Abstract:
A method for setting a gap in a hydrodynamic bearing of a disc drive spindle motors and a motor having such a bearing gap. The method comprises mounting a rotor hub having a central journal sleeve onto a shaft that has been secured to a support after having had a lower thrust bearing pressed onto the shaft in communication with said support, adding an amount of hydrodynamic fluid into the rotor hub&#39;s journal sleeve, pressing an upper thrust bearing onto the rotor shaft until contact is made with the rotor hub, and rotating the hub until axial forces balance and set the bearing gap.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is based on U.S. Provisional Application, Ser. No. 60/247,099, entitled “OPPOSED FDB FOR SETTING BEARING GAP”, filed Nov. 9, 2000, by Herndon, et al., which is hereby incorporated by reference in its entirety. 
     
    
     
       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 the gaps in the hydrodynamic bearings in electric motors utilized in a disk drive system.  
         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 using 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 using 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 using two ball or hydrodynamic bearing systems to a motor shaft disposed in the center of the hub.  
           [0005]    In a hydrodynamic bearing, a lubricating fluid such as air, gas or oil provides a bearing surface between two relatively rotating members, typically a shaft and surrounding sleeve. A volume containing the lubricating fluid is typically held between the hydrodynamic bearings. Each bearing is positioned proximate an end of the shaft and is spaced apart from the rotor hub by a small gap. To effectively form hydrodynamic bearings, the volume of fluid must be consistently and accurately formed. As such, the gap between the bearings and the rotor hub must be repeatable from disc drive to disc drive in the manufacturing process.  
           [0006]    Therefore, there is a need in the art for a method that can accurately and repeatably set these gaps while allowing for high-speed assembly.  
         SUMMARY OF THE INVENTION  
         [0007]    A method for setting a gap in a hydrodynamic bearing of a disc drive spindle motor is provided. The invention comprises mounting a rotor hub having a central journal sleeve onto a shaft that has been secured to a support after having had a lower thrust bearing pressed onto the shaft in communication with the support, adding an amount of hydrodynamic fluid into the rotor hub&#39;s journal sleeve, pressing an upper thrust bearing onto the rotor shaft until contact is made with the rotor hub, and rotating the hub until axial forces balance and set the air and fluid bearing.  
           [0008]    While rotating, the rotor hub generates a pressure force that forces the hydrodynamic fluid disposed around the base of the shaft to move up the shaft. As the pressure force builds, a layer of air between the upper bearing and the fluid builds up pressure, the layer of air is forced between the upper shaft bearing and the rotor hub. The pressure forces the rotor hub to move until the air pressure equals the fluid pressure. This invention is especially useful in disc drive spindle motors, in that it provides a more efficient way of assembling a disc drive spindle motor because of the significant reduction of steps for manufacturing the prior art hydrodynamic bearings commonly found in disc drive spindle motors. Additionally, the invention may be useful for setting gaps in the hydrodynamic bearings of other types of motors. The invention can be used in both conical type hydrodynamic bearings and flat/thrust plate bearings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The teachings of the invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0010]    [0010]FIG. 1 is a top plan view of a disc drive, in accordance with the present invention;  
         [0011]    [0011]FIG. 2 is a sectional view of an isolated hydrodynamic bearing spindle motor in accordance with the present invention;  
         [0012]    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 the present invention; and  
         [0013]    [0013]FIG. 4 is a flow chart of the steps required to set bearing gaps in an electric motor according to the present invention.  
         [0014]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    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 disc drive  10  wherein the invention is useful. The disc drive  10  comprises 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.  
         [0016]    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  14 .  
         [0017]    The disc drive  10  further comprises a disc pack  16  that is mounted for rotation on a spindle motor (not shown) by a disc clamp  18 . The disc pack  16  includes one or more of individual discs that are mounted for co-rotation about a central axis. Each disc surface has an associated head  20  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  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 disc drives having other types of actuators, such as linear actuators.  
         [0018]    [0018]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 fixed and attached to a base  208 . The hub  204  is supported by the shaft  202  through bearings  210 A and  210 B for rotation about the shaft  202 . The bearings  210 A and  210 B are, for example, hydrodynamic bearings.  
         [0019]    The bearings  210 A and  210 B depicted in FIG. 2 are conical type hydrodynamic bearings as contrasted to the “flat plate” or “thrust” design as illustrated in FIG. 3. The present invention covers both designs, and as such, both designs have been represented.  
         [0020]    The hub  204  includes a disc carrier flange  212  that supports a disc pack (not shown) for rotation about the shaft  202 . The disc pack is held on the disc carrier flange  212  by a disc clamp (not shown). A plurality of permanent magnets  214  are attached to a first inner surface  216  of the hub  204 , with the hub  204  and the magnets  214  acting as a rotor for the spindle motor  200 .  
         [0021]    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.  
         [0022]    In accordance with the invention, the hub  204  is initially assembled without any air gaps between the hub  204  and the bearings  210 A and  210 B. The invention sets the air gap. The invention may be practiced regardless of the hydrodynamic bearing type specified.  
         [0023]    The reader may find it useful to simultaneously refer to FIGS. 3 and 4. FIGS.  3 A- 3 E are a series of sectional views of the assembly process according to the present invention. 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 chart representing a method  400  of motor assembly and setting of gaps in the hydrodynamic bearings of an electric motor according to the present invention.  
         [0024]    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 .  
         [0025]    [0025]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  305  in communication with elements of a shaft  304  and a lower thrust bearing  306 . Typically, a shaft  304  is mounted or affixed, at step  403 , to a rotor hub support  305  by conventional methods such as press fitting or use of fasteners, epoxy, etc., while the lower thrust bearing is generally press-fit into place, although other arrangements may be used.  
         [0026]    [0026]FIG. 3B depicts, at step  404 , a progression in the assembly process  400  wherein a rotor hub  308  is aligned coaxially with the center of the shaft  304  and affixed onto the shaft  304  such that the lower recess  310  of the rotor hub  308  rests on the lower thrust bearing  306 . The bearing face  314  of the lower thrust bearing  306  is perpendicular to the shaft  304 , while the body of the lower thrust bearing  306  is coaxially aligned with the central axis of the shaft  304 . The journal sleeve  314  of the rotor hub  308  acts as a fluid transmission conduit as well as an axial bearing surface.  
         [0027]    Once the rotor hub  308  is seated on the lower thrust bearing  306 , a hydrodynamic fluid  316 , at step  406 , is deposited into the rotor hub  308  before the upper thrust bearing  318  is installed. The fluid  316  is placed into the rotor hub&#39;s journal  312  where it flows down around the lower thrust bearing  306  and forms a meniscus  320  between the rotor hub  308  and the lower thrust bearing  306 .  
         [0028]    After the hydrodynamic fluid  316  has been added, the upper thrust bearing  318  is ready to be installed at step  408 . The upper thrust bearing  318  is aligned coaxial with the shaft  304  and press fit into an upper recess  332  formed in the top of the rotor hub  308  as shown in FIG. 3D. No gap is left between the top of the rotor hub  308  and the upper thrust bearing plate  318 , however, a gap  334  does exist between the rotor hub  308  and the lower thrust bearing  306 . The gap  334  is a fluid gap where hydrodynamic fluid  316  has accumulated via capillary action after having been added in FIG. 3C. The fluid 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. The present invention focuses on setting an upper air bearing gap  330  between the rotor hub  308  and the upper thrust bearing plate  318 . After the upper thrust bearing plate  318  has been set, the fit between the rotor hub  308  and the thrust bearing plate  318  is checked to make certain contact is achieved as noted in step  410  of FIG. 4.  
         [0029]    The upper air bearing gap  330  is not created until the rotor hub  308  is rotated at the normal operating speed of the motor during use (known as duty cycle speed). Hub rotation is denoted in FIG. 3E and FIG. 3F as arrow  324 . Before being rotated, the unit is assembled as a complete motor as noted in step  412  of FIG. 4. (Illustrations of a completed motor assembly have been omitted from the FIG. 3 series for clarity.) At step  414 , the rotor hub  308  is rotated about the shaft  304  to pressurize the hydrodynamic fluid  316 . The pressure may vary from a few pounds per square inch gas (PSIG) to multihundreds of PSIG (gauge pressure). The pressurized hydrodynamic force generated creates an air gap  330  that forms between the upper thrust plate  318  and the rotor hub  308 . The rotor hub  308  will ride at a gap between the lower thrust bearing  306  and the upper thrust bearing  318  where the axial forces shown by arrows  328 A and  328 B are balanced. Typically, in order for an air bearing gap to generate enough equal and opposite force to counteract the axial force generated by the hydrodynamic fluid riding on the bottom thrust bearing, the upper thrust bearing  318  will have to ride at less than a  0 . 001  mm gap. The motor is checked in step  416  for proper adjusted function.  
         [0030]    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 thrust gaps  330  and  334  as seen in FIG. 3F.  
         [0031]    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.