Motor including hydrodynamic bearings with pair of thrust plates

[0002] 1. Technical Field

[0003] The present invention relates to motors for driving storage disks and, in particular, pertains to a motor provided with hydrodynamic bearings in which distribution of lubricating oil for the bearings is regulated.

[0004] 2. Description of Related Art

[0005] It is known to furnish motors with hydrodynamic bearing mechanisms for rotatably supporting the rotor in the motor. Such hydrodynamic bearing mechanisms include a pair of thrust dynamic-pressure bearings and a pair of radial dynamic-pressure bearings. A pair of thrust plates along with a sleeve in the motor composes the thrust dynamic bearings. Lubricating oil is retained in the thrust dynamic bearings in a gap between the axially inward surface of each thrust plate and each of axially outward surfaces of the sleeve adjacent to the thrust plate inward surfaces. Striations for generating dynamic pressure in the lubricating oil are formed on either the axially inward surfaces of the thrust plates, the outward surfaces of the sleeve, or both. The substantially cylindrical inner surface of the sleeve surrounds the motor shaft to form a gap between the sleeve and the outer circumferential surface of the shaft radially opposing the sleeve inner surface. Lubricating oil is retained in the gap, and striations for generating dynamic pressure in the lubricating oil are formed on either of the opposing sleeve inner and shaft outer surfaces, or both, to compose the radial dynamic bearings.

[0006] Japanese Laid-open patent application No. 09-217735 discloses a motor furnished with a conventional hydrodynamic bearing mechanist including a pair of upper and lower thrust dynamic-pressure bearings, and a pair of upper and lower radial dynamic-pressure bearings. Thrust plates secured on either end of the motor shaft form a pair whose axially inward surfaces together with respectively adjacent axially outward surfaces of the sleeve in the motor form the pair of upper and lower thrust dynamic bearings. The pair of upper and lower radial dynamic bearings is formed by portions of the shaft outer circumferential surface and the sleeve inner circumferential surface, respectively adjacent the upper and lower thrust bearings. The radial bearings are axially separated by an air-filled space for letting air bubbles generated in the lubricant go out of the bearing mechanism. Thus this conventional hydrodynamic bearing mechanism has an upper bearing section, composed of the upper thrust and the upper radial dynamic-pressure bearings, and a lower bearing section, composed of the lower thrust and the lower radial dynamic-pressure bearings. The upper bearing section retains lubricating oil that is continuous throughout the upper thrust and radial dynamic-pressure bearings. Likewise, the lower bearing section retains lubricating oil that is continuous throughout the lower thrust and radial dynamic-pressure bearings. Accordingly, once the motor has been assembled the lubricating oil retained in the upper bearing section is completely separated by the air-filled space from the lubricating oil retained in the lower bearing section.

[0007] In the foregoing conventional motor having the conventional hydrodynamic pressure bearing mechanism, the particular configuration of the dynamic-pressure-generating striations in the thrust and radial bearings, together with taper seals formed adjacent the upper and lower bearing sections, prevents the lubricating oil from leaking outside the bearings. The upper and lower thrust bearing gaps taper radially inward to form the taper seals. The striations for the thrust dynamic-pressure bearings are configured as spiral grooves in a formation to urge the lubricating oil radially inward. The striations for the radial dynamic-pressure bearings are configured as herringbone grooves in an axially asymmetrical formation to urge the lubricating oil axially outward from the radial bearings toward the respectively adjacent thrust bearings.

[0008] In the motor furnished with foregoing conventional hydrodynamic pressure bearing mechanism, the upper and lower taper seals serve to retain the lubricating oil within the respective upper and lower bearing sections. However, due to characteristics of the lubricating oil at its molecular level, an oil migration phenomenon occurs in which the lubricating oil tends to spread from the bearing sections to areas where the oil normally is not present: along surfaces of the thrust-bearing thrust plates and of the rotor, and along surfaces of the shaft and sleeve in the radial bearings.

[0009] The conditions of the foregoing oil migration phenomenon change depending on the material, surface smoothness and like factors of the members in contact with the lubricating oil, such that the lubricating oil disperses by different amounts in different dynamic pressure bearings. Accordingly, when lubricating oil is retained in amounts completely separated from each other in the upper and in the lower bearing sections once the motor has been assembled—as in the case of the foregoing conventional motor—a difference, and therefore an imbalance, occurs between the upper and lower amounts of lubricating oil retained. In addition, wherein the motor assumes its normal or upright posture with its base plate underneath, gravity acting on the lubricating oil in the upper bearing section accelerates the oil migration phenomenon, which increases leakage and dispersion of the lubricating oil.

[0010] Moreover, while the motor is rotating, the lubricating oil retained in the upper dynamic bearing section is displaced toward the upper thrust dynamic-pressure bearing under centrifugal force, and by a pumping action created by the axially asymmetric herringbone grooves of the upper radial dynamic-pressure bearing. Consequently, the lower boundary surface of the lubricating oil in the upper radial dynamic-pressure bearing is displaced upward, which exposes the lower ends of the herringbone grooves formed on the shaft or sleeve to the air in the air-filled space. Thus the border of the lubricating oil boundary surface comes to lie on the herringbone grooves, such that the border rises and falls along the corrugated contour of the grooves, which sets up vibrations during rotation of the motor. Motor vibrations thus caused, as well as external vibrations or impact on the motor, dislodge lubricating oil retained in the upper radial dynamic-pressure bearing, such that it drips down into the lower radial dynamic-pressure bearing. This increases the difference in the amount of lubricating oil retained in the upper and lower bearing sections.

[0011] When an imbalance occurs between the amount of lubricating oil retained in the upper and lower bearing sections, the dynamic-pressure-generating striations in the bearing that retains the lesser amount of lubricating oil are partially exposed to air, reducing the dynamic pressure produced therein. Consequently bearing rigidity between the upper and lower bearing sections differs, which tends to destabilize the motor rotation. Further, durability and reliability of the motor are impaired by early depletion of lubricating oil from the bearings due to such causes as oil migration leakage.

[0012] In the conventional motor, it has in fact been impossible to redistribute the lubricating oil between the upper and lower bearing sections to equalize the retained amounts when there is a difference between the upper and lower sections, since the pair of radial bearings is completely separated by the air-filled space.

[0013] Also to be noted is that the above-described imbalance occurs in the amounts of lubricating oil retained in the upper and lower dynamic-pressure bearing sections due to errors in the operation of injecting lubricating oil into the bearing sections.

[0014] An object of the present invention is to increase the usable life span of hydrodynamic-bearing-equipped disk-drive motors.

[0015] Another object is to improve the reliability and endurance of such disk-drive motors.

[0016] A still further object of the invention is, in disk-drive motors equipped with hydrodynamic bearings having bearing sections separated by an air intervention, to equalize the amount of lubricant held in the bearing sections and to take up and re-circulate again to the bearing sections lubricant leaking out of the radial dynamic-pressure bearing portions, in order to maximize rotation stability toward increasing usable life span.

[0017] A motor of the present invention is furnished with two hydrodynamic bearing sections separated with respect to the shaft by an air intervention, each composed of a radial and a thrust dynamic-pressure bearing portion, and in each of which lubricant is retained continuously throughout the radial and thrust bearing portions.

[0018] The boundaries of the lubricant retained in the two bearing sections front on air. The lubricant boundaries are meniscuses formed by the balancing of the energy of the lubricant itself (the surface tension and intermolecular forces of the lubricant) and of external energy (air pressure of the air contacting the lubricant boundaries and the surface energy of the motor bearing components).

[0019] At least one communicating pathway is formed in the sleeve, axially communicating the thrust faces that are constituents of the thrust dynamic-pressure bearing portions of the two bearing sections, the communicating pathway retains lubricant continuously with the pair of thrust bearing portions.

[0020] Via the communicating pathway, the lubricant retained in the two hydrodynamic bearing sections shifts mutually from the one section to the other, such that the radii of curvature of the meniscuses forming the respective boundaries are equalized.

[0021] A quantitative imbalance occurring between the amounts of lubricant retained in the two bearing sections, for example, when lubricant dislodged from one of the radial dynamic-pressure bearing portions is displaced to the other, means that a discrepancy in the radii of curvature of the meniscuses will arise. When this happens, a discrepancy by the pressure difference between energy of the lubricant itself and the external energy occurs in the lubricant retained in the two bearing sections The discrepancy from the pressure difference sets up an imbalance in the internal pressure of the lubricant retained in the two bearing sections. Due to this internal pressure imbalance, however, a pressure shift occurs via the communicating pathway from that bearing section in which the amount of lubricant retained is greater to that in which the amount is lesser. Consequently, redistribution that equalizes the retained amounts of lubricant in the two hydrodynamic bearing sections takes place through the communicating pathway, eliminating the quantitative imbalance in the lubricant retained in the two bearing sections.

[0022] From the following detailed description in conjunction with the accompanying drawings, the foregoing and other objects, features, aspects and advantages of the present invention will become readily apparent to those skilled in the art.

[0023] FIG. 1 is a sectional view typifying an outline configuration of a motor of the present invention;

[0024] FIG. 2A is an oblique view of an instance in which communicating pathways are made by cutting off sections of the outer periphery of the sleeve to form planes, and

[0025] FIG. 2B is an oblique view of an instance in which communicating pathways are made by forming V-shaped grooves in sections of the outer periphery of the sleeve.

[0026] The storage-disk drive motor 1 depicted in FIG. 1 is furnished with a bracket 2, a shaft 4 the exterior of one end of which is fixedly fitted into a central opening 2ain the bracket 2, and a rotor 6 rotative relative to the shaft 4. The rotor 6 is furnished with a rotor hub 6aon the outer periphery of which storage disks D are loaded, and a sleeve 6bfitted to the inner circumferential surface of a central hole 6a1 that is a round bore formed through the rotor hub 6a. Further, rotor magnets 10 are fastened to the inner periphery of the rotor hub 6aby adhesive means, and a stator 12 is fitted to the bracket 2, radially opposing the rotor magnets 10.

[0027] A bore 6cis formed in the approximate center of the sleeve 6b. The inner circumferential surface of the bore 6caxially extending through the sleeve 6btherein is at an interval from the outer circumferential surface of the shaft 4 to form a micro-gapin which lubricant 8 is retained. Radially outwardly projecting disk-shaped upper thrust plate 4aand lower thrust plate 4bare mounted respectively to the upper portion and the lower portion of the shaft 4.

[0028] It should be understood that “upper” and “lower” herein are relative terms used for the sake of clarity in reference to the drawings, and are not meant to imply that the motor described is functional only in the illustrated orientation. Further, “axially inward” herein means “heading from either end (of the shaft or sleeve, for example) axially toward the lengthwise middle,” while “axially outward” means “heading from the lengthwise middle axially toward either end (of the shaft or sleeve, for example).”

[0029] The inner circumferential surface of the central hole 6a1, and upper and lower thrust faces 6fand 6gthat are either axial end face of the sleeve, define upper and lower openings 6dand 6ein regions of the central hole 6a1 that correspond to the upper and lower thrust plates 4aand 4b. The upper and lower openings 6dand 6eare diametrically larger than the outer diameter of the thrust plates 4aand 4b, and are blocked off by ring-shaped upper and lower counter-plates 7aand 7b. Apertures 7a2, 7b2, centrally through which the shaft 4 pierces, are formed in the respective upper and lower counter-plates 7aand 7b.

[0030] An upper thrust dynamic-pressure bearing portion 16 is configured by an upper micro-gap in which lubricant 8 is retained, and by an upper set of spiral grooves 14. The upper micro-gap is formed in between the upper thrust face 6fof the sleeve 6band the lower face (axially inward face) of the upper thrust plate 4a, and extends from the inner periphery of the bore 6cto the outer periphery of the upper opening 6d. The upper spiral grooves 14 are formed in rows on the lower face of the upper thrust plate 4afor generating dynamic pressure in the lubricant 8 attending rotation of the rotor 6.

[0031] In turn, a lower thrust dynamic-pressure bearing portion 18 is configured by a lower micro-gap in which lubricant 8 is retained, and by a lower set of spiral grooves 14. The lower micro-gap is formed in between the lower thrust face 6gof the sleeve 6b, and the lower face (axially inward face) of the upper thrust plate 4b, and extends from the inner periphery of the bore 6cto the outer periphery of the lower opening 6e. The lower spiral grooves 14 are formed in rows on the upper face of the lower thrust plate 4bfor generating dynamic pressure in the lubricant 8 attending rotation of the rotor 6.

[0032] The spiral grooves 14 formed in the upper and lower thrust dynamic-pressure bearing portions 16, 18 develop a pumping action in the lubricant 8 that pressure-sends it heading radially inward.

[0033] The outer circumferential surface 4a1 of the upper thrust plate 4ais formed into a taper such that heading axially outward, the gap between it and the radially opposing inner circumferential surface of the upper opening 6dof the rotor 6 expands. When the motor 1 is at a standstill, a meniscus (interface between the lubricant 8 and the outer air) is formed in lubricant 8 retained in the upper thrust-bearing portion 16, within this tapered gap between the surface 4a1 and the inner circumferential surface of the upper opening 6d. The energy of the lubricant 8 itself (surface tension of and intermolecular forces in the lubricant 8) and external energy (atmospheric pressure of air contacting the lubricant boundary surface and surface energy of the motor 1 components the lubricant contacts) balance to form this meniscus. Accordingly, the gap defined between the outer circumferential surface 4a1 of the upper thrust plate 4aand the inner circumferential surface of the upper opening 6din the rotor 6 functions as an upper first tapered seal 17a.

[0034] Likewise, the outer circumferential surface 4b1 of the lower thrust plate 4bis formed into a taper such that heading axially outward, the gap in between it and the radially opposing inner circumferential surface of the lower opening 6eof the rotor 6 expands. When the motor 1 is at a standstill, a meniscus is formed in lubricant 8 retained in a the lower thrust dynamic-pressure bearing portion 18, within the tapered gap between the surface 4b1 and the inner circumferential surface of the lower opening 6ein the rotor 6. Accordingly, the gap defined in between the outer circumferential surface 4b1 of the lower thrust plate 4band the inner circumferential surface of the lower opening 6ein the rotor 6 functions as a lower first tapered seal 19a.

[0035] The lubricant 8 held in the upper and lower thrust dynamic-pressure bearing portions 16, 18 forms meniscuses and is retained in the upper and lower first tapered seals 17a, 19a. Therein, even if the action of centrifugal force during motor 1 rotation presses and shifts lubricant 8 radially outward, the lubricant is blocked from travelling further by the inner circumferential surfaces of the upper opening 6dand the lower opening 6e. Further, lubricant 8 that due to the oil migration phenomenon has diffused onto the surfaces of the upper and lower thrust plates 4a, 4band of the rotor 6 is pressure-sent radially outward under the action of centrifugal force, and re-circulated to lubricant 8 retained in the upper and lower first tapered seals 17a, 19a, preventing leakage of the lubricant 8 to the exterior of the motor 1.

[0036] The bottom face 7a1 of the upper counter-plate 7aas it heads radially inward forms a taper in which the axial gap at the interval with the upper face (axially outward face) of the upper thrust plate 4aexpands, by which an upper second tapered seal 17bis formed. The upper second tapered seal 17bthrough the gap between the aperture 7a2 and the outer circumferential surface of the shaft 4 is communicated open to the motor 1 external air. During rotation of the motor 1, the lubricant 8 forms a meniscus and is retained in the upper second tapered seal 17b. In turn, upper face 7b1 of the lower counter-plate 7bas it heads radially inward forms a taper in which the axial gap at the interval with the lower face (axially outward face) of the lower thrust plate 4bexpands, by which a lower second tapered seal 19bis formed. Likewise, the lower second tapered seal 19bthrough the gap between the aperture 7b2 and the outer circumferential surface of the shaft 4 is also communicated open to the motor 1 external air. During rotation of the motor 1, the lubricant 8 forms a meniscus and is retained in the lower second tapered seal 19b.

[0037] Thus, during rotation of the motor 1, the meniscuses in the lubricant 8 head radially inward to lie within the second tapered seals 17b, 19b. Consequently, when the motor 1 rotates lubricant 8 that has dispersed due to the oil migration phenomenon is pressure-sent heading radially outward under centrifugal force acting on the lubricant 8, and is re-circulated to lubricant 8 retained in the second tapered seals 17b, 19b. This further prevents leakage to the exterior of the motor 1.

[0038] An annular recess 4cis formed in approximately the mid-portion of the outer circumferential surface of the shaft 4 with respect to the sleeve 6b. The annular recess 4cconstitutes a gap expansion in which the radial dimension of the gap at the interval with the inner circumferential surface of the bore 6cexpands heading from either end axially to the mid-portion of the shaft 4. Opening on the recess 4cis a through-hole 20 formed within the shaft 4, which communicates with air external to the motor 1. Air taken into the micro-gaps from this opening forms an annular gas intervention 22 in between the recess 4cand the inner circumferential surface of the bore 6c.

[0039] Upper and lower radial dynamic-pressure bearing portions 26 and 28 are configured by sets of herringbone grooves 24 formed in rows in regions on the inner circumferential surface of the bore 6ccorresponding to where the lubricant 8 is retained sectioned axially above and below by air held in the recess 4c. The herringbone grooves 24 generate dynamic pressure in the lubricant 8 attending rotation of the rotor 6.

[0040] The herringbone grooves 24 formed in the upper and lower radial dynamic-pressure bearing portions 26, 28 are spiral striations joined mutually contrariwise, configured in an axially asymmetrical form in which the spiral striations located axially outward are short compared to the spiral striations located axially inward Thus pressure peaks in the dynamic pressure that develops in the herringbone grooves 24 formed in the upper and lower radial dynamic-pressure bearing portions 26, 28 appear biased axially outward. That is, the purpling action on the lubricant 8 by the herringbone grooves 24 acts heading toward the upper and lower thrust dynamic-pressure bearing portions 16, 18. Meniscuses are formed in the lubricant 8 held in the upper and lower radial dynamic-pressure bearing portions 26 and 28 respectively. The meniscuses are retained within the gap in which the gas intervention 22, defined between the recess 4cin the shaft 4 and the inner circumferential surface of the bore 6c, varies in radial dimensions.

[0041] Because the dynamic-pressure generating means formed in the thrust dynamic-pressure bearing portions 16, 18 are the spiral grooves, necessary axial load-supporting pressure is not generated by them alone. Nonetheless, the herringbone grooves 24 in the neighboring radial dynamic-pressure bearing portions 26, 28 are formed to bias axially outward (toward the thrust dynamic-pressure bearing portions 16, 18) the pressure peaks in the dynamic pressure that is generated. Through cooperation of the two bearing sections, this enables the necessary dynamic pressure for supporting loads borne by the rotor 6 to be generated. The herringbone grooves 24 are herein set to balance the dynamic pressure of the thrust dynamic-pressure bearing portions 16, 18 during rotation of the rotor 6, by shifting the lubricant 8 boundary interfaces in the vicinity of the radial dynamic-pressure bearing portions 26, 28 on the gas intervention 22 side into the radial dynamic-pressure bearing portions 26, 28. This exposes a portion of the asymmetrical herringbone grooves 24 into the air.

[0042] Again, the upper and lower radial dynamic-pressure bearing portions 26, 28 configure respectively an upper dynamic-pressure bearing section and a lower dynamic-pressure bearing section in which lubricant 8 is retained continuously with the adjacent thrust dynamic-pressure bearing portions 16, 18. In the dynamic-pressure bearing sections, from the lubricant 8 boundaries on the one hand (the meniscuses formed in the gas intervention 22) to the lubricant 8 boundaries on the other (the meniscuses formed in the first tapered seals 17a, 19aor in the second tapered seals 17b, 19b), the dynamic pressure becomes greatest at only one point during rotation of the rotor 6; no minimum point exists. This accordingly makes for a configuration that readily rids air bubbles into the air from the boundaries of the lubricant 8, where the pressure automatically becomes minimal even if they have become included in the lubricant 8.

[0043] Thus, air bubbles that have arisen in the lubricant 6 retained in the dynamic-pressure bearing sections shift successively to the low-pressure side, and are released to the air from the boundary surfaces of the lubricant 8. Therefore the air bubbles do not stay in the lubricant 8, which prevents lubricant 8 from leaking to the bearing exterior due to thermal expansion of air bubbles during temperature elevation in the motor 1.

[0044] Further communicating pathways 9 are formed on the inner circumferential surface of the sleeve 6b, positioned in between the upper and lower thrust dynamic-pressure bearing sections 16, 18, making connecting passages between the pair of thrust dynamic-pressure bearing sections 16, 18. Either ends of the communicating pathways 9 open in the vicinity of the outer peripheral ends of the thrust dynamic-pressure bearing portions 16, 18 wherein lubricant 8 continuous with the lubricant 8 retained in the thrust dynamic-pressure bearing portions 16, 18 is sustained by the capillary effect.

[0045] If a quantitative imbalance has occurred between the lubricant 8 retained in the upper dynamic-pressure bearing section and the lubricant 8 retained in the lower dynamic-pressure bearing section, separated by the gas intervention 22, the lubricant 8 meniscus in the dynamic-pressure bearing section on the side retaining the greater lubricant 8 will position to that section among the gas intervention 22 and the tapered seals 17a, 17b, 19aand 19bwhere the gap is more expanded. Consequently, a difference arises between the radius of curvature of the lubricant 8 meniscuses in the upper dynamic-pressure bearing section within the gas intervention 22 and the upper first tapered seal 17aor upper second tapered seal 17b, and the radius of curvature of the lubricant 8 lower dynamic-pressure bearing section meniscuses in the gas intervention 22 and the lower first tapered seal 19aor lower second tapered seal 19b.

[0046] The meniscuses on the side in which the greater lubricant 8 is retained will naturally be in sections where the gaps are larger than will be the meniscuses on the lesser lubricant 8 side. This makes the pressure difference between the external energy acting on the meniscuses in the section where the gaps are smaller—i.e., where their radius of curvature is smaller—and the energy of the lubricant 8 itself, greater than the pressure difference between the external energy acting on the meniscuses in the section where the gaps are larger—i.e., where their radius of curvature is larger—and the energy of the lubricant 8 itself. An imbalance therefore occurs in the internal pressure of the lubricant 8.

[0047] In response, a pressure shift from the bearing section side in which the retained amount of lubricant 8 is greater to the side in which it is lesser takes place until the imbalance in the internal pressure of the lubricant 8 retained in the two bearing sections is eliminated. The pressure shift makes the radii of curvature of the lubricant 8 meniscuses formed within the gas intervention 22 and the tapered seals 17a, 19a, 17b, and 19bequivalent. In other words, redistribution of the lubricant 8 through the communicating pathways 9 occurs until the lubricant 8 meniscuses among the gas intervention 22 and the tapered seals 17a, 19a, 17b, and 19bare in equivalent gap positions. This eliminates the quantitative imbalance between the lubricant 8 retained in the upper bearing section and that retained in the lower bearing section.

[0048] In particular, lubricant 8 is always retained within the communicating pathways 9 by capillary action, continues with lubricant 8 held in the thrust dynamic-pressure bearing portions 16, 18. This means that during rotation of the motor 1, in addition to the above-described external energy, centrifugal force due to the motor 1 rotation acts as a compressive force from the second tapered seals 17b, 19btoward the thrust dynamic-pressure bearing portions 16, 18. Accordingly, forming the communicating pathways 9 so as to open within the upper/lower thrust faces 6f, 6gin the vicinity of the thrust dynamic-pressure bearing portion 16, 18 outer peripheral boundaries means that a larger pressure shift is impated to the lubricant 8. As a consequence, even if, stemming from external vibrations and shocks applied to the motor 1, a considerable amount of lubricant 8 that had been retained in a bearing section on the one hand shifts to the bearing section on the other hand, lubricant 8 will be quickly redistributed to the bearing section side short of lubricant 8.

[0049] The lubricant 8 retained in the upper and lower, air-separated bearing sections is thus made mutually shiftable between then. Wherein a quantitative imbalance in the retained lubricant 8 has occurred, redistribution of lubricant 8 takes place through the communicating pathways 9, and the retained amounts of lubricant 8 held in the upper and in the lower bearing section are equalized, thereby to attain stabilized rotation of the motor 1.

[0050] The communicating pathways 9 may be configured, as is indicated in FIG. 2A, by forming planes 9aon the sleeve 6bfrom the upper thrust face 6fto the lower thrust face 6g. The sleeve 6bis then inserted into the central hole 6a1 that is the round bore in the rotor hub 6a, fitted therein to define air slots between the planes 9aand the inner circumferential surface of the central hole 6a1. The planes 9aare formed by shaving portions approximately 50 to 100 m in thickness dimension off the outer circumferential surface of the sleeve 6b. Alternatively, the communicating pathways 9 may be configured, as is indicated in FIG. 2B, by forming axially extending grooves 9bin the outer circumferential surface of the sleeve 6b. The grooves 9bare roughly V-shaped in cross-section and approximately 50 to 100 m in depth. The sleeve 6bis inserted into the central hole 6a1 to define air slots between the grooves 9band the central hole 6a1.

[0051] It should be understood that wherein the rotor 6 is formed of a single member, the communicating pathways 9 may also be formed by bores approximately 50 to 100 m in diameter. The bores extend from the upper to the lower thrust faces 6f, 6gthat are components of the thrust dynamic-pressure bearing portions 16, 18. Furthermore, in FIG. 1 and FIG. 2A, 2B an instance is illustrated in which the communicating pathways 9 are formed in two places circumferentially, but the configuration is not thus limited. A plural configuration of communicating pathways 9 evenly spaced circumferentially is possible, or the configuration may be only a single communicating pathways 9.

[0052] The radial gap between the inner circumferential surfaces of the apertures 7a2, 7b2 in the upper and lower counter-plates 7a, 7band the outer circumferential surface of the shaft 4 is set to be as small as possible. Therefore, a difference arises between the flow speeds of air current generated during rotation of the motor 1 in the axial gap defined between the axially outward faces of the thrust plates 4a, 4band the top face 7a1 and bottom face 7b1 of the counter-plates 7a, 7b, and of air current in the radial gap defined between the outer circumferential surface of the shaft 4 and the inner circumferential surfaces of the apertures 7a2, 7b2. This consequently makes greater the resistance to outflow to the motor 1 exterior of vapor (oil mist) that has arisen due to the lubricant 8 gasifying, and keeps pressure in the vicinity of the lubricant 8 interfaces high, which therefore prevents further vaporization of the lubricant 8. Also, applying an oil repellent made from for example a fluoric material onto each of these faces, prevents the lubricant 8 from leaking to the exterior of the motor 1 when at a standstill wherein centrifugal force does not act on the lubricant 8. This is in addition to the effect of the seals described above. The lubricant 8 might otherwise leak through the radial gap between the inner circumferential surfaces 7a2, 7b2 of the upper and lower counter-plates 7a, 7band the outer circumferential surface of the shaft 4 due to the oil migration phenomenon when the motor 1 is at a standstill.

[0053] Further, because employing the spiral grooves 14 enables the outside diameter of the thrust plates 4a, 4bto be made diametrically smaller, the thrust plate configuration is abbreviated and the number of manufacturing steps is reduced, which contributes to motor cost reduction. At the same time the peripheral speed of the thrust plates 4a, 4bduring rotation of the motor is lessened, and bearing damage that is due to viscous resistance of the lubricant 8 is restrained, which elevates the electrical efficiency of the motor and curbs its power consumption.

[0054] In addition, by enabling the thrust plates 4a, 4bto be made diametrically smaller, sufficient space can be secured for the magnetic circuit consisting of the rotor magnet 10 and the stator 12, even wherein the magnetic circuit is positioned in the vicinity of the lower thrust dynamic-pressure bearing portion 18. High drive torque can therefore be attained.

[0055] While only selected embodiments have been chosen to illustrate the present invention, to those skilled in the art it will be apparent from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing description of the embodiments according to the present invention is provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.