Abstract:
The counterplate and/or sleeve are modified to reduce or eliminate distortion in the counterplate in a laser welded fluid bearing design. It is known to directly weld the counterplate which lies across the end of the shaft and the thrust plate to a wall or shoulder of the sleeve in which the counterplate is fit. The radial stiffness of the sleeve is weakened by cutting grooves or the like at or near the interface between the sleeve wall and the counterplate, or on the outer diameter of the sleeve wall to essentially weaken the sleeve wall. This weakening of the sleeve wall prevents bowing or distortion of the counterplate being imposed by the sleeve.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims benefit of U.S. Provisional Application No. 60/247,097, entitled DISTORTION FREE LASER WELDED FLUID BEARING DESIGN, filed Nov. 9, 2000 by Le et al., which is hereby incorporated by reference in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to the field of hydrodynamic bearing assemblies of the type that cooperates with high-speed spindle elements. More specifically, the invention relates to a design that reduces stress and distortion in parts which are incorporated into a hydrodynamic bearing.  
         BACKGROUND OF THE INVENTION  
         [0003]    Disc drive memory systems have been used in computers 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 is accessed by means of read/write heads generally located on a pivoting arm that 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.  
           [0004]    During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor generally located inside a hub that supports the discs. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of two ball or hydrodynamic bearing systems to a motor shaft disposed in the center of the hub. Generally, such motors include a stator comprising a plurality of teeth arranged in a circle. Each of the teeth support a plurality of coils or windings that may be sequentially energized to polarize the stator. A plurality of permanent magnets are disposed in alternating polarity adjacent the stators. As the coils disposed on the stators are sequentially energized in alternating polarity, the magnetic attraction and repulsion of each stator to the adjacent magnets cause the spindle to rotate, thereby rotating the disc and passing the information storage tracks beneath the head.  
           [0005]    The use of hydrodynamic bearing assemblies in such drive systems has become preferred due to desirable reductions in drive size and noise generation as compared to conventional ball bearing drive systems. In hydrodynamic bearings, a lubricating fluid, such as oil or air, functions as the bearing surface between a base or housing and a spindle or hub. As the lubricating fluids require small gaps between the stationary and rotating members in order to provide the support, stiffness and lubricity required for proper bearing operation, conventional drive components and assemblies typically require tight tolerances and demand precision assembly methods. Such demanding tolerance and assembly control results in increased part and assembly costs along with an increased level of quality control to ensure proper drive operation.  
           [0006]    Thus, the problem presented is to reliably set close bearing gaps without requiring excessive or burdensome part or manufacturing tolerances.  
           [0007]    However, a related problem arises from the fact that to maintain the integrity of the thrust plate style hydrodynamic bearing, the technology has adopted the approach of laser welding the counterplate which overlies the thrust plate and is supported adjacent the thrust plate an upraised wall of the sleeve. This seals the fluid dynamic bearing, maintaining the fluid within the bearing without the necessity of using an o-ring or the like between the counterplate and sleeve to prevent any loss of fluid. However, thermal contraction forces caused by cooling of the weld nugget cause the counterplate to bow (typically outward), which may increase the gap between thrust plate and counterplate and thereby impact stiffness and wear contact conditions for the thrust bearing. Therefore, the problem presented is to adopt a design which eliminates or diminishes the bowing of the counterplate during welding.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention seeks to provide a method and apparatus for eliminating bowing or other distortion of the counterplate incorporated into a thrust plate design hydrodynamic bearing.  
           [0009]    More specifically, the present invention seeks to improve the method of welding the counterplate to the surrounding supporting sleeve by making the effect on the counterplate substantially distortion free.  
           [0010]    In summary, according to the present invention, the counterplate and/or sleeve are modified to reduce or eliminate distortion in the counterplate in a laser welded fluid bearing design. More specifically, according to the present invention, which comprises a shaft with a thrust plate at one end, surrounded by a sleeve, and a counterplate supported in the sleeve adjacent the thrust plate to define a gap with the thrust plate. It is known to directly weld the counterplate which lies across the end of the shaft and the thrust plate to a wall or shoulder of the sleeve in which the counterplate is fit. According to the invention, the radial stiffness of the sleeve is weakened by cutting grooves or the like at or near the interface between the sleeve wall and the counterplate, or on the outer diameter of the sleeve wall to essentially weaken the sleeve wall. This weakening of the sleeve wall prevents bowing or distortion of the counterplate being imposed by the sleeve.  
           [0011]    More specifically, in one embodiment, the distortion is eliminated by cutting a groove at the interface of counterplate and sleeve. The groove may be solely in the sleeve shoulder or may also intrude into the counterplate. In a preferred embodiment, the depth is about half of the counterplate thickness.  
           [0012]    In yet another alternative embodiment, a groove is cut on the sleeve outer diameter inward toward the counterplate, approximately radially parallel with the counterplate. In yet another approach, the sleeve may simply cut away at the axially or grooved and radical outer end of the shoulder which supports the counterplate.  
           [0013]    In yet a further alternative embodiment, an undercut may be imposed on the inner surface of the sleeve shoulder at or near where a corner of the counterplate would rest against the sleeve shoulder.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    So that the manner in which the above recited features of the present 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.  
         [0015]    [0015]FIG. 1 is a top plan view of a disc drive data storage device, in accordance with the invention; and  
         [0016]    [0016]FIG. 2 is a vertical sectional view of a typical disc drive spindle motor in which the present invention is especially useful.  
         [0017]    [0017]FIGS. 3A, 3B,  4 A,  4 B and  5  are vertical sectional views of alternative embodiments of this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    The invention comprises a spindle motor for a disc drive data storage device having a thrust plate type hydrodynamic bearing. FIG. 1 is a plan view of a typical disc drive  10  wherein the invention is useful. Disc 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 from outside the sealed environment.  
         [0019]    The base and top cover arrangement shown in FIG. 1 is common in the industry. However, other arrangements of the housing components have been frequently used, and the invention is not limited to the configuration of the disc drive housing. For example, disc drives have been manufactured using a vertical split between two housing members. In such drives, that portion of the housing half that connects to the lower end of the spindle motor is analogous to base  12 , while the opposite side of the same housing member, that is connected to or adjacent the top of the spindle motor, is functionally the same as the top cover  24 .  
         [0020]    Disc drive  10  further includes a disc pack  16  that is mounted for rotation on a spindle motor (not shown) by a disc clamp  14 . Disc pack  16  includes one or more individual discs that are mounted for co-rotation about a central axis. Each disc surface has an associated head  20  that is mounted to disc drive  10  for communicating with the disc surface. In the example shown in FIG. 1, heads  20  are supported by flexures  22  that are in turn attached to head mounting arms  18  of an actuator body  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 body  26  with its attached heads  20  about a pivot shaft  30  to position heads  20  over a desired data track along an arcuate path  32 . While a rotary actuator is shown in FIG. 1, the invention is also useful in disc drives having other types of actuators, such as linear actuators.  
         [0021]    [0021]FIG. 2 shows a rotating shaft  100  spindle motor design in which the shaft is integrated with the hub  102  which carries flange  103  which functions as a disc support surface. The shaft with the hub  102  supports a magnet  104  on its inner axial surface, facing stator  106  whose energization causes stable rotation of the hub. The stator in turn is supported on an axial extension  108  of base casting  110 . A sleeve  112  which supports the shaft  100  and its associated thrust plate  116  is incorporated into the axial extension  108  of the base  110 . This sleeve  112  has axial surface  120  that faces a surface of the shaft. These two surfaces define a journal bearing which is of standard design and not further shown. Further, the thrust plate at surfaces  122  and  124  define in cooperation with the sleeve  112  and the counterplate  130  thrust bearings of the fluid dynamic type which further support the shaft against axial forces. Each of these journal and thrust bearings require fluid in the gap between the facing surfaces. This fluid may either recirculate through an internal channel  134  which either passes through the thrust plate or between the thrust plate and shaft, or through a central bore. To prevent the escape of any fluid between the surface  140  of the sleeve and the complementary surface  142  of the thrust plate, a laser weld has been applied at the junction at the axially outer edge of the counter plate  130  and the sleeve  112 . This laser weld is applied using well-known techniques and technology but by its very simplicity enhances the reliability.  
         [0022]    Given the welded counterplate to sleeve shoulder scheme of FIG. 2, this effectively seals the hydrodynamic bearing fluid. However, thermal contraction forces caused by cooling of the weld nugget at the seam along the counterplate/sleeve interface results, under certain circumstances, in a bowing of the counterplate  130 . This bowing can increase the gap which is intended to be carefully defined between the counterplate and sleeve, thereby impacting the stiffness and wear contact conditions of the thrust bearing.  
         [0023]    The succeeding figures show a plurality of solutions to eliminate counterplate distortion from fluid bearing design due to laser welding. Referring for example to FIG. 3A, we see an example wherein a groove is cut at the weld interface  300  into both the counterplate  302  and the radially inner edge of the shoulder  304  of sleeve  306 . Preferably, the groove depth should be about half of the counterplate thickness. Further, as is shown, the radial width of the groove as cut into the shoulder  304  is about half of the total width  310  of the sleeve. Testing has demonstrated that this is highly effective in relieving the stress imposed by the weld  300  at the seam  315  between counterplate  302  and sleeve  306 .  
         [0024]    In an alternative shown in FIG. 3B, a groove  320  is cut into the interface between counterplate  302  and sleeve  306  at the seam  315 . In this case, rather than cutting a rectangular cross-sectional groove into the shoulder  304  of the sleeve  306 , a cut which is at first a continuation of the cut imposed on the counterplate  302  and then gradually tapers  310  axially away to the outer edge of the shoulder  304  is provided. In this situation, the diminishing of the stress created at the weld  300  is still achieved, while the groove is somewhat easier to fabricate under certain circumstances.  
         [0025]    In yet another alternative shown in FIG. 4A, a small groove  420  is cut into the outer surface of the shoulder  404  of sleeve  406 . No cut is made into the counterplate  402  in order to maintain its rigidity. This weakening of the shoulder  404  makes it easier to impose the weld  400  at the seam  415  between the counterplate  402  and the sleeve  406  without distortion of the counterplate due to the forces imposed by heating and later contraction of the weld  400 .  
         [0026]    Groove  420  can be anywhere along the axially outer surface of shoulder  404 ; is preferably imposed roughly along the plane where the lower edge  430  of the counterplate will rest on an upper surface  432  of the sleeve  406 . As shown by the dotted lines  440 , the groove may be axially extended even below this imaginary line which is defined by this intersection between counterplate surface  430  and sleeve surface  432  to further weaken the sleeve and diminish the possibility of distortion.  
         [0027]    A further application of this same principle is shown in FIG. 4B where the outer edge region  450  of the sleeve  406  is cut away, again leaving the counterplate  402  undisturbed. The axial depth of this cut, or cut-away portion  450  of the shoulder  404  need only be far enough to weaken shoulder  404 ; in an exemplary approach, it extends preferably at least half the axial depth of the counterplate  402  in order to diminish the radial forces imposed on the counterplate  402  which could bow the counterplate.  
         [0028]    Yet another approach to the solution to this problem is what is referred hereto as an undercut  501  shown in FIG. 5. In this figure, an undercut region  501  eliminates a portion of the junction between the inner surface  470  of shoulder  404  and the radially outer wall  472  of counterplate  402 . This undercut  501  preferably starts from what would be the corner where the outer corner  480  of the counterplate would rest against the corner defined by the inner surface  470  and the surface  432  on which the counterplate rests. This also has the effect of reducing the stress imposed by the weld  500  on the seam  515  by both weakening the shoulder  404  so to provide less resistance to the contraction of the weld  500 , as well as diminishing the amount of surface contact between the inner surface  470  of the sleeve  404  and the radially outer surface  472  of the counterplate  402 .  
         [0029]    Other features and advantages of this invention will be apparent to a person of skill in the art who studies the disclosure provided above. Therefore, the scope of the present invention is to be limited only by the following claims.