Abstract:
Disclosed herein is a spindle motor which is capable of more easily controlling an axial gap and levelness between the thrust plate of a rotating shaft and a sealing cap. The spindle motor includes a rotating shaft having a thrust plate which is perpendicularly inserted into the upper portion of the rotating shaft. A sleeve accommodates the rotating shaft and rotatably supports the rotating shaft. The sleeve is secured to a plate. A sealing cap is secured to the sleeve through laser welding in such a way as to face the upper surface of the thrust plate. At least part of the sealing cap is stepped towards the thrust plate to correspond to a degree of deformation occurring during the laser welding.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2009-0006582, filed on Jan. 28, 2009, entitled “SPINDLE MOTOR”, which is hereby incorporated by reference in its entirety into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to a spindle motor and, more particularly, to a spindle motor which is capable of more easily controlling an axial gap and levelness between the thrust plate of a rotating shaft and a sealing cap. 
         [0004]    2. Description of the Related Art 
         [0005]    Generally, a spindle motor maintains the rotational characteristics of high precision, because a bearing housing a rotating shaft therein rotatably supports the rotating shaft. Because of these characteristics, the spindle motor has been widely used as the drive means of hard-disk drives, optical disk drives, magnetic disk drives and other recording media requiring high-speed rotation. 
         [0006]    In such a spindle motor, a hydrodynamic bearing is generally used to inject a predetermined fluid between a rotating shaft and a sleeve for the axial support of the rotating shaft so that the rotating shaft may easily rotate, and to generate dynamic pressure when the rotating shaft rotates. 
         [0007]    The hydrodynamic bearing may have a dynamic pressure-generating groove so as to generate dynamic pressure of the fluid during the rotation of the rotating shaft. Such a dynamic pressure-generating groove may be formed in each of the inner circumferential part of the sleeve which rotatably supports the rotating shaft and a thrust plate which is installed perpendicular to the axial direction of the rotating shaft. One example of the conventional spindle motor is illustrated in  FIG. 6 . 
         [0008]    As shown in  FIG. 6 , the conventional spindle motor includes a plate  10 , a sleeve  20 , an armature  30 , a rotating shaft  40 , a thrust plate  50 , a hub  60  and a sealing cap  70 . 
         [0009]    The plate  10  is mounted to a device such as a hard-disk drive, and the sleeve  20  is secured to the central portion of the plate  10  through press-fitting. 
         [0010]    The sleeve  20  rotatably accommodates the rotating shaft  40  therein, and the sealing cap  70  is secured to the upper portion of the sleeve so as to prevent the removal of the thrust plate  50  and the rotating shaft  40 . Further, the sleeve  20  has hydrodynamic bearings on the inner circumference facing the rotating shaft  40  and a portion facing the thrust plate  50 . 
         [0011]    When external power is applied to the armature  30 , the armature  30  forms an electric field so as to rotate the hub  60  on which an optical or magnetic disk is mounted. The armature  30  includes a core  31  which is formed by laminating a plurality of metal sheets and a coil  32  which is wound several times on the core  31 . 
         [0012]    The rotating shaft  40  axially supports the hub  60 , and is inserted into the sleeve  20  to be rotatably supported by the sleeve  20 . The thrust plate  50  is secured to the upper portion of the rotating shaft  40 . 
         [0013]    The thrust plate  50  is secured to the rotating shaft  40 . An upper thrust bearing is provided between the thrust plate  50  and the sealing cap  70 , and a lower thrust bearing is provided between the thrust plate  50  and the sleeve  20 . Here, the lower thrust bearing generates fluid dynamic pressure using a fluid stored between the sleeve  20  and the thrust plate  50  during the rotation of the rotating shaft  40 , thus floating the thrust plate  50  from the sleeve  20 . That is, owing to the lower thrust hydrodynamic bearing, the thrust plate  50  is not in contact with the sleeve  20  during the rotation of the rotating shaft  40 . Further, the upper thrust bearing generates fluid dynamic pressure using fluid between the thrust plate  50  and the sealing cap  70  during the rotation of the rotating shaft  40 , so that the non-contact state between the thrust plate  50  and the sealing cap  70  is maintained. 
         [0014]    The hub  60  mounts the optical or magnetic disk (not shown) thereon to rotate it. A magnet  61  which forms a magnetic force is secured to the inner circumference of the hub  60  in such a way as to face the armature  30 . 
         [0015]    The sealing cap  70  is secured to the sleeve  20  in such a way as to face the thrust plate  50 . A fluid sealing part  71  is formed between the sealing cap  70  and the thrust plate  50  to store fluid. Further, a gap must be maintained between the sealing cap  70  and the thrust plate  50  to form the upper thrust bearing. 
         [0016]    Meanwhile, in the conventional spindle motor having the above construction, the sealing cap  70  is welded to the sleeve  20  through laser welding or the like and a predetermined gap is maintained between the sealing cap  70  and the thrust plate  50 . However, during the laser welding process for the coupling of the sealing cap  70 , the sealing cap  70  may become deformed or curved due to the hardening of a weld part  80  and the residual stress applied to the sealing cap  70 . 
         [0017]    In detail, as shown in  FIGS. 7 and 8 , in the case where the sealing cap  70  is seated on the sleeve  20  and thereafter a junction between the sealing cap  70  and the sleeve  20  is welded using a laser welding machine or the like, as shown in  FIG. 8 , the inner circumference of the sealing cap  70  may be bent upwards (the direction shown by the arrow) or damaged. 
         [0018]    That is, during the laser welding of the sealing cap  70 , the gap and the levelness between the sealing cap  70  and the thrust plate  50  cannot be kept constant. Hence, it is difficult to obtain the stable drive characteristics of the spindle motor. 
       SUMMARY OF THE INVENTION 
       [0019]    The present invention has been made in an effort to provide a spindle motor, in which the lower portion of a sealing cap is stepped by a distance between the sealing cap and a thrust plate which is increased by laser welding, thus maintaining a desired gap between the sealing cap and the thrust plate even after the laser welding process has been completed. 
         [0020]    In a spindle motor according to an embodiment of the present invention, a spindle motor includes a rotating shaft having a thrust plate which is perpendicularly inserted into the upper portion of the rotating shaft. A sleeve accommodates the rotating shaft and rotatably supports the rotating shaft. The sleeve is secured to a plate. A sealing cap is secured to the sleeve through laser welding in such a way as to face the upper surface of the thrust plate. At least part of the sealing cap is stepped towards the thrust plate to correspond to a degree of deformation occurring during the laser welding. 
         [0021]    The sealing cap includes a seating part seated on the sleeve, a stepped part extending to the thrust plate in such a way as to form a step between the stepped part and a bottom surface of the seating part, and a fluid sealing part extending from the stepped part and holding fluid therein. 
         [0022]    The stepped part of the sealing cap is stepped in proportion to a distance between the sealing cap and the thrust plate which is increased by residual stress applied to the sealing cap during the laser welding. 
         [0023]    Further, the stepped part is formed such that a step of 15 to 20 μm is formed between the stepped part and the seating part. 
         [0024]    The bottom surface of the stepped part is spaced apart from the thrust plate by 30 μm after the laser welding. 
         [0025]    The bottom surface of the stepped part is parallel to the bottom surface of the seating part. 
         [0026]    The stepped part includes an inclined surface which is inclined to a degree corresponding to a change of angle between the sealing cap and the thrust plate due to residual stress applied to the sealing cap during the laser welding. 
         [0027]    The inclined surface is formed in such a way as to be inclined from the seating part to the thrust plate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description, taken in conjunction with the accompanying drawings, in which: 
           [0029]      FIG. 1  is a schematic sectional view illustrating a spindle motor according to an embodiment of the present invention; 
           [0030]      FIG. 2  is a schematic sectional view illustrating a sealing cap included in the spindle motor of  FIG. 1 ; 
           [0031]      FIGS. 3 and 4  are schematic views illustrating the laser welding for the sealing cap of  FIG. 1 ; 
           [0032]      FIG. 5  is a schematic sectional view illustrating a sealing cap according to another embodiment of the present invention; 
           [0033]      FIG. 6  is a schematic sectional view illustrating a conventional spindle motor; and 
           [0034]      FIGS. 7 and 8  are schematic views illustrating the laser welding for a conventional sealing cap. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    Hereinafter, spindle motors according to the preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
         [0036]    As shown in  FIG. 1 , a spindle motor  100  according to the preferred embodiment of the present invention includes a plate  110 , a sleeve  120 , an armature  130 , a rotating shaft  140 , a thrust plate  150 , a hub  160  and a sealing cap  170 . 
         [0037]    The plate  110  functions to support the entire spindle motor  100  and is mounted to a device such as a hard disk drive to which the spindle motor  100  is to be installed. Here, the plate  110  is manufactured using a light material such as an aluminum or aluminum alloy plate. However, the plate  110  may be manufactured using a steel plate. 
         [0038]    Further, a sleeve coupling part  111  protrudes from the plate  110 , so that the sleeve  120  is coupled to the sleeve coupling part  111 . The sleeve coupling part  111  has in a central portion thereof a coupling hole having a diameter which is the same as the outer diameter of the sleeve  120 , so that the sleeve  120  is inserted into the central portion of the sleeve coupling part  111 . That is, the sleeve  120  is inserted into the coupling hole, so that the sleeve  120  is held at a predetermined position. Here, in order to secure the sleeve  120  to the sleeve coupling part  111 , an adhesion process using an additional adhesive or a laser welding process may be performed. However, the sleeve  120  may be secured to the sleeve coupling part  111  by press-fitting the sleeve  120  into the coupling hole with a predetermined pressure. 
         [0039]    The sleeve  120  functions to rotatably support the rotating shaft  140  and has a hollow cylindrical shape, with hydrodynamic bearings provided on the inner circumferential part  121  which faces the rotating shaft  140  and a bearing surface  122  which faces the thrust plate  150 . In a detailed description, a radial dynamic pressure-generating groove (not shown) is formed in the inner circumferential part  121  of the sleeve  120  to form the radial hydrodynamic bearing between the sleeve  120  and the rotating shaft  140 . Fluid is stored between the inner circumferential part  121  and the rotating shaft  140 . The radial dynamic pressure-generating groove generates fluid dynamic pressure using the fluid stored between the sleeve  120  and the rotating shaft  140  during the rotation of the rotating shaft  140 , thus maintaining the non-contact state between the rotating shaft  140  and the sleeve  120 . According to this embodiment, the radial dynamic pressure-generating groove is formed in the inner circumferential part  121  of the sleeve  120 . However, the radial dynamic pressure-generating groove may be formed in the outer circumference of the rotating shaft  140 . 
         [0040]    Here, a lower thrust bearing is formed between the bearing surface  122  of the sleeve  120  and the lower surface of the thrust plate  150 . The lower thrust bearing generates fluid dynamic pressure using the fluid stored between the thrust plate  150  and the bearing surface  122  of the sleeve  120 , thus maintaining the non-contact state between the thrust plate  150  and the sleeve  120 . 
         [0041]    Further, the cap coupling part  123  of the sleeve  120  to which the sealing cap  170  is secured extends perpendicularly to the bearing surface  122  and forms a step. 
         [0042]    The cap coupling part  123  extends perpendicularly to the bearing surface  122  by the height of the thrust plate  150 , and includes a seating surface  123   a  on which the sealing cap  170  is seated, and a weld surface  123   b  which extends perpendicularly from the seating surface  123   a  by the height of the sealing cap  170  and is welded to an end of the sealing cap  170 . Here, a weld part  180  is formed between the weld surface  123   b  and the sealing cap  170  through laser welding. 
         [0043]    The armature  130  forms an electric field using external power applied to the armature  130  so as to rotate the hub  160  on which the optical or magnetic disk is mounted. The armature  130  includes a core  131  which is formed by laminating a plurality of metal sheets and a coil  132  which is wound several times on the core  131 . 
         [0044]    The core  131  is secured to the outer circumference of the sleeve coupling part  111  of the plate  110 , and the coil  132  is wound on the core  131 . Here, the coil  132  forms an electric field using an external current applied to the coil  132 , thus rotating the hub  160  using electromagnetic force generated between the coil  132  and the magnet  163  of the hub  160 . 
         [0045]    The rotating shaft  140  functions to axially support the hub  160 . The rotating shaft  140  is inserted into and rotatably supported by the sleeve  120 . Meanwhile, the thrust plate  150  is secured to the upper portion of the rotating shaft  140 . Here, in order to secure the thrust plate  150  inserted into the upper portion of the rotating shaft  140  to the rotating shaft  140 , an additional laser welding process may be performed. However, by applying a predetermined pressure to the thrust plate  150 , the thrust plate  150  may be press-fitted into the rotating shaft  140 . 
         [0046]    The thrust plate  150  is secured to the rotating shaft  140 . An upper thrust bearing is formed between the thrust plate  150  and the sealing cap  170 , and a lower thrust bearing is formed between the thrust plate  150  and the bearing surface  122  of the sleeve  120 . Upper and lower thrust dynamic pressure-generating grooves (not shown) are formed in a portion of the thrust plate  150  facing the sealing cap  170  and a portion of the thrust plate  150  facing the sleeve  120 , respectively. The lower thrust dynamic pressure-generating groove generates fluid dynamic pressure using fluid stored between the sleeve  120  and the thrust plate  150  during the rotation of the rotating shaft  140 , thus floating the thrust plate  150  from the bearing surface  122  of the sleeve  120  by a predetermined height. Further, the upper thrust dynamic pressure-generating groove generates fluid dynamic pressure using fluid stored between the sealing cap  170  and the thrust plate  150  during the rotation of the rotating shaft  140 , thus generating a force pushing the thrust plate  150  from the sealing cap  170 . That is, by the upper and lower thrust hydrodynamic bearings, the thrust plate  150  is not in contact with the sealing cap  170  and the sleeve  120  during the rotation of the rotating shaft  140 . According to this embodiment, the thrust dynamic pressure-generating grooves are formed in the thrust plate  150 . However, the thrust dynamic pressure-generating grooves may be formed in the bearing surface  122  of the sleeve  120  and the sealing cap  170 . 
         [0047]    The hub  160  mounts the optical or magnetic disk thereon to rotate it. The hub  160  includes a disk part  161  to which the rotating shaft  140  is secured, and an annular edge part  162  which extends from an end of the disk part  161 . 
         [0048]    The rotating shaft  140  is inserted into the central portion of the disk part  161 , and the edge part  162  extends axially along the rotating shaft  140  in such a way that the inner circumferential surface of the edge part  162  faces the armature  130 . The magnet  163  is attached to the inner circumference of the edge part  162  and forms a magnetic field so as to generate an electromagnetic force in cooperation with the electric field formed by the coil  132 . 
         [0049]    The sealing cap  170  functions to support the thrust plate  150 , thus preventing the removal of the hub  160  and the rotating shaft  140 . After the sealing cap  170  is seated on the seating surface  123   a  of the cap coupling part  123  of the sleeve  120  in such a way as to face the upper surface of the thrust plate  150 , the sealing cap  170  is secured to the sleeve  120  through laser welding. 
         [0050]    Here, the sealing cap  170  has the shape of an annular disk. As shown in  FIG. 2 , the sealing cap  170  includes a seating part  171  which is seated on the seating surface  123   a  of the cap coupling part  123 , a stepped part  172  which extends from the seating part  171  to the thrust plate  150  in such a way as to form a step, and a fluid sealing part  173  which extends from the stepped part  172  in such a way as to be tapered. 
         [0051]    As shown in  FIG. 2 , the bottom surface of the seating part  171  and the bottom surface of the stepped part  172  are stepped such that they have a difference in height h 1 . Here, the height h 1  corresponds to the upward deformation of the sealing cap  170  by residual stress during the mounting of the sealing cap  170  through laser welding. That is, after the degree of deformation caused by the laser welding is calculated, the height h 1  is determined. Thus, even if the sealing cap  170  is deformed by the laser welding, a constant gap can be maintained between the bottom surface of the stepped part  172  and the thrust plate  150 . 
         [0052]    That is, as shown in  FIGS. 3 and 4 , even if the sealing cap  170  becomes deformed in the arrow direction during the laser welding for mounting the sealing cap  170 , the stepped part  172  extends to the thrust plate  150  to form a step in accordance with the degree of deformation. Thus, a constant gap can be maintained for fluid dynamic pressure between the stepped part  172  and the thrust plate  150 . 
         [0053]    For example, in order to stably drive the spindle motor  100  even at a low temperature, the gap of about 30 μm must be maintained between the sealing cap  170  and the thrust plate  150 . But, considering that the sealing cap  170  is deformed upwards by about 15 to 20 μm during the laser welding of the sealing cap  170 , the height h 1  of the stepped part  172  is determined to be 15 to 20 μm, so that a proper gap can be maintained between the sealing cap  170  and the thrust plate  150 . 
         [0054]      FIG. 5  illustrates a sealing cap  270  according to another embodiment of the present invention. The sealing cap  270  according to another embodiment of the present invention has the shape of an annular disk. The sealing cap  270  includes a seating part  271  which is seated on the seating surface  123   a  of the cap coupling part  123 , a stepped part  272  which extends from the seating part  271  to the thrust plate  150  in such a way as to form a step, a fluid sealing part  273  which extends from the stepped part  272  in such a way as to be tapered, and an inclined surface  274  which is formed on the stepped part  272  in such a way as to face the thrust plate  150 . 
         [0055]    As shown in  FIG. 5 , the seating part  271  and the stepped part  272  are stepped such that they have a difference in height h 2 . Here, the height h 2  corresponds to the upward deformation of the sealing cap  270  by residual stress during the mounting of the sealing cap  270  through laser welding. That is, after the degree of deformation caused by the laser welding is calculated, the height h 2  is determined. Thus, even if the sealing cap  270  is deformed by the laser welding, a constant gap can be maintained between the inclined surface  274  of the stepped part  272  and the thrust plate  250 . 
         [0056]    For example, in order to stably drive the spindle motor  100  even at a low temperature, the gap of about 30 μm must be maintained between the sealing cap  270  and the thrust plate  150 . But, considering that the sealing cap  270  is deformed upwards by about 15 to 20 μm during the laser welding of the sealing cap  270 , the height h 2  of the stepped part  272  is determined to be 15 to 20 μm, so that a proper gap can be maintained between the sealing cap  270  and the thrust plate  150 . 
         [0057]    Further, the inclined surface  274  is inclined towards the thrust plate  150  while an angle a is formed between the inclined surface  274  and the seating part  271 . Here, the angle a may correspond to the degree of bending of the sealing cap  270  during laser welding. That is, after the degree of bending through laser welding is calculated, the angle a is determined. Therefore, even if the sealing cap  270  is deformed or curved through laser welding, constant levelness can be maintained between the sealing cap  270  and the thrust plate  150 . 
         [0058]    Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 
         [0059]    As described above, the present invention provides a spindle motor, in which the stepped part of a sealing cap extends towards a thrust plate by a distance corresponding to a degree of deformation occurring during laser welding, thus maintaining a constant gap between the sealing cap and the thrust plate even after the laser welding of the sealing cap. Further, the bottom surface of the stepped part is inclined by the degree of deformation resulting from the laser welding, thus maintaining a constant gap between the sealing cap and the thrust plate and providing excellent levelness.