Dynamic Bearing Device

Abstract

A seal member is fixed to a predetermined position on an outer peripheral surface of a shaft portion. During rotation of a shaft member, a lower end surface of the seal member is opposed to an upper end surface of a bearing sleeve through an intermediation of a thrust bearing gap to form a second thrust bearing gap. An outer peripheral surface of the seal member defines between itself and an inner peripheral surface of an upper end portion of a housing a seal space having a predetermined volume.

Claims

1 . A dynamic bearing device, comprising: a housing; a bearing sleeve fixed inside the housing; a shaft member making a relative rotation with respect to the housing and the bearing sleeve; a seal member situated at one end of the housing; and a radial bearing portion supporting the shaft member radially in a non-contact manner by a dynamic pressure action of a fluid generated in a radial bearing gap between the bearing sleeve and the shaft member, characterized in that the seal member is provided on the shaft member, with a seal space being provided beside an outer peripheral surface of the seal member. 2 . A dynamic bearing device, comprising: a housing; a bearing sleeve fixed inside the housing; a shaft member making a relative rotation with respect to the housing and the bearing sleeve; a seal member situated at one end of the housing; and a radial bearing portion supporting the shaft member radially in a non-contact manner by a dynamic pressure action of a fluid generated in a radial bearing gap between the bearing sleeve and the shaft member, characterized in that the shaft member has a shaft portion inserted into an inner peripheral surface of the bearing sleeve and a flange portion provided on the shaft portion, that the seal member is fixed to the shaft member, with a seal space being defined beside an outer peripheral surface of the seal member, that the dynamic bearing device includes a first thrust bearing portion provided between one end surface of the seal member and one end surface of the bearing sleeve opposed thereto, with the first thrust bearing portion supporting the seal member and the shaft member in a thrust direction in a non-contact manner by a dynamic pressure action of a fluid generated in a thrust bearing gap, and that the dynamic bearing device includes a second thrust bearing portion provided between one end surface of the flange portion and another end surface of the bearing sleeve opposed thereto, with the second thrust bearing portion supporting the shaft member in the thrust direction in a non-contact manner by a dynamic pressure action of the fluid generated in a thrust bearing gap. 3 . A dynamic bearing device according to claim 1 , characterized in that a tapered surface gradually diminished in diameter toward outside of the housing is formed on the outer peripheral surface of the seal member. 4 . A dynamic bearing device according to claim 1 , characterized in that the seal member is fixed to the shaft member by an adhesive, and that an adhesion position of at least one of the seal member and the shaft member is provided with a recess to be filled with the adhesive. 5 . A dynamic bearing device, comprising: a housing; a bearing sleeve fixed inside the housing; a shaft member making a relative rotation with respect to the housing and the bearing sleeve; a seal member situated at one end of the housing; and a radial bearing portion supporting the shaft member radially in a non-contact manner by a dynamic pressure action of a fluid generated in a radial bearing gap between the bearing sleeve and the shaft member, characterized in that the seal member is provided on the shaft member, that one end surface of the seal member is opposed to one end surface of the bearing sleeve through an intermediation of a thrust bearing gap, and that an outer peripheral surface of the seal member is provided with a tapered surface gradually diminished in diameter toward outside of the housing and facing a seal space. 6 . A dynamic bearing device according to claim 1 , characterized in that the radial bearing portion has a dynamic pressure groove as a dynamic pressure generating means. 7 . A dynamic bearing device according to claim 1 , characterized in that the radial bearing portion is formed by a multi-arc bearing. 8 . A spindle motor for a disk device, comprising the dynamic bearing device according to claim 1 . 9 . A method of manufacturing a dynamic bearing device comprising: a housing; a bearing sleeve fixed inside the housing; a shaft member having a shaft portion inserted into an inner peripheral surface of the bearing sleeve and a flange portion provided on the shaft portion; a seal member fixed to the shaft member; a radial bearing portion supporting the shaft member radially in a non-contact manner by a dynamic pressure action of a fluid generated in a radial bearing gap between an inner peripheral surface of the bearing sleeve and an outer peripheral surface of the shaft member; a first thrust bearing portion supporting the seal member and the shaft member in a thrust direction in a non-contact manner by a dynamic pressure action of the fluid generated in a thrust bearing gap between one end surface of the seal member and one end surface of the bearing sleeve; and a second thrust bearing portion supporting the shaft member in the thrust direction in a non-contact manner by a dynamic pressure action of the fluid generated in a thrust bearing gap between one end surface of the flange portion and another end surface of the bearing sleeve, the method comprising the steps of: inserting the shaft portion of the shaft member into the inner peripheral surface of the bearing sleeve and attaching the seal member to the shaft portion to thereby interpose the bearing sleeve between the one end surface of the seal member and the one end surface of the flange portion; adjusting, after the step, an axial relative position of the shaft portion and the seal member to define, between the bearing sleeve and the one end surface of the seal member and between the bearing sleeve and the one end surface of the flange portion, gaps of an amount corresponding to a sum total of the thrust bearing gaps of the first thrust bearing portion and the second thrust bearing portion; fixing, after the step, the seal member to the shaft portion; and accommodating an assembly including the bearing sleeve, the shaft member, and the seal member assembled together by the step in the housing. 10 . A dynamic bearing device according to claim 2 , characterized in that a tapered surface gradually diminished in diameter toward outside of the housing is formed on the outer peripheral surface of the seal member. 11 . A dynamic bearing device according to claim 2 , characterized in that the seal member is fixed to the shaft member by an adhesive, and that an adhesion position of at least one of the seal member and the shaft member is provided with a recess to be filled with the adhesive. 12 . A dynamic bearing device according to claim 2 , characterized in that the radial bearing portion has a dynamic pressure groove as a dynamic pressure generating means. 13 . A dynamic bearing device according to claim 5 , characterized in that the radial bearing portion has a dynamic pressure groove as a dynamic pressure generating means. 14 . A dynamic bearing device according to claim 2 , characterized in that the radial bearing portion is formed by a multi-arc bearing. 15 . A dynamic bearing device according to claim 5 , characterized in that the radial bearing portion is formed by a multi-arc bearing. 16 . A spindle motor for a disk device, comprising the dynamic bearing device according to claim 2 . 17 . A spindle motor for a disk device, comprising the dynamic bearing device according to claim 5.
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a dynamic bearing device for supporting a rotary member in a non-contact manner by a dynamic pressure action of a fluid (i.e., lubricating fluid) generated in a bearing gap. The dynamic bearing device is suitable for use in: a spindle motor for an information apparatus, for example, a magnetic disk device, such as an HDD or an FDD, an optical disk device, such as a CD-ROM, a CD-R/RW, or a DVD-ROM/RAM, or a magneto-optical disk device, such as an MD or MO; a polygon scanner motor for a laser beam printer (LBP); or a small motor for an electrical apparatus such as an axial fan. [0003] 2. Description of the Related Art [0004] Apart from high rotational accuracy, an improvement in speed, a reduction in cost, a reduction in noise, etc. are required of the various motors mentioned above. One of the factors determining such the requisite performances is a bearing for supporting a spindle of the motor. Recently, as a bearing of this type, use of a dynamic bearing superior in the above-mentioned requisite performances has been considered, or such the dynamic bearing has been actually put into practical use. [0005] For example, in a dynamic bearing device to be incorporated into the spindle motor of a disk drive apparatus, such as an HDD, there are provided a radial bearing portion supporting a shaft member radially in a non-contact manner and a thrust bearing portion supporting the shaft member in a thrust direction in a non-contact manner. As the radial bearing portion, there is used a dynamic bearing provided with grooves (i.e., dynamic pressure grooves) for dynamic pressure generation provided in an inner peripheral surface of a bearing sleeve or in an outer peripheral surface of the shaft member. As the thrust bearing portion, there is used a dynamic bearing provided with dynamic pressure grooves in, for example, both end surfaces of a flange portion of the shaft member or in surfaces opposed thereto (e.g., an end surface of the bearing sleeve and an end surface of a thrust member fixed to a housing (see, for example, Patent Documents 1 and 2). Alternatively, as the thrust bearing portion, there may be used a bearing (i.e., a so-called pivot bearing) of a structure in which one end surface of the shaft member is contact-supported by a thrust plate (see, for example, FIG. 4 of Patent Document 2). [0006] In general, the bearing sleeve is fixed to a predetermined position of an inner periphery of the housing and, to prevent leakage of the fluid (e.g., a lubricating oil) poured into an inner space of the housing to the outside, a seal member is arranged at an opening of the housing in many cases. The inner peripheral surface of the seal member defines a seal space between itself and the outer peripheral surface of the shaft member, and the volume of the seal space is set to be larger than the amount by which the lubricating oil filling the inner space of the housing undergoes a change in volume through thermal expansion/contraction within a temperature range of use. Thus, even when there is a change in the volume of the lubricating oil as a result of a temperature change, an oil level of the lubricant is always maintained within the seal space (see Patent Document 1). [0000] Patent Document 1: JP 2003-65324 A [0000] Patent Document 2: JP 2003-336636 A [0007] As described above, in the conventional dynamic bearing device, the seal space is formed between the inner peripheral surface of the seal member fixed at the opening of the housing and the outer peripheral surface of the shaft member; if the seal space is to have a function to absorb a change in the volume of the lubricating oil due to a temperature change, it is necessary to secure a relatively large axial dimension for the seal space (i.e., the seal member). Thus, from the design standpoint, it is necessary to lower, within the housing, the position of the axial center of the bearing sleeve relatively toward a bottom side of the housing, with the result that the distance between the bearing center of the radial bearing portion and the center of gravity of the rotary member increases, which, depending upon the condition of use, etc., can lead to a shortage of load capacity with respect to a moment load. Further, in a construction in which thrust bearing portions are provided on both sides of the flange portion of the shaft member, the axial distance between the two thrust bearing portions becomes relatively smaller, with the result that the load capacity of the thrust bearing portions with respect to the moment load tends to be so much the lower. In particular, in a case of a dynamic bearing device for use in a disk drive apparatus, as a rotor (i.e., a rotary member to which a rotor hub, a rotor magnet, a disk, a clamper, etc. are assembled) rotates, a relatively large moment load acts on the shaft member, so the moment load resistance is an important characteristic. [0008] Further, in a dynamic bearing of this type, the thrust bearing gap of the thrust bearing portion is under the influence of component precision, assembly precision, etc., so it is difficult to control the thrust bearing gap to a desired value. Under the circumstances, there is nothing for it but to perform a complicated assembly operation. [0009] It is an object of the present invention to make it possible to reduce the axial dimension of the above-mentioned seal space of a dynamic bearing device of this type, thereby enhancing the load capacity of the dynamic bearing device with respect to the moment load or reducing the axial dimension of the dynamic bearing device. [0010] Another object of the present invention is to enhance the load capacity of the thrust bearing portion with respect to the moment load. [0011] Still another object of the present invention is to provide a method which makes it possible to easily set the thrust bearing gaps of a dynamic bearing device of this type with high accuracy. SUMMARY OF THE INVENTION [0012] To attain the above-mentioned objects, the present invention provides a dynamic bearing device, including: a housing; a bearing sleeve fixed inside the housing; a shaft member making a relative rotation with respect to the housing and the bearing sleeve; a seal member situated at one end of the housing; and a radial bearing portion supporting the shaft member radially in a non-contact manner by a dynamic pressure action of a lubricating oil generated in a radial bearing gap between the bearing sleeve and the shaft member, characterized in that the seal member is provided on the shaft member, with a seal space being provided beside an outer peripheral surface of the seal member. [0013] Here, as the fluid (i.e., lubricating fluid), it is also possible to use a gas, such as air, apart from a liquid, such as a lubricating oil (or lubricating grease) or a magnetic fluid. [0014] In the above-mentioned construction, the seal space is provided between the outer peripheral surface of the seal member provided on the shaft portion and the inner peripheral surface of one end portion of the housing, so, in securing, in the seal space, a volume capable of absorbing a change in volume due to a change in the temperature of the lubricating oil filling the inner space of the housing, it is possible to make the axial dimension of the seal space (i.e., seal member) smaller than that in the prior art. Thus, it is possible to set, within the housing, the axial center position of the bearing sleeve nearer to one end portion of the housing than in the prior art (that is, to arrange the bearing sleeve nearer to one end portion of the housing than in the prior art, or to make the axial dimension of the bearing sleeve larger than that in the prior art), whereby the distance between the bearing center of the radial bearing portion and the center of gravity of the rotary member is reduced, thereby enhancing the load capacity with respect to the moment load. Further, in a case of arranging the bearing sleeve nearer to one end portion of the housing than in the prior art, it is possible to make the axial dimension of the dynamic bearing device smaller than that in the prior art. [0015] TO attain the above-mentioned objects, the present invention further provides a dynamic bearing device, including: a housing; a bearing sleeve fixed inside the housing; a shaft member making a relative rotation with respect to the housing and the bearing sleeve; a seal member situated at one end of the housing; and a radial bearing portion supporting the shaft member radially in a non-contact manner by a dynamic pressure action of a fluid generated in a radial bearing gap between the bearing sleeve and the shaft member, characterized in that the shaft member has a shaft portion inserted into an inner peripheral surface of the bearing sleeve and a flange portion provided on the shaft portion, that the seal member is fixed to the shaft member, with a seal space being defined beside an outer peripheral surface of the seal member, that the dynamic bearing device includes a first thrust bearing portion provided between one end surface of the seal member and one end surface of the bearing sleeve opposed thereto, with the first thrust bearing portion supporting the seal member and the shaft member in a thrust direction in a non-contact manner by a dynamic pressure action of a fluid generated in a thrust bearing gap, and that the dynamic bearing device includes a second thrust bearing portion provided between one end surface of the flange portion and another end surface of the bearing sleeve opposed thereto, with the second thrust bearing portion supporting the shaft member in the thrust direction in a non-contact manner by a dynamic pressure action of the fluid generated in a thrust bearing gap. [0016] In addition to the above-mentioned effects, due to the provision of the first thrust bearing portion and the second thrust bearing portion so as to sandwich the bearing sleeve axially from both sides, the axial distance between the two thrust bearing portions is increased as compared with the construction in which the thrust bearing portions are provided on both sides of the flange portion, and the load capacity of the thrust bearing portions with respect to the moment load is so much the higher. [0017] The fixation of the seal member to the shaft member can be effected by an appropriate fixing means, such as adhesion, a combination of adhesion and press-fitting, or welding (ultrasonic welding). When adopting adhesion (or a combination of adhesion and press-fitting) as the fixing means, it is possible to provide a recess to be filled with adhesive at an adhesion position of at least one of the seal member and the shaft member. The recess may be provided in the form of a circumferential groove or in the form of one or a plurality of recesses arranged circumferentially. The adhesive also fills the recess at the adhesion position and solidifies, whereby the strength with which the seal member is fixed to the shaft member is enhanced. [0018] In the above-mentioned construction, the width (i.e., radial dimension) of the seal space may be axially uniform; however, from the viewpoint of enhancing the sealing property, it is desirable for the seal space to be of a tapered configuration gradually diminished in size toward the interior of the housing. That is, when the seal space has the tapered configuration, the fluid in the seal space is drawn in by capillary attraction in the direction in which the seal space is diminished in size (i.e., toward the interior of the housing). As a result, it is possible to effectively prevent leakage of the fluid from the interior to the exterior of the housing. As means for realizing such the construction, there are available a means for forming, on the outer peripheral surface of the seal member, a tapered surface gradually diminished in diameter toward the exterior of the housing, and a means for forming a tapered surface gradually diminished in diameter toward the exterior of the housing on a surface opposed to the outer peripheral surface of the seal member through the intermediation of the seal space, for example, on the inner peripheral surface of one end portion of the housing. In particular, with the former means, the seal member rotates together with the shaft member, whereby, in addition to the drawing-in action due to the capillary attraction, it is also possible to obtain a drawing-in action due to the centrifugal force during rotation (i.e., a so-called centrifugal seal), so leakage of the fluid from the interior to the exterior of the housing is more effectively prevented. [0019] To attain the above-mentioned objects, the present invention further provides a dynamic bearing device, including: a housing; a bearing sleeve fixed inside the housing; a shaft member making a relative rotation with respect to the housing and the bearing sleeve; a seal member situated at one end of the housing; and a radial bearing portion supporting the shaft member radially in a non-contact manner by a dynamic pressure action of a fluid generated in a radial bearing gap between the bearing sleeve and the shaft member, characterized in that the seal member is provided on the shaft member, that one end surface of the seal member is opposed to one end surface of the bearing sleeve through an intermediation of a thrust bearing gap, and that an outer peripheral surface of the seal member is provided with a tapered surface gradually diminished in diameter toward outside of the housing and facing a seal space. [0020] In the dynamic bearing device constructed as described above, the radial bearing portion may be formed by a dynamic bearing provided with dynamic pressure grooves of an axially inclined configuration, such as a herringbone-like configuration or a spiral configuration, a dynamic bearing (i.e., multi-arc bearing) in which the radial bearing gap is diminished in a wedge-like fashion in one or both circumferential directions, or a dynamic bearing (i.e., step bearing) in which a plurality of dynamic pressure grooves in the form of axial grooves are provided at predetermined circumferential intervals. [0021] The dynamic bearing device constructed as described above can preferably be used as a dynamic bearing device for a spindle motor for use in an information apparatus, such as a disk device. [0022] To attain the above-mentioned objects, the present invention further provides a method of manufacturing a dynamic bearing device, including: a housing; a bearing sleeve fixed inside the housing; a shaft member having a shaft portion inserted into an inner peripheral surface of the bearing sleeve and a flange portion provided on the shaft portion; a seal member fixed to the shaft member; a radial bearing portion supporting the shaft member radially in a non-contact manner by a dynamic pressure action of a fluid generated in a radial bearing gap between an inner peripheral surface of the bearing sleeve and an outer peripheral surface of the shaft member; a first thrust bearing portion supporting the seal member and the shaft member in a thrust direction in a non-contact manner by a dynamic pressure action of the fluid generated in a thrust bearing gap between one end surface of the seal member and one end surface of the bearing sleeve; and a second thrust bearing portion supporting the shaft member in the thrust direction in a non-contact manner by a dynamic pressure action of the fluid generated in a thrust bearing gap between one end surface of the flange portion and another end surface of the bearing sleeve, the method including the steps of: inserting the shaft portion of the shaft member into the inner peripheral surface of the bearing sleeve and attaching the seal member to the shaft portion to thereby interpose the bearing sleeve between the one end surface of the seal member and the one end surface of the flange portion; adjusting, after the step, an axial relative position of the shaft portion and the seal member to define, between the bearing sleeve and the one end surface of the seal member and between the bearing sleeve and the one end surface of the flange portion, gaps of an amount corresponding to a sum total of the thrust bearing gaps of the first thrust bearing portion and the second thrust bearing portion; fixing, after the step, the seal member to the shaft portion; and accommodating an assembly including the bearing sleeve, the shaft member, and the seal member assembled together by the step in the housing. [0023] In the above-mentioned construction, the thrust bearing gaps are set at the stage of previously assembling together the bearing sleeve, the shaft member, and the seal member, so the thrust bearing gaps can easily be set with high accuracy. After the setting of the thrust bearing gaps, the operation of assembling the components to each other is completed when the assembly including the bearing sleeve, the shaft member, and the seal member is accommodated in the housing, thus simplifying the assembling operation. [0024] According to the present invention, it is possible to enhance the load capacity of a dynamic bearing device with respect to the moment load, or to make the axial dimension of a dynamic bearing device compact. Thus, it is possible to achieve a reduction in the size of a spindle motor equipped with the dynamic bearing device for use in an information apparatus, such as a disk device. [0025] Further, according to the present invention, it is possible to enhance the load capacity of the thrust bearing portion with respect to the moment load. [0026] Further, according to the present invention, it is possible to set a thrust bearing gap in a dynamic bearing device of this type easily and with high accuracy. BRIEF DESCRIPTION OF THE DRAWINGS [0027] FIG. 1 is a sectional view of a spindle motor for an information apparatus into which a dynamic bearing device according to an embodiment of the present invention is incorporated. [0028] FIG. 2 is a sectional view of a dynamic bearing device according to a first embodiment of the present invention. [0029] FIG. 3 is a sectional view of a bearing sleeve and diagrams showing a lower end surface and an upper end surface thereof. [0030] FIG. 4 is a diagram illustrating an assembly process. [0031] FIG. 5 is a diagram illustrating an assembly process. [0032] FIG. 6 is a diagram illustrating an assembly process. [0033] FIG. 7 is a sectional view of a dynamic bearing device according to a second embodiment of the present invention. [0034] FIG. 8 is a sectional view of a dynamic bearing device according to a third embodiment of the present invention. [0035] FIG. 9 is a sectional view of a bearing sleeve and a diagram showing a lower end surface thereof. [0036] FIG. 10 is a top view of a housing. [0037] FIG. 11 is a sectional view of another example of a radial bearing portion. [0038] FIG. 12 is a sectional view of still another example of the radial bearing portion. [0039] FIG. 13 is a sectional view of yet another example of the radial bearing portion. [0040] FIG. 14 is a sectional view of another example of the radial bearing portion. DETAILED DESCRIPTION OF THE INVENTION [0041] In the following, an embodiment of the present invention will be described with reference to the drawings. [0042] FIG. 1 is a diagram conceptually showing a construction example of an information apparatus spindle motor into which the dynamic bearing device (i.e., fluid dynamic bearing device) 1 according to this embodiment is incorporated. The spindle motor is used in a disk drive apparatus, such as an HDD, and is equipped with: the dynamic bearing device 1 for rotatably supporting a shaft member 2 in a non-contact manner; a rotor (i.e., disk hub) 3 mounted to the shaft member 2 ; and a stator coil 4 and a rotor magnet 5 opposed to each other through the intermediation of, for example, a radial gap. The stator coil 4 is mounted to the outer periphery of a bracket 6 , and the rotor magnet 5 is mounted to the inner periphery of the disk hub 3 . A housing 7 of the dynamic bearing device 1 is fixed to the inner periphery of the bracket 6 . One or a plurality of disks D such as magnetic disks are retained by the disk hub 3 . When electricity is supplied to the stator coil 4 , the rotor magnet 5 is rotated by an electromagnetic force generated between the stator coil 4 and the rotor magnet 5 , and with this rotation, the disk hub 3 is rotated integrally with the shaft 2 . [0043] FIG. 2 shows the dynamic bearing device 1 of a first embodiment of the present invention. The dynamic bearing device is composed of the housing 7 , a bearing sleeve 8 fixed to the housing 7 , the shaft member 2 , and a seal member 9 fixed to the shaft member 2 . [0044] Between an inner peripheral surface 8 a of the bearing sleeve 8 and an outer peripheral surface 2 a 1 of a shaft portion 2 a of the shaft member 2 , there are provided a first radial bearing portion R 1 and a second radial bearing portion R 2 which are axially spaced apart from each other. Further, a first thrust bearing portion T 1 is provided between an upper end surface 8 b of the bearing sleeve 8 and a lower end surface 9 b of the seal member 9 , and a second thrust bearing portion T 2 is provided between a lower end surface 8 c of the bearing sleeve 8 and an upper end surface 2 b 1 of a flange portion 2 b of the shaft member 2 . For the sake of convenience in illustration, in the following description, a side on which a bottom portion 7 b of the housing 7 is situated will be referred to as the lower side, and a side on which an opening of the housing 7 is situated (i.e., side opposite to the bottom portion 7 b ) will be referred to as the upper side. [0045] The housing 7 is formed as a bottomed cylinder, for example, by injection molding of a resin material, and is equipped with a cylindrical side portion 7 a and the bottom portion 7 b provided integrally at the lower end of the side portion 7 a . Further, a step portion 7 d is integrally formed at a position spaced apart axially upwards from an inner bottom surface of the bottom portion 7 b by a predetermined dimension. [0046] A resin forming the housing is mainly made of a thermoplastic resin; for example, it is possible to use an amorphous resin, such as polysulfone (PSF), polyether sulfone (PES), polyphenyl sulfone (PPSU), or polyether imide (PEI), or a crystalline resin, such as liquid crystal polymer (LCP), polyetherether ketone (PEEK), polybutyrene terephthalate (PBT), or polyphenylene sulfide (PPS). There are no particular limitations regarding the filler to be used for the above resin; for example, it is possible to use, as the filler, a fibrous filler, such as glass fiber, a whisker-like filler, such as potassium titanate, a scale-like filler, such as mica, or a fibrous or powdered conductive filler, such as carbon fiber, carbon black, graphite, carbon nanomaterial, or metal powder. These fillers may be used singly or in the form of a mixture of two or more them. This embodiment employs, as the material forming the housing 7 , a resin material obtained by mixing 2 to 8 wt % of carbon fiber or carbon nanotube as the conductive filler with liquid crystal polymer (LCP) as the crystalline resin. [0047] The shaft member 2 is formed, for example, of a metal material, such as stainless steel, or in a hybrid structure composed of metal and a resin, and is equipped with the shaft portion 2 a and the flange portion 2 b provided integrally or separately at the lower end of the shaft portion 2 a . Further, in this embodiment, a recess, for example, a circumferential groove 2 a 2 , is formed at a predetermined position of the outer peripheral surface 2 a 1 of the shaft portion 2 a to which the seal member 9 is fixed. [0048] The bearing sleeve 8 is formed as a cylinder of a porous material composed of a sintered metal, in particular, a porous material composed of a sintered metal whose main component is copper. The bearing sleeve 8 is fixed to a predetermined position of an inner peripheral surface 7 c of the housing 7 . It is also possible for the bearing sleeve 8 to be formed not only of the sintered metal, but also of some other metal material, which is not the porous material, for example, of a soft metal such as brass. [0049] On an inner peripheral surface 8 a of the bearing sleeve 8 , there are provided axially separated upper and lower two regions constituting the respective radial bearing surfaces of the first radial bearing portion R 1 and the second radial bearing portion R 2 . In the two regions, there are respectively formed dynamic pressure grooves 8 a 1 and 8 a 2 of a herringbone configuration as shown, for example, in FIG. 3 . The upper dynamic pressure grooves 8 a 1 are formed axially asymmetrically with respect to an axial center m (i.e., axial center of the region between the upper and lower inclined groves), and an axial dimension X 1 of the region on the upper side of the axial center m is larger than an axial dimension X 2 of the region on the lower side of the axial center m. Further, in an outer peripheral surface 8 d of the bearing sleeve 8 , there is formed one or a plurality of axial grooves 8 d 1 extending over the entire axial length of the bearing sleeve. In this example, three axial grooves 8 d 1 are formed at constant intervals in a circumferential direction. [0050] Dynamic pressure grooves 8 b 1 of a spiral configuration as shown, for example, in FIG. 3 , are formed in an upper end surface 8 b of the bearing sleeve 8 constituting the thrust bearing surface of the first thrust bearing portion T 1 . Similarly, dynamic pressure grooves 8 c 1 of a spiral configuration as shown, for example, in FIG. 3 , are formed in a lower end surface 8 c of the bearing sleeve 8 constituting the thrust bearing surface of the second thrust bearing portion T 2 . [0051] The seal member 9 is formed in a ring-like shape, for example, of a soft metal material, such as brass, or some other metal material, or a resin material, and is fixed, for example, by adhesive, to a predetermined position of the outer peripheral surface 2 a 1 of the shaft portion 2 a . During rotation of the shaft member 2 , a lower end surface 9 b of the seal member 9 is opposed to the upper end surface 8 b of the bearing sleeve 8 through the intermediation of a predetermined thrust bearing gap to form the first thrust bearing portion T 1 . An outer peripheral surface 9 a of the seal member 9 forms between itself and an inner peripheral surface 7 a 1 of the upper end portion (i.e., opening) of the housing 7 a seal space S having a predetermined volume. The seal space S is formed beside the outer peripheral surface 9 a of the seal member 9 , so, in securing in the seal space S a volume capable of absorbing a change in volume due to a change in the temperature of the fluid filling the inner space of the housing 7 , it is possible to make an axial dimension of the seal space S (i.e., the seal member 9 ) smaller than that in the prior art. Thus, it is possible, for example, to make the axial length of the bearing sleeve 8 larger than that in the prior art and transfer the axial center m of the dynamic pressure grooves 8 a 1 of the first radial bearing portion R 1 to the upper end surface 8 b side, or to make an axial dimension of the bearing sleeve 8 smaller than that in the prior art. When the former measure is taken, an axial distance between the axial center m of the dynamic pressure grooves 8 a 1 of the first radial portion R 1 and the axial center of the dynamic pressure grooves 8 a 2 of the second radial bearing portion R 2 increases, so it is possible to achieve an enhancement in the load capacity with respect to a moment load. On the other hand, when the latter measure is taken, it is possible to make the axial dimension of the dynamic bearing device smaller than that in the prior art. [0052] In this embodiment, the outer peripheral surface 9 a of the seal member 9 is equipped with a tapered surface 9 a 1 gradually diminished in diameter toward the exterior of the housing 7 , so the seal space S exhibits a tapered configuration gradually diminished in size toward the interior of the housing 7 . During rotation of the shaft member 2 , the fluid in the seal space S is drawn in a direction in which the seal space S is narrowed (i.e., in a direction of the interior of the housing) by a drawing-in action due to capillary attraction and a drawing-in action due to a centrifugal force during the rotation. As a result, leakage of a lubricating oil from the interior of the housing 7 is effectively prevented. [0053] The dynamic bearing device 1 of this embodiment is assembled, for example, by the following process. [0054] First, the shaft member 2 , the bearing sleeve 8 , and the seal member 9 are assembled together. As shown, for example, in FIG. 4 , the bearing sleeve 8 is attached to the shaft portion 2 a of the shaft member 2 placed on the upper surface of a base 10 , and the lower end surface 8 c of the bearing sleeve 8 is brought into contact with the upper end surface 2 b 1 of the flange portion 2 b . Then, after applying an adhesive, for example, a thermosetting adhesive, to the shaft portion 2 a , the seal member 9 is attached to the shaft portion 2 a , and the lower end surface 9 b of the seal member 9 is brought into contact with the upper end surface 8 b of the bearing sleeve 8 , whereby the bearing sleeve 8 is interposed between the lower end surface 9 b of the seal member 9 and the upper end surface 2 b 1 of the flange portion 2 b. [0055] Next, the thrust bearing gaps are set. The setting of the thrust bearing gaps is effected by adjusting the relative axial positions of the shaft member 2 and the seal member 9 . For example, in the above-mentioned state, that is, in a state in which the lower end surface 8 c of the bearing sleeve 8 is in contact with the upper end surface 2 b 1 of the flange portion 2 b and the lower end surface 9 b of the seal member 9 is in contact with the upper end surface 8 b of the bearing sleeve 8 (i.e., state in which there is no thrust bearing gap), the shaft member 2 is caused to make a relative movement in the axial direction with respect to the bearing sleeve 8 and the seal member 9 by an amount corresponding to the sum total δ of the thrust bearing gap of the first thrust bearing portion T 1 (indicated by symbol δ 1 ) and the thrust bearing gap of the second thrust bearing portion T 2 (indicated by symbol δ 2 ) (=δ 1 +δ 2 ). [0056] To be more specific, as shown, for example, in FIG. 5 , an assembly put together in the state as described above is placed on an upper surface of a jig 11 provided with a step portion 11 a of a predetermined depth W 2 , with the lower end surface 8 c of the bearing sleeve 8 being in contact with the upper surface of the jig 11 and the flange portion 2 b being accommodated in the step portion 11 a . In this state, the shaft member 2 is pressed from above to be allowed to make a relative movement in the axial direction with respect to the bearing sleeve 8 and the seal member 9 by the predetermined amount 5 . In this case, when the depth W 2 of the step portion 11 a is accurately controlled with respect to an axial dimension W 1 of the flange portion 2 b such that a relationship of W 2 =W 1 +δ is obtained, it is possible to set the thrust bearing gap δ(=δ 1 +δ 2 ) easily and with high accuracy solely by pushing the shaft member 2 forward until the lower end surface 2 b 2 of the flange portion 2 b abuts a bottom surface 11 a 1 of the step portion 11 a . Thus, the operation and device related to the setting of the thrust bearing gap are simplified. Alternatively, it is also possible to set the thrust bearing gap δ(=δ 1 +δ 2 ) by setting the depth of the step portion such that a relationship of W 2 >W 1 +δ is obtained and by controlling an axial relative movement amount of the shaft member 2 . [0057] Alternatively, the setting of the thrust bearing gap can be effected by using as a reference the upper end surface 8 b of the bearing sleeve 8 when the lower end surface 8 c of the bearing sleeve 8 is in contact with the upper end surface 2 b 1 of the flange portion 2 b and adjusting the axial position of the seal member 9 such that the lower end surface 9 b of the seal member 9 is brought to a position where the lower end surface 9 b is axially spaced apart from the upper end surface 8 b by an amount corresponding to the above-mentioned sum total δ(=δ 1 +δ 2 ). Such the positional adjustment in the axial direction of the seal member 9 can be effected easily and with high accuracy, for example, by allowing a spacer, whose width is accurately controlled to a dimension equal to the above-mentioned sum total δ, to be interposed between the upper end surface 8 b of the bearing sleeve 8 and the lower end surface 9 b of the seal member 9 . [0058] After setting the thrust bearing gap (δ) through adjustment of the axial relative position of the shaft portion 2 and the seal member 9 , the seal member 9 is fixed to the shaft portion 2 a at that position. In this embodiment, the seal member 9 is fixed by adhesion to the shaft portion 2 a through heat treatment (baking) of the thermosetting adhesive applied to the shaft portion 2 a . At this time, the adhesive applied to the shaft portion 2 a also fills a circumferential groove 2 a 2 in the outer peripheral surface 2 a 1 and solidifies therein, whereby the adhesion strength for the seal member 9 with respect to the shaft member 2 is enhanced. [0059] Next, as shown in FIG. 6 , the assembly put together by the above process and composed of the shaft member 2 , the bearing sleeve 8 , and the seal member 9 is inserted into the inner peripheral surface 7 c of the housing 7 , and the lower end surface 8 c is brought into contact with the step portion 7 d of the housing 7 ; in this state, the outer peripheral surface 8 d of the bearing sleeve 8 is fixed to the inner peripheral surface 7 c of the housing 7 . The fixation of the bearing sleeve 8 to the housing 7 can be effected by an appropriate means, such as adhesion, press-fitting, a combination of adhesion and press-fitting, or welding (such as ultrasonic welding). In the drawing, 5 is considerably exaggerated in dimension. [0060] When the assembly is completed as described above, the shaft portion 2 a of the shaft member 2 is inserted into the inner peripheral surface 8 a of the bearing sleeve 8 , and the flange portion 2 b is accommodated in the space portion between the lower end surface 8 c of the bearing sleeve 8 and the inner bottom surface of the bottom portion 7 b of the housing 7 . Further, the seal space S having a predetermined volume is defined between the outer peripheral surface 9 a of the seal member 9 and the inner peripheral surface 7 a 1 of the upper end portion of the housing 7 . After that, the inner space of the housing 7 sealed by the seal member 9 , inclusive of inner voids of the bearing sleeve 8 , is filled with a fluid, such as a lubricating oil. An oil level of the lubricating oil is constantly maintained within a range of the seal space S. [0061] During rotation of the shaft member 2 , regions of the inner peripheral surface 8 a of the bearing sleeve 8 constituting the radial bearing surfaces (i.e., two upper and lower regions) are respectively opposed to the outer peripheral surface 2 a 1 of the shaft portion 2 a through the intermediation of a radial bearing gap. Further, a region of the upper end surface 8 b of the bearing sleeve 8 constituting the thrust bearing surface is opposed to the lower end surface 9 b of the seal member 9 through the intermediation of a thrust bearing gap, and a region of the lower end surface 8 c of the bearing sleeve 8 constituting the thrust bearing surface is opposed to the upper end surface 2 b 1 of the flange portion 2 b through the intermediation of a thrust bearing gap. As the shaft member 2 rotates, a dynamic pressure of the lubricating oil is generated in the radial bearing gap, and the shaft portion 2 a of the shaft member 2 is supported radially and rotatably in a non-contact manner by an oil film of the lubricating oil formed in the radial bearing gap. As a result, there are formed the first radial bearing portion R 1 and the second radial bearing portion R 2 supporting the shaft member 2 radially and rotatably in a non-contact manner. At the same time, a dynamic pressure of the lubricating oil is generated in the thrust bearing gaps, and the shaft member 2 and the seal member 9 are supported rotatably in the thrust direction in a non-contact manner by oil films of the lubricating oil formed in the thrust bearing gaps. As a result, there are formed the first thrust bearing portion T 1 and the second thrust bearing portion T 2 supporting the shaft member 2 rotatably in the thrust direction in a non-contact manner. The thrust bearing gap (indicated by δ 1 ) of the first thrust bearing portion T 1 and the thrust bearing gap (indicated by δ 2 ) of the second thrust bearing portion T 2 are controlled accurately in the above-described assembly step such that a relationship of δ=δ 1 +δ 2 is obtained, so it is possible to obtain a stable thrust bearing function. [0062] Further, as described above, the dynamic pressure grooves 8 a 1 of the first radial bearing portion R 1 are formed asymmetrically in the axial direction with respect to the axial center m, with the axial dimension X 1 of the region on the upper side of the axial center m being larger than the axial dimension X 2 of the region on the lower side thereof (see FIG. 3 ). Thus, during rotation of the shaft member 2 , the lubricating oil drawing force (i.e., pumping force) due to the dynamic pressure grooves 8 a 1 is larger in the upper region than in the lower region. Due to this difference in drawing force, the lubricating oil filling the gap between the inner peripheral surface 8 a of the bearing sleeve 8 and the outer peripheral surface 2 a 1 of the shaft portion 2 a flows downwards, and circulates through the route: the thrust bearing gap of the second thrust bearing portion T 2 →the axial grooves 8 d 1 →the thrust bearing gap of the first thrust bearing portion T 1 before being drawn into the radial bearing gap of the first radial bearing portion R 1 again. In this way, the lubricating oil flows and circulates through the inner space of the housing 7 , whereby it is possible to prevent a phenomenon in which the pressure of the lubricating oil in the inner space locally becomes a negative pressure, making it possible to eliminate the generation of bubbles as a result of the generation of a negative pressure, and leakage of the lubricating oil, generation of vibration, etc. due to the generation of bubbles. Further, if, for some reason, bubbles are allowed to be mixed into the lubricating oil, the bubbles are discharged into the atmosphere through the oil surface (gas-liquid interface) of the lubricating oil in the seal space S when they circulate with the lubricating oil, so it is possible to more effectively prevent the adverse effects of the bubbles. [0063] Further, the inward drawing force (i.e., pumping force) for the lubricating oil due to the dynamic pressure grooves 8 b 1 of the first thrust bearing portion T 1 also acts on the lubricating oil in the radial bearing gap of the first radial bearing portion R 1 , so even if the above-mentioned difference in drawing force in the first radial bearing portion R 1 is relatively small, it is possible to secure a satisfactory flowing circulation of the lubricating oil. As a result, it is possible to reduce the axial asymmetry of the dynamic pressure grooves 8 a 1 of the first radial bearing portion R 1 as compared with that in the prior art; for example, it is possible to diminish the axial dimension X 1 of the upper region of the dynamic pressure grooves 8 a 1 as compared with that in the prior art and transfer the axial center m of the dynamic pressure grooves 8 a 1 to the upper end surface 8 b side, or to diminish the axial dimension of the bearing sleeve 8 . When the former measure is taken, the axial distance between the axial center m of the dynamic pressure grooves 8 a 1 of the first radial bearing portion R 1 and the axial center of the dynamic pressure grooves 8 a 2 of second radial bearing portion R 2 increases, so it is possible to achieve an enhancement in the load capacity with respect to the moment load. On the other hand, when the latter measure is taken, it is possible to reduce the axial dimension of the dynamic bearing device as compared with that in the prior art. [0064] FIG. 7 shows a dynamic bearing device 21 according to a second embodiment of the present invention. The dynamic bearing device 21 of this embodiment differs from the dynamic bearing device 1 of the first embodiment described above in that the bearing sleeve is composed of an upper bearing sleeve 81 and a lower bearing sleeve 82 , with a spacer member 83 being interposed between them. The spacer member 83 is formed in a ring-like shape of a soft metal material, such as brass, or some other metal material, or a resin material, and, unlike the upper bearing sleeve 81 and the lower bearing sleeve 82 , has no porous texture. [0065] The first radial bearing portion R 1 is provided between an inner peripheral surface 81 a of the upper bearing sleeve 81 and the outer peripheral surface 2 a 1 of the shaft portion 2 a of the shaft member 2 , and the second radial bearing portion R 2 is provided between an inner peripheral surface 82 a of the lower bearing sleeve 82 and the outer peripheral surface 2 a 1 of the shaft portion 2 a . Further, the first thrust bearing portion T 1 is provided between an upper end surface 81 b of the upper bearing sleeve 81 and the lower end surface 9 b of the seal member 9 , and the second thrust bearing portion T 2 is provided between a lower end surface 82 c of the lower bearing sleeve 82 and the upper end surface 2 b 1 of the flange portion 2 b of the shaft member 2 . Formed in the lower end surface of the upper bearing sleeve 81 is an annular groove (i.e., V-groove) for distinguishing the upper bearing sleeve from the lower bearing sleeve 82 . In an outer peripheral surface 81 d of the upper bearing sleeve 81 , an outer peripheral surface 82 d of the lower bearing sleeve 82 , and the outer peripheral surface of the spacer member 83 , there are respectively formed one or a plurality of axial grooves 81 d 1 , 82 d 1 , and 83 d which respectively extend over the entire axial lengths of the axial grooves. The axial grooves 81 d 1 , 82 d 1 , and 83 d are formed in alignment in circumferential phase, and communicate axially with each other. [0066] Since the spacer member 83 with no porous texture is interposed between the upper bearing sleeve 81 and the lower bearing sleeve 82 , the total amount of lubricating oil filling the inner space of the housing 7 can be less than that in the dynamic bearing device 1 of the above-mentioned embodiment (for, the interior of the spacer member 83 is impregnated with no lubricating oil). On the other hand, the change in volume as a result of thermal expansion/contraction of the lubricating oil is proportional to the total amount of lubricating oil filling the inner space of the housing 7 , so, due to the reduction in the total oil amount, it is possible to reduce the volume of the seal space S. Thus, in the dynamic bearing device 21 of this embodiment, it is possible to further reduce the axial dimension of the seal space S (thus, the seal member 9 ). Otherwise, this embodiment is the same as the first embodiment, so a redundant description thereof will be omitted. [0067] FIG. 8 shows a dynamic bearing device 31 according to a third embodiment of the present invention. The dynamic bearing device 31 of this embodiment differs from the dynamic bearing device 1 of the first embodiment in that the first thrust bearing portion T 1 is provided between the lower end surface 8 c of the bearing sleeve 8 and the upper end surface 2 b 1 of the flange portion 2 b of the shaft member 2 , and that the second thrust bearing portion T 2 is provided between the inner bottom surface 7 b 1 of the bottom portion 7 b of the housing 7 and the lower end surface 2 b 2 of the flange portion 2 b. [0068] As shown in FIG. 10 , dynamic pressure grooves 7 b 2 of, for example, a spiral configuration, are formed in the inner bottom surface 7 b 1 of the bottom portion 7 b constituting the thrust bearing surface of the second thrust bearing portion T 2 . The dynamic pressure grooves 7 b 2 are formed at the time of the injection molding of the housing 7 . That is, a groove pattern for forming the dynamic pressure grooves 7 b 2 is formed by machining at a predetermined position (i.e., position where the inner bottom surface 7 b 1 is to be formed) of the mold for molding the housing 7 , and the configuration of the groove pattern is transferred to the inner bottom surface 7 b 1 of the housing 7 at the time of the injection molding of the housing 7 , whereby it is possible to form the dynamic pressure grooves 7 b 2 simultaneously with the molding of the housing 7 . Further, the step portion 7 d is integrally formed at a position spaced apart from the inner bottom surface (thrust bearing surface) 7 b 1 axially upwards by a predetermined distance x. [0069] Further, as shown in FIG. 9 , dynamic pressure grooves 8 c 1 of a spiral configuration as shown, for example, in FIG. 3 ( b ), are formed in the lower end surface 8 c of the bearing sleeve 8 constituting the thrust bearing surface of the first thrust bearing portion T 1 . No dynamic pressure grooves are formed in the upper end surface 8 b of the bearing sleeve 8 . The upper end surface 8 b of the bearing sleeve 8 is opposed to the lower end surface 9 b of the seal member 9 through the intermediation of a gap larger than the thrust bearing gap. [0070] The thrust bearing gap of the first thrust bearing portion T 1 (indicated by symbol δ 1 ) and the thrust bearing gap of the second thrust bearing portion T 2 (indicated by symbol 52 ) can be controlled with high accuracy such that a relationship of x−w=δ 1 +δ 2 is obtained, where x is the axial dimension from the inner bottom surface 7 b 1 to the step portion 7 d of the housing 7 , and w is the axial dimension of the flange portion 2 b of the shaft member 2 . [0071] In the third embodiment, when the second bearing portion T 2 is formed by a so-called pivot bearing, the shaft member 2 used is one having no flange portion 2 b (i.e., one with a straight configuration). In this case, it is also possible for the seal member 9 to be formed integrally with the shaft portion 2 a of the shaft member 2 . Otherwise, this embodiment is the same as the first embodiment, so a redundant description thereof will be omitted. [0072] The above-mentioned embodiments adopt a construction in which a dynamic pressure action of a lubricant oil is generated by dynamic pressure grooves of a herringbone-like or a spiral configuration formed in the radial bearing portions R 1 , R 2 and the thrust bearing portions T 1 , T 2 ; the present invention, however, is not restricted to this construction. [0073] For example, it is also possible to adopt a so-called step bearing or a multi-arc bearing as the radial bearing portions R 1 , R 2 . [0074] FIG. 11 shows an example of a case in which one or both of the radial bearing portions R 1 , R 2 are formed by step bearings. In this example, a plurality of dynamic pressure grooves 8 a 3 in the form of axial grooves are provided at predetermined circumferential intervals in the region of the inner peripheral surface 8 a of the bearing sleeve 8 constituting the radial bearing surface. [0075] FIG. 12 shows an example of a case in which one or both of the radial bearing portions R 1 , R 2 are formed by multi-arc bearings. In this example, the region of the inner peripheral surface 8 a of the bearing sleeve 8 constituting the radial bearing surface is composed of three arcuate surfaces 8 a 4 , 8 a 5 , and 8 a 6 (i.e, so-called three-arc bearing). Respective centers of curvature of the three arcuate surfaces 8 a 4 , 8 a 5 , and 8 a 6 are offset by the same distance from the axial center O of the bearing sleeve 8 (i.e., the shaft portion 2 a ). In each of regions defined by the three arcuate surfaces 8 a 4 , 8 a 5 , and 8 a 6 , the radial bearing gap has a configuration gradually diminished in a wedge-like fashion in both circumferential directions. Thus, when the bearing sleeve 8 and the shaft portion 2 a make a relative rotation, the lubricating oil in the radial bearing gap is forced into the gradually diminished minimum gaps according to the direction of the relative rotation to undergo an increase in the pressure thereof. Due to the dynamic pressure action of the lubricating oil, the bearing sleeve 8 and the shaft portion 2 a are supported in a non-contact manner. It is also possible to form, in the border portions between the three arcuate surfaces 8 a 4 , 8 a 5 , and 8 a 6 , axial grooves called separation grooves, which are one step deeper. [0076] FIG. 13 shows another example of the case in which one or both of the radial bearing portions R 1 , R 2 are formed by multi-arc bearings. In this example also, the region of the inner peripheral surface 8 a of the bearing sleeve 8 constituting the radial bearing surface is formed by three arcuate surfaces 8 a 7 , 8 a 8 , and 8 a 9 (i.e., so-called three-arc bearing); in each of regions defined by the three arcuate surfaces 8 a 7 , 8 a 8 , and 8 a 9 , the radial bearing gap has a configuration gradually diminished in a wedge-like fashion in one circumferential direction. A multi-arc bearing of this construction is sometimes referred to as a tapered bearing. Further, in the border portions between the three arcuate surfaces 8 a 7 , 8 a 8 , and 8 a 9 , there are formed axial grooves 8 a 10 , 8 a 11 , and 8 a 12 called separation grooves, which are one step deeper. Thus, when the bearing sleeve 8 and the shaft portion 2 a make a relative rotation in a predetermined direction, the lubricating oil in the radial bearing gap is forced into a minimum gap diminished in wedge-like fashion to undergo an increase in the pressure thereof. Due to the dynamic pressure action of the lubricating oil, the bearing sleeve 8 and the shaft portion 2 a are supported in a non-contact manner. [0077] FIG. 14 shows another example of the case in which one or both of the radial bearing portions R 1 , R 2 are formed by multi-arc bearings. In this example, in the construction shown in FIG. 10 , predetermined regions 0 on the minimum gap side of the three arcuate surfaces 8 a 7 , 8 a 8 , and 8 a 9 are formed by concentric arcs whose center of curvature coincides with the axial center O of the bearing sleeve 8 (i.e., the shaft portion 2 a ). Thus, in each predetermined region θ, the radial bearing gap (i.e., minimum gap) is constant. A multi-arc bearing of this construction is sometimes referred to as a tapered flat bearing. [0078] While the multi-arc bearings of the above-mentioned examples are so-called three-arc bearings, this should not be construed restrictively; it is also possible to adopt a so-called four-arc bearing, a five-arc bearing, and further, a multi-arc bearing formed by six or more arcuate surfaces. In the case in which the radial bearing portion is formed by a step bearing or a multi-arc bearing, it is possible to adopt, apart from the construction in which two radial bearing portions are axially spaced apart from each other as in the case of the radial bearing portions R 1 , R 2 , a construction in which one radial bearing portion is provided to extend over the vertical region of the inner peripheral surface 8 a of the bearing sleeve 8 . [0079] Further, one or both of the thrust bearing portions T 1 , T 2 may be formed, for example, by a so-called step bearing, a so-called wave-type bearing (with an undulated step pattern), etc., in which a plurality of dynamic pressure grooves in the form of radial grooves are provided at predetermined circumferential intervals in the region constituting the thrust bearing surface. [0080] While in the above-mentioned embodiments a lubricating oil is used as the fluid filling the interior of the dynamic bearing device 1 and generating a dynamic pressure in the radial bearing gap between the bearing sleeve 8 and the shaft member 2 and in the thrust bearing gaps between the bearing sleeve 8 , the shaft member 2 , and the seal member 9 , it is also possible to use some other fluid capable of generating a dynamic pressure in the bearing gaps, for example, a gas such as air, or a magnetic fluid. [0081] Further, while in the above-mentioned embodiments the radial bearing surface is formed on the inner peripheral surface 8 a of the bearing sleeve 8 , it is also possible to form the radial bearing surface on the surface opposed thereto through the intermediation of the radial bearing gap, that is, on the outer peripheral surface 2 a 1 of the shaft portion 2 a . Further, while in the above examples the thrust bearing surfaces having dynamic pressure grooves are formed on the end surfaces 8 b , 8 c of the bearing sleeve, it is also possible to form the thrust bearing surfaces on the surfaces opposed thereto through the intermediation of the thrust bearing gaps, that is, on the lower end surface 9 b of the seal member 9 and on the upper end surface 2 b 1 of the flange portion 2 b of the shaft member 2 .

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