust bearing, generally identified as oil-filled bearing  116 , generally comprises the aforementioned upper and lower bearing plates  140  and  144 , and the thrust plate  168 . The upper and lower bearing plates  140  and  144  rotate relative to the thrust plate  168 , which substantially occupies the space  152  between the upper and lower bearing plates  140  and  144 . The space  152  that is not occupied by the thrust plate  168  is filled with oil  188  that prevents direct contact between the bearing plates  140  and  144  and the thrust plate  168 . Although the space between the upper and lower bearing plates  140  and  144  and the thrust plate  168  is relatively small towards an outside diameter of the thrust plate  168 , the space increases towards the inside diameter of the upper and lower bearing plates  140  and  144 . These spaces are the result of tapers  192  on the opposing faces of the upper and lower bearing plates  140  and  144 . These tapers create capillaries  196 , which serve to retain the oil  188  in the space between the upper and lower bearing plates  140  and  144  and the thrust plate  168 . In addition to lubricating the oil bearing  116  components, the oil  188  creates a seal between the air bearing  120  region and the top of the hybrid bearing  104 . In one embodiment, the oil  188  may be electrically conductive. 
     The capillaries  196  retain the oil  188  in position by taking advantage of the surface tension of the oil  188 . In addition, when the drive is in use and the rotating hub  108  is rotating relative to the base assembly  112 , centripetal forces tend to force the oil  188  towards the outer diameter of the oil filled bearing  116 . Since this area is completely sealed by the upper and lower thrust plates  140  and  144  and the spacer  148 , the oil  188  will be prevented from leaking out of the oil-filled bearing  116 . 
     As loads, such as external shocks, are applied to the oil-filled bearing  116 , the pressure exerted on the oil  188  will differ depending on the area under consideration. For example, if the rotating hub  108  is forced down, towards the base  156 , the pressure on the oil  188  will be at a maximum between the upper bearing plate  140  and the top of the thrust plate  168 . Typically, in response to such a pressure, some of the oil  188  would be forced out of the oil-filled bearing  116  between the upper bearing plate  140  and the thrust plate  168 . To prevent oil  188  from leaking out of the oil-filled bearing  116  due to such occurrences, axial or through-hole passageways  200  are provided in the thrust bearing  168 . The axial ports  200  allow oil to flow from an area of relatively high pressure on one side of the thrust plate  168  to an area of relatively low pressure on the other side of the thrust plate  168 . In addition, the thrust plate  168  may be provided with radial passageways  204  to aid in the flow of oil  188  from areas of higher pressure to areas of lower pressure. According to one embodiment of the present invention, a plurality of oil-filled thrust bearings may be provided. 
     Referring now to FIG. 2, a top view of thrust bearing  168  is illustrated. In FIG. 2, an arrangement of axial through-hole passageways  200  and radial passageways  204  according to one embodiment of the present invention is illustrated. Of course, the particular arrangement of passageways  200  and  204  S can be varied according to where high pressure areas are expected to occur, and the number of passageways  200  and  204  may also be varied. Additionally, the size and configuration of the passageways  200  and  204  may be varied in balancing the stiffness of the oil-filled bearing  116  against allowing the oil  188  to flow freely within the oil-filled bearing  116 . 
     The oil  188  of the oil-filled bearing  116  may be any viscous oil suitable for lubrication. According to one embodiment of the present invention, the oil  188  may be electrically conductive. In yet another embodiment, the oil  188  may be magnetically conductive. The oil  188  may be synthetic or mineral based. Furthermore, the oil  188  may be any viscous fluid. Preferably, the oil  188  has a viscosity that remains relatively constant as the temperature of the oil  188  changes, and that has lubricating properties to reduce wear on the oil-filled bearing  116  components. Also, it is preferable that the oil  188  exhibit low outgassing. 
     Referring now to FIG. 3, an air bearing element  164 , according to one embodiment of the present invention is illustrated. The air bearing element  164  may be provided with grooves  300  designed to increase or generate air pressure towards middle portions of the air bearing element  164 ,and away from the top  304  and bottom  308  edges of the air bearing element  164 . In the illustrated embodiment, the grooves  300  are shown as parallel rows of chevron-shaped grooves. However, any design suitable for drawing air away from the edges  304  and  308  of the air bearing element  164  and thereby increasing the air pressure along intermediate portions of the air bearing element  164  is suitable. For example, alternative grooves may be in the form of generally arcuate grooves, one row of chevron-shaped grooves, staggered grooves in parallel rows, etc. In yet another preferred embodiment, the width of the groove  300  is equal to the width of the land area  312  between adjacent grooves  300 . In another preferred embodiment, the cross section of the grooves  300  is semicircular. In a further preferred embodiment, the cross section of the grooves  300  is square. 
     As an alternative to grooves  300  formed in the surface of the air bearing element  164 , grooves may be formed in the surface of the air bearing sleeve  180 . However, if formed in the sleeve  180  the grooves would generally point in the direction opposite of how they would point if formed on air bearing element  164 . This is because the relative air flow against air bearing element  164  is opposite that of the air flow relative to air bearing sleeve  180 . According to yet another embodiment of the present invention, the air pressure towards the medial portions of air bearing element  164  is increased or generated by means of protrusions on the surface of the air bearing element  164 . As yet another embodiment of the present invention, protrusions may be formed on the interior surface of sleeve  180 . As with grooves, the configuration and size of protrusions can be varied, so long as they generate increased air pressure along intermediate portions of the air bearing. 
     Regardless of whether grooves or protrusions are used to increase air pressure, they generally should not be formed on both the air bearing element  164  and the air bearing sleeve  180 . This is because having such features on both surfaces prevents the proper development of high pressure areas. In addition, providing air pressure generating features on both surfaces increases manufacturing costs. 
     Referring again to FIG. 1, the stator assembly  172  includes laminations  208  and coils  212 . When the coils  212  are supplied with an electrical current, a magnetic field is produced. This magnetic field is directed by the positioning of the coils  212  and by the positioning of the laminations  208 . Generally, the magnetic field is directed towards magnets  216  that are affixed to the air bearing sleeve  160 . The magnetic field generated by the stator assembly  172  interacts with the magnets  216  that are interconnected to the hub assembly  108 . This interaction causes the rotation of the hub assembly  108  relative to the base assembly  112 . The stator assembly  172  is generally arranged in a circle about the magnets  216  of the hub assembly  108 . 
     According to the above-described embodiment, the air bearing sleeve  180  is preferably constructed from magnetic steel. Steel is desirable in this application because it can be machined to fine tolerances, assisting in the accurate formation of the gap  184  between the air bearing element  164  and the sleeve  180 . In addition, the magnetic properties of steel enhance the interaction between the magnetic field produced by the coils  212  of the stator assembly  172  and the magnets  216 . The air bearing element  164 , and the upper and lower bearing plates  140  and  144 , the thrust plate  168 , and the spindle  160  may also be constructed from steel according to an embodiment of the present invention. Preferably, the hub  124  and the base  156  are constructed of aluminum. Aluminum is desirable in these components because it is easily machined, lightweight, can be machined to fine tolerances and is not magnetically conductive. Alternatively, the above-described components may be constructed from a ceramic or composite material. 
     In a preferred embodiment, the diameter of the spindle  160  is from about 4 mm to about 6 mm, the diameter of the air bearing element  164  is from about 14 mm to about 20 mm, the gap  184  between the air bearing element  164  and the sleeve  180  is from about 0.001 mm to about 0.005 mm, and the height of the air bearing element  164  is from about 10 mm to about 18 mm. The diameter of the thrust plate  168  may be from about 7 mm to about 12 mm, the thickness of the thrust plate  168  may be from about 1 mm to about 4 mm. The height of the spacer  148  is from about 1 mm to about 4 mm and the tapers  192  of the upper and lower bearing plates  140  and  144  may be from about 2° to about 10°. 
     Referring now to FIG. 4, another embodiment of the hybrid bearing of the present invention, identified generally as hybrid bearing  404 , is illustrated. The hybrid bearing  404  generally includes a rotatable hub assembly  408  and a stationary base assembly  412 . The bearing elements of hybrid bearing  404  are the oil-filled bearing assembly  416  and the air-filled bearing assembly  420 . Many of the structural elements and features of the embodiment identified as hybrid bearing  404  are the same or similar to the features of the hybrid bearing  104 . Therefore, the following discussion will concentrate on the areas in which these embodiments differ. 
     The rotatable hub assembly  408  generally includes the hub  424 , which features a retaining clamp groove  428  and a flange  432 . The hub  424  may include a constant diameter outer mounting surface  436  to locate storage disks (not shown) in a radial direction. 
     The rotating oil-filled bearing  416  comprises upper  440  and lower  444  thrust bearing plates, and a spacer  448 . A space  452  is maintained between the upper  440  and lower  444  bearing plates by the spacer  448 . 
     The base plate  456  is interconnected to a first end of a spindle  460  through an air bearing element  464 . Thus, according to this embodiment, a portion of the circumference of the air bearing element  464  is in closely-fitting contact with the base plate  456 . A stationary thrust plate  468  is affixed to a second end of the spindle  460 . A threaded hole  476  may be provided in the second end of the spindle  460  for securing the second end of the spindle to the top of the disk drive enclosure (not shown). 
     A rotating air bearing sleeve  480  is interconnected to the interior of the hub  424 . According to the hybrid bearing  404  of the embodiment illustrated in FIG. 4, the air bearing sleeve  480  is constructed from a ceramic or composite material, or from some other material that is not magnetically conductive. Therefore, hybrid bearing  404  includes a back iron component  484 . The back iron  484  is constructed from a magnetically conductive material, such as steel, and is affixed to either the air bearing sleeve  480  or the hub  424 . Affixed to the outer circumference of the back iron  484  are magnets  488 . The magnets  488  interact with the stator  472  of the base assembly  412 . In another embodiment, the air bearing sleeve may be constructed from a magnetically conductive material and the back iron may then be omitted. 
     The hybrid bearing  404  also features an upper thrust bearing plate  440  that is integral to the portion of the hub  424  that includes the retaining clamp groove  428 . Although the spacer  448  is illustrated as a separate component, it could also be combined with upper thrust bearing plate  440 . Alternatively, spacer  448  could be integral to the hub  424 , or to the lower thrust bearing plate  444 . Combining these components into one integral unit can increase the strength of the hub assembly  408 . Of course, additional divisions or combinations of components are possible. However, it is important that the upper bearing plate  440  and the lower bearing plate  444  not be integral to each other, to allow insertion of the thrust plate  468  into the oil-filled bearing  416  as it is being assembled. 
     The invention in its broader aspects relates to a bearing apparatus used to support a rotating assembly. The apparatus is suitable for use with any rotating assembly, and in particular with computer storage devices, such as disk drives. The apparatus provides suitable levels of bearing stiffness, while being inexpensive to manufacture and consuming relatively little power. The apparatus also features very little nonrepeatable runout, allowing data to be stored on rotating storage media in high densities. Furthermore, the apparatus is inexpensive to manufacture and it has been designed to operate reliably. 
     The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, within the skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain the best mode presently known for practicing the invention and to enable others skilled in the art to utilize the invention and such, or in other, embodiments and with various modifications required by the particular application or use of the invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.