Fluid dynamic bearing device and motor equipped with the same

A fluid dynamic bearing device is mainly composed of a housing, a bearing sleeve, and a rotary member. The housing is formed by a side portion formed through injection molding of a resin material, and a bottom member fixed to a fixing portion provided in the inner periphery at one axial end of the side portion and adapted to close an opening at one axial end of the side portion. The fixing portion is a molding surface formed through solidification, in conformity with the inner surface of the mold, of the resin injected by the injection molding.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fluid dynamic bearing device rotatably supporting a shaft member by a dynamic pressure action in a non-contact support through the intermediation of a fluid (lubricating fluid) generated in a radial bearing gap. This bearing device is suitable for use in a spindle motor of an information apparatus, for example, a magnetic disc apparatus, such as an HDD or an FDD, an optical disc apparatus, such as a CD-ROM, a CD-R/RW, or a DVD-ROM/RAM, or a magneto-optical disc apparatus, such as an MD or an MO, the polygon scanner motor of a laser beam printer (LBP), or the small motor of an electric apparatus, such as an axial flow fan.

2. Description of the Related Art

Apart from high rotational accuracy, an improvement in speed, a reduction in cost, a reduction in noise, etc. are required of the motors as mentioned above. One of the factors determining such requisite performances is the bearing supporting the spindle of the motor. Recently, as such the bearing, use of a fluid dynamic bearing superior in the above requisite performances is being considered, or such a fluid bearing has been actually put into practical use.

For example, in a fluid dynamic bearing device to be incorporated into the spindle motor of a disk apparatus, such as an HDD, there is used a dynamic pressure bearing that is equipped with a radial bearing portion supporting a rotary member, which has a shaft portion and a flange portion, in the radial direction in a non-contact support, and a thrust bearing portion supporting the rotary member in the thrust direction in a non-contact support (see, for example, JP2002-61641A). In the dynamic pressure bearing, dynamic pressure grooves as dynamic pressure generating means are provided in the inner peripheral surface of a bearing sleeve constituting the radial bearing portion or in the outer peripheral surface of the shaft portion opposed thereto. Further, dynamic pressure grooves are provided in both end surfaces of the flange portion constituting the thrust bearing portions or in surfaces opposed thereto (e.g., an end surface of the bearing sleeve, or an end surface at the bottom of a housing).

As information apparatuses are improved in terms of performance, efforts are being made, regarding fluid dynamic bearing devices of this type, to enhance the machining precision and assembly precision of their components in order to secure the high bearing performance as required. On the other hand, as a result of the marked reduction in the price of information apparatuses, there is an increasingly strict demand for a reduction in the cost of fluid dynamic bearing devices of this type. Further, today, there is a fierce competition to develop information apparatuses reduced in size and weight, and as a result, there is also a demand for a reduction in the size and weight of fluid dynamic bearing devices of this type.

A housing is equipped with a substantially cylindrical side portion and a bottom portion closing an opening at one end of the side portion. The bottom portion may be formed integrally with the side portion, or separately from the side portion. In the latter case, a fixing portion is previously provided in the inner periphery of one end portion of the side portion, and a member (bottom member) constituting the bottom portion is fixed to this fixing portion by press-fitting, adhesive, etc.

In recent years, an attempt has been made to replace the housing, which has been a machined metal material, with an injection molding formed of a resin material in order to achieve a reduction in the cost and weight of fluid dynamic bearing devices. In particular, in the case of a housing whose bottom portion is formed as a separate bottom member, the realization of a resin housing is often attained by forming exclusively the side portion of a resin molding while forming the bottom member of a metal material, such as brass or an aluminum alloy as in the prior art. When thus forming exclusively the side portion of a resin molding, it is common practice to provide the gate of the injection resin molding at the portion constituting the fixing portion after injection molding. In this case, the fixing portion is formed by machining after cutting the gate.

It should be noted, however, that the dimensional accuracy required of the fixing portion of the side portion is generally strict, and the machining thereof must be conducted meticulously. Thus, an increase in machining cost is unavoidable, which means the advantage in terms of cost attained by using resin cannot be enjoyed to a sufficient degree. Further, there is a possibility of foreign matter generated as a result of machining remaining at the fixing portion. In that case, such foreign matter may enter the interior of the bearing device after the press-fitting of the bottom member and constitute a contaminant, thereby deteriorating the bearing performance.

SUMMARY OF THE INVENTION

It is an object of the present invention to achieve a reduction in the production cost of a fluid dynamic bearing device of this type, there by providing a less expensive fluid dynamic bearing device.

To achieve the above object, there is provided a fluid dynamic bearing device including: a housing formed as a bottomed cylinder; a bearing sleeve retained in an inner periphery of the housing; a rotary member having a shaft portion inserted into the bearing sleeve and adapted to rotate around a rotation center axis; and a radial bearing portion supporting the rotary member in a radial direction in a non-contact support by a dynamic pressure action of a fluid generated in a radial bearing gap between the bearing sleeve and the shaft portion of the rotary member, characterized in that: the housing has a side portion formed in a substantially cylindrical configuration of a resin material by injection molding, and a bottom member fixed to a fixing portion provided in an inner periphery of one axial end of the side portion and closing an opening at one axial end of the side portion; and the fixing portion is a molding surface formed through solidification, in conformity with an inner surface of a mold, of the resin material injected by the injection molding.

Here, the above-mentioned fluid (lubricating fluid) may be a liquid such as a lubricating oil (or lubricating grease) or a magnetic fluid, or a gas such as air.

By forming the side portion, which constitutes the housing, of an injection molding of a resin material, it is possible to produce a housing of higher precision and less weight as compared with the case in which it is formed of metal by machining, such as cutting, or press working. Further, since the fixing portion formed in the side portion and used to fix the bottom member is a molding surface formed through solidification, in conformity with the inner surface of the mold, of the resin injected by the injection molding, it is possible to form a high precision fixing portion easily and at low cost. Since the fixing portion is a smooth surface where no filler in the resin material is exposed, there is generated no minute foreign matter or the like which leads to contamination through friction when press-fitting the bottom member.

The gate through which the resin material (molten resin) is injected into the mold (cavity) may be provided at a position in the cavity other than the position corresponding to the fixing portion. In particular, it is desirable for the gate to be provided at a position corresponding to the inner peripheral edge portion on the axially opposite side of the fixing portion. With this construction, it is possible to fill the cavity uniformly with resin, thereby making it possible to perform high precision component molding. In this process, a gate cut portion as a gate mark is formed on the inner peripheral edge portion at one end of the side portion. This portion, however, is not directly related to the fixation of other components, so there is no need to separately perform finishing or the like. Thus, it is possible to reduce the number of steps and to achieve a reduction in the production cost of the housing. Regarding the gate configuration, there are no particular limitations as long as it allows the cavity to be uniformly filled with molten resin. It may be selected from a group consisting of a film gate, a point gate (inclusive of a multi-point gate), a disc gate, etc.

As stated above, on the inner peripheral edge portion of the portion on the side axially opposite to the fixing portion of the side portion, there is formed the gate cut portion as the gate mark. In this regard, when the axial dimension of the gate cut portion is larger than that of a chamfered portion provided on an outer peripheral edge of the bearing sleeve, resistance is offered when fixing the bearing sleeve to the housing, for example, when inserting the bearing sleeve into the housing. Further, abrasion powder attributable to the friction between the gate cut portion and the chamfered portion may be allowed to intrude into the bearing device to cause a deterioration in bearing performance. Thus, it is desirable for the axial dimension of the gate cut portion to be not larger than the axial dimension of the chamfered portion of the bearing sleeve.

Even after the housing and the bearing sleeve have been fixed to each other, the gate cut portion remains in the state in which it has been automatically cut by the opening/closing movement of the mold, or in the state in which it has been stamped by a punch after molding, that is, burrs, etc. are allowed to remain. It is not desirable to leave this state to stand since there is a fear of contamination due to falling of burrs, etc. On the other hand, an attempt to completely remove the burrs requires a meticulous machining operation, resulting in high cost.

In view of this, in the present invention, the gate cut portion is covered with an adhesive. This makes it possible to prevent falling of burrs, etc. by a simple measure, making it possible to solve the above problem at low cost.

There are no particular limitations regarding the resin material forming the housing as long as it is a thermoplastic resin allowing injection molding, and both an amorphous resin and a crystalline resin can be used. The resin material may be mixed as appropriate with a filler for imparting characteristics thereto, such as strength, moldability, and conductivity.

There are no particular limitations regarding the form of the radial bearing portion as long as it can generate pressure through the dynamic pressure action of a fluid. For example, it may be a dynamic pressure bearing equipped with dynamic pressure grooves of an axially inclined configuration, such as a herringbone-like configuration, a dynamic pressure bearing (multi-lobed bearing) having a plurality of wedge-like gaps in the radial bearing gap, or a dynamic pressure bearing (step bearing) in which a plurality of dynamic pressure grooves formed as axial grooves are provided at equal intervals in the circumferential direction.

The fluid dynamic bearing device, constructed as described above, can be suitably used in a motor having a rotor magnet and a stator coil, for example, a spindle motor for a disk apparatus, such as an HDD.

As described above, according to the present invention, it is possible to achieve a reduction in the production cost of the housing, and to enhance the bearing performance of a fluid dynamic bearing device without involving a problem such as contamination.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1shows a construction example of an information apparatus spindle motor in which a fluid dynamic bearing device1according to this embodiment is incorporated. This spindle motor is used in a disk drive apparatus, such as an HDD, and is equipped with the fluid dynamic bearing device1supporting rotatably in a non-contact support a rotary member3equipped with a shaft portion2, a stator coil4and a rotor magnet5opposed to each other through the intermediation, for example, of a radial gap, and a motor bracket (retaining member)6. The stator coil4is mounted to the outer periphery of the motor bracket6, and the rotor magnet5is mounted to the outer periphery of the rotary member3. A housing7of the fluid dynamic bearing device1is fixed to the inner periphery of the motor bracket6by press-fitting/adhesion, etc. One or a plurality of disc-like information recording media, such as magnetic disks, are retained by the rotary member3. When the stator coil4is energized, the rotor magnet5is rotated by an electromagnetic force generated between the stator coil4and the rotor magnet5, whereby the rotary member3and the shaft portion2rotate integrally.

As shown, for example, inFIG. 2, the fluid dynamic bearing device1has, as its main components, the housing7composed of a side portion7aand a bottom member10separate from the side portion7aand closing an opening at one axial end of the side portion7a, a bearing sleeve8retained inside the housing7, and the rotary member3making relative rotation with respect to the housing7and the bearing sleeve8. In the following description, for the sake of convenience, the bottom member10side will be referred to as the lower side, and the side axially opposite to the bottom member10will be referred to as the upper side.

The rotary member3is composed of a hub portion9covering, for example, the upper side of the housing7, and the shaft portion2inserted into the inner periphery of the bearing sleeve8.

The hub portion9is equipped with a disc portion9aprovided on top of the housing7, a cylindrical portion9bextending axially downwards from the outer peripheral portion of the disc portion9a, and a disk mounting surface9cand a flange portion9dthat are provided in the outer periphery of the cylindrical portion9b. A disc-like information recording medium (not shown) is fitted onto the outer periphery of the disc portion9a, and is placed on the disk mounting surface9c. The disc-like information recording medium is retained on the hub portion9by appropriate retaining means (not shown).

In this embodiment, the shaft portion2is formed integrally with the hub portion9, and has at its lower end a separate flange portion11as a detachment prevention member. The flange portion11is formed of metal, and is fixed to the shaft portion2by screws, etc.

The bearing sleeve8is 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. On the outer peripheral edges at both ends of the bearing sleeve8, there are formed chamfered portions8dwith an axial dimension of δ2 (seeFIG. 5). The material of the bearing sleeve8is not restricted to a sintered metal. For example, it is also possible to form the bearing sleeve8of a soft metal, such as brass.

As shown inFIG. 2, on an inner peripheral surface8aof the bearing sleeve8, there are provided, so as to be axially spaced apart from each other, two upper and lower regions constituting a first radial bearing portion R1and a second radial bearing portion R2. As shown, for example, inFIG. 3A, in the above-mentioned two regions, there are formed herringbone-shaped dynamic pressure grooves8a1and8a2, respectively. The upper dynamic pressure grooves8a1are formed asymmetrically with respect to an axial center m (axial center of the region between the upper and lower inclined grooves), with the axial dimension X1of the region on the upper side of the axial center m being larger than the axial dimension X2of the region on the lower side thereof. Further, in an outer peripheral surface8bof the bearing sleeve8, there are formed one or a plurality of axial grooves8b1so as to extend over the entire axial length. In this embodiment, there are formed three axial grooves8b1at equal circumferential intervals.

Further, as shown, for example, inFIG. 3B, in a partially annular region of the lower end surface8cof the bearing sleeve8constituting the thrust bearing surface of the thrust bearing portion T2, there are formed a plurality of dynamic pressure grooves8c1arranged, for example, in a spiral configuration.

The housing7is formed by the substantially cylindrical side portion7a, and the bottom member10serving as the bottom portion closing the lower opening of the side portion7a. The side portion7ais formed by injection molding of a resin material. Although not shown, in a partially annular region constituting the thrust bearing surface of the thrust bearing portion T1at the upper end surface7a1of the side portion7a, there are formed a plurality of dynamic pressure grooves arranged, for example, in a spiral configuration. A form corresponding to these dynamic pressure grooves is imparted to the surface of the mold for molding the side portion7a, and these grooves are formed simultaneously with the molding of the side portion7a.

As shown inFIG. 2, in the outer periphery of the side portion7a, there is formed a tapered outer wall7bgradually diverging upwards. Between the tapered outer wall7band the inner peripheral surface9b1of the cylindrical portion9b, there is formed an annular seal space S whose radial dimension gradually diminishes upwardly from the lower end of the housing7. During rotation of the shaft portion2and the hub portion9, the seal space S communicates with the outer periphery of the thrust bearing gap of the thrust bearing portion T1.

In the inner periphery of the lower end of the side portion7a, there is formed a fixing portion7c, to which the bottom member10is fixed. The inner peripheral surface7c1of the fixing portion7chas a larger diameter than the inner peripheral surface7dretaining the bearing sleeve8, and the wall thickness of the fixing portion7cis smaller than that of the disc portion of the side portion7a(the region other than the fixing portion7c). Further, in the inner periphery of the upper end of the side portion7a, there is formed an inner peripheral edge portion7a2. As shown inFIG. 4B, the inner peripheral edge portion7a2means the portion of the inner peripheral surface7dof the side portion7anear the upper end thereof, and includes the upper end of the inner peripheral surface7dforming the axial cylindrical surface, and a tapered portion7fadjacent thereto.

As stated above, the housing7(the side portion7a) is formed by injection molding of a resin material, so that it can be reduced in weight as compared, for example, with a conventional metal housing formed by machining, and further, each portion thereof can be formed with high precision. Further, it helps to achieve a reduction in the cost for forming the housing. Further, the dynamic pressure grooves are formed simultaneously with the injection molding, so that it is possible to achieve a reduction in the cost for forming the dynamic pressure grooves.

There are no particular limitations regarding the resin material for forming the side portion7aas long as it is a thermoplastic resin allowing injection molding. Examples of amorphous resins that can be used include polysulfone (PSU), polyether sulfone (PES), polyphenyl sulfone (PPSU), and polyether imide (PEI). Examples of crystalline resins that can be used include liquid crystal polymer (LCP), polyetherether ketone (PEEK), polybutylene terephthalate (PBT), and polyphenylene sulfide (PPS).

It is possible to mix a filler as appropriate into the above-mentioned resin materials to impart various characteristics thereto. For example, in order to achieve an improvement in strength and moldability, it is possible to mix a fibrous filler such as glass fiber, a whisker filler such as potassium titanate, or a scaly filler such as mica. Further, to impart conductivity to the resin material, it is possible to mix, for example, a fibrous or powdered conductive filler such as carbon fiber, carbon black, graphite, carbon nanomaterial, or metal powder. Such filler may be used singly, or two or more kinds of filler may be used in a mixed state.

The bottom member10is formed of a metal material, such as brass or aluminum alloy, and is fixed to the inner periphery of the fixing portion7cformed in the inner periphery of the lower end of the side portion7aby press-fitting, press-fitting/adhesion, etc.

FIGS. 4A and 4Bare conceptual drawings illustrating the injection molding process for forming the housing7(the side portion7a). In the mold, which is composed of a stationary mold and a movable mold, there are provided a cavity17, a gate17a, and a runner17b. The gate17a, which has an axial dimension of δ1, is provided at a position of the mold corresponding to the inner peripheral edge portion7a2of the side portion7a. As described below, in the example shown in the drawings, the gate17ais provided at a position corresponding to the upper end of the inner peripheral surface7dof the side portion7ain view of the convenience in stamping the gate17ain the axial direction with a punch. When, for example, some other gate cutting method is adopted, it is also possible to provide the gate17aat a position corresponding to the tapered portion7f.

A resin material (molten resin) P injected from the nozzle of an injection molding machine (not shown) passes the runner17band the gate17aof the mold to fill the cavity17. In this way, by filling the cavity17with molten resin P from the gate17aprovided at a position corresponding to the inner peripheral edge portion7a2in the inner periphery of the upper end of the side portion7a, the molten resin P fills the cavity17uniformly in the circumferential direction and in the axial direction, so it is possible to form the side portion7awith high precision. There are no particular limitations regarding the configuration of the gate17aas long as it allows the cavity17to be uniformly filled with the molten resin P. For example, it is possible to adopt, apart from a point gate (multi-point gate) provided at one or a plurality of positions in the circumference at a position corresponding to the inner peripheral edge portion7a2, an annular film gate, a disc gate, etc.

After the molten resin P filling the cavity17has been cooled and solidified, the movable mold is moved to open the mold. Then, the molding is extracted from the mold, and the gate17ais stamped with a punch, whereby, as shown inFIG. 4B, a gate cut portion7eas a gate mark of the gate17ais formed on the inner peripheral edge portion7a2. Like the gate17a, the gate cut portion7ehas an axial dimension of δ1.

Conventionally, the gate is provided at a position in the cavity17corresponding to the fixing portion7c. To fix the bottom member10to the fixing portion7cwith high accuracy by press-fitting, the dimensional precision required is rather strict, so finishing is performed through meticulous machining. As a result, an increase in processing cost is unavoidable, and the advantage in terms of cost achieved by using resin cannot be enjoyed to a sufficient degree. As is apparent fromFIG. 4A, in this embodiment, in contrast, the fixing portion7cis a molding surface obtained through solidification of the injection-molding resin in conformity with the inner surface of the mold. Thus, it is possible to form the fixing portion7cwith much higher accuracy and at a much lower cost than in the case of the conventional fixing portion obtained by machining.

Further, when machining is performed, the filler in the resin material, for example, is exposed on the surface to generate foreign matter. When such foreign matter remains at the fixing portion7c, the foreign matter may enter the interior of the bearing device to constitute a contaminant after press-fitting the bottom member10, thereby deteriorating the bearing performance. In this embodiment, in contrast, the fixing portion7cis a molding surface, so no filler is exposed on the surface, and generation of foreign matter attributable thereto is suppressed, thus making it possible to avoid danger of a deterioration in bearing performance.

Next, the process for assembling the fluid dynamic bearing device1of this embodiment will be described.

First, the bearing sleeve8is fixed to the inner peripheral surface7dof the side portion7aconstituting the housing7by press-fitting/adhesion, etc. As shown inFIG. 5, the axial dimension δ1 of the gate cut portion7eformed on the inner peripheral edge portion7a2at the upper end of the side portion7ais not larger than the axial dimension δ2 of the chamfered portion8dprovided on the outer peripheral edge at either end of the bearing sleeve8(δ1≦δ2). If δ1>δ2, resistance is offered when, for example, inserting the bearing sleeve8into the housing7. Further, there is a fear of the bearing sleeve8not being fixed at a predetermined position. Further, there is a fear of a deterioration in bearing performance due to generation of foreign matter attributable to the friction between the gate cut portion7eand the chamfered portion8d. In the construction of this embodiment, in contrast, no such problem is involved, and the bearing sleeve8can be press-fitted easily.

After the bearing sleeve8is fixed to the side portion7a, an adhesive M is caused to fill the space formed by the chamfered portion8dformed on the bearing sleeve8and the inner peripheral edge portion7a2of the side portion7aopposed thereto so as to cover the gate cut portion7e.

Even after the fixation, the gate cut portion7eremains in the state in which the gate17ahas been stamped with the punch and cut, that is, in the state in which burrs are allowed to stay. When the bearing device is operated with this state being left to stand, a portion (burrs) of the gate cut portion7emay enter the interior of the bearing device to constitute a contaminant, thereby deteriorating the bearing performance. If finishing is performed on the gate cut portion7eby machining or the like, it is necessary to meticulously conduct polishing, foreign matter removing operation after the polishing, etc., resulting in a rather high cost. In this embodiment, in contrast, it is possible to prevent falling of burrs and, by extension, generation of contaminant by the very simple and inexpensive means of filling with the adhesive M.

Next, the shaft portion2formed integrally with the hub portion9is inserted into the bearing sleeve8fixed to the side portion7a. After that, the flange portion11is mounted to the shaft portion2by, for example, screws, and then the bottom member10is fixed to the inner periphery of the fixing portion7cof the side portion7aby press-fitting or press-fitting/adhesion.

When the assembly is completed as described above, the shaft portion2of the rotary member3is inserted into the bore defined by the inner peripheral surface8aof the bearing sleeve8, and the flange portion11is accommodated in the space between the lower end surface8cof the bearing sleeve8and the upper end surface10aof the bottom member10. After that, the inner space of the fluid dynamic bearing device1, inclusive of the internal voids of the bearing sleeve8, is filled with a fluid (lubricating fluid), such as a lubricating oil. At this time, the oil level of the lubricating oil is maintained within the range of the seal space S.

In the fluid dynamic bearing1, constructed as described above, when the rotary member3(the shaft portion2) rotates, the upper and lower two regions of the inner peripheral surface8aof the bearing sleeve8constituting the radial bearing surfaces are opposed to the outer peripheral surface2aof the shaft portion2through the intermediation of the radial bearing gap. As the shaft portion2rotates, there is generated a dynamic pressure action due to the lubricating oil filling the radial bearing gap, and by this pressure, there are formed the first radial bearing portion R1and the second radial bearing portion R2supporting the shaft portion2rotatably in a non-contact support in the radial direction.

Further, between the upper end surface7a1of the side portion7aof the housing7and the lower end surface9a1of the hub portion9formed integrally with the shaft portion2, there is formed a thrust bearing gap (not shown). As the rotary member3rotates, there is generated in this thrust bearing gap a dynamic pressure action due to the lubricating oil, and by this pressure, there is formed the first thrust bearing portion T1supporting the rotary member3rotatably in a non-contact support in the thrust direction. Similarly, between the lower end surface8cof the bearing sleeve8and the upper end surface11aof the flange portion11, there is formed a thrust bearing gap, and in this bearing gap, there is generated a dynamic pressure action due to the lubricating oil, thus forming the second thrust bearing portion T2supporting the rotary member3in a non-contact support in the thrust direction.

As described above, in the construction of the present invention, when forming the housing7and assembling the fluid dynamic bearing device1, it is possible to eliminate all kinds of machining operation, so it is possible to achieve a substantial reduction in production cost. In the fluid dynamic bearing device1, constructed as described above, a thrust bearing gap (the thrust bearing portion T1) is formed between the upper end surface7a1of the housing7and the lower end surface9a1of the rotary member3(the hub portion9), whereby the seal space S is formed in the outer periphery of the housing7, thereby achieving a reduction in the size (thickness) of the bearing device. Further, due to the reduction in weight through formation of the housing7of a resin material, the bearing device is suitably applicable, in particular, to a spindle motor for a portable information apparatus.

The present invention is not restricted to the above-described construction. It is suitably applicable to any construction in which there is provided a fixing portion7cfor fixing a bottom member10to the inner periphery of one end portion of a resin side portion7a.FIG. 6shows an example of such a construction, in which, as in the above-described one, it is possible to form a light-weight and high-precision housing7while achieving a reduction in production cost. In the drawing, the components and elements that are the same as those of the embodiment shown inFIG. 2are indicated by the same reference symbols, and a redundant description thereof will be omitted.

The fluid dynamic bearing device1shown inFIG. 6has, as its main components, a shaft member12having a shaft portion12aat its rotation center and an outwardly protruding flange portion12bat one end thereof, a bearing sleeve8whose inner periphery allows insertion of the shaft portion12a, a housing7to the inner periphery of which the bearing sleeve8is fixed, and a seal member13sealing an opening at one end of the housing7. In the fluid dynamic bearing device1, a hub portion9(not shown), which is separate from the shaft member12, is fixed by press-fitting or the like to constitute the rotary member after the completion of the bearing device.

In this embodiment, the first thrust bearing portion T1is formed by the upper end surface12b1of the flange portion12band the lower end surface8cof the bearing sleeve8opposed thereto, and the second trust bearing portion T2is formed by the lower end surface12b2of the flange portion12band the upper end surface10aof the bottom member10opposed thereto. Further, a seal space S is formed between the outer peripheral surface12a1of the shaft portion12aand the inner peripheral surface13aof the seal member13. A radial groove13b1is formed in the lower end surface13bof the seal member13.

While in the constructions described above the dynamic pressure action of a fluid is generated by herringbone-shaped or spiral dynamic pressure grooves in the radial bearing portions R1and R2and the thrust bearing portions T1and T2, the present invention is not restricted to such constructions.

For example, it is also possible to adopt a so-called multi-lobed bearing or a step bearing as the radial bearing portion R1, R2.

FIG. 7shows an example in which one or both of the radial bearing portions R1and R2is formed by a multi-lobed bearing. In this example, the region of the inner peripheral surface8aof the bearing sleeve8constituting the radial bearing surface is composed of three arcuate surfaces8a3,8a4, and8a5(to form a so-called three-arc bearing). The centers of curvature of the three arcuate surfaces8a3,8a4, and8a5are offset from the axial center O of the bearing sleeve8by the same distance. In each of the regions defined by the three arcuate surfaces8a3,8a4, and8a5, the radial bearing gap gradually diminishes in a wedge-like fashion in both circumferential directions. Thus, when the bearing sleeve8and the shaft portion2(the rotary member3) make relative rotation, the lubricating fluid in the radial bearing gap is forced toward either of the minimum gap sides diminished in a wedge-like fashion according to the direction of the relative rotation, and undergoes an increase in pressure. Due to the dynamic pressure action of this lubricating fluid, the bearing sleeve8and the shaft portion2are supported in a non-contact support. It is also possible to form axial grooves that are one step deeper and called separation grooves in the boundary portions between the three arcuate surfaces8a3,8a4, and8a5.

FIG. 8shows another example of the case in which one or both of the radial bearing portions R1and R2are formed by multi-lobed bearings. In this example also, the region constituting the radial bearing surface of the inner peripheral surface8aof the bearing sleeve8are formed by three arcuate surfaces8a6,8a7, and8a8(to form a so-called three-arc bearing). In each of the regions defined by the three arcuate surfaces8a6,8a7, and8a8, the radial bearing gap is configured so as to gradually diminish in a wedge-like fashion in one circumferential direction. A multi-lobed bearing thus constructed is sometimes called a tapered bearing. Further, in the boundary portions between the three arcuate surfaces8a6,8a7, and8a8, there are formed axial grooves8a9,8a10, and8a11that are called separation grooves and that are one step deeper. Thus, when the bearing sleeve8and the shaft portion2make relative rotation in a predetermined direction, the lubricating fluid in the radial bearing gap is forced into the minimum gap portions diminished in a wedge-like fashion, and undergoes an increase in pressure. By the dynamic pressure action of this lubricating fluid, the bearing sleeve8and the shaft portion2are supported in a non-contact support.

FIG. 9shows another example of the case in which one or both of the radial bearing portions R1and R2are formed by multi-lobed bearings. In this example, predetermined regions α on the minimum gap side of the three arcuate surfaces8a6,8a7, and8a8in the construction shown inFIG. 8are formed by concentric arcs whose centers of curvature coincide with the axial center O of the bearing sleeve8. Thus, in each predetermined region α, the radial bearing gap (minimum gap) is constant. A multi-lobed bearing thus constructed is sometimes called a tapered flat bearing.

While each of the multi-lobed bearings of the above examples is a so-called three-arc bearing, this should not be construed restrictively. It is also possible to adopt a so-called four-arc bearing or five-arc bearing, or a multi-lobed bearing formed by six or more arcuate surfaces. Further, in forming the radial bearing portion by a multi-lobed 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 R1and R2, a construction in which a single radial bearing portion is provided to extend over the vertical region of the inner peripheral surface8aof the bearing sleeve8.

It is also possible for one or both of the radial bearing portions R1and R2to be formed by step bearings (not shown). A step bearing is a bearing in which, for example, in the region of the inner peripheral surface8aof the bearing sleeve8constituting a radial bearing surface, there are provided a plurality of dynamic pressure grooves in the form of a plurality of axial grooves provided at predetermined circumferential intervals.

Further, although not shown, one or both of the thrust bearing portions T1and T2may be formed by so-called step bearings, so-called undulated bearings (undulated step bearings) or the like, in which, for example, in the regions constituting thrust bearing surfaces, there are provided a plurality of dynamic pressure grooves in the form of radial grooves arranged at predetermined circumferential intervals.

While in the above embodiments a lubricating oil is used as the lubricating fluid filling the interior of the fluid dynamic bearing device1, it is also possible to use some other fluid capable of generating dynamic pressure in each bearing gap, for example, a magnetic fluid or a gas, such as air.

While the above-described dynamic pressure bearing has dynamic pressure generating means, such as dynamic pressure grooves, in the radial bearing portion, the construction of the present invention is also suitably applicable, apart from such a dynamic pressure bearing, to a fluid lubrication•bearing adopting a so-called cylindrical bearing having no dynamic pressure generating means in the radial bearing portion.