Patent Publication Number: US-2006002642-A1

Title: Fluid dynamic pressure bearing apparatus

Description:
DETAILED DESCRIPTION OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a fluid dynamic pressure bearing apparatus and more particularly, to a fluid dynamic pressure bearing apparatus having a rotating shaft member and a bearing sleeve member wherein the rotating shaft member and the bearing sleeve member are supported in a relatively rotatable state by a dynamic pressure generated from a lubrication fluid disposed between a dynamic pressure surface of the rotating shaft member and a dynamic pressure surface of the bearing sleeve member.  
      2. Related Art  
      In recent years, various fluid bearing apparatuses have been proposed for rotatably supporting a rotator body such as magnetic disks, optical disks, or polygon mirrors in a high-speed rotation.  
      For example, a dynamic pressure bearing apparatus is used in a spindle motor such as a hard disk drive (HDD) shown in  FIG. 7 . A bearing sleeve  13  acts as a bearing member and is mounted for supporting a rotating shaft member  21 . A dynamic pressure surface provided on an outer peripheral surface of the rotating shaft member  21  and a dynamic pressure surface provided on the internal circumference of the bearing sleeve  13  radially face each other in proximity to form a narrow gap. A radial dynamic pressure bearing RB is formed by filling a lubricating fluid such as oil or air within the narrow gap. In addition, dynamic pressure surfaces provided on both sides of a thrust plate  23  fixed to the rotating shaft member  21  and a dynamic pressure surface of the bearing sleeve  13  or a dynamic pressure surface of a counter plate  16  mounted in an opening portion of the bearing sleeve  13  face each other in proximity to form a narrow gap in an axial direction. The lubricating fluid is filled to form thrust dynamic pressure bearings SBa and SBb.  
      At least one surface of the dynamic pressure surfaces of the radial dynamic pressure bearing RB and the thrust dynamic pressure bearings SBa or SBb is provided with a fluid pressuring means such as dynamic pressure generating grooves. A lubrication fluid is pressurized by means of a pumping operation created by the dynamic pressure generating grooves (not shown) to generate a dynamic pressure. Due to the generated dynamic pressure, a rotating member is rotatably supported with respect to a stationary member in an elevated state in a radial and a thrust direction, respectively.  
      In such a fluid bearing device described above, when the rotating member rotates, the rotating member moves in a non-contact position with respect to the stationary fixed member by the lubrication fluid. When the rotating member stops rotating, one side of the two thrust bearing portions comes in contact with the rotating member and the fixed member. For example, in a spindle motor for HDDs, after a hard disk supported by the rotating member rotates at a particular speed, a recording/reproducing head moves over a disk via a guide member and recording/reproducing is performed in a non-contact state. When the motor stops, the rotating member moves downward from an elevated position to a contacting position due to gravitational forces.  
      While this operation is performed, the hard disk is separated from the guide member having the recording/reproducing magnetic head so that the hard disk does not come into contact with the guide member. However, when the motor is placed in an upside down position or an extremely large shock is added to the motor from the outside, the rotating member becomes displaced to a larger extent than the elevated height and the hard disk may come into contact with the guide member, which damages the disk and/or the guide member.  
      In order to solve this problem, a motor having a magnetic plate is attached to the fixed member so as to face a motor drive magnet provided on the rotor. The magnetic plate and the motor drive magnet are magnetically attracted to each other by the magnetic attraction force of the drive magnet. As a result, the rotating member is attracted towards the fixed member.  
      However, the gap dimension between the magnetic plate and the motor drive magnet varies according to the assembling process of the motor or errors in the dimensions of various component parts. Thus, the magnetic attraction force also varies. The variation of the magnetic attraction force results in a fluctuation of the spaces of the thrust dynamic pressure bearings SBa and SBb. Hence, the required thrust dynamic pressure may not be obtained. In addition, when the gap space of the thrust dynamic pressure bearing is made smaller due to a stronger magnetic attraction force during rotation, the viscosity of the lubrication fluid increases and a loss of torque in the motor also increases.  
      Generally, the amount a bearing wears is proportional to the product of the rotational speed and the contacting time of the rotating member. Therefore, the amount the bearing wears can be reduced by making the rotating member elevate even at a low speed of rotation such as when the motor first starts. However, when the rotating member and the fixed member are magnetically attracted to each other, the needed time for elevating the rotating member will increase because it is necessary to generate a force of dynamic pressure greater than the magnetic attraction force. That is, the time both members are in contact with each other increases and this causes the bearing to wear down.  
      The present invention provides a fluid dynamic pressure bearing apparatus with a high reliability, wherein the restrictions as to the position of the motor in use is eliminated. The present invention also provides a fluid dynamic pressure bearing apparatus wherein a required dynamic pressure can be generated readily in a thrust dynamic pressure bearing and the wearing of the thrust dynamic pressure bearing is reduced.  
     SUMMARY OF THE INVENTION  
      A fluid dynamic pressure bearing apparatus includes a radial dynamic pressure bearing formed in a gap portion between a bearing member and a shaft member. The apparatus also includes a thrust dynamic pressure bearing having a first thrust bearing portion formed between a top surface of the thrust plate and a first facing member opposing thereto in the axial direction and a second thrust bearing portion formed between a bottom surface of the thrust plate and a second facing member opposing thereto in an axial direction. Dynamic pressure generating grooves are formed on the radial dynamic pressure bearing and the thrust dynamic pressure bearing. The shaft member and the bearing member are rotated together as a rotation member, such that the rotation member is supported in a position that a gap space L 1  of the first thrust bearing portion is larger than a gap space L 2  of the second thrust bearing portion and the depth of the dynamic pressure generating grooves where the gap space is smaller is formed shallower than that where the gap space is larger.  
      In this configuration, the shallower depth of the thrust dynamic pressure generating grooves generates larger dynamic pressure at a low speed of rotation. As a result, the rotating member is able to elevate from the fixed member in the thrust bearing portion even at a low speed of rotation such that the wearing of the contact sliding of both members is decreased. Accordingly, a fluid dynamic pressure bearing which is superior to its durability is obtained and a larger thrust dynamic pressure is obtained even in the thrust bearing portion in which the opposing gap space is small.  
      In accordance with one embodiment of the present invention, the depth of the dynamic pressure generating grooves in the thrust bearing portion in which the gap space is smaller is determined in such a manner that the coefficient of elasticity of the thrust bearing portion is set to be generally at a maximum value. Therefore, a large bearing rigidity is obtained.  
      For this purpose, it is preferable to establish the depth of the dynamic pressure generating grooves in the dimension of 0.8 times to 2.8 times of its gap space.  
      In accordance with one embodiment of the present invention, each of the depths of the dynamic pressure generating grooves in the first and the second thrust bearing portions is determined in such a manner that each of the coefficients of elasticity of the thrust bearing portions is respectively set to be a generally maximum value. Therefore, a large bearing rigidity is obtained and further, a desired coefficient of elasticity can be obtained even if the real peak value of the coefficient of elasticity is displaced by residual stress or distortion produced in the thrust plate.  
      In accordance with one embodiment of the present invention, the second facing member is formed from a material of greater hardness than that of the first facing member and a biasing means is provided for urging the thrust plate to elevate from the second thrust bearing portion.  
      In this configuration, the thrust plate elevates from the side of the material of greater hardness and thus the wear of the thrust dynamic pressure bearing is restrained, regardless of the position of the motor in use, and the reliability of the bearing is improved.  
      Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the present invention. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a sectional view of a spindle motor provided with a fluid dynamic pressure bearing apparatus according to an embodiment of the present invention.  
       FIG. 2  is a plane explanatory view of one example of dynamic pressure generating grooves formed in a thrust plate.  
       FIG. 3  is a partially cross-sectional view of the spindle motor provided with a fluid dynamic pressure bearing apparatus in a suspended state according to an embodiment of the present invention.  
       FIG. 4  is a partially cross-sectional view of the spindle motor provided with a fluid dynamic pressure bearing apparatus in a rotation state according to an embodiment of the present invention.  
       FIG. 5  is a schematic illustration which shows a simulation result of the coefficient of elasticity in the thrust dynamic pressure bearing with respect to the depth of the thrust dynamic pressure generating groove SG as a parameter of the floating amount of the thrust plate.  
       FIG. 6  is a cross-sectional view of a fluid dynamic pressure bearing apparatus according to another embodiment of the present invention.  
       FIG. 7  is a sectional view of a conventional fluid bearing apparatus. 
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION  
      Various embodiments of the present invention will be explained below. First, an overall structure of a hard disk drive (HDD) device to which the present invention may be applied will be explained in reference to the accompanying drawings.  
      A shaft rotation-type spindle motor for a HDD shown in  FIG. 1  generally includes a stator assembly  10 , which is a fixed member, and a rotor assembly  20 , which is a rotating member assembled on top of the stator assembly  10 . The stator assembly  10  has a fixed frame  11  screwed onto a fixed base or chassis of the drive apparatus (not shown). The fixed frame  11  is formed of an aluminum metal material to reduce its weight. A cylindrical sleeve holding portion  12  is formed upright in the generally center area of the fixed frame  11 . A bearing sleeve  13  in a hollow cylinder shape, which is a fixed bearing member, is attached to an inner circumference of the bearing holder  12  and joined to the bearing holder  12  through press fit or shrink fit. The bearing sleeve  13  is formed from a copper alloy material, such as phosphorous bronze, in order to facilitate the machining of holes with small diameter.  
      On the outer circumference mounting surface of the bearing holder  12  is mounted a stator core  14  including a stacked layered body of electromagnetic steel plates. A drive coil  15  is wound on each of the salient pole sections provided on the stator core  14 .  
      A bearing hole (not shown) is provided in the center of the bearing sleeve  13 , and a part of the rotor assembly  20 , which is a rotating shaft  21 , is inserted inside the bearing hole. The rotating shaft  21  in the present embodiment is formed from stainless steel. This means that the bearing sleeve  13  which is a bearing member, is formed from a material with more resilience than the rotating shaft  21 , which is a shaft member. On the inner circumference surface of the bearing hole of the bearing sleeve  13  is formed a dynamic pressure surface, which is positioned to face in the radial direction a dynamic pressure surface formed on the outer circumference surface of the rotating shaft  21 , such that a radial dynamic pressure bearing section RB is created in minute bearing gap between the dynamic pressure surfaces. More specifically, the dynamic pressure surface on the bearing sleeve  13  side and the dynamic pressure surface on the rotating shaft  21  side in the radial dynamic pressure bearing section RB face each other across a minute gap of a few μm; this minute gap forms a bearing space into which lubricating fluid is continuously charged in the axial direction. The lubricating fluid is, for example, lubricating oil such as from an ester series or a poly-α-olefin series, magnetic fluid, or air.  
      Radial dynamic pressure generating grooves (not shown) in a herringbone shape, for example, are provided on at least one of the dynamic pressure surfaces on the bearing sleeve  13  and the rotating shaft  21  in two blocks of concave ring shapes separated in the axial direction. When the rotation takes place, the pumping action of the grooves pressurizes the lubricating fluid to generate a dynamic pressure, so that the rotating shaft  21  and the rotating hub  22  are shaft-supported in the radial direction.  
      A capillary seal portion RS is provided at the top of the bearing space in the figure that forms the radial dynamic pressure bearing section RB. The capillary sealing section RS is structured by an angle surface formed on the rotating shaft  21  or on the bearing sleeve  13  that gradually widens the bearing gaps towards the outside of the bearing, and has a gap dimension of about 20 μm to about 300 μm, for example. The surface level of the lubricating fluid is positioned within the capillary sealing section RS in both the motor rotation and stop states.  
      A rotating hub  22  that along with the rotating shaft  21  forms the rotor assembly  20  is formed from a generally cup-shaped member made of an aluminum metal, so that a recording medium such as a magnetic disk (not shown) can be mounted on the rotating hub  22 . In the center area of the rotating hub  22  is provided a joint hole  22   d , which is joined in a unitary fashion through press fit or shrink fit with the top area of the rotating shaft  21 .  
      The rotating hub  22  has a generally cylinder-shaped body section  22   a  on whose outer circumference a disk is mounted, and on the bottom inner circumference wall of the body section  22   a  is mounted via a back yoke  22   b  and a ring-shaped drive magnet  22   c . The magnet  22   c  is positioned to face the outer circumference end surface of the stator core  14 .  
      At the bottom end of the rotating shaft  21  is fixed a disk-shaped thrust plate  23 . The thrust plate  23  is contained in a cylinder-shaped concave recess formed at the bottom center of the bearing sleeve  13 . In the recess of the bearing sleeve  13 , the dynamic pressure surface provided on the top surface of the thrust plate  23  faces in close proximity the dynamic pressure surface provided on the bearing sleeve  13 . A dynamic pressure generating groove is formed on at least one of the two facing dynamic pressure surfaces, and a top thrust dynamic pressure bearing section Sba is formed in the gap between the dynamic pressure surfaces of the thrust plate  23  and the bearing sleeve  13  facing each other.  
      In close proximity to the bottom dynamic pressure surface of the thrust plate  23  is a counter plate  16 , which is formed from a disk-shaped member with a relatively large diameter. The counter plate  16  closes off the bottom opening area of the bearing sleeve  13 . A dynamic pressure generating groove is also formed between the dynamic pressure surface provided at the top of the counter plate  16  and the dynamic pressure surface on the bottom of the thrust plate  23 , which forms a bottom thrust dynamic pressure bearing section SBb.  
      The two dynamic pressure surfaces of the thrust plate  23  and the dynamic pressure surface of the bearing sleeve  13  and of the counter plate  16  that faces them together constitute a set of thrust dynamic pressure bearing sections SBa and SBb next to each other in the axial direction and are in each case arranged so that the opposing dynamic pressure surfaces face each other across a minute gap of a few μm; and the lubricating fluid is charged continuously into the minute gaps in the axial direction via a path provided on the outer circumference of the thrust plate  23 .  
      Furthermore, normal herringbone-shaped thrust dynamic pressure generating grooves in a ring shape are provided on at least one of the dynamic pressure surfaces of the thrust plate  23  and that of the bearing sleeve  13 , and on at least one of the dynamic pressure surfaces of the thrust plate  23  and that of the counter plate  16 . As a result, when rotation takes place, the pumping action of the thrust dynamic pressure generating grooves pressurizes the lubricating fluid to generate dynamic pressure and the rotating shaft  21  and the rotating hub  22  are supported in the thrust direction. A further option for the thrust dynamic pressure generating grooves is that they may be formed on the dynamic pressure surface of the bearing sleeve  13  at the top thrust dynamic pressure bearing section SBa and may be formed on the dynamic pressure surface of the counter plate  16  at the bottom thrust pressure bearing section SBb.  
      In the spindle motor shown in  FIG. 1 , a ring shaped magnetic attraction plate  17  made of magnetic material is mounted on fixed frame  11 . The magnetic attraction plate  17  is fixed on the fixed frame  11  to face onto the bottom surface of the drive magnet  22   c  and therefore includes a biasing means. The magnetic attraction plate  17  attracts the rotor assembly  20  towards the frame  11  by a magnetic attraction force between the drive magnet  22   c  and the magnetic attraction plate  17 . The magnitude of the force is larger than the gravitational force applied on the entire rotor assembly  20  when the motor is placed upside down.  
      Therefore, the magnetic attraction plate  17  can always pull the rotor assembly  20  towards the fixed frame  11 , even if the motor is placed upside down and the gravitational force is applied to the rotor assembly  20 . Thus, when the rotor assembly  20  does not rotate, the counter plate  16  contacts with the bottom surface of the thrust plate  23  of the bottom thrust dynamic pressure bearing section SBb.  
      When a current is applied to the drive coil  15  and the rotor assembly  20  rotates by the electromagnetic action between the stator core  14  and the drive magnet  22   c , the thrust plate  23  elevates from the counter plate  16 . When rotation is suspended, the thrust plate  23  returns to be in contact with the counter plate  16  again.  
      In addition, regarding the top thrust dynamic pressure bearing section SBa which includes the upper surface of the thrust plate  23  and the bottom surface of the bearing sleeve  13 , the bottom surface of the bearing sleeve  13  does not form a flat surface because the surface is provided with cut streaks. Furthermore, the bearing sleeve  13  is formed from a comparatively soft metal such as copper or copper alloy. This is for easy workability of the top thrust dynamic pressure bearing section SBa as well as radial dynamic pressure generation grooves formed on its internal circumference surface.  
      On the other hand, regarding the bottom dynamic pressure bearing section SBb which includes the under surface of the thrust plate  23  and the upper surface of the counter plate  16 , the upper surface of the counter plate  16  has a flat surface which is achieved by lapping work or polishing work. Further, the counter plate  16  is formed of a hard material which is a heat-treated rustless steel.  
      As described above, when the rotor assembly  20  stops rotating, the bottom face of the thrust plate  23  remains in contact with the counter plate  16  of the bottom thrust dynamic pressure bearing surface SBb. In a low-speed rotating state such as just after starting or before stopping, both members remain slightly in contact with each other. However, since the upper surface of the counter plate  16  is flat and hard, the wearing of the dynamic pressure surfaces of the counter plate  16  and the thrust plate  23  can be greatly reduced.  
      The structure of the thrust dynamic pressure bearing according to an embodiment of the present invention is described below in detail with reference to  FIG. 3  and  FIG. 4 .  FIG. 3  is a sectional view of a spindle motor for a HDD in a stopped state in an embodiment of the present invention.  FIG. 4  is a sectional view of the motor in a rotating state.  
      In  FIG. 3 , the thrust plate  23  is fixed to one end of the rotating shaft  21  and is provided within the recessed portion  13   a  formed within the bearing sleeve  13 . Thrust dynamic pressure generating grooves SGa having a depth d 1  are formed on the upper surface  23   a  of the thrust plate  23 . The upper surface  23   a  faces onto a dynamic pressure surface of the bearing sleeve  13  (the first facing member) and the top thrust dynamic pressure bearing section SBa is formed by the bearing gap space between the upper surface  23   a  and the bearing sleeve  13 . Also, thrust dynamic pressure generating grooves SGb having a depth d 2  are formed on the bottom surface  23   b  of the thrust plate  23 . The bottom surface  23   b  faces onto a dynamic pressure surface of the counter plate  16  (the second facing member) and the bottom thrust dynamic pressure bearing surface SBb is formed by the bearing gap space between the bottom surface  23   b  and the counter plate  16 . When the motor stops, the bottom surface  23   b  of the thrust plate  23  and the dynamic pressure surface of the counter plate  16  make contact by a force of the biasing means including that of the magnetic attraction plate  17 .  
      The depth d 1  of the thrust dynamic pressure generating grooves SGa formed on the upper surface  23   a  and the depth d 2  of the thrust dynamic pressure generating grooves SGb formed on the bottom surface  23   b  are so determined as to satisfy the relationship of d 1 &gt;d 2 , that is, the depth d 1  is deeper than the depth d 2  in the thrust plate  23 .  
      When the rotating shaft  21  and the thrust plate  23  start rotating, the dynamic pressure in the bottom thrust dynamic pressure bearing surface SBb increases to make the thrust plate  23  elevate. At a specified number of rotations, the dynamic pressure of the bottom thrust dynamic pressure bearing surface SBb and the dynamic pressure of the top thrust dynamic pressure bearing surface SBa balance each other so that the thrust plate  23  will continue to rotate in a state that the thrust plate  23  maintains a specified elevation as shown in  FIG. 4 .  
      In this case, the relationship between the gap dimension L 1  in the top thrust dynamic pressure bearing section SBa and the gap dimension L 2  in the bottom thrust dynamic pressure bearing section SBb is as follows; L 1 &gt;L 2 , that is, the rotation is performed in a state that the gap L 2  is smaller than the gap L 1 . This is because the rotor assembly  20  is attracted to the frame  11  by the biasing means described above.  
       FIG. 5  is a schematic illustration which shows the simulation results of the coefficient of elasticity in the thrust dynamic pressure bearing, that is, the magnitude of the repulsive force by the dynamic pressure with respect to the depth of the thrust dynamic pressure generating groove SG as a parameter of the elevated amount of the thrust plate  23 . In  FIG. 5 , three cases of the elevated amount (gap dimension) of the thrust plate  23  as 5.0 μm, 7.5 μm, and 10.0 μm are plotted to show how the coefficient of elasticity in the thrust dynamic pressure bearing varies depending on the depth of the thrust dynamic pressure generating groove SG.  
      As a result, when the groove&#39;s depth is progressively increased by 1 m with the elevated amount or gap dimension maintaining a constant value of 5.0 μm, the coefficient of elasticity rapidly increases within the range of about 8 μm of the depth of the dynamic pressure generating groove and the peak value of the coefficient of elasticity reaches about 110,000 N/m.  
      Especially, when the groove&#39;s depth is in the range of 1 times to 2 times of the elevated amount (5.0 μm), that is, the groove&#39;s depth is in the range of about 5 μm to 10 μm, a sufficient coefficient of elasticity around 90% of the coefficient of elasticity of the peak can be obtained. In addition, when the groove&#39;s depth is in the range of about 0.8 times to 2.8 times of the elevated amount (5.0 μm), that is, the groove&#39;s depth is in the range of about 4 μm to 14 μm, the coefficient of elasticity reaches a value larger than 70% of the peak. On the other hand, when the depth of the dynamic pressure generating groove is larger than about 8 μm, the coefficient of elasticity decreases gradually, and when the groove&#39;s depth is about 20 μm, the coefficient of elasticity reaches approximately one-third of the peak.  
      Next, when the elevated amount is a constant value of 7.5 μm and the depth of the dynamic pressure generating groove is progressively increased, the coefficient of elasticity increases gradually up to around 12 μm and the peak value of the coefficient of elasticity reaches about 33,000 N/m. That is a similar result to the case of the elevated amount of 5.0 μm. When the groove&#39;s depth is in the range of about one times to two times of the elevated amount, that is, when the groove&#39;s depth is in the range of about 7.5 μm to 15 μm, the coefficient of elasticity is reached to a value of about 90% of the peak. Also, when the groove&#39;s depth is in the range of about 0.8 times to 2.8 times of the elevated amount, that is, the groove&#39;s depth is in the range of 6 μm to 21 μm, the coefficient of elasticity reaches to a value larger than 70% of the peak. The curve of the elevated amount of 7.5 μm in  FIG. 5  varies more gradually than the curve of 5.0 μm, and after the peak of the coefficient of elasticity, the coefficient of elasticity decreases according to the increase of the groove&#39;s depth.  
      When the elevated amount is a constant value of 10.0 μm and the depth of the dynamic pressure generating groove is progressively increased, the coefficient of elasticity increases gradually up to around 16 μm and the peak value of the coefficient of elasticity reaches about 15,000 N/m. Similar to the case when the elevated amount is 5.0 μm, when the groove&#39;s depth is in the range of about one times to two times of the elevated amount, that is, when the groove&#39;s depth is in the range of about 10 μm to 20 μm, the coefficient of elasticity reaches a value about 90% of the peak. When the groove&#39;s depth is in the range of about 0.8 times to 2.8 times of the elevated amount, that is, when the groove&#39;s depth is in the range of about 8 μm to 28 μm, the coefficient of elasticity reaches a value larger than 70% of the peak. The curve of the elevated amount of 10.0 μm varies more gradually than the curve of 7.5 μm in the figure, and after the peak of the coefficient of elasticity, the coefficient of elasticity decreases slightly according to the increase of the groove&#39;s depth.  
      From the results described above in  FIG. 5 , it is understood that when the amount of the thrust plate&#39;s elevation is smaller, the coefficient of elasticity is larger with relatively smaller depths of the dynamic pressure generating grooves. On the otherhand, when the amount of the thrust plate&#39;s elevation is larger, the coefficient of elasticity is larger with relatively larger depths of the dynamic pressure generating grooves.  
      As described above and as illustrated in  FIG. 4 , the thrust dynamic pressure bearing is structured in such a manner that the gap dimension L 2  of the bottom thrust dynamic pressure bearing section SBb is narrower than the gap dimension L 1  of the top thrust dynamic pressure bearing section SBa in a rated rotation state. (Please define the term “rated rotation state”.) Further, a larger coefficient of elasticity and a bearing rigidity can be achieved by setting the depth of a dynamic pressure generating groove in the smaller elevation amount side of the thrust plate  23  (the bottom thrust dynamic pressure bearing section SBb) smaller than the depth in the larger elevation amount side. At this time, the depth of the dynamic pressure generating groove is preferably in the range of about 0.8 times to 2.8 times of the elevation amount (gap dimension) of the thrust plate  23  in a rated number of rotations. Further, when the depth of the dynamic pressure generating groove is set in the range of about one times to two times of the floating amount (gap dimension), the coefficient of elasticity can be achieved near the peak coefficient of elasticity in the bottom thrust dynamic pressure bearing section SBb.  
      On the other hand, a larger coefficient of elasticity and a bearing rigidity can be achieved by setting the depth of a dynamic pressure generating groove in the larger elevation amount side of the thrust plate  23  (the top thrust dynamic pressure bearing section SBa) larger than the depth in the smaller elevated amount side. At this time, the depth of the dynamic pressure generating groove is desirably in the range of about 0.8 times to 2.8 times of the elevated amount of the thrust plate  23  in a rated number of rotations. Further, when the depth of the dynamic pressure generating groove is set in the range of about one times to two times of the elevated amount, the coefficient of elasticity can be achieved near the peak coefficient of elasticity in the top thrust dynamic pressure bearing section SBa.  
      As described above, when the coefficient of elasticity is set to be near to the maximum value of the coefficient of elasticity, close to the desired value of the coefficient of elasticity can be obtained in each of the thrust bearing portions SBa and SBb respectively, even if the real value of the coefficient of elasticity is shifted a little from the peak value by the residual stress or the distortion applied to the thrust plate  23  when the dynamic pressure generating grooves are formed on the thrust plate  23  or when the thrust plate  23  is fitted to the rotor shaft  21 . Therefore, when the rotating member begins to rotate, a dynamic pressure with a high coefficient of elasticity occurs even at a slow speed and the thrust plate  23  begins to elevate. As a result, a sliding period with the counter plate  16  can be reduced to a short time and wear in the thrust dynamic pressure bearing can be reduced. Consequently, a fluid dynamic pressure bearing apparatus having a high reliability can be provided.  
      Next, another embodiment according to the present invention is described with reference to  FIG. 6 , which is a sectional view of a shaft fixed-type HDD spindle motor. In  FIG. 6 , the same reference symbols are used for the same members as the shaft rotation-type HDD spindle motor shown in  FIG. 1 , and its description is omitted.  
      The HDD spindle motor shown in  FIG. 6  includes a stator assembly  30  as a fixed member and a rotor assembly  40  as a rotation member which is rotatably supported to the stator assembly  30 . The stator assembly  30  is provided with a frame  31  fixed to a main chassis of the drive apparatus (not shown) by a screw, etc. A fixed shaft  35  as a shaft member is mounted in a center area of the frame  31 , and its upper end portion is provided with a tapped hole for a screw to the drive chassis. This construction is known as a motor where both ends of the shaft are fixed.  
      A cup-shaped hub  22  formed unitarily with a bearing sleeve  41  is rotatably mounted to an outer periphery of the fixed shaft  35  via the bearing sleeve  41  as a bearing member, including the rotor assembly  40 . A ring-shaped drive magnet  22   c  is mounted on an inner wall surface of the hub  22  via a back yoke  22   b.    
      In an internal peripheral surface of a center hole of the bearing sleeve  41 , a pair of radial bearing portions are formed apart from each other in an axial direction. These radial bearing portions face opposite to an outer peripheral surface of the fixed shaft  35 . A pair of radial dynamic pressure bearings RB are provided between the dynamic pressure surfaces which are formed on the internal peripheral surface of these bearing portions of the bearing sleeve  41  and the dynamic pressure surface formed on the outer peripheral surface of the fixed shaft  35 . The hub  22  is rotatably supported to the fixed shaft  35  in a radial direction by these radial dynamic pressure bearings RB.  
      A thrust plate  36  is fixed to the upper end portion of the shaft  35  and is disposed in a recessed portion formed in an upper central part of the bearing sleeve  41 . The top thrust dynamic pressure bearing section SBa is formed between a dynamic pressure surface, an upper surface of the bearing sleeve  41  as a first facing member and a dynamic pressure surface provided on a bottom surface of the thrust plate  36  in a proximate state in an axial direction.  
      In addition, the counter plate  44  (the second facing member) having a larger diameter than the thrust plate  36  is mounted in an opening portion of the bearing sleeve  41  so as to oppose a dynamic pressure surface of the upper side of the thrust plate  36  in a proximate state. The bottom thrust dynamic pressure bearing section SBb is positioned between the dynamic pressure surface provided on an under surface of the counter plate  44  and the dynamic pressure surface provided on the upper surface of the thrust plate  36 . A lubricating fluid such as lubricating oil, magnetic fluid, or air is filled into the top and the bottom thrust dynamic pressure bearing surfaces SBa and SBb and the radial dynamic pressure bearings RB.  
      In a spindle motor shown in  FIG. 6 , a biasing means, such as a ring shaped magnetic attraction plate  17  made of a magnetic material, is fixed on the frame  31  so as to oppose a bottom end surface of the drive magnet  22   c . The magnetic attraction plate  17  attracts the rotor assembly  40  to the side of the frame  31  with the magnetic attraction force that is larger than the magnitude of the gravitational force relating to the rotor assembly  40 . Therefore, even when the motor is used in an upside down manner, the magnetic attraction plate  17  still attracts the rotor assembly  40  to the frame  31  given the gravity applied to the rotor assembly  40 . Thus, when the rotor assembly  40  does not rotate, the upper surface of the thrust plate  36  and the bottom surface of the counter plate  44  of the bottom thrust dynamic pressure bearing section SBb are in contact with each other.  
      In a shaft fixed-type HDD spindle motor described above, the counter plate  44  and the bearing sleeve  41  are formed to rotate with respect to the thrust plate  36  attached to the fixed shaft  35 . To the contrary, in a shaft rotation-type HDD spindle motor described above, the thrust plate  23  is fixed to the rotating shaft  21  which rotates with respect to the bearing sleeve  13 , and the counter plate  16  is mounted to the fixed member. These two embodiments differ from each other in the fixed member and the rotating member, but they are similar in basic construction according to the present invention.  
      That is, in a shaft fixed-type HDD spindle motor shown in  FIG. 6 , when the rotor assembly  40  rotates with respect to the stator assembly  30  in a rated speed, the gap dimension L 2  of the bottom thrust dynamic pressure bearing section SBb is constructed so as to be smaller than the gap dimension L 1  of the top thrust dynamic pressure bearing section SBa. The depth of the dynamic pressure generating grooves in the bottom thrust dynamic pressure bearing section SBb is formed shallower than the depth of the dynamic pressure generating grooves in the top thrust dynamic pressure bearing section SBa.  
      In addition, the depth of the dynamic pressure generating grooves in the bottom thrust dynamic pressure bearing section SBb, of which the gap dimension L 2  is smaller than the gap dimension L 1 , is established so that the coefficient of elasticity of the bottom thrust dynamic pressure bearing section SBb has generally the greatest value. More specifically, the depth of the dynamic pressure generating grooves of the bottom thrust dynamic pressure bearing section SBb is established in the dimension of 0.8 times-2.8 times with respect to the gap dimension L 2 . For example, when the gap dimension L 2 , that is, the elevated amount of the thrust plate  36  in a rated rotation is 5 μm, the depth of the dynamic pressure generating grooves of the bottom thrust dynamic pressure bearing section SBb is established between 4 μm and 14 μm. More preferably, the coefficient of elasticity of near the maximum value can be obtained by setting the groove depth between 5 μm and 10 μm.  
      In addition, the depth of the dynamic pressure generating grooves in the top thrust dynamic pressure bearing section SBa, which is the larger side in the gap dimension, is preferably set so that the coefficient of elasticity of the top thrust dynamic pressure bearing section SBa has generally the greatest value.  
      In this embodiment, a magnetic attraction plate  17  is provided on the frame  31  for urging the rotation member to the frame  31 . Thus, the counter plate  44  as a second facing member is formed by a material of greater hardness than the bearing sleeve  41  as a first facing member, and the thrust plate  36  can always elevate from the bottom thrust dynamic pressure bearing surface SBb. In addition, the surface roughness of the dynamic pressure surface of the counter plate  44  is formed more smoothly than the surface roughness of the dynamic pressure surface of the bearing sleeve  41 .  
      Therefore, even when the counter plate  44  and the thrust plate  36  slide together at a slow speed of rotation such as when starting to rotate, the wear of the dynamic pressure surfaces of the counter plate  44  and the thrust plate  36  can be extremely reduced because the dynamic pressure surface of the counter plate  44  can be formed smoothly and formed by a material of greater hardness.  
      The embodiments of the invention are described above. However, the present invention is not limited to the embodiments described above, and many modifications can be made without departing from the subject matter of the present invention.  
      In the above-mentioned embodiments, for example, the thrust dynamic pressure generating grooves SG are formed on both surfaces of the thrust plate  23  and  36  in an axial direction. However, the thrust dynamic pressure generating grooves SG may be formed on the dynamic pressure surface of the bearing sleeves  13  or  41  as the first facing member, and on the dynamic pressure surface of the counter plates  16  or  44  as the second facing member in an axial direction.  
      Also, in the above-mentioned embodiments, the magnetic attraction plate  17  is arranged at a position so as to oppose against the drive magnet  22   c  as a biasing means in an axial direction. However, instead of arranging the magnetic attraction plate  17 , the rotor assembly  20  or  40  may be attracted to the frame  11  or  31  by shifting a magnetic center of the drive magnet  22   c  with respect to a magnetic center of the stator core  14  in a reverse direction of the frame.  
      In addition, the shaft member  21  or  35  and thrust plate  23  or  36  may be formed as one member. The thrust dynamic pressure generating grooves SG formed on the thrust dynamic pressure bearing SB may be configured as a shape of well-known spiral type grooves instead of the herringbone configuration grooves shown in  FIG. 2 .  
      Furthermore, the present invention can be similarly applied to motors of one side shaft fixed-type motor instead of both side shaft fixed-type motor shown in  FIG. 6 .  
      While the description above refers to particular embodiments of the invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention.  
      The disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive. The scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.