Hydrodynamic bearing device and disk rotating apparatus

In a hydrodynamic bearing device in which a radial bearing face having a dynamic pressure generating groove on a shaft or an inner periphery of a sleeve is provided and a clearance between the shaft and the sleeve is filled with lubricant, an annular depression is provided on one end face of the sleeve adjacent to a rotor hub and a cover plate for covering the depression is attached to the sleeve so as to define a reservoir for the lubricant or air for the purpose of preventing such a risk that absence of an oil film occurs in clearances of a bearing of the hydrodynamic bearing device due to outflow of oil upon forcing of the oil by air received into the bearing. A step portion is provided on the other end face of the sleeve such that the step portion and the reservoir are communicated with each other by a communication hole. During operation of the hydrodynamic bearing device, air in the hydrodynamic bearing device reaches the reservoir via the communication hole so as to be discharged from the reservoir.

TECHNICAL FIELD

The present invention relates to a hydrodynamic bearing device for use in a main shaft portion of a rotary device requiring high-speed and high precision rotations and a disk rotating apparatus provided with the hydrodynamic bearing device.

BACKGROUND ART

In recent rotational recording apparatuses employing a magnetic disk, etc., data-transfer velocity is rising upon increase of their storage capacity. Hence, since a disk rotating apparatus employed in such recording apparatuses requires high-speed and high precision rotations, a hydrodynamic bearing device is used in a rotary main shaft portion of the disk rotating apparatus.

Hereinafter, a conventional hydrodynamic bearing device is described with reference toFIGS. 18 and 19. InFIG. 18, a shaft31is rotatably inserted into a bearing bore32A of a sleeve32mounted on a base35. InFIG. 18, the shaft31has a flange33formed integrally at its lower end portion. The flange is received in a step portion32K of the sleeve32so as to rotatably confront a thrust plate34. A rotor hub36to which a rotor magnet38is secured to is attached to the shaft31. A plurality of disk39held by a spacer40and a damper41are mounted on the rotor hub36. A motor stator37confronting the rotor magnet38is mounted on the base35. Dynamic pressure generating grooves32B and32C are provided on an inner peripheral surface of the bearing bore32A of the sleeve32. Dynamic pressure generating grooves33A are provided on one face of the flange33, which confronts the step portion32K of the sleeve32, while dynamic pressure generating grooves33B are provided on the other face of the flange33, which confronts the thrust plate34. Clearances between the shaft31and the flange33on one hand and the sleeve32on the other hand, which include the dynamic pressure generating grooves32B,32C,33A and33B, are filled with oil42. One or more vent holes32E are provided on the sleeve32substantially in parallel with an axis of the sleeve32. A lower end of the vent holes32E communicates with a space which is disposed at a lower end portion of the sleeve32so as to contain the flange33. An upper end of the vent holes32E opens to an upper end face of the sleeve32.

Operation of the conventional hydrodynamic bearing device of the above described arrangement is described by referring toFIGS. 18 and 19. InFIG. 18, if the motor stator37is energized, a rotary magnetic field is generated and thus, the rotor magnet38, the rotor hub36, the shaft31and the flange33start rotations. At this time, a pumping pressure is generated in the oil42by the dynamic pressure generating grooves32B,32C,33A and33B. Thus, the shaft31is raised and is rotated without coming into contact with the thrust plate34and the inner peripheral surface of the bearing bore32A while being lubricated by the oil42. A magnetic head (not shown) is brought into contact with the disks39so as to perform recording and reproduction of electrical signals.

The above conventional hydrodynamic bearing device has the following problems.FIG. 19is a fragmentary sectional view including the shaft31and the sleeve32ofFIG. 18. As shown inFIG. 19, the shaft31is rotated in the bearing bore32A of the sleeve32while being lubricated by the oil42. When the hydrodynamic bearing device has been assembled or while the hydrodynamic bearing device is being transported, air lumps or air bubbles (hereinafter, referred to as “air43A or43B”) may penetrate into the oil42in the bearing bore32A. For example, in case ambient pressure has changed during transport in an aircraft, penetration of air bubbles may happen. If volume of the air43A penetrating into the vicinity of the dynamic pressure generating grooves32B and32cis expanded by rise of temperature or drop of atmospheric pressure, a portion of the dynamic pressure generating grooves32bis covered by air, thereby resulting in absence of the oil film. Meanwhile, a portion of the oil may leak out of the hydrodynamic bearing device as indicated by oil42B. Meanwhile, if the air43B penetrating into the vicinity of the flange33is expanded, the hatched oil42A in the vent hole32E may be pushed upwardly by expanded air43C so as to leak out of the hydrodynamic bearing device as shown by oil42D. If the oil42leaks outwardly, shortage of quantity of the oil in the bearing occurs. As a result, there is a risk of extreme aggravation of reliability due to contact of the shaft31with the sleeve32during rotation.

Meanwhile, also in case a drop impact load (acceleration) is applied to the conventional hydrodynamic bearing device in the direction of the arrow G1as shown inFIG. 19, there is a risk that the oil42leaks outwardly as shown by the oil42B.

DISCLOSURE OF INVENTION

The present invention has for its object to provide a hydrodynamic bearing device which is highly reliable by preventing lubricant such as oil filled in the hydrodynamic bearing device from leaking out of a bearing, and a disk rotating apparatus including the hydrodynamic bearing device.

A hydrodynamic bearing device of the present invention includes a sleeve having a bearing bore into which a shaft is rotatably inserted and a cover plate which is provided such that a reservoir for storing lubricant or air is defined in the vicinity of one end portion of the bearing bore. A substantially disklike flange is secured to one end portion of the shaft and has one face confronting one end face of the sleeve in the vicinity of the other end portion of the bearing bore. A thrust plate is provided so as to confront the other face of the flange and seal a region including the one end face of the sleeve. A communication path is formed for establishing communication between the reservoir and the region. First and second dynamic pressure generating grooves of a herringbone pattern are arranged in a direction along an axis of the shaft on at least one of an inner peripheral surface of the bearing bore of the sleeve and an outer peripheral surface of the shaft. A third dynamic pressure generating groove of a herringbone pattern is provided on at least one of opposed faces of the flange and the thrust plate and a fourth dynamic pressure generating groove of a herringbone pattern is provided on at least one of the one face of the flange and the one end face of the sleeve. Clearances between the shaft and the sleeve and between the flange and the thrust plate including the first, second, third and fourth dynamic pressure generating grooves are filled with lubricant. One of the sleeve and the shaft is mounted on a stationary base and the other of the sleeve and the shaft is mounted on a rotary member.

In the present invention, the reservoir and the covered region are communicated with each other by the communication path. Thus, the lubricant filled in the clearance between the shaft and the bearing bore of the sleeve is circulated by way of the communication path during operation of the hydrodynamic bearing device. By circulation of the lubricant, air such as air bubbles mixed into the lubricant is also circulated together with the lubricant. When the air bubbles contained in the lubricant have reached the reservoir during the circulation, the air bubbles are separated from the lubricant and are discharged outwardly. Since the reservoir is covered by the cover plate, the lubricant does not leak outwardly. Since the air in the lubricant is automatically removed during operation of the hydrodynamic bearing device in this way, air mixed into the lubricant during assembly of the hydrodynamic bearing device is also removed gradually and thus, only the lubricant is left in the hydrodynamic bearing device. The lubricant flows into the clearance between the shaft and the sleeve from the reservoir but does not leak outwardly. Hence, shortage of the lubricant or absence of the oil film does not occur between the shaft and the sleeve and thus, the hydrodynamic bearing device operates stably. Accordingly, the hydrodynamic bearing device having high long-term reliability can be materialized.

A hydrodynamic bearing device in another aspect of the present invention includes a shaft which has, at its one end portion, a thrust bearing face perpendicular to an axis of the shaft and a sleeve having a bearing bore into which the shaft is rotatably inserted such that the bearing bore acts as a radial bearing. A cover plate is provided such that a reservoir for storing lubricant or air is defined in the vicinity of one end portion of the bearing bore. A thrust plate is provided so as to seal the other end portion of the bearing bore and confront the thrust bearing face of the shaft. A communication path is formed for establishing communication between the reservoir and a region of the other end portion of the bearing bore. First and second dynamic pressure generating grooves of a herringbone pattern are arranged in a direction along the axis of the shaft on at least one of an inner peripheral surface of the bearing bore of the sleeve and an outer peripheral surface of the shaft. A third dynamic pressure generating groove of a herringbone pattern is provided on at least one of the thrust bearing face and one face of the thrust plate confronting the thrust bearing face. Clearances between the shaft and the sleeve and between the thrust bearing face and the thrust plate including the first, second and third dynamic pressure generating grooves are filled with lubricant. One of the sleeve and the shaft is mounted on a stationary base and the other of the sleeve and the shaft is mounted on a rotary member.

In the present invention, the reservoir and the covered region are communicated with each other by the communication path. Thus, the lubricant filled in the clearance between the shaft and the bearing bore of the sleeve is circulated by way of the communication path during operation of the hydrodynamic bearing device. By circulation of the lubricant, air such as air bubbles mixed into the lubricant is also circulated together with the lubricant. When the air bubbles contained in the lubricant have reached the reservoir during the circulation, the air bubbles are separated from the lubricant and are discharged outwardly. Since the air in the lubricant is automatically removed during operation of the hydrodynamic bearing device in this way, air mixed into the lubricant during assembly of the hydrodynamic bearing device is also removed gradually and thus, only the lubricant is left in the hydrodynamic bearing device. The lubricant flows into the clearance between the shaft and the sleeve from the reservoir but does not leak outwardly. Hence, shortage of the lubricant or absence of the oil film does not occur between the shaft and the sleeve and thus, the hydrodynamic bearing device operates stably. Accordingly, the hydrodynamic bearing device having high long-term reliability can be materialized.

In the present invention, since the third dynamic pressure generating groove is provided on the thrust bearing face of the shaft so as to form a thrust bearing portion, the construction is simplified without the need for provision of the flange.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of a hydrodynamic bearing device and a disk rotating apparatus having the hydrodynamic bearing device in the present invention are described with reference toFIGS. 1 to 17.

First Embodiment

A hydrodynamic bearing device according to a first embodiment of the present invention is described with reference toFIGS. 1 to 7.FIG. 1is a sectional view of the hydrodynamic bearing device of the first embodiment of the present invention, whileFIG. 2is an enlarged fragmentary sectional view showing a shaft1and a sleeve2. InFIG. 1, the sleeve2has a bearing bore2A and the cylindrical shaft1is rotatably inserted into the bearing bore2A. There is a minute clearance between an outer peripheral surface of the shaft1and an inner peripheral surface of the bearing bore2A of the sleeve2. Dynamic pressure generating grooves1B and1C of a known herringbone pattern in which each groove is bent at an angular portion are formed on at least one of the outer peripheral surface of the shaft1and the inner peripheral surface of the bearing bore2A of the sleeve2so as to act as a radial bearing portion. The radial bearing portion supports the shaft1in a radial direction of the shaft1. In the example ofFIG. 1, the dynamic pressure generating grooves1B and1C are formed on the inner peripheral surface of the bearing bore2A. Each of the dynamic pressure generating grooves1B and1C has the herringbone pattern. InFIG. 1, at least one of the dynamic pressure generating grooves1B and1C (the dynamic pressure generating grooves1B in the example ofFIG. 1) are formed such that a length of a lower groove1M from an angular portion1K is smaller than that of an upper groove1L from the angular portion1K as shown inFIG. 2.

InFIG. 1, a rotor hub12having a rotor magnet8is mounted on an upper end of the shaft1. A flange3having faces orthogonal to an axis of the shaft1and a diameter larger than that of the shaft1is integrally formed at a lower end of the shaft1. A thrust bearing face3F disposed at the lower face of the flange3confronts a thrust plate4fixed to the sleeve2. The thrust plate4seals an end portion region of the thrust bore2A of the sleeve2, which includes the flange3. Dynamic pressure generating grooves3B of a helical shape or a herringbone pattern are formed on one of a lower face of the flange3and an upper face of the thrust plate4(the lower face of the flange3inFIG. 1) so as to act as a thrust bearing portion.

Dynamic pressure generating grooves3A are formed on one of an outer peripheral portion of an upper face of the flange3and a step portion2D of the sleeve2, which confronts the outer peripheral portion of the upper face of the flange3(the upper face of the flange3inFIG. 1). A known large clearance portion2B is formed at an axially intermediate portion of the bearing bore2A of the sleeve2but is not relevant to the present invention directly, so that its description is abbreviated. The flange3is received in the step portion2D of the sleeve2. A clearance or a recess3C for storing oil is provided on the lower face of the flange3.

An annular upper depression2C surrounding the bearing bore2A is provided on an upper end face of the sleeve2. A ringlike cover plate5is attached to the sleeve2so as to cover the upper depression2C. An outer peripheral portion of the cover plate5is secured to an outer peripheral portion of the sleeve2by caulking or the like. An inner peripheral portion of the cover plate5is mounted so as to hold a small clearance with an upper end portion of the bearing bore2A of the sleeve2as will be described later in detail. A space or a clearance defined by the upper depression2C and the cover plate5is referred to as an upper reservoir15. Oil is stored in the upper reservoir15as necessary. In the upper reservoir15, dimension of the clearance interposed between the cover plate5and the sleeve2is not constant in the radial direction. Namely, the dimension is made sufficiently small at the opening15A confronting the outer peripheral surface of the shaft1, i.e., at an inner peripheral portion of the upper reservoir15and is made large at an outer peripheral portion of the upper reservoir15.

A first communication hole2E extending substantially in parallel with an axis of the bearing bore2A is provided on the sleeve2. The first communication hole2E communicates, at its upper end, with the upper reservoir15and communicates, at its lower end, with a space including the step portion2D of the sleeve2so as to form a communication path. The sleeve2is secured to a base6on which a motor stator7is mounted. A gap between the shaft1and the bearing bore2A of the sleeve2including a clearance between the shaft1and the sleeve2and a clearance between the flange3and the thrust plate4is filled with lubricant (hereinafter, referred to as “oil”)13. Since the oil13has a certain viscosity, air bubbles14may penetrate in between the shaft1and the bearing bore2A as shown inFIG. 2. The oil enters also the first communication hole2E and the upper reservoir15but a small amount of air (air bubbles)14are present in the first communication hole2E and the upper reservoir15. As shown inFIG. 1, a plurality of disks9are mounted on the rotor hub12by a spacer10and a damper11such that a disk rotating apparatus is constituted.

Operation of the hydrodynamic bearing device of the above described arrangement is described with reference toFIGS. 1 to 7, hereinafter. InFIG. 1, if the motor stator7is energized from a power source (not shown), a rotary magnetic field is generated and thus, the rotor hub12on which the rotor magnet7is mounted starts rotation together with the shaft1, the flange3, the disks9, the damper11and the spacer10. If the rotation is started, the dynamic pressure generating grooves1B,1C,3A and3B collect the oil13to predetermined locations so as to generate a known pumping pressure. Hence, the shaft1is raised and is rotated at high speed without coming into contact with the sleeve2and the thrust plate4.FIG. 2shows a state in which the air14mixes into the oil13during rotation of the hydrodynamic bearing device.

FIG. 3is a top plan view showing an example of the known dynamic pressure generating grooves3A provided on the upper face of the flange3, which confront the step portion2D of the sleeve2.FIG. 4is a top plan view showing an example of the known dynamic pressure generating grooves3B provided on the lower face of the flange3. The radially bent dynamic pressure generating grooves3A and3B shown inFIGS. 3 and 4collect the oil13so as to generate a thrust force parallel to the axis of the shaft1.

FIG. 5is an enlarged fragmentary sectional view showing the shaft1and the sleeve2of the hydrodynamic bearing device of this embodiment. InFIG. 5, “S1” denotes a dimension of a radial clearance of the dynamic pressure generating grooves1B and “S2” denotes a dimension of a radial clearance between the outer periphery of the shaft1and the cover plate5. An upper end portion2H of the bearing bore2A of the sleeve2has a diameter larger than that of the bearing bore2A. The “dimension of the radial clearance” is defined as a dimension of a clearance between the outer periphery of the shaft1and the inner periphery of the bearing bore2A at the time the axis of the shaft1is held in alignment with a central axis of the bearing bore2A of the sleeve2. “S3” denotes a dimension of the clearance15A of a portion of the upper reservoir15confronting the shaft1, i.e., an inner peripheral portion. “S4” denotes a dimension of a clearance at an outer peripheral portion of the upper reservoir15. In this embodiment, the dimensions S1and S2of the radial clearances and the dimensions S3and S4of the clearances are set so as to have the following relations.
S1<S2, S1<S3 and S3<S4

By setting the clearances as described above, the oil13stored in the upper reservoir15moves, by its surface tension, to the neighborhood of the opening15A of the clearance dimension S3smaller than the clearance dimension S4. From the opening15A of the clearance dimension S3, the oil13further enters the smaller radial clearance of the dimension S1between the shaft1and the bearing bore2A so as to flow, as shown by the arrow13A, to a region of the dynamic pressure generating grooves1B acting as the radial bearing portion.

In the dynamic pressure generating grooves1B and1C of the radial bearing, each of the dynamic pressure generating grooves1B has an upper groove1L and a lower groove1M from an angular portion1K and a length L of the upper groove1L is made larger than a length M of the lower groove1M such that a vertically asymmetric herringbone pattern is formed. Therefore, the oil13flowing into the radial clearance of the dimension S2between the shaft1and the upper end portion2H of the bearing bore2A is sucked, by pumping action at the time of start of the hydrodynamic bearing device and during rotation of the hydrodynamic bearing device, into the radial bearing between the bearing bore2A and the shaft1including the dynamic pressure generating grooves1B and1C. Thus, the oil13present in the upper reservoir15flows into the radial bearing as shown by the arrow13A. As a result, in the clearance between the shaft1and the bearing bore2A, the oil13flows in a direction indicated by the arrow13C. Therefore, the oil13disposed adjacent to the flange3is delivered so as to flow into the communication hole2E and reaches the upper reservoir15. Then, the oil13again flows from the opening15A between the cover plate5and the sleeve2into the radial bearing portion between the shaft1and the bearing bore2A so as to circulate through the hydrodynamic bearing device. By circulation of the oil13, the air bubbles14in the oil13also passes through the communication hole2E together with the oil13so as to reach the upper reservoir15. The air bubbles14which have reached the upper reservoir15are discharged outwardly from the clearance between the cover plate5and the sleeve2.

Discharge of air is described in more detail with reference toFIG. 6.FIG. 6is a fragmentary sectional view showing state of air which has entered the oil13in the hydrodynamic bearing device. If quantity of air14A formed by air bubbles or air lumps present in the hydrodynamic bearing device increases, internal pressure of the air14A rises upon rise of ambient temperature or the air14A is expanded by drop of atmospheric pressure, volume of the air14A increases. In such a case, the air14A enters the first communication hole2E from a lower inlet2F of the first communication hole2E and proceeds upwardly together with the oil13as indicated by air14D. The air14D which has reached an upper end2G of the first communication hole2E enters the upper reservoir15and is discharged outwardly from the small clearance between the cover plate5and the sleeve2as shown by the arrow C. In the first communication hole2E, the oil13also proceeds upwardly together with the air14D. However, after the oil13has been carried to the upper reservoir15, the oil13remains in the upper reservoir15due to its surface tension. Thus, only the air14D is discharged. Therefore, since the oil13is neither forced nor leaked out of the hydrodynamic bearing device, such a phenomenon that absence of the oil film is caused by shortage of the oil13does not happen, so that the hydrodynamic bearing device is rotated stably.

In a concrete example of the embodiment shown inFIG. 5, the shaft1has a diameter of 1 to 20 mm. The clearance dimension S3ranges from 30 to 150 microns. The dimension S1of the radial clearance of the radial bearing ranges from 1 to 10 microns. The first communication hole2E has a diameter of 0.3 to 1.0 mm. Experiments conducted by the present inventors have revealed that if the diameter of the shaft1, the clearance dimension S3, the dimension S1of the radial clearance and the diameter of the first communication hole2E fall in the respective ranges referred to above, the oil13is held in the hydrodynamic bearing device without leaking outwardly and only the air14is discharged outwardly.

As shown inFIG. 7which is a fragmentary sectional view similar toFIG. 6, the present inventors have made various tests in which a drop impact load or vibrations are applied in a direction of the arrow G2. The test results have shown that the oil13stored in the upper reservoir15is held in the upper reservoir15due to its surface tension without flowing out of the hydrodynamic bearing device. In the experiments, it has been found that even if an acceleration of 2,500 G is applied to the hydrodynamic bearing device for 1 to 10 msec by setting both of the clearance dimensions S2and S3to about 50 microns, the oil13does not leak.

In this embodiment, air such as air bubbles, which has entered the oil13in the hydrodynamic bearing device, proceeds to the upper reservoir15of the sleeve2by way of the first communication hole2E during operation of the hydrodynamic bearing device and is discharged out of the hydrodynamic bearing device therefrom. However, the oil13remains in the upper reservoir15without leaking outwardly. For example, since air which has entered the oil during manufacture of the hydrodynamic bearing device is also removed during use of the hydrodynamic bearing device, long-term reliability of the hydrodynamic bearing device is upgraded. Meanwhile, the single first communication hole2E is illustrated inFIG. 1but a plurality of the first communication holes2E may be provided on the sleeve2. As shown by the dotted lines inFIG. 6, a communication hole2Q for establishing communication between the upper reservoir15and space of the step portion2D may be provided between the outer periphery of the sleeve2and the base6in place of the first communication hole2E. In this case, a vertical groove may be provided on the outer periphery of the sleeve2as a portion of the communication hole2Q corresponding to the outer peripheral portion of the sleeve2.

Second Embodiment

FIG. 8is a fragmentary sectional view showing the shaft1and a sleeve20of a hydrodynamic bearing device according to a second embodiment of the present invention. InFIG. 8, a second communication hole20J for establishing communication between the first communication hole2E and a large clearance portion20B is provided at a central portion of the sleeve20. Other constructions of this hydrodynamic bearing device are similar to those of the hydrodynamic bearing device of the first embodiment shown inFIG. 1.

In order to form the second communication hole20J, there is, for example, a method in which as shown inFIG. 8, a hole is formed on the sleeve20in a direction of the arrow20H with a drill. After the hole has been formed on the sleeve20, a hole20K on the outer periphery of the sleeve20is sealed with a plug17.

In the hydrodynamic bearing device of this embodiment, the first communication hole2E communicates with space between the dynamic pressure generating grooves1B and the dynamic pressure generating grooves1C via the second communication hole20J. Thus, the oil13flows from the upper reservoir15into a portion of the dynamic pressure generating grooves1B as shown by the arrow13A and flows also from the second communication hole20J into the portion of the dynamic pressure generating grooves1B as shown by the arrow13D. The oil13which has flowed into the portion of the dynamic pressure generating grooves1B in the direction of the arrow13D flows together with the oil13having flowed into the portion of the dynamic pressure generating grooves1B in the direction of the arrow13A and returns from a lower inlet20F to the first communication hole2E through a space between the shaft1and the bearing bore20A including the dynamic pressure generating grooves1B and1C. Air mixed into the oil13is separated from the oil13when the oil13flows into the second communication hole20J as shown by the arrow13G. The separated air14proceeds in a direction of the arrow14F and is discharged outwardly by way of the upper reservoir15.

In this embodiment, since the oil13is displaced vigorously by providing the second communication hole20J, removal of the air14from the oil13is performed efficiently. As a result, reliability of the hydrodynamic bearing device is upgraded further. Meanwhile, even if the air14has entered the oil13in the hydrodynamic bearing device for some reason, the air14is discharged out of the hydrodynamic bearing device rapidly, so that reliability of the hydrodynamic bearing device becomes high.

Third Embodiment

FIG. 9is a fragmentary sectional view showing a shaft30and the sleeve2of a hydrodynamic bearing device according to a third embodiment of the present invention. InFIG. 9, a small diameter portion30A having a diameter smaller than that of the shaft30is provided on the shaft30in the vicinity of an end portion of the shaft30coupled with the rotor hub12. A diameter of an inner peripheral edge25A of a ringlike cover plate25is larger than that of the small diameter portion30A but is smaller than that of the shaft30. Namely, the cover plate25is arranged to cover the clearance between the shaft30and the sleeve2. Other constructions of this hydrodynamic bearing device are similar to those of the first embodiment shown inFIG. 1. By this arrangement, it is possible to further positively prevent outward leakage of the oil13from the clearance between the shaft30and the cover plate25. Meanwhile, since the diameter of the inner peripheral edge25A of the cover plate25is smaller than that of the shaft30, the shaft30is not detached from the bearing bore2A of the sleeve2. Namely, the cover plate25functions to prevent detachment of the shaft30.

InFIG. 9, it is supposed that “S1” denotes a dimension of a radial clearance in the neighborhood of the dynamic pressure generating grooves1B and “S2” denotes a dimension of a radial clearance between the shaft30and the upper end portion2H of the sleeve2. The upper end portion2H of the bearing bore2A of the sleeve2has a diameter lager than that of the bearing bore2A. In the upper reservoir15defined by the cover plate25and the upper depression2C of the sleeve2, it is supposed that “S3” denotes a dimension of a clearance of an inner peripheral portion of the upper reservoir15and “S5” denotes a dimension of an axial clearance between the cover plate25and an upper end of the shaft30. It is supposed that “S6” denotes a dimension of a radial clearance between the small diameter portion30A of the shaft30and the inner peripheral edge25A of the cover plate25. In this embodiment, the dimension S1is set to be smaller than the dimensions S2, S3, S5and S6, i.e., S1<S2, S1<S3, S1<S5and S1<S6. Oil has a property to flow into a smallest clearance by its surface tension. Thus, by setting the dimensions S1, S2, S3, S5and S6as described above, the oil13stored in the upper reservoir15flows into the clearance of the smallest dimension S1between the shaft30and the bearing bore2A. As a result, since the oil13flows to regions of the dynamic pressure generating grooves1B and1C sufficiently, absence of the oil film does not occur. Meanwhile, the dimensions S2, S3, S5and S6are set such that the dimension S2is smaller than the dimension S6, the dimension S3is smaller than the dimension S6and the dimension S5is smaller than the dimension S6, i.e., S2<S6, S3<S6and S5<S6. By setting the dimensions S2, S3, S5and S6as described above, the oil13does not flow out of the clearance of the largest dimension S6between the small diameter portion30A and the inner peripheral edge25A of the cover plate25

In the hydrodynamic bearing device of this embodiment, a ventilation port25B is provided on the cover plate25. In a plane containing the cover plate25, the ventilation port25B is disposed so as to circumferentially deviate by 180 degrees inFIG. 1from a mouth of the first communication hole2E opening to the upper reservoir15. If the ventilation port25B is aligned with the mouth of the first communication hole2E, such an incident may happen that when air rising through the first communication hole2E is discharged from the ventilation port25B, the oil13is also expelled outwardly. This expulsion of the oil13can be prevented by circumferentially shifting the ventilation port25B and the first communication hole2E from each other as described above. Air which has flowed out of the upper end of the first communication hole2E travels circumferentially in the upper reservoir15along the cover plate25and runs outwardly when the air has reached the ventilation port25B.

Fourth Embodiment

FIG. 10is a fragmentary sectional view showing a shaft35and the sleeve2of a hydrodynamic bearing device according to a fourth embodiment of the present invention. InFIG. 10, dynamic pressure generating grooves35D are formed on a lower end face35C of the shaft35. Therefore, the flange3of the hydrodynamic bearing device of the third embodiment shown inFIG. 9is not provided on the shaft35. Other constructions of this hydrodynamic bearing device are substantially similar to those of the hydrodynamic bearing device ofFIG. 9. At an end portion of the shaft35, on which the rotor hub12is mounted, the shaft35has a small diameter portion35A. A diameter of the inner peripheral edge25A of the cover plate25is set to be larger than an outside diameter of the small diameter portion35A and smaller than an outside diameter of the shaft35. Namely, the inner peripheral edge25A of the cover plate25is arranged to cover a clearance between the shaft35and the bearing bore2A of the sleeve2. Thus, it is possible to positively prevent leakage of the oil13from the upper clearance between the shaft35and the sleeve2inFIG. 10.

The dynamic pressure generating grooves35D which may be formed on one of the lower end face35C of the shaft35and an upper face of the thrust plate4are formed on the lower end face35C of the shaft35inFIG. 10and confront the thrust plate4so as to constitute a thrust bearing with the thrust plate4. InFIG. 10, the step portion2D is formed on the underside of the sleeve2. An end portion of the bearing bore2A including the step portion2D of the sleeve2is sealed by the thrust plate4. A space between the step portion2D and the thrust plate4communicates with the first communication hole2E at the lower inlet2F. The first communication hole2E acts as a communication path for establishing communication between the step portion2D and the upper reservoir15.

In the hydrodynamic bearing device of this embodiment, the dynamic pressure generating grooves35D are provided on the lower end face35C of the shaft35without providing the flange on the shaft35. Hence, in comparison with the foregoing embodiments, the construction is simpler and thus, is cheaper.

Also in the hydrodynamic bearing device of this embodiment, dynamic pressure generating grooves2B and2C formed by shallow grooves of a herringbone pattern are provided on at least one of the outer peripheral face of the shaft35and the inner peripheral surface of the sleeve2(on the inner peripheral surface of the sleeve2inFIG. 10) and the clearance between the shaft35and the sleeve2is filled with the oil13in the same manner as the foregoing embodiments. The upper reservoir15is provided on the sleeve2in the vicinity of the upper end face of the sleeve2and communicates with a space adjacent to the lower end face35C of the shaft35through the first communication hole2E. Thus, the oil13circulates in a path in which the oil13flows from the upper reservoir15into the clearance between the shaft35and the sleeve2and returns from the lower portion of the sleeve2to the upper reservoir15via the first communication hole2E. Since air mixed into the oil13is discharged outwardly from the ventilation port25B of the cover plate25during operation of the hydrodynamic bearing device, the air in the oil13is eliminated and thus, absence of the oil film does not occur in the clearance around the shaft35. Thus, the hydrodynamic bearing device of this embodiment can preserve high reliability for a long term. Meanwhile, a disk rotating apparatus employing the hydrodynamic bearing device of this embodiment has high long-term reliability.

Also in the hydrodynamic bearing device of this embodiment, since air mixed into the oil13is readily discharged outwardly, absence of the oil film, which is apt to happen in the hydrodynamic bearing device, is prevented, so that long service life and high long-term reliability are obtained.

Fifth Embodiment

FIG. 11is a fragmentary sectional view showing the shaft35and the sleeve2of a hydrodynamic bearing device according to a fifth embodiment of the present invention. InFIG. 11, the hydrodynamic bearing device of this embodiment has a construction similar to that of the hydrodynamic bearing device of the fourth embodiment shown inFIG. 10except that a cover plate27is different from the cover plate25of the fourth embodiment.

The cover plate27of this embodiment is shown in a perspective view ofFIG. 12aand a sectional view ofFIG. 12balong the line XIIb-XIIb. As shown inFIGS. 12aand12b, the cover plate27has, on its lower face, at least one recess27E. A boss27H is formed at a portion of an upper face of the cover plate27, which portion corresponds to the recess27E. A ventilation port27F is provided at a substantially central portion of the recess27E. The cover plate27is attached to the sleeve2such that the recess27E confronts the upper reservoir15.

In this embodiment, the clearance between the cover plate27and the upper reservoir15becomes large at the recess27E of the cover plate27. The oil13in the upper reservoir15is least likely to flow into the large clearance below the recess27E due to its surface tension and thus, remains in a portion of a small clearance surrounding the recess27E. Hence, since the vetilation port27F disposed at the central portion of the recess27E is not covered by the oil13, air in the upper reservoir15is discharged smoothly from the vetilation port27F.

In the hydrodynamic bearing device of this embodiment, supposing that “S1” denotes a dimension of a radial clearance between the bearing bore2A of the dynamic pressure generating grooves1B and the shaft35, “S2” denotes a dimension of a radial clearance between an outer periphery of the shaft35and an inner periphery of the upper end portion2H of the sleeve2, “S3” denotes a dimension of a clearance between the cover plate27and an end portion of the shaft35and “S4” denotes a dimension of a clearance at an outer peripheral portion of the upper reservoir15, the dimensions S2and S3are set to be larger than the dimension S1, i.e., S1<S2and S1<S3. Meanwhile, the dimension S4is set to be larger than the dimension S3, i.e., S4>S3. As a result, the oil13in the upper reservoir15gathers to a vicinity of an opening27A having the clearance of the small dimension S3due to its surface tension and then, flows into the clearance (radial bearing portion) having the smaller dimension S1between the shaft35and the bearing bore2A.

Each of the dynamic pressure generating grooves1B has the upper groove1L and the lower groove1M, which have the dimensions L and M from the angular portion1K such that the upper groove1L is larger than the lower groove1M. Hence, the oil13which has flowed into the clearance of the dimension S2between the upper end portion2H of the sleeve2and the shaft35is drawn into the radial clearance of the dimension S1between the shaft35and the bearing bore2A of the sleeve2by pumping action of the dynamic pressure generating grooves1B at the time of start of operation of the hydrodynamic bearing device and during operation of the hydrodynamic bearing device. By this action, the oil13in the upper reservoir15is caused to flow into the radial bearing positively.

Air in the form of minute air bubbles is mixed into the lubricant such as the oil13filled in the clearance between the sleeve2and the shaft35. If quantity of the air mixed into the oil13is large, the air bubbles are expanded by rise of internal pressure of the air bubbles upon rise of ambient temperature or the air bubbles are expanded in an environment of low pressure, volume of the air increases. The air whose volume has increased enters the first communication hole2E from the lower inlet2F of the first communication hole2E as indicated by air14. InFIG. 11, the air14proceeds further upwardly so as to enter the upper reservoir15. The air14which has entered the upper reservoir15travels circumferentially and is discharged outwardly from the vetilation port27F as shown by the arrow C when the air14has reached the recess27E. At this time, the oil13is also displaced together with the air14in the first communication hole2E. However, since the oil13which has been carried to the upper reservoir15is separated from the air14, only the oil13remains in the upper reservoir15due to its surface tension, while the air14is discharged from the vetilation port27F as shown by the arrow C. Hence, the oil13is neither forced nor leaked out of the hydrodynamic bearing device. Therefore, the hydrodynamic bearing device can be rotated stably without being subjected to absence of the oil film.

In this embodiment, the shaft35has a diameter of 1 to 20 mm and the dimension S3of the clearance ranges from 30 to 150 microns. The dimension S1of the radial clearance of the radial bearing ranges from 1 to 10 microns and the first communication hole2E has a diameter of 0.3 to 1.0 mm. In the hydrodynamic bearing device having the dimensions set in the above ranges, it has been confirmed that the oil13is satisfactorily held in the respective clearances of the hydrodynamic bearing device and the air14is discharged favorably.

InFIG. 11, even if a drop impact load or vibrations are applied in a direction of the arrow G2, the oil13stored in the upper reservoir15is held in the upper reservoir15due to its surface tension without flowing outwardly.

Experiments conducted by the present inventor have revealed that even if an acceleration of 2,500 G is applied for 1 to 10 msec by setting the dimensions S2and S3to about 50 microns, the oil13does not leak and rotation of the hydrodynamic bearing device can be continued by holding the shaft35and the sleeve3out of contact with each other.

Sixth Embodiment

FIG. 13is a fragmentary sectional view showing the shaft35and the sleeve2of a hydrodynamic bearing device according to a sixth embodiment of the present invention. In this embodiment, a cover plate28is different from the cover plate27of the hydrodynamic bearing device of the fifth embodiment shown inFIG. 12. Other constructions of this hydrodynamic bearing device are similar to those shown inFIG. 1. As shown in a perspective view ofFIG. 14aand a sectional view ofFIG. 14b, the cover plate28of this embodiment is formed with a bulge portion28D by partially bulging an inner peripheral portion of the ringlike cover plate28. A recess28E is formed on one face of the cover plate28opposite to the bulge portion28D. The cover plate28is attached to the sleeve2such that the recess28E confronts the upper end portion2H on the end face of the sleeve2. In an area in which the recess28E and the end face of the sleeve2confront each other, clearance between the recess28E and the end face of the sleeve2increases by a size of the recess28E. Thus, the oil13does not gather to a vicinity of the recess28E due to its surface tension. Therefore, air which has reached the upper reservoir15by way of the first communication hole2E travels in the annular upper reservoir15circumferentially and is smoothly discharged from the recess28E when the air has reached the recess28E. Since the oil13proceeds to the small clearance due to its surface tension and does not gather to a vicinity of the recess28E having the large clearance, leakage of the oil13out of the recess28E does not occur.

In this embodiment, the recess28E can be formed by such a simple working in which the inner peripheral portion of the cover plate28is recessed.

FIG. 15is a perspective view showing a cover plate40as another example of the cover plate28of the hydrodynamic bearing device of this embodiment. Other constructions than the cover plate40are similar to those ofFIG. 13. InFIG. 15, the ringlike cover plate40is formed, at its inner peripheral portion, with a notch40E. Since a part of the upper end portion2H of the inner peripheral portion of the sleeve2is connected to an outside by the notch40E, air is discharged smoothly through the notch40E. The notch40E can be formed by performing simple working, for example, simultaneously with press working of the cover plate40. Since the notch40E can be worked easily, manufacturing cost of the cover plate40is also reduced.

Seventh Embodiment

FIG. 16is a fragmentary sectional view showing the shaft35and the sleeve2of a hydrodynamic bearing device according to a seventh embodiment of the present invention.FIG. 17is a sectional view along the line XVII-XVII inFIG. 16. In this embodiment, a cover plate41is structurally different from that of the sixth embodiment and other constructions are similar to those of the sixth embodiment.

The cover plate41of this embodiment has a disklike portion41A and a cylindrical portion41B which are formed integrally with each other. An upper portion of the sleeve2is desirably press fitted into the cylindrical portion41B. The sleeve2may also be inserted into the cylindrical portion41B so as to be attached thereto.

In the hydrodynamic bearing devices of the foregoing embodiments, the shaft35is formed by an iron-series material having high rigidity. Meanwhile, the sleeve2is formed by a copper-series material such as free-cutting brass in which high machining accuracy is obtained easily by its quite excellent machinability. The cover plate41is at least made of a material whose coefficient of linear expansion is smaller than that of the sleeve2. For example, it is desirable that the cover plate41is made of an iron-series material having high rigidity in the same manner as the shaft35.

However, if the shaft35and the sleeve2of the hydrodynamic bearing device of this embodiment are made of the materials referred to above, the sleeve2is expanded due to difference in the coefficients of linear expansion of the respective materials when the hydrodynamic bearing device has reached a high temperature. Consequently, the dimension S1of the radial clearance between the shaft35and the sleeve2increases as shown inFIG. 17, thereby possibly resulting in drop of pressure generated by the hydrodynamic bearing device and drop of rigidity of the oil film.

Therefore, in this embodiment, the shaft35is formed by an iron-series material such as a ferrite-series or martensite-series stainless steel having a coefficient of linear expansion of 1.03×10−5/° C. and the sleeve2is formed by a copper alloy having a coefficient of linear expansion of 2.05×10−5/° C. Meanwhile, the cover plate41is formed by a martensite-series stainless steel having a coefficient of linear expansion of 1.03×10−5/° C. If the materials are selected as described above, inside diameter of the cylindrical portion41B of the cover plate41does not increase so much due to small amount of expansion of the cylindrical portion41B. On the other hand, the sleeve2is made of the material having the coefficient of linear expansion larger than that of the cover plate41. Hence, when temperature rises, amount of expansion of the outside diameter of the sleeve2is larger than that of an inside diameter of the cylindrical portion41B of the cover plate41. However, since the less expandable cylindrical portion41B of the cover plate41grasps the outside diameter of the sleeve2, thermal expansion of the outside diameter of the sleeve2is restrained by the cylindrical portion41B of the cover plate41. Namely, the cylindrical portion41B of the cover plate41is capable of restricting expansion of the inside and outside diameters of the sleeve2by applying pressure to the outer periphery of the sleeve2.

In this embodiment, even at high temperatures, amount of thermal expansion of the inside and outside diameters of the sleeve2is small and is not so different from that of the shaft35. Thus, it can be arranged that the dimension S1of the radial clearance of the radial bearing does not change greatly upon changes in temperature. As a result, changes of performance of the hydrodynamic bearing device with temperature are restrained. Meanwhile, since the cylindrical portion41B of the cover plate41is fixed to the outer periphery of the sleeve2, the cover plate41is securely mounted on the sleeve2and thus, there is no risk that the shaft35is detached from the sleeve2.

In this embodiment, since air mixed into the oil13of the hydrodynamic bearing device is readily discharged, absence of the oil film, which is often associated with prior art bearings, is prevented and it is possible to minimize change of the radial clearance between the shaft35and the sleeve2upon changes in temperature of the hydrodynamic bearing device. Thus, it is possible to materialize the hydrodynamic bearing device operating with high precision for a long service life even in a use environment having changes in temperature. By employing this hydrodynamic bearing device, it is possible to obtain the disk rotating apparatus operating with high precision for a service life.

INDUSTRIAL APPLICABILITY

The present invention is applicable to not only the hydrodynamic bearing device which is highly reliable for a long service life by preventing leakage of the lubricant but the disk rotating apparatus employing this hydrodynamic bearing device.