Abstract:
A bearing having a conical hydrodynamic bearing section and a pivot bearing section. The bearing can be used with a spindle motor. The conical hydrodynamic bearing section is formed by the conical portion of the shaft and a correspondingly shaped cavity of the bearing sleeve. The pivot bearing section is located adjacently to the conical bearing section and is formed by the curved end face of the shaft and an endplate inserted into the cavity of the bearing sleeve.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from Fed. Rep. Of Germany Patent Application No. DE10239886.0, filed on Aug. 29, 2002, and from U.S. Provisional Patent Application Ser. No. 60/363,784, filed on Mar. 12, 2002. 

   BACKGROUND OF THE INVENTION 
   The following invention relates to electronic spindle motors of the type used in disk drives and in particular relates to improvements in fluid bearings for such motors. 
   Disc drive systems have been used in computers and other electronic devices for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle, the information being accessed by means of transducers located on a pivoting arm which moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information; thus the discs must be rotationally stable. 
   Electric spindle motors of the type used in disk drives conventionally rely on ball bearings to support a rotary member, such as a rotating hub, on a stationary member, such as a shaft. Ball bearings are wear parts and in time friction will cause failure of the motor. In addition, ball bearings create debris in the form of dust or fine particles that can find their way into “clean” chambers housing the rotary magnetic disks which are driven by the motor. The mechanical friction inherent in ball bearings also generates heat and noise, both of which are undesirable in a disk drive motor. 
   Fluid dynamic bearings represent a considerable improvement over conventional ball bearings in spindle drive motors. In these types of systems, lubricating fluid—either gas or liquid—functions as the actual bearing surface between a stationary base or housing in the rotating spindle or rotating hub of the motor. For example, liquid lubricants comprising oil, more complex ferro-magnetic fluids or even air have been utilized in hydrodynamic bearing systems. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a spindle motor with a fluid dynamic pivot bearing which saves run-current and, therefore, reduces power consumption of the spindle motor. The present inventions combines the benefit of increased stability provided by hydrodynamic bearings with the benefit of low power consumption provided by pivot bearings. 
   The present invention provides these benefits by providing a fluid dynamic conical bearing with a pivot bearing for use in a spindle motor. The fluid dynamic conical bearing resists both horizontal motion of the shaft and upward motion of the shaft, while the pivot bearing resists downward motion of the shaft. 
   The above and other objects, aspects, features and advantages of the invention will be more readily apparent from the description of the preferred embodiments thereof taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is illustrated by way of example and not limitation and the figures of the accompanying drawings in which like references denote like or corresponding parts, and in which: 
       FIG. 1  is a side cut-away view of an electronic spindle motor having a rotational shaft, a fluid dynamic conical bearing, and a pivot bearing according to the first embodiment of the present invention. 
       FIG. 2  is a side cut-away view of a bearing according to the second embodiment of the present invention. 
       FIG. 3  is a side cut-away view of a bearing according to the third embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that the described embodiments are not intended to limit the invention specifically to those embodiments. 
   The first embodiment of the present invention is shown in  FIG. 1. A  Spindle motor  2  includes a bracket  4  which is to be mounted on a disk drive device (not shown). A rotor  6  is arranged for rotation relative to bracket  4 . A sleeve  8  and a stator  10  are fixedly mounted on bracket  4 . 
   Rotor  6  comprises a rotor hub  18  and a tubular shaft  20  fixed coaxially to rotor hub  18 . A rotor magnet  12  is bonded to the inner side of a circumferential wall of rotor hub  18 . The outer side of the circumferential wall of the rotor hub  18  is shaped to hold a magnetic disk (not shown). 
   Stator  10  comprises a core  52  fixedly fitted to bracket  4  and coils  54  wound on the core  52 . The stator  10  is radially spaced by a small gap from and arranged opposite to the rotor magnet  12 . 
   Sleeve  8  is a tubular member comprising four sections that are distinguished from each other based upon their inner radii. First sleeve section  81  has a constant inner radius A, except that, the inner radius of first sleeve section  81  increases slightly near the top surface of sleeve  8  to form a capillary seal. Second sleeve section  82  has an inner radius that increases linearly as the depth of the sleeve  8  increases so as to form a frustum shaped conic section. The inner radius of second sleeve section  82  can be described by the following equation: r sl2 =A+mY, where r sl2  is the inner radius of second sleeve section  82 , A is the inner radius of first sleeve section  81 , m is the rate of increase (slope) of the radius of second sleeve section  82 , and Y is the vertical distance below first sleeve section  81 . Third sleeve section  83  has a constant inner radius B that is equal to the maximum radius of second sleeve section  82 . Fourth sleeve section  84  has a constant inner radius C that is greater than the inner radius of third sleeve section  83 . End plate  42  is fit into fourth sleeve section  84  of Sleeve  8 . 
   Shaft  20  is also comprised of four sections. First shaft section  201  fits inside of and is rigidly connected to hub  18 . Second shaft section  202  fits inside of first sleeve section  81 , although second shaft section  202  extends slightly below first sleeve section  202 . Second shaft section  202  has a constant radius F that is slightly less than the radius A of first sleeve section  81 . Third shaft section  203  extends below second shaft section  202  and it fits within second sleeve section  82 . The radius of third shaft section  203  is less than the radius of second sleeve section  82 . However, the radius of third shaft section  203  increases faster than the rate of increase of the radius of second sleeve section  82  until approximately the mid point of second sleeve section  82 , after which the radius of third shaft section  203  increases slower than the rate of increase of the radius of second sleeve section  82 . Hence, the distance between second sleeve section  82  and third shaft section  203  reaches a minimum at approximately the mid point of second sleeve section  82 . Fourth shaft section  204  extends below third shaft section  203  and it begins at approximately the same height as third sleeve section  83 . The radius of fourth shaft section  204  decreases rapidly and it goes to zero at the bottom of third sleeve section  83  at which point fourth shaft section  204  touches end plate  42 . 
   The radius of third shaft section  203  can be described by the following equation: r s3 =D+f s3 (Z), where r s3  is the radius of third shaft section  203 , D is the radius of second shaft section  202 , Z is the vertical distance below second shaft section  202 , and f s3 (Z) is a constantly increasing function of Z. The derivative of f s3 (Z) with respect to Z, which is equal to the derivative of the radius of third shaft section  203  with respect to Z, is always positive (df s3 (Z)/dZ=dr s3 /dZ&gt;0). Additionally, in the first embodiment, the second derivative of f s3 (Z) with respect to Z is always negative (d 2 f s3 (Z)/d 2 Z=d 2 r s3 /d 2 Z&lt;0) and it is a continuous function. 
   The distance between second sleeve section  82  and third shaft section  203  is at its minimum when the rate of increase of the radius of third shaft section  203  as a function of the vertical distance below second shaft section  202  (the derivative of f s3 (Z) with respect to Z) is equal to the rate of increase of the radius of second sleeve section  82  (r sl2 −r s3  is a minimum when df s3 (Z)/dZ=m). Bearing stiffness can be adjusted by varying the derivative of the rate of increase of the radius of third shaft section  203 . The closer that d 2 f s3 (Z)/d 2 Z is to zero the stiffer the bearing will be, provided that the minimum distance between third shaft section  203  and second sleeve section  82  remains constant. However, such increased stiffness results in greater energy losses. 
   The gap comprised of the spaces between sleeve  8 , end plate  42  and shaft  20  is filled with an appropriate lubricating fluid. Pressure generating grooves are formed either onto the surface of second sleeve section  82  or onto the surface of third shaft section  203  to create a conical bearing. The grooves are formed such that they are centered approximately at the point where the distance between second sleeve section  82  and third shaft section  203  is a minimum (df s3 (Z)/dZ=m). Hence, the maximum pressure is generated at that point. 
   Forth shaft section  204 , end plate  42 , and third sleeve section  203  form a pivot bearing. Downward motion of the rotor is resisted by the physical contact of shaft  20  and end plate  42 . Additionally, the conical bearing provides stabilization of the rotor in the horizontal plane and it also resists upward motion of the rotor. Hence, thrust bearings and journal bearings are generally not required for this embodiment of the invention. However, one or two journal bearings may be added to this embodiment by forming pressure generating grooves onto the surface of either first sleeve section  81  or second shaft section  202 , if additional horizontal stabilization is required. Additionally, pressure generating grooves can be placed on the bottom of the fourth shaft section  204  or on the top of the end plate  42  to minimize material contact between the shaft  20  and the end plate  42 . 
   This first embodiment can be modified to allow for a fixed shaft rotating hub arrangement. In such an arrangement, shaft  20  is press-fit into the end plate  42  and sleeve  8  is affixed to the hub  18 . 
   The second embodiment of the present invention is shown in FIG.  2 .  FIG. 2  shows the bearing portion of a spindle motor. It is comprised of sleeve  8 , shaft  20 , and end plate  42 . 
   As shown in  FIG. 2 , sleeve  8  is a tubular member comprising four sections that are distinguished from each other based upon their inner radii. First sleeve section  81  has a constant inner radius A, except that the inner radius of first sleeve section  81  increases slightly near the top surface of sleeve  8  to form a capillary seal. Second sleeve section  82  has an inner radius that increases linearly as the depth of the sleeve  8  increases so as to form a conic section. The inner radius of second sleeve section  82  can be described by the following equation: r Sl2 =A+mY, where r Sl2  is the inner radius of second sleeve section  82 , A is the inner radius of first sleeve section  81 , m is the rate of increase (slope) of the radius of second sleeve section  82 , and Y is the vertical distance below first sleeve section  81 . Third sleeve section  83  has a constant inner radius B that is equal to the maximum radius of second sleeve section  82 . Fourth sleeve section  84  has a constant inner radius C that is greater than the inner radius of third sleeve section  83 . End plate  42  is fit into fourth sleeve section  84  of Sleeve  8 . 
   As shown in  FIG. 2 , shaft  20  is also comprised of four sections. First shaft section  201  fits inside of and is rigidly connected to hub  18 . Second shaft section  202  fits inside of first sleeve section  81 , although second shaft section  202  extends slightly below first sleeve section  202 . Second shaft section  202  has a constant radius F that is slightly less than the radius A of first sleeve section  81 . Third shaft section  203  extends below second shaft section  202  and it fits within second sleeve section  82 . The radius of third shaft section  203  is less than the radius of second sleeve section  82  by a constant amount. Hence, the distance between second sleeve section  82  and third shaft section  203  is constant over the entire length of third shaft section  203 . Fourth shaft section  204  extends below third shaft section  203  and it begins at approximately the same height as third sleeve section  83 . The radius of fourth shaft section  204  decreases rapidly and it goes to zero at the bottom of third sleeve section  83  at which point fourth shaft section  204  touches end plate  42 . 
   The radius of third shaft section  203  can be described by the following equation: r s3 =D+f s3 (Z), where r s3  is the radius of third shaft section  203 , D is the radius of second shaft section  202 , Z is the vertical distance below second shaft section  202 , and f s3 (Z) is a constantly increasing function of Z. The derivative of f s3 (Z) with respect to Z, which is equal to the derivative of the radius of third shaft section  203  with respect to Z, is a constant positive value (df s3 (Z)/dZ=dr s3 /dZ&gt;0). Additionally, in the second embodiment, the second derivative of f s3 (Z) with respect to Z is always zero (d 2 f s3 (Z)/d 2 Z=d 2 r s3 /d 2 Z=0). 
   The gap comprised of the spaces between sleeve  8 , end plate  42  and shaft  20  is filled with an appropriate lubricating fluid. Pressure generating grooves are formed on the surface of second sleeve section  82  or onto the surface of third shaft section  203  to create a conical bearing. Fourth shaft section  204 , end plate  42 , and third sleeve section  83  form a pivot bearing. In the second embodiment, the conical bearing provides stabilization to the rotor in the horizontal plane and it also resists upward motion of the rotor. Downward motion of the rotor is resisted by the pivot bearing. Hence, thrust bearings and journal bearings are generally not required for this embodiment of the invention. However, as shown on  FIG. 2 , pressure generating grooves  21  are included on second shaft section  202  to form a journal bearing and provide additional horizontal stabilization. Additionally, pressure generating grooves may be placed on the bottom of the fourth shaft section  204  or on the top of the end plate  42  to minimize material contact between the shaft  20  and the end plate  42 . 
   The primary difference between embodiment 2 and the bearing of embodiment 1 is that the derivative of the rate of increase of the radius of third shaft section  203  with respect to the vertical distance below second shaft section  202  is equal to zero (d 2 f s3 (Z)/d 2 Z=d 2 r s3 /d 2 Z=0), which is the limiting case from embodiment 1. 
   The third embodiment of the present invention is shown in FIG.  3 .  FIG. 3  shows the bearing portion of a spindle motor. It is comprised of sleeve  8 , shaft  20 , and end plate  42 . 
   As shown in  FIG. 3 , sleeve  8  is a tubular member comprising four sections that are distinguished from each other based upon their inner radii. First sleeve section  81  has a constant inner radius A, except that the inner radius of first sleeve section  81  increases slightly near the top surface of sleeve  8  to form a capillary seal. Second sleeve section  82  has an inner radius that increases as the depth of sleeve  8  increases. However, the rate of increase of the inner radius of second sleeve section  82  decreases as the depth of sleeve  8  increases. Third sleeve section  83  has a constant inner radius B that is equal to the maximum radius of second sleeve section  82 . Fourth sleeve section  84  has a constant inner radius C that is greater than the inner radius of third sleeve section  83 . End plate  42  is fit into fourth sleeve section  84  of Sleeve  8 . 
   The inner radius of second sleeve section  82  can be described by the following equation: r sl2 =D+f sl2 (Y), where r Sl2  is the radius of second sleeve section  82 , D is the radius of first sleeve section  81 , Y is the vertical distance below first sleeve section  81 , and f sl2 (Y) is a constantly increasing function of Y. The derivative of f s3 (Y) with respect to Y, which is equal to the derivative of the radius of second sleeve section  82  with respect to Y, is always positive (df sl2 (Y)/dY=dr sl2 /dY&gt;0). Additionally, in the third embodiment, the second derivative of f sl2 (Y) with respect to Y is always negative (d 2 f s3 (Z)/d 2 Z=d 2 r s3 /d 2 Z&lt;0) and it is a continuous function. 
   As shown in  FIG. 3 , shaft  20  is also comprised of four sections. First shaft section  201  fits inside of and is rigidly connected to hub  18 . Second shaft section  202  fits inside of first sleeve section  81 , although second shaft section  202  extends slightly below first sleeve section  202 . Second shaft section  202  has a constant radius F that is slightly less than the radius A of first sleeve section  81 . Third shaft section  203  extends below second shaft section  202  and it fits within second sleeve section  82 . The radius of third shaft section  203  increases as the depth of third shaft section  203  increases. However, the rate of increase of the radius of third shaft section  203  decreases as the depth of third shaft section  203  increases. The radius of third shaft section  203  is less than the radius of second sleeve section  82  by a constant amount for the entire length of third shaft section  203 . Fourth shaft section  204  extends below third shaft section  203  and it begins at approximately the same height as third sleeve section  83 . The radius of fourth shaft section  204  decreases rapidly and it goes to zero at the bottom of third sleeve section  83  at which point fourth shaft section  204  touches end plate  42 . 
   The radius of third shaft section  203  can be described by the following equation: r s3 =D+f s3 (Z), where r s3  is the radius of third shaft section  203 , D is the radius of second shaft section  202 , Z is the vertical distance below second shaft section  202 , and f s3 (Z) is a constantly increasing function of Z. The derivative of f s3 (Z) with respect to Z, which is equal to the derivative of the radius of third shaft section  203  with respect to Z, is always positive (df s3 (Z)/dZ=dr s3 /dZ&gt;0). Additionally, in the first embodiment, the second derivative of f s3 (Z) with respect to Z is always negative (d 2 f s3 (Z)/d 2 Z=d 2 r s3 /d 2 Z&lt;0) and it is a continuous function. 
   In the third embodiment, the gap comprised of the spaces between sleeve  8 , end plate  42  and shaft  20  is filled with an appropriate lubricating fluid. Pressure generating grooves are formed either onto the surface of second sleeve section  82  or onto the surface of third shaft section  203  to create a conical bearing. The conical bearing provides stabilization to the rotor in the horizontal plane and it also resists upward motion of the rotor. The placement of the grooves determines the relative strength of the horizontal stabilization and the upward stabilization (the thrust and journal components of the conical bearing). A higher placement of the grooves results in relatively greater resistance to upward movement of the shaft, while a lower placement of the grooves results in relatively greater horizontal stabilization. Additionally, fourth shaft section  204 , end plate  42 , and third sleeve section  83  form a pivot bearing, which resists downward motion of the shaft by the physical contact of shaft  20  with end plate  42 . Hence, thrust bearings and journal bearings are generally not required for this embodiment of the invention. However, as shown on  FIG. 3 , pressure generating grooves  21  are included on second shaft section  202  to form a journal bearing and to provide additional horizontal stabilization. Additionally, pressure generating grooves can be placed on the bottom of the fourth shaft section  204  or on the top of the end plate  42  to minimize material contact between the shaft  20  and the end plate  42 .