Patent Publication Number: US-2006001939-A1

Title: Dynamic bearing and beam deflecting apparatus employing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit under 35 U.S.C. § 119(a) of Korean Patent Application No. 10-2004-0051518, filed on Jul. 2, 2004, the entire disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a fluid dynamic bearing for supporting a rotating body and a beam deflecting apparatus employing the same. More particularly, the present invention relates to a fluid dynamic bearing with an improved thrust bearing portion and a beam deflecting apparatus employing the same.  
      2. Description of the Related Art  
      In general, fluid dynamic bearings are employed in high-speed motors. A fluid dynamic bearing supports a rotating shaft by generating fluid (or air) dynamic pressure so that the rotating shaft is stable at high speeds. Fluid dynamic bearings are widely used in such applications as beam deflecting apparatuses in laser printers, optical storage devices, brushless DC motors, and similar applications.  
      As technology develops, the printing speed of laser printers tends to gradually become faster to meet user&#39;s requirements. Accordingly, in a beam deflecting apparatus, a polygon mirror (which is a kind of beam deflecting device) must be rotated at high speeds. The beam deflecting apparatus must also operate reliably for long periods of time.  
      Referring to  FIG.1 , a conventional fluid dynamic bearing supports a rotating shaft  1  so that the rotating shaft  1  can rotate. The bearing includes a housing  10  having a hollow cavity  13  partially filled with oil  1   1 , a sleeve  15  inserted into the hollow cavity  13 , and a thrust plate  17  supporting the rotating shaft  1  in an axial direction.  
      Grooves (not shown) having a herringbone shape are formed on the inner surface of the sleeve  15 , that is, the surface facing the circumferential surface of the rotating shaft  1 . The grooves generate dynamic pressure during rotational movement. In addition, the oil  11  forms a lubrication film between the sleeve  15  and the rotating shaft  1 . The lower end of the rotating shaft  1  has a round shape to minimize the contact area between the rotating shaft  1  and the thrust plate  17 . Therefore, when the rotating shaft  1  rotates at high speed, the sleeve  15  supports the rotating shaft  1  in radial directions, and the thrust plate  17  supports the rotating shaft  1  in an axial direction.  
      The physical contact between the thrust plate  17  and the lower end of the rotating shaft  1  causes problems in that it shortens the service life of the dynamic bearing, and generates undesirable, abrasive substances due to abrasion between the rotating shaft  1  and the thrust plate  17 . In addition, when the conventional dynamic bearing described above is employed in a beam deflecting apparatus, the friction between the thrust plate  17  and the rotating shaft  1  limits the ability to increase the rotational speed of the polygon mirror.  
      Accordingly, there is a need for an improved dynamic bearing.  
     SUMMARY OF THE INVENTION  
      An aspect of the present invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an object of the present invention is to provide a dynamic bearing with an improved thrust bearing that supports a rotating shaft in an axial direction without contact, and, in addition, prevents the rotating shaft from unstable shaking in radial directions, and a beam deflecting apparatus employing the same.  
      According to an aspect of the present invention, a dynamic bearing includes a bearing housing having a hollow cavity. A shaft is provided in the hollow cavity and is installed so that it can rotate with respect to the bearing housing. A first magnet is disposed at one end of the shaft, and a second magnet is disposed in the hollow cavity. The magnets are spaced a predetermined gap from each other and face each other. The magnets support the shaft in both axial and radial directions without contact due to magnetic, repulsive forces between the first and second magnets.  
      According to another aspect of the present invention, a beam deflecting apparatus includes a bearing housing having a hollow cavity. A shaft is provided in the hollow cavity and is installed so that it can rotate with respect to the bearing housing. A first magnet is disposed at one end of the shaft, and a second magnet is disposed in the hollow cavity. The magnets are spaced a predetermined gap from each other and face each other. The magnets support the shaft in both axial and radial directions without contact due to magnetic, repulsive forces between the first and second magnets. A driving source is installed, in parts, at the bearing housing and the rotating shaft, and rotates the rotating shaft by electromagnetic forces. A beam deflecting device is installed on the rotating shaft for deflecting an incident beam and performing a scanning operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features, and advantages of certain embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a schematic, sectional view of a conventional dynamic bearing;  
       FIG. 2  is a schematic, sectional view of a dynamic bearing according to a first embodiment of the present invention;  
       FIG. 3  is an enlarged, schematic sectional view of the lower portion of the dynamic bearing illustrated in  FIG. 2 ;  
       FIG. 4  is a schematic, sectional view of a variation of the dynamic bearing shown in  FIG. 2 ;  
       FIG. 5  is a schematic view of the surface of the sleeve of the dynamic bearing shown in  FIG. 2 ;  
       FIG. 6  is a schematic, sectional view of a dynamic bearing according to a second embodiment of the present invention;  
       FIG. 7  is an enlarged, schematic sectional view of the lower portion of the dynamic bearing illustrated in  FIG. 4 ;  
       FIG. 8  is a schematic, sectional view of a dynamic bearing according to a third embodiment of the present invention;  
       FIG. 9  is a schematic, sectional view of a dynamic bearing according to a fourth embodiment of the present invention; and  
       FIG. 10  is a schematic, sectional view of a beam deflecting apparatus according to an embodiment of the present invention. 
    
    
     Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features, and structures.  
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of the embodiments of the invention. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness.  
      Referring to  FIGS. 2 and 3 , a fluid dynamic bearing according to a first embodiment of the present invention supports a rotating body (for example, a polygon mirror of an optical scanning apparatus, or a turntable of an optical recording and reproducing apparatus) by dynamic pressure of fluid or air to allow the rotating body to rotate. The dynamic bearing includes a bearing housing  110 , a shaft  120 , and a thrust bearing portion  130 .  
      The bearing housing  110  has a hollow cavity  111 . The shaft  120  is installed in the hollow cavity  111  so that the shaft  120  and the bearing housing  110  can rotate with respect to one another. Either the shaft  120  or the bearing housing  110  rotates together with the rotating body, while the other supports the rotating body in a static state so that the rotating body can rotate.  
      As illustrated in  FIG. 4 , the bearing housing  110  may have a structure provided within the hollow cavity  111  for directly supporting the shaft  120  so that the shaft  120  and bearing housing  110  can rotate with respect to one another. To directly support the shaft, grooves (not shown) having a predetermined shape, for example, a herringbone shape, can be formed at the inner surface of the hollow cavity  111 .  
      Alternatively, as illustrated in  FIGS. 2-3 , one or more sleeves  115  for supporting the shaft  120  may be included in the hollow cavity  111 . The sleeves  115  surround the shaft  120  in the hollow cavity  111 . Grooves  116  (schematically illustrated in  FIG. 5 ) having a predetermined shape are formed on the inner surfaces  117  (that is, the surfaces face the shaft  120 ) of the sleeves  115  so that dynamic pressure is generated during the rotating operation.  
      The thrust bearing portion  130  supports the shaft  120  so that the shaft  120  does not shake in the axial direction when the shaft  120  rotates with respect to the bearing housing  110 . In addition, the thrust bearing portion  130  supports the end of the shaft  120  so that the end does not shake in the radial direction. As will be explained below, the thrust bearing portion  130  supports the shaft  120  without contact, thereby minimizing the abrasion of the shaft  120  due to friction and the generation of undesired, abrasive substances.  
      The thrust bearing portion  130  includes a first magnet  131  disposed at the end of the shaft  120 . A second magnet  135  is disposed at a corresponding inner portion of the hollow cavity  111 . The magnets face each other and are spaced a predetermined gap from each other. The first and second magnets  131  and  135  support the shaft  120  in axial and radial directions without contact due to magnetic repulsion between the magnets. To accomplish this, the poles and the shapes of the first and second magnets  131  and  135  are arranged in a specific disposition.  
      Referring to  FIG. 3 , the first and second magnets  131  and  135  are permanent magnets having respective S and N poles. Preferably, the first and second magnets  131  and  135  are disposed so that the N poles face each other, thereby generating magnetic, repulsive forces. As a result, the end of the shaft  120  does not contact the bearing housing  110 . Alternatively, the first and second magnets  131  and  135  can be disposed so that the S poles face each other.  
      In addition, the first and second magnets  131  and  135  are shaped so that the magnetic, repulsive forces generated between the first and second magnets  131  and  135  have components in both the axial and radial directions of the shaft  120 . To accomplish this, in the embodiment illustrated in  FIGS. 2 and 3 , the first magnet  131  has a truncated cone shape, and forms a taper at one end of the shaft  120 . The second magnet  135  has a shape corresponding to that of the first magnet  131 , and has a recessed portion  137 . The first magnet  131  fits into the recessed portion  137  while being spaced a predetermined gap from the recess portion  137 .  
      When the first and second magnets  131  and  135  are configured in this manner, the interaction between the first magnet  131  and the second magnet  135  supports the end portion of the shaft  120  in the axial and radial directions of the shaft  120 . That is, the magnetic, repulsive forces at two arbitrary points a and b on the first magnet  131  can be expressed by Fa and Fb, respectively. The repulsive forces Fa and Fb are vector quantities, and are comprised of the sums of the respective vectors Fax and Fay, and vectors Fbx and Fby. The forces Fax and Fbx acting in the x-direction support the shaft  120  in the radial direction of the shaft  120 , and the forces Fay and Fby acting in the y-direction support the shaft  120  in the axial direction of the shaft  120 . Therefore, the thrust bearing portion  130  supports the end portion of the shaft  120  in the hollow cavity  111  in both the axial and radial directions without contact. Thus, the thrust bearing portion  130  stably supports the shaft  120  while preventing the formation of undesirable, abrasive substances by frictional contact between the thrust bearing portion  130  and the end of the shaft  120 .  
      Referring to  FIGS. 6 and 7 , a dynamic bearing according to a second embodiment of the present invention includes a bearing housing  210  having a hollow cavity  211 , a shaft  220  and a thrust bearing portion  230 . The structure of the bearing housing  210  and the shaft  220  are substantially the same as those of the dynamic bearing of the first embodiment of the invention, so a detailed description is omitted for clarity and conciseness.  
      The thrust bearing portion  230  includes a first magnet  231  disposed at the end of the shaft  220 . A second magnet  235  is disposed at a corresponding inner portion of the hollow cavity  211 . The magnets face each other and are spaced a predetermined gap from each other. The first and second magnets  231  and  235  support the shaft  220  in axial and radial directions without contact due to magnetic repulsion between the magnets. The structure and disposition of the first and second magnets  231  and  235  in this second embodiment are substantially the same as the first and second magnets  131  and  135  in the first embodiment, except for their shapes.  
      Referring to  FIG. 7 , the first magnet  231  is hemispherically shaped, and projects from one end of the shaft  220 . The second magnet  235  has a shape corresponding to that of the first magnet  231 , that is, a hemispherically shaped recessed portion  233 . The end of the first magnet  231  fits into the recessed portion  233  while being spaced a predetermined gap from the recessed portion  233 .  
      When the first magnet  231  and the second magnet  235  are configured as described above, the interaction between the first magnet  231  and the second magnet  235  support the end of the shaft  220  in the axial and radial directions of the shaft  220 . That is, the magnetic, repulsive forces at two arbitrary points c and d on the first magnet  231  can be expressed by Fc and Fd, respectively. The repulsive forces Fc and Fd are vector quantities, and are comprised of the sums of respective vectors Fcx and Fcy, and vectors Fdx and Fdy. The forces Fcx and Fdx acting in the x-direction support the shaft  220  in the radial direction of the shaft  220 , and the forces Fcy and Fdy acting in the y-direction support the shaft  220  in the axial direction of the shaft  220 .  
      Referring to  FIG. 8 , a dynamic bearing according to a third embodiment of the present invention includes a bearing housing  310 , a shaft  320  and a thrust bearing portion  330 . The third embodiment is similar to the first embodiment, except for the shape of the magnets  331  and  335 . Therefore, for clarity and conciseness, the modified portions are described while the description of unmodified portions is omitted.  
      Referring to  FIG. 8 , the first magnet  331  is formed at one end of the shaft  320 , and includes a tapered recess  333 . That is, a recess is formed at the end of the shaft  320 , and the first magnet  331  is installed on the inner surfaces of the recess. The second magnet  335  is provided in a hollow cavity  311  of the bearing housing  310 , and projects from the bottom surface of the cavity. The second magnet  335  has a truncated cone shape corresponding to the shape of the recess  333 . Therefore, the magnetic, repulsive forces between the first and second magnets  331  and  335  support the shaft  320  without contact so that the shaft  320  can rotate.  
      Referring to  FIG. 9 , a dynamic bearing according to a fourth embodiment of the present invention includes a bearing housing  410 , a shaft  420 , and a thrust bearing portion  430 . The dynamic bearing of this embodiment is similar to the second embodiment except for the shape of the first and second magnets  431  and  435 . Therefore, for clarity and conciseness, the modified portions are described while the description of unmodified portions is omitted.  
      Referring to  FIG. 9 , the first magnet  431  is formed at one end of the shaft  420 , and includes a hemispherically shaped recess  433 . That is, a hemispherically shaped recess is formed at one end of the shaft  420 , and the first magnet  431  is installed on the inner surfaces of the recess. The second magnet  435  is provided in a hollow cavity  411  of the bearing housing  410 , and projects from the bottom surface of the cavity. The second magnet is hemispherically shaped and corresponds to the recess  433 . Therefore, the magnetic, repulsive forces between the first and second magnets  431  and  435  support the shaft  420  without contact so that the shaft  420  can rotate.  
      Referring to  FIG. 10 , a beam deflecting apparatus according to an embodiment of the present invention includes a bearing housing  510 . The bearing housing  510  is fixed to a base  500  and has a hollow cavity  511 . A rotating shaft  520  is provided in the hollow cavity  511  and rotates with respect to the bearing housing  510 . A thrust bearing portion  530  supports the rotating shaft  520 , a driving source  540 , and a beam deflecting device  550 .  
      The structures of the bearing housing  510 , the rotating shaft  520 , and the thrust bearing portion  530  are substantially the same as the respective structures of the bearing housings, the shafts, and the thrust bearing portions of the dynamic bearings according to the first through fourth embodiments of the present invention described with reference to  FIGS. 2 through 9 . Detailed descriptions are therefore omitted for clarity and conciseness.  
      The driving source  540  is installed, in parts, at the bearing housing  510  and the rotating shaft  520 . The electromagnetic force of the driving source  540  rotates the rotating shaft  520 . The driving source  540  includes a stator core  541 , a rotor frame  543 , a rotor housing  545 , and a magnet  547 . The stator core  541  is fixedly installed at the outer circumferential surface of the bearing housing  510 , and includes a coil  542  wound around the stator core  541 . The rotor frame  543  is installed at the outer circumferential surface of the rotating shaft  520 , and the beam deflecting device  550  and the rotor housing  545  are installed at outer circumferential portions of the rotor frame  543 . The rotor housing  545  is joined to the rotor frame  543 , and encircles the circumference of the stator core  541 . The magnet  547  is joined to and installed in the rotor housing  545 , and is positioned to face the stator core  542 .  
      The beam deflecting device  550  deflects an incident beam and performs a scanning operation while being rotated by the driving source  540 . In this embodiment, the exemplary beam deflecting device  550  is a polygon mirror  551  having a plurality of reflecting mirrors on its side walls. Rather than a polygon mirror, the beam deflecting device  550  may include a hologon disk for deflecting an incident beam and performing a scanning operation according to a diffraction hologram pattern. Since the structures of the polygon mirror and the hologon disk are well-known, detailed descriptions are omitted for clarity and conciseness.  
      The dynamic bearing described above with respect to exemplary embodiments of present invention employs a thrust bearing structure that supports the end of the shaft without contact, in both the axial and radial directions, thereby preventing the shaft from shaking. Since the shaft is supported without contact, the thrust bearing structure prevents the generation of undesired abrasive substances due to contact between a static body and a rotating body.  
      In addition, a beam deflecting apparatus according to an exemplary embodiment of the present invention employs the dynamic bearing that supports one end of the rotating shaft in both axial and radial directions without contact. The beam deflecting device is therefore stably supported while rotating at high speed.  
      While the invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. In particular, the dynamic bearing according to the present invention can be widely applied to not only the exemplary beam deflecting apparatus described above, but also all apparatuses that have a rotating body, for example, hard disk drives, optical disk drives, and the like. Accordingly, it should be understood that the scope of the present invention is defined by the appended claims rather than the foregoing description.