Hydrodynamic bearing device and disk rotating apparatus

This application is a divisional of U.S. patent application Ser. No. 10/554,473 filed on May 26, 2006, which is a national stage of International Application No. PCT/JP2004/005277 filed Apr. 13, 2004. This application claims the benefit and priority of JP 2003-120025 filed Apr. 24, 2003 and JP 2003-176481 filed Jun. 20, 2003. The entire disclosure of each of the above applications are incorporated herein by reference.

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 to FIGS. 18 and 19 . In FIG. 18 , a shaft 31 is rotatably inserted into a bearing bore 32 A of a sleeve 32 mounted on a base 35 . In FIG. 18 , the shaft 31 has a flange 33 formed integrally at its lower end portion. The flange is received in a step portion 32 K of the sleeve 32 so as to rotatably confront a thrust plate 34 . A rotor hub 36 to which a rotor magnet 38 is secured to is attached to the shaft 31 . A plurality of disk 39 held by a spacer 40 and a clamper 41 are mounted on the rotor hub 36 . A motor stator 37 confronting the rotor magnet 38 is mounted on the base 35 . Dynamic pressure generating grooves 32 B and 32 C are provided on an inner peripheral surface of the bearing bore 32 A of the sleeve 32 . Dynamic pressure generating grooves 33 A are provided on one face of the flange 33 , which confronts the step portion 32 K of the sleeve 32 , while dynamic pressure generating grooves 33 B are provided on the other face of the flange 33 , which confronts the thrust plate 34 . Clearances between the shaft 31 and the flange 33 on one hand and the sleeve 32 on the other hand, which include the dynamic pressure generating grooves 32 B, 32 C, 33 A and 33 B, are filled with oil 42 . One or more vent holes 32 E are provided on the sleeve 32 substantially in parallel with an axis of the sleeve 32 . A lower end of the vent holes 32 E communicates with a space which is disposed at a lower end portion of the sleeve 32 so as to contain the flange 33 . An upper end of the vent holes 32 E opens to an upper end face of the sleeve 32 .

Operation of the conventional hydrodynamic bearing device of the above described arrangement is described by referring to FIGS. 18 and 19 . In FIG. 18 , if the motor stator 37 is energized, a rotary magnetic field is generated and thus, the rotor magnet 38 , the rotor hub 36 , the shaft 31 and the flange 33 start rotations. At this time, a pumping pressure is generated in the oil 42 by the dynamic pressure generating grooves 32 B, 32 C, 33 A and 33 B. Thus, the shaft 31 is raised and is rotated without coming into contact with the thrust plate 34 and the inner peripheral surface of the bearing bore 32 A while being lubricated by the oil 42 . A magnetic head (not shown) is brought into contact with the disks 39 so as to perform recording and reproduction of electrical signals.

The above conventional hydrodynamic bearing device has the following problems. FIG. 19 is a fragmentary sectional view including the shaft 31 and the sleeve 32 of FIG. 18 . As shown in FIG. 19 , the shaft 31 is rotated in the bearing bore 32 A of the sleeve 32 while being lubricated by the oil 42 . 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 “air 43 A or 43 B”) may penetrate into the oil 42 in the bearing bore 32 A. For example, in case ambient pressure has changed during transport in an aircraft, penetration of air bubbles may happen. If volume of the air 43 A penetrating into the vicinity of the dynamic pressure generating grooves 32 B and 32 c is expanded by rise of temperature or drop of atmospheric pressure, a portion of the dynamic pressure generating grooves 32 b is 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 oil 42 B. Meanwhile, if the air 43 B penetrating into the vicinity of the flange 33 is expanded, the hatched oil 42 A in the vent hole 32 E may be pushed upwardly by expanded air 43 C so as to leak out of the hydrodynamic bearing device as shown by oil 42 D. If the oil 42 leaks 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 shaft 31 with the sleeve 32 during rotation.

Meanwhile, also in case a drop impact load (acceleration) is applied to the conventional hydrodynamic bearing device in the direction of the arrow G 1 as shown in FIG. 19 , there is a risk that the oil 42 leaks outwardly as shown by the oil 42 B.

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 and 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 and 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.

FIG. 1 is a sectional view of a hydrodynamic bearing device according to a first embodiment of the present invention.

FIG. 2 is an enlarged fragmentary sectional view showing a shaft and a sleeve of the hydrodynamic bearing device of the first embodiment of the first embodiment.

FIG. 3 is a top plan view of a flange 3 of the hydrodynamic bearing device of the first embodiment of the present invention.

FIG. 4 is a bottom plan view of the flange 3 of the hydrodynamic bearing device of the first embodiment of the present invention.

FIG. 5 is a fragmentary sectional view showing an operation of the hydrodynamic bearing device of the first embodiment of the present invention including the shaft and the sleeve.

FIG. 6 is a fragmentary sectional view showing a further operation of the hydrodynamic bearing device of the first embodiment of the present invention including the shaft and the sleeve.

FIG. 7 is a fragmentary sectional view showing a still further operation of the hydrodynamic bearing device of the first embodiment of the present invention including the shaft and the sleeve.

FIG. 8 is a fragmentary sectional view showing a shaft and a sleeve of a hydrodynamic bearing device according to a second embodiment of the present invention.

FIG. 9 is a fragmentary sectional view showing a shaft and a sleeve of a hydrodynamic bearing device according to a third embodiment of the present invention.

FIG. 10 is a fragmentary sectional view showing a shaft and a sleeve of a hydrodynamic bearing device according to a fourth embodiment of the present invention.

FIG. 11 is a fragmentary sectional view showing a shaft and a sleeve of a hydrodynamic bearing device according to a fifth embodiment of the present invention.

FIG. 12 a is a perspective view of a cover plate of the hydrodynamic bearing device of the fifth embodiment of the present invention.

FIG. 12 b is a sectional view of the cover plate of the hydrodynamic bearing device of the fifth embodiment of the present invention.

FIG. 13 is a fragmentary sectional view showing a shaft and a sleeve of a hydrodynamic bearing device according to a sixth embodiment of the present invention.

FIG. 14 a is a perspective view of a cover plate of the hydrodynamic bearing device of the sixth embodiment of the present invention.

FIG. 14 b is a sectional view of the cover plate of the hydrodynamic bearing device of the sixth embodiment of the present invention.

FIG. 15 is a perspective view of a further cover plate of the hydrodynamic bearing device of the sixth embodiment of the present invention.

FIG. 16 is a fragmentary sectional view showing a shaft and a sleeve of a hydrodynamic bearing device according to a seventh embodiment of the present invention.

FIG. 17 is a sectional view showing a clearance of a radial bearing of the hydrodynamic bearing device of the seventh embodiment of the present invention.

FIG. 18 is a sectional view of a prior art hydrodynamic bearing device.

FIG. 19 is a fragmentary sectional view showing a shaft and sleeve of the prior art hydrodynamic bearing device.

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 to FIGS. 1 to 17 .

First Embodiment

A hydrodynamic bearing device according to a first embodiment of the present invention is described with reference to FIGS. 1 to 7 . FIG. 1 is a sectional view of the hydrodynamic bearing device of the first embodiment of the present invention, while FIG. 2 is an enlarged fragmentary sectional view showing a shaft 1 and a sleeve 2 . In FIG. 1 , the sleeve 2 has a bearing bore 2 A and the cylindrical shaft 1 is rotatably inserted into the bearing bore 2 A. There is a minute clearance between an outer peripheral surface of the shaft 1 and an inner peripheral surface of the bearing bore 2 A of the sleeve 2 . Dynamic pressure generating grooves 1 B and 1 C 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 shaft 1 and the inner peripheral surface of the bearing bore 2 A of the sleeve 2 so as to act as a radial bearing portion. The radial bearing portion supports the shaft 1 in a radial direction of the shaft 1 . In the example of FIG. 1 , the dynamic pressure generating grooves 1 B and 1 C are formed on the inner peripheral surface of the bearing bore 2 A. Each of the dynamic pressure generating grooves 1 B and 1 C has the herringbone pattern. In FIG. 1 , at least one of the dynamic pressure generating grooves 1 B and 1 C (the dynamic pressure generating grooves 1 B in the example of FIG. 1 ) are formed such that a length of a lower groove 1 M from an angular portion 1 K is smaller than that of an upper groove 1 L from the angular portion 1 K as shown in FIG. 2 .

In FIG. 1 , a rotor hub 12 having a rotor magnet 8 is mounted on an upper end of the shaft 1 . A flange 3 having faces orthogonal to an axis of the shaft 1 and a diameter larger than that of the shaft 1 is integrally formed at a lower end of the shaft 1 . A thrust bearing face 3 F disposed at the lower face of the flange 3 confronts a thrust plate 4 fixed to the sleeve 2 . The thrust plate 4 seals an end portion region of the thrust bore 2 A of the sleeve 2 , which includes the flange 3 . Dynamic pressure generating grooves 3 B of a helical shape or a herringbone pattern are formed on one of a lower face of the flange 3 and an upper face of the thrust plate 4 (the lower face of the flange 3 in FIG. 1 ) so as to act as a thrust bearing portion.

Dynamic pressure generating grooves 3 A are formed on one of an outer peripheral portion of an upper face of the flange 3 and a step portion 2 D of the sleeve 2 , which confronts the outer peripheral portion of the upper face of the flange 3 (the upper face of the flange 3 in FIG. 1 ). A known large clearance portion 2 B is formed at an axially intermediate portion of the bearing bore 2 A of the sleeve 2 but is not relevant to the present invention directly, so that its description is abbreviated. The flange 3 is received in the step portion 2 D of the sleeve 2 . A clearance or a recess 3 C for storing oil is provided on the lower face of the flange 3 .

An annular upper depression 2 C surrounding the bearing bore 2 A is provided on an upper end face of the sleeve 2 . A ringlike cover plate 5 is attached to the sleeve 2 so as to cover the upper depression 2 C. An outer peripheral portion of the cover plate 5 is secured to an outer peripheral portion of the sleeve 2 by caulking or the like. An inner peripheral portion of the cover plate 5 is mounted so as to hold a small clearance with an upper end portion of the bearing bore 2 A of the sleeve 2 as will be described later in detail. A space or a clearance defined by the upper depression 2 C and the cover plate 5 is referred to as an upper reservoir 15 . Oil is stored in the upper reservoir 15 as necessary. In the upper reservoir 15 , dimension of the clearance interposed between the cover plate 5 and the sleeve 2 is not constant in the radial direction. Namely, the dimension is made sufficiently small at the opening 15 A confronting the outer peripheral surface of the shaft 1 , i.e., at an inner peripheral portion of the upper reservoir 15 and is made large at an outer peripheral portion of the upper reservoir 15 .

A first communication hole 2 E extending substantially in parallel with an axis of the bearing bore 2 A is provided on the sleeve 2 . The first communication hole 2 E communicates, at its upper end, with the upper reservoir 15 and communicates, at its lower end, with a space including the step portion 2 D of the sleeve 2 so as to form a communication path. The sleeve 2 is secured to a base 6 on which a motor stator 7 is mounted. A gap between the shaft 1 and the bearing bore 2 A of the sleeve 2 including a clearance between the shaft 1 and the sleeve 2 and a clearance between the flange 3 and the thrust plate 4 is filled with lubricant (hereinafter, referred to as “oil”) 13 . Since the oil 13 has a certain viscosity, air bubbles 14 may penetrate in between the shaft 1 and the bearing bore 2 A as shown in FIG. 2 . The oil enters also the first communication hole 2 E and the upper reservoir 15 but a small amount of air (air bubbles) 14 are present in the first communication hole 2 E and the upper reservoir 15 . As shown in FIG. 1 , a plurality of disks 9 are mounted on the rotor hub 12 by a spacer 10 and a clamper 11 such that a disk rotating apparatus is constituted.

Operation of the hydrodynamic bearing device of the above described arrangement is described with reference to FIGS. 1 to 7 , hereinafter. In FIG. 1 , if the motor stator 7 is energized from a power source (not shown), a rotary magnetic field is generated and thus, the rotor hub 12 on which the rotor magnet 7 is mounted starts rotation together with the shaft 1 , the flange 3 , the disks 9 , the clamper 11 and the spacer 10 . If the rotation is started, the dynamic pressure generating grooves 1 B, 1 C, 3 A and 3 B collect the oil 13 to predetermined locations so as to generate a known pumping pressure. Hence, the shaft 1 is raised and is rotated at high speed without coming into contact with the sleeve 2 and the thrust plate 4 . FIG. 2 shows a state in which the air 14 mixes into the oil 13 during rotation of the hydrodynamic bearing device.

FIG. 3 is a top plan view showing an example of the known dynamic pressure generating grooves 3 A provided on the upper face of the flange 3 , which confront the step portion 2 D of the sleeve 2 . FIG. 4 is a top plan view showing an example of the known dynamic pressure generating grooves 3 B provided on the lower face of the flange 3 . The radially bent dynamic pressure generating grooves 3 A and 3 B shown in FIGS. 3 and 4 collect the oil 13 so as to generate a thrust force parallel to the axis of the shaft 1 .

FIG. 5 is an enlarged fragmentary sectional view showing the shaft 1 and the sleeve 2 of the hydrodynamic bearing device of this embodiment. In FIG. 5 , “S 1 ” denotes a dimension of a radial clearance of the dynamic pressure generating grooves 1 B and “S 2 ” denotes a dimension of a radial clearance between the outer periphery of the shaft 1 and the cover plate 5 . An upper end portion 2 H of the bearing bore 2 A of the sleeve 2 has a diameter larger than that of the bearing bore 2 A. The “dimension of the radial clearance” is defined as a dimension of a clearance between the outer periphery of the shaft 1 and the inner periphery of the bearing bore 2 A at the time the axis of the shaft 1 is held in alignment with a central axis of the bearing bore 2 A of the sleeve 2 . “S 3 ” denotes a dimension of the clearance 15 A of a portion of the upper reservoir 15 confronting the shaft 1 , i.e., an inner peripheral portion. “S 4 ” denotes a dimension of a clearance at an outer peripheral portion of the upper reservoir 15 . In this embodiment, the dimensions S 1 and S 2 of the radial clearances and the dimensions S 3 and S 4 of the clearances are set so as to have the following relations.

S 1 <S 2 , S 1 <S 3 and S 3 <S 4

By setting the clearances as described above, the oil 13 stored in the upper reservoir 15 moves, by its surface tension, to the neighborhood of the opening 15 A of the clearance dimension S 3 smaller than the clearance dimension S 4 . From the opening 15 A of the clearance dimension S 3 , the oil 13 further enters the smaller radial clearance of the dimension S 1 between the shaft 1 and the bearing bore 2 A so as to flow, as shown by the arrow 13 A, to a region of the dynamic pressure generating grooves 1 B acting as the radial bearing portion.

In the dynamic pressure generating grooves 1 B and 1 C of the radial bearing, each of the dynamic pressure generating grooves 1 B has an upper groove 1 L and a lower groove 1 M from an angular portion 1 K and a length L of the upper groove 1 L is made larger than a length M of the lower groove 1 M such that a vertically asymmetric herringbone pattern is formed. Therefore, the oil 13 flowing into the radial clearance of the dimension S 2 between the shaft 1 and the upper end portion 2 H of the bearing bore 2 A 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 bore 2 A and the shaft 1 including the dynamic pressure generating grooves 1 B and 1 C. Thus, the oil 13 present in the upper reservoir 15 flows into the radial bearing as shown by the arrow 13 A. As a result, in the clearance between the shaft 1 and the bearing bore 2 A, the oil 13 flows in a direction indicated by the arrow 13 C. Therefore, the oil 13 disposed adjacent to the flange 3 is delivered so as to flow into the communication hole 2 E and reaches the upper reservoir 15 . Then, the oil 13 again flows from the opening 15 A between the cover plate 5 and the sleeve 2 into the radial bearing portion between the shaft 1 and the bearing bore 2 A so as to circulate through the hydrodynamic bearing device. By circulation of the oil 13 , the air bubbles 14 in the oil 13 also passes through the communication hole 2 E together with the oil 13 so as to reach the upper reservoir 15 . The air bubbles 14 which have reached the upper reservoir 15 are discharged outwardly from the clearance between the cover plate 5 and the sleeve 2 .

Discharge of air is described in more detail with reference to FIG. 6 . FIG. 6 is a fragmentary sectional view showing state of air which has entered the oil 13 in the hydrodynamic bearing device. If quantity of air 14 A formed by air bubbles or air lumps present in the hydrodynamic bearing device increases, internal pressure of the air 14 A rises upon rise of ambient temperature or the air 14 A is expanded by drop of atmospheric pressure, volume of the air 14 A increases. In such a case, the air 14 A enters the first communication hole 2 E from a lower inlet 2 F of the first communication hole 2 E and proceeds upwardly together with the oil 13 as indicated by air 14 D. The air 14 D which has reached an upper end 2 G of the first communication hole 2 E enters the upper reservoir 15 and is discharged outwardly from the small clearance between the cover plate 5 and the sleeve 2 as shown by the arrow C. In the first communication hole 2 E, the oil 13 also proceeds upwardly together with the air 14 D. However, after the oil 13 has been carried to the upper reservoir 15 , the oil 13 remains in the upper reservoir 15 due to its surface tension. Thus, only the air 14 D is discharged. Therefore, since the oil 13 is 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 oil 13 does not happen, so that the hydrodynamic bearing device is rotated stably.

In a concrete example of the embodiment shown in FIG. 5 , the shaft 1 has a diameter of 1 to 20 mm. The clearance dimension S 3 ranges from 30 to 150 microns. The dimension S 1 of the radial clearance of the radial bearing ranges from 1 to 10 microns. The first communication hole 2 E has a diameter of 0.3 to 1.0 mm. Experiments conducted by the present inventors have revealed that if the diameter of the shaft 1 , the clearance dimension S 3 , the dimension S 1 of the radial clearance and the diameter of the first communication hole 2 E fall in the respective ranges referred to above, the oil 13 is held in the hydrodynamic bearing device without leaking outwardly and only the air 14 is discharged outwardly.

As shown in FIG. 7 which is a fragmentary sectional view similar to FIG. 6 , the present inventors have made various tests in which a drop impact load or vibrations are applied in a direction of the arrow G 2 . The test results have shown that the oil 13 stored in the upper reservoir 15 is held in the upper reservoir 15 due 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 S 2 and S 3 to about 50 microns, the oil 13 does not leak.

In this embodiment, air such as air bubbles, which has entered the oil 13 in the hydrodynamic bearing device, proceeds to the upper reservoir 15 of the sleeve 2 by way of the first communication hole 2 E during operation of the hydrodynamic bearing device and is discharged out of the hydrodynamic bearing device therefrom. However, the oil 13 remains in the upper reservoir 15 without 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 hole 2 E is illustrated in FIG. 1 but a plurality of the first communication holes 2 E may be provided on the sleeve 2 . As shown by the dotted lines in FIG. 6 , a communication hole 2 Q for establishing communication between the upper reservoir 15 and space of the step portion 2 D may be provided between the outer periphery of the sleeve 2 and the base 6 in place of the first communication hole 2 E. In this case, a vertical groove may be provided on the outer periphery of the sleeve 2 as a portion of the communication hole 2 Q corresponding to the outer peripheral portion of the sleeve 2 .

Second Embodiment

FIG. 8 is a fragmentary sectional view showing the shaft 1 and a sleeve 20 of a hydrodynamic bearing device according to a second embodiment of the present invention. In FIG. 8 , a second communication hole 20 J for establishing communication between the first communication hole 2 E and a large clearance portion 20 B is provided at a central portion of the sleeve 20 . Other constructions of this hydrodynamic bearing device are similar to those of the hydrodynamic bearing device of the first embodiment shown in FIG. 1 .

In order to form the second communication hole 20 J, there is, for example, a method in which as shown in FIG. 8 , a hole is formed on the sleeve 20 in a direction of the arrow 20 H with a drill. After the hole has been formed on the sleeve 20 , a hole 20 K on the outer periphery of the sleeve 20 is sealed with a plug 17 .

In the hydrodynamic bearing device of this embodiment, the first communication hole 2 E communicates with space between the dynamic pressure generating grooves 1 B and the dynamic pressure generating grooves 1 C via the second communication hole 20 J. Thus, the oil 13 flows from the upper reservoir 15 into a portion of the dynamic pressure generating grooves 1 B as shown by the arrow 13 A and flows also from the second communication hole 20 J into the portion of the dynamic pressure generating grooves 1 B as shown by the arrow 13 D. The oil 13 which has flowed into the portion of the dynamic pressure generating grooves 1 B in the direction of the arrow 13 D flows together with the oil 13 having flowed into the portion of the dynamic pressure generating grooves 1 B in the direction of the arrow 13 A and returns from a lower inlet 20 F to the first communication hole 2 E through a space between the shaft 1 and the bearing bore 20 A including the dynamic pressure generating grooves 1 B and 1 C. Air mixed into the oil 13 is separated from the oil 13 when the oil 13 flows into the second communication hole 20 J as shown by the arrow 13 G. The separated air 14 proceeds in a direction of the arrow 14 F and is discharged outwardly by way of the upper reservoir 15 .

In this embodiment, since the oil 13 is displaced vigorously by providing the second communication hole 20 J, removal of the air 14 from the oil 13 is performed efficiently. As a result, reliability of the hydrodynamic bearing device is upgraded further. Meanwhile, even if the air 14 has entered the oil 13 in the hydrodynamic bearing device for some reason, the air 14 is discharged out of the hydrodynamic bearing device rapidly, so that reliability of the hydrodynamic bearing device becomes high.

Third Embodiment

FIG. 9 is a fragmentary sectional view showing a shaft 30 and the sleeve 2 of a hydrodynamic bearing device according to a third embodiment of the present invention. In FIG. 9 , a small diameter portion 30 A having a diameter smaller than that of the shaft 30 is provided on the shaft 30 in the vicinity of an end portion of the shaft 30 coupled with the rotor hub 12 . A diameter of an inner peripheral edge 25 A of a ringlike cover plate 25 is larger than that of the small diameter portion 30 A but is smaller than that of the shaft 30 . Namely, the cover plate 25 is arranged to cover the clearance between the shaft 30 and the sleeve 2 . Other constructions of this hydrodynamic bearing device are similar to those of the first embodiment shown in FIG. 1 . By this arrangement, it is possible to further positively prevent outward leakage of the oil 13 from the clearance between the shaft 30 and the cover plate 25 . Meanwhile, since the diameter of the inner peripheral edge 25 A of the cover plate 25 is smaller than that of the shaft 30 , the shaft 30 is not detached from the bearing bore 2 A of the sleeve 2 . Namely, the cover plate 25 functions to prevent detachment of the shaft 30 .

In FIG. 9 , it is supposed that “S 1 ” denotes a dimension of a radial clearance in the neighborhood of the dynamic pressure generating grooves 1 B and “S 2 ” denotes a dimension of a radial clearance between the shaft 30 and the upper end portion 2 H of the sleeve 2 . The upper end portion 2 H of the bearing bore 2 A of the sleeve 2 has a diameter lager than that of the bearing bore 2 A. In the upper reservoir 15 defined by the cover plate 25 and the upper depression 2 C of the sleeve 2 , it is supposed that “S 3 ” denotes a dimension of a clearance of an inner peripheral portion of the upper reservoir 15 and “S 5 ” denotes a dimension of an axial clearance between the cover plate 25 and an upper end of the shaft 30 . It is supposed that “S 6 ” denotes a dimension of a radial clearance between the small diameter portion 30 A of the shaft 30 and the inner peripheral edge 25 A of the cover plate 25 . In this embodiment, the dimension S 1 is set to be smaller than the dimensions S 2 , S 3 , S 5 and S 6 , i.e., S 1 <S 2 , S 1 <S 3 , S 1 <S 5 and S 1 <S 6 . Oil has a property to flow into a smallest clearance by its surface tension. Thus, by setting the dimensions S 1 , S 2 , S 3 , S 5 and S 6 as described above, the oil 13 stored in the upper reservoir 15 flows into the clearance of the smallest dimension S 1 between the shaft 30 and the bearing bore 2 A. As a result, since the oil 13 flows to regions of the dynamic pressure generating grooves 1 B and 1 C sufficiently, absence of the oil film does not occur. Meanwhile, the dimensions S 2 , S 3 , S 5 and S 6 are set such that the dimension S 2 is smaller than the dimension S 6 , the dimension S 3 is smaller than the dimension S 6 and the dimension S 5 is smaller than the dimension S 6 , i.e., S 2 <S 6 , S 3 <S 6 and S 5 <S 6 . By setting the dimensions S 2 , S 3 , S 5 and S 6 as described above, the oil 13 does not flow out of the clearance of the largest dimension S 6 between the small diameter portion 30 A and the inner peripheral edge 25 A of the cover plate 25 .

In the hydrodynamic bearing device of this embodiment, a ventilation port 25 B is provided on the cover plate 25 . In a plane containing the cover plate 25 , the ventilation port 25 B is disposed so as to circumferentially deviate by 180 degrees in FIG. 1 from a mouth of the first communication hole 2 E opening to the upper reservoir 15 . If the ventilation port 25 B is aligned with the mouth of the first communication hole 2 E, such an incident may happen that when air rising through the first communication hole 2 E is discharged from the ventilation port 25 B, the oil 13 is also expelled outwardly. This expulsion of the oil 13 can be prevented by circumferentially shifting the ventilation port 25 B and the first communication hole 2 E from each other as described above. Air which has flowed out of the upper end of the first communication hole 2 E travels circumferentially in the upper reservoir 15 along the cover plate 25 and runs outwardly when the air has reached the ventilation port 25 B.

Fourth Embodiment

FIG. 10 is a fragmentary sectional view showing a shaft 35 and the sleeve 2 of a hydrodynamic bearing device according to a fourth embodiment of the present invention. In FIG. 10 , dynamic pressure generating grooves 35 D are formed on a lower end face 35 C of the shaft 35 . Therefore, the flange 3 of the hydrodynamic bearing device of the third embodiment shown in FIG. 9 is not provided on the shaft 35 . Other constructions of this hydrodynamic bearing device are substantially similar to those of the hydrodynamic bearing device of FIG. 9 . At an end portion of the shaft 35 , on which the rotor hub 12 is mounted, the shaft 35 has a small diameter portion 35 A. A diameter of the inner peripheral edge 25 A of the cover plate 25 is set to be larger than an outside diameter of the small diameter portion 35 A and smaller than an outside diameter of the shaft 35 . Namely, the inner peripheral edge 25 A of the cover plate 25 is arranged to cover a clearance between the shaft 35 and the bearing bore 2 A of the sleeve 2 . Thus, it is possible to positively prevent leakage of the oil 13 from the upper clearance between the shaft 35 and the sleeve 2 in FIG. 10 .

The dynamic pressure generating grooves 35 D which may be formed on one of the lower end face 35 C of the shaft 35 and an upper face of the thrust plate 4 are formed on the lower end face 35 C of the shaft 35 in FIG. 10 and confront the thrust plate 4 so as to constitute a thrust bearing with the thrust plate 4 . In FIG. 10 , the step portion 2 D is formed on the underside of the sleeve 2 . An end portion of the bearing bore 2 A including the step portion 2 D of the sleeve 2 is sealed by the thrust plate 4 . A space between the step portion 2 D and the thrust plate 4 communicates with the first communication hole 2 E at the lower inlet 2 F. The first communication hole 2 E acts as a communication path for establishing communication between the step portion 2 D and the upper reservoir 15 .

In the hydrodynamic bearing device of this embodiment, the dynamic pressure generating grooves 35 D are provided on the lower end face 35 C of the shaft 35 without providing the flange on the shaft 35 . 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 grooves 2 B and 2 C formed by shallow grooves of a herringbone pattern are provided on at least one of the outer peripheral face of the shaft 35 and the inner peripheral surface of the sleeve 2 (on the inner peripheral surface of the sleeve 2 in FIG. 10 ) and the clearance between the shaft 35 and the sleeve 2 is filled with the oil 13 in the same manner as the foregoing embodiments. The upper reservoir 15 is provided on the sleeve 2 in the vicinity of the upper end face of the sleeve 2 and communicates with a space adjacent to the lower end face 35 C of the shaft 35 through the first communication hole 2 E. Thus, the oil 13 circulates in a path in which the oil 13 flows from the upper reservoir 15 into the clearance between the shaft 35 and the sleeve 2 and returns from the lower portion of the sleeve 2 to the upper reservoir 15 via the first communication hole 2 E. Since air mixed into the oil 13 is discharged outwardly from the ventilation port 25 B of the cover plate 25 during operation of the hydrodynamic bearing device, the air in the oil 13 is eliminated and thus, absence of the oil film does not occur in the clearance around the shaft 35 . 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 oil 13 is 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. 11 is a fragmentary sectional view showing the shaft 35 and the sleeve 2 of a hydrodynamic bearing device according to a fifth embodiment of the present invention. In FIG. 11 , the hydrodynamic bearing device of this embodiment has a construction similar to that of the hydrodynamic bearing device of the fourth embodiment shown in FIG. 10 except that a cover plate 27 is different from the cover plate 25 of the fourth embodiment.

The cover plate 27 of this embodiment is shown in a perspective view of FIG. 12 a and a sectional view of FIG. 12 b along the line XIIb-XIIb. As shown in FIGS. 12 a and 12 b , the cover plate 27 has, on its lower face, at least one recess 27 E. A boss 27 H is formed at a portion of an upper face of the cover plate 27 , which portion corresponds to the recess 27 E. A ventilation port 27 F is provided at a substantially central portion of the recess 27 E. The cover plate 27 is attached to the sleeve 2 such that the recess 27 E confronts the upper reservoir 15 .

In this embodiment, the clearance between the cover plate 27 and the upper reservoir 15 becomes large at the recess 27 E of the cover plate 27 . The oil 13 in the upper reservoir 15 is least likely to flow into the large clearance below the recess 27 E due to its surface tension and thus, remains in a portion of a small clearance surrounding the recess 27 E. Hence, since the ventilation port 27 F disposed at the central portion of the recess 27 E is not covered by the oil 13 , air in the upper reservoir 15 is discharged smoothly from the ventilation port 27 F.

In the hydrodynamic bearing device of this embodiment, supposing that “S 1 ” denotes a dimension of a radial clearance between the bearing bore 2 A of the dynamic pressure generating grooves 1 B and the shaft 35 , “S 2 ” denotes a dimension of a radial clearance between an outer periphery of the shaft 35 and an inner periphery of the upper end portion 2 H of the sleeve 2 , “S 3 ” denotes a dimension of a clearance between the cover plate 27 and an end portion of the shaft 35 and “S 4 ” denotes a dimension of a clearance at an outer peripheral portion of the upper reservoir 15 , the dimensions S 2 and S 3 are set to be larger than the dimension S 1 , i.e., S 1 <S 2 and S 1 <S 3 . Meanwhile, the dimension S 4 is set to be larger than the dimension S 3 , i.e., S 4 >S 3 . As a result, the oil 13 in the upper reservoir 15 gathers to a vicinity of an opening 27 A having the clearance of the small dimension S 3 due to its surface tension and then, flows into the clearance (radial bearing portion) having the smaller dimension S 1 between the shaft 35 and the bearing bore 2 A.

Each of the dynamic pressure generating grooves 1 B has the upper groove 1 L and the lower groove 1 M, which have the dimensions L and M from the angular portion 1 K such that the upper groove 1 L is larger than the lower groove 1 M. Hence, the oil 13 which has flowed into the clearance of the dimension S 2 between the upper end portion 2 H of the sleeve 2 and the shaft 35 is drawn into the radial clearance of the dimension S 1 between the shaft 35 and the bearing bore 2 A of the sleeve 2 by pumping action of the dynamic pressure generating grooves 1 B at the time of start of operation of the hydrodynamic bearing device and during operation of the hydrodynamic bearing device. By this action, the oil 13 in the upper reservoir 15 is caused to flow into the radial bearing positively.

Air in the form of minute air bubbles is mixed into the lubricant such as the oil 13 filled in the clearance between the sleeve 2 and the shaft 35 . If quantity of the air mixed into the oil 13 is 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 hole 2 E from the lower inlet 2 F of the first communication hole 2 E as indicated by air 14 . In FIG. 11 , the air 14 proceeds further upwardly so as to enter the upper reservoir 15 . The air 14 which has entered the upper reservoir 15 travels circumferentially and is discharged outwardly from the ventilation port 27 F as shown by the arrow C when the air 14 has reached the recess 27 E. At this time, the oil 13 is also displaced together with the air 14 in the first communication hole 2 E. However, since the oil 13 which has been carried to the upper reservoir 15 is separated from the air 14 , only the oil 13 remains in the upper reservoir 15 due to its surface tension, while the air 14 is discharged from the ventilation port 27 F as shown by the arrow C. Hence, the oil 13 is 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 shaft 35 has a diameter of 1 to 20 mm and the dimension S 3 of the clearance ranges from 30 to 150 microns. The dimension S 1 of the radial clearance of the radial bearing ranges from 1 to 10 microns and the first communication hole 2 E 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 oil 13 is satisfactorily held in the respective clearances of the hydrodynamic bearing device and the air 14 is discharged favorably.

In FIG. 11 , even if a drop impact load or vibrations are applied in a direction of the arrow G 2 , the oil 13 stored in the upper reservoir 15 is held in the upper reservoir 15 due 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 S 2 and S 3 to about 50 microns, the oil 13 does not leak and rotation of the hydrodynamic bearing device can be continued by holding the shaft 35 and the sleeve 3 out of contact with each other.

Sixth Embodiment

FIG. 13 is a fragmentary sectional view showing the shaft 35 and the sleeve 2 of a hydrodynamic bearing device according to a sixth embodiment of the present invention. In this embodiment, a cover plate 28 is different from the cover plate 27 of the hydrodynamic bearing device of the fifth embodiment shown in FIG. 12 . Other constructions of this hydrodynamic bearing device are similar to those shown in FIG. 1 . As shown in a perspective view of FIG. 14 a and a sectional view of FIG. 14 b , the cover plate 28 of this embodiment is formed with a bulge portion 28 D by partially bulging an inner peripheral portion of the ringlike cover plate 28 . A recess 28 E is formed on one face of the cover plate 28 opposite to the bulge portion 28 D. The cover plate 28 is attached to the sleeve 2 such that the recess 28 E confronts the upper end portion 2 H on the end face of the sleeve 2 . In an area in which the recess 28 E and the end face of the sleeve 2 confront each other, clearance between the recess 28 E and the end face of the sleeve 2 increases by a size of the recess 28 E. Thus, the oil 13 does not gather to a vicinity of the recess 28 E due to its surface tension. Therefore, air which has reached the upper reservoir 15 by way of the first communication hole 2 E travels in the annular upper reservoir 15 circumferentially and is smoothly discharged from the recess 28 E when the air has reached the recess 28 E. Since the oil 13 proceeds to the small clearance due to its surface tension and does not gather to a vicinity of the recess 28 E having the large clearance, leakage of the oil 13 out of the recess 28 E does not occur.

In this embodiment, the recess 28 E can be formed by such a simple working in which the inner peripheral portion of the cover plate 28 is recessed.

FIG. 15 is a perspective view showing a cover plate 40 as another example of the cover plate 28 of the hydrodynamic bearing device of this embodiment. Other constructions than the cover plate 40 are similar to those of FIG. 13 . In FIG. 15 , the ringlike cover plate 40 is formed, at its inner peripheral portion, with a notch 40 E. Since a part of the upper end portion 2 H of the inner peripheral portion of the sleeve 2 is connected to an outside by the notch 40 E, air is discharged smoothly through the notch 40 E. The notch 40 E can be formed by performing simple working, for example, simultaneously with press working of the cover plate 40 . Since the notch 40 E can be worked easily, manufacturing cost of the cover plate 40 is also reduced.

Seventh Embodiment

FIG. 16 is a fragmentary sectional view showing the shaft 35 and the sleeve 2 of a hydrodynamic bearing device according to a seventh embodiment of the present invention. FIG. 17 is a sectional view along the line XVII-XVII in FIG. 16 . In this embodiment, a cover plate 41 is structurally different from that of the sixth embodiment and other constructions are similar to those of the sixth embodiment.

The cover plate 41 of this embodiment has a disklike portion 41 A and a cylindrical portion 41 B which are formed integrally with each other. An upper portion of the sleeve 2 is desirably press fitted into the cylindrical portion 41 B. The sleeve 2 may also be inserted into the cylindrical portion 41 B so as to be attached thereto.

In the hydrodynamic bearing devices of the foregoing embodiments, the shaft 35 is formed by an iron-series material having high rigidity. Meanwhile, the sleeve 2 is 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 plate 41 is at least made of a material whose coefficient of linear expansion is smaller than that of the sleeve 2 . For example, it is desirable that the cover plate 41 is made of an iron-series material having high rigidity in the same manner as the shaft 35 .

However, if the shaft 35 and the sleeve 2 of the hydrodynamic bearing device of this embodiment are made of the materials referred to above, the sleeve 2 is 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 S 1 of the radial clearance between the shaft 35 and the sleeve 2 increases as shown in FIG. 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 shaft 35 is 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 sleeve 2 is formed by a copper alloy having a coefficient of linear expansion of 2.05×10 −5 /° C. Meanwhile, the cover plate 41 is 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 portion 41 B of the cover plate 41 does not increase so much due to small amount of expansion of the cylindrical portion 41 B. On the other hand, the sleeve 2 is made of the material having the coefficient of linear expansion larger than that of the cover plate 41 . Hence, when temperature rises, amount of expansion of the outside diameter of the sleeve 2 is larger than that of an inside diameter of the cylindrical portion 41 B of the cover plate 41 . However, since the less expandable cylindrical portion 41 B of the cover plate 41 grasps the outside diameter of the sleeve 2 , thermal expansion of the outside diameter of the sleeve 2 is restrained by the cylindrical portion 41 B of the cover plate 41 . Namely, the cylindrical portion 41 B of the cover plate 41 is capable of restricting expansion of the inside and outside diameters of the sleeve 2 by applying pressure to the outer periphery of the sleeve 2 .

In this embodiment, even at high temperatures, amount of thermal expansion of the inside and outside diameters of the sleeve 2 is small and is not so different from that of the shaft 35 . Thus, it can be arranged that the dimension S 1 of 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 portion 41 B of the cover plate 41 is fixed to the outer periphery of the sleeve 2 , the cover plate 41 is securely mounted on the sleeve 2 and thus, there is no risk that the shaft 35 is detached from the sleeve 2 .

In this embodiment, since air mixed into the oil 13 of 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 shaft 35 and the sleeve 2 upon 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.

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.