Abstract:
A disk drive spindle air bearing is disclosed having increased bearing stiffness, while being capable of manufacture using conventional tolerances. The invention therefore allows the construction of a disk drive spindle bearing without the need for oil or grease that may potentially contaminate the storage disks. The disclosed disk drive spindle air bearing also provides an air bearing having low acoustical noise and power consumption characteristics.

Description:
FIELD OF INVENTION 
     The present invention relates to air bearings, and in particular to air-bearings used in conjunction with hard disk drive spindle motors. The invention further relates to air bearings used in disk drive spindle motors having an insideout motor design, or alternatively, an underslung motor design. 
     BACKGROUND OF THE INVENTION 
     Disk drive memory systems store digital information on magnetic disks. The information is stored in concentric tracks divided into sectors. The disks themselves are rotatably mounted on a spindle, and information is accessed by means of read/write heads mounted on pivoting arms able to move radially over the surface of the disk. This radial movement of the transducer heads allows different tracks to be accessed. Rotation of the disk allows the read/write head to access different sectors on the disk. 
     In operation, the disk or disks comprising the magnetic media are rotated at very high speeds by means of an electric motor generally located inside the hub that supports the individual disks. Bearings mounted inside the hub allow the hub to rotate about a fixed shaft. These 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 run out and low acoustic noise. However, these bearings suffer from several shortcomings. For instance, the oil used to provide the fluid bearing has a tendency to leak and outgas. Therefore, such bearings may lead to contamination of the interior of the disk drive. Such contamination may cause a failure of the drive in the form of data errors. Bearing systems incorporating an oil lubricant also have a limited maximum rotational speed due to their large power consumption at high speeds. 
     Alternative designs have utilized air bearings having grooved surfaces to generate areas of increased pressure when the surfaces of the bearing move in opposition to each other. However, such designs have typically had only 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, previous designs have featured relatively small-diameter radial bearing surfaces, resulting in bearings that have inadequate stiffness. 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. 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 lubricants. 
     Other bearing designs have utilized pressurized gas as a lubricating fluid. Such designs require an external supply of pressurized air and so would not be suitable for a disk drive application. 
     Air is desirable as a bearing lubricant because its use removes concerns about leakage and outgassing. In addition, the viscosity of air does not vary with changes in temperature as much as does the viscosity of oil or other lubricants. Furthermore, air bearings provide lower acoustical noise characteristics and less non-repeatable run out than ball-bearing designs and lower power consumption due to decreased friction than oil-filled bearings. However, known designs using air as a lubricant have used extremely high rotational speeds or extremely tight internal clearances or both to increase the stiffness of the bearing in order to achieve stiffness levels that are comparable to the stiffness of oil filled bearings. A bearing that lacks stiffness will allow the rotating disks to deviate from the desired alignment when the drive is subjected to external forces. High rotational speeds and tight clearances have been necessary in conventional air bearings because the viscosity of air is approximately 1/700 that 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 tremendously. 
     Other air bearing designs are physically larger in size than conventional oil filled bearings, and are therefore unsuitable for small form factor drives. Also, these other designs have a relatively large number of parts, increasing manufacturing costs. 
     It would be desirable to provide a bearing system for a disk drive motor assembly that utilized air as the fluid medium between bearing surfaces. In addition, it would be desirable that such a device be easy to manufacture in large volumes and at low cost. Furthermore, it would be advantageous to provide a bearing having adequate stiffness, while providing enhanced performance, lowered power consumption and wear and tear, and having a longer life than conventional bearings. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an air bearing apparatus for use in hard disk drive spindle motors. In particular, the invention provides an air bearing having a large surface area, to increase the stiffness of the bearing, while allowing the bearing to be manufactured with conventional oil filled bearing type tolerances. In a preferred embodiment, the air bearing is used in conjunction with an inside out underslung motor to further increase the area of the bearing. In addition, the present invention includes a method for providing a disk drive device with a bearing having air as its lubricating fluid, and providing adequate levels of stiffness while being capable of manufacture using conventional tolerances. 
     The device includes a computer hard disk drive having a base. Affixed to the base is a stationary shaft having an enlarged bearing portion and a spindle portion. The diameter of the bearing portion of the shaft is approximately four times greater than that of the spindle portion. Enveloping the bearing portion of the stationary shaft is a hub having an internal cylindrical bore that is concentric to the stationary shaft and adjacent to the bearing portion of that shaft. The top portion of the cylindrical bore is adjacent to the top of the bearing. Also interconnected to the hub is a thrust plate, concentric to the stationary shaft and adjacent to a bottom of the bearing. Between the cylindrical bore in the hub and the bearing portion of the stationary shaft, and between the thrust plate and the bottom of the bearing, are fluid filled gaps. In a preferred embodiment, the fluid filling these gaps is air. In a further preferred embodiment of the device, the bearing has a plurality of grooves on the top, side and bottom surfaces of the bearing. In a most preferred embodiment, the device further includes grooves on the top, side, and bottom surfaces of the bearing that are arrayed in a herring bone pattern, and that have a square or semi-circular cross section. 
     In a further embodiment, a disk storage drive is disclosed having a stationary shaft with a bearing portion having a length that is less than about 90% of its diameter. The device further has a hub portion defining an interior volume, and a sleeve interconnected to the hub. The sleeve is concentric to the stationary shaft and adjacent to the bearing, and has an annular top portion concentric to the stationary shaft and adjacent to a top of the bearing. An annular thrust plate is also interconnected to the hub such that it is concentric to the stationary shaft and adjacent to a bottom of the bearing. Between the sleeve and the bearing portion and between the annular thrust plate and the bearing portion are fluid filled gaps. According to this embodiment, the bearing portion of the stationary shaft substantially occupies the internal volume of the hub. 
     In an additional embodiment of the present invention, a motor assembly for use in a magnetic disk drive system is disclosed. The assembly features a base, a cylindrical bearing interconnected to the base, a rotatable hub disposed about and concentric to the bearing, a stator interconnected to the base and disposed radially about an axis of rotation of the hub, and magnetic means interconnected to the hub. The interior of the hub has a surface defining a cylindrical volume that is substantially filled by the bearing. An annular thrust plate is adjacent to a bottom of the bearing. In a preferred embodiment, a cylindrical sleeve member is affixed to the hub and interposed between the cylindrical volume of the hub and the bearing. In a further preferred embodiment of the invention, the motor assembly stator defines an inner diameter, and the magnetic means is disposed about and outside of that diameter. In an alternative preferred embodiment, the stator defines an outer diameter, and the magnetic means is disposed within the diameter of the stator. In a most preferred embodiment, the sleeve member is made from a ferromagnetic material. 
     In another embodiment, the present invention provides an air bearing motor assembly having a base, a stationary shaft affixed to the base, and a stationary annular bearing disposed about the shaft, wherein the bearing has an outer diameter that is at least about four times the diameter of the shaft. The air bearing motor further has a rotatable hub disposed about the shaft, and a sleeve affixed to the inside of the hub. The sleeve has an upper annular portion and a cylindrical side portion, with a diameter that is slightly greater than the diameter of the bearing. The bottom portion of the sleeve extends beyond a bottom portion of the hub. Interconnected to the hub is an annular thrust plate adjacent to a bottom of the sleeve. A stator is affixed to the base such that it can interact with magnetic means interconnected to the hub. 
     In yet another embodiment, a disk storage unit is provided having a cylindrical bearing. The cylindrical bearing has a top, a side, a bottom and a diameter. A hub having a cylindrical inner surface with a diameter that is larger than the diameter of the bearing encloses the top and side of the cylindrical bearing such that a fluid filled gap is formed. The volume defined by the cylindrical inner surface is substantially equal to a second volume defined by the bearing. Furthermore, the volume of the cylindrical inner surface of the hub is substantially equal to a volume described by an outer surface of the hub. An annular thrust plate interconnected to the hub is positioned such that a fluid filled gap is formed between the thrust plate and the bottom of the bearing. In a preferred embodiment, the volume of the cylindrical inner surface of the hub is at least about 80% of the volume described by the outer surface of the hub. 
     In a further embodiment of the present invention, a method is provided for supplying an air filled bearing for use in a disk drive spindle motor. The bearing is enclosed within a closely fitting surface interconnected to a rotatable hub. The volume enclosed by the hub is substantially filled by the bearing to maximize the surface area of the bearing. In a preferred embodiment, the side, top and bottom surfaces of the cylindrical bearing are provided with grooves to increase air pressure in the medial portions of the bearing when the hub is rotating about the stationary shaft. 
     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 drive spindle air bearing having an underslung motor design in accordance with one embodiment of the present invention; 
     FIG. 2 is a side view of a spindle bearing having a grooved surface in accordance with one embodiment of the present invention; 
     FIG. 3 is a top view of a spindle bearing having a grooved surface in accordance with one embodiment of the present invention; 
     FIG. 4 is a detail illustrating the geometry of an individual groove comprising the grooved surface illustrated in FIG. 3 in accordance with one embodiment of the present invention; and 
     FIG. 5 is a cutaway view of a disk drive spindle air bearing having an inside-out underslung motor in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, a disk drive spindle air bearing is provided. 
     With reference to FIG. 1, an air bearing having an underslung motor design constructed in accordance with one embodiment of the present invention is generally identified as air bearing  104 . The air bearing  104  generally comprises a base assembly  108  and a hub assembly  112 . 
     The base assembly  108  generally comprises a base  116 , only the center portion of which is illustrated in FIG. 1, a stator assembly  120  and a spindle  124 . The spindle  124  includes an enlarged bearing portion  128 . The stator assembly  120  generally comprises laminations  132  and coils  136 . The laminations  132  are affixed to the base  116  and arranged radially about the longitudinal axis  140  of the spindle  124 , which is itself affixed to the base  116 . The coils  136  are disposed about the laminations  132 . In a preferred embodiment, the base  116  comprises cast aluminum, the spindle  124  comprises machined steel, the laminations  132  comprise thin sheets of a ferromagnetic material stacked on top of one another, and the coils  136  comprise an electrically conductive wire having an insulating exterior, wound about the laminations  132 . 
     The hub assembly  112  comprises the hub  144 , sleeve  148 , thrust plate  152 , back iron  156 , and magnets  160 . The hub  144  has an internal cavity enclosing the bearing portion  128  of the spindle  124  and the stator assembly  120 . The hub features a flange  164  onto which magnetic storage disks (not shown) may be stacked and supported. The hub  144  also features a clamp  168  to which a retainer (not shown) may be affixed to retain the magnetic disks (not shown). The upper portion of the internal cavity of the hub  144  defines a first cylindrical space having a length and a first diameter. Affixed to this upper portion of the cavity of the hub  144  is the sleeve  148 . The interior of the sleeve  148  has an annular upper surface  172  and a cylindrical side surface  176 . The exterior of the sleeve  148  is sized such that it can be securely mounted within the upper portion of the internal cavity of the hub  144 , while the interior of the sleeve  148 , which generally defines a cylindrical volume, has dimensions that are slightly larger than the length and diameter of the spindle bearing  128 . Thrust plate  152  has an annular shape, and is adjacent to the bottom of the sleeve  148 . The thrust plate  152  may be affixed to the hub  144 , or to the sleeve  148 . 
     A skirt portion  180  of the hub  144  defines a second cylindrical space in the interior of the hub  144 . Affixed to the hub  144 , and located within the skirt portion  180  of the hub  144  is the back iron  156 . The back iron  156  is generally cylindrical in shape and is preferably made from a ferromagnetic material such as iron. Affixed to the back iron  156  are a plurality of magnets  160 . The positioning of the magnets on the interior surface of the back iron  156  and within a circumference generally defined by the skirt portion  180  of the hub  144  positions them radially about the stator assembly  120 . 
     In operation, the hub assembly  112  rotates about the longitudinal axis  140  of the spindle  124 . The impetus for this rotation is provided by the motor  182 , which generally comprises the stator  120 , and the magnets (or rotor)  160 . Energy to impart this motion is provided by an electrical current sent through the coils  136  of the stator assembly  120 , which creates a magnetic field about and through the laminations  132 . The interaction of this magnetic field with the magnetic field of the magnets  160  of the hub assembly  112  causes the hub assembly  112  to rotate relative to the base assembly  108 . 
     While the hub assembly  112  is rotating relative to the base  116 , resistance to forces along the longitudinal axis  140  of the spindle  124  is provided by high pressure air in the upper  184  and the lower  188  annular spaces. These areas of high pressure air are created by a laminar air flow that is created when the upper surface  172  of the sleeve  148  rotates relative to the upper annular surface  192  of the spindle bearing  128 , and the thrust plate  152  rotates relative to the lower annular surface  196  of the spindle bearing  128 . an Resistance to radial forces is provided by high pressure air in the cylindrical gap between the side of the spindle bearing  128  and the cylindrical side surface  176  of the sleeve  148 . This high pressure air is created by a laminar air flow created when the sleeve  148  rotates relative to the spindle bearing  128 . 
     As can be seen from the embodiment illustrated in FIG. 1, the spindle bearing  128  is relatively large, and it substantially fills the enclosed volume defined by the upper interior surfaces of the hub  144 . Furthermore, the spindle bearing  128  has a volume slightly less than the enclosed volume defined by the interior surfaces located between the sleeve  148  and the thrust plate  152  on the one hand, and the spindle bearing  128  on the other hand. This large size is advantageous, because it increases the stiffness of the bearing. The relatively large size of the bearing allows it to have a stiffness that approximates the stiffness of a conventional oil bearing, even when the fluid filling the bearing is air. This is so even though the viscosity of air is approximately 1/700 the viscosity of oil. Furthermore, the disclosed design allows an air bearing having suitable stiffness characteristics to be manufactured using conventional oil-filled bearing tolerances. In addition, the disclosed design provides adequate stiffness even at conventional disk drive rotational speeds (e.g., 7200 rpm). 
     In accordance with one embodiment of the present invention, the side surface  200  of the spindle bearing  128  is grooved. With reference now to FIG. 2, the radial pressure grooves  204  provided according to this embodiment generally comprise parallel rows of grooves having a herring-bone shaped pattern. Preferably, the cross-section of the radial pressure grooves  204  is square, although suitable radial pressure grooves  204  can be constructed using other profiles, such as semi-circular or triangular. In a preferred embodiment, the ratio of the width of the radial pressure grooves  204  to the land  208  between the grooves is 1:1. 
     The radial pressure grooves  204  increase the air pressure in the annular space defined by the gap between the cylindrical side surface  176  of the sleeve  148  and the side surface  200  of the spindle bearing  128  when the hub assembly  112  rotates relative to the base assembly  108 . In the illustrated embodiment, the grooves are designed so that the air pressure in the aforementioned annular space is increased when hub assembly  112  rotates about the spindle bearing  128  in the direction in which the herring-bone pattern points. Specifically, the rotation of the sleeve  148  relative to the spindle bearing  128  creates a flow of air about the spindle bearing  128  in the same direction that the sleeve  148  is rotating. The radial bearing grooves  204  tend to pull air towards the center of each row of grooves  204 , thus creating areas of high pressure. Because of the increased air pressure along the center lines of each row of radial bearing grooves  204 , the radial stiffness of the bearing itself is improved. 
     In addition to the embodiment illustrated in FIG. 2, the present invention encompasses radial bearing grooves  204  having other configurations. Thus, radial bearing grooves could be provided in any pattern generally adapted to drawing air to a center of the side surface  200  of the spindle bearing  128 , so that an area of high pressure air is created. Accordingly, acceptable groove patterns include a single row a of grooves in a herring-bone shaped pattern, opposing arrays of diagonal grooves, a spiraling pattern of grooves, or varying arrangements of arcuate grooves. In addition, the present invention includes within its scope the use of vanes or other raised areas on the side surface  200  of the spindle bearing  128  to perform the same function of pumping air to an intermediate area of the side surface  200  of the spindle bearing  128  as do the grooves in the illustrated embodiment. Any pattern or arrangement of grooves or raised surfaces suitable for increasing air pressure along the side surface  200  of the spindle bearing  128  may be used. Furthermore, grooves and vanes or protrusions may be used in combination. 
     Although the embodiment illustrated in FIG. 2 shows grooves on the side surface  200  of the spindle bearing  128 , the grooves may alternatively be provided on the interior of the side surface  176  of the sleeve  148 . As described above, the function of the grooves is to create high pressure areas in a middle portion or portions of the side surface  200  of the spindle bearing  128  to increase the stiffness of the bearing in a radial direction. Therefore, the shape and pattern of provided grooves may be similar to those that would be provided on the spindle bearing  128 . However, the direction of, for example, a herring-bone pattern, would be opposite that of grooves provided on the spindle bearing  128 . Therefore, the herring-bone pattern would point away from the direction of rotation of the hub assembly  112  about the spindle  124  of the base assembly  108 . Again, this is to draw air to an intermediate portion or portions of the side surface  176  of the sleeve  148 . Furthermore, as described above, the features provided to pump air to the intermediate portions of the sleeve  148  need not be grooves, but may also be vanes or other protrusions. 
     Referring now to FIG. 3, the lower annular surface  196  of spindle bearing  128  according to an embodiment of the present invention is illustrated. According to the illustrated embodiment, a plurality of thrust bearing grooves  304  are provided on the lower annular surface  196  of the spindle bearing to increase the air pressure in the lower annular space  188  when the hub assembly  112  rotates relative to the base assembly  108 . In the illustrated embodiment, the grooves are designed so that the air pressure in the lower  188  annular space is increased when the hub assembly  112  rotates about the spindle bearing  128  in the direction in which the herring-bone pattern points. In a preferred embodiment, similar grooves are also provided on the upper annular surface  192  of the spindle bearing  128 . 
     The grooves  304  described above may be substituted by vanes or other raised areas on the upper  192  and lower  196  annular surfaces of the spindle bearing  128 . As with grooves, the purpose of any such vanes or protrusions is to pump air to an intermediate or inner circumference of the upper  192  and lower  196  annular surfaces of the spindle bearing  128 , thereby increasing the stiffness of the air bearing  104  in a direction along the longitudinal axis  140  of the spindle  124 . 
     In an alternative embodiment, the grooves illustrated in FIG. 3 may be provided on the upper annular surface  172  of the sleeve  148 , adjacent to the upper annular surface  192  of the spindle bearing  128 , and on the surface of the thrust plate  152  that is adjacent to the lower annular surface  196  of the spindle bearing  128 . Suitable groove designs are similar to those used when the grooves are provided on the upper  192  and lower  196  annular surfaces of the spindle bearing  128 , however, the direction of such grooves would be reversed. Therefore, for example, when a herring-bone pattern is used, the herring-bone elements will point in a direction opposite that of the rotation of the sleeve  148  with respect to the spindle bearing  128  of the base assembly  108 . Also, the grooves may be replaced by vanes or protrusions which serve the purpose of pumping air to an intermediate circumference of the upper  184  and lower  188  annular spaces. 
     A detail of one of the thrust bearing grooves  304  is shown in FIG.  4 . As can be seen from that figure, each thrust bearing groove  304  is generally comprised of two arcuate grooves joined at their ends to form one larger groove generally having an arrow-head shape. Radii  404  of the annular surfaces  192  and  196  of the spindle bearing  128  are shown in FIG. 4 for illustration purposes. The radii  404  emanate from the longitudinal axis  140  (or center line) of the spindle  124 . The inner groove portion  408  of the thrust bearing groove  304  can be seen to intersect each radius  404  at an angle a  412 . According to the illustrated embodiment, the angle a  412  is equal at any point along inner groove portion  408  through which a radius  404  of the annular surfaces  192  and  196  of the spindle bearing  128  is drawn. The upper groove portion  416  is also shown with radii  404  of the spindle bearing  128  passing through it for illustration purposes. The angle β  420  between the upper groove portion  416  at the radii  404  is, according to the illustrated embodiment of the invention, the same, regardless of the point along upper groove portion  416  that a radius  404  of the annular surfaces  192  and  196  of the spindle bearing  128  is drawn. Furthermore, in a preferred embodiment of the present invention, the angles α  412  and β  420  are equal. Most preferably, the angles α  412  and β  420  are in a range of from about 20° to about 30°. 
     Although the grooves  204  and  304  or vanes used to draw air to intermediate or inner portions of the bearing surfaces may be positioned on either the spindle bearing  128  or the bearing surfaces of the hub assembly  112  (i.e. the sleeve  148  and the thrust plate  152 ), they generally should not be placed on both the spindle bearing  128  and the bearing surfaces of the hub assembly  112 . If grooves are provided on opposing surfaces, air pressure is not developed properly. 
     In a preferred embodiment, the length of the spindle bearing  128  is about 8 mm, the diameter of the spindle bearing  128  is about 20 mm, and the inside diameter of the sleeve  148  is about 21.5 mm. The spindle  124  has a diameter of about 5 mm. The radial clearance between the upper annular surface  192  of the spindle bearing  128  and the upper annular surface  172  of the sleeve  148 , and between the lower annular surface  196  of the spindle bearing  128  and the thrust plate  152 , is about 9.0 μm. The hub  144  extends vertically from the flange  164  for about 12 mm, and has an outer diameter of about 25 mm over that distance to allow the hub to accept a stack of magnetic storage disks. The inside diameter of the hub  144  between about the flange  164  and the clamp  168  has a diameter of about 23.5 mm and defines an upper inner cylindrical volume. The sleeve  148  fitted within this upper inner cylindrical volume has an inside diameter of about 21.5 mm. 
     In FIG. 5, an air bearing having an inside-out underslung motor design constructed in accordance with another embodiment of the present invention is identified as air bearing  504 . In general terms, the air bearing  504  differs from the embodiment of the present invention illustrated in FIG. 1 in that the bearing area of air bearing  504  is increased. This is because, for a given height of the hub  544  in FIG. 5, as measured from the clamp  564  to the flange  560 , the spindle bearing  528  and the sleeve  548  are about 60% longer than those in the air bearing  104  shown in FIG. 1 having a hub  144  with an equal height, as measured from the clamp  168  to the flange  164 . This increased bearing size is the result of the inside-out underslung motor design of the air bearing  504 , which offers increased radial bearing stiffness over the embodiment of FIG. 1, while maintaining a compact overall size. Indeed, in a preferred embodiment, for a given disk drive size format, the external dimensions of air bearing  504  are no larger than the external dimensions of air bearing  104 . 
     The air bearing  504  is generally comprised of a base assembly  508  and a hub assembly  512 . The base assembly  508  of the present embodiment is similar to the base assembly  108  of the embodiment illustrated in FIG. 1 in that it generally comprises a base  516 , only a portion of which is illustrated in FIG. 5, a stator assembly  520 , and a spindle  524 . The spindle  524  includes an enlarged bearing portion  528 . 
     The stator assembly  520  is comprised of laminations  532  and coils  536 . The laminations  532  are affixed to the base  516  and are arranged radially about the longitudinal axis  540  of the spindle  524 . Being a part of the base assembly  508 , the spindle  524  is affixed to the base portion  516 . The coils  536  of the stator assembly  520  are disposed about the laminations  532 . In a preferred embodiment, the laminations  532  comprise thin sheets of a ferromagnetic material stacked on top of one another, and the coils  536  comprise an electrically conductive wire having an insulating exterior, wound about the laminations  532 . Also in a preferred embodiment, the base  516  comprises cast aluminum, and the spindle  524  comprises machined steel. 
     The hub assembly  512  comprises the hub  544 , sleeve  548 , thrust plate  552 , and magnets  556 . The hub  544  has an internal cavity that is substantially filled by the bearing portion  528  of the spindle  524 . The sleeve  548  according to this embodiment of the present invention extends beyond the lower extreme of the hub  544 . At the lower extreme of the hub  544  is a flange  560  onto which magnetic storage disks (not shown) may be stacked. The hub  544  also features a clamp  564  to which a retainer (not shown) may be affixed to retain the magnetic disks (not shown). 
     The internal cavity of the hub  544  is generally cylindrical in shape. Affixed to this internal cavity of the hub  544  is the sleeve  548 . The interior of the sleeve  548  has an annular upper surface  568  and a cylindrical side surface  572 . The interior of the sleeve  548  is sized such that the inner diameter of the cylindrical side surface  572  is slightly larger than the diameter of the spindle bearing  528 . Thrust plate  552  has an annular shape, and is located adjacent to the lower annular surface  576  of the spindle bearing  528 . The thrust plate  552  is affixed to the lower portion of the sleeve  548 . The cylindrical side surface  572  of the sleeve  548  is slightly longer than the length of the spindle bearing  528 . Therefore, when the thrust plate  552  is affixed to the sleeve  548 , there is a thin upper annular space  580  and a similarly dimensioned lower annular space  584  between the spindle bearing  528  and the interior bearing surfaces  568 ,  572  and  552  of the hub assembly  512 . 
     A portion of the cylindrical side surface  572  of the sleeve  548  is adapted to receive a plurality of magnets  556  on its outer circumference. Accordingly, the magnets  556  are located radially about the longitudinal axis of the spindle  540 . Furthermore, the magnets  556  are positioned so that they are within a circumference described by the stator assembly  520 , and adjacent to the laminations  532  of the stator assembly  520 . Therefore, there is no need for a separate back iron component according to this embodiment of the present invention. In addition, the air bearing  504  having an inside-out underslung motor design features greater resistance to radial movement caused by magnetic forces than does the air bearing  104  having an underslung motor of conventional design. This is so because the air bearing  504  has a spindle bearing  528  that extends to at least the center line of the magnets  556  that interact with the stator assembly  520  when the hub assembly  512  is being rotated relative to the base  516 . 
     The motor  588  of this embodiment of the present invention is generally comprised of the laminations  532 , the coils  536 , and the magnets  556 . When the motor  588  is in operation, an electrical current is supplied to the coils  536 , which creates a magnetic field about and through the laminations  532 . This magnetic force causes the hub assembly  512  to rotate relative to the base  516  through its interaction with the magnetic force of the magnets  556 . 
     The rotation of the hub assembly  512  and the associated sleeve  548  and thrust plate  552 , relative to the spindle bearing  528  of the hub assembly  508 , creates a flow of air in the upper annular space  580 , the lower annular space  584 , and the cylindrical space  592  formed between the spindle bearing  528  and the side surface  572  of the sleeve  548 . This air flow creates higher air pressures in the spaces between the spindle bearing  528 , and the sleeve  548  and thrust plate  552  of the hub assembly  512 . This high pressure air then serves to prevent direct contact between the spindle bearing  528  and the bearing surfaces of the hub assembly  512 . Because of the greater spindle bearing  528  length of the air bearing  504  having an inside-out underslung motor design, the radial stiffness (i.e. the resistance of the bearing to forces along a radius of the hub  512 ) of the air bearing  504  assembly is increased. Thus, the embodiment of FIG. 5 offers greater resistance to radial forces, and/or allows lower bearing tolerances while achieving acceptable amounts of bearing stiffness. 
     In a preferred embodiment, the air bearing  504  has grooves on the upper  580  and lower  576  annular surfaces, and on the cylindrical side surface  572  of the sleeve  548 . The general design and arrangement of these grooves may be as discussed above with respect to the air bearing  104  having an underslung motor design. Also, the air bearing  504  may similarly utilize vanes rather than grooves in the bearing surfaces, and the vanes or grooves may be provided on the interior surfaces of the thrust plate  552  and sleeve  548  rather than on the spindle bearing  528 . 
     In a preferred embodiment, the length of the spindle bearing  528  is about 13.5 mm, the diameter of the spindle bearing  528  is about 20 mm, and the inside diameter of the sleeve  548  is about 21.5 mm. The spindle  524  has a diameter of about 5.0 mm. The radial clearance between the lower annular surface  576  and the thrust plate  552 , and between the upper annular surface  580  and the upper surface of the sleeve  568  is about 9.0μm. The upper portion of the hub  544  defines an inner cylindrical volume having a diameter of about 23.5 mm. The sleeve  548  fitted in this inner cylindrical volume has an inside diameter of about 21.5 mm. 
     The foregoing discussion of the invention has been presented for purposes of illustration and description. Further, 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 of practicing the invention and to enable others skilled in the art to utilize the invention in such or in other embodiments and with various modifications required by their 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.