Abstract:
A method and apparatus for assembling a hydrodynamic bearing in a motor is provided. The method comprises affixing a first bearing upon a shaft, applying an axial tension force to the shaft, and affixing a second bearing upon the shaft in a spaced apart relation to the first bearing, as the axial force is applied to the shaft.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the priority of U.S. Provisional Application No. 60/368,488, filed Mar. 29, 2002 (entitled “Improved Gap Setting Tool Design”), which is herein incorporated by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to the field of hydrodynamic bearings in electric motors. More specifically, the invention relates to a method for setting a gaps in a hydrodynamic bearing of an electric motor.  
         BACKGROUND OF THE INVENTION  
         [0003]    Disk 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 disk medium, the actual information being stored in the form of magnetic transitions within the medium. The disks themselves are rotatably mounted on a spindle. The information is accessed by using read/write heads generally located on a pivoting arm that moves radially over the surface of the 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 that is generally located inside a hub that supports the disks. 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 using two ball or hydrodynamic bearings mounted to a motor shaft disposed in the center of the hub.  
           [0005]    In a hydrodynamic bearing, a bearing has two spaced-apart surfaces, mounted respectively on two relatively rotating members (typically a shaft and a surrounding sleeve), with a lubricating fluid such as air, gas or oil providing a bearing between them. In one design, a bearing surface is positioned proximate each end of the shaft and is spaced apart from another bearing surface mounted on the rotor hub. A volume containing the lubricating fluid (a gap) is therefore formed between the bearing surfaces. The gap between the bearing surfaces must be repeatable from disk drive to disk drive in the manufacturing process.  
           [0006]    The conventional technique for setting a gap in a bearing comprises mounting a lower bearing component onto a shaft after the shaft has been secured to a support. A rotor hub having a central journal sleeve and a bearing surface affixed thereon is then mounted onto the shaft in a spaced apart relation to the lower bearing component, and an amount of hydrodynamic fluid is added into the rotor hub&#39;s journal sleeve. An upper bearing is then pressed onto the rotor shaft in a spaced apart relation to the rotor hub to complete the assembly.  
           [0007]    One problem with this conventional technique, however, is that it compresses the shaft between the bearing surfaces, enlarging the shaft diameter and causing additional stiction between the shaft and the bearing surfaces. The problem is further complicated when the shaft is compressed to an enlarged diameter that is greater than the actual gap that must be set; consequently the hydrodynamic bearing cannot be properly formed.  
           [0008]    Therefore, there is a need in the art for a method that can accurately and repeatably set these gaps while allowing for rapid motor assembly.  
         SUMMARY OF THE INVENTION  
         [0009]    The invention provides a method and apparatus for assembling a hydrodynamic bearing of a disk drive spindle motor. This invention provides for the pulling of the shaft by applying an axial force to its free end while installing the bearing components, rather than compressing the shaft between the components. This elongates the free end of the shaft and eliminates compression in the shaft during motor assembly. Additionally, the invention may be useful for setting gaps in the hydrodynamic bearings of other types of motors. The invention can be used for installing conical type hydrodynamic bearings and flat/thrust plate bearings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The teachings of the invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0011]    [0011]FIG. 1 is a top plan view of a disk drive;  
         [0012]    [0012]FIG. 2 is a sectional view of an isolated hydrodynamic bearing spindle motor;  
         [0013]    FIGS.  3 A- 3 F are a series of sectional motor assembly views of a sequence of steps to set bearing gaps in an electric motor according to one embodiment of the present invention; and  
         [0014]    [0014]FIG. 4 is a flow diagram of a method for setting bearing gaps in an electric motor according to one embodiment of the present invention.  
         [0015]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     
    
     DETAILED DESCRIPTION  
       [0016]    The invention comprises a method for setting bearing gaps for hydrodynamic bearings in an electric motor. FIG. 1 is a plan view of a typical disk drive  10  wherein the invention is used. The disk drive  10  comprises a housing base  12  and a top cover  14 . The housing base  12  is combined with the top cover  14  to form a sealed environment to protect the internal components from contamination by elements from outside the sealed environment.  
         [0017]    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 disk drive housing. For example, disk 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 the 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  14 .  
         [0018]    The disk drive  10  further comprises a disk pack  16  that is mounted for rotation on a spindle motor (not shown) by a disk clamp  18 . The 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 head  20  for communicating with the disk surface. In the example shown in FIG. 1, heads  20  are supported by flexures  22  that are in turn attached to head mounting arms  24  of an actuator body  26 . The actuator body  26  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 . The voice coil motor  28  rotates the actuator body  26  with its attached heads  20  about a pivot shaft  30  to position the heads  20  over a desired data track along an arcuate path  32 . While a rotary actuator is illustratively shown in FIG. 1, the invention is also useful in disk drives having other types of actuators, such as linear actuators.  
         [0019]    [0019]FIG. 2 is a sectional view of a hydrodynamic bearing spindle motor  200 . The spindle motor  200  includes a stationary shaft  202 , a hub  204  and a stator  206 . The shaft  202  is disposed through a base  208 . The shaft  202  includes an inlet conduit  224  with outlet ports  226 A and  226 B that branch off and exit the shaft  202  at a point on the length of the shaft  202  that is between the bearing components  210 A and  210 B. The hub  204  is supported by the shaft  202  through bearing components  210 A,  210 B and  211  for rotation about the shaft  202 . The bearing components  210 A,  210 B and  211  comprise, for example, a hydrodynamic bearing.  
         [0020]    The bearing (comprising components  210 A,  210 B and  211 ) depicted in FIG. 2 is a conical type hydrodynamic bearing as contrasted to the “flat plate” or “thrust” design; the present invention may be used in conjunction with either bearing design. Specifically, bearing components  210 A and  210 B are “male” bearing components that are spaced apart from a “female” bearing component  211  bya gap  222 .  
         [0021]    The hub  204  includes a plurality of permanent magnets  214  attached to a first inner surface  216  of the hub  204 , with the hub  204  and the magnets  214  operating as a rotor for the spindle motor  200 .  
         [0022]    The stator  206  is generally formed of a stack of stator laminations  218  that form a plurality of stator “teeth” that are each wound with an associated stator winding  220 . The stator  206  is generally retained in the base  208  by fasteners, adhesives or other conventional methods.  
         [0023]    In accordance with the invention, the hub  204  is initially assembled without any air gaps between the female bearing component  211  and the male bearing component  210 A and  210 B. The inventive method described below sets the air gap between the bearing components.  
         [0024]    The reader may find it useful to simultaneously refer to FIGS.  3 A-F and  4 . FIGS.  3 A- 3 F are a series of schematic, sectional views of the assembly process according to one embodiment of the present invention. FIGS.  3 A-F illustrate a “static” technique for setting a bearing gap; however it should be appreciated that the inventive approach described may be used in “dynamic” setting techniques as well to yield additional and alternative benefits. This series has been simplified in order to emphasize the unique features of the present invention. The series as such depicts only the necessary elements needed to fully describe the present invention. FIG. 4 is a flow diagram representing a method  400  of motor assembly and setting of gaps in the hydrodynamic bearings of an electric motor according to one embodiment of the present invention.  
         [0025]    The first step in the method for setting gaps in hydrodynamic bearings begins with FIG. 3A. The assembly process  400  begins by providing a motor housing in step  402 .  
         [0026]    [0026]FIG. 3A depicts a rotor hub support housing  302 . Those skilled in the art will appreciate that a rotor hub support housing  302  may comprise a range of elements from the essential components necessary to support a rotor hub to the entire assembly surrounding the entire motor. FIG. 3A depicts the rotor hub support housing  302  in communication with elements of a shaft  304  and a lower “male” conical bearing component  210 B. Typically, a shaft  304  is mounted or affixed, at step  402 , to a rotor hub support housing  302  by conventional methods such as press fitting or use of fasteners, epoxy, etc., while a lower male bearing component  210 B is generally press-fit into place, although other arrangements may be used. A free end  305  of the shaft  304  extends beyond the bottom surface of the rotor hub support  302 .  
         [0027]    [0027]FIG. 3B depicts the motor at step  406 , in which the rotor hub  308  is set into the motor housing  302 . The rotor hub  308  is fixedly attached, for example by press fitting, to a “female” conical bearing component  211  that is disposed on the surface  313  of the rotor hub  308 . The rotor hub  308  is aligned coaxially with the center of the shaft  304  and rests such that a small volume of air (gap)  311  remains in between the female bearing component  211  and lower male bearing component  210 B. The angled face  314  of the lower male bearing component  210 B is roughly parallel to the angled face  315  of female bearing component  211 , while the body of the lower male bearing component  210 B is coaxially aligned with the central axis of the shaft  304 . The journal sleeve  312  of the rotor hub  308  is spaced from the shaft  304  and acts as a fluid transmission conduit as well as an axial bearing surface.  
         [0028]    The upper male bearing component  210 A is installed at step  410  (see FIG. 3D). The upper male bearing component  210 A is aligned coaxial with the shaft  304  and press fit (arrows  322 ) into an upper recess  332  defined in the top of the rotor hub  308 . Little or no gap is left between the female bearing component  211  at top of the rotor hub  308  and the upper male bearing component  210 A, however, a gap  334  does exist between the female bearing component  211  and the lower male bearing component  210 B. The gap  334  may be set or adjusted by conventional methods such as tooling, etc., and is set within the range of 0.003 mm to 0.02 mm.  
         [0029]    [0029]FIG. 3E depicts the final step in the gap setting method, step  412 . A capacitance probe  340  is coupled to the assembly to monitor the distance between the female bearing component  211  and the upper male bearing component  210 A. An axial force F is applied to the free end  305  (i.e. the end that extends through the bottom of the support housing  302 ) of the shaft  304  in a direction away from the support housing  302 , essentially pulling the free end  305  of the shaft  304  downward. This is accomplished by securing a pulling means  309  to the free end  305  of the shaft  304  and applying force F. The pulling means  309  may be, for example, a clamp, though it should be appreciated that any sort of device capable of securedly grasping the free end  305  may be used. The value of the force F that is applied to the free end  305  of the shaft  304  is, for example, in the range of 50 to 250 lbs. Simultaneously, lower male bearing component  210 B is held stationary to allow the shaft to be stretched.  
         [0030]    As the axial force F is applied to the free end  305  of the shaft  304 , the rotor hub  308  slides downward on the shaft  304 , narrowing the gap  334  between the female bearing component  211  and the lower male bearing component  210 B. Simultaneously, a gap ( 330  in FIG. 3F) is formed between the female bearing component  211  and the upper male bearing component  210 A as the rotor hub  308  moves away from the upper male bearing component  210 A. The capacitance probe  340  is used to monitor the progress of the gap  330  until it is properly set. FIG. 3F illustrates the resulting bearing gaps  330  and  334 : bearing gap  330  is formed between the upper male bearing component  210 A and the female bearing component  211 , while bearing gap  334  is formed between the lower male bearing component  210 B and the female bearing component  211 .  
         [0031]    Step  412  is essential to avoid compression (and resulting increase in diameter) or tension of the shaft  304  in the region between male bearing components  210 A and  210 B when the gap is set. Pulling on the shaft  304  serves to elongate the free end  305  and make the shaft  304  “neck down” or decrease in diameter, which also reduces stiction between the shaft  304  and the bearing surfaces.  
         [0032]    Once balanced, there are three translational axis of freedom. Two of the axis&#39; absolute motion are constrained by the journal gap size  336 , while the axial absolute movement is constrained by the bearing gaps  330  and  334  as seen in FIG. 3F. The ends of the shaft  304  are resolved, for example, by mounting or screwing them into the base and top cover of the disk drive (shown in FIG. 1), but may be disposed of otherwise in assembling the drive.  
         [0033]    Alternatively, the same concept of applying an axial force F to a free end  305  of the shaft  304  may be used in dynamic gap setting techniques (i.e. rotation of the hub). For example, the rotor hub  308  may be rotated to build pressure within the journal aperture  336 , creating an air gap between the female bearing component  211  and the upper male bearing component  210 A. An additional benefit of using such a method here is that it allows the female bearing component  211  to be moved axially while the gap is being set. This is real-time feedback of the actual gap, which allows the gap to be measured, for example by a capacitance probe, while it is being set.  
         [0034]    Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings. What is claimed is: