Abstract:
Approaches for a fluid dynamic bearing (FDB) system for use within a hard-disk drive. A fluid dynamic bearing (FDB) system may comprise an upper conical bearing and a lower conical bearing that are both disposed along a stationary shaft on which a magnetic-recording disk is rotatably mounted. The upper conical bearing and the lower conical bearing may have different cone angles, diameters, and/or lubricants to produce a desired difference in stiffness between the first conical bearing and the second conical bearing. By adjusting characteristics of the fluid dynamic bearing (FDB) system to achieve the desired bearing stiffness ratio, the tendency for the magnetic-recording disks to experience a sustained vibration when the hard-disk drive receives a mechanical shock is reduced. By preventing the magnetic-recording disks from sustained vibration after a mechanical shock, data may be written to and read from the magnetic-recording disks with greater reliability.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the invention relate to conical fluid dynamic bearings (FDB) in a hard-disk drive (HDD). 
       BACKGROUND OF THE INVENTION 
       [0002]    A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disks having magnetic surfaces (a disk may also be referred to as a platter). When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head which is positioned over a specific location of a disk by an actuator. 
         [0003]    A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. As a magnetic dipole field decreases rapidly with distance from a magnetic pole, the distance between a read/write head and the surface of a magnetic-recording disk must be tightly controlled. An actuator relies on suspension&#39;s force on the read/write head to provide the proper distance between the read/write head and the surface of the magnetic-recording disk while the magnetic-recording disk rotates. A read/write head therefore is said to “fly” over the surface of the magnetic-recording disk. When the magnetic-recording disk stops spinning, a read/write head must either “land” or be pulled away onto a mechanical landing ramp from the disk surface. 
         [0004]    As the distance between the read/write head and the surface of the magnetic-recording disk must be tightly controlled, it is desirable that the magnetic-recording disks vibrate or oscillate as little as possible. To address this concern, a tied-shaft fluid dynamic bearings (FDB) spindle motor may be used to increase the rigidity of the motor structure and to minimize, and potentially eliminate, any vibration or oscillation of the magnetic-recording disks, especially when the HDD is bumped or otherwise experiences a mechanical shock. 
         [0005]      FIG. 1  illustrates a tied-shaft fluid dynamic bearings spindle motor according to known approaches.  FIG. 1  is taken from and discussed in U.S. Pat. No. 6,118,620. As illustrated in  FIG. 1 , upper symmetrical cone  10  and lower symmetrical cone  12  are symmetrical and serve to stabilize the magnetic-recording disk, both when magnetic-recording disk is rotating and when the HDD is bumped or otherwise experiences a mechanical shock. The dimensions of upper symmetrical cone  10  and lower symmetrical cone  12  are designed to be the same so that upper symmetrical cone  10  and lower symmetrical cone  12  have the same bearing stiffness. If the upper symmetrical cone  10  and lower symmetrical cone  12  have the same bearing stiffness, then it is expected that any force applied to the magnetic-record disk (perhaps due to the HDD being bumped) should be equally dampened by upper symmetrical cone  10  and lower symmetrical cone  12 , thereby reducing or eliminating the tendency for the magnetic-recording disk to oscillate. 
       SUMMARY OF THE INVENTION 
       [0006]    It is observed that, in practice, the upper cone and the lower cone of a fluid dynamic bearing (FDB) in a hard-disk drive (HDD) may bear different amounts of weight. For example, lower symmetrical cone  12  of  FIG. 1  bears more weight than upper symmetrical cone  10  as lower symmetrical cone  12  bears more of the weight of the magnetic-recording disk(s) and motor structure. As a fluid dynamic bearing bears more weight, the fluid within the bearing compresses, thereby causing the bearing to become stiffer. As a result, when a HDD is oriented as shown in  FIG. 1 , lower symmetrical cone  12  will have a higher bearing stiffness than upper symmetrical cone  10 , even though upper symmetrical cone  10  and lower symmetrical cone  12  have the same or substantially similar dimensions. The bearing stiffness between upper symmetrical cone  10  and lower symmetrical cone  12  may differ around 10% due to the disparity in how much weight is born by each bearing. FDB systems with symmetric cone designs can generally accommodate this disparity in stiffness between the upper and lower cones. 
         [0007]    However, certain spindle motors may require FDB designs that support a larger disparity in bearing stiffness (for example, a 200% difference) between the upper and lower conical bearings. This need may arise due to an uncertainty in how much external force each conical bearing will eventually bear (which may be compounded because it may be unknown how the HDD will be oriented in the field). A FDB system with a symmetric or substantially symmetrical conical bearing design may not be able to accommodate such difference in the bearing stiffness between the upper and lower conical bearings. Also, as the form factor of HDDs becomes smaller and more compact, the space limitations within the HDD may render the use of symmetrical conical bearings impractical or impossible. 
         [0008]    Embodiments of the invention provide for a fluid dynamic bearing (FDB) system that supports a greater range of difference in the bearing stiffness between the conical bearings of the FDB system. The difference in stiffness between the conical bearings of the FDB system of an embodiment may be achieved through a variety of different approaches. In an embodiment, the upper and lower conical bearings may have different cone angles. A cone angle is the angle measured from the axis of the conical bearing to the lateral surface of the conical bearing. By changing the cone angle of a conical bearing, the bearing stiffness of the conical bearing may be adjusted. Similarly, the cone diameter of a conical bearing may also be adjusted to produce a desired change in the bearing stiffness of a conical bearing. Naturally, these approaches may be used individually, or in combination, so that each conical bearing of the FDB system has a desired stiffness. 
         [0009]    In another embodiment, the lubricant or fluid used within a fluid dynamic bearing may be designed to have a viscosity which results in a desired bearing stiffness ratio between the upper and lower conical bearings. To illustrate, an upper conical bearing may use a lubricant or fluid having a different viscosity than the lubricant used with the lower conical bearing. Alternatively, either the upper conical bearing or the lower conical bearing may change the temperature of the same lubricant or fluid to adjust the viscosity of the lubricant or fluid to configure the stiffness of that conical bearing. Using a lubricant or fluid with a certain viscosity may be, but need not be, performed in concert or conjunction with one or more of adjusting the cone angle or the cone diameter of a conical bearing to realize a desired stiffness in the conical bearing. 
         [0010]    Embodiments discussed in the Summary of the Invention section are not meant to suggest, describe, or teach all the embodiments discussed herein. Thus, embodiments of the invention may contain additional or different features than those discussed in this section. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Embodiments of the invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
           [0012]      FIG. 1  illustrates a tied-shaft fluid dynamic bearing (FDB) spindle motor according to known approaches; 
           [0013]      FIG. 2  is a plan view of an HDD according to an embodiment of the invention; 
           [0014]      FIG. 3  is a plan view of a head-arm-assembly (HAA) according to an embodiment of the invention; 
           [0015]      FIG. 4A  is a diagram of a fluid dynamic bearing (FDB) system according to one embodiment of the invention; 
           [0016]      FIG. 4B  is a diagram of a fluid dynamic bearing (FDB) system according to another embodiment of the invention; 
           [0017]      FIG. 4C  is a diagram of a fluid dynamic bearing (FDB) system according to another embodiment of the invention; and 
           [0018]      FIG. 5  is a diagram of the stiffness vectors produced by conical bearings of an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Approaches for a fluid dynamic bearing (FDB) system that supports a greater disparity in the bearing stiffness between the conical bearings of the FDB system are described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the invention described herein. It will be apparent, however, that the embodiments of the invention described herein may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the embodiments of the invention described herein. 
       Physical Description of Illustrative Embodiments of the Invention 
       [0020]    A fluid dynamic bearing (FDB) system according to embodiments of the invention supports a greater disparity in the bearing stiffness between the conical bearings of the FDB system.  FIG. 5  is a diagram that shows the bearing stiffness produced by conical bearings according to embodiments of the invention. As shown in  FIG. 5 , upper bearing  410  and lower bearing  412  each produce a stiffness force perpendicular to the lateral surface of the bearing. The stiffness force (or vector) may be decomposed into an axial stiffness component and a radial stiffness component as shown in  FIG. 5 . Embodiments of the invention support a greater disparity in the radial stiffness between conical bearings of a FDB system. 
         [0021]    The fluid dynamic bearing (FDB) system of embodiments may be used in conjunction with any type of spindle motor system. For purposes of providing a concrete example, particular embodiments of the invention shall be described with reference to a hard-disk drive (HDD) using a tied-shaft spindle motor. 
         [0022]    In accordance with an embodiment of the invention, a plan view of a HDD  100  is shown in  FIG. 2 .  FIG. 2  illustrates the functional arrangement of components of the HDD including a slider  110   b  that includes a magnetic-recording head  110   a.  The HDD  100  includes at least one head gimbal assembly (HGA)  110  including the head  110   a,  a lead suspension  110   c  attached to the head  110   a,  and a load beam  110   d  attached to the slider  110   b,  which includes the head  110   a  at a distal end of the slider  110   b;  the slider  110   b  is attached at the distal end of the load beam  110   d  to a gimbal portion of the load beam  110   d.  The HDD  100  also includes at least one magnetic-recording disk  120  rotatably mounted on a spindle  124  and a drive motor (not shown) attached to the spindle  124  for rotating the disk  120 . The head  110   a  includes a write element and a read element for respectively writing and reading information stored on the disk  120  of the HDD  100 . The disk  120  or a plurality (not shown) of disks may be affixed to the spindle  124  with a disk clamp  128 . The HDD  100  further includes an arm  132  attached to the HGA  110 , a carriage  134 , a voice-coil motor (VCM) that includes an armature  136  including a voice coil  140  attached to the carriage  134 ; and a stator  144  including a voice-coil magnet (not shown); the armature  136  of the VCM is attached to the carriage  134  and is configured to move the arm  132  and the HGA  110  to access portions of the disk  120  being mounted on a pivot-shaft  148  with an interposed pivot-bearing assembly  152 . 
         [0023]    With further reference to  FIG. 2 , in accordance with an embodiment of the present invention, electrical signals, for example, current to the voice coil  140  of the VCM, write signal to and read signal from the PMR head  110   a,  are provided by a flexible cable  156 . Interconnection between the flexible cable  156  and the head  110   a  may be provided by an arm-electronics (AE) module  160 , which may have an on-board pre-amplifier for the read signal, as well as other read-channel and write-channel electronic components. The flexible cable  156  is coupled to an electrical-connector block  164 , which provides electrical communication through electrical feedthroughs (not shown) provided by an HDD housing  168 . The HDD housing  168 , also referred to as a casting, depending upon whether the HDD housing is cast, in conjunction with an HDD cover (not shown) provides a sealed, protective enclosure for the information storage components of the HDD  100 . 
         [0024]    With further reference to  FIG. 2 , in accordance with an embodiment of the present invention, other electronic components (not shown), including a disk controller and servo electronics including a digital-signal processor (DSP), provide electrical signals to the drive motor, the voice coil  140  of the VCM and the head  110   a  of the HGA  110 . The electrical signal provided to the drive motor enables the drive motor to spin providing a torque to the spindle  124  which is in turn transmitted to the disk  120  that is affixed to the spindle  124  by the disk clamp  128 ; as a result, the disk  120  spins in a direction  172 . The spinning disk  120  creates a cushion of air that acts as an air-bearing on which the air-bearing surface (ABS) of the slider  110   b  rides so that the slider  110   b  flies above the surface of the disk  120  without making contact with a thin magnetic-recording medium of the disk  120  in which information is recorded. The electrical signal provided to the voice coil  140  of the VCM enables the head  110   a  of the HGA  110  to access a track  176  on which information is recorded. Thus, the armature  136  of the VCM swings through an arc  180  which enables the HGA  110  attached to the armature  136  by the arm  132  to access various tracks on the disk  120 . Information is stored on the disk  120  in a plurality of concentric tracks (not shown) arranged in sectors on the disk  120 , for example, sector  184 . Correspondingly, each track is composed of a plurality of sectored track portions, for example, sectored track portion  188 . Each sectored track portion  188  is composed of recorded data and a header containing a servo-burst-signal pattern, for example, an ABCD-servo-burst-signal pattern, information that identifies the track  176 , and error correction code information. In accessing the track  176 , the read element of the head  110   a  of the HGA  110  reads the servo-burst-signal pattern which provides a position-error-signal (PES) to the servo electronics, which controls the electrical signal provided to the voice coil  140  of the VCM, enabling the head  110   a  to follow the track  176 . Upon finding the track  176  and identifying a particular sectored track portion  188 , the head  110   a  either reads data from the track  176  or writes data to the track  176  depending on instructions received by the disk controller from an external agent, for example, a microprocessor of a computer system. 
         [0025]    Embodiments of the invention also encompass HDD  100  that includes the HGA  110 , the disk  120  rotatably mounted on the spindle  124 , the arm  132  attached to the HGA  110  including the slider  110   b  including the head  110   a.    
         [0026]    With reference now to  FIG. 3 , in accordance with an embodiment of the present invention, a plan view of a head-arm-assembly (HAA) including the HGA  110  is shown.  FIG. 2  illustrates the functional arrangement of the HAA with respect to the HGA  110 . The HAA includes the arm  132  and HGA  110  including the slider  110   b  including the head  110   a.  The HAA is attached at the arm  132  to the carriage  134 . In the case of an HDD having multiple disks, or platters as disks are sometimes referred to in the art, the carriage  134  is called an “E-block,” or comb, because the carriage is arranged to carry a ganged array of arms that gives it the appearance of a comb. As shown in  FIG. 2 , the armature  136  of the VCM is attached to the carriage  134  and the voice coil  140  is attached to the armature  136 . The AE  160  may be attached to the carriage  134  as shown. The carriage  134  is mounted on the pivot-shaft  148  with the interposed pivot-bearing assembly  152 . 
       Adjusting the Stiffness of a Fluid Dynamic Bearing 
       [0027]    It may be desirable for a fluid dynamic bearing (FDB) system to support a wide disparity in the bearing stiffness, and in particular the radial bearing stiffness, between the conical bearings of the FDB system. As used herein, the bearing center of a FDB system is the middle point in the distance between the two conical bearings of the FDB system. The distance between the two conical bearings may be determined by measuring the distance between the ends of the two bearings in a direction parallel to the stationary shaft of the FDB system. The bearing center of a FDB system is depicted in  FIG. 5 . If the bearing center of the upper and lower conical bearings is a sufficient distance away from the center of gravity of the HDD, then when the disks in the HDD experience radial motion, the disks will experience a different responsive dampening movement from each side of the bearing, which will tend to prolong or increase the time in which the disks oscillate. Therefore, it would be advantageous to configure the stiffness of the conical bearings so that the disks receive a similar dampening movement in response to any radial motion by the disks. Providing a similar dampening movement form each side of the bearing will stabilize the disk and discourage or minimize angular motion. 
         [0028]    Embodiments of the invention provide for a fluid dynamic bearing (FDB) system that supports a greater range of difference in the bearing stiffness, and in particular the radial bearing stiffness, between the conical bearings of the FDB system. For example, in embodiments, either the upper conical bearing or the lower conical bearing may be at least twice as stiff as the other. A FDB system that supports a greater range of difference in the bearing stiffness between the conical bearings is beneficial in systems with a small form factor or which have a center or gravity that is not near the bearing center of the FDB system. 
         [0029]    The difference in stiffness between the conical bearings of the FDB system may be achieved through a variety of different approaches by embodiments. Embodiments may customize the bearing stiffness, and in particular the radial bearing stiffness, between conical bearings of a FDB system by adjusting a factor that contributes to the overall bearing stiffness. For example, the bearing stiffness may be affected by adjusting one or more of: (a) the cone angle of the bearing, (b) the height of the bearing, (c) the length of the gap between the bearing and the stationary shaft, and (d) the viscosity of the lubricant. Generally speaking, the stiffness of a conical bearing increases with (a) an increase in the cone angle of the bearing, (b) an increase in the height of the bearing, (c) a reduction in the length of the gap between the bearing and the stationary shaft, and (d) an increase in the viscosity of the lubricant used with the bearing. 
         [0030]      FIG. 4A  is a diagram of a fluid dynamic bearing (FDB) system according to one embodiment of the invention. As depicted in  FIG. 4A , upper conical bearing  410  and lower conical bearing  412  have different cone angles. A cone angle is the angle measured from the axis of the conical bearing to the lateral surface of the conical bearing. As shown in  FIG. 4A , lower conical bearing  412  has a greater cone angle than upper conical bearing  410 . 
         [0031]    By changing the cone angles of a conical bearing and keeping all other factors the same, the bearing stiffness of the conical bearing may be increased. Generally, the wider the cone angle, the stiffer a conical bearing will be. 
         [0032]    There are different ways in which the cone angle of a conical bearing may be changed. For example,  FIG. 4A  depicts an embodiment where the cone angle is adjusted by changing the height of the conical bearings, but the cone diameter of conical bearings  410  and  412  are the same. In this context, the term cone diameter refers to the length of the base of the conical bearing. The cone angle may also be adjusted by changing the cone diameter rather than the height of the conical bearing. To illustrate this approach, consider  FIG. 4B , which is a diagram of a fluid dynamic bearing (FDB) system according to another embodiment of the invention. As shown in  FIG. 4B , upper bearing  420  and lower bearing  420  have the same height, but have different cone angles and cone diameters. 
         [0033]    In another embodiment, the lubricant used with a conical bearing of the FDB system may be designed to have a viscosity which results in a desired stiffness for the conical bearing. To illustrate, an upper conical bearing may use a lubricant having a different viscosity than the lubricant used with the lower conical bearing. Alternatively, either the upper conical bearing or the lower conical bearing may employer a heating apparatus to change the temperature of the lubricant. A change in the temperature in the lubricant will cause a change in the viscosity of the lubricant. A lubricant which is more viscous will be less stiff than a lubricant which is less viscous. Thus, by changing the temperature of the lubricant used in a conical bearing, the stiffness of the conical bearing may be configured. Using a lubricant with a certain viscosity may be, but need not be, performed in concert or conjunction with one or more of adjusting the opening angle or the height of a conical bearing to adjust the stiffness of the conical bearing. 
         [0034]      FIG. 4C  is a diagram of a fluid dynamic bearing (FDB) system according to such an embodiment of the invention. The viscosity of the lubricant is upper conical bearing  440  is different than the viscosity of the lubricant is lower conical bearing  442 . Naturally, these approaches may be used individually, or in combination, so that each conical bearing of the FDB system has a desired stiffness. One bearing or more bearings may include a heating element to heat the lubricant, e.g.,  FIG. 4C  depicts heating element  444  to heat the lubricant used with upper bearing  440 . Heating the lubricant used with upper bearing  440  will cause upper bearing  440  to be less stiff (including a lower radial stiffness) than lower bearing  442 . 
         [0035]    Embodiments may also use two or more of the above techniques to adjust the difference in bearing stiffness in the conical bearing of the FDB system. To illustrate, embodiments may employ an upper conical bearing and a lower conical bearing that have (a) different cone angles and (b) different cone diameters to produce a desired bearing stiffness ratio between the upper conical bearing and the lower conical bearing. 
         [0036]    In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is the invention, and is intended by the applicants to be the invention, is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Any definitions expressly set forth herein for terms contained in such claims shall govern the meaning of such terms as used in the claims. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.