Abstract:
According to the present invention spindle bearings are assembled with at least one annular gimbal to compensate for undesired components of bearing compression force. Specific devices and methods are directed to compensating for either (a) operational force variations such as those caused by temperature variation or (b) assembly-related force variations such as those caused by misalignment.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/169,014 filed on Dec. 3, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to spindle bearing assemblies, and more particularly to those including a plurality of raceways containing rolling members compressed with a controlled preload force. 
     BACKGROUND OF THE INVENTION 
     Although the use of such bearings is common in devices incorporating small electric motors, such as disc drives, preload force variations in such bearings are difficult to control in practice. Variations that reduce the preload force can cause play between the rotating and stationary members and/or undesired oscillations. Variations that increase the preload force can cause other problems, such as excessive or uneven wear in the bearings and/or balls. 
     Although some control mechanisms exist within the systems that apply the preload force, the need for spindle bearings having an internal control mechanism remains to be satisfied. 
     SUMMARY OF THE INVENTION 
     Spindle bearings are assembled with at least one annular gimbal to compensate for undesired components of bearing compression force. Spindle bearings are provided with a pair of coaxial raceways that are separated so that a first assembly can rotate with respect to a second. Balls rollingly engage the inner and outer races to maintain the races in coaxial alignment, typically with an offset preload so that the balls are kept in compression. 
     A preferred gimbal of the present invention has a somewhat oblong cross section along a radial half-plane and is formed integral to the assembly by cutting at least one groove about a rigid portion to make a deformable layer about 0.5 millimeters thick. Alternatively, the gimbals may be pre-formed and affixed to a rigid member to form the assembly. 
     Type I embodiments of the present invention compensate for operational force variations such as those caused by temperature variation. Type I devices include gimbals on one or both assemblies, compensating for variations in these forces that might otherwise become excessive. Some Type I devices are disc drives using stainless steel spindle bearings with balls made of ceramic. Ceramic balls typically have a thermal coefficient of expansion less than a fourth that of steel, often resulting in unacceptably large force variations in response to thermal variations less than 40 degrees Centigrade. Ceramic balls are much harder than stainless steel, however, resulting in favorable durability characteristics for applications such as disc drives. 
     A “rigid” element as used herein is a continuous mass of hard material (such as steel) of which no portion will be displaced from the rest by more than a few nanometers by ball bearing preloads less than 6 pounds. An “annular gimbal” as used herein is an annular mass of resilient material(s) such as steel arranged about an axis of symmetry. Gimbals of the present invention typically have a thickness Less than the diameter of the balls. Preferred disc drives of the present invention feature at least one spindle bearing gimbal with a spring constant 1 to 4 times larger (stiffer) than the balls in the spindle bearing assembly, under nominal normal operating conditions. 
     Type II embodiments of the present invention compensate for force variations that can occur during assembly, such as those caused by misalignment during the application of a preload. Gimbals of the present invention, when partially compressed or stretched, exert a restoring force that tends to equalize the preload force about the bearings. Virtually all conventional preload application mechanisms have enough give that this restorative force provides a helpful repositioning mechanism. 
     Additional features and benefits will become apparent to those skilled in the art upon reviewing the following figures and the accompanying detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art disc drive comprising a disc stack mounted onto the hub of a spindle bearing assembly. 
     FIG. 2 shows a method of the present invention for making an improved spindle motor. 
     FIG. 3 shows a disc drive having a spindle bearing exemplifying the present invention with an X-type preload. 
     FIG. 4 shows another preferred method of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Numerous aspects of disc drive or spindle bearing technology that are not a part of the present invention (or are well known in the art) are omitted for brevity. These include (1) detailed design or assembly of motor components; (2) the operation of recording discs, disc clamping mechanisms, or other technologies specific to disc drives; and (3) specific structures of basic bearing assemblies or preload application mechanisms. Although the examples below show more than enough detail to allow those skilled in the art to practice the present invention, subject matter regarded as the invention is broader than any single example below. The scope of the present invention is distinctly defined, however, in the claims at the end of this document. 
     FIG. 1 shows a prior art disc drive  200  comprising discs  105  mounted onto a hub  114  of a spindle bearing assembly  100 . Two coaxial ball bearing raceways are defined by outer bearing races  111  mounted to a rigid cylindrical support  112  and inner bearing races  113  mounted to a rigid shaft  115 . An armature core  116  is mounted on the outer peripheral surface of the support  112 . A drive magnet  117  is affixed onto the inner surface of the hub  114 . Armature core  116  and the drive magnet  117  and other parts make up a motor, which rotates the drive magnet  117  so as to rotate the hub  114  together with the drive magnet  117 . 
     FIG. 2 shows a method of the present invention for making an improved spindle motor, including steps  210  through  230 . Two raceways are constructed  212 , each comprising first and second race members. Suitable races are readily available for use in constructing race members of the present invention. A “race member” as used herein is an annular race or a rigid or gimbaled assembly that includes at least one annular race. As will become clearer from a review of FIG. 4, step  212  of constructing is preferably accomplished by gluing, welding, shrink-fitting, or integrally forming extensions onto at least one of the ordinary races. 
     Next, the second members of each raceway are affixed together into a common assembly having at least one gimbal between the second members  218 . The rolling members are then preloaded  222  so that the gimbal(s) are partially deformed as the first members are affixed into a common assembly  225 . Note that at steps  222  and  225 , gimbals are partially deformed so that they tend to compensate for any non-uniformity in the axial preloading force. 
     FIG. 3 shows a disc drive  400  having a spindle bearing assembly  300  of the present invention. Discs  390  are mounted in alternation with disc spacers  392  to form a disc stack having an axis of rotation  305 . A first set of balls  310  is positioned for movement along a first circle  312 , which is defined by the rotation of radius  317  about axis  305 . Upper races  313 , 314  compress the balls  310  along one of the axes of compression  315  as they roll. Each of the axes of compression  315  forms an acute angle  306  with axis  305  that is preferably less than about 80 degrees. The angle  306  may be inward as shown for an “X-type” preload, or may be outward for a “diamond-type” preload. It will be seen that the angle  306  and the preload magnitude each interact with the axial gimbal-deflecting force of the present invention. 
     A second set of balls  320  is positioned for movement along a second circle  322  defined by the rotation of radius  327  about axis  305 . Upper outer race  314 , backiron  330 , magnet  332 , hub  334 , and an outer vertical portion  351  of grooved member  350  are coupled together in a first rigid assembly that is configured for rolling engagement with the first set of balls  310 . Lower outer race  324  is coupled with an inner vertical portion  353  of grooved member  350  in a second rigid assembly that is configured for rolling engagement with the second set of balls  320 . 
     In addition to the vertical portions  351 , 353 , grooved member  350  includes an annular gimbal  352 . Gimbal  352  is operatively coupled between the first and second rigid assemblies, able to bend so that an axial force of less than 6 npounds between the rigid assemblies can produce an appreciable gimbal deformation. As gimbal deformation will be “appreciable,” for clarity as used herein, if it effects a ball bearing preload reduction of at least 0.1% as compared with the force that would exist in the absence of deformation. Gimbal deformation(s) allow the first rigid assembly to move axially with respect to the second rigid assembly, even after the inner races  313 , 323  are coupled together to form a complete rigid assembly. 
     Extending “substantially along” major surface  358  (e.g. best fit by least squares method) is a reference line  318  that passes through the axis of rotation  305  and forms a hinge angle  308  therebetween which will shift as gimbal  352  deforms. Annular gimbal  352  has a thickness  355  (measured perpendicular to the reference line  318 ) that is desirably about about 0.2 to 0.8 millimeters, and a width  356  (along reference line  318 ) that is desirably about 2 to 10 times larger. The axes of compression  315  and the reference line  318  desirably form a compression transfer angle  305  (in each plane passing through axis of rotation  305 ). A preferred gimbal  352  of the present invention has a compression transfer angel  305  in the range of about 10 to 25 degrees. 
     Alternatively, the reference line  318  of a given half-plane may be defined to maximize the ratio of the gimbal width  356  to the average gimbal thickness  355  perpendicular to that width  356 . This definition is also exemplified by FIG.  3 . 
     To increase the gimbal&#39;s deflection, gimbal  352  has a major surface  358  that is substantially perpendicular (i.e. within a few degrees) to the axis of rotation  305 . Note that gimbal  352  need not be a uniform layer but may take other shapes that will allow a deflection having an appreciable axial deflection such as a section of a bowl, cone shape, or toroid. In some cases, gimbal thickness will vary greatly. In the general case, a reference line is desirably constructed which is parallel to a line “substantially along” a surface midway between opposite major surface, of the gimbal. Reference line  318  meets this definition. Whatever variation in materials and geometry is used in the practice of the present invention, it is recommended that each gimbal generally have a minimum thickness that is less than the diameter of the rolling elements. 
     In a preferred embodiment, the balls  310 , 320  and the rigid assemblies essentially comprise a common alloy such as a steel, so that they expand fairly uniformly with temperature. Suitable steel balls  310 , 320  (e.g. SAE 52100) and rigid components optionally have a Rockwell Hardness (HRC) of about 56 to 59. In a most preferred embodiment, the balls  310 , 320  are instead made of a ceramic. Suitable ceramics, are readily commercially available that are significantly harder and more durable than steel. Unfortunately, ceramics generally have smaller coefficients of thermal expansion than hard alloys suitable for the rigid assemblies of a disc drive spindle bearing assembly. So that temperature variation will not cause large preload force variation, structures of this embodiment use a gimbal designed for preload force compensation. 
     FIG. 4 shows another preferred method of the present invention, comprising steps  410  through  475 . At least one annular groove is machined into a bearing housing to provide a predetermined gimbal thickness  415 . For a single-layer stainless steel gimbal such as that of FIG. 3 for use in a typical disc drive, the gimbal is desirably about 0.6 millimeters thick (with a tolerance of about 0.02 to 0.10 mm) over at least half of the gimbal&#39;s width. A nominal gimbal thickness greater than about 0.2 to 0.3 millimeters is preferred, because lesser gimbal thicknesses will require tolerances smaller than about 0.02 to 0.05 mm for a satisfactory degree of predictability in the gimbal&#39;s restorative force (i.e. modulus of elasticity). Such precise tolerances can increase manufacturing costs significantly. 
     Other materials may readily be substituted for part or all of the gimbal structure, so long as their dimensions are selected for similar resilience (i.e., within a few orders of magnitude). Lesser thicknesses may increase manufacturing costs because of the necessity of restrictive machining tolerances. Greater thicknesses, however, may reduce the axial range of gimbal deflection excessively. 
     Before or after machining the gimbal  415 , the bearing housing is glued onto the first raceway&#39;s outer member  420 . A large inner race element is constructed by gluing the shaft onto the first raceway&#39;s inner race  425  and onto the stator  430 . 
     After wiring the stator  435 , a large outer race element is constructed by affixing the backiron to the magnet  440 , to the hub  445 , and to the second raceway&#39;s outer member  450 . Next, glue is applied to the bearing housing/backiron joint  455  and to the shaft/second inner race member element  460 . Construction of the spindle bearing is completed by applying an axial preload while allowing the glue to cure  470 . The spindle bearing can then be assembled into a disc drive, and the disc(s) can be mounted onto the hub  470 . 
     In FIG. 3, the gimbal&#39;s movement is substantially axial (i.e. within about 1 degree of the axis of rotation) within its range. The angle between the axes of ball compression and of gimbal compression is desirably at least 5-15 degrees over the gimbal&#39;s range of motion, so that the gimbal can deflect significantly in response to ball bearing compression values less than 5 pounds. 
     Note that the structure of FIG. 3 can be obtained by methods other than those of FIG. 2 or  4 , such as by fully deflecting the gimbal before completing the assembly. Conversely, the distinct methods of FIGS. 2 &amp; 4 can each be used to make structures unlike that of FIG. 3, such as those having a gimbal on each of the two assemblies configured for relative rotation. 
     Referring again to the example of FIG. 3, Type I embodiments are presented above with a spindle bearing  300  part of which is configured for rotation about an axis  305 . A first set of balls  310  is positioned for movement along a first circle  312  within a raceway about the axis  305 . A second set of balls  320  is positioned for movement along a second circle  322  about the axis  305 . A first member (which includes outer race  314 ) is configured for rolling engagement with the first set of balls  310 , and a second member (which includes outer race  324 ) is configured for rolling engagement with the second set of balls  320 . This structure is improved by the inclusion of at least one annular gimbal  352  operatively coupled between the first and second members and able to bend so that the first member (including race  314 ) moves axially with respect to the second member (including race  324 ). After placing the gimbal(s), methods of the present invention include a step  225 , 465  of completing one or both assemblies for relative rotation. 
     FIG. 3 also exemplifies preferred Type I embodiments in which each ball of at least one set  320  has an axis of compression  315  forming an angle  306  with the axis of rotation  305  that is less than about 80 degrees. FIG. 3 defines a radial half plane extending to the right of axis  305 , which typifies radial half planes of the disc drive  400 . A reference line  318  is shown that intersects the axis of rotation  305  at an acute angle  308  greater than 45 degrees. Each ball of at least one set  320  also has an axis of compression  315  that intersects its respective reference line  318  to form a compression transfer angle  308  that is desirably less than about 25 degrees. 
     Referring again to the examples of FIGS. 2 &amp; 4, Type II embodiments are presented above as methods of assembling a spindle bearing from components including first and second bearing assemblies each comprising a set of balls in raceways. An annular gimbal on the first raceway&#39;s second member is constructed  415 , to which the other raceway&#39;s “second member” is affixed  218 , 455 , 465 . While urging the second members away from one another so as to deform the gimbal partially  222 , the “first members” are then assembled into a common fixed or gimbaled assembly  226 , 465 . This preload configuration will result in an X-type preload. Alternatively, step  465  can be performed with second members being urged toward one another so that a diamond-type preload will result. 
     All of the structures described above will be understood to one of ordinary skill in the art, and would enable the practice of the present invention without undue experimentation. It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only. Changes may be made in the details, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. For example, steps of the above methods can be reordered while maintaining substantially the same functionality, without departing from the scope and spirit of the present invention. In addition, although the preferred embodiments described herein are largely directed to spindle bearing configurations especially suitable in magnetic disc drives, it will be appreciated by those skilled in the art that many teachings of the present invention can be applied to other systems without departing from the scope and spirit of the present invention.