Patent Publication Number: US-7220054-B2

Title: Dynamic pressure bearing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a divisional of U.S. patent application Ser. No. 10/473,543 filed Sep. 30, 2003 and now U.S. Pat. No. 7,101,084, which claims priority benefits of International Patent Application PCT/JP02/03696 filed Apr. 12, 2002, both of which are incorporated here by reference, and which further claims priority rights from Japanese Patent Applications 2001-115618 and 2001-115324 both filed on Apr. 13, 2001, which priority claims are repeated here. 

   TECHNICAL FIELD 
   The present invention relates to a bearing apparatus. More particularly, the present invention relates to a dynamic pressure bearing apparatus which supports a shaft and a bearing by a dynamic pressure of a lubricating fluid so as to be capable of relatively rotating. 
   BACKGROUND ART 
   In recent years, there have been made various kinds of proposals concerning a dynamic pressure bearing apparatus used to support various kinds of rotors in a device having a portion which rotates at a high speed, which is typified by a magnetic disc, a polygon mirror or an optical disc. For example, in a dynamic pressure bearing apparatus adopted in a hard disk drive apparatus (which will be referred to as an HDD hereinafter) shown in  FIG. 21 , a rotary shaft (shaft)  2  is rotatably inserted in a fixed bearing sleeve (bearing)  1 , and a lubricating fluid F such as an oil or a magnetic fluid is injected into a minute gap between an inner peripheral surface of the bearing sleeve  1  and an outer peripheral surface of the rotary shaft  2 , thereby constituting two radial bearing portions RB and RB provided at respective positions so as to be separated from each other in the axial direction. It is to be noted that, in this HDD, a rotary hub  5  used to hold a recording disk (not shown) is joined to a part of the rotary shaft  2  which is protruding from the bearing sleeve  1  by press fitting or shrink fitting, and the recording disk is held by a clamper (not shown) screwed by utilizing a screw hole  7  at an upper end portion of the rotary hub  5 . 
   In such a dynamic pressure bearing apparatus, a thrust plate  3  is provided at least one end of the rotary shaft  2  as well as the above-described radial bearing portions RB and RB, and thrust bearing portions SBa and SBb are constituted between the bearing sleeve  1  and a counter plate  4  attached to the bearing sleeve  1 . The lubricating fluid F is continuously injected into the thrust bearing portions SBa and SBb from the radial bearing portions RB. 
   Here, as to attachment of the thrust plate  3  to the rotary shaft  2 , the structure that joining is performed by a fastening force of such a headed screw  6  as shown in  FIG. 21  is conventionally known from, e.g., Japanese Registered Utility Model Publication No. 256791, Japanese patent Application Laid-open Publication No. 74841/1996 and others. Such a screwing structure holds the thrust plate  3  so as to be sandwiched in the axial direction between a head portion  6 a of a headed screw  6  screwed at a lower end portion of the rotary shaft  2  and an end surface of the rotary shaft  2 . 
   The screwing structure of this thrust plate  3  corresponds to a request or the like for a reduction in thickness of the dynamic pressure bearing apparatus. That is, when a height of the dynamic pressure bearing apparatus in the axial direction is greatly suppressed owing to a reduction in thickness of the dynamic pressure bearing apparatus, a joint length of the thrust plate  3  relative to the rotary shaft  2  in the axial direction is reduced, and the joint strength of the thrust plate  3  may be possibly lowered. In particular, in an apparatus such as a mobile computer which is presupposed to be carried, the impact resistance at the time of falling or the like must be sufficiently assured, and hence the joint strength of the thrust plate  3  is a serious problem. Thus, adoption of the above-described screwing structure is used to firmly fix the thrust plate  3 . 
   Further, as another attachment method of the thrust plate  3  relative to the rotary shaft  2 , there is a fixing technique such as press fitting or shrink fitting as shown in  FIG. 22 , for example. In this case, the thrust bearing portions SBa and SBb are configured above and below the thrust plate  3 ′ between the thrust plate  3 ′ fixed at the lower end portion of the rotary shaft  2 ′ by press fitting or shrink fitting and the bearing sleeve  1  and between the thrust plate  3 ′ and the counter plate  4 ′ attached to the bearing sleeve  1 . It is to be noted that a rotary hub  5 ′ used to hold the recording disk is joined to a part of the rotary shaft  2 ′ which is protruding from the bearing sleeve  1  by press fitting, shrink fitting or the like, and a damper  9  is fixed at the upper end portion of the rotary shaft  2 ′ by the headed screw  6 ′ so that the recording disk  8  is held by a pressing force of the damper  9  in the axial direction. It is to be noted that the reference numeral  6   a ′ denotes a screw head portion,  6   b ′ designates a male screw portion and  2   a ′ denotes a female screw portion formed from the upper end surface of the rotary shaft  2 ′ toward the lower end surface of the same in the drawing. 
   In the fixing structure of the thrust plate  3  using screwing shown in  FIG. 21 , however, the lubricating fluid F gradually penetrates by the capillarity generated in a gap, to the gap of the screw portion where the female screw portion formed to the shaft  2  and the male screw portion on the headed screw  6  side are screwed. As a result, the lubricating fluid F in the thrust dynamic pressure bearing portion is substantially decreased and becomes insufficient, thereby possibly reducing the duration of life of the bearing. 
   Furthermore, since a hole which is preparedly provided for thread cutting is opened longer than the female screw portion, a space (which is called a screw non-occupied space) is generated between the end of the headed screw  6  and the bottom of the screw hole. Since air remains in the screw non-occupied space, when the remaining air is expanded by an increase in temperature, the expanded air pushes the lubricating fluid F toward the outside, which results in the leak. Such leak of the lubricating fluid F may possibly lead to a fatal problem in the HDD or the like requiring the cleanness. 
   In the dynamic pressure bearing apparatus shown in  FIG. 22 , the female screw portion  2   a ′ used to fix the damper  9  formed to the rotary shaft  2 ′ is configured so as to be cut by a bottomed hole. Therefore, foreign matters such as chips generated at the time of thread cutting or the like are readily deposited on the bottom of the hole in which the female screw portion  2   a ′ is cut. Moreover, the foreign matters once deposited on the bottom of the hole of the female screw portion  2   a ′ cannot be completely removed even if a cleansing step or the like is carried out. If these foreign matters spatter to the outside during use of the apparatus, they adhere to the recording disk which must be cleaned, which may possibly result in a fatal error problem such that recording/reproduction of information is disabled, for example. 
   Additionally, if a reduction in thickness of the apparatus further advances, a length of the lower hole of the female screw portion  2   a ′ formed to the above-described rotary shaft  2 ′ is also decreased. In such a case, the female screw portion  2   a ′ is subjected to thread cutting in the shortened lower hole by using the similarly shortened threading tap tool. However, by doing so, a large stress is loaded to the tap tool when shaping the screw, a problem such that the tap tool is readily bent occurs, thereby possibly lowering the productivity. 
   It is an object of the present invention to provide a dynamic pressure bearing apparatus which can readily increase the joint strength of the thrust plate relative to the shaft and simultaneously obtain the excellent dynamic pressure characteristic and high productivity. It is another object of the present invention to provide a dynamic pressure bearing apparatus excellent in the cleanliness and the processability. 
   DISCLOSURE OF INVENTION  
   To achieve this aim, according to the present invention, there is provided a dynamic pressure bearing apparatus in which a radial dynamic pressure bearing portion is constituted between an inner peripheral surface of a bearing and an outer peripheral surface of a shaft, a thrust dynamic pressure bearing portion is constituted between a thrust plate attached to at least one end surface side of the shaft and the bearing or a counter plate fixed to the bearing and the bearing and the shaft are supported by generating a dynamic pressure to a lubricating fluid injected into each dynamic pressure bearing portion by relative rotation produced between the bearing and the shaft, wherein a bottomed screw hole used to screw a headed screw which attaches the thrust plate is provided to at least one end of the shaft, the lubricating fluid is filled in a gap of a screw portion between a screw and a screw hole or a screw non-occupied space between a bottom portion of the screw hole and a screw portion end of the headed screw, and an interposing member which reduces a void cubic content in the screw non-occupied space is attached to the screw non-occupied space. 
   According to the present invention, therefore, since air does not remain in the screw hole when the lubricating fluid is filled in the screw gap in the screw hole or the screw non-occupied space, risk of leak of the lubricating fluid to the outside of the bearing by expansion of the remaining air can be excellently avoided. Further, the leakage phenomenon of the lubricating fluid having the larger thermal expansion property than the metal member constituting each member on the bearing side can be further reduced by decreasing a quantity of the lubricating fluid to be filled in the screw non-occupied space by an amount corresponding to a cubic content of the interposing member, thereby improving the above-described advantage. Furthermore, although this structure is simple but it can readily increase the joint strength of the thrust plate relative to the shaft and obtain the excellent dynamic pressure characteristic and the high productivity at the same time. 
   Moreover, in the dynamic pressure bearing apparatus according to the present invention, it is preferable that the interposing member is formed as a member provided separately from the headed screw or formed of a resin material which is injected into the screw non-occupied space and hardened there. It this case, since a general headed screw can be used as the headed screw, thereby decreasing the component cost. 
   Additionally, in the dynamic pressure bearing apparatus according to the present invention, it is preferable that the interposing member is formed as a member integral with the headed screw. In this case, since attachment of the interposing member is completed simultaneously with screwing of the headed screw, i.e., fixation of the thrust plate to the shaft, the assembling step becomes efficient, thereby improving the productivity. 
   Further, in the dynamic bearing apparatus according to the present invention, a fluid injection path which causes the gap of the screw portion in the screw hole and the screw non-occupied space to communicate with the outside of the shaft is provided at a contact portion between the thrust plate and the screw head portion of the headed screw or the thrust plate and the end surface of the shaft. In this case, the lubricating fluid can be injected into the gap of the screw portion in the screw hole and the screw non-occupied space through the injection path in a short time, thereby improving the productivity. 
   Furthermore, it is preferable that the fluid injection path includes a thrust fluid path which causes thrust dynamic pressure bearing portions respectively formed on both sides of the thrust plate in the axial direction to communicate with each other. In this case, even if a pressure difference is generated between the thrust dynamic pressure bearing portions with the thrust plate therebetween during rotation, the lubricating fluid can be appropriately reciprocated through the thrust fluid path, thereby obtaining the stable dynamic pressure characteristic. 
   Moreover, in the dynamic pressure bearing apparatus according to the present invention, a tool engagement concave portion for fastening provided at the screw head portion of the headed screw is filled with an adhesive having the oil resistance. In this case, since chip pieces such as a flash generated during the fastening operation of the headed screw can be assuredly prevented from scattering to the outside and the excellent cleanliness can be obtained, thereby further improving the dynamic pressure characteristic. 
   Further, in the dynamic pressure bearing apparatus according to the present invention, a through hole piercing in the axial direction is provided to the shaft, a female screw portion is formed, and the thrust plate is fixed to at least one end of the shaft by the headed screw which engages with the female screw portion. An adhesive, which seals a gap in the engagement portion and prevents the lubricating fluid from leaking outside, is filled in at least a part of the screwing portion of the headed screw and the female screw portion of the shaft. 
   Therefore, since the screw portion with the necessary length can be readily obtained even if the axial length is shortened (the height direction of the dynamic pressure bearing apparatus is reduced), the fastening force of the headed screw which fixes the thrust plate can be assured, thereby obtaining the sufficient joint strength of the thrust plate. Further, the female screw portion can be readily and efficiently processed with respect to the lower hole consisting of the through hole having a maximum length by using a thread cutting tool having an enough processing length. Furthermore, foreign matters such as chips which have entered in the shaft when processing the female screw portion can be easily removed to the outside via the through hole, thus obtaining the excellent cleanliness. Moreover, since there is provided a simple structure which can readily perform injection of the adhesive into the screw end by utilizing the through hole and the looseness of the headed screw which fixes the thrust plate can be avoided while preventing the lubricating fluid which tends to flow out to the outside via the through hole, the reliability of the strength, the cleanliness and the workability of the dynamic pressure bearing apparatus can be improved with the simple structure. 
   Additionally, in the dynamic pressure bearing apparatus according to the present invention, a ventilation path which causes a gap which reaches an adhesive filled part of the screw portion in the screw hole to communicate with the outside of the shaft is provided at a contact portion between the thrust plate and the screw portion of the headed screw or the thrust plate and the end surface of the shaft. In this case, when injecting the adhesive into the screw end portion from the open portion of the through hole, since air in the through hole and in the gap of the screw portion can be removed to the outside via the ventilation path, the adhesive injection operation can be very excellently and rapidly carried out. 
   Further, in the dynamic bearing apparatus according to the present invention, a tool engagement concave portion for fastening which is provided to the screw head portion of the headed screw which is screwed into the female screw portion of the through hole is filled with an adhesive with the oil resistance. In this case, since chip pieces such as the flash generated in the fastening operation of the headed screw can be assuredly prevented from scattering to the outside and the excellent cleanliness can be obtained, thus further improving the dynamic pressure characteristic. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a vertical cross-sectional view showing an entire structure of an HDD including an axial rotation type dynamic pressure bearing apparatus according to an embodiment of a dynamic pressure bearing apparatus of the present invention; 
       FIG. 2  is an enlarged vertical cross-sectional view showing a dynamic pressure bearing apparatus portion of the HDD depicted in  FIG. 1 ; 
       FIG. 3  is a vertical cross-sectional view showing another embodiment of the dynamic pressure bearing apparatus according to the present invention; 
       FIG. 4  is a vertical cross-sectional view showing still another embodiment of the dynamic pressure bearing apparatus according to the present invention; 
       FIG. 5  is a vertical cross-sectional view showing yet another embodiment, in which an injection communication hole is provided, of the dynamic pressure bearing apparatus according to the present invention; 
       FIG. 6  is an enlarged vertical cross-sectional view showing a thrust plate used in the embodiment illustrated in  FIG. 5 ; 
       FIG. 7  is an enlarged plane view of the thrust plate used in the embodiment depicted in  FIG. 5 , showing grooves for generating a dynamic pressure by hatching; 
       FIG. 8  is a vertical cross-sectional view showing yet another embodiment of the dynamic pressure bearing apparatus according to the present invention; 
       FIG. 9  is an enlarged vertical cross sectional view of the thrust plate used in the embodiment depicted in  FIG. 8 ; 
       FIG. 10  is an enlarged plane view of the thrust plate used in the embodiment depicted in  FIG. 8 , showing grooves for generating a dynamic pressure by hatching; 
       FIG. 11  is a vertical cross-sectional view showing a further embodiment of the dynamic pressure bearing apparatus according to the present invention; 
       FIG. 12  is a vertical cross-sectional view showing a still further embodiment of the dynamic pressure bearing apparatus according to the present invention using a resin solid body as an interposing member; 
       FIG. 13  is a vertical cross-sectional view showing an embodiment of an HDD using the dynamic pressure bearing apparatus according to the embodiment having a through screw hole according to the present invention; 
       FIG. 14  is an enlarged vertical cross-sectional view of a dynamic pressure bearing apparatus depicted in  FIG. 13 ; 
       FIG. 15  is a bottom view showing a screw fixing structure of the dynamic pressure bearing apparatus depicted in  FIG. 14 , showing grooves for generating a dynamic pressure and a filling resin by hatching; 
       FIG. 16  is a plane view of the dynamic pressure bearing apparatus depicted in  FIG. 14 , showing grooves for generating a dynamic pressure and a ventilation path by hatching; 
       FIG. 17  is a vertical cross sectional view showing a yet further embodiment of the dynamic pressure bearing apparatus having a through screw hole according to the present invention; 
       FIG. 18  is a plane view of the dynamic pressure bearing apparatus depicted in  FIG. 17 , showing grooves for generating a dynamic pressure and a ventilation path by hatching; 
       FIG. 19  is a vertical cross-sectional view showing another embodiment of the dynamic pressure bearing apparatus according to the present invention; 
       FIG. 20  is a vertical cross-sectional view showing an entire structure of an HDD including the shaft fixing type dynamic pressure bearing apparatus according to an embodiment of the dynamic pressure bearing apparatus of the present invention; 
       FIG. 21  is a vertical cross-sectional view showing an entire structure of an HDD including a conventional dynamic pressure bearing apparatus; and 
       FIG. 22  is a vertical cross-sectional view showing an entire structure of an HDD including another conventional dynamic pressure bearing apparatus. 
   

   BEST MODE FOR CARRYING OUT OF THE INVENTION  
   A structure of the present invention will now be described in detail based on the illustrated best mode. 
     FIG. 1  shows an HDD using an embodiment of a dynamic pressure bearing apparatus according to the present invention. This HDD is of an axial rotation type and constituted by a stator set  10  as a fixing member and a rotor set  20  as a rotation member assembled to the stator set  10  from the upper side. The stator set  10  has a fixed frame  11  screwed to a non-illustrated fixed base side. Although this fixed frame  11  is formed of, e.g., an aluminium alloy in order to reduce the weight thereof, a bearing sleeve  13  as a bearing formed into a hollow cylindrical shape is joined to a bearing holder  12  by press fitting or shrink fitting on the inner peripheral surface side of the cylindrical bearing holder  12  formed so as to be erected at a substantially central portion of the fixed frame  11 . This bearing sleeve  13  is formed of a copper alloy such as phosphorous bronze in order to facilitate boring of a small diameter or the like. 
   Further, a stator core  14  consisting of a laminated body of an electromagnetic steel plate is fitted to the outer peripheral attachment surface of the bearing holder  12 . A drive coil  15  is wound around each salient pole provided to the stator core  14 . 
   Furthermore, a shaft (which will be referred to as a rotary shaft hereinafter) constituting the rotor set  20  is rotatably inserted into the bearing sleeve  13 . That is, a dynamic pressure surface formed at the inner peripheral wall portion of the bearing sleeve  13  is arranged so as to be opposed to the dynamic pressure surface formed on the outer peripheral surface of the rotary shaft  21  in the radial direction, and a radial dynamic pressure bearing portion RB is constituted at a small gap portion. In detail, the dynamic pressure surface on the bearing sleeve  13  side in the radial dynamic pressure bearing portion RB and the dynamic pressure surface on the rotary shaft  21  side are arranged so as to be opposed to each other in the circumferential form with a small gap of several μm therebetween, a lubricating fluid F such as a lubricating oil or a magnetic fluid (air in some cases) is continuously injected or interposed in the bearing space consisting of the small gap in the axial direction. 
   Furthermore, non-illustrated radial dynamic pressure generating grooves consisting of a herringbone shape or the like are divided into two blocks in the axial direction and annularly provided at least one side of the dynamic pressure surfaces of the bearing sleeve  13  and the rotary shaft  21 . Moreover, during rotation, a pressure is applied to the lubricating fluid F by a pumping effect of the radial dynamic pressure generation grooves, a dynamic pressure is generated between the bearing sleeve  13  and the rotary shaft  21 , and a rotary hub  22  is supported together with the rotary shaft  21  with respect to the bearing sleeve  13  in the radial direction in the non-contact state. 
   Additionally, a capillary seal portion RS is arranged at an open part at the upper end of the bearing space constituting each radial dynamic pressure bearing portion RB. This capillary seal portion RS is obtained by gradually enlarging a gap between the rotary shaft  21  and the bearing sleeve  13  toward the outside of the bearing by forming an inclined surface to at least one of the rotary shaft  21  and the bearing sleeve  13 , and it is set in such a manner that the gap is gradually enlarged from, e.g., 20 μm to 300 μm. A liquid level of the lubricating fluid F is positioned to the capillary seal portion RS in both rotation and stop of the motor. 
   Moreover, the rotary hub  22  constituting the rotor set  20  together with the rotary shaft  21  consists of a substantially cup-shaped member consisting of an aluminium-based metal, and a junction hole  22   a  provided at the central part of the rotary hub  22  is integrally coupled with a part of the rotary shaft  21  protruding from the bearing sleeve  13  by press fitting or shrink fitting. Additionally, a recording medium such as a magnetic disk is fixed to the rotary hub  22  by a non-illustrated camper. 
   The rotary hub  22  has a substantially cylindrical trunk portion  22   b  used to mount a recording medium disk on the outer peripheral part, and an annular drive magnet  22   d  is attached on the inner peripheral wall surface side on the lower side of the trunk portion  22   b  through a back yoke  22   c . The annular drive magnet  22   d  is arranged in close proximity so as to be annularly opposed to the outer peripheral side end surface of the stator core  14 . 
   As shown in  FIG. 2  in the enlarged manner, a discoid thrust plate  23  is fixed to the lower end portion of the rotary shaft  21  by a plate fixing screw  24 . This thrust plate  23  is arranged in such a manner that it is accommodated in a circular concave portion  13   a  provided so as to be dimpled at the inner side part at the lower end of the bearing sleeve  13 . In the concave portion  13   a  of the bearing sleeve  13 , a dynamic pressure surface provided on the upper surface of the thrust plate  23  is oppositely arranged so as to come close to the dynamic pressure surface provided to the bearing sleeve  13  in the axial direction. Additionally, non-illustrated thrust dynamic pressure generating grooves consisting of a herringbone shape or the like are formed on the upper dynamic pressure surface of the thrust plate  23 , and an upper thrust dynamic pressure bearing portion SBa is formed at the gap portion between the opposed dynamic pressure surfaces of the thrust plate  23  and the bearing sleeve  13 . 
   Further, a counter plate  16  consisting of a discoid member having a relatively large diameter is arranged in close proximity to the lower dynamic pressure surface of the thrust plate  23 . This counter plate  16  is arranged so as to close the open part on the lower end side of the bearing sleeve  13 , and the outer peripheral side part of the counter plate  16  is fixed to the bearing sleeve  13  side by caulking or the like. 
   Furthermore, non-illustrated thrust dynamic pressure generating grooves consisting of a herringbone shape or the like are formed on the lower dynamic pressure surface of the thrust plate  23 , and a lower thrust dynamic pressure bearing portion SBb is thereby formed. 
   Both of the dynamic pressure surfaces on the thrust plate  23  side constituting one set of the thrust dynamic pressure bearing portions SBa and SBb arranged so as to be adjacent to each other in the axial direction and both of the dynamic pressure surfaces on the bearing sleeve  13  and counter plate  16  side opposed to the former surfaces in close proximity are respectively oppositely arranged through small gaps of several μm in the axial direction, and the lubricating fluid F such as an oil or a magnetic fluid is continuously injected or interposed in the bearing space formed by this small gap in the axial direction through the outer peripheral side path of the thrust plate  23 . During rotation, a pressure is applied to the lubricating fluid F by the pumping effect of the thrust dynamic pressure generating grooves provided to the thrust plate  23 , and a dynamic pressure is thereby generated. The rotary shaft  21  and the rotary hub  22  are axially supported in the non-contact state that they are lifted in the thrust direction by the dynamic pressure of the lubricating fluid F. 
   Here, the embodiment according to the present invention will now be described in more detail. A cylindrical screw hole  21   a  is formed on the lower end side of the rotary shaft  21  along the central axis of the rotary shaft  21 . This screw hole  21   a  extends so as to have an appropriate length from the lower end side toward the upper end side of the rotary shaft  21 , and the upper end side part of the screw hole  21   a  is formed into a closed bottomed hole. Furthermore, a female screw portion  21   b  is formed on the inner peripheral surface of the screw hole  21   a  over an appropriate axial length from the lower end side, and a part from the upper end part of the female screw portion  21   b  in the screw hole  21   a  to the closed bottom portion is formed into an incomplete screw portion in the lower hole state. Moreover, a screw non-occupied space  17  is formed between the end of a plate fixing screw  24  and the bottom of the lower hole of the screw hole  21   a.    
   The plate fixing screw  24  as a headed screw used to fix the thrust plate  23  is screwed to the female screw portion  21   b  provided to the screw hole  21   a . The plate fixing screw  24  has a male screw portion  24   a  which meshes with the female screw portion  21   b  of the rotary shaft  21 . The male screw portion  24   a  extends from the screw head portion  24   b  in the axial direction (upper direction in  FIG. 2 ). Additionally, the male screw portion  24   a  of the plate fixing screw  24  is inserted into a through hole  21   a  of the rotary shaft  21  from the lower end side and screwed to the female screw portion  21   b . As a result, the screw head portion  24   b  is pressure-welded to the lower end surface side of the thrust plate  23  in  FIG. 2 . Consequently, the thrust plate  23  is sandwiched and fixed between the screw head portion  24   b  of the plate fixing screw  24  and the lower end surface of the rotary shaft  21  in the axial direction. 
   It is to be noted that the female screw portion  21   b  is formed so as to constitute a right screw when a recording medium such as a magnetic disk held by the rotary hub  22  rotates in the counterclockwise direction as seen from the upper side in  FIG. 1 . On the contrary, when the recording medium such as a magnetic disk rotates in the clockwise direction as seen from the upper side in  FIG. 2 , the female screw portion  21   b  is formed so as to constitute a left screw. This prevents the clamp fixing screw  24  from becoming loose due to a torque in activation of rotation of the motor. 
   In such a structure, although there is formed the screw non-occupied space  17  including the incomplete screw portion from the end portion of the female screw portion  24   a  of the plate fixing screw  24  to the closed bottom portion of the screw hole  21   a  and a part of the female screw portion  21   b , an interposing member  25  which reduces a void cubic content of the screw non-occupied space  17  is attached in the screw non-occupied space  17 . This interposing member  25  is formed of a substantially cylindrical member different from the plate fixing screw  24  and constituted so as to occupy the screw non-occupied space  17  as much as possible. 
   Further, the lubricating fluid F injected into each dynamic pressure bearing portion is filled in the void part extending from the gap of the screw portion where the male screw portion  24   a  of the plate fixing screw  24  engages with the female screw portion  21   b  of the screw hole  21   a  to the screw non-occupied space  17  around the interposing member  25 . Injection of the lubricating fluid F is carried out through an injection path  27  provided so as to form a groove at the lower end portion of the rotary shaft  21 , and the lubricating fluid F injected into each dynamic pressure bearing portion is led from the outside of the rotary shaft  21  in the radial direction to the screw portion in the screw hole  21   a  and the screw non-occupied space  17  around the interposing member  25  through the injection path  27 . 
   In more detail, a plurality of the injection paths  27  are provided at the lower end surface part of the rotary shaft  21 , which is in contact with the upper surface of the thrust plate  23 , as radial grooves which pierce in the radial direction. Each of the injection paths  27  extends from the screw hole  21   a  of the rotary shaft  21  to the outer peripheral surface of the rotary shaft  21  toward the outside in the radial direction, and it is provided so as to cause the space in the screw hole  21   a , i.e., the screw gap between the male screw portion  24   a  and the female screw portion  21   b  which engage with each other and the screw non-occupied space  17  around the interposing member  25  to communicate with the outside of the rotary shaft  21 . 
   Furthermore, a non-illustrated tool engagement concave portion whose plane shape is a star shape and which is used for screw fastening is formed at the axial central part of the outer surface of the screw head portion  24   b  of the plate fixing screw  24 . This tool engagement concave portion is formed in such a manner that its lateral cross section is recessed in the substantially triangular shape, and an adhesive having the oil resistance is filled therein after screw fastening. The adhesive is used to embed foreign particles such as a flash which may be possibly generated in the tool engagement concave portion by a scraping phenomenon when fastening the plate fixing screw  24 , captures foreign particles such as a flash and prevents them from flowing out or scattering to the outside. 
   According to the dynamic pressure bearing apparatus having such a structure, since the thrust plate  23  is firmly fixed by the fastening force of the plate fixing screw  24  relative to the rotary shaft  21 , the joint strength of the thrust plate  23  is greatly improved. 
   Moreover, the lubricating fluid F is filled in the void part extending from the gap between the male screws and the female screws of the plate fixing screw  24  and the screw hole  21   a  of the rotary shaft  21  to the screw non-occupied space  27  including the incomplete screw portion. Since the lubricating fluid F is used so that the air does not remain in the screw non-occupied space  17 , the lubricating fluid F is excellently prevented from leaking outside by expansion of the residual air. Here, when filling the lubricating fluid F in the screw non-occupied space  17 , the air can be prevented from remaining by forming a vacuum in the screw non-occupied space  17  in advance and then sucking and filling the lubricating fluid F. 
   Additionally, in this embodiment, since the interposing member  25  which reduces the void cubic content is attached to the screw non-occupied space  17 , a filling quantity of the lubricating fluid F in the screw non-occupied space  17  is reduced by an amount of the interposing member, and leakage of the lubricating fluid F which has the thermal expansion property larger than that of each metal member constituting the dynamic pressure bearing portion can be further reduced. 
   Further, in the apparatus according to this embodiment, since the interposing member  25  formed as a member different from the plate fixing screw  24  is used, a general inexpensive headed screw can be used as the plate fixing screw  24 . 
   Furthermore,  FIG. 3  shows a second embodiment. In this embodiment, besides the injection paths  27 , there is provided a communication hole  37  which causes thrust bearing portions SBa and SBb constituted above and below the thrust plate  23  to communicate with each other. The communication hole  37  is a hole which pierces both front and back surfaces of the thrust plate in the axial direction in the middle of the thrust plate in the radial direction. It is to be noted that like reference numerals denote structures equal to those in the first embodiment. 
   Although this embodiment can obtain the same effects/advantages as those in the first embodiment, it is possible to obtain the stable dynamic pressure characteristic in movement of the lubricating fluid F through the communication hole  37  even if a pressure difference is generated between both thrust bearing portions SBa and SBb in rotation of the motor by the communication hole  37 . 
   Furthermore,  FIG. 4  shows a third embodiment. In this embodiment, the injection path  27  is provided on the lower end surface of the shaft  21  like each of the foregoing embodiments. In addition to this, an injection path  47  consisting of a groove having the same shape as that of the injection path  27  is formed on the surface of the thrust plate  23  which comes into contact with the head portion  24   b  of the plate fixing screw  24 . Moreover, both injection paths  27  and  47  are constituted so as to also function as the communication hole  37  (see  FIG. 3 ) in the second embodiment. 
   That is, both injection paths  27  and  47  extend to the outer peripheral surface of the rotary shaft  21  from the screw hole  21   a  of the rotary shaft  21  toward the outside in the radial direction, and they are provided so as to cause the gap of the screw portion in the screw hole  21   a  and the screw non-occupied space  17  to communicate with the outside of the rotary shaft  21 . 
   In such an embodiment according to the present invention, it is possible to obtain the same effects/advantages as those in each of the foregoing embodiments. In addition to this, since both injection paths  27  and  47  communicate with each other on the outer side of the rotary shaft  21 , the lubricating fluid F appropriately moves through both injection paths  27  and  47  even if a pressure difference is generated between the upper and lower thrust bearing portions SBa and SBb, thereby obtaining the stable dynamic pressure characteristic. 
   Moreover,  FIGS. 5 to 7  show a fourth embodiment. In this embodiment, in place of the injection path in the first to third embodiments, an injection communication groove  57  is formed so as to notch the inner peripheral surface of the thrust plate  23  in the radial direction. The injection communication grooves  57  are provided at four positions in the circumferential direction, and each injection communication groove  57  is formed in such a manner that its plane has a substantial U shape. Additionally, each injection communication groove  57  likewise extends to the outer peripheral surface of the rotary shaft  21  from the screw hole  21   a  of the rotary shaft  21  toward the outside in the radial direction, and it is provided so as to cause the gap of the screw portion in the screw hole  21   a  and the screw non-occupied space  17  to communicate with the outside of the rotary shaft  21 , thereby facilitating injection of the lubricating fluid F. It is to be noted that reference numeral  23   a  in the drawing denotes a thrust dynamic pressure generating grooves. 
   Further, since the thrust bearing portions SBa and SBb communicate with each other in the axial direction by each injection communication groove  57 , the lubricating fluid F appropriately moves through each injection communication groove  57  even if a pressure difference is generated between the upper and lower thrust bearing portions SBa and SBb during rotation of the motor, thereby obtaining the stable dynamic pressure characteristic. 
     FIGS. 8 to 10  show a fifth embodiment. In this embodiment, in place of the injection path or the communication groove in each of the first to fourth embodiments, an injection communication hole  67  is pierced and formed at the inner peripheral side part of the thrust plate  23  in the axial direction. The injection communication holes  67  are provided at four positions in the circumferential direction, and each of the injection communication holes  67  is formed so as to have a large-diameter shape which extends from the screw hole  21   a  of the rotary shaft  21  toward the outer peripheral surface of the rotary shaft  21  toward the outside in the radial direction. As a result, the screw portion gap in the screw hole  21   a  and the screw non-occupied space  17  are caused to communicate with the outside of the rotary shaft  21 . 
   Furthermore, since the thrust bearing portions SBa and SBb communicate with each other in the axial direction by the injection communication holes  67 , the lubricating fluid F appropriately moves through each injection communication hole  67  even if a pressure difference is generated between the upper and lower thrust bearing portions SBa and SBb during rotation of the motor, thereby obtaining the stable dynamic pressure characteristic. 
   At this moment, an interposing member  68  which reduces an internal cubic content of the injection communication hole  67  is inserted into each injection communication hole  67  having a large diameter. This interposing member  68  is formed so as to have a diameter which occupies a larger space in the injection communication hole  67 . Arranging such an interposing member  68  can reduce a quantity of the lubricating fluid which enters the injection communication hole  67 , thereby excellently avoiding flowing out of the lubricating fluid when expanded. Moreover, changing a size of the interposing member  68  can adjust a passage quantity of the lubricating fluid F. 
   Additionally,  FIG. 11  shows a sixth embodiment. This embodiment adopts an interposing member  25 ′ integrally provided with the plate fixing screw  24  in place of the interposing member  25  in the first embodiment. The interposing member  25 ′ is formed so as to integrally protrude from the end of the plate fixing screw  24 . 
   In this embodiment, the same effects/advantages as those in each of the foregoing embodiments can be obtained. However, in this embodiment in particular, attachment of the interposing member  25 ′ is carried out simultaneously with fastening of the plate fixing screw  24 , which facilitates handling of the components and simplifying the assembling steps. 
   Further,  FIG. 12  shows a seventh embodiment. In this embodiment, a resin material  75  is used in place of the interposing member  25  in the first embodiment. This resin material  75  is injected into the screw non-occupied space  17  formed between the illustrated upper end portion of the plate fixing screw  24  and the closed bottom portion of the screw hole  21   a  on the rotary shaft  21  side, and it is hardened in the screw non-occupied space  17 . 
   In such an embodiment according to the present invention, the same effects/advantages as those in each of the foregoing embodiments can be obtained. However, in this embodiment particularly, since the screw non-occupied space  17  is substantially completely filled with the resin material  75 , a large quantity of the lubricating fluid F in the thrust dynamic pressure bearing portions SBa and SBb does not enter the screw hole  21   a , it is possible to eliminate the possibility that the lubricating fluid F is substantially insufficient in the thrust dynamic pressure bearing portions SBa and SBb and the duration of life of the bearing is shortened. Furthermore, since a general headed screw can be used as a headed screw, the component cost can be reduced. 
   Moreover, it is possible to form the large surface roughness at parts corresponding to the injection path, the communication hole, the injection communication groove or the injection communication hole in each of the embodiments in place of these paths. In such a case, the gaps generated by irregularities of the large surface roughness can be formed into substantial paths or the like. 
     FIG. 13  shows an eighth embodiment. In this embodiment, a screw hole for the plate fixing screw  24  is provided by piercing the shaft  121  in the axial direction. It is to be noted that the entire structure of the HDD to which the present invention is applied is equivalent to that illustrated in  FIG. 1 . Therefore, in this example, explanation of the structure of the HDD is eliminated, and like reference numerals denote members having the same functions, thereby omitting the detailed description. 
   To the rotary shaft  121  in this embodiment is formed a through hole  121   a  so as to be parallel with the central axis of the rotary shaft  121 , and the upper and lower ends of the rotary shaft  121  in the axial direction are caused to communicate in the axial direction by the through hole  121   a . A female screw portion  121   b  is formed at the inner wall portion forming the cylindrical shape of the through hole  121   a , and a non-illustrated clamp fixing screw (see reference numeral  6 ′ in  FIG. 22 ) is screwed at the part on the upper end side of the female screw portion  121   b . The screw head portion of the clamp fixing screw is pressure-welded to a disk fixing damper (see reference numeral  9  in  FIG. 22 ), and the damper is fixed by the fastening force of the clamp fixing screw. 
   Additionally, a plate fixing screw  24  as a headed screw used for fixing the thrust plate  23  is screwed on the lower end side of the female screw portion  121   b  formed on the inner wall of the through hole  121   a  of the rotary shaft  121 . This plate fixing screw  24  has a male screw portion  24   a  which meshes with the female screw portion  121   b  of the rotary shaft  121 , and the male screw portion  24   a  is provided so as to extend from the screw head portion  24   a  in the axial direction. Further, the male screw portion  24   a  of the plate fixing screw  24  is inserted into the through hole  121   a  of the rotary shaft  121  from the lower side and screwed to the female screw portion  121   b . As a result, the screw head portion  24   b  is pressure-welded to the lower end of the thrust plate  23 . 
   At this moment, the thrust plate  23  is sandwiched in the axial direction between the screw head portion  24   b  of the plate fixing screw  24  and the lower end surface of the rotary shaft  121 , thereby fixing the thrust plate  23 . 
   Further, a washer  125  is interposed between the screw head portion  24   b  of the plate fixing screw  24  and the thrust plate  23 . This washer  125  is used to eliminate foreign particles such as a flash which may be possibly generated by a scraping phenomenon when fastening the plate fixing screw  24 . Thus, as the washer  25 , a washer made of a resin such as PTFE or PEEK or a metal washer having the high hardness and the excellent smoothness is used. 
   Furthermore, at a screwing portion between the male screw portion  24   a  of the plate fixing screw  24  and the female screw portion  121   b  on the rotary shaft  121  side, i.e., meshed screw portion, an adhesive  126  having the oil resistance such as an epoxy resin is filled at a part extending from the substantially entire length of the screwing portion to the end portion of the plate fixing screw  24 . This adhesive  126  is injected so as to enter the gap generated in the screwing portion of the plate fixing screw  24  from the upper side after fixation using the plate fixing screw  24 , and it is filled so as to seal the through hole  121   a  in the axial direction. Therefore, the adhesive  126  is used to completely prevent the lubricating fluid F from leaking to the outside, and looseness of the plate fixing screw  24  is avoided. 
   At this moment, the adhesive  126  is injected to the inner side of the through hole  121   a  through the open portion on the upper end side of the through hole  121   a  provided to the rotary shaft  121 . Therefore, the air pressurized on the inner side of the through hole  121   a  when the adhesive  126  is injected is discharged toward the outer side in the radial direction of the rotary shaft  121  through ventilation paths  127  provided so as to form groove shapes at the lower end part of the rotary shaft  121 . 
   In more detail, the ventilation paths  127  are recessed at four positions in the circumferential direction at a part of the thrust plate  23  which comes into contact with the lower end portion of the rotary shaft  121 . Each ventilation path  127  extends toward the outside in the radial direction from the edge on the inner peripheral side of the central hole portion  23   c  of the thrust plate  23 , and it is formed so as to reach the outer position from the outer peripheral surface of the rotary shaft  121 . Further, the internal space of the through hole  121   a  formed at the part extending from the contact portion of the thrust plate  23  and the lower end portion of the rotary shaft  121  to the part that the adhesive  126  is filled in is caused to communicate with the outside of the rotary shaft  121  by each ventilation path  127 . 
   Furthermore, as shown in  FIGS. 14 and 15  in particular, at the screw head portion  24   b  of the plate fixing screw  24 , a tool engagement concave portion  24   c  which is used for screw fastening and whose plane has a star shape is formed at the axial central part on the outer surface of the screw head portion  24   b . This tool engagement concave portion  24   c  is provided in such a manner that its vertical cross section is recessed in the substantially triangular shape, and an adhesive  24   d  having the oil resistance is filled therein after screw fastening. The adhesive  24   d  is used to embed foreign matters such as a flash which may be possibly generated in the tool engagement concave portion  24   c  by the scraping phenomenon or the like when fastening the plate fixing screw  24 , and this adhesive  24   d  prevents foreign matters such as a flash from flowing out/scattering to the outside. 
   In this embodiment having such a configuration, when the plate fixing screw  24  is fastened to the rotary shaft  121 , the thrust plate  23  is firmly fixed. Therefore, the joint strength of the thrust plate  23  can be greatly improved as compared with that obtained by conventional press fitting and the like. 
   Moreover, in this embodiment, since the through hole  121   a  used to form the female screw portion  121   b  to the rotary shaft  121  is formed, the lower hole with a maximum length is provided in the axial direction. Additionally, the female screw portion  121   b  is formed to the lower hole consisting of the through hole  121   a  with the maximum length by a thread cutting tool having an extra machining length, thereby more easily and efficiently performing machining. 
   Further, foreign particles such as flashes generated in machining or the like of the female screw portion  121   b  can be readily discharged to the outside through the open portion of the through hole  121   a , thereby obtaining the excellent cleanliness. 
   Furthermore, in this embodiment, since the adhesive  126  which seals the gap in the screw portion and prevents the lubricating fluid from leaking to the outside is filled in the screwing portion between the male screw portion  24   a  of the plate fixing screw  24  and the female screw portion  121   b  of the rotary shaft  121 , the lubricating fluid which tends to flow out via the through hole  121   a  can be sealed, and looseness of the plate fixing screw  24  can be assuredly avoided by the adhesive  126 . 
   Furthermore, when injecting the adhesive  126  into the through hole  121   a  of the rotary shaft  121 , the injection operation can be very easily carried out through the open portion of the through hole  121   a , and the air in the through hole  121   a  which is pressurized at the time of injection of the adhesive  126  is discharged to the outside through the ventilation path  127 , thus excellently effecting the filling operation of the adhesive  126 . 
   Moreover, in this embodiment, since the tool engagement concave portion  24   c  for fastening provided at the screw head portion  24   b  of the plate fixing screw  24  is filled with the adhesive  24   d  having the oil resistance, chip pieces such as flashes are held on the inner side of the adhesive  24   d  even if such chip pieces are generated in the tool engagement concave portion  24   c  of the screw head portion  24   b  in the screw fastening operation relative to the plate fixing screw  24 , and scattering to the outside is assuredly avoided, thereby obtaining the excellent cleanliness. 
     FIGS. 17 and 18  show a ninth embodiment. In this embodiment, ventilation paths  127 ′ are provided to the thrust plate  23 . That is, the ventilation paths  127 ′ in this embodiment are formed at parts where the thrust plate  23  is in contact with the screw head portion  24   b  of the plate fixing screw  24 . 
     FIG. 19  shows a tenth embodiment. In this embodiment, ventilation paths  127 ″ are provided on the lower end surface side of the rotary shaft  121  rather than the thrust plate  23  side. In such an embodiment, the same effects/advantages as those in the eighth embodiment can be demonstrated. 
   Further,  FIG. 20  shows an embodiment where the present invention is applied to a fixed shaft type dynamic pressure bearing apparatus. In this embodiment, a bearing hole of the bearing sleeve  41  as a bearing constituting the rotor set  40  is rotatably inserted into the outer peripheral of the fixed shaft  31  fixed so as to be erected relative to the base frame  30   a  constituting the stator set  30 . Furthermore, a dynamic pressure surface formed on the inner peripheral surface of the bearing hole in the bearing sleeve  41  is arranged so as to face the dynamic pressure surface formed on the outer peripheral surface of the fixed shaft  31  in the radial direction, and two radial dynamic pressure bearing portions RB and RB are formed in a small gap between the dynamic pressure surfaces so as to be separated from each other in the axial direction. 
   The dynamic pressure surface on the bearing sleeve  41  side in each radial dynamic pressure bearing portion RB and the dynamic pressure surface on the fixed shaft  31  side are arranged so as to be opposed to each other in the circumferential form with a small gap of approximately several μm therebetween, and a predetermined lubricating fluid consisting of a lubricating oil, a magnetic fluid or the like is continuously injected into a small gap which includes the radial dynamic pressure bearing portion RB formed between these surfaces and extends in the axial direction. Furthermore, the lubricating fluid F is pressurized by the pumping effect of the non-illustrated radial dynamic pressure generating grooves during rotation, and a dynamic pressure is generated. A rotary hub  42  integrally formed with the bearing sleeve  41  is axially supported while being lifted in the radial direction by the dynamic pressure of the lubricating fluid F. 
   Moreover, a discoid thrust plate  33  is fixed at the illustrated upper end portion of the fixed shaft  31  by a later-described plate fixing screw  34 . This thrust plate  33  is arranged so as to be accommodated in the annular concave portion  41   a  provided at the central part on the upper end side of the bearing sleeve  41 , and the dynamic pressure surface provided on the illustrated upper side surface of the thrust plate  33  is arranged so as to be opposed to the dynamic pressure surface provided to the bearing sleeve  41  in close proximity in axial direction in the concave portion  41   a  of the bearing sleeve  41 . Additionally, although not shown, for example, herringbone-shaped thrust dynamic pressure generating grooves are formed on the dynamic pressure surface on the illustrated upper side of the thrust plate  33 , and the lower thrust dynamic pressure bearing portion SBa is formed at the gap portion between the opposed dynamic pressure surfaces of the thrust plate  33  and the bearing sleeve  41 . 
   Further, a counter plate  43  consisting of a discoid member having a relatively large diameter is arranged in close proximity to the upper dynamic pressure surface of the thrust plate  33 . This counter plate  43  is arranged so as to close the open portion on the upper end side of the bearing sleeve  41 , and the outer peripheral side portion of the counter plate  43  is fixed to the bearing sleeve  41  side. 
   Furthermore, although not shown, for example, herringbone-shaped thrust dynamic pressure generating grooves are formed on the upper dynamic pressure surface of the thrust plate  33 , and the upper thrust dynamic pressure bearing portion SBb is thereby formed. 
   Both dynamic pressure surfaces on the thrust plate  33  side constituting one set of the thrust dynamic pressure bearing portions SBa and SBb arranged so as to be adjacent to each other in the axial direction and both dynamic pressure surfaces of the bearing sleeve  41  and counter plate  43  side opposed thereto in close proximity are oppositely arranged in the axial direction through each small gap of several μm, and a lubricating fluid such as a lubricating oil, a magnetic fluid or air is continuously injected or interposed in the bearing space consisting of the small gaps through the outer peripheral side path of the thrust plate  33 . In rotation, the lubricating fluid is pressurized by the pumping effect of the thrust dynamic pressure generating grooves provided to the thrust plate  33 , and the dynamic pressure is generated. The bearing sleeve  41  and the rotary hub  42  are axially supported in a non-contact manner while being lifted in the thrust direction by the dynamic pressure of the lubricating fluid. 
   At this moment, a through hole  31   a  is formed so as to be parallel to the axial direction of the rotary shaft  31 , and the illustrated upper and lower both ends of the rotary shaft  31  in the axial direction are caused to communicate with each other by the through hole  31   a . A female screw portion  31   b  is formed at the inner wall portion forming the cylindrical shape of the through hole  31   a , and a plate fixing screw  34  as a headed screw used to fix the thrust plate  33  is screwed to the upper end side of the female screw portion  31   b . This plate fixing screw  34  has a male screw portion  34   a  which meshes with the female screw portion  31   b  on the fixed shaft  31  side. Further, the male screw portion  34   a  has a male screw portion  34   a  which engages with the screw head portion  31   b . Moreover, the male screw portion  34   a  protrudes from the screw head portion  34   b  in the axial direction. Additionally, the male screw portion  34   a  of the plate fixing screw  34  is inserted into the through hole  31   a  of the fixed shaft  31  and screwed to the female screw portion  31   b . As a result, the screw head portion  34   b  has the thrust plate  33  sandwiched in the axial direction. In this state, the plate fixing screw  34  is fastened, thereby fixing the thrust plate  33 . 
   Additionally, an adhesive  36  having the oil resistance such as an epoxy resin is filled in a part extending from the substantially entire length of the screwing portion of the male screw portion  34   a  of the plate fixing screw  34  and the female screw portion  31   b  on the fixed shaft  31  side to the end of the male screw portion  34   a  of the plate fixing screw  34 . This adhesive  36  is filled in such a manner that it enters the gap in the screwing portion of the plate fixing screw  34  and seals the through hole  31   a  in the axial direction, and the adhesive  36  completely prevents the lubricating fluid from leaking to the outside and avoids looseness of the plate fixing screw  34 . 
   In the fixed shaft type dynamic pressure bearing apparatus having such a structure, it is possible to obtain substantially the same effects/advantages as those in each of the foregoing embodiments. 
   It is to be noted that the foregoing embodiments are just preferred embodiments according to the present invention, but they are not restricted thereto, and various modifications can be carried out without departing from the scope of the present invention. For example, the present invention is not restricted to the axial rotation type dynamic pressure bearing apparatus like the foregoing embodiments, and it can be likewise applied to the fixed shaft type dynamic pressure bearing apparatus. Additionally, the present invention can be likewise applied to the dynamic pressure bearing apparatus used in various apparatuses such as a polygon mirror drive motor or a CD-ROM drive motor other than the HDD motors such as described in the foregoing embodiments.