CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is based on a provisional application 60/483,029, filed Jun. 27, 2003, attorney docket number STL 3411.01, entitled Through Hub Oil Fill And Vent For Fluid Dynamic Motors, and assigned to the Assignee of this application and incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates generally to spindle motors, and more particularly to filling and venting a fluid dynamic bearing for use with disc drive data storage systems.  
         BACKGROUND OF THE INVENTION  
         [0003]    The recent new environments for usage of disc drive memory systems have intensified design and performance needs including needs for heightened robustness. Besides traditional computing environments, disc drive memory systems are used more recently by devices including digital cameras, digital video recorders, laser printers, photo copiers, jukeboxes, video games and personal music players. Disc drive memory systems store digital information that is recorded on concentric tracks of a magnetic disc medium. Several discs are rotatably mounted on a spindle, and the information, which can be stored in the form of magnetic transitions within the discs, is accessed using read/write heads or transducers. A drive controller is conventionally used for controlling the disc drive system based on commands received from a host system. The drive controller controls the disc drive to store and retrieve information from the magnetic discs. The read/write heads are located on a pivoting arm that moves radially over the surface of the disc. The discs are rotated at high speeds during operation using an electric motor located inside a hub or below the discs. Magnets on the hub interact with a stator to cause rotation of the hub relative to the stator. One type of motor is known as an in-hub or in-spindle motor, which typically has a spindle mounted by means of a bearing system to a motor shaft disposed in the center of the hub. The bearings permit rotational movement between the shaft and the sleeve, while maintaining alignment of the spindle to the shaft. The read/write heads must be accurately aligned with the storage tracks on the disc to ensure the proper reading and writing of information.  
           [0004]    Spindle motors have in the past used conventional ball bearings between the sleeve and the shaft. However, the demand for increased storage capacity and smaller disc drives has led to the design of higher recording area density such that the read/write heads are placed increasingly closer to the disc surface. A slight wobble or run-out in disc rotation can cause the disc to strike the read/write head, possibly damaging the disc drive and resulting in loss of data. Conventional ball bearings exhibit shortcomings in regard to these concerns. Imperfections in the raceways and ball bearing spheres result in vibrations. Also, resistance to mechanical shock and vibration is poor in the case of ball bearings, because of low damping. Vibrations and mechanical shock can result in misalignment between data tracks and the read/write transducer. These shortcomings limit the data track density and overall performance of the disc drive system. Because this rotational accuracy cannot be achieved using ball bearings, disc drives currently utilize a spindle motor having fluid dynamic bearings between a shaft and sleeve to support a hub and the disc for rotation. One alternative bearing design is a hydrodynamic bearing.  
           [0005]    In a hydrodynamic bearing, a lubricating fluid such as gas or liquid or air provides a bearing surface between a fixed member and a rotating member of the disc drive. Hydrodynamic bearings eliminate mechanical contact vibration problems experienced by ball bearing systems. Further, hydrodynamic bearings can be scaled to smaller sizes whereas ball bearings have smallness limitations. However, hydrodynamic bearings suffer from sensitivity to external loads or mechanical shock events. Fluid can in some cases be jarred out of the bearing by vibration or shock events. Further, bearing fluid is susceptible to evaporation over time. Bearing fluids can give off vaporous components that could diffuse into a disc chamber. This vapor can transport particles such as material abraded from bearings or other components. These particles can deposit on the read/write heads and the surfaces of the discs, causing damage to the discs and the read/write heads as they pass over the discs. It is critical to avoid outgassing of contaminants into the sealed area of the head/disc housing.  
           [0006]    Proper sealing is critical in the case of hydrodynamic bearings, and efforts have been made to address these problems. A capillary seal is typically employed to ensure fluid is maintained within a bearing. Here, a fluid meniscus is formed between two walls and capillary attraction retains the fluid. However, tests show that recent radial capillary seal designs fail at about 500 Gs of shock, and fluid leaks through fill holes at about 500 Gs of shock. Additionally, mobile applications require higher resilience to shock events than desktop or enterprise products. Laptop or portable computers can be subjected to large magnitudes of mechanical shock as a result of handling. It has become essential in the industry to require disc drives to be capable of withstanding substantial mechanical shock.  
           [0007]    Fluid must be accurately filled into the journal gap and bearing. If excessive fluid is loaded into the bearing, the fluid will escape into the surrounding atmosphere landing on the surface of the disc and degrade the performance of the disc drive. If insufficient fluid is loaded into the bearing, then the physical bearing surfaces could contact, leading to increased wear and eventual failure of the bearing system.  
           [0008]    Further, current oil fill and air evacuation methods for fluid dynamic bearings are relatively complex and costly due to the often awkward filling angles and tight clearances. It can be difficult to consistently accurately load fluid into the sharp corners of a shield hole. Further, current oil filling methods can leave a considerable amount of excess oil on the surfaces of the sleeve, which must be subsequently removed through an arduous post-cleaning process. The cleaning process can amount to ten percent of the total assembly cost of the motor.  
         SUMMARY OF THE INVENTION  
         [0009]    A through hub oil fill and air vent is provided for spindle motors. Oil leakage and evaporation from a motor is reduced, potentially extending motor life. In an embodiment, the present invention provides for oil retention under conditions of a shock event of at least 1000 G. The present invention may be used with top cover attach motors, additionally providing a more robust motor.  
           [0010]    The process of filling oil into a spindle motor is made easier from a motor set up and tooling perspective. In conventional designs, the hub typically is removed or oil fill is performed prior to installation of the hub. Additionally, in an embodiment, bottom shield motor designs can be filled in a normal orientation, rather than filled at an angle or filled in an inverted orientation. Further, the through hub oil fill design allows for use with a relatively large diameter oil jet fill dispenser head, and further allows for subambient fill methods, ambient fill methods, injection fill methods or micro dispenser fill methods. A measured and controlled amount of oil or hydrofluid can therefore be dispensed into the motor, reducing any variability in the motor filling process.  
           [0011]    Features of the invention are achieved in part by forming an oil fill and air vent passageway through a hub. As compared to previous designs, a longer oil diffusion path from within the motor is provided. In previous designs, oil is filled through a shield having a fill hole that extends a shorter length than a hub fill hole. The present invention eliminates the shield as a potential oil leakage source by utilizing a shield without a fill or vent hole.  
           [0012]    Moreover, as compared to conventional designs, the present invention positions the oil fill passageway a greater distance from the motor oil reservoir, further reducing oil loss. In an embodiment, the hub oil channel and vent hole have a varying diameter and geometry, and can be angled, utilizing centrifugal forces, further reducing oil leakage. In an embodiment, an additional cavity is employed within the hub, substantially opposite the hub oil fill channel, for maintaining rotor rotational balance.  
           [0013]    Other features and advantages of this invention will be apparent to a person of skill in the art who studies the invention disclosure. Therefore, the scope of the invention will be better understood by reference to an example of an embodiment, given with respect to the following figures.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0015]    [0015]FIG. 1 is a top plan view of a disc drive data storage system in which the present invention is useful, in an embodiment of the present invention;  
         [0016]    [0016]FIG. 2 is a sectional side view of a hydrodynamic bearing spindle motor with a rotating hub and an attached shield, in which the present invention is useful;  
         [0017]    [0017]FIG. 3 is another sectional side view of the hydrodynamic bearing spindle motor of FIG. 2, illustrating a previously employed fluid filling method through a shield;  
         [0018]    [0018]FIG. 4 is a further sectional side view of the hydrodynamic bearing spindle motor of FIG. 2, with an enlarged view of the hub and shield illustrating a fluid fill hole through the hub, in an embodiment of the present invention;  
         [0019]    [0019]FIG. 5 is a hydrodynamic bearing spindle motor with a shield attached to a thrust plate, in which the present invention is additionally useful; and  
         [0020]    [0020]FIG. 6 illustrate various diameters and geometries that can be utilized for a fluid fill hole and a balancing hole, in an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Exemplary embodiments are described with reference to specific configurations. Those of ordinary skill in the art will appreciate that various changes and modifications can be made while remaining within the scope of the appended claims. Additionally, well-known elements, devices, components, methods, process steps and the like may not be set forth in detail in order to avoid obscuring the invention.  
         [0022]    An apparatus and method is described herein for filling and venting a fluid dynamic bearing motor and other spindle motors. By employing a hub having a fill hole and vent hole, oil leakage and oil evaporation is reduced, and the oil filling process is simplified. The present invention is especially useful with motor designs where a shield is employed adjacent to a sleeve having a fluid reservoir therebetween.  
         [0023]    It will be apparent that features of the discussion and claims may be utilized with disc drives, low profile disc drive memory systems (including one-inch disc drive designs), spindle motors, various fluid dynamic bearing designs including hydrodynamic and hydrostatic bearings, and other motors employing a stationary and a rotatable component. Further, embodiments of the present invention may be employed with a fixed shaft and a rotating shaft.  
         [0024]    As used herein, the terms “axially” or “axial direction” refers to a direction along a centerline axis length of the shaft (i.e., along axis  440  shown in FIG. 4), and “radially” or “radial direction” refers to a direction perpendicular to the centerline length of the shaft.  
         [0025]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 illustrates a typical disc drive data storage device  110  in which the present invention is useful. Clearly, features of the discussion and claims are not limited to this particular design, which is shown only for purposes of the example. Disc drive  110  includes housing base  112  that is combined with cover  114  forming a sealed environment to protect the internal components from contamination by elements outside the sealed environment. Disc drive  110  further includes disc pack  116 , which is mounted for rotation on a spindle motor (described in FIG. 2) by disc clamp  118 . Disc pack  116  includes a plurality of individual discs, which are mounted for co-rotation about a central axis. Each disc surface has an associated head  120  (read head and write head), which is mounted to disc drive  110  for communicating with the disc surface. In the example shown in FIG. 1, heads  120  are supported by flexures  122 , which are in turn attached to head mounting arms  124  of actuator body  126 . The actuator shown in FIG. 1 is a rotary moving coil actuator and includes a voice coil motor, shown generally at  128 . Voice coil motor  128  rotates actuator body  126  with its attached heads  120  about pivot shaft  130  to position heads  120  over a desired data track along arc path  132 . This allows heads  120  to read and write magnetically encoded information on the surfaces of discs  116  at selected locations.  
         [0026]    A flex assembly provides the requisite electrical connection paths for the actuator assembly while allowing pivotal movement of the actuator body  126  during operation. The flex assembly (not shown) terminates at a flex bracket for communication to a printed circuit board mounted to the bottom side of disc drive  110  to which head wires are connected; the head wires being routed along the actuator arms  124  and the flexures  122  to the heads  120 . The printed circuit board typically includes circuitry for controlling the write currents applied to the heads  120  during a write operation and a preamplifier for amplifying read signals generated by the heads  120  during a read operation.  
         [0027]    [0027]FIG. 2 is a sectional side view of a hydrodynamic bearing spindle motor  255  used in disc drives  110  in which the present invention is useful. Again, the present invention is not limited to use with a hydrodynamic spindle motor design of a disc drive, which is shown only for purposes of example. Typically, spindle motor  255  includes a stationary component and a relatively rotatable component, defining a journal gap there between. The stationary component includes shaft  275  that is fixed and attached to base  210 . In an embodiment, shaft  275  is attached to top cover  256 , providing stability to shaft  275  and improving dynamic performance. Thus, in a fixed shaft motor, both upper and lower ends of shaft  275  can be fastened to base  210  and to top cover  256  of the housing, so that the stiffness of the motor and its resistance to shock as well as its alignment to the rest of the system is enhanced.  
         [0028]    The rotatable components include sleeve  280  and hub  260  having one or more magnets  265  attached to a periphery thereof. The magnets  265  interact with a stator winding  270  attached to the base  210  to cause the hub  260  to rotate. Magnet  265  can be formed as a unitary, annular ring or can be formed of a plurality of individual magnets that are spaced about the periphery of hub  260 . Magnet  265  is magnetized to form one or more magnetic poles.  
         [0029]    The hub  260  is supported on a shaft  275  having a thrust plate  283  on one end. Thrust plate  283  can be an integral part of the shaft  275 , or it can be a separate piece that is attached to the shaft, for example, by a press fit. Thrust plate  283  engages with base  210  at interface  290 . Hub  260  includes a disc carrier member  214 , which supports disc pack  116  (shown in FIG. 1) for rotation about shaft  275 . Disc pack  116  is held on disc carrier member  214  by disc clamp  118 . Hub  260 , positioned for rotation about shaft  275 , is situated adjacent to shaft  275  across journal bearing  262 . A fluid, such as lubricating oil or a ferromagnetic fluid fills interfacial regions between shaft  275  and sleeve  280 , thrust plate  283  and sleeve  280 , thrust plate  283  and shield  282 , and between shield  282  and sleeve  280 . While the present figure is described herein with a lubricating fluid, those skilled in the art will appreciate that useable fluids include a lubricating liquid and gas.  
         [0030]    Typically, one of shaft  275  and sleeve  280  includes sections of pressure generating grooves, including asymmetric grooves  242 , and symmetric grooves  246 . Asymmetric grooves  242  and symmetric grooves  246  having a pattern including one of a herringbone pattern and a sinusoidal pattern induces fluid flow in the interfacial region and generates a localized region of dynamic high pressure and radial stiffness. As sleeve  280  rotates, pressure is built up in each of its grooved regions and shaft  275  supports hub  260  for constant high speed rotation.  
         [0031]    A Shield  282  is radially self-aligned into sleeve  280 . On one end (adjacent to thrust plate  283 ) sleeve  280  locates shield  282  radially, and on another end shield  282  is attached to hub  260  (i.e., laser welded). A constant gap of about 20 to 30 microns is formed between thrust plate  283  and shield  282 . A fluid reservoir  284  is formed between shield  282  and sleeve  280 . Embodiments of the present invention can be utilized with motor designs wherein shield  282  is attached to hub  260 , or alternatively wherein shield  282  is attached to thrust plate  283 , as shown in FIG. 5. A fluid recirculation path (sleeve passageway  286 ) is formed through sleeve  280  to pass and recirculate fluid through journal bearing  262 . Sleeve passageway  286  is positioned such that one end is placed generally adjacent to a midpoint along shaft  275  and a second end joins recirculation plenum  432  (shown in FIG. 4) such that, in one situation, fluid and air may travel along channels on shield  282  toward and along fluid reservoir  284 .  
         [0032]    [0032]FIG. 3 shows a fluid fill hole previously employed with a fluid dynamic bearing motor design. As illustrated, fluid fill hole  302  is formed through shield  282 . The length of fluid fill hole  302  is determined by the distance across shield  282 . In some cases, a shorter length fill hole is more vulnerable to leakage and evaporation, as well as to a shock event. As shown in FIG. 4, the present invention provides a longer fluid fill hole  450  than previous designs. Further, fluid fill hole  302  is positioned closer to fluid and a fluid meniscus  316  situated in fluid reservoir  284 , as compared to fluid fill hole  450  of the present invention as shown in FIG. 4. Therefore, fluid fill hole  302  presents an added opportunity, by reason of closer proximity to a fluid situated in fluid reservoir  284 , for loss of fluid out the fill hole.  
         [0033]    Fluid fill and air evacuation processes for fluid dynamic bearings can be relatively complex and costly due in part to the often awkward filling angles and tight clearances. Further, positioning of a spindle motor for fluid filling can be complicated by gravitational effects, thus requiring abnormal or restrictive filling orientations. As shown on the right half of FIG. 3, components including base  210  are absent in order to position a filling apparatus  310  and filler extension  312  within fluid fill hole  302 . Base  210  and additional components must be removed or the components installed subsequent to filling fluid into spindle motor  255 . A tight filling angle exists and is apparent from the illustrated angle of filling apparatus  310  in FIG. 3, even with a number of components removed.  
         [0034]    Referring to FIG. 4, another sectional side view of the hydrodynamic bearing spindle motor of FIG. 2 is shown, with an enlarged view of components for focusing on components near fill hole  450  and fluid reservoir  284 . Due to a lower flow resistance and lower pressure in fluid reservoir  284 , compared with other fluid containing areas, fluid is received and retained within fluid reservoir  284  during operating or non-operating shock events. When the motor is spinning and forcing fluid by centrifugal force from reservoir  284 , pumping grooves  424  on thrust plate  283  generate pumping pressure and drive fluid recirculation through the motor. However, when the motor is not spinning and centrifugal force subsides, or during shock events, reservoir  284  can receive fluid from areas including the outer diameter gap  446  of thrust plate  283  and from the journal between shaft  275  and sleeve  280 .  
         [0035]    Grooved pumping is employed along the inside diameter (ID) and the outside diameter (OD) of thrust plate  283 . In the case of the ID, spiral pumping grooves  424  generate pumping pressure to drive fluid recirculation and to pump fluid from thrust plate bearing passageway (adjacent to the thrust plate ID) toward shaft  275 , into the journal bearing  262 , when shaft  275  and sleeve  280  are in relative rotational motion. In an embodiment, when the motor is spinning, the fluid flow direction is inward from the bearing of the thrust plate ID  430 , along the journal bearing  262  to journal plenum  412 , through sleeve passageway  286 , to recirculation plenum  432  and then returning to the bearing of the thrust plate ID  430 . Recirculation plenum  432  is defined by a junction joining fluid reservoir  284 , sleeve passageway  286 , thrust plate ID  430  and thrust plate OD gap  346 . The fluid flow direction, in an example, is illustrated by solid lines shown in FIG. 3. A grooved pumping seal (GPS)  418  is employed in outer diameter gap  446  defined between shield  282  and an OD of thrust plate  283 . GPS  418  pumps fluid from outer diameter gap  446  serving to prevent fluid leakage from the motor. Further, a centrifugal capillary seal (CCS)  316  is employed between sleeve  280  and shield  282 . In an embodiment, the adjacent surfaces of shield  282  and sleeve  280  have relatively tapered surfaces that converge toward recirculation plenum  432 . A meniscus  316  is formed between the tapered surfaces, and fluid within reservoir  284  is forced toward recirculation plenum  432  by centrifugal force when shaft  275  and sleeve  280  are in relative rotational motion.  
         [0036]    An embodiment of the present invention is illustrated by fill hole  450  and balancing hole  452 . Fill hole  450 , being extended, withstands a shock event and prevents any fluid from leaking, evaporating or wicking from the motor. That is, fill hole  450  being formed through hub  260  has a longer length than previous fill hole designs through shield  282 , hub  260  having a greater length than shield  282  for forming a fill hole. As spindle motor  255  proceeds through operational cycles, fluid is better retained with an extended fluid fill hole  450 , especially during an air purge cycle. Fill hole  450  provides a longer oil diffusion path from within the motor, extending motor life through improved fluid retainment. In an embodiment, the extended fill hole  450  provides for oil retention under conditions of a shock event of at least 1000 G.  
         [0037]    Further, fluid fill hole  450  is positioned a greater distance from fluid and a fluid meniscus situated in fluid reservoir  284 , as compared to fluid fill hole  302  of previous designs as shown in FIG. 3. Therefore, fluid fill hole  450  further reduces a chance for fluid loss out a fill hole by reason of greater distance from fluid situated in fluid reservoir  284 .  
         [0038]    Shield  282 , forming a sealed location, is attached to sleeve  280  at attachment location  402 , in an embodiment of the invention. Fill hole  450  is positioned adjacent to attachment location  402 . Fill hole  450  is positioned without making an angle with a surface of hub  260 . In another embodiment, fill hole  450  is positioned to make a 30 degree angle or an alternative angle with a surface of hub  260 . An angled fill hole opposes escape of fluid during shock since the fluid follows a path of least resistance and an angled fill hole presents greater resistance in comparison to capillary force gradients. In an embodiment, fill hole  450  is angled through hub  260  toward shaft  275  such that when the motor is spinning, centrifugal force aids to retain fluid. Further, the thickness of hub  260  supports various angles and geometries for fill hole  450 . In an embodiment, fill hole  450  is positioned between channels formed on shield  282  (not shown).  
         [0039]    Fill hole  450  (also an air vent hole) provides a means to fill a fluid dynamic bearing with fluid by injecting a predetermined amount of fluid into fill hole  450  above capillary seal  316 . In an embodiment, fill hole  450  supports both ambient fluid fill and subambient fluid fill processes for dispensing fluid to spindle motor  255 . In an ambient fill process, fluid is dispensed through, for example, a high precision, neumatically controlled syringe. In a subambient fill process, the fluid dynamic bearing is under vacuum and the fluid is dispensed. Fluid volume is controllable through these fill processes, which is critical for issues including performance and motor life in the case of hydrodynamic bearing spindle motor  255 .  
         [0040]    Fluid fill hole  450  allows fluid filling the motor with hub  260 , base  210  and other components in place. Further, fluid fill hole  450  allows spindle motors, including bottom shield motors to be fluid filled in a normal orientation, rather than an angled fluid fill process with an inverted spindle motor orientation as in previous designs such as that shown in FIG. 3. Further, added space for positioning a fluid dispenser head is provided with the fluid fill hole  450  as compared to previous designs shown in FIG. 3. As shown, in an embodiment of the present invention, filling apparatus  456  is positioned over the top of the spindle motor, the spindle motor being in a normal orientation, and spindle motor components including the base being previously installed and present during the fill process. Further, filling apparatus  456  is positioned in a non-angled orientation over the spindle motor and filling extension  454  is inserted into fluid fill hole  450 .  
         [0041]    [0041]FIG. 5 shows a further embodiment of the invention wherein spindle motor  500  employs a shield  520  attached to thrust plate  552 , attached at shield attachment  522 . Hub  554  and sleeve  556  rotate relative to stationary shield  520 , stationary shaft  575  and base  550 . As in previously discussed spindle motor designs, a fluid recirculation path, including sleeve passageway  526 , is formed through sleeve  556  to pass and recirculate fluid through the journal bearing. Also, a fluid reservoir  524  is formed between shield  520  and sleeve  556 .  
         [0042]    Fill hole  510  (or air vent hole) provides a means to fill the fluid dynamic bearing motor with fluid. Similar advantages as discussed above are provided by the positioning of fill hole  510  through hub  554 , including an extended fill hole, reduced oil leakage and evaporation from the motor, as well as a simplified oil filling process.  
         [0043]    In an embodiment, fill hole  510  further supports a micro dispenser system including a MicroDrop™ fluid fill process, which fills a predetermined volume of fluid with a tightly controlled volume tolerance for spindle motor designs. The MicroDrop™ fill process utilizes a nozzle  530  with a frequency controlled electric element for controlling fluid drop volume (i.e., droplets of 30 μm to 100 μm). Fluid  532  is dispensed from the MicroDrop™ process on an individual droplet sequence and drops are expelled and fly at a velocity of 1.5 to 3 meters per second or more. Thus, fluid from the MicroDrop™ process may be dispensed from a distance, rather than requiring embedding a syringe into the fluid reservoir of the spindle motor. The MicroDrop™ process offers a further advantage by expelling fluid from a non-contact nozzle, rather than from a syringe. With a syringe having a fluid adhering surface, a fluid drop can be undesirably removed from a spindle motor and contaminate areas outside a fluid reservoir.  
         [0044]    Referring to FIG. 6, various diameters and geometries may be utilized for fluid fill hole  450  and balancing hole  452 . Additionally, in an embodiment, two fill holes are employed through hub  260 , and it is to be appreciated that additional numbers of fill hole  450  and balancing hole  452  can be utilized. The through passageway as described herein is one of a fluid fill-hole and an air vent. The various geometries or shapes for fluid fill hole  450  include a rounded end, a rectangular end, and a triangular end, with a smaller diameter passageway extended through hub  260  to the opposite end. It is to be appreciated that the diameter of both fluid fill hole  450  and balancing hole  452  can be varied through the length of hub  260  or can remain a constant diameter. In an embodiment, balancing hole  452  is similarly shaped as fluid fill hole  450 . Balancing hole  452  can either form an opening completely through hub  260  or be formed some length into hub  260  without making an opening completely through hub  260 . In an embodiment, balancing hole  452  is employed for rotor rotational balance.  
         [0045]    Further, in an embodiment, fluid fill hole  450  is shaped such that a narrow passageway is positioned distant to the fluid reservoir  284 , and a geometry such as a rounded end is positioned adjacent to the fluid reservoir  284 . This allows any air bubble to burst into the rounded end to retain residual fluid, rather than burst externally from the motor.  
         [0046]    Other features and advantages of this invention will be apparent to a person of skill in the art who studies this disclosure. For example, those skilled in the art will appreciate that features of the present invention allows various fluid filling processes including the MicroDrop™ fluid dispenser process. Further, fill hole  450  and balancing hole  452 , having an extended length and allowing various diameters and geometries, may be utilized to provide rotational balance where rotational balance difficulties arise with a spindle motor. Thus, exemplary embodiments, modifications and variations may be made to the disclosed embodiments while remaining within the spirit and scope of the invention as defined by the appended claims.
A through hub fill hole and air vent having an enlarged fluid diffusion path is provided for spindle motors. Oil leakage and evaporation from a motor is reduced. In an aspect, oil is retained under conditions of at least a 1000 G shock event. In an aspect, the hub fill hole has a varying diameter and geometry, and is angled, further reducing oil leakage. In an aspect, an additional cavity is employed within the hub, for maintaining rotor rotational balance. The process of filling oil into a spindle motor is made easier from a motor set up and tooling perspective. Removal of the hub and other motor components is not necessary for filling a motor. Large diameter oil fill dispenser heads, subambient and ambient fill processes, and micro dispenser fill processes may be utilized. A measured and controlled amount of oil can be dispensed, reducing variability in the motor filling process.