Patent Publication Number: US-2003234589-A1

Title: Rotor limiter for fluid dynamic bearing motor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
     [0001] This application claims the priority of United States Provisional Application No. 60/390,382, filed Jun. 21, 2002 by Parsoneault et al. (entitled “Rotor Limiter for FDB Motor”), which is herein incorporated by reference. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The invention relates to fluid dynamic bearing motors, and more specifically to fluid dynamic spindle motors with limited axial movement.  
       BACKGROUND OF THE INVENTION  
       [0003] Disk drives are capable of storing large amounts of digital data in a relatively small area. Disk drives store information on one or more recording media, which conventionally take the form of circular storage disks (e.g. media) having a plurality of concentric circular recording tracks. A typical disk drive has one or more disks for storing information. This information is written to and read from the disks using read/write heads mounted on actuator arms that are moved from track to track across the surfaces of the disks by an actuator mechanism.  
       [0004] Generally, the disks are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the disks under the read/write heads. The spindle motor generally includes a shaft and a hub, to which one or more discs are attached, and a sleeve defining a bore for the shaft. Permanent magnets attached to the hub interact with a stator winding to rotate the hub and disc. In order to facilitate rotation, one or more bearings are usually disposed between the sleeve and the shaft.  
       [0005] Over the years, storage density has tended to increase, and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage disks.  
       [0006] From the foregoing discussion, it can be seen that the bearing assembly that supports the hub and storage disk is of critical importance. One typical bearing assembly comprises ball bearings supported between a pair of races that allow a hub of a storage disk to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems, such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor because of low damping.  
       [0007] One alternative bearing design is a fluid dynamic bearing. In a fluid dynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disk hub. In addition to air, typical lubricants include gas, oil, or other fluids. Fluid dynamic bearings spread the bearing surface over a large surface area, as opposed to a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or runout between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing, which helps to reduce non-repeatable run-out.  
       [0008] One embodiment of a fluid dynamic bearing motor is magnetically biased. That is, the bearing design cooperates with a magnetically biased circuit or element to establish and maintain fluid pressure in the bearing areas, especially in designs where the thrust bearing is defined in the gap at the end of the shaft. This eliminates the need to provide hydrodynamic grooves on one or more motor elements in order to accomplish the same, which in turn reduces the power consumed by the motor. However, this means that the only thing holding the rotating portion of the motor in place is the axial magnetic force; therefore, if under shock axial forces exceed magnetic forces in the motor, the rotor can shift and the disk drive will fail.  
       [0009] Therefore, there is a need for a fluid dynamic spindle motor in which axial movement of the rotor is restricted.  
       SUMMARY OF THE INVENTION  
       [0010] The invention is a motor comprising a rotor, a stationary sleeve disposed about the rotor, a fluid dynamic bearing between the rotor and sleeve, and a limiter for restricting axial movement of the rotor relative to the stationary sleeve. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011] So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
     [0012]FIG. 1 depicts a plan view of one embodiment of a disk drive having a motor in accordance with the present invention;  
     [0013]FIG. 2 is a vertical sectional view depicting a magnetically biased fluid dynamic spindle motor according to one embodiment of the present invention;  
     [0014]FIG. 3 is a vertical sectional view depicting a magnetically biased fluid dynamic spindle motor according to a second embodiment of the present invention;  
     [0015]FIG. 4 is a vertical sectional view depicting a magnetically biased fluid dynamic spindle motor according to a third embodiment of the present invention; and  
     [0016]FIG. 5 is a vertical sectional view depicting a magnetically biased fluid dynamic spindle motor according to a fourth embodiment of the present invention.  
     [0017]FIG. 6 is a vertical sectional view of a motor embodying a further embodiment of the present invention.  
     [0018]FIG. 7 is a vertical sectional view of a rotating shaft motor embodying a further embodiment of the invention.  
     [0019]FIGS. 8A and 8B are partial vertical sectional view and an exploded view of another embodiment of the limiter used in a fixed shaft motor.  
     [0020]FIG. 9 is an exploded view of a limiter useful in the embodiment of FIG. 8A.  
     [0021]FIGS. 10A and 10B are a partial vertical sectional view and an exploded view of a stationary shaft motor incorporating an alternative embodiment of the present invention.  
     [0022]FIG. 11A is a vertical sectional view of a limiter useful in a rotating shaft motor such as shown in FIG. 6; FIGS. 11B and 11D are plan views of the limiter shown in FIG. 11A; and FIG. 11C is a cross-sectional view of the limiter of FIGS. 11B and 11D;  
     [0023] And FIGS. 12A and 12B are partial vertical sectional views alternate embodiments of a limiter especially useful with a rotating shaft motor.  
     [0024] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
    
    
     DETAILED DESCRIPTION  
     [0025]FIG. 1 depicts a plan view of one embodiment of a disk drive  10  for use with embodiments of the invention. Referring to FIG. 1, the disk drive  10  includes a housing base  12  and a top cover  14 . The housing base  12  is combined with top cover  14  to form a sealed environment to protect the internal components from contamination by elements outside the sealed environment. The base and top cover arrangement shown in FIG. 1 is well known in the industry; however, other arrangements of the housing components have frequently been used, and aspects of the invention are not limited by the particular configuration of the disk drive housing. Disk drive  10  further includes a disk pack  16  that is mounted on a hub  202  (see FIG. 2) for rotation on a spindle motor (not shown) by a disk clamp  18 . Disk pack  16  includes one or more of individual disks that are mounted for co-rotation about a central axis. Each disk surface has an associated read/write head  20  that is mounted to the disk drive  10  for communicating with the disk surface. In the example shown in FIG. 1, read/write heads  20  are supported by flexures  22  that are in turn attached to head mounting arms  24  of an actuator  26 . The actuator shown in FIG. 1 is of the type known as a rotary moving coil actuator and includes a voice coil motor (VCM), shown generally at  28 . Voice coil motor  28  rotates actuator  26  with its attached read/write heads  20  about a pivot shaft  30  to position read/write heads  20  over a desired data track along a path  32 .  
     [0026]FIG. 2 is a vertical sectional view of one embodiment of a magnetically biased fluid dynamic bearing spindle motor  200  according to the present invention.  
     [0027] The motor  200  includes a base  12  and a rotating assembly  201  comprising a hub  202  mounted to a shaft  204  for rotatably supporting one or more disks  205 . The rotating assembly  201  further comprises a magnet  206  affixed to back iron  207 . The hub  202  comprises a disk-mounting flange  230  that supports one or more disks  205  and disk spacers  207 , if necessary.  
     [0028] A stationary assembly  203  comprises a sleeve  208  with a recess  210  defined therethrough to receive the shaft  204 . Mounted upon the sleeve  208  is a stator  212  that, when energized, communicates with the magnet  206  in the hub  202  and, when the stator  212  is energized, induces rotation of the shaft  204  and hub  202  about the stationary sleeve  208 . The stator  212  comprises a plurality of “teeth”  215  formed of a magnetic material where each of the teeth  215  is wound with a winding or wire  217 .  
     [0029] A bearing assembly  232  is further provided for stable rotational support of the shaft  204  and hub  202  relative to the stationary sleeve  208 . A hydrodynamic bearing on the rotating assembly—here shown as a conical bearing  214  integrally formed with the shaft  204 —together with a bearing on the bore of the stationary journal sleeve  208 , forms a bearing surface. Alternatively, journal bearings may be formed on the outer surface of the shaft  204  and the bore of the journal sleeve  208  by grooving the facing surfaces. Fluid  216  such as oil, air, or gas is introduced between the outer surface of the shaft  204  and the bore of the journal sleeve  208 . Additionally, grooved surfaces on a thrust or counterplate (not shown in the figure) may provide additional bearing surfaces.  
     [0030] To establish and maintain pressure in the fluid  216 , and to bias the rotating assembly, a constant force magnetic circuit is provided comprising a magnet  206  supported on the rotating assembly (here mounted on the hub  202 ), located across a gap from a magnetically conducting steel ring  218  supported on the stationary assembly (here mounted on the base  12 ). Other magnetic circuits or placements are of course possible. Such a configuration recognizes many advantages; however, a significant disadvantage to magnetically biased fluid dynamic bearing motors of the prior art is that the axial magnetic force is the only force holding the rotating assembly in place in the motor. If other axial forces such as a shock should exceed the magnetic force, then the rotating assembly will fall out of the motor, and the disk drive will fail.  
     [0031] It is therefore the aim of the present invention to restrict the axial movement of the rotating assembly by means other than the axial magnetic forces acting alone. One embodiment of the invention is limiter pin  220 , which is mounted, for example by press fitting or epoxy, into a bore  209  in the stationary sleeve  208 . Limiter pin  220  protrudes at an angle substantially perpendicular to the hub  202 , from approximately the midpoint of the sleeve  208  and into an annular recess  222  on the outer surface of the shaft  204 . The annular recess  222  is defined by a decreased diameter on the shaft surface and has a depth of, for example, one quarter of the shaft diameter. The width of the recess  222  on the shaft surface dictates the extent to which the shaft  204  may move axially when engaged by the limiter pin  220 .  
     [0032] In another embodiment (shown in FIG. 3), a limiter screw  320  is screwed into a threaded bore  321  in the base  12 . Alternatively, the screw  320  could be epoxied or press fit into the bore  321 . In one embodiment, the limiter screw  320  protrudes from the base  12  at an angle of approximately 45 degrees and extends into a recess  322  defined by an annular, angular cut on a lower portion of the hub&#39;s outer surface. The recess  322  is cut at an angle of approximately 45 degrees from a bottom surface  325  of the hub  202  and comprises a face  323  that is substantially parallel to an end of the screw  320 . Alternatively, the screw  320  could extend at an angle substantially perpendicular to the outer surface of the hub  202 , into a recess  322  that is substantially perpendicular to the shaft  204 . As in FIG. 2, the depth of the recess  322  defines the axial range in which the hub  202  is free to move.  
     [0033] In yet another embodiment (shown in FIGS. 4A and B), a limiter block  420  is mounted, for example by press fit or epoxy, into a bore  419  in the base  12  and extends at an angle substantially perpendicular to the hub  202 . Limiter block  420  protrudes from a portion of the base  12  located across a gap  421  from a lower portion  423  of the hub  202 . Limiter block  420  extends into an annular recess  422  defined in the lower portion  423  of the hub  202  by a decreased diameter on the outer hub surface. Limiter block  420  is, for example, trapezoidal in shape as shown in FIG. 4B, but may admit to other equally effective geometries. As in FIGS. 2 and 3, the width of the recess  422  defines the axial range in which the hub  202  is free to move.  
     [0034]FIG. 5 represents an alternate embodiment of a limiter comprising a flange  522  on the upper portion  507  the stationary sleeve  508 . The flange  522  and sleeve  508  define a capillary seal (here a centrifugal capillary seal)  530  therebetween to prevent fluid from escaping the motor  500 . The flange  522  is substantially perpendicular to the shaft  504  and extends inward from an outer surface  509  of the sleeve  508 . The end  521  of the flange  522  extends over a lip  532  that extends around the circumference of the shaft  504 . The lip  532  is defined by a decrease in the diameter of the upper portion of the shaft  504 . The distance between the lip  532  on the shaft  504  and the lower edge  534  of the flange  522  defines the axial range in which the shaft  504  is free to move.  
     [0035] A further alternative embodiment is shown in FIG. 6 which is a motor with a rotating shaft  610  supporting a hub  612 . Rotation of the motor is caused by the interaction between the magnet  614  and the stator  616 , the magnet being support from the hub  612  and the stator from the base  620 . An axial electromagnetic bias is established by an axial offset of the magnet  614  from the stator  616 . The limiter in this embodiment comprises a step  630  defined at an end of the shaft closest to the base  620  or counterplate  622 . This step  630  may be either integrated with the shaft  610  or be press fit thereon. The step extends axially a limited distance under the sleeve  640  which is supported from the base  620  and in turn supports the counterplate  622 . As shown, it may extend at least part way into the recirculation path  642  which is defined axially through the shaft  640 ; typically, there is grooving on one of the end of the shaft  610  or the facing surface of counterplate  622  to drive fluid toward the center axis  650  of the shaft causing any air bubbles to move toward the outer edge of the shaft and step  630  and then into the recirculation path; therefore this intrusion of the step into the recirculation path does not hinder the successful operation on this embodiment.  
     [0036]FIG. 7 shows an alternative embodiment wherein as before, a rotating shaft  700  supports a hub  702  for rotation supporting one or more disks. The hub supports a magnet  704  on its interior surface, generally aligned with but in this embodiment axially offset from a stator  706 . In this embodiment, the motor is shown supported from a base  710  of a disc drive, and within and below a top cover  720  of the housing for the disc drive. According to this approach, a screw  730  is threaded through the cover  720 , and just out of contact with a top surface  740  of the shaft. The distance between the bottom surface  750  of the screw  730  and the upper surface  740  of the shaft  700  sets the limit of travel under sharp conditions and the like for the shaft. Thus, in a very straight forward fashion the system is guaranteed against unnecessary disengagement or misalignment of the shaft from the sleeve.  
     [0037]FIG. 8A is a partial sectional view of a stationary shaft motor cooperating with a limiter supported from the surrounding sleeve, and FIG. 8B, is an exploded view of a section of the same design. A stationary shaft  800  has two sets of grooves  802 ,  804  spaced axially along the shaft which form a fluid dynamic bearing for supporting the sleeve  810  for rotation around the shaft. This support is provided by fluid  812  which lies in the gap between the surface of the shaft  800  and the surface of the sleeve  810  and is pressurized by the grooves to form a support for the rotating sleeve.  
     [0038] One or more disks (not shown) are supported from an outer surface of the hub  810  and are held in place using a clamp  820  which is screwed or otherwise fastened to the hub  810  by screw  824 . Also not shown, magnetic biasing is established, preferably by axially offsetting the magnet and stator which cause rotation of the hub similar to FIGS. 6 and 7.  
     [0039] To prevent axial disengagement of the hub from the stationary shaft, a limiter  850  which is shown especially clearly in the enlarged view in FIG. 8B is fastened with adhesive or by welding or any other useable fastening system to the sleeve  810 . The limiter  850  extends axially beneath a shoulder  860  defined on the shaft  800 . The gap  842  between the rotor  810  and the shaft  800  continues between the upper surface  862  of the limiter  850  and the facing surface  864  of the shoulder  860 . The surfaces  862 ,  864  diverge axially, thereby forming a radial capillary seal between the fixed shaft and the rotating hub to retain the fluid in the gap  842  which supports the relative rotation. Thus, dual benefits are achieved by the limiter design of FIGS. 8A and 8B.  
     [0040] A further alternative embodiment is shown in FIG. 9. FIG. 9 shows a variation on the embodiment of FIG. 8 wherein the limiter  910  is supported from the stationary shaft  900  within the rotating sleeve  902 . In this embodiment, the limiter  910  is supported from the shoulder  930  of the shaft  900  and extends radially underneath the shoulder  930  and under a shoulder  940  of sleeve  902 . As with other embodiments, to support relative rotation of the shaft and sleeve, a fluid filled gap  945  is provided between the shaft and sleeve, with grooves on one of the surfaces that define the gap  945  pressurizing the fluid to support the smooth rotation in order to maintain the fluid in the gap  945 . A reservoir  950  is defined by the facing and axially diverging surfaces of the limiter  910  and the shaft  900 . This is most easily achieved by providing a flat axial surface  952  on the limiter  910 , and an axially diverging surface  954  facing the limiter across the reservoir. An air opening  956  is also provided comprising one or more openings cut through the limiter in an axial direction to support the formation of the meniscus  958  which is the end of the fluid filled gap and maintains the fluid within the gap. In a further modification, in this embodiment a limiter shield  960  is provided supported from the rotating sleeve  902  and extending generally radially toward the shaft, ending across a small air gap  962  from the shaft. This limiter shield is provided so that a fluid gap can extend around the surface of the limiter  910  where it underlies the sleeve  902  so that the fluid in this gap  965  will support relative rotation of the limiter and the sleeve. The gap  965  must extend around the limiter past the radial and axially facing surfaces of the sleeve and then between the limiter and the limiter shield  960  in order to provide an appropriate termination for the fluid in the gap. The surfaces  966 ,  968  of the limiter and shield respectively diverge as shown, provided another oil reservoir  970  ending in a meniscus  972 . An air opening  980  should also be provided though the limiter into the gap between the limiter and the limiter shield to support establishment and maintenance of the meniscus  972 .  
     [0041] A further alternative embodiment and a variation on the embodiment of FIG. 9 is shown in FIG. 10A and 10B. FIG. 10A shows a stationary shaft  1010  surrounded by a rotating sleeve  1012 . As with FIGS. 8 and 9, a limiter is supported from the shaft  1010 , details of which are shown in the enlarged view of FIG. 10B. The embodiment of FIG. 10B shows a limiter  1020  supported from and extending radially from the base  1010 , spaced underneath a shoulder  1025  of the shaft  1010  and extending underneath a shoulder  1030  of the sleeve  1012 . As with the preceding embodiment, the fluid filled gap  1040  between the shaft and the sleeve is extended radially in both directions, to both surround the radial end of the limiter  1020  and to extend inwardly toward the central axis to end in a reservoir  1040 . One or more openings  1050  are formed through the limiter  1020  to aid in the formation of the meniscus  1055  at the end of the reservoir  1040 .  
     [0042] According to this embodiment, a secondary limiter  1060  extends radially inwardly from the sleeve  1012  and axially beneath the shaft supported limiter  1020 . Both of these limiters are placed above the base  1075  so that the limiter  1020  intervenes between the sleeve limiter  1060  and the shoulder  1030  of the sleeve. In this way, the limiter  1020  is held securely below the rotating sleeve  1012  and above the sleeve supported limiter  1060  so that it is very difficult for the sleeve to move axially under shock in either direction relative to the shaft.  
     [0043]FIGS. 11A, B, and C show alternative approaches to capturing the axial location of the shaft relative to the sleeve in a motor such as shown in FIG. 6. Specifically, referring to FIG. 11A, we see a sleeve  1110  and a shaft  1120  rotating within the bore  1125  defined by the sleeve  1110 . A recirculation path as previously described in FIG. 6 provides for fluid recirculation within the system. To hold the shaft axially in place under shock conditions, a retaining ring such as shown in FIG. 11B or  11 D is provided, with  11 D being a sectional view along the line AA of the ring  1130  shown in FIG. 11B. The ring is especially useful for this function because as clearly appears in FIGS. 11B and 11C, it is a part which is easily installed and inexpensive to fabricate, and comprises a floating design which is not necessarily connected to the shaft or hub. The ring is simply an L shaped cross section as appears in FIG. 11C, is designed for the ring to both rest on the counterplate  1140  and to be of a sufficient radial extent to extend into a slot or groove  1150  in the shaft. Thus, the retaining ring lends itself to a simple assembly by virtual of putting the shaft  1120  into the bore  1125  of the sleeve  1110  with the two sections of the ring  1130  inserted into the shaft recess  1150  from either side, and the counterplate  1140  then being wedged, welded or otherwise fastened in place. Thereby capturing the shaft axially using the ring sections  1130 .  
     [0044] In an alternative embodiment, the ring  1135  (FIG. 11D) may be used, Although the assembly sequence would be substantially the same, this offers the benefit of providing the slot  1170  on one side which can be oriented to the return hole  1128 . By using this approach, the chance of any interference with the free flow of the lubricating fluid from the opening  1125  between the sleeve and the shaft to the return hole  128  as known in the art is diminished.  
     [0045] Yet another alternative as shown in. FIGS. 12A and 12B. In both of these figures, the shaft  1200  is shown inserted in a bore  1210  defined by a sleeve  1220 . In both FIGS. 12A and 12B, a flexible ring  1250  or  1260  is provided. The difference in the two embodiments is that the ring  1250  in FIG. 12A is circular in cross section; the ring  1260  of FIG. 12B is generally rectangular. Referring specifically to  12 A, it can be seen that there is a groove  1270  in the outer surface of the shaft  1200  which is roughly similar in cross section to the outer surface of the ring  1250  so that the ring may be compressed into the groove while the shaft slides through the sleeve  1220 . When it reaches a locking groove  1272 , the ring snaps in place, being lodged partly in the locking groove  1272  and partly in the groove  1270  of the shaft thereby holding the shaft axially in place. The design of the embodiment of FIG. 12B is similar, with the ring  1260  being compressed into the generally rectangular groove  1280 , and then snapping into place in the slot  1282  of sleeve  1220 .  
     [0046] Therefore, the present invention accomplishes the task of restricting axial movement of the rotating assembly in a magnetically biased fluid dynamic bearing motor. The advantages of such a motor may exploited despite the presence of axial forces that may exceed magnetic forces in the motor.  
     [0047] While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.