Fluid dynamic pressure bearing and spindle motor

A fluid dynamic bearing includes a shaft, a stationary portion supporting the shaft in a rotatable manner, a lubricating oil arranged in a radial gap defined between an outer circumferential surface of the shaft and a surface of the stationary portion, opposing each other. The fluid dynamic bearing includes a first seal portion arranged radially outside of the radial gap, and a second seal portion arranged at an axially upper end of the radial gap. The axial length of the first seal portion is greater than that of the second seal portion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fan for sending air in which a rotor of a motor is arranged in an impeller cup.

2. Description of the Related Art

Bearing devices using fluid dynamic pressure (hereinafter referred to as a dynamic pressure bearing) are conventionally used for a spindle motor of a signal recording and reproducing device such as a hard disk drive.

These days, the signal recording and reproducing devices such as hard disk drives are installed in portable devices such as portable music players, and there is a great demand for a signal recording and reproducing device which has more storage capacity, and reduced thickness and dimensions. In order to reduce the thickness and the dimensions of the signal recording and reproducing device, it is desirable to reduce the thickness and dimensions of the spindle motor.

By reducing the thickness of the spindle motor, however, an axial height of the dynamic pressure bearing is reduced as well, thereby reducing resistance of the dynamic pressure bearing against an external force.

SUMMARY OF THE INVENTION

In order to overcome the problems above, preferred embodiments of the present invention provide a fluid dynamic pressure bearing including a shaft centered on a center axis, a stationary portion having a bearing hole into which the shaft is inserted and arranged to support the shaft in a rotatable manner about the center axis, a radial gap located between opposing surfaces of the shaft and the stationary portion, a lubricating oil with which the radial gap is filled, a first seal portion at least partially in the form of an axially extending space and arranged within the stationary portion outside the radial gap in a radial direction that is substantially perpendicular to the center axis and connected to the radial gap, a second seal portion at least partially in the form of an axially extending space and arranged at an upper end of the radial gap and connected to the radial gap and the first seal portion, and a radial fluid dynamic pressure bearing located in a first gap as a portion of the radial gap and having a plurality of dynamic pressure generation grooves arranged to generate a dynamic pressure of the lubricating oil during rotation of the shaft. In the fluid dynamic pressure bearing, an axial length of the first seal portion is longer than that of the second seal portion.

Another preferred embodiment of the present invention provides a fluid dynamic pressure bearing including a shaft centered on a center axis, a stationary portion having a bearing hole into which the shaft is inserted and arranged to support the shaft in a rotatable manner about the center axis, a radial gap located between opposing surfaces of the shaft and the stationary portion, a lubricating oil with which the radial gap is filled, a first seal portion at least partially in the form of an axially extending space and arranged within the stationary portion outside the radial gap in a radial direction that is substantially perpendicular to the center axis and connected to the radial gap, a second seal portion at least partially in the form of an axially extending space and arranged at an upper end of the radial gap and connected to the radial gap and the first seal portion, and a radial fluid dynamic pressure bearing located in a first gap as a portion of the radial gap and having a plurality of dynamic pressure generation grooves arranged to generate a dynamic pressure of the lubricating oil during rotation of the shaft. In the fluid dynamic pressure bearing, a first boundary between the lubricating oil and air is located in the first seal portion, and a second boundary between the lubricating oil and air is located in the second seal portion axially above the first boundary.

Yet another preferred embodiment of the present invention provides a fluid dynamic pressure bearing including a shaft centered on a center axis, a stationary portion having a bearing hole into which the shaft is inserted and arranged to support the shaft in a rotatable manner about the center axis, a radial gap located between an outer surface of the shaft and an inner surface of the stationary portion, and a lubricating oil with which the radial gap is filled. In the fluid dynamic pressure bearing, the stationary portion includes a generally cylindrical sleeve having the bearing hole, a housing surrounding the sleeve, and a cover member having a plate-like portion located above the sleeve and a wall portion extending downward from an outer periphery of the plate-like portion. The plate-like portion has an inner surface opposed to the outer surface of the shaft. A first seal portion, at least partially in the form of an axially extending space, is located between the wall portion of the cover member and a portion of the stationary portion opposed thereto to be connected to the radial gap. A second seal portion, at least partially in the form of an axially extending space, is located at an upper end of the radial gap between the inner surface of the plate-like portion and the outer surface of the shaft to be connected to the radial gap and the first seal portion. A radial fluid dynamic pressure bearing formed in a first gap as a portion of the radial gap and having a plurality of dynamic pressure generation grooves arranged to generate a dynamic pressure of the lubricating oil during rotation of the shaft.

Further, another preferred embodiment of the present invention provides a fluid dynamic pressure bearing including a shaft centered on a center axis, a stationary portion having a bearing hole into which the shaft is inserted and arranged to support the shaft in a rotatable manner about the center axis, a radial gap located between an outer surface of the shaft and an inner surface of the stationary portion opposed thereto, and a lubricating oil with which the radial gap is filled. In the fluid dynamic pressure bearing, a first seal portion, at least partially in the form of a circumferentially extending space, is arranged outside the radial gap in a radial direction that is substantially perpendicular to the center axis and connected to the radial gap. In addition, a second seal portion, at least partially in the form of an axially extending space, is arranged at an upper end of the radial gap and connected to the radial gap and the first seal portion. A radial fluid dynamic pressure bearing is located in a first gap as a portion of the radial gap and has a plurality of dynamic pressure generation grooves arranged to generate a dynamic pressure of the lubricating oil during rotation of the shaft.

Other features, elements, steps, processes, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference toFIGS. 1A through 18, a bearing device and a spindle motor according to the preferred embodiments of the present invention will be described in detail.

In the description of the preferred embodiments of the present invention, words such as upper, lower, left, right, upward, downward, top, and bottom for describing positional relationships between respective members and directions merely indicate positional relationships and directions in the drawings. Such words do not indicate positional relationships and directions of the members mounted in an actual device. Additionally, in the following description, an axial direction indicates a longitudinal direction of a rotation axis, and a radial direction indicates a direction that is perpendicular or substantially perpendicular to the center axis. A radially outward direction indicates a direction departing from the center axis along the radial direction, and a radially inside direction indicates a direction approaching to the center axis along the radial direction.

First Preferred Embodiment

1-1 Description of Spindle Motor

FIG. 2is a cross-sectional view of a spindle motor1according to a first preferred embodiment of the present invention. The spindle motor1preferably includes a base plate2, a bearing device3attached to the base plate2, a stator4arranged on the base plate2so as to radially surround the bearing device3, and a rotor5attached to one end of a shaft31. The rotor5includes a hub6and a rotor magnet7, and the rotor magnet7is attached to an inner circumferential surface of a cylindrical portion of the hub6so as to oppose a magnetic pole of the stator4. By energizing the stator4, a rotary drive force is generated. The base plate2of the spindle motor1is attached to a case of a hard disk drive or the like.

1-2 Entire Structure of Bearing Device

FIG. 3Ais a cross-sectional view of the bearing device3andFIG. 3Bis a perspective view thereof.

The bearing device3includes a stationary portion having a substantially cylindrical sleeve33defining a bearing hole radially inside thereof and a cylindrical housing32with a base, and a rotational portion having a shaft31disposed radially inside of the sleeve33(i.e., the shaft31is inserted into the bearing hole).

As illustrated inFIG. 3A, a thrust plate34is arranged at an axially lower end portion of the shaft31such that an axially upper surface of the thrust plate34axially opposes a lower end surface of the sleeve33. In the present preferred embodiment of the present invention illustrated inFIG. 3A, a step41is arranged at a lower portion of an inner circumferential surface of the housing32such that the thrust plate34is housed at an inside bottom of the housing32. It should be noted that the step41may be omitted from the housing32when an outer diameter of the thrust plate34and an outer diameter of the sleeve33are approximately the same.

1-3 Lubricating Oil and Communicating Path

A first gap91awhich is a portion of a radial gap91is defined between an outer circumferential surface of the shaft31and a stationary portion (an inner circumferential surface of the sleeve33in the present preferred embodiment of the present invention). Similarly, gaps are provided between the thrust plate34, and the inner circumferential surface of the housing32, and between the lower end surface of the sleeve33and the axially upper surface of the thrust plate34. These gaps are filled with a lubricating fluid (i.e., lubricating oil) without interruption. In addition, an upper portion of the radial gap91and a lower portion of the radial gap91are communicated by a path42, described below.

A cover member35is fitted to an upper end side of the sleeve33. The cover member35includes a plate-like portion35aand a wall portion35baxially downwardly extending from a radially outer end of the plate-like portion35a. A radially inner surface of the plate-like portion35aradially opposes the outer circumferential surface of the shaft31. Between an outer circumferential surface of the wall portion35band the inner circumferential surface of the housing32opposing each other via an axially extending space, a first seal portion40in which a clearance therebetween is enlarged toward an opening end. In other words, the first seal portion40is arranged within the stationary portion outside the radial gap91in a radial direction that is perpendicular or substantially perpendicular to the center axis. Furthermore, the cover member35includes convex portions43(FIG. 3B) at which a portion of the plate-like portion35ais axially upwardly raised, formed by pressing, for example. Each of the convex portions43extends in a radial direction. When the cover member is attached to the bearing device, a clearance between the upper end surface of the sleeve33and the plate-like portion35ain the axial direction gradually increases toward a radially outer direction at a location axially below each of the convex portions43, defining first paths42ain which the lubricating oil may pass through. At least one of the upper end surface of the sleeve33and an axially lower surface of the plate-like member35amay have a tapered surface slanted toward each other. The first paths42aare a part of the path42, connecting the upper portion and the lower portion of the radial gap91.

On a radially outside surface of the sleeve33, three substantially planar areas44extending in an axial direction and are preferably arranged in a circumferentially spaced manner (seeFIG. 4). By fitting the sleeve33into the housing32, second paths42bare defined with clearances between the radially outside surface of the sleeve33and the inner circumferential surface of the housing32.

FIG. 5Ais a top plan view illustrating the bearing device3viewed from an axially upper side thereof.FIG. 5Bis a top plan view illustrating the bearing device from which the cover member35is removed. Dashed lines inFIG. 5Aindicate the locations of the planar areas44of the sleeve33. Radially outer ends of the convex portions43are respectively arranged axially above the corresponding planar areas44. With this configuration, each of the first paths42aand the second paths42bare connected, defining together the path42that is arranged to continuously connect the axially upper portion and the axially lower portion of the bearing device. In the bearing device, the path42and the radial gap91are filled with the lubricating oil. With this configuration, the lubricating oil smoothly circulates between the upper and lower portions of the radial gap.

As illustrated inFIG. 3A, when the step41arranged at the inner bottom portion of the housing32is abutted against the lower end surface of the sleeve33, the second paths42bmay be closed at the step41, thus failing to connect the path42with the radial gap91. In the present preferred embodiment of the present invention, a concave portion is arranged in an upper surface of the step41to connect the second paths42band the radial gap91at a lower end of the second paths42b, such that the lubricating oil smoothly circulates inside the bearing device.

The plate-like portion35aincludes a hole46(FIG. 6A) in which the shaft31is inserted. When the shaft31is inserted therein, the inner circumferential surface of the plate-like portion35aradially opposes the outer circumferential surface of the shaft31via a minute gap (i.e., an axially extending space) therebetween, defining a second seal portion. In the second seal portion, a second boundary51between the lubricating oil and air is arranged. An oil repellent agent preferably composed of a fluorocarbon resin may be applied to the outer circumferential surface of the shaft31and the surface of the cover member35which are adjacent to the second boundary51to form an oil repellent region.

As illustrated inFIGS. 1A and 1B, exemplary cross-sectional views of a portion of the bearing device near the second boundary, the oil-repelling region is formed (illustrated by double lines on the shaft31and the sleeve33) in the vicinity of the second boundary51. When the oil-repellant region extends into the clearance, as shown inFIG. 1B, the contact angle of the lubricating oil on the surface defining the second seal portion becomes greater, making the second boundary51unstable. By arranging the oil repellent region as illustrated inFIG. 1A, the contact angle of the lubricating oil becomes smaller, making the second boundary51stable. In preferred embodiments of the present invention, the contact angle is referred to as the equilibrium angle of contact of a liquid on a rigid surface, measured within the liquid at the contact line where three phases (liquid, solid, gas) meet.

Meanwhile, the first seal portion40is connected to the first paths42ain a lower end side of the plate-like portion35aof the cover member35. Up to the middle in the axial length of the first seal portion40is filled with the lubricating oil and thus the first boundary52is formed in the first seal portion51. In addition, as illustrated in drawings, an axial length of the first seal portion40is greater than that of the second seal portion.

In the bearing device3according to present preferred embodiment of the present invention, the lubricating oil filling the gaps between the outer circumferential surface of the shaft31and the inner circumferential surface of the sleeve33comes in contact with ambient air only at these first and second boundaries. The second boundary may be located axially above the first boundary. In order to customize this feature, radial widths of the first seal portion40and the second seal portion may be preferably modified.

1-4 Dynamic Pressure Bearing

As illustrated inFIG. 3A, in the inner circumferential surface of the sleeve33, a plurality of dynamic pressure generating grooves37,38are arrayed at two positions axially separated from each other, defining upper and lower radial dynamic pressure bearing mechanisms, respectively. Each of these dynamic pressure generating grooves preferably includes two portions. One of the two portions acts so as to increase a downward pressure in the lubricating oil, and the other acts so as to increase an upward pressure in the lubricating oil, during the rotation of the shaft31. The former portion is arranged at upper side of the latter, and thus high dynamic pressure is generated between them (e.g., at an axially middle portion between the two portions). With the high dynamic pressure, the shaft31is stably supported.

In the drawings, a double line drawn obliquely with respect to the axial direction indicates the existence of the dynamic pressure generating groove, and indicates that the pressure of the lubricating oil is increased toward the side on which the double line is spaced away from the bearing surface. The double line in the figure is cornered, and is farthest from the bearing surface at the corner, which indicates that at this portion, the highest pressure is generated.

Out of the two sets of radial dynamic pressure generating grooves37and38, groove37located on the upper side is not vertically symmetrical, but the portion increasing the pressure downward is preferably larger. Therefore, the dynamic pressure generating grooves37act so as to push the lubricating oil downward of the bearing, while it generates a shaft supporting force in the radial direction. The other set of radial dynamic pressure generating grooves38is symmetrical and two sets of thrust dynamic pressure generating grooves36are arranged to be symmetrical in the radial direction.

In the entire dynamic pressure generating grooves, the radial dynamic pressure generating grooves37generate a flow of lubricating oil, in which the lubricating oil flows downward through the radial gap and flows through the communicating path42back to a vicinity of the second boundary51which is the upper end portion of the radial bearing surface. This flow inhibits leakage of the lubricating oil from the second boundary51. Furthermore, it helps air bubbles or the like generated inside of the bearing to be discharged to the outside of the bearing via the communicating path42and the first seal portion40.

1-5 Manufacturing Method

Next, the cover member35will be described with reference toFIGS. 6A,6B,6C and7.

FIGS. 6A to 6Cillustrate the cover member35in the manufacturing process thereof. Firstly, a metal plate or a thermoplastic plate is pressed to form the plate into an approximately cylindrical shape with a lid, and a portion of the lid is punched out to form the hole46. During the pressing, the convex portions43extending in the radial direction are formed on the lid (seeFIG. 6A).

Then, a radially side surface of the cover member35is cut and lifted at three lifted positions48preferably in a circumferentially equally spaced manner. In the present preferred embodiment of the present invention, the three convex portions43preferably are circumferentially equally spaced, and the lifted portions48at which the radially side surface is cut and lifted are arranged circumferentially between the adjacent convex portions43. In other words, the lifted portions48and the convex portions43are alternately arranged at every 60 degrees, approximately. By lifting the lifted portions48, slits47(i.e., circumferential spaces) are formed in a circumferentially substantially equally spaced manner in the radially side surface of the cover member35(seeFIG. 6B).

The lifted portion48is then cut and connecting portions49are formed. Each of the connecting portions49radially protrudes so as to come in contact with the axially upper surface of the housing32when the cover member35is installed in the bearing device (seeFIG. 6C).

After the processing of the cover member35, the oil repellent agent is applied to an axially upper surface of the lid of the cover member35at an area around the hole46(i.e., on the plate-like portion35aat an area around the hole46) to provide an oil repellent property. It should be noted that the oil repellent agent is not applied to the inner circumferential surface defining the hole46and an axially lower surface of the plate-like portion35a.

The cover member35is arranged on the sleeve such that the connecting portions49come in contact with the housing32, and the connecting portions49are welded to fix the cover member35to the housing32. The connecting portion49may be welded by radiating converged energy beam thereto (e.g., laser welding and electron beam welding may be applicable). Alternatively, the connecting portion49may be fixed to the housing32by adhesion or fitting.

For injection of the lubricating oil into the bearing device, the lubricating oil may be dropped into the first seal portion after the cover member35is welded to the housing32. Alternatively, the cover member35may be welded after dropping the lubricating oil (seeFIG. 7).

The bearing device according to the present preferred embodiment of the present invention preferably includes two air-lubricating oil boundaries in the first and second seal portions. Through this configuration, the lubricating oil smoothly fills the gaps provided in the bearing device since air in the gaps is released via either one of first or second seal portions when the lubricating oil is injected via the other seal portion.

In a shaft31bof the bearing device illustrated inFIG. 7, an axially upper portion axially upwardly projecting from the hole46of the cover member35is slightly reduced in its diameter beginning at portion80. Through this configuration, when attaching the cover member, the outer circumferential surface of the shaft31band the inner circumferential surface50of the hole46do not easily come into contact with each other. The diameter of the shaft is smaller than that of the hole even if the diameter is not reduced at the axially upper portion of the shaft31b. It should be noted, however, with a small difference in the diameters of the shaft31band the hole46, the inner circumferential surface50of the cover member35may be damaged during the attachment process of the cover member35. This problem can be avoided by partially reducing the diameter of the shaft. The reduced diameter portion of the shaft31bmay not be necessarily continued across the region from an axially upper end of the shaft31bto a vicinity of the opening of the hole46. Even when it ends at a position located slightly axially upwardly away from the opening, the effect of preventing the contact between them may be attained. However, it is preferable that the diameter reduced portion of the shaft31bextends to vicinity of the opening as illustrated inFIG. 7. Through this configuration, the lubricating oil leakage is reduced even in case the oil-repellent layer in the vicinity of the opening is somehow removed. With the diameter reduced portion axially extending vicinity of the opening, the clearance between the shaft31band the cover member35becomes greater, and thus the second boundary becomes large, restricting the outflow of the lubricating oil.

Any preferable material (e.g., metal material) may be used for constructing the bearing device according to the present preferred embodiment of the present invention.

In the present preferred embodiment of the present invention, the housing is preferably made of aluminum alloy. The sleeve is preferably made of free-machining stainless steel, and after machining, any intervening substances may be removed by surface chemical treatment. Furthermore, the shaft is preferably made of martensitic stainless steel, and the cover member is preferably made of copper alloy or synthetic resin material such as liquid crystal polymer. Treatment for increasing attractiveness may be applied to the inner circumferential surface50defining the hole46of the cover member35as needed.

For the oil-repellent layer arranged on the cover member35at an area around the hole46, perfluoro resin may preferably be used. Furthermore, for the lubricating oil, an ester based compound may preferably be used as a base oil. It should be noted, however, the materials are not limited to those described above, and various modifications may be made.

Variation of the First Preferred Embodiment

With reference toFIGS. 8A,8B and9, the bearing device3′ according to a variant of the first preferred embodiment of the present invention will be described.FIG. 8Ais a top plan view illustrating the bearing device3′, andFIG. 8Bis a top plan view illustrating the bearing device3′ from which the cover member35′ is removed.

In the present variant, the connection portion49is located directly above the planar areas44of the sleeve33. In addition, a pair of convex portions43′, that is, convex portions43′aand43′b, radially extends on the plate-like portion35afrom portions above circumferential ends of each of the planner areas44.

FIG. 9Ais a perspective view illustrating a cover member35′ according to the variant of the first preferred embodiment of the present invention.FIG. 9Bis a cross sectional view illustrating the first boundary arranged between the cover member35′ and the sleeve33of the bearing device3′ according to the variant of the first preferred embodiment of the present invention.

As illustrated inFIG. 9B, the additional seal portions40′ are provided in a circumferentially extending space defined between an inner circumferential surface of the cover member35′ and the outer circumferential surface of the sleeve33, other than the first seal portion40. A portion of the first boundary52′ appears at the additional seal portions40′.

The lubricating oil circulating inside the bearing device flows from a portion with a narrow width of the seal portion40′ into first paths42a, and flows back to the upper portion of the bearing. Even if air bubbles are contained in the lubricating oil, it is difficult for the air bubbles to intrude into the region of the additional seal portion where the width is narrow. Thus, the air bubbles are shoved to the boundary52′ side located in the additional seal portion part40′ and discharged through the slit47. In this manner, since the first seal portion part40′ functions as an efficient sieve separating the air bubbles in the lubricating oil, the bearing device3′ can discharge the air bubbles in the lubricating oil to outside of the bearing device3′ efficiently.

Second and Third Preferred Embodiments

With reference toFIGS. 10A and 10B, a bearing device according to the second and third preferred embodiments of the present invention will be described.FIG. 10Ais a cross sectional view illustrating a bearing device83according to the second preferred embodiment of the present invention.FIG. 10Bis a cross sectional view illustrating the bearing device93according to the third preferred embodiment of the present invention.

As illustrated inFIG. 10A, by reducing an external diameter of a sleeve33′ at an axially upper portion thereof, the first seal portion may be formed without providing a slanted surface in a radially inner side of a housing32′. The sleeve33′ may be a copper based porous sintered body, and the tapered shape is formed simultaneously when the powder material is pressed.

InFIG. 10B, a path42′ may be defined with a through hole axially penetrating the sleeve33. Since the length of circulation route of the lubricating oil becomes short, the lubricating liquid smoothly circulates in the bearing device93, stabilizing the bearing property.

Fourth Preferred Embodiment

With reference toFIGS. 11A,11B, and11C, a bearing device according to a fourth preferred embodiment of the present invention will be described in detail.

FIG. 11Ais a cross sectional view illustrating a principle portion of a bearing device73according to the fourth preferred embodiment of the present invention.FIG. 11Bis a cross sectional view illustrating a portion of the bearing device73near the first boundary52in a magnified manner.FIG. 11Cis a cross sectional view illustrating a portion of the bearing device73near the second boundary51in a magnified manner.

As illustrated inFIG. 11A, the inner circumferential surface of the cover member35is chamfered. The inner circumferential surface opposes the outer circumferential surface of the shaft31to form a second seal portion39. In the present preferred embodiment of the present invention, a taper angle of θa of the second seal portion39is greater than a taper seal portion θb of the first taper seal portion. For example, a taper angle θa of the second seal portion39is about 34 degrees, and a taper angle θb of the first seal portion40is about 5 degrees. The angle θa and θb may be variously modified. θa may preferably be within the range of about 15 degrees to about 50 degrees, and θb may preferably be within the range of about 3 degrees to about 10 degrees, for example.

In order to stabilize the boundary, it is desirable that the surfaces defining the seal portion have sufficient wettability. The width W1of the second boundary51is narrower than a width W2of the first boundary52.

The oil-repellent layers60,62are arranged radially outside and axially upside of the second tapered seal portion. As illustrated inFIG. 11A, portions where the oil-repellent layers60,62are arranged are indicated by double lines. In the outer circumferential surface of the shaft31, an annular concave portion81is formed, and the oil-repellent layer62is arranged in the annular concave portion81. The effect of providing the annular concave portion81is analogous to that of the diameter reduced portion illustrated inFIG. 7. By providing the oil-repellent layer62in the annular concave portion, it is possible to prevent the oil-repellent layer62from being removed due to the contact between the cover member35and the shaft31.

As illustratedFIG. 11A, the oil-repellent layer60does not extend into the second seal portion39.

In the bearing device73, the outflow of the lubricating oil from the radial gap is inhibited by the second seal portion. The oil-repellent layer inhibits further movement toward the outside in the case where the boundary axially upwardly moves to an axially upper end of the seal portion.

Fifth Preferred Embodiment

With reference toFIGS. 12A and 12B, a fifth preferred embodiment of the present invention will be described in detail.FIG. 12Ais a cross sectional view illustrating a bearing device3baccording to the fifth preferred embodiment of the present invention.FIG. 12Bis a cross sectional view illustrating a portion of the bearing device3cnear the second boundary51in a magnified manner.

Unlike the bearing device3as illustrated inFIG. 3, the bearing device3cdoes not include the cover member35. As illustrated inFIG. 12A, an upper end portion of a radial gap91is connected to the path42dvia through a connecting path42c. In the upper end portion of the radial gap91, the second boundary51is provided. The oil repellent area90is arranged on a portion of the axially upper portion of the sleeve33′ at an area around the bearing cavity. A thrust plate34′ is arranged at an axially lower end portion of the shaft31such that an axially upper surface of the thrust plate34′ axially opposes a lower end surface of the sleeve33′.

The radial dynamic pressure bearing mechanisms are arranged so as to be axially separated from each other. An upper end of the upper radial dynamic pressure bearing mechanism37is arranged in the vicinity of the upper end of the radial gap91. Thus, the radial dynamic pressure bearing mechanism37can be arranged so as to be axially separated from the lower radial dynamic pressure bearing mechanism38. Since the distance between supporting points (at each point, the dynamic pressure is highest in each bearing mechanism) of the radial bearings37,38is enlarged, rigidity against an external force applied in a direction inclining the shaft31is increased. Unlike the bearing device3, the bearing device3cdoes not include the cover member35, thus the distance between the supporting points of the radial bearings becomes longer in an amount corresponding to the axial thickness of the cover member35.

The thickness of the cover member35is not significant in the case where a height of the bearing is comparatively large. However, in the thin bearing having a reduced axial height, the thickness of the cover member has an influence that cannot be ignored. In such an application, the bearing device3bmay be preferably applied.

Sixth Preferred Embodiment

Hereinafter, with reference toFIGS. 13A,13B, and13C, a bearing device according to a sixth preferred embodiment of the present invention will be described in detail. Note that inFIGS. 13A to 13C, elements similar to those illustrated in the foregoing description are denoted by similar or the same reference numerals and description thereof is omitted.FIG. 13Ais a cross sectional view illustrating a bearing device according to the sixth preferred embodiment of the present invention.FIG. 13Bis a cross sectional view illustrating a portion of the bearing device near a first boundary in a magnified manner.FIG. 13Cis a cross sectional view illustrating a portion of the bearing device near a second boundary in a magnified manner.

As illustrated inFIG. 13B, a first seal portion440is defined between a side wall portion435bof the cover member435and an upper portion of an outer circumferential surface of the housing432, radially opposing each other via a gap. A clearance of the gap in the radial direction gradually increases toward an axially lower direction. The lubricating oil meets the outside air in the first seal portion440, defining a first boundary452in the first seal portion440.

As illustrated inFIG. 13C, a second seal portion439is defined between an inner circumferential surface of a plate-like portion435aof the cover member435and an outer circumferential surface of the shaft431, radially opposing each other via a gap. A clearance of the gap in the radial direction gradually increases toward an axially upper direction. The lubricating oil meets the outside air in the second seal portion439, defining a second boundary451in the second seal portion439.

The first seal portion440and the second seal portion439are connected via first paths442a. The first paths442aare defined by a gap between an axially lower surface of the plate-like portion435aof the cover member435and the axially upper surface of the sleeve433.

Seventh Preferred Embodiment

Hereinafter, with reference toFIGS. 14A, and14B, a bearing device according to a seventh preferred embodiment of the present invention will be described in detail. Note that inFIGS. 14A and 14B, elements similar to those illustrated in the foregoing description are denoted by similar or the same reference numerals, and description thereof is omitted.FIG. 14Ais a cross sectional view illustrating a bearing device according to the seventh preferred embodiment of the present invention.FIG. 14Bis a cross sectional view illustrating a portion of the bearing device near a first boundary in a magnified manner.

As illustrated inFIG. 14B, a first seal portion540is defined between a side wall portion535bof the cover member535and an upper portion of an outer circumferential surface of the sleeve533, radially opposing each other via a gap. A clearance of the gap in the radial direction gradually increases toward an axially lower direction. The lubricating oil meets the outside air in the first seal portion540, defining a first boundary552in the first seal portion540.

A second seal portion539is defined between an inner circumferential surface of a plate-like portion535aof the cover member535and an outer circumferential surface of the shaft531provided in the housing532, radially opposing each other via a gap. A clearance of the gap in the radial direction gradually increases toward an axially upper direction. The lubricating oil meets the outside air in the second seal portion539, defining a second boundary551in the second seal portion539.

The first seal portion540and the second seal portion539are connected via first paths542a. The first paths542aare defined by a gap between an axially lower surface of the plate-like portion535aof the cover member535and the axially upper surface of the sleeve533.

Eighth Preferred Embodiment

Hereinafter, with reference toFIGS. 15A,15B, and15C, a bearing device according to an eighth preferred embodiment of the present invention will be described in detail. Note that inFIGS. 15A to 15C, elements similar to those illustrated in the foregoing description are denoted by similar or the same reference numerals, and description thereof is omitted.FIG. 15Ais a cross sectional view illustrating a bearing device according to the eighth preferred embodiment of the present invention.FIG. 15Bis a cross sectional view illustrating a portion of the bearing device near a first boundary in a magnified manner.FIG. 15Cis a cross sectional view illustrating a portion of the bearing device near a second boundary in a magnified manner.

As illustrated inFIG. 15B, a first seal portion640is defined between an outer circumferential surface of a side wall portion635bof the cover member635and a circumferential surface of the sleeve633radially opposing the outer circumferential surface of the side wall portion635avia a gap. A clearance of the gap in the radial direction gradually increases toward an axially upper direction. The lubricating oil meets the outside air in the first seal portion640, defining a first boundary652in the first seal portion640.

In the present preferred embodiment of the present invention, the cover member635includes a through hole635caxially penetrating the plate-like portion635aof the cover member635. The through hole635caxially connects the first seal portion and outside of the bearing device, and via the through hole635c, the lubricating oil is injected into the bearing device. The cover member635is fixed to the sleeve633and/or the housing632.

As illustrated inFIG. 15C, a second seal portion639is defined between an inner circumferential surface of a plate-like portion635aof the cover member635and an outer circumferential surface of the shaft631, radially opposing each other via a gap. A clearance of the gap in the radial direction gradually increases toward an axially upper direction. The lubricating oil meets the outside air in the second seal portion639, defining a second boundary651in the second seal portion639.

The first seal portion640and the second seal portion639are connected via a first path642a. The first path642ais defined by a gap between an axially lower surface of the plate-like portion635aof the cover member635and the axially upper surface of the sleeve633, and another gap between an inner circumferential surface of the wall portion635band a circumferential surface of the sleeve633radially opposing each other.

Ninth Preferred Embodiment

Hereinafter, with reference toFIGS. 16A,16B, and16C, a bearing device according to a ninth preferred embodiment of the present invention will be described in detail. Note that inFIGS. 16A to 16C, elements similar to those illustrated in the foregoing description are denoted by similar or the same reference numerals, and description thereof is omitted. FIG.16A is a cross sectional view illustrating a bearing device according to the ninth preferred embodiment of the present invention.FIG. 16Bis a cross sectional view illustrating a portion of the bearing device near a first boundary in a magnified manner.FIG. 16Cis a cross sectional view illustrating a portion of the bearing device near a second boundary in a magnified manner.

As illustrated inFIG. 16B, the sleeve733includes an inner cylindrical portion, an discoid portion radially outwardly extending from an axially upper portion of the inner cylindrical portion, and an outer cylindrical portion733daxially downwardly extending from a radially outer end of the discoid portion, adjacent a wall portion735bextending axially downwardly from a radially outer end of a plate-like portion735a. The outer cylindrical portion733dradially opposes the inner cylindrical portion of the inner cylindrical portion via a space therebetween, and an axially upper portion of the housing732is arranged in the space. A first seal portion740is defined between the outer cylindrical portion733dand an axially upper portion of the housing732, radially opposing each other via a gap. A clearance of the gap in the radial direction gradually increases toward an axially lower direction. The lubricating oil meets the outside air in the first seal portion740, defining a first boundary752in the first seal portion740.

As illustrated inFIG. 16C, a second seal portion739is defined between an inner circumferential surface of a plate-like portion735aof the cover member735and an outer circumferential surface of the shaft731, radially opposing each other via a gap. A clearance of the gap in the radial direction gradually increases toward an axially upper direction. The lubricating oil meets the outside air in the second seal portion739, defining a second boundary751in the second seal portion739.

The first seal portion740and the second seal portion739are connected via a first path742a. The first path742ais defined by a through hole733eand a gap between an axially lower surface of the plate-like portion735aof the cover member735and the axially upper surface of the sleeve733.

Variations of the Preferred Embodiments

Hereinafter, with reference toFIGS. 17A,17B, and17C, a bearing device according to variants of the first preferred embodiment of the present invention will be described in detail. Note that inFIGS. 17A to 17C, elements similar to those illustrated in the foregoing description are denoted by similar or the same reference numerals, and description thereof is omitted.FIG. 17Ais a cross sectional view illustrating a portion of the bearing device near a second boundary in a magnified manner according to a variant of the first preferred embodiment of the present invention.FIG. 17Bis a cross sectional view illustrating a portion of the bearing device near the second boundary in a magnified manner according to another variant of the first preferred embodiment of the present invention.FIG. 17Cis a cross sectional view illustrating a portion of the bearing device near the second boundary in a magnified manner according to yet another variant of the first preferred embodiment of the present invention.

As illustrated inFIG. 17A, a second seal portion839is defined between the inner circumferential surface of the plate-like portion835aand the outer circumferential surface of the shaft831, radially opposing each other via a gap. In the variant of the present preferred embodiment of the present invention, the clearance of the gap is substantially the same across the second seal portion839.

A concave portion70(a step70) is located at a radially inner end of the axially upper surface of the plate-like portion835a, at which the axially upper surface of the plate-like portion835ais indented axially downwardly. A second boundary851may be arranged in the second seal portion839or may be arranged in the concave portion70. With this configuration, outflow of the lubricating oil of the bearing device may be restricted. By providing another concave portion71radially outside of the concave portion70, the outflow of the lubricating oil of the bearing device is even more reliably restricted.

As illustrated inFIG. 17B, the oil-repellent layer860may be arranged on the plate-like portion835ato restrict the outflow of the lubricating oil.

As illustrated inFIG. 17C, an annular concave portion881may be arranged at a portion axially above the second seal portion839of the outer circumferential surface of the shaft831, and the oil-repellent layer may be arranged in the annular concave portion881. A second boundary862may be arranged in the second seal portion839or may be arranged in the concave portion70. The configuration restricts the axial flow of the lubricating oil along the outer circumferential surface of the shaft839, preventing the outflow of the lubricating oil.

With reference toFIG. 18, another variant of the preferred embodiments of the present invention will be described in detail. Note that inFIG. 18, elements similar to those illustrated in the foregoing description are denoted by similar or the same reference numerals, and description thereof is omitted.

As illustrated inFIG. 18, a second seal portion939is defined between an inner circumferential surface of the plate-like portion935aand a portion of an outer circumferential surface of the shaft931, radially opposing each other via a gap therebetween. The portion of the outer circumferential surface is slanted at portion931arelative to the center axis such that a clearance of the gap in the radial direction gradually increases toward axially upper direction. The lubricating oil meets the outside air in the second seal portion939, defining a second boundary951in the second seal portion939. Additionally, the inner circumferential surface may be slanted such that the clearance of the gap in the radial direction gradually increases toward an axially upper direction. An oil-repellent layer960may be arranged on the plate-like portion935ato restrict the outflow of the lubricating oil.

While various preferred embodiments of the present invention have been described in the foregoing, the present invention is not limited to the preferred embodiments detailed above, and various modifications are possible. To those skilled in the art, it will be apparent from the foregoing disclosure that various changes and modifications can be made herein without departing from the scope of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.