Abstract:
A conical hydrodynamic bearing gauging and seating system that enables the efficient seating of conical hydrodynamic bearings to a rotor shaft and hub while the rotor assembly remains in a pressing fixture. The invention provides a conical hydrodynamic bearing gauging and seating apparatus comprising a capacitive probe assembly disposed within a hollow pressing fixture in communication with a rotor shaft.

Description:
This application claims benefit of U.S. Provisional Application No. 60/185,820 filed Feb. 29, 2000, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of electric motor assembly. More specifically, the present invention relates to an apparatus and method for assembling conical hydrodynamic bearings in electric motors. 
     2. Description of the Background Art 
     Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric 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, while the information is accessed by read/write heads generally located on a pivoting arm which moves radially over the surface of the rotating 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. 
     During operation, the discs are rotated at very high speeds within an enclosed housing using an electric motor generally located inside the hub or below the discs. One type of motor in common use is known as an in-hub or in-spindle motor. Such known in-spindle motors typically have a spindle mounted by two ball bearing systems to a motor shaft disposed in the center of the hub. One of the bearings is located near the top of the spindle and the other near the bottom. These bearings allow for rotational movement between the shaft and the hub while maintaining accurate alignment of the spindle to the shaft. The bearings themselves are normally lubricated by highly refined grease or oil. 
     The conventional bearing system described above is prone, however, to several shortcomings. First is the problem of vibration generated by the balls rolling on the bearing raceways. Ball bearings used in hard disk drive spindles run under conditions that generally guarantee physical contact between raceways and balls, this in spite of the lubrication layer provided by the bearing oil or grease. Hence, bearing balls running on the generally even and smooth, but microscopically uneven and rough raceways. The ball bearings transmit the rough surface structure as well as their imperfections in sphericity in the form of vibration to the rotating disk. This vibration results in misalignment between the data tracks and the read/write transducer. This source of vibration limits, therefore, the data track density and the overall performance of the disc drive system. 
     Moreover, mechanical bearings are not always scaleable to smaller dimensions. This is a significant drawback since the tendency in the disc drive industry has been to continually shrink the physical dimensions of the disc drive unit. 
     As an alternative to conventional ball bearing spindle systems, researchers have concentrated much of their efforts on developing a hydrodynamic bearing. In these types of systems, lubricating fluid—either gas or liquid—functions as the actual bearing surface between a stationary base of the housing and the rotating spindle or rotating hub. For example, liquid lubricants comprising oil, more complex ferromagnetic fluids, or even air have been utilized for use in hydrodynamic bearing systems. The reason for the popularity of the use of air is the importance of avoiding the outgassing of contaminants into the sealed area of the housing. However, air does not provide the lubricating qualities of oil. Its low viscosity requires smaller bearing gaps and therefore higher tolerance standards to achieve similar dynamic performance. 
     Therefore, there is a need in the art for an apparatus and method that enables conical bearings to be press fit to motor shafts that is expedient and precise in order to increase throughput of the motor assembly process. More specifically, there is a need to be able to press conical bearings (cones) onto a shaft, precisely measure remaining axial play, and then adjust/press cone or cones a second time to final position. 
     SUMMARY OF THE INVENTION 
     A method for measuring axial play within a cone pressing apparatus for conical hydrodynamic bearings is provided. The apparatus includes a hollow nest motor mounting fixture that is sandwiched between a rotor assembly and a capacitive gauging apparatus. The hollow nest provides a fixed area that engages the lower or bottom conical hydrodynamic bearing while allowing the motor shaft to be pressed in place. The capacitive gauging apparatus measures the distance the shaft has been pressed and measures the distance the shaft may be moved in rectilinear motion. This distance corresponds to the axial play of the conical hydrodynamic bearings and allows for axial play to be reduced without removing the motor from the hollow nest fixture. 
     Several embodiments of the present invention are included. These embodiments vary in the degree of complexity and automation of the capacitive gauging apparatus. The first embodiment includes a target element affixed to the rotor shaft, and a capacitive element is used to gauge the displacement of the shaft as it is moved up and down by hand. Both of the elements are coupled to a hollow nest fixture. 
     In another embodiment, the gauging apparatus includes an automatic actuating device that reciprocates the motor shaft and measures the distance the shaft can be moved. Again, this is used in conjunction with a capacitive element, a rotor shaft and a hollow nest fixture. 
     In yet another embodiment, a plurality of actuators, similar to that of the previous embodiment, are employed in order to reciprocate the motor shaft in an axial direction to determine the axial play of the shaft. Here, too, the actuators are coupled to a capacitive gauge and a hollow nest fixture that relays information to a display device. 
     The method provided includes in all embodiments, the mounting of a rotor, having a shaft fit between two conical hydrodynamic bearings, to a hollow nest fixture. Once mounted upon the hollow nest, the rotor shaft is moved linearly in a rectilinear fashion along its axis wherein a measurement of the axial play is taken by the capacitive gauging element. The shaft is then pressed a distance into the rotor. The shaft is again linearly reciprocated while the axial play is measured by the capacitive gauging element. If axial play is still not within the desired range, the procedure is repeated. Once the conical hydrodynamic bearings are properly positioned such that the axial play is within the desired specification, the rotor is moved to the next stage in the motor assembly process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a top plan view of a known disk drive in which a motor fabricated and have the features of the present invention is useful; 
         FIG. 2  is a vertical sectional view of a spindle motor having two conical hydrodynamic bearings.; 
         FIGS. 3A-3F  are schematics of a conical bearing assembly and gauging apparatus; 
         FIG. 4  is a partial vertical sectional view of another embodiment of the present invention; 
         FIG. 5  is a vertical sectional view of the magnetic shaft and coil as shown in  FIG. 4 ; 
         FIG. 6  is a vertical sectional view of yet another embodiment of the present invention. 
       To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     
    
    
     DETAILED DESCRIPTION 
     The following description discloses the assembly of a spindle motor incorporating a pair of conical hydrodynamic bearings and a rotary shaft which carries a hub for supporting one or more disks. This form of spindle motor is especially useful in a disk drive for a computer system. The present invention allows for measurement of the axial play of the shaft during the final cone pressing process. Adopting this approach allows the conical hydrodynamic bearings to be pressed precisely into place in less time than assembly currently takes while reducing handling and minimizing associated errors. 
     A simple plan view of a disk drive of the type in which this motor is useful is shown in FIG.  1 . This view illustrates the basic elements of the drive, including a rotating disk or disks  19  that are rotated by a spindle motor  21  to be described in further detail below. As the disks  19  rotate, a transducer  23  mounted on an end of an actuator arm  27  is selectively positioned by a voice coil motor  29 . The VC motor  29  rotates about a pivot  31  to move the transducer  23  from track to track over the surface of the disk  19 . 
       FIG. 2  is a vertical sectional view of a prior art spindle motor including a set of conical hydrodynamic bearings which support the shaft  204  and the hub  202  for relative rotation. The motor is a brushless direct current motor  200  having a hub  202  rotatably mounted about stationary shaft  204  by upper and lower bearings  206  and  208 , respectively. The hub  202  is formed in a generally inverted U-shape in cross-section and has an inner annular arm  210 , and outer annular arm  212  and a top portion  214 . Outer annular arm  212  includes a shoulder  216  for supporting a storage disk in a contaminant free environment. A plurality of storage disks separated by spacers or washers may be stacked along the length of outer annular arm  212 . 
     The interior portion of hub  202  operably receives a stator, generally designated  220  including stator lamination stack  224  and stator windings  222 . A permanent magnet  228  is mounted on a back iron  229  supported from outer annular arm  212  for magnetically interacting with magnetically active stator lamination stack  224  and stator windings  222 . It is to be understood that a plurality of permanent magnet segments may be substituted for permanent magnet  228 . 
     The disk drive motor  200  is mounted to a frame or base member  230  of disc drive assembly  200  by inserting member  230 . 
     A stator support  240  surrounds stationary shaft  204  and supports stator  220  in a substantially vertical position. Stator support  240  comprises a boss  242  formed in base plate member  230  which serves to maintain disc drive motor  200  in a spaced relation with respect to base member  230 . The stator  220  is bonded to the base  230 . 
     A printed circuit connector  244  is mounted to a lower surface  246  of the base member  230 . Printed circuit connector  244  is electronically connected to stator windings  222  by a wire  248  for electrical communication between the stator windings and a printed circuit board (not shown). Circuitry is etched on a lower surface of circuit connector  244  for transmitting electrical signals from drive electronics or speed control circuits carried on external printed circuit boards or the like. 
     The present invention provides an apparatus and method that allows an electric spindle motor with conical hydrodynamic bearings to be quickly and easily assembled. 
       FIG. 4  is a vertical sectional view of an embodiment  400  of the present invention and specifically the apparatus for carrying out the measurement of axial play in the shaft illustrated in FIG.  3 D. The linear actuator and capacitive gauging apparatus  402  includes a shaft actuator  408  device for moving the rotor shaft  404 . The shaft actuator  408  moves the rotor shaft  404  in a reciprocating axial motion. The shaft actuator  408  includes a magnetic shaft  410 , two other shafts  412 A and  412 B, linear bearings  414 A and  414 B and two oppositely wound coils  416 . The shafts  412 A and  412 B are held in place by a pair of linear bearings  414 A and  414 B. The linear bearings  414 A and  414 B position the magnetic shaft  410  such that it is disposed through the center of coils  416 . The linear actuator and gauging apparatus  402  is disposed within a housing  418  and coupled on one end to a nest fixture  420 . The assembly  400  may be permanently affixed or removably affixed by any method, including but not limited to, welding, bolting, gluing or threading Because the shaft assembly is directly coupled to the rotor shaft  404 , any movement of the magnetic shaft  410  will translate into axial play of the rotor shaft  404 . Thus, the axial play can be measured by monitoring the motion of the lower shaft in shaft actuator  412 B using the capacitive probe  406  as described above with reference to  FIGS. 3D-3F . 
     Furthermore, the upward force and the downward force can be set to different levels so as to offset the weight of rotor shaft  404  and the conical shaft assembly  424 . The technique of inputting a sign wave into the magnetic coil assembly  416  causes the magnetic shaft  410  to reciprocate; thus, a large number of axial play measurements can be taken on a rotor assembly  424  in a short amount of time using this technique. This method produces statistically more reliable measurements of the axial play in a particular rotor assembly than could be obtained previously. 
       FIG. 5  is an enlarged view of a vertical sectional view of the magnetic coil assembly of the shaft actuator  408  of FIG.  4 . In this figure, a magnetic shaft  502  is disposed between two rows of wound linear voice coils  504 A and  504 B. Surrounding the outside of the linear voice coils is the back iron section  506 . The two coils  504 A and  504 B are wound in opposite directions and placed on the inside of the back iron  506 . When a voltage is applied to the coils, the current in the upper coil  504 A is moving clockwise when viewed from above. If the magnetic field at the top of the magnetic shaft  502  is moving out radially, through the coil  504 A and back iron  506 , the force produced will be orthogonal to the magnetic field and current vectors. These forces translate into up or down movements of the magnetic shaft  502  in the Y direction. The resulting magnetic field in the return path at the bottom will be in the opposite direction of the top. To produce a force in the same direction, the lower coil  504 B must be wound in the opposite direction. 
       FIG. 6  is a vertical sectional view of another embodiment  600  of the present invention. This apparatus measures axial play of a spindle motor by using a system similar to that of the previous embodiment. In this embodiment, however, dual shaft actuators  602  and  602 B are employed to apply force to the rotor&#39;s shaft  604  to obtain a measurement of axial play of the shaft  604 . 
     This design incorporates flanged magnetic shafts  608 A and  608 B. Each of the shafts has a magnetic flange  610 A and  610 B located near the center and extending radially outward. The magnetic shafts  608 A and  608 B for both the upper and the lower linear voice coil actuators  602  and  602 B are substantially similar. The actuators  602  and  602 B each engage the rotor shaft  604  axially along a geometric center line. Each of the magnetic voice coil actuators  602  and  602 B has a pair of magnetic-wound coils  612  and  614  above and below the flanges  610 A,  610 B of the magnetic shafts  608 A and  608 B. The magnetic coils  612  and  614  consist of an upper set  612 A and  614 A and a lower set  612 B and  614 B for each of the actuators  602  and  602 B. The magnetic coils  612  and  614  in each of the actuators  602  and  602 B consist of the lower coil  612 A and  614 B wound in a first direction and the upper coil  612 A and  614 A wound in a second direction. When a current is applied to the actuators  602  and  602 B, the magnetic shafts  608 A and  608 B are forced to move. As in the previous embodiment, a capacitive probe  620  is located under the shaft actuator  608 A for measuring the axial movement of the rotor shaft  604 . 
       FIGS. 3A through 3F  illustrate a method for assembling the rotor assembly, such as used in the motor of FIG.  2 .  FIG. 3A  depicts a rotor shaft  802  for a spindle motor as used in accordance with a first embodiment of the present invention. In  FIG. 3A , the lower male cone  804  is pressed onto the rotor shaft  802 . (Note that the shaft and hub assembly depicted in this and the following figures is upside down from the way it will be incorporated into a furnished motor.) The lower male cone  804  is pressed a distance onto the rotor shaft  802  such that it allows the rotor shaft  802  to pass through the rotor assembly  808 . Enough space is allowed on the rotor shaft  802  so that the upper cone  806  may be pressed onto the rotor shaft  802  to cooperate with the opposite portion of the rotor  808  as shown in FIG.  3 B. 
       FIG. 3B  shows a further step in the construction of the rotor assembly  800 . The rotor shaft  802  is inserted into the central aperture  805  of the rotor  808 . The rotor shaft  802  is pushed through the rotor  808  until the lower male cone  804  comes in contact with the lower female cone  803  of the rotor  808 . An upper male cone  806  is pressed onto the rotor shaft  802  from the lower side as shown, thus capturing the rotor shaft  802  within the rotor  808 . 
       FIG. 3C  shows a further step in the construction of the rotor assembly  800  wherein the rotor  808  is supported on the nest fixture  812 . For proper support to occur, the rotor  808  must be aligned with the nest  812  so that the rotor shaft  802  may enter the nest aperture  813 . The alignment is critical because the rotor shaft  802  must enter the nest aperture  813  in order for the shoulders  818  to support the male cone  806 . 
     Once properly mounted as shown in  FIG. 3D , the rotor shaft  802  is in communication with the capacitive probe  816  by means of target  811  fixed to shaft  802 . At this stage of assembly, the rotor shaft  802  is caused to move in a reciprocating fashion along its geometric axis as illustrated by double arrow  820  by apparatus to be described with respect to  FIGS. 4  et seq. The movement is sensed by the capacitive probe  816  which relays data regarding the amount of movement sensed to a display device (not shown). The capacitive probe  816  is able to sense the distance of the target element  811  to the capacitive probe  816  through the use of an electrical impulse. The capacitive probe  816  sends and receives signals when energized such that the capacitance can be measured between the probe  816  and the surface of the target elements  811 , so that even very small capacitances can be measured accurately. The capacitance measured is then converted to a distance measurement. 
     After the data has been interpreted and a solution calculated, a force, as illustrated by arrow  822 , is applied to the top of the shaft, as seen in  FIG. 3E , in order to move the male portion of the conical hydrodynamic bearings  806  closer to the female portion. 
     The rotor shaft  802  is again moved in a reciprocating fashion, illustrated by double arrow  824 , as shown in FIG.  3 F. Movement of the rotor shaft  802  will be detected by the capacitive probe  816  and converted into a measurement representing the total axial play available to shaft  802 , this measurement thus accurately represents the total gap in the two hydrodynamic bearings  804 ,  806 . Another pressing and measurement can then take place until a target measurement is achieved. The rotor assembly  808  is then ready to go onto other stages of preparation as commonly known in the art in order to produce an electric motor. 
     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.