Patent Publication Number: US-2007103020-A1

Title: Motor

Description:
This is a Divisional of application Ser. No. 10/756,339, filed Jan. 14, 2004, which in turn is a Divisional of application Ser. No. 10/123,228, filed Apr. 17, 2002. The disclosure of the prior applications is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND  
      The present invention relates to a motor for driving magnetic disks such as a spindle motor used in the hard disk drive device of the computer.  
      Recently, the field of the hard disk drive device has been making steady progress in increasing capacity thereof. In order to optimize such a progress in increasing capacity, there is a growing need for higher rotational speed for the motor used in the hard disk drive device. As a bearing for such a motor, a ball bearing has been generally used so far. However, in order to optimize the need for higher rotational speed, application of fluid dynamic bearings has been introduced.  
      As an example of the motor used in the hard disk drive device and comprising a fluid dynamic bearing, there is shown in  FIG. 20 a  spindle motor for driving magnetic disks. The spindle motor  1  for driving magnetic disks (hereinafter, referred to as a spindle motor) is provided with a magnet  5  on the rotor  4  so as to face toward the stator  3  provided on the flange  2 .  
      The flange  2  generally comprises a flange body  6  for holding the stator  3 , and a sleeve  7  to be press-fitted into the hole (sleeve fitting hole  6   a ) formed on the flange body  6 .  
      The sleeve  7  generally comprises a cylindrical sleeve body  9  and a disk-shaped counter plate  11 .  
      The sleeve body  9  comprises a hole (no reference numeral is assigned) extending from one side (the upper side in  FIG. 20 ) to the other side (the lower side in  FIG. 20 ) for inserting a shaft  12  therein, and the hole is constructed of a hole formed on one side (hereinafter, referred to as a sleeve hole)  7   a  and an annular stepped portion  8  formed concentrically and in communication with the sleeve hole  7   a  via a step.  
      As shown in  FIG. 21  and  FIG. 22 , the annular stepped portion  8  comprises an annular hole  8   a  having a larger inner diameter in comparison with the sleeve hole  7   a  and formed in communication with the sleeve hole  7   a  via a step (hereinafter, referred to as a medium diameter annular hole), and an annular hole having a larger inner diameter in comparison with the medium diameter annular hole  8   a  and formed in communication with the medium diameter annular hole  8   a  via a step (hereinafter, referred to as large diameter annular hole). The large diameter annular hole  8   b  opens at one end (the lower side in  FIG. 21 ) of the sleeve body  9 . The counter plate  11  is disposed at the large diameter annular hole  8   b , and the counter plate  11  and the sleeve body  9  are hermetically connected by welding or the like.  
      The shaft  12  comprises a shaft body  12   a , and an annular body  10  fitted on one end (the lower portion in  FIG. 20 ) of the shaft body  12   a . The annular body  10  of the shaft  12  is disposed in the medium diameter annular hole  8   a  and the shaft body  12   a  of the shaft  12  is inserted into the sleeve hole  7   a.    
      As described above, the annular body  10  of the shaft  12  is disposed in the medium annular hole  8   a  and the shaft body  12   a  of the shaft  12  is inserted into the sleeve hole  7   a , and the sleeve  7  constitutes a fluid dynamic bearing  13  with the shaft  12 . Though oil  14  is generally used as a fluid for the fluid dynamic bearing  13 , it may be constructed to use gas such as air.  
      In other words, a plurality of rows of grooves  15  are formed on the inner wall (sleeve hole  7   a ) of the sleeve body  9 , and a plurality of rows of grooves (not shown) are formed on the end portion of the annular body  10  that touches the stepped wall surface of the medium annular hole  8   a  of the sleeve body  9  and the portion of the upper surface of the counter plate  11  that touches the annular body  10 . Oil  14  is filled and reserved in the gap between the sleeve  7  including the grooves  15  and the shaft  12 , and in the grooves that are not shown in the figure. The inner peripheral surface of the annular body  10  is formed with a fluid circulating groove  10   a  so as to facilitate circulation of the fluid. The annular body  10  slightly projects toward the counter plate  11  with respect to the shaft  12 , so as to facilitate inflow and outflow of fluid from and to the fluid circulating groove  10   a.    
      The annular body  10  of the shaft  12  is disposed at the medium diameter annular hole  8   a , that is, between the wall surface of the medium diameter annular hole  8   a  that faces in the axial direction (the upper side in  FIG. 20 ) and the counter plate  11 , so that the axial movement (vertical movement in  FIG. 20 ) of the shaft  12  is controlled via the annular body  10 .  
      The dynamic pressure generated by the pumping action in association with rotation of the shaft  12  forces a fluid layer to be formed between the sleeve  7  and the shaft  12 , and the shaft  12  that touched the counter plate  11  as shown in  FIG. 21  during the rest time rises from the counter plate  11  as shown in  FIG. 22 , so that the shaft  12  can rotate with respect to the sleeve  7  via the fluid layer. The fluid dynamic bearing  13  forms a fluid layer by the dynamic pressure and forms a gap between the shaft  12  and the counter plate  11  to support a thrust load of the shaft  12  as described above [in other words, the counter plate  11  supports a thrust load applied downwardly of the shaft  12  (in the direction of the arrow D in  FIG. 20 ), and the ceiling wall of the medium diameter annular hole portion  8   a  supports a thrust load applied upwardly of the shaft  12  (annular body  10 )(in the direction of the arrow U in  FIG. 20 )], and a radial load of the shaft  12  is supported by the portion of the sleeve  7  where the sleeve hole  7   a  is formed.  
      Referring now to  FIG. 21  and  FIG. 22 , the operation of the fluid dynamic bearing of the related art will be described.  
       FIG. 22  shows a state in which the shaft  12  is rotated and the dynamic pressure of a fluid is generated.  
      In  FIG. 22 , when the spindle motor  1  is actuated and the shaft  12  starts rotating, the dynamic pressure is generated and thus a fluid layer is formed in the gap formed between the inner diameter surface of the sleeve  7  that is a fixed body and the outer peripheral surface of the shaft  12  that is a rotating body, between the stepped end surface (annular stepped portion  8 ) of the sleeve  7  and the opposing end surface of the annual body  10 , between the wall surface of the medium diameter annular hole  8   a  of the sleeve  7  and the outer diameter surface of the annular body  10 , and between the upper surface  11   a  (inner end surface) of the counter plate  11  that is fitted into the sleeve  7  and the end surface  10   b  of the annular body  10  and the end surface  12   b  of the shaft body  12   a , so that the rotating portion can rotate without touching the stationary portion, thereby forming a fluid dynamic bearing.  
      In  FIG. 22 , G 07  designates an axial distance of the gap formed between the end surface  10   b  of the annular body  10  and the upper surface  11   a  of the counter plate  11  when the rotor  4  (shaft  12 ) is rotated at a specified rotational speed.  
       FIG. 21  shows the state of the end portion of the shaft when the spindle motor  1  is oriented in such a manner that the counter plate  11  faces downward when the rotation of the shaft  12  is stopped and remained at rest.  
      In  FIG. 21 , loads of the hub  32 , the yolk  41 , and the magnet  5  assembled to the shaft  12  shown in  FIG. 20  are applied downward, and thus the shaft  12  on which the annular body  10  is fitted moves downward, whereby the end surface  10   b  of the annular body  10  touches the upper surface  11   a  of the counter plate  11  via a thin fluid layer. Since the fluid layer interposed between the upper surface  11   a  of the counter plate  11  and the end surface  10   b  of the annular body  10  is extremely thin, a gap G 17  between the upper surface  11   a  of the counter plate  11  and the end surface  10   b  of the annular body  10  becomes extremely small value, or otherwise they may touch each other.  
      In the spindle motor  1 , as shown in  FIG. 20 , when the shaft  12  is oriented in the vertical direction and disposed on the counter plate  11 , a load is applied to the lower end of the shaft  12 , and thus when an impact or vibrations is applied, the fluid layer on the contact surface is susceptible to mechanical damages such as breakage or scratch.  
      For example, when rotation of the shaft  12  is started, so-called fluid circularity blocking action is effected because circulation of a fluid is slow due to narrow gap G 17 . As a consequent, the fluid layer cannot be formed quickly, and thus the body of revolution (shaft  12 ) cannot rise quickly or sufficiently, which may result in difficulty in performing the function of the fluid layer as a fluid dynamic bearing. In a state where the shaft  12  is not rotating, there is no rising action effected by the fluid dynamic pressure, and thus the lower end surface of the shaft  12  (the end surface  10   b  of the annular body  10 ) touches the upper surface  11   a  of the counter plate  11  as shown in  FIG. 21 , which results in scratch on both contact surfaces.  
      Especially, during transportation or handling, it is susceptible to a large impact. In such a case, damages on the contact surface may increase and may cause failure in the performance of the apparatus.  
     SUMMARY  
      In view of such circumstances, it is an object of the present invention to provide a motor that can prevent damages to the fluid dynamic bearing.  
      A motor according to the first aspect of the present invention has a rotating member supported on a stationary portion via a fluid dynamic bearing for supporting both of a thrust load and a radial load, and comprises one or more projections provided on one of the opposing generally flat surfaces at the end of the shaft of the fluid dynamic bearing each as a separate unit, wherein the projections are capable of abutting against the other surface when the rotating member is at rest.  
      Preferably, one of the surfaces is an end surface of the shaft provided on the rotating member and the other one of the surfaces is the portion on the surface of the stationary portion facing toward the end surface of the shaft, or one of the surfaces is the portion on the surface of the stationary portion facing toward the end surface of the shaft and the other one of the surfaces is an end surface of the shaft.  
      A motor according to the second aspect of the present invention comprises a shaft fitted with an annular body on one end of the shaft body, a rotating member supported on the stationary portion via a fluid dynamic bearing for supporting both of a thrust load and a radial load, and one or more projections provided on the end surface of the shaft body each as a separate unit, wherein the projection is provided in such a manner that the tip portion thereof comes to the position higher than the end surface of the annular body.  
      A motor according to the third aspect of the present invention comprises a shaft fitted with an annular body on one end of the shaft body, a rotating member supported on the stationary portion via a fluid dynamic bearing for supporting both of a thrust load and a radial load, and one or more projections provided on the end surface of the annular body each as a separate unit. A motor according to the forth aspect of the present invention comprises a shaft fitted with an annular body on one end of the shaft body, a rotating member supported on the stationary portion via a fluid dynamic bearing for supporting both of a thrust load and a radial load, and one or more projections provided on the portion on the surface of the stationary portion facing toward the end surface of the annular body each as a separate unit.  
      A motor according to the fifth aspect of the present invention comprises a shaft fitted with an annular body on one end of the shaft body, a rotating member supported on the stationary portion via a fluid dynamic bearing for supporting both of a thrust load and a radial load, and one or more projections provided on the portion on the surface of the stationary portion facing toward the end surface of the shaft each as a separate unit, wherein the height of the projection from the mounted portion is larger than the distance from the end surface of the shaft body to the end surface of the annular body.  
      Preferably, the projection is press-fitted into the member on which the projection is to be provided.  
      Preferably, the projection has a spherical shape.  
      Preferably, the projection is formed of ceramic.  
      Preferably, the projection is a member made of a high hardness material formed by a sputtering.  
      Preferably, the member is formed of a base member containing silicon or chromium as a main component and a secondary member made of a high hardness material placed thereon, and both of the members are formed by the sputtering.  
      Preferably, the member made of a high hardness material is amorphous carbon or DLC (Diamond-like Carbon). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross sectional view showing an embodiment of the present invention;  
       FIG. 2  is a cross sectional view showing a spindle motor shown in  FIG. 1  remained at rest;  
       FIG. 3  is a cross sectional view of the spindle motor shown in  FIG. 1  in the state of being rotated;  
       FIG. 4  is a cross sectional view explaining the setting of the height of the ball shown in  FIG. 1 ;  
       FIG. 5  is a cross sectional view showing the second embodiment of the present invention;  
       FIG. 6  is a cross sectional view of the spindle motor according to the third embodiment remained at rest;  
       FIG. 7  is a cross sectional view showing the spindle motor shown in  FIG. 6  in the state of being rotated;  
       FIG. 8  is a cross sectional view explaining the setting of the height of the ball shown in  FIG. 6 ;  
       FIG. 9  is a cross sectional view explaining the setting of the height of the ball for the spindle motor according to the fourth embodiment of the present invention;  
       FIG. 10  is a cross sectional view explaining the setting of the ball for the spindle motor according to the fifth embodiment;  
       FIG. 11  is a cross sectional view showing an example of the present invention which a conical projection is provided on the shaft body;  
       FIG. 12  is a cross sectional view showing an example of the present invention in which a conical projection is provided on the counter plate;  
       FIG. 13  is a cross sectional view showing the six embodiment of the present invention;  
       FIG. 14  is a cross sectional view showing the seventh embodiment of the present invention.  
       FIG. 15  is a cross sectional view showing the eighth embodiment of the present invention;  
       FIG. 16  is a cross sectional view showing the ninth embodiment of the present invention;  
       FIG. 17  is a cross sectional view showing the tenth embodiment of the present invention;  
       FIG. 18  is a cross sectional view showing the eleventh embodiment of the present invention;  
       FIG. 19  is a cross sectional view showing the twelfth embodiment of the present invention;  
       FIG. 20  is a cross sectional view showing an example of the conventional spindle motor;  
       FIG. 21  is a cross sectional view showing the spindle motor of  FIG. 20  at rest; and  
       FIG. 22  is a cross sectional view showing the spindle motor of  FIG. 20  in the state of being rotated. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
      Referring now to FIGS.  1  to  3 , a first embodiment of the present invention will be described. The first embodiment corresponds to the second aspect of the present invention.  
      The same parts as in FIGS.  20  to  22  are designated by the same reference numerals and the description thereof will be omitted as appropriate.  
      In the spindle motor  1 A (a spindle motor for driving magnetic disks), a rotor  4  is provided with a magnet  5  facing toward the stator  3  provided on the flange  2  as shown in  FIG. 1 .  
      The flange  2  is generally constructed of a flange body  6  formed of aluminum or of stainless material holding the stator  3 , and a sleeve  7  to be press-fitted into a hole (sleeve fitting hole  6   a ) formed on the flange body  6 .  
      The flange body  6  generally comprises a cylindrical central cylindrical portion  20  having the sleeve fitting hole  6   a , and a frame  21  provided on the proximal side of the central cylindrical portion  20  so as to extend radially outwardly.  
      The frame  21  generally comprises an annular base portion  22  integrally extending from the central cylindrical portion  20 , a cylindrical outer peripheral wall portion  23  extending upwardly from the outer peripheral edge of the base portion  22 , and an extension  24  extending radially outwardly from the upper end of the outer peripheral wall  23 , and there is provided an annular space  25  between the central cylindrical portion  20  and the outer peripheral wall  23 .  
      The stator  3  comprises a stator stack  27  and a coil  28  wound by the stator stack  27 , and disposed in the annular space  25  with the stator stack  27  supported by the outer peripheral surface of the central cylindrical portion  20 . The coil  28  is connected to the outer circuit via the connector  30  to which the outgoing line  29  is connected. In  FIG. 1 , the reference numeral  31  designates a sealing member.  
      The rotor  4  generally comprises a hub  32  formed of aluminum or stainless materials, and a shaft  12  fixed to the hub  32 .  
      The hub  32  has a cup shaped configuration with three steps with their opened sides down in such a manner that the diameters of which sequentially increases from the top toward the bottom. Hereinafter, these cylindrical bodies are referred to as the first, the second, and the third hub cylindrical bodies  32   a ,  32   b ,  32   c , in ascending order for the sake of convenience.  
      The shaft  12  is fitted to the hole  34  formed on the bottom of the first hub cylindrical body  32   a.    
      A magnetic disk  36  is fitted on the outer peripheral surface of the outer peripheral wall  35  of the first hub cylindrical body  32   a , and the first hub cylindrical body  32   a  is formed with a female screw  37  for fixing the cover for holding the magnetic disk  36  on the outer peripheral wall  35  thereof. The first hub cylindrical body  32   a  is formed with a plurality of holes  39  on the outer peripheral wall  35  along the circumference thereof, so that a balance weight  40  can be selectively mounted to these holes  39 .  
      As shown in  FIG. 1  and  FIG. 2 , a plurality of rows of grooves  15  are formed on the inner peripheral wall of the sleeve body  9  (sleeve hole  7   a ), and a plurality of rows of grooves (not shown) are formed on the end portion of the annular body  10  that touches the wall surface of the annular stepped portion  8  of the sleeve body  9 , and the portion that touches the annular body  10  of the counter plate  11 . Oil  14  is filled and reserved in the gap between the sleeve  7  including the grooves  15  and the shaft  12 , and in the grooves that are not shown in the figure. In this embodiment, the shaft  12  is constructed of a shaft body  12   a  that is a body of the shaft, and an annular body  10 .  
      The dynamic pressure generated by the pumping action in association with rotation of the shaft  12  forces a fluid layer to be formed between the sleeve  7  and the shaft  12 , whereby the shaft  12  rises with respect to the counter plate  11  and the shaft  12  rotates with respect to the sleeve  7  via a fluid layer as shown in  FIG. 3 . In other words, the fluid dynamic bearing  13  forms a fluid layer by the dynamic pressure as described above to form a gap between the shaft  12  and the counter plate  11  (stationary portion) to support a thrust load of the shaft  12  (in other words, the counter plate  11  supports a thrust load applied downwardly of the shaft  12  (in the direction of the arrow D in  FIG. 1 ) and the ceiling wall of the medium diameter hole portion  8   a  supports a thrust load applied upwardly of the shaft  12  (annular body  10 ) (in the direction of the arrow U in  FIG. 1 ), and a radial load of the shaft  12  is supported by the portion of the sleeve  7  where the sleeve hole  7   a  is formed.  
      The inner peripheral surface of the annular body  10  is formed with one or more fluid circulating groove  10   a  so as to facilitate circulation of the fluid. The annular body  10  slightly projects toward the counter plate  11  with respect to the shaft  12 , so as to facilitate inflow and outflow of fluid from and to the fluid circulating groove  10   a . It is also possible to provide the annular body  10  so as not to project toward the counter plate  11  with respect to the shaft body  12   a  to form a flat surface (or to be flush with the shaft body  12   a ).  
      The annular body  10  of the shaft  12  is disposed at the medium diameter annular hole  8   a , that is, between the wall surface of the medium diameter annular hole  8   a  that faces in the axial direction (the upper side in  FIG. 1 ) and the counter plate  11 , so that the axial movement (vertical movement in  FIG. 1 ) of the shaft  12  is controlled via the annular body  10 .  
      The central position of the end surface  12   b  of the shaft body  12   a  is press-fitted with a ball (projection)  51  formed from ceramic. The ball  51  is provided in such a manner that the tip portion (not designated by the reference numeral) comes to the position higher than the end surface  10   b  of the annular body  10 .  
      The position to which the ball  51  is mounted is not limited to the central position of the end surface  12   b  of the shaft body  12   a , but it may be any positions other than the central position as far as it is on the end surface  12   b  of the shaft body  12   a . There may be provided a plurality of balls  51 . When a plurality of balls  51  are provided, it is preferably to arrange the plurality of balls  51  so that a load of the shaft  12  can be supported in a balanced manner.  
      The ball  51  is provided in such a manner that the tip portion thereof comes to the position higher than the end surface  10   b  of the annular body  10 . More specifically, the projection measurement h of the ball  51  from the end surface  12   b  of the shaft body  12   a  or the height ht of the ball  51  is determined as follows.  
      The distance G 2 , the distance G 3 , and the distance G 0  shown in  FIG. 4  are determined as follows, and the projection measurement h or the height ht of the ball  51  is determined so that the sum of the distance G 2  and the distance G 3  is equal to the distance G 0 ; (G 2 +G 3 =G 0 ).  
      The distance G 2 : The axial distance between the end surface  10   b  of the annular body  10  and the upper surface  11   a  of the counter plate  11  when the tip portion of the ball  51  touches the upper portion  11   a  of the counter plate  11  facing toward the shaft  12  with the shaft  12  (rotor  4 ) remained at rest.  FIG. 4  is a cross section taken when the shaft  12  is being rotated, and the distance G 2  shown in  FIG. 4  is marked just for the sake of convenience.  
      The distance G 3 : The axial distance between the tip portion of the ball  51  and the upper surface  11   a  of the counter plate  11  when the shaft  12  is rotated at a specified rotational speed.  
      The distance G 0 : The axial distance between the end surface  10   b  of the annual body  10  and the upper surface  11   a  of the counter plate  11  when the shaft  12  is rotated at a specified rotational speed.  
      In this embodiment, a ball  51  is provided on the end surface  12   b  of the shaft body  12   a  in such a manner that the tip portion of the ball  51  comes to the position higher than the end surface  10   b  of the annual body  10  as described above. In this arrangement, when the shaft  12  is at rest, the ball  51  abuts against the upper portion  11   a  of the counter plate  11 , and the end surface  10   b  of the annual body  10  is brought into a state of being raised from the upper surface  11   a  of the counter plate  11 , so that the situation in which the end surface  10   b  of the annular body  10  and the upper surface  11   a  of the counter plate  11  are brought into almost fully touch each other, which could be occurred in the related art described above, can be avoided. Therefore, a specified gap is formed between the end surface  10   b  of the annular body  10  and the upper surface  11   a  of the counter plate  11 , and thus the circulation speed of a fluid is increased when rotation is started. As a consequent, a fluid layer is quickly formed and thus the shaft  12  rises quickly and sufficiently.  
      As described above, when the shaft  12  is at rest, the ball  51  abuts against the upper surface  11   a  of the counter plate  1 , and the end surface  10   b  of the annular body  10  is brought into a state of being raised from the upper surface  11   a  of the counter plate  11 , and the shaft  12  is raised sufficiently and quickly. Therefore, a fluid circularity blocking action caused by adhesion in the tightly sticked state or by a small clearance, which could be occurred in the related art described above, can be avoided, and generation of scratch caused by starting rotation in the tightly sticked state can be positively prevented.  
      Since the ball  51  is formed of porous ceramic that can impregnate oil, lubricity can be further improved.  
      In the spindle motor disclosed in Japanese Unexamined Patent Application Publication No. 11-311245, as shown in  FIG. 1  and the paragraphs [0016] to [0017] of the same publication, in a state in which the free end of the shaft body touches the closed end surface (upper side in the figure) of the cylindrical member, a gap is formed between the end surface of the cylindrical member on the side of the opening and the upper surface of the support (lower side in the figure), so that the free end of the shaft body is configured into a curved surface. In this spindle motor, the curved surface (projecting portion) is formed of the same material as the cylindrical member. Therefore, manufacturing of the shaft body is constrained, which results in lowering of versatility correspondingly. On the other hand, in this embodiment, since the projection (ball  51 ) is provided separately from the member on which the projection is provided (shaft boy  12   a ), the member on which the projection is provided (shaft body  12   a ) may be used widely to various types of the motor, thereby improving productivity correspondingly.  
      In the embodiment described above, there is shown an example in which the ball  51  is provided on the end surface  12   b  of the shaft body  12   a . Alternatively, as shown in  FIG. 5 , the ball  51  may be press-fitted to the end surface  10   b  of the annular body  10  (second embodiment). The second embodiment corresponds to the third aspect of the present invention. In the second embodiment, the height h of the ball  51  (dimension of the annular body  10  projecting from the end surface  10   b ) is specifically determined as follows.  
      The distance G 22 , the distance G 32 , and the distance G 02  shown in  FIG. 5  are determined as follows, and the height h 1  of the ball  51  is determined so that the sum of the distance G 22  and the distance G 32  is equal to the distance G 02 ; (G 22 +G 32 =G 02 ).  
      The distance G 22 : The axial distance between the end surface  10   b  of the annular body  10  and the upper surface  11   a  of the counter plate  11  when the tip portion of the ball  51  touches the upper surface  11   a  of the counter plate  11  with the shaft  12  remained at rest.  FIG. 5  is a cross section taken when the shaft  12  is being rotated, and the distance G 22  shown in  FIG. 5  is marked just for the sake of convenience.  
      The distance G 32 : The axial distance between the tip portion of the ball  51  and the upper surface  11   a  of the counter plate  11  when the shaft  12  is rotated at a specified rotational speed.  
      The distance G 02 : The axial distance between the end surface  10   b  of the annular body  10  and the upper surface  11   a  of the counter plate  11  when the shaft  12  is rotated at a specified rotational speed.  
      In the second embodiment, when the shaft  12  is remained at rest, the ball  51  abuts against the upper surface  11   a  of the counter plate  11 , and as in the first embodiment, the end surface  10   b  of the annular body  10  is brought into a state of being raised from the upper surface  11   a  of the counter plate  11 , so that the situation in which the end surface  10   b  of the annular body  10  and the upper surface  11   a  of the counter plate  11  are brought into almost fully touch each other can be avoided. Therefore, a fluid circularity blocking action that could be occurred in the related art can be avoided and generation of scratch caused by starting rotation in the tightly sticked state can be positively prevented.  
      In the first and second embodiment, there is shown an example in which the ball  51  is provided on the shaft  12  side (the shaft body  12   a  or the annular body  10 ). Alternatively, as shown in FIGS.  6  to  8 , the ball  51  may be press-fitted to the portion  11   b  on the upper surface  11   a  of the counter plate  11  facing toward the end surface  12   b  of the shaft body  12   a  (the surface on the stationary portion facing toward the shaft body)(third embodiment). The third embodiment corresponds to the fifth aspect of the present invention. In the third embodiment, the height h of the ball  51  (dimension projecting from the upper surface  11   a  of the counter plate  11 ) is determined to be larger than the dimension from the end surface  12   b  of the shaft body  12   a  to the end surface  10   b  of the annular body  10 , and specifically it is determined as follows.  
      The distance G 23 , the distance G 33 , and the distance G 03  shown in  FIG. 8  are determined as follows, and the height h of the ball  51  is determined so that the sum of the distance G 23  and the distance G 33  is equal to the distance G 03 ; (G 23 +G 33 =G 03 ).  
      The distance G 23 : The axial distance between the end surface  10   b  of the annular body  10  and the upper surface  11   a  of the counter plate  11  when the tip portion of the ball  51  touches the end surface  12   b  of the shaft body  12   a  with the shaft  12  remained at rest.  FIG. 8  is a cross section taken when the shaft  12  is being rotated, and the distance G 23  shown in  FIG. 8  is marked just for the sake of convenience.  
      The distance G 33 : The axial distance between the tip portion of the ball  51  and the end surface  12   b  of the shaft body  12   a  when the shaft  12  is rotated at a specified rotational speed.  
      The distance G 03 : The axial distance between the end surface  12   b  of the shaft body  12   a  and the upper surface  11   a  of the counter plate  11  when the shaft  12  is rotated at a specified rotational speed.  
      In the third embodiment, when the shaft  12  is remained at rest, the ball  51  abuts against the end surface  12   b  of the shaft body  12   a , and as in the first embodiment, the end surface  10   b  of the annular body  10  is brought into a state of being raised from the upper surface  11   a  of the counter plate  11 , so that the situation in which the end surface  10   b  of the annular body  10  and the upper surface  11   a  of the counter plate  11  are brought into almost fully touch each other can be avoided. Therefore, a fluid circularity blocking action that could be occurred in the related art can be avoided and generation of scratch caused by staring rotation in the tightly sticked state can be positively prevented.  
      In the third embodiment, there is shown an example in which the ball  51  is press-fitted into the portion on the surface  11   b  of the counter plate facing toward the shaft body (the portion on the surface of the stationary portion facing toward the shaft body). Alternatively, as shown in  FIG. 9 , the ball  51  may be press-fitted to the portion  11   c  on the upper surface  11   a  of the counter plate  11  facing toward the end surface  10   b  of the annular body  10  (the portion on the surface of the stationary portion facing toward the annular body)(fourth embodiment). The fourth embodiment corresponds to the fourth aspect of the present invention. In the fourth embodiment, the height h of the ball  51  (dimension projecting from the upper surface  11   a  of the counter plate  11 ) is determined as follows.  
      The distance G 24 , the distance G 34 , and the distance G 04  shown in  FIG. 9  are determined as follows, and the height h 1  of the ball  51  is determined so that the sum of the distance G 24  and the distance G 34  is equal to the distance G 04 ; (G 24 +G 34 =G 04 ).  
      The distance G 24 : The axial distance between the end surface  10   b  of the annular body  10  and the upper surface  11   a  of the counter plate  11  when the tip portion of the ball  51  touches the end surface  10   b  of the annular body  10  with the shaft  12  remained at rest.  FIG. 9  is a cross section taken when the shaft  12  is being rotated, and the distance G 24  shown in  FIG. 10  is marked just for the sake of convenience.  
      The distance G 34 : The axial distance between the tip portion of the ball  51  and the end surface  10   b  of the annular body  10  when the shaft  12  is rotated at a specified rotational speed.  
      The distance G 04 : The axial distance between the end surface  10   b  of the annular body  10  and the upper surface  11   a  of the counter plate  11  when the shaft  12  is rotated at a specified rotational speed.  
      In the fourth embodiment, when the shaft  12  is remained at rest, the ball  51  abuts against the end surface  10   b  of the annular body  10 , and as in the first embodiment, the end surface  10   b  of the annular body  10  is brought into a state of being raised from the upper surface  11   a  of the counter plate  11 , so that the situation in which the end surface  10   b  of the annular body  10  and the upper surface  11   a  of the counter plate  11  are brought into almost fully touch each other can be avoided. Therefore, a fluid circularity blocking action that could be occurred in the related art can be avoided and generation of scratch caused by starting rotation in the tightly sticked state can be positively prevented.  
      In the first to fourth embodiments, there is shown an example in which the shaft  12  constructed of the shaft body  12  and the annular body  10  is used. Alternatively, as shown in  FIG. 10 , it is also possible to use a shaft that is not provided with the annular body  10  (hereinafter referred to as a single shaft for the sake of convenience)  12 T, and the ball  51  is press-fitted into the end surface  12 T 1  of the single shaft  12 T (fifth embodiment). The fifth embodiment corresponds to the first aspect of the present invention. In the fifth embodiment, the height h 1  of the ball  51  (dimension of the single shaft  12 T projecting from the end surface  12 T 1 ) is determined as follows.  
      The distance G 25 , the distance G 35 , and the distance G 05  shown in  FIG. 10  are determined as follows, and the height h 1  of the ball  51  is determined so that the sum of the distance G 25  and the distance G 35  is equal to the distance G 05 ; (G 25 +G 35 =G 05 ).  
      The distance G 25 : The axial distance between the end surface  12 T 1  of the shingle shaft  12 T and the upper surface  11   a  of the counter plate  11  when the tip portion of the ball  51  touches the upper surface  11   a  of the counter plate  11  with the shaft  12  remained at rest.  FIG. 10  is a cross section taken when the shaft  12  is being rotated, and the distance G 25  shown in  FIG. 9  is marked just for the sake of convenience.  
      The distance G 35 : The axial distance between the tip portion of the ball  51  and the upper surface  11   a  of the counter plate  11  when the single shaft  12 T is rotated at a specified rotational speed.  
      The distance G 05 : The axial distance between the end surface  12 T 1  of the single shaft  12 T and the upper surface  11   a  of the counter plate  11  when the single shaft  12 T is rotated at a specified rotational speed.  
      In the fifth embodiment, when the single shaft  12 T is remained at rest, the ball  51  abuts against the upper surface  11   a  of the counter plate  11 , and as in the first embodiment, the end surface  12 T 1  of the single shaft  12 T is brought into a state of being raised from the upper surface  11   a  of the counter plate  11 , so that the situation in which the end surface  12 T 1  of the single shaft  12 T 1  and the upper surface  11   a  of the counter plate  11  are brought into almost fully touch each other can be avoided.  
      Therefore, a fluid circularity blocking action that could be occurred in the related art can be avoided. Therefore, a fluid circularity blocking action that could be occurred in the related art can be avoided and generation of scratch caused by starting rotation in the tightly sticked state can be positively prevented.  
      In the fifth embodiment, there is shown an example in which the ball  51  is press-fitted to the end surface  12 T 1  of the single shaft  12 T. Alternatively, it is also possible press-fit the ball  51  into the upper surface  11   a  of the counter plate  11  so that the portion on the tip side projects from the upper surface  11   a  of the counter plate  111  (corresponding to the invention according to the claim  1  or claim  2 ).  
      In each embodiment described above, there is shown an example in which the ball  51  is press-fitted into the member on which the ball  51  is to be provided (shaft body  12   a , the annular body  10 , or the counter plate  11 ). However, it is also possible to fix the ball  51  on the member on which the ball  51  is to be provided (shaft body  12   a , the annular body  10  or the counter plate  11 ) with fixing means such as adhesives. In this case, the fixing means such as adhesives should be compatible with the fluid.  
      In each of the embodiment described above, there is shown an example in which the projection is a ball  51  formed of ceramic. Alternatively, it may be a steel ball.  
      The projection is not limited to the spherical shape (ball  51 ) described in the above-described embodiments, but it may be a conical projection as shown in  FIG. 11 , or may be other shapes such as a shaft shape and tapered shape. When the tapered shape is employed, the tip portion is preferably formed into a convex curved shape so as not to set down the mated surface that touches it.  
      In addition, in the embodiments described above, there is shown an example in which the projection (ball  51 ) is formed separately from the member on which the projection (ball  51 ) is provided (shaft body  12   a , annular body  10  or the counter plate  11 ). Alternatively, the projection may be formed integrally with the member on which the projection is provided (shaft body  12   a , annular body  10  or the counter plate  11 ). For example, as shown in  FIG. 12 , it is also possible to form the projection  52  of conical shape on the upper surface  11   a  of the counter plate  11 .  
      In the embodiments described above, there are shown examples in which the ball  51  is press-fitted into the member (shaft body  12   a , annular body  10  or counter plate  11 ) or fixed thereon with the fixing means such as adhesives. As an alternative thereto, as shown in  FIGS. 13-19 , the projection may be a member made of a high hardness material formed by the sputtering (corresponding to claims  10  to  12 ).  
      In the sixth embodiment, as shown in  FIG. 13 , a single disk-like projection  55  is provided at the center of the end surface  12   b  of the shaft body  12   a . The diameter and the height h 1  of the projection  55  is 0.5 mm to 5 mm and 2 μm, respectively. The projection  55  is composed of a base member  56  containing silicon or chromium as a main component and being 0.5 μm in its height and a secondary member made of a high hardness material  57  (hereinafter a secondary member  57 ) placed thereon and being 1.5 μm in its height, and both of the members are formed by the sputtering. The secondary member  57  is made of DLC, which is formed by being crystallized in an atmosphere of hydrogen or methane, characterized in that the hardness or smoothness thereof is more superior to amorphous carbon (crystal body made by which carbon is crystallized in a vacuum).  
      As described above, the height h 1  of the projection  55  is determined to be 2 μm and the tip portion thereof is made to be higher than the end surface  10   b  of the annular body  10 . To be specific, the ball  51  as shown in  FIG. 4  is replaced by the projection  55 , and each specific measurement is determined as same as the first embodiment. And, the measurement ht projected from the end surface  12   b  of the shaft body  12   a  (hereinafter projection measurement of the projection  55 ) is determined in a state that the height h 1  of the projection  55  is set to be 2 μm. That is, when considering the projection measurement ht or the height h 1  of the projection  55  (2 μm), the sum of the distance G 2  and the distance G 3  is equal to the distance G 0  (G 2 +G 3 □G 0 ).  
      In the sixth embodiment thus constructed, when the shaft  12  remains at rest, the projection  55  abuts against the upper surface  11   a  of the counter plate  1 , and the end surface  10   b  of the annular body  10  is brought into a state of being raised from the upper surface  11   a  of the counter plate  11 . Moreover, since the shaft  12  is sufficiently raised to a specific level and in a quick motion, problems occurred in prior arts can be effectively prevented. That is, an adhesion occurred in a cohered state or a fluid circularity blocking action due to a small aperture can be prevented. Moreover, scratches caused when a rotor is started to rotate in the cohered state can be prevented in a certain manner.  
      In addition, durability is improved due to that the secondary member  57  of the projection  55  is made by DLC which is characteristically superior in a high hardness and a surface smoothness. In this case, because the sputtering is not a complicated method, obtaining of the projection  55  is easy. For example, by forming an aperture on a stainless mask and conducting the sputtering thereover, the projection can be formed. Alternatively, in case that a plurality of projections are formed at a time, apertures corresponding to the projections should be made on the mask, then a plurality of projections can be formed by conducting only one sputtering. In addition, a conical or a hemisphere projection can be formed by adjusting the shape of the mask.  
      Further, the projection  55  provided at the end surface  12   b  of the shaft body  12   a  comprises the base member  56  containing silicon or chromium as a main component and secondary member  57  made of a high hardness material placed thereon, and both of the members are formed by the sputtering. That is, since the base member  56  is placed between the end surface  12   b  of the shaft body  12   a  and the secondary member  57 , the secondary member  57  and the end surface  12   b  can be made a certain attachment.  
      Because the projection  55  is provided at the center of the end surface  12   b  of the shaft body  12   a , a starting torque can be reduced. However, the portion at where the projection  55  is provided is not limited to the center of the end surface  12   b  of the shaft body  12   a . Instead, the projection  55  can be provided at any point as long as that is at the end surface  12   b  of the shaft body  12   a . Alternatively, the projection  55  can be provided in a plural number. In this case a plurality of the projections  55  should be provided in such a manner that a load of the shaft  12  is most effectively supported.  
      Furthermore, since the projection  55  abuts against the upper surface  11   a  of the counter plate  11  giving more gap between the upper surface  11   a  of the counter plate  11  and the end surface  12   b  of the shaft body  12   a , more amount of oil can be filled and reserved therein.  
      Still further, in the sixth embodiment there is shown example in which the height h 1  of the projection  55  is set to be 2 μm, but this is not limited thereto. Instead, the height h 1  can be set within the range from 0.02 μm to 5 μm, and this can be also applied to the seventh to twelfth embodiments described hereinafter.  
      And, in the sixth embodiment there is shown example in which the secondary member  57  of the projection  55  is made of DLC, but this is not limited thereto. Instead, the secondary member  57  can be made of amorphous carbon, and this can be applied to the seventh to twelfth embodiments described hereinafter.  
      Furthermore, in the sixth embodiment there is shown example in which the projection  55  comprises the base member  56  being 0.5 μm in its height and the secondary member  57  being 1.5 μm in its height, but this is not limited thereto. Instead, the projection  55  can be composed only of the secondary member  57  being 2.0 μm in its height without providing any of the base member  56 . In this case the height of the secondary member  57  (or the projection  55 ) is not limited to 2.0 μm but can be set within the range from 0.02 μm to 5 μm (corresponding to claim  10 ), and this can be applied to the seventh to twelfth embodiments described hereinafter.  
      In the sixth embodiment ( FIG. 13 ) there is also shown example in which the projection  55  is provided on the end surface  12   b  of the shaft body  12   a , but this is not limited thereto. Instead, in the seventh embodiment as shown in  FIG. 14 , the projection  55  is provided on the end surface  10   b  of the annular body  10 . In this seventh embodiment the height h 1  of the projection  55  (projection measurement from the end surface  10   b  of the annular body  10 ) is set to be 2 μm. Moreover, in the seventh embodiment each measurement of G  22 , G  32  and G  02  is determined as same as the case of the second embodiment by replacing the ball  51  with the projection  55 . That is, the sum of the distance G  22  and the distance G  32  is equal to the distance G  02  (G 22 +G  32 =G  02 ).  
      In the sixth and seventh embodiment there is shown example in which the projection  55  is provided on the side of the shaft  12  (shaft body  12   a  or annular body  10 ), but this is not limited thereto. Instead, as shown in  FIG. 15  (corresponding to the eighth embodiment), the projection  55  can be provided on the surface  11   b  of the counter plate  11  facing the end surface  12   b  of the shaft body  12   a . In the eighth embodiment the height h 1  of the projection  55  (projection measurement from the upper surface  11   a  of the counter plate  11 ) is set to be 2 μm. Moreover, each measurement of G  23 , G  33  and G  03  as shown in  FIG. 15  is determined as same as the case of the third embodiment by replacing the ball  51  with the projection  55 . That is, the sum of the distance G  23  and the distance G  33  is equal to the distance G  03  (G  23 +G  33 =G  03 ).  
      In the above eighth embodiment the projection  55  is provided on the surface  11   b  of the counter plate  11 , but this is not limited thereto. Instead, as shown in  FIG. 16  (corresponding to the ninth embodiment), the projection  55  can be provided on the portion  11   c  of the counter plate  11  facing the end surface  10   b  of the annular body  10 . In the ninth embodiment the height h 1  of the projection  55  (projection measurement from the upper surface  11   a  of the counter plate  11 ) is set to be 2 μm. Moreover, each measurement of G  24 , G  34  and G  04  as shown in  FIG. 16  is determined as same as the case of the forth embodiment by replacing the ball  51  with the projection  55 . That is, the sum of the distance G  24  and the distance G 34  is equal to the distance G  04  (G 24 +G 34 =G 04 ).  
      In the sixth to ninth embodiments there is shown example in which the shaft  12  is composed of the shaft body  12   a  and the annular body  10 , but this is not limited thereto. Instead, as shown in  FIG. 17  (corresponding to the tenth embodiment) the projection  55  can be provided on the end surface  12 T 1  of the single shaft  12 T meaning the shaft not having the annular body  10 . In the tenth embodiment the height h 1  of the projection  55  (projection measurement from the end surface  12 T 1  of the single shaft  12 T) is set to be 2 μm. Moreover, each measurement of G  25 , G  35  and G  05  as shown in  FIG. 17  is determined as same as the case of the fifth embodiment by replacing the ball  51  with the projection  55 . That is, the sum of the distance G  25  and the distance G  35  is equal to the distance G 05  (G 25 +G 35 =G 05 ).  
      Furthermore, in the sixth to tenth embodiments there is shown example in which the projection  55  is plate-like shape, but this is not limited thereto. Instead, as shown in  FIG. 18 , the projection  55  can be conical shape or hemisphere shape as shown in  FIG. 18  (corresponding to the eleventh embodiment) and  FIG. 19  (corresponding to the twelfth embodiment), respectively.  
      According to the present invention, when the rotating member is at rest, a gap is generated between the opposing generally flat end surfaces of the fluid dynamic bearing by the abutment of the projection, and one of these opposing end surfaces is brought into a state of being raised from the other end surface, so that the situation in which both of the end surfaces are brought into almost fully touch each other can be avoided. Therefore, a fluid circularity blocking action that could be occurred in the related art can be avoided and generation of scratch caused by starting rotation in the tightly sticked state can be positively prevented.