Abstract:
Hard disk drive track density is increased by selectively increasing the stiffness of a fluid dynamic bearing (FDB) motor during servo write. Applying a load to the shaft of a fixed shaft FDB motor to close the bearing gaps increases stiffness. Alternatively, the disk drive is cooled to increase bearing fluid viscosity or the motor is operated at an increased rotational velocity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. provisional patent application Ser. No. 60/338,412, filed on Dec. 5, 2001, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to field of disk drives and more particularly to the writing of servo tracks onto the disks during manufacture. 
     2. Description of the Related Art 
     Disk drive servo tracks are typically written onto a blank magnetic disk after the disk drive has been substantially assembled. While there are many known methods for writing servo tracks onto a blank magnetic disk, the most common methods include the use of a laser interferometer to control a picker that attaches to the ARM assembly of the disk drive and steps the arms and the heads, attached to the end of the arms, across the disk while writing the servo patterns. 
     In a mechanically perfect world, servo tracks would be written in perfectly concentric circles as the servo track writer steps the ARM assembly across the disk. However, servo track writers are not mechanically perfect. The resultant imperfections, from whatever source, result in the servo tracks “wandering” from an idealized track center. Tracks spacing is limited by degree to which the written circle track “wanders” from the idealized track center in a nonuniform way. 
     A significant mechanical source for imperfections in the servo write process is the “run out” of the spindle motor. “Run out” is the amount of radial excursion of the motor in response to dynamic forces on the motor. In a fluid dynamic bearing (“FDB”) motor, the degree of run out is primarily related to the lack of radial stiffness of the fluid dynamic bearing. The stiffness of a fluid bearing is generally related to 1) the size of the gap between bearing surfaces, and 2) the viscosity of the bearing fluid and 3) to the rotational velocity of the motor: The larger the gap, the less viscous the fluid or the slower the rotational velocity (to a point), the looser the bearing. 
       FIG. 1  shows the general relationship between bearing stiffness and disk rotational velocity of a disc drive FDB motor. It charts the inverse of stiffness (e.g., microinches per 1-g excitation), 1/k, vs. the frequency, f, of rotation for two different bearings. The top curve  10  shows the profile of a relatively loose bearing. The bottom curve  20  shows the profile of a stiffer bearing. Both have peaks,  12  and  22  respectively, and at approximately half the frequency of rotation of the motor. 
     Increasing radial stiffness of the FDB bearing therefore reduces FDB run out. However, there is a trade-off between bearing stiffness and power consumption: the greater the stiffness, the higher the power consumption. Higher power consumption is extremely undesirable in disk drives for a variety of reasons. 
     Therefore there is a need to increase FDB bearing radial stiffness without increasing power consumption in order to permit the writing of servo tracks at higher track densities. 
     SUMMARY OF THE INVENTION 
     The invention comprises selectively increasing the stiffness of the FDB bearing of an FDB spindle motor only during servo write. 
     When the FDB bearing structure is of the conical, spool or spherical type, etc., it typically includes a fixed shaft. With these configurations, increased stiffness is preferably provided by selectively compressing the shaft while servo writing. As a shaft compresses, the bearing gap(s) decreases thereby increasing bearing stiffness. A preferred form for providing a compressive load on the shaft is clamping the disk drive into the servo-writing fixture by means of a clamp abutting the disk drive casing at both ends of the shaft. The clamp provides a predetermined load to compress the shaft by a predetermined amount. 
     When the FDB motor is of a structure that does not have a fixed shaft that cannot be compressed through an externally applied clamping force, bearing stiffness can be increased by substantially reducing the temperature of the disk drive during servo write. This reduction in temperature increases the viscosity of the bearing fluid, which increases bearing stiffness. 
     Finally, as bearing stiffness is also related to motor rotational velocity, the invention comprises significantly increasing the rotational velocity of the motor during servo write. 
     An alternate manner of reducing the bearing gaps is to provide an electromagnetic structure affixed between the rotating and fixed elements of the FDB motor. The electromagnet can be selectively energized during servo write. The electromagnet attracts the rotating elements towards the fixed elements thereby increasing the stiffness of the bearing. 
     In one form, an armature in the form of an annular steel plate is attached to one end of the FDB motor&#39;s rotating hub. An annular U-shaped stator is mounted on the fixed elements of the motor, or alternatively on the disk drive casing, facing the steel plate. Coils are wound within the U-shaped armature. When energized, magnetic flux flows between the ends of this U-shaped stator through the steel armature thereby attracting it toward the U-shaped stator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a chart showing the relationship of the inverse of stiffness vs. disk rotational frequency of two different FDB bearings. 
         FIG. 2  is a partial cross-sectional view of a disk drive having a conical bearing FDB motor. The disk drive is shown mounted in a fixture by means of clamps abutting the disk drive at both ends of the motor&#39;s fixed shaft. 
         FIG. 3  is a chart showing the axial deflection vs. load of the fixed shaft of an embodiment of the invention according to FIG.  2 . 
         FIG. 4  is a partial cross-sectional view of a disk drive having a conical bearing FDB motor including and alternate embodiment of the present invention. And electromagnet is shown mounted at the lower end of the disk-mounting hub. 
         FIG. 5  is a flow diagram illustrating a method for increasing FDB bearing stiffness, according to one embodiment of the present invention. 
         FIG. 6  is a flow diagram illustrating a method  600  for increasing FDB bearing stiffness, according to another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The disk drives and FDB motors shown and described in connection with figures are set forth only with sufficient detail necessary to understand the invention. 
     Referring to  FIG. 2 , disk drive  30  is shown mounted in a servo writer (most of whose structure is omitted for sake of clarity) by means of a clamp  50  and  52 . Clamp members  50  and  52  abut the disk drive  30  at both ends of the FDB motor&#39;s  40  fixed shaft  60 . 
     The FDB motor  40  is of the conical variety. It includes a fixed shaft  60  (typically stainless steel) interference mounted into a boss  84  of a base casting  80  (sheet metal) of the disk drive  30 . The other end of the shaft  60  is affixed, typically by a screw (not shown) to the top cover  82  (sheet metal) of the disk drive casing. The FDB motor  40  further includes hub  110  (stainless steel) rotably mounted on the shaft by means of conical bearings  70  and  72 . The conical bearings  70  and  72  are typically fixedly mounted on shaft  60 . The interface between bearings  70  and  72  and the conical facing surfaces  76  and  78  of the rotable hub  110  provide respective bearing surfaces. One or both of these surfaces are grooved (not shown). These bearing surfaces are separated by gaps  74  and  75  conventionally filled with a fluid (oil, air) to provide lubrication between the surfaces of the bearing. When the motor  40  rotates, the grooves of the bearing surfaces increase the pressure of the fluid in the gaps  74  and  75  and form a bearing: the greater the pressure, the stiffer the bearing. 
     Also shown in the figure are stator elements  100  mounted on the aforesaid boss  84  of the base casting  80 . Annular magnet  102  is shown mounted on the rotating hub  110  facing stator  100 . Magnetic disks  90  and  92  are mounted on hub  110  by means of a disk clamping means (not shown) and separated from each other by a disk spacer  94 . 
     Clamp lower member  52  directly abuts the bottom end the shaft  60 . Clamp upper member  50  abuts the top cover  82  of the disk drive adjacent the point of attachment of the shaft  60  to top cover  80 . Clamps  50  and  52  cooperate to apply a predetermined compressive force on shaft  60 . This compressive force compresses the shaft  60  by a predetermined amount. When shaft  60  compresses, the bearing gaps  76  and  75  between conical bearings  70  and  72  and the respective bearing surfaces  76  and  78  on hub  110  also compress. This gap compression increases the stiffness of the bearings. 
     Conical bearings  70  and  72  have a bearing surface at an angle to the shaft  60 . This angle provides stiffness in both the axial, that is, along the shaft  60 , and in the radial, that is, perpendicular to shaft and parallel to the disks  90  and  92 , directions. TIncreased stiffness in the radial direction directly reduces radial runout of the motor  40  and thereby permits improved servo write performance. However, increased stiffness in the axial direction also reduce runout because it reduces vibrational modes excited by axial movement that contribute to the unbalancing forces that contribute runout. 
       FIG. 3  is a graph of the axial deflection in mm vs. load in foot pounds of a column of  440 C SST stainless steel (modulus of elasticity=2E+11 Pa, yield strength=1.9E+09 Pa) having the following dimensions: O.D. 2.8=mm; length=20 mm; cross-sectional area=6.1575216 square mm. Axial deflection at 175 foot-pounds is 0.0126 mm. As illustrated, shaft compression is a linear function of load. 
       FIG. 4  illustrates an alternate embodiment of the present invention. This embodiment is useful whenever an external clamp is not available to compress shaft  60 . Here the compressive load is provided by an electromagnet  118  located between one end of the hub  110  and base  80 . Electromagnet  118 , when actuated, attracts the hub  110  towards base  80 . This operates to compress bearing gap  75 , which increases its radial stiffness. 
     In the figure, hub  110  is comprised of aluminum and is fixed to a member  112  composed of stainless steel. Such a configuration provides stainless-steel bearing surfaces,  76  and  78 , while permitting disks  90  and  92 , which are typically composed of aluminum, to be mounted on a hub of the same material for the purpose of matching coefficients of thermal expansion. (This embodiment will work in the FDB motor configuration of  FIG. 1  if the stainless steel of that embodiment&#39;s hub  110  is magnetic.) 
     With this hub configuration, the electromagnet is composed of a annular steel ring  120  (armature) mounted on hub  110 , opposed by a U-shaped ring  122  (stator) mounted on the base  80 . Coils  124  are mounted within the prongs of the “U”. When actuated, current flowing in coils  124  induces magnetic flux between the prongs of the “U,” which flows through ring  120 . This causes ring  120  to be drawn toward U-shaped ring  122 . 
     This configuration can also be used in rotating shaft FDB motors such a shown in, for example, U.S. Pat. No. 6,183,385, which is hereby incorporated by reference. 
       FIG. 5  is a flow diagram illustrating a method  500  for increasing FDB bearing stiffness, according to one embodiment of the present invention. The method  500  is initialized at step  502  and proceeds to step  504 , where the method  500  commences to servo write process. In step  506 , the method  500  reduces the temperature of the disk drive during servo write. This reduced temperature increases the viscosity of the bearing&#39;s fluid, which thereby increases bearing stiffness. The method  500  terminates at step  508 . 
       FIG. 6  is a flow diagram illustrating a method  600  for increasing FDB bearing stiffness, according to another embodiment of the present invention. The method  600  is initialized at step  602  and proceeds to step  604 , where the method  600  commences the servo write process. In step  606 , the method  600  increases the rotational velocity of the motor during servo write. The method  600  terminates at step  608 . 
     These, and in general, all methods for selectively increasing FDB bearing stiffness during servo write are contemplated to be within the scope of the present invention. Depending on the construction of the bearing, which can be any type of thrust, conical, spool or spherical bearing, increased stiffness need not only be applied to those bearing components that provide radial stiffness. If possible, increasing the stiffness of all the bearing components in a nonselective manner will automatically reduce runout.