Abstract:
The present invention relates to the field of fluid dynamic bearings. Specifically, the present invention provides an apparatus and method useful for constraining axial movement of a motor hub in a high speed spindle motor assembly.

Description:
CROSS REFERENCE TO A RELATED APPLICATION  
       [0001]    This application claims priority to provisional application Serial No. 60/383,993, filed May 28, 2002, entitled “Spindle Motor Stator/Magnet Axial Bias” invented by Jim-Po Wang and Paco Flores, and incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to the field of computer disk drives, specifically, those having fluid dynamic bearings.  
         BACKGROUND OF THE INVENTION  
         [0003]    Disk drive memory systems have been used in computers for many years for the storage of digital information. Information is recorded on concentric tracks of a magnetic disk medium, the actual information being stored in the forward magnetic transitions within the medium. The disks themselves are rotatably mounted on a spindle. Information is accessed by a read/write transducer located on a pivoting arm that moves radially over the surface of the rotating disk. The read/write heads or transducers must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.  
           [0004]    During operation, the disks are rotated at very high speeds within an enclosed housing using an electric motor generally located inside a hub or below the disks. Such spindle motors may have a spindle mounted by two ball bearing systems to a motor shaft disposed in the center of the hub. The bearing systems are spaced apart, with one located near the top of the spindle and the other spaced a distance away. These bearings allow support the spindle or hub about the shaft, and allow for a stable rotational relative movement between the shaft and the spindle or hub while maintaining accurate alignment of the spindle and shaft. The bearings themselves are normally lubricated by highly refined grease or oil.  
           [0005]    The conventional ball bearing system described above is prone to several shortcomings. First is the problem of vibration generated by the balls rolling on the bearing raceways. This is one of the conditions that generally guarantees physical contact between raceways and balls, in spite of the lubrication provided by the bearing oil or grease. Bearing balls running on the microscopically uneven and rough raceways transmit the vibration induced by the rough surface structure to the rotating disk. This vibration results in misalignment between the data tracks and the read/write transducer, limiting the data track density and the overall performance of the disk drive system. Further, mechanical bearings are not always scalable to smaller dimensions. This is a significant drawback, since the tendency in the disk drive industry has been to shrink the physical dimensions of the disk drive unit.  
           [0006]    As an alternative to conventional ball bearing spindle systems, much effort has been focused on developing a fluid dynamic bearing (FDB). In these types of systems, lubricating fluid, either gas or liquid, functions as the actual bearing surface between a shaft and a sleeve or hub. Liquid lubricants comprising oil, more complex fluids, or other lubricants have been utilized in such fluid dynamic bearings.  
           [0007]    The reason for the popularity of the use of such fluids is the elimination of the vibrations caused by mechanical contact in a ball bearing system and the ability to scale the fluid dynamic bearing to smaller and smaller sizes. In designs such as the single plate FDB, two thrust surfaces generally are used to maintain the axial position of the spindle/motor shaft assembly. Such a configuration maintains axial position; however, this configuration does not aid in reducing the power required by the FDB at start up.  
           [0008]    In such designs, the changing viscosity of the fluid with changing operating temperature of the bearing and/or motor imposes a significant restraint on available designs. As the temperature changes, the power required to spin the motor will vary—if the gap remains constant; further, the stiffness of the system will diminish as the system heats and fluid viscosity diminishes.  
           [0009]    Another approach to assure axial position of the spindle/motor shaft assembly and to address varying viscosity of the fluid is to remove one of the thrust surfaces from the FDB and replace it with a magnetic force to constrain the motor&#39;s axial movement. This typically involves adding a magnetic circuit to the assembly consisting of a magnet fixed to the hub, sleeve or base that attracts (or repels) the facing motor hub, sleeve or base. Though effective, this additional magnetic configuration requires additional parts, machining and assembly.  
           [0010]    Other efforts to address the problems of axial positioning and fluid viscosity have included using different metals in the shaft and sleeve so that the gap would change with changes in temperature; however, such solutions are typically relatively expensive. Accordingly, it would be advantageous to design a disk drive assembly that maintains axial positioning which minimizing the power required at start-up and constant speed rotation even as the viscosity of the fluid undergoes substantial changes.  
           [0011]    Thus, there is an interest in the art to assure proper axial positioning of the spindle/motor shaft assembly and reduce the power required at start up without additional parts, machining and assembly.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention is intended to provide reduced power in a fluid dynamic bearing assembly and constrained axial movement of the motor hub, without additional parts or re-design of currently used parts.  
           [0013]    These and other advantages and objectives are achieved by providing a fluid bearing design where a fluid bearing supports the shaft for rotation, with its positioning being axially compensated by a magnetic preload. By this combination, as the motor speeds up and heats up, which otherwise would cause the fluid pressure in the bearing gap to change, the magnetic preload maintains the pressure in the fluid between relatively rotating rotor and stator.  
           [0014]    In a first exemplary embodiment, the shaft is supported for rotation by a bearing rotating within a sleeve and upon a counter plate. To prevent misalignment of the rotor and stator as the motor heats up and fluid viscosity changes and to prevent upward movement of the shaft due to an upward force while spinning, a magnetic preload is established; in a preferred embodiment, the magnetic preload is achieved using a stator magnet offset with the stator.  
           [0015]    Thus, the present invention provides a fluid dynamic bearing comprising a sleeve and a shaft supported for rotation within the sleeve and upon a counter plate. The shaft supports a hub at one end for rotation with the shaft, has an outer surface facing an inner surface of the sleeve, and a bottom surface adjacent to a counter plate. Either the outer surface of the shaft or the sleeve has a set of grooves defined thereon. Also, either the bottom surface of the shaft or the top surface of the counter plate has a set of grooves defined thereon. The shaft further is supported for rotation relative to the sleeve by fluid in a gap between the shaft and the sleeve and the shaft and the counter plate. In addition, there is a stator supported on an outer surface of the sleeve. A stator magnet is supported on an inner surface of the hub and is offset vertically relative to the stator. The shaft is axially biased by the stator magnet being vertically offset to the stator. In addition, there is a base supporting the sleeve.  
           [0016]    In sum, according to the present arrangement, proper axial position of the spindle/motor shaft assembly is maintained and power is reduced even as the temperature changes.  
           [0017]    It can further be seen that the design will be relatively easy to assemble requiring simply a vertical offset of the stator with the stator magnet. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    For a fuller understanding of the present invention, reference is made to the accompanying drawings in the following detailed description.  
         [0019]    [0019]FIG. 1 illustrates an example of a magnetic disk drive in which the invention may be employed;  
         [0020]    [0020]FIG. 2 is a vertical sectional view of a prior art constant pressure magnetic preload fluid dynamic bearing;  
         [0021]    [0021]FIG. 3 is a vertical sectional view of an embodiment of the magnetically compensated constant pressure fluid dynamic bearing of the present invention; and  
         [0022]    [0022]FIG. 4A shows the configuration of a stator/magnet offset; and FIG. 4B is a graph showing test results of rotor axial force versus magnet/stator offset. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    Reference will now be made in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with these embodiments, it is to be understood that the described embodiments are not intended to limit the invention solely and specifically to only those embodiments, or to use the invention solely in the disk drive which is illustrated. On the contrary, the invention is intended to cover alternatives, modifications and equivalents that may be included within the spirit and scope of the invention as defined by the attached claims. Further, both hard disk drives and spindle motors are both well known to those of skill in this field. In order to avoid confusion while enabling those skilled in the art to practice the claimed invention, this specification omits such details with respect to known items.  
         [0024]    The embodiments of the present invention are intended to minimize power consumption and maintain stability of the rotating hub. The problem is complicated by the fact that the relative rotation of hub/sleeve/shaft combinations is typically supported by fluid whose viscosity changes with temperature. Moreover, the power consumption also changes with the change in viscosity of the fluid. At low temperature the viscosity is high and the power consumption is also relatively high. The larger the grooved areas, the greater the power consumption. The power consumption and also stiffness change with the width of the gap in which the bearing is established. In typical designs, the gap is constant, and therefore the power consumption and stiffness vary as the viscosity of the fluid changes. In addition, axial positioning of the spindle assembly must be maintained to reduce power and maintain fidelity of the system.  
         [0025]    [0025]FIG. 1 illustrates an example of a magnetic disk drive in which the invention may be employed. At least one magnetic disk  60  having a plurality of concentric tracks for recording information is mounted on a spindle  10 . The spindle is mounted on spindle support shaft  25  for rotation about a central axis. As the disks are rotated by the motor, a transducer  64  mounted on the end of an actuator end  65  is selectively positioned by a voice coil motor  66  rotating about a pivot axis  67  to move the transducer  64  from track to track across the surface of the disk  60 . The elements of the disk drive are mounted on base  40  in a housing  70  that is typically sealed to prevent contamination (a top or cover of housing  70  is not shown). The disks  60  are mounted on spindle  10 .  
         [0026]    [0026]FIG. 2 shows a fluid bearing comprising a sleeve  200  and a shaft  202  supporting a hub  204  for rotation. The hub supports one or more disks (not shown). The design includes a fluid dynamic bearing  210  comprising a gap between the outer surface  212  of shaft  202  and the inner surface  214  of sleeve  200 . One of those two surfaces has grooves to maintain the pressure of a fluid  216  maintained in this gap to support the relative rotation of the shaft and sleeve. In addition, there is an additional fluid dynamic bearing  242  comprising a gap between the bottom  244  of the shaft  202 , and the top  246  of counter plate  248 . One of the bottom surface  244  of shaft  202  or the top  246  of counter plate  246  also has grooves to maintain pressure of fluid  216  maintained in the gap.  
         [0027]    The design shown includes a stator  222  supported on the outer surface of the base  224 , and cooperating with stator magnet  226  so that appropriate energization of the stator causes high speed rotation of the hub  204  and therefore the disks. Stator  222  and stator magnet  226  are level vertically at their respective midpoints  260 . A biasing magnet or magnet preload  232  is mounted on an axially facing surface of the sleeve  220 . This is an approach known in the art used to establish a magnetic axial bias against the shaft; that is, to axially position the shaft  202  relative to sleeve  200 .  
         [0028]    The directional force of the system when in operation without magnetic biasing is shown at  240 . Spinning of the shaft with the fluid dynamic bearings  210  and  242  imposes an upward directional force that can misalign the assembly. Magnet preload  232  prevents such misalignment.  
         [0029]    [0029]FIG. 3 shows a fluid bearing comprising a sleeve  300  and a shaft  302  supporting a hub  304  for rotation in which the design is modified to maintain stiffness with changes in viscosity. The hub  304  supports one or more disks (not shown). The design includes a fluid dynamic bearing  310  comprising a gap between the outer surface  312  of shaft  302  and the inner surface  314  of sleeve  300 . One of those two surfaces has grooves to maintain the pressure of a fluid  316  maintained in this gap to support the relative rotation of the shaft and sleeve. It should be recognized that although conical-shaped bearing are shown, bearing of other shapes and/or configurations may be used as well.  
         [0030]    In addition, there is an additional fluid dynamic bearing  342  comprising a gap between the bottom  344  of the shaft  302 , and the top  346  of counter plate  348 . One of the bottom surface  344  of shaft  302  or the top  346  of counter plate  346  has grooves to maintain pressure of fluid  316  in the gap.  
         [0031]    The directional force of the system when in operation without magnetic biasing is shown at  340 . Spinning of the shaft with the fluid dynamic bearings  310  and  342  imposes an upward directional force that can misalign the assembly. A magnet preload prevents such misalignment.  
         [0032]    The design shown includes a stator  322  supported on the outer surface of the base  324 , and cooperating with stator magnet  326  so that appropriate energization of the stator causes high speed rotation of the hub  304  and, therefore, the disks. However, in the present embodiment, an additional biasing magnet is not required (see magnet  232  of FIG. 2). Instead, the stator magnet  326  is offset vertically from the stator  322  (at  360 ). This approach establishes a magnetic axial bias against the shaft using the stator magnet; that is, the stator magnet not only energizes the stator to cause rotation of the hub  304 , but the stator magnet additionally serves the purpose of axially positioning the shaft  302  relative to sleeve  300  without the addition of additional magnet to the disk drive assembly.  
         [0033]    Once the axial bias is established, as the temperature changes and the viscosity of the fluid changes, the fluid bearing gap will adjust so that the axial force across the gap remains substantially stable with changes in temperature. Further, with the use of the FDB conical design, which provides both axial and radial support for the relatively rotating parts, good misalignment stiffness is established.  
         [0034]    It is necessary to calibrate the axial bias due to the offset of stator magnet  326  to establish and maintain the pressure in the gap  312  with changes in temperature of the fluid so that the fluid bearing is properly temperature compensated. To reproduce the motor in high volume production, the gap  312  should be set accurately so that by utilizing the offset stator magnet  326 , a constant force can be established, which in turn establishes the parameters for the rest of the motor so that a constant force is established across the bearing gap.  
         [0035]    It should be noted that in this particular embodiment, a further fluid bearing  350  is defined between the outer surface of the shaft  302  and the inner surface of the sleeve  300 . This bearing is defined using well-established technology, imposing grooves on either the outer surface of the shaft or the  302  or the inner surface of sleeve  300  with fluid in the gap supporting the relative rotation of the shaft and sleeve.  
       EXAMPLE  
       [0036]    [0036]FIG. 4A shows the configuration of a stator/magnet offset, where offset is equal to Zs−Zm. Zm is half magnet height from Datum and Zs is half stator height from Datum. FIG. 4B is a graph showing rotor axial force versus magnet/stator offset for a particular stator/magnet configuration, though one skilled in the art will note that the actual value for magnet offset will vary on the size and strength of the stator and the magnet used.  
         [0037]    Other features and advantages of the invention will become apparent to a person of skill in the art who studies the following disklosure of preferred embodiments.