FLUID DYNAMIC PRESSURE BEARING LUBRICANT OIL COMPOSITION, FLUID DYNAMIC PRESSURE BEARING, AND FLUID DYNAMIC PRESSURE BEARING APPARATUS

Abstract

A lubricating-oil composition for a fluid-dynamic bearing, which is to be used for a fluid-dynamic bearing (8) including radial dynamic pressure generating portions (A1, A2) provided on an inner peripheral surface (Sa) of a porous body, has the following makeup. That is, a base oil having a kinematic viscosity of more than 30 mm.sup.2/s and 80 mm.sup.2/s or less at a temperature of 40 C. is used as a base oil of the lubricating-oil composition, and the makeup of the lubricating-oil composition is adjusted so that the lubricating-oil composition has a kinematic viscosity of 90 mm.sup.2/s or more and 140 mm.sup.2/s or less at a temperature of 40 C.

Claims

1. A lubricating-oil composition for a fluid-dynamic bearing, the lubricating-oil composition being configured to be used for the fluid-dynamic bearing, which is formed of a porous body having inner pores, and comprises a radial dynamic pressure generating portion on an inner peripheral surface of the porous body, wherein a base oil having a kinematic viscosity of more than 30 mm.sup.2/s and 80 mm.sup.2/s or less at a temperature of 40 C. is used as a base oil, and wherein the lubricating-oil composition has a kinematic viscosity of 90 mm.sup.2/s or more and 140 mm.sup.2/s or less at a temperature of 40 C.

2. The lubricating-oil composition for a fluid-dynamic bearing according to claim 1, wherein the base oil is a mixture of a non-polar oil and a polar oil, and a ratio of the non-polar oil to an entirety of the base oil is 20 wt % or more and 30 wt % or less.

3. The lubricating-oil composition for a fluid-dynamic bearing according to claim 2, wherein the non-polar oil is a synthetic hydrocarbon oil made from poly--olefin or a hydride of poly--olefin.

4. The lubricating-oil composition for a fluid-dynamic bearing according to claim 2, wherein the polar oil is an ester oil.

5. A fluid-dynamic bearing obtained through impregnation of the inner pores with the lubricating-oil composition of claim 1.

6. The fluid-dynamic bearing according to claim 5, wherein a dynamic pressure generating groove array region is formed as the radial dynamic pressure generating portion on the inner peripheral surface.

7. The fluid-dynamic bearing according to claim 5, wherein an inner diameter dimension is 2.0 mm or less, and an axial dimension is 3.5 mm or less.

8. A fluid-dynamic bearing device, comprising: the fluid-dynamic bearing of claim 5; a housing having an inner periphery to which the fluid-dynamic bearing is fixed; a rotator having a shaft portion inserted along an inner periphery of the fluid-dynamic bearing; and a radial bearing portion configured to radially support the shaft portion in a non-contact manner through an oil film of the lubricating-oil composition, which is formed in a radial bearing gap between the inner peripheral surface of the fluid-dynamic bearing and an outer peripheral surface of the shaft portion.

9. The fluid-dynamic bearing device according to claim 8, wherein the shaft portion being rotatable with respect to the fluid-dynamic bearing in a non-contact manner has an eccentricity ratio of up to 98%.

10. A motor comprising the fluid-dynamic bearing device of claim 8.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0027] FIG. 1 is a sectional view of a fan motor according to one embodiment of the present invention.

[0028] FIG. 2 is a sectional view of a fluid-dynamic bearing device illustrated in FIG. 1.

[0029] FIG. 3 is a sectional view of a fluid-dynamic bearing illustrated in FIG. 2.

[0030] FIG. 4 is a plan view of the fluid-dynamic bearing illustrated in FIG. 2.

[0031] FIG. 5 is a bottom view of the fluid-dynamic bearing illustrated in FIG. 2.

[0032] FIG. 6 is a sectional view of a fluid-dynamic bearing device according to another embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

[0033] Now, one embodiment of the present invention is described with reference to the drawings. Note that, in the following description, with respect to a fluid-dynamic bearing, a disc portion side of a hub portion is referred to as upper side, and a bottom portion side of a housing is referred to as lower side. As a matter of course, the upper and lower sides as defined above do not limit a mode of installation and a mode of use of actual products.

[0034] FIG. 1 is a view for schematically illustrating one configuration example of a fan motor 1 according to this embodiment. The fan motor 1 comprises a fluid-dynamic bearing device 2, a plurality of fans 4 provided to a rotator 3 of the fluid-dynamic bearing device 2, and a drive portion 5 configured to rotate the fans 4 together with the rotator 3. The drive portion 5 comprises, for example, coils 5a and magnets 5b which are opposed to each other across a radial gap. In this embodiment, the coils 8a are fixed to a base portion 6 corresponding to a stationary side of the fan motor 1, and the magnets 5b are fixed to the rotator 3 corresponding to a rotary side of the fan motor 1.

[0035] In the fan motor 1 configured as described above, when the coils 5a are energized, an excitation force is generated between the coils 5a and the magnets 8b so that the magnets 5b are rotated. With this rotation, the plurality of fans 4 arranged upright along an outer rim of the rotator 3 are rotated together with the rotator 3. This rotation causes the fans 4 to generate air flow in a direction determined by their shapes (for example, air flow to a radially outer side in this case). In a manner of being drawn by the air flow, air flow from an axially upper side toward an axially lower side of the fan motor 1 is secondarily generated. With the air flow generated around the fan motor 1 in this manner, an information apparatus (not shown) onto which the fan motor 1 is mounted can be cooled.

[0036] Further, when the air flow is generated in the axial direction of the fan motor 1 as described above, a force (reaction force) in a direction opposite to the direction of the air flow is generated around the rotator 3 of the fluid-dynamic bearing device 2. A magnetic force (repulsive force) in a direction of cancelling the reaction force is exerted between the coils 5a and the magnets Sb. A thrust load generated due to a difference in magnitude between the reaction force and the magnetic force is applied on thrust bearing portions T1 and T2 (refer to FIG. 2 described later) of the fluid-dynamic bearing device 2. The magnetic force in the direction of cancelling the reaction force can be generated by, for example, arranging the coils Sa and the magnets Sb in an axially shifted manner (detailed illustration is omitted). Further, during the rotation of the rotator 3, a radial load is applied to a shaft portion 10 of the fluid-dynamic bearing device 2, which is described later. This radial load is applied to radial bearing portions R1 and R2 of the fluid-dynamic bearing device 2.

[0037] FIG. 2 is a sectional view of the fluid-dynamic bearing device 2 built in the fan motor 1. The fluid-dynamic bearing device 2 comprises a housing 7, a fluid-dynamic bearing 8 fixed to an inner periphery of the housing 7, and the rotator 3 to be rotated relative to the fluid-dynamic bearing 8.

[0038] In this embodiment, the rotator 3 comprises a hub portion 9 arranged on an upper-end opening side of the housing 7, and the shaft portion 10 inserted along an inner periphery of the fluid-dynamic bearing 8.

[0039] As illustrated in FIG. 1 and FIG. 2, the hub portion 9 comprises a disc portion 9a covering the upper-end opening side of the housing 7, a first cylindrical portion 9b extending from the disc portion 9a to the axially lower side, a second cylindrical portion 9c located on the radially outer side with respect to the first cylindrical portion 9b and extending from the disc portion 9a to the axially lower side, and a flange portion 9d further extending from an axially lower end of the second cylindrical portion 9c to the radially outer side. The disc portion 9a is opposed to one end surface (upper end surface 8b) of the fluid-dynamic bearing 8 fixed to the inner periphery of the housing 7. Further, the plurality of fans 4 are formed integrally with the hub portion 9 in an upright posture along an outer rim of the flange portion 9d.

[0040] In this embodiment, the shaft portion 10 is formed as a body separate from the hub portion 9, and an upper end thereof is fixed into a mounting hole 9e formed in the hub portion 9. In this case, the shaft portion 10 has an outer peripheral surface 10a having a constant outer diameter dimension, and a lower end portion 10b being continuous with a lower end of the outer peripheral surface 10a and having, for example, a partially spherical shape. That is, the shaft portion 10 has a shape insertable from one axial side along an inner periphery of the fluid-dynamic bearing 8. As a matter of course, the shaft portion 10 and the hub portion 9 may be formed integrally of the same material. Alternatively, one of the shaft portion 10 and the hub portion 9 to be made of materials different from each other may be formed through injection molding of a metal or a resin with another one of the shaft portion 10 and the hub portion 9 being used as an insert component.

[0041] The housing 7 is formed into a shape with its upper end being opened and its lower end being closed. Further, the fluid-dynamic bearing 8 is fixed to an inner peripheral surface 7a of the housing 7, and an outer peripheral surface 7b of the housing 7 is fixed to the base portion 6 (refer to FIG. 1). An axial opposing clearance between an upper end surface 7c of the housing 7 and a lower end surface 9a1 of the disc portion 9a of the hub portion 9 is larger than an opposing clearance between the upper end surface 8b of the fluid-dynamic bearing 8 and the lower end surface 9a1 of the disc portion 9a. In this case, the opposing clearances are set so as to have sizes that have substantially no influence on an increase in torque loss during rotational drive.

[0042] A tapered sealing surface 7d increased in outer diameter dimension as approaching to the upper side is formed on an upper side of an outer periphery of the housing 7. An annular sealing space S gradually reduced in radial opposing clearance from a closed side (lower side) toward an opening side (upper side) of the housing 7 is formed between the tapered sealing surface 7d and an inner peripheral surface 9b1 of the first cylindrical portion 9b. During rotation of the shaft portion 10 and the hub portion 9, the sealing space S is in communication with a radially outer side of a thrust bearing gap in the first thrust bearing portion T1 described later, thereby allowing the lubricating-oil composition to flow in a bearing interior space comprising bearing gaps. Further, a charging amount of the lubricating-oil composition is adjusted so that an oil surface (gas-liquid interface) of the lubricating-oil composition is constantly maintained within the sealing space S under a state in which inner pores of the fluid-dynamic bearing 8 are impregnated with the lubricating-oil composition and the bearing interior space is filled with the same lubricating-oil composition (refer to FIG. 2).

[0043] Further, when the housing 7 has a sealing structure as described above, the coils Sa are arranged so as to be positioned on a radially outer side with respect to the first cylindrical portion 9b of the hub portion 9 and so as to partially overlap with the first cylindrical portion 9b in the axial direction. In this manner, thinning (reduction in the axial dimension) of the housing 7, and in turn, of the fluid-dynamic bearing device 2 is achieved.

[0044] Further, in this embodiment, a thrust receiving portion 11 configured to receive the lower end portion 10b of the shaft portion 10, which has a spherical shape, is formed in a bottom portion 7e of the housing 7. That is, the thrust receiving portion 11 is constantly in contact with the lower end portion 10b of the shaft portion 10 under a state after the fluid-dynamic bearing device 2 is completed so that the shall portion 10 can be rotationally supported thereby. It is preferred that an abutment position of the thrust receiving portion 11 in the up-and-down direction with respect to a bearing abutment surface 7f of the housing 7 be set, for example, so as to fall within a region of an inner chamfered portion 8e of the fluid-dynamic bearing 8 in the up-and-down direction.

[0045] The housing 7 may be made of any appropriate material and may have any appropriate makeup. For example, a publicly known material such as a resin or a metal can be appropriately used in accordance with a fixing method for the fluid-dynamic bearing 8 to the housing 7, which is described later.

[0046] The fluid-dynamic bearing 8 is formed of a porous body of a sintered metal obtained through compression molding and sintering of metal powder having predetermined makeup or raw-material powder containing the metal powder as a main component. The fluid-dynamic bearing 8 is formed into a tubular shape. In this embodiment, the fluid-dynamic bearing 8 is formed into a circular cylindrical shape as illustrated in FIG. 4 and the like. In an entirely or a part of an inner peripheral surface 8a of the fluid-dynamic bearing 8, an array region of a plurality of dynamic pressure generating grooves Sal is formed as a radial dynamic pressure generating portion. In other words, the fluid-dynamic bearing 8 comprises the porous body configured as described above and the array region of the dynamic pressure generating grooves Sal formed on the inner peripheral surface 8a of the porous body. In this embodiment, as illustrated in FIG. 3, in the array region of the dynamic pressure generating grooves 8a1, the plurality of dynamic pressure generating grooves 8a1 inclined at a predetermined angle with respect to a circumferential direction, inclined ridge portions 8a2 partitioning those dynamic pressure generating grooves Sal from each other in the circumferential direction, and belt portions 8a3 extending in the circumferential direction and partitioning the dynamic pressure generating grooves 8a1 from each other in the axial direction are arrayed in a herringbone pattern (both of the inclined ridge portions 8a2 and the belt portions 8a3 are indicated by cross-hatching in FIG. 3). Two dynamic pressure generating groove array regions are formed continuously to each other in the axial direction. In this case, both an upper dynamic pressure generating groove array region A1 and a lower dynamic pressure generating groove array region A2 are formed so as to be axially symmetrical with respect to an axial center line (imaginary line connecting axial centers of the belt portion 8a3 to each other in the circumferential direction), and are equal to each other in axial dimension.

[0047] In this embodiment, in an entirety or a part of the upper end surface 8b of the fluid-dynamic bearing 8, an array region of a plurality of dynamic pressure generating grooves 8b1 is formed as a thrust dynamic pressure generating portion. For example, as illustrated in FIG. 4, the array region of the dynamic pressure generating grooves 8b1 is formed so that the plurality of dynamic pressure generating grooves 8b1 extending in a spiral pattern are arrayed in the circumferential direction. In this case, an orientation of the spiral of the dynamic pressure generating grooves 8b1 is set to an orientation corresponding to a rotation direction of the rotator 3. Under a state in which the fluid-dynamic bearing device 2 illustrated in FIG. 2 is driven to rotate, the thrust bearing gap of the first thrust bearing portion T1 described later is formed between the array region of the dynamic pressure generating grooves 8b1 configured as described above and the opposed lower end surface 9a1 of the disc portion 9a of the hub portion 9.

[0048] Meanwhile, a thrust dynamic pressure generating portion is not formed in a lower end surface 8c of the fluid-dynamic bearing 8. That is, in this embodiment, as illustrated in FIG. 5, the lower end surface 8c has a flat shape. The lower end surface 8c is in abutment against the housing 7 on a radially outer side of the housing 7 with respect to the thrust receiving portion 11. A position of the bearing abutment surface 7f of the housing 7 in the up-and-down direction is appropriately set within a range in which a gap between the upper end surface 8b of the fluid-dynamic bearing 8 and the lower end surface 9ai of the hub portion 9 can function as the thrust bearing gap.

[0049] In an outer peripheral surface 8d of the fluid-dynamic bearing 8, one or a plurality of (five in this embodiment) axial grooves 8d1 are formed (refer to, for example. FIG. 4). Under a state in which the fluid-dynamic bearing 8 is fixed to the housing 7, passages for the lubricating-oil composition are formed between the axial grooves 8d1 and the inner peripheral surface 7a of the housing 7 (refer to FIG. 2).

[0050] Next, referring to FIG. 3, various dimensions of the fluid-dynamic bearing 8 are described. An axial dimension L (axial distance between both the end surfaces 8b and 8c) of the fluid-dynamic bearing 8 is set to 4.8 mm or less, preferably 3.5 mm or less, more preferably 1.8 mm or less in terms of thinning of the fluid-dynamic bearing device 2, and in turn, of the fan motor 1. Meanwhile, in terms of ensuring required radial bearing rigidity, the axial dimension L is set to 0.8 mm or more, preferably 1.1 mm or more.

[0051] An inner diameter dimension D1 (strictly, inner diameter dimension of each of the belt portions 8a3 that are smallest diameter portions as well as the inclined ridge portions 8a2 on the inner peripheral surface Sa) may basically be any appropriate dimension. In terms of, for example, ensuring biting performance of a sizing pin into the inner peripheral surface 8a at the time of dynamic pressure generating groove sizing described later, the inner diameter dimension D1 is desirably 1.2 mm or more, more desirably 1.5 mm or more. Meanwhile, in terms of avoiding a risk of difficulty in reliable transfer of a press-fit force to a surface layer portion of the inner peripheral surface 8a at the time of the dynamic pressure generating groove sizing due to a resulting increase in thickness dimension t, which is larger than required, the inner diameter dimension D1 is desirably 2.5 mm or less, more desirably 2.0 mm or less.

[0052] An outer diameter dimension D2 of the fluid-dynamic bearing 8 may also basically be any appropriate dimension. In consideration of a relationship with, for example, required inner diameter dimension D1 and thickness dimension t, the outer diameter dimension D2 is desirably 2.5 mm or more, more desirably 3.0 mm or more. From a similar point of view, the outer diameter dimension D2 is desirably 5.0 mm or less, more desirably 4.5 mm or less.

[0053] The thickness dimension t {=(D2D1)/2} of the fluid-dynamic bearing 8 may basically be any appropriate dimension. For example, in terms of ensuring a required thrust bearing area on the upper end surface 8b, it is preferred that the thickness dimension t be set to 0.5 mm or more. Meanwhile, in terms of enabling transfer of a sufficient compressing force from a die to a surface layer portion of an inner peripheral surface of a sintered compact at the time of dynamic pressure generating groove sizing, it is preferred that the thickness dimension t be set to 1.5 mm or less.

[0054] Next, description is given of makeup when the fluid-dynamic bearing 8 is formed of a porous body of a sintered metal. The fluid-dynamic bearing 8 is obtained, for example, through compression molding and sintering of raw-material powder containing one of copper-based powder and iron-based powder in the largest amount and another one thereof in second largest amount. In other words, the porous body of the fluid-dynamic bearing 8 substantially has makeup with one of copper and iron as a main component and another one thereof as a second component (component contained in the second largest amount). The copper-based powder as used herein comprises not only pure copper powder but also copper alloy powder. Further, pure copper comprises not only copper at a purity of 100% but also copper at a purity of 99.99% or more, which is industrially accepted as pure copper. Similarly, the iron-based powder as used herein comprises not only pure iron powder but also iron alloy powder of stainless steel or the like. Further, pure iron as used herein comprises not only pure iron at a purity of 100% but also iron at a purity of 99.99% or more, which is industrially accepted as pure iron. A kind and a blending ratio of powder corresponding to a third or subsequent component may be determined appropriately as long as the above-mentioned makeup (powder blending ratio) is established.

[0055] As makeup (powder blending ratio) of the raw-material powder, for example, [copper-based powder: from 50% by weight to 70% by weight, iron-based powder: from 30% by weight to 48% by weight, tin powder: from 0% to 5%] can be used. A specific example thereof comprises: [pure iron powder of 140 mesh or less: from 38% by weight to 42% by weight, tin powder of 330 mesh or less: from 1% to 3%, pure copper powder of 200 mesh or less: balance].

[0056] Next, a density ratio of the fluid-dynamic bearing 8 is described. The density ratio of the fluid-dynamic bearing 8 as a whole bearing is set to, for example, 80% or more and 95% or less. The density ratio can be set by, for example, adjusting a material, a particle diameter (distribution), a blending ratio, or the like of metal powder serving as a raw material. Note that, the fluctuation of the density ratio in this case can be evaluated with a microporosity that has a certain correlation with the density ratio. The microporosity as used herein is represented in ratio (percentage) of a volume of micropores per unit volume of the bearing, and empirically has a negative correlation with the density ratio (correlation coefficient of 1).

[0057] Further, a surface porosity of the inner peripheral surface 8a, particularly, surface porosities of inner peripheral surfaces of the inclined ridge portions 8a2 and the belt portions 8a3, which serve as a radial bearing surface, are adjusted to, for example, 15% or less. The surface porosity can be adjusted by, for example, rotary sizing described later.

[0058] As a matter of course, the makeup and the blending ratio of the raw-material powder, the density ratio, and the surface porosity described above are merely examples, and may have appropriate values in accordance with usage or an individual request.

[0059] The fluid-dynamic bearing 8 configured as described above is manufactured through, for example, a compacting step S1, a sintering step S2, a dynamic pressure generating groove sizing step S3, and an oil impregnation step S4. In this case, a dimension sizing step S21 and a rotary sizing step S22 may be set after the sintering step S2 and before the dynamic pressure generating groove sizing step S3.

(S1) Compacting Step

[0060] First, the raw-material powder is prepared as a material for the fluid-dynamic bearing 8 to be finished into a complete product, and then is compressed into a predetermined shape through die press molding. Specifically, although not shown, the compression molding of the raw-material powder is performed by using a molding die set comprising a die, a core pin to be inserted and arranged into a hole of the die, a lower punch arranged between the die and the core pin, and configured to be capable of being raised and lowered relative to the die, and an upper punch configured to be capable of being displaced (raised and lowered) relative to both the die and the lower punch. In this case, the raw-material powder is charged into a space defined by an inner peripheral surface of the die, an outer peripheral surface of the core pin, and an upper end surface of the lower punch. Under a state in which the lower punch is fixed, the upper punch is lowered so that the charged raw-material powder is pressurized in the axial direction. Then, under the pressurized state, the upper punch is lowered to a predetermined position so that the raw-material powder is compressed into a predetermined axial dimension. In this way, a green compact is obtained.

(S2) Sintering Step

[0061] After the green compact is obtained in the above-mentioned manner, the green compact is sintered at a temperature appropriate to a kind of the raw-material powder (for example, a melting point of a metal contained as a main component). With this, a sintered compact is obtained.

(S21) Dimension Sizing Step and (S22) Rotary Sizing Step

[0062] Then, dimension sizing is performed on the sintered compact so that an outer diameter dimension, an inner diameter dimension, and an axial dimension of the sintered compact are corrected into dimensions in accordance with those of a complete product. In addition, the surface porosity of the inner peripheral surface 8a is adjusted to a ratio (for example, within the above-mentioned numerical range of 15% or less) appropriate to a fluid-dynamic bearing having the dynamic pressure generating portion on the inner peripheral surface. At this stage, the array regions A1 and A2 of the predetermined dynamic pressure generating grooves Kal have not yet been formed in the inner peripheral surface 8a of the sintered compact. Similarly, although not shown, the array regions of the predetermined dynamic pressure generating grooves 8b1 have not yet been formed in the upper end surface 8b of the sintered compact.

(S3) Dynamic Pressure Generating Groove Sizing Step

[0063] Through predetermined dynamic pressure generating groove sizing on the sintered compact that is obtained through the series of steps described above, the array regions A1 and A2 of the dynamic pressure generating grooves 8a1 are molded in the inner peripheral surface 8a of the sintered compact. Specifically, although not shown, a molding apparatus comprising a die having a press-fit hole for the sintered compact, a sizing pin arranged so as to be insertable into the press-fit hole of the die, a lower punch arranged between the die and the sizing pin, and configured to be capable of being raised and lowered relative to the die, and an upper punch configured to be capable of being raised and lowered relative to both the die and the lower punch is used to perform predetermined molding on the sintered compact. In this case, on an outer peripheral surface of the sizing pin, there is formed a molding die in conformity with the patterns of the dynamic pressure generating groove array regions A1 and A2 (FIG. 3) on the inner peripheral surface 8a to be molded. On a lower end surface of the upper punch, there is formed a molding die in conformity with the patterns of the array region of the dynamic pressure generating grooves 8b1 (FIG. 4) of the upper end surface 8b to be molded. Thus, under a state in which the sintered compact is arranged on an upper end surface of the die and the sizing pin is inserted along an inner periphery of the sintered compact, the upper punch is lowered to press an upper end surface of the sintered compact. With this, the sintered compact is pressed into the press-fit hole of the die so that an outer peripheral surface of the sintered compact is compressed, and the molding die of the sizing pin inserted along the inner periphery in advance is caused to bite into the inner peripheral surface of the sintered compact. In this way, the pattern of the molding die is transferred onto the inner peripheral surface of the sintered compact so that the array regions A1 and A2 of the dynamic pressure generating grooves 8a1 are molded on the inner peripheral surface. Further, at this time, the molding die formed on the lower end surface of the upper punch is caused to bite into the upper end surface of the sintered compact. With this, the pattern of the molding die is transferred onto the upper end surface. In this way, the array region of the dynamic pressure generating grooves 8b1 is correspondingly molded.

[0064] After the predetermined array regions of the dynamic pressure generating grooves 8a1 and 8b1 are molded respectively in the inner peripheral surface and the upper end surface of the sintered compact in this way, the die is lowered relative to the lower punch so that the sintered compact is released from the restriction by the die. With this, the sintered compact is caused to spring back radially outward. As a result, the sintered compact can be removed from the sizing pin.

(S4) Oil Impregnation Step

[0065] The lubricating-oil composition is impregnated into inner pores of the sintered compact (porous body) obtained in the above-mentioned manner to thereby complete the fluid-dynamic bearing 8.

[0066] In this case, as the lubricating-oil composition for the fluid-dynamic bearing 8, a lubricating-oil composition having the following makeup is used. That is, as the lubricating-oil composition according to the present invention, there is used a lubricating-oil composition that contains a base oil having a kinematic viscosity of more than 30 mm.sup.2/s and 80 mm.sup.2/s or less at a temperature of 40 C. and has makeup adjusted so as to have a kinematic viscosity of 90 mm.sup.2/s or more and 140 mm.sup.2/s or less at a temperature of 40 C.

[0067] The base oil may have any appropriate makeup as long as the above-mentioned condition of the kinematic viscosity is satisfied. However, when the base oil is a mixture of a non-polar oil and a polar oil, it is preferred that a ratio of the non-polar oil to an entirety of the base oil be 20 wt % or more and 30 wt % or less.

[0068] In this case, any appropriate kind of non-polar oil can basically be used as the non-polar oil. For example, a synthetic hydrocarbon oil made from poly--olefin or a hydride thereof, which can provide excellent characteristics as the fluid-dynamic bearing, for example, a wide temperature range of use, excellent lubricity, desirable initial conformability, and desirable durability, is suitable. In this case, it is preferred that a ratio of the synthetic hydrocarbon oil made from poly--olefin or a hydride thereof to the entirety of the base oil be 20 wt % or more and 30 wt % or less.

[0069] In this case, poly--olefin has an average molecular weight of from 200 to 1,600, preferably, from 400 to 800. Poly--olefin obtained by polymerizing decene-1, isobutene, or the like with a Lewis acid complex, an aluminum oxide catalyst, or the like is appropriate. The hydride of poly--olefin is obtained by, for example, hydrogenating poly--olefin under the presence of a hydrogenation catalyst.

[0070] Any appropriate kind of polar oil can basically be used as the polar oil, and, for example, an ester oil excellent in evaporation characteristic, lubricity, and abrasion resistance is suitably used. In this case, an ester to be used may be any of a monoester (ester of a monohydric alcohol and a monovalent fatty acid), a diester (ester of a monohydric alcohol and a divalent fatty acid), a polyol ester (such as an ester of an alcohol having a neopentyl skeleton and a monovalent fatty acid), and a complex ester (oligomer ester obtained by adding a polyvalent fatty acid to a polyol ester being a raw material and cross-linking polyol).

[0071] Alternatively, to the lubricating-oil composition according to this embodiment, publicly known viscosity index improver, metal deactivator, antioxidant, rust inhibitor, pour-point depressant, ashless dispersant, metal-based detergent, surfactant, friction modifier, and the like can be added as a component other than the base oil.

[0072] For any kind of the additives descried above, it is important to set a kind of additive, an addition ratio, the number of additives, and the like so that the kinematic viscosity of the lubricating-oil composition satisfies the above-mentioned numerical range (90 mm.sup.2/s or more and 140 mm.sup.2/s or less) at a temperature of 40 C.

[0073] The fluid-dynamic bearing 8 is fixed to the inner periphery of the housing 7 by a publicly-known method such as press-fitting, press-fitting with bonding, bonding, or welding. After that, the shaft portion 10 of the rotator 3 is inserted along the inner periphery of the fluid-dynamic bearing 8. Then, as a final step, an internal space of the bearing is charged with (filled with) the lubricating-oil composition having the makeup described above to thereby complete the fluid-dynamic bearing device 2 under a state in which an interface of the lubricating-oil composition is held in the sealing space S (refer to FIG. 2).

[0074] The oil impregnation step S4 may be carried out, for example, after the sintered compact for forming the fluid-dynamic bearing 8 is fixed to the inner periphery of the housing 7 so that an impregnation operation for the fluid-dynamic bearing 8 with the lubricating-oil composition and a filling operation for the internal space of the fluid-dynamic bearing device 2 with the lubricating-oil composition are performed simultaneously.

[0075] In the fluid-dynamic bearing device 2 configured as described above, during the rotation of the shaft portion 10 (rotator 3), the regions on the inner peripheral surface 8a of the fluid-dynamic bearing 8 (two upper and lower array regions A1 and A2 of the dynamic pressure generating grooves 8a1), which serve as the radial bearing surface, are opposed to the outer peripheral surface 10a of the shaft portion 10 across the radial bearing gap. Then, along with the rotation of the shaft portion 10, the lubricating-oil composition in the radial bearing gap is forced toward an axial center in each of the array regions A1 and A2 of the dynamic pressure generating grooves 8a1. With this, a pressure of the lubricating-oil composition is increased in a region on an axial center side (belt portion 8a3 in this case). Due to such a dynamic pressure action of the dynamic pressure generating grooves 8a1, the oil film of the lubricating-oil composition is formed in the radial bearing gap, and the first radial bearing portion R1 and the second radial bearing portion R2 configured to radially support the shaft portion 10 in a freely rotatable and non-contact manner are formed separately from each other in the axial direction.

[0076] Further, in the thrust bearing gap between the upper end surface 8b of the fluid-dynamic bearing 8 (array region of the dynamic pressure generating grooves 8b1) and the opposed lower end surface 9a1 of the hub portion 9, an oil film of the lubricating-oil composition is formed due to a dynamic pressure action of the dynamic pressure generating grooves 8b1. Then, due to a pressure of the oil film, the first thrust bearing portion T1 configured to support the rotator 3 in a non-contact manner in thrust directions is formed. Further, the lower end portion 10b of the shaft portion 10 is supported in a rotatable and contact manner by the thrust receiving portion 11 formed in the bottom portion 7e of the housing 7. As a result, the second thrust bearing portion T2 configured to support the rotator 3 in the thrust directions in a contact manner is formed.

[0077] As described above, in the lubricating-oil composition for the fluid-dynamic bearing 8 according to this embodiment, as the base oil, a base oil having the kinematic viscosity of more than 30 mm.sup.2/s and 80 mm.sup.2/s or less at a temperature of 40 C. is used, and the makeup of the lubricating-oil composition is adjusted so that the lubricating-oil composition has the kinematic viscosity of 90 mm.sup.2/s or more and 140 mm/s or less at a temperature of 40 C. Further, in this embodiment, for the adjustment of the viscosity, the amount of addition of a polymer-based viscosity index improver is adjusted to 4 wt % or less. With the adjustment of the kinematic viscosities of the base oil and the lubricating-oil composition as described above, the oil film can be stably formed in the radial bearing gap regardless of a size of the radial bearing gap. Thus, the fluid-dynamic bearing 8 can have a high load-carrying capacity. Further, as a result of large eccentricity of the shaft portion 10, the oil film of the lubricating-oil composition can be stably formed in the radial bearing gap even when the radial bearing gap is reduced. With the adjustment of the makeup of the lubricating-oil composition as described above, for example, addition of the viscosity index improver as little as possible, a viscosity at a low temperature at the time of start of activation can be prevented from being increased more than required. Thus, a required high load-carrying capacity can be stably achieved without increasing the bearing torque.

[0078] Further, in this embodiment, the mixture of a non-polar oil and a polar oil is used as the base oil, and the ratio of the non-polar oil to the base oil as a whole is set to 20 wt % or more and 30 wt % or less. In this way, when the ratio of the non-polar oil having a relatively high electrical resistivity is set to 20 wt % or more, an electrical resistivity of the entire lubricating-oil composition can be held down to such a degree that the energization of the shaft portion 10 and the fluid-dynamic bearing 8 via the lubricating-oil composition can be prevented as much as possible. Thus, an erroneous determination in an evaluation process for the load-carrying capacity based on the electrical resistance method (refer to a description given later for details) can be prevented as much as possible to thereby enable appropriate evaluation of the load-carrying capacity of the fluid-dynamic bearing 8. Further, when the ratio of the non-polar oil is held down to 30 wt % or less, an increase in viscosity of the lubricating-oil composition in a low temperature state can be prevented and hence, an increase in evaporation amount can be prevented.

[0079] Further, in this embodiment, the synthetic hydrocarbon oil made from poly--olefin or a hydride thereof is used as the non-polar oil, and hence a temperature range of use for the lubricating-oil composition can be broadened. Further, the fluid-dynamic bearing can be provided with desirable initial conformability together with excellent lubricity. Further, improvement in durability can be achieved.

[0080] Further, in this embodiment, an ester oil is used as the polar oil, and hence an evaporation characteristic, the lubricity, and abrasion resistance of the lubricating-oil composition can be improved. In particular, when a synthetic hydrocarbon oil made from poly--olefin or a hydride thereof is used as the non-polar oil, the use of the ester oil as a polar oil allows a problem in solubility, which is a disadvantage of polyolefins, to be overcome. Further, with mixing of an ester oil with the synthetic hydrocarbon oil made from poly--olefin or a hydride thereof, desirable bearing performance of the fluid-dynamic bearing can be stably maintained over a long period of time.

[0081] One embodiment of the present invention has been described above. The lubricating-oil composition for a fluid-dynamic bearing according to the present invention, the fluid-dynamic bearing obtained through impregnation of the lubricating-oil composition, and the fluid-dynamic bearing device comprising this bearing are not limited to those exemplified above, and may have any appropriate configurations within the scope of the present invention.

[0082] FIG. 6 is a sectional view of a fluid-dynamic bearing device 12 according to another embodiment of the present invention. As illustrated in FIG. 6, the fluid-dynamic bearing device 12 according to this embodiment differs from the fluid-dynamic bearing device 2 illustrated in FIG. 2 in that the fluid-dynamic bearing device 12 has only a first thrust bearing portion T1. More specifically, in the fluid-dynamic bearing device 12 according to this embodiment, a predetermined thrust-direction gap is constantly defined between a lower end surface 10c of a shaft portion 10 and an upper end surface 7e1 of a bottom portion 7e of a housing 7. In this case, a size of an opposing clearance (thrust-direction gap) between the upper end surface 7e1 of the bottom portion 7e and the lower end surface 10c of the shaft portion 10 is larger than an opposing clearance between an upper end surface 8b of a fluid-dynamic bearing 8 and a lower end surface 9a1 of a disc portion 9a, and, in this case, is set to such a size that is considered not to substantially affect an increase in loss torque during rotational drive. Other configurations are the same as those of the fluid-dynamic bearing device 2 illustrated in FIG. 2 and the like, and hence a detailed description thereof is omitted.

[0083] Also in the fluid-dynamic bearing device 12 according to this embodiment, a base oil having a kinematic viscosity of more than 30 mm.sup.2/s and 80 mm.sup.2/s or less at a temperature of 40 C. is used as a base oil, and a lubricating-oil composition for a fluid-dynamic bearing 8, which is obtained by adjusting makeup so that the lubricating-oil composition has a kinematic viscosity of 90 mm.sup.2/s or more and 140 mm.sup.2/s or less at a temperature of 40 C., is used. In this manner, an oil film can be stably formed in a radial bearing gap regardless of a size of the radial bearing gap. Thus, the fluid-dynamic bearing 8 can have a high load-carrying capacity. Further, a required thrust bearing area can be ensured on an upper end surface 8b of the fluid-dynamic bearing 8. Thus, even when a thrust bearing portion (first thrust bearing portion T1) is formed only on the upper end surface 8b side of the fluid-dynamic bearing 8, excellent rotational accuracy can be achieved. Accordingly, as illustrated in FIG. 6, sufficient thrust bearing performance can be provided without supporting the lower end surface 10c of the shaft portion 10 in the thrust directions. Thus, costs can be reduced correspondingly to simplification of a lower end shape of the shaft portion 10, such as omission of a thrust receiving portion 11.

[0084] As a matter of course, a publicly known radial dynamic pressure generating portion having a shape other than a dynamic pressure generating groove, such as so-called multi-arc shape, step-like shape, or corrugated shape can be used for any one or both of the radial dynamic pressure generating portions (array regions A1 and A2 of the dynamic pressure generating grooves 8a1 in FIG. 3) that form the radial bearing portions R1 and R2 in cooperation with an outer peripheral surface 10a of the shaft portion 10.

[0085] In the description given above, the housing 7 of the fluid-dynamic bearing device 2 is fixed to an inner periphery of the base portion 6 of the fan motor 1. However, the housing 7 may be directly mounted to, for example, a base portion of an information apparatus (not shown) to which the fan motor 1 is to be mounted. Alternatively, apart corresponding to those base portions may be formed integrally with the housing 7.

[0086] Further, in the description given above, an external force for forcing the shaft portion 10 toward the bottom portion 7e of the housing 7 is exerted on the shaft portion 10 (rotator 3) by arranging the magnets Sb and the coils 5a in an axially shifted manner. However, a method of exerting such an external force on the shaft portion 10 is not limited to that described above. Although not shown, the magnetic force can be exerted on the rotator 3 by, for example, arranging magnetic members that can attract the magnets 5b so that the magnetic members are opposed to the magnets Sb in the axial direction. Further, when a thrust force as a reaction force to an air-sending action is sufficiently large and the shaft portion 10 can be forced downward only with the thrust force, a magnetic force (magnetic attraction force) as the external force for forcing the shaft portion 10 downward is not required to be used.

[0087] Further, the description has been given above of a case in which the present invention is applied to the fluid-dynamic bearing device 2 in which the rotator 3 having the fan 4 is fixed to the shaft portion 10. However, the present invention is also preferably applicable to the fluid-dynamic bearing device 2 in which a disc hub having a disc mounting surface, or a polygon mirror is fixed as the rotator 3 to the shaft portion 10. That is, the present invention is also preferably applicable to the fluid-dynamic bearing device 2 to be built not only in the fan motor 1 as illustrated in FIG. 1 but also in other electrical apparatus such as a spindle motor for a disc device or a polygon scanner motor for a laser beam printer (LBP).

EXAMPLE

[0088] Now, an example for verifying actions and effects of the present invention is described.

[0089] In this example, lubricating-oil compositions having the makeup according to the present invention (Examples 1 to 3) and lubricating-oil compositions having makeup other than that according to the present invention (Comparative Examples 1 to 4) were prepared to evaluate a kinematic viscosity and a viscosity of each of the lubricating-oil compositions. Further, a characteristic relating to a load-carrying capacity was evaluated for a fluid-dynamic bearing device obtained by impregnating a fluid-dynamic bearing with each of the lubricating-oil compositions so that a bearing internal space was filled with the lubricating-oil composition.

[0090] In this example, a lubricating-oil composition containing a mixture of a non-polar oil and a polar oil as a base oil was used as each of the lubricating-oil compositions to be evaluated. Specifically, a synthetic hydrocarbon oil made from poly--olefin (PAO) was used as the non-polar oil, a polyol ester oil and a diester oil were used as the polar oil, and a polymethacrylate-based viscosity index improver was used as a viscosity index improver. Further, all the used lubricating-oil compositions contained an antioxidant and a rust inhibitor. In Table 1, there are shown ratios (blending ratios) [wt %] of the non-polar oil, the polar oil, and the viscosity index improver to the lubricating-oil compositions according to Examples 1 to 3 and Comparative Examples 1 to 4.

TABLE-US-00001 TABLE 1 Comparative Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 1 Example 2 Example 3 Example 4 Polar oil, wt % 70 70 70 75 75 70 73 Non-polar oil, wt % 25 25 25 16 17 25 23 Viscosity index 3 3 3 7 6 3 2 improver, wt % Other additive(s), wt % 2 2 2 2 2 2 2 Total, wt % 100 100 100 100 100 100 100

[0091] Further, the kinematic viscosity [mm.sup.2/s] of the base oil at a temperature of 40 C. was acquired based on JIS K 2283. The kinematic viscosities of the base oils of Examples 1 to 3 and Comparative Examples 1 to 4 at a temperature of 40 C. are as shown in Table 2.

[0092] Further, the kinematic viscosity [mm.sup.2/s] of each of the lubricating-oil compositions at a temperature of 40 C. was acquired based on JIS K 2283. The kinematic viscosities of the lubricating-oil compositions of Examples 1 to 3 and Comparative Examples 1 to 4 at a temperature of 40 C. are as shown in Table 2.

[0093] Further, a viscosity [mPa.Math.s] of each of the lubricating-oil compositions at a temperature of 40 C. was acquired by using a cone-and-plate rotational viscometer (shear rate: 300 s.sup.1). The viscosities of the lubricating-oil compositions of Examples 1 to 3 and Comparative Examples 1 to 4 at a temperature of 40 C. are as shown in Table 2.

[0094] Further, for the characteristic relating to the load-carrying capacity of the fluid-dynamic bearing device obtained through impregnation of and filling with each of the lubricating-oil compositions, a floating minimum rotation speed based on an electrical resistance method was measured. Further, a maximum value of an eccentricity ratio of the shaft portion being rotatable with respect to the fluid-dynamic bearing in a non-contact manner was obtained based on the result of the measurement.

[0095] Among the above-mentioned values, the floating minimum rotation speed was measured in the following manner. That is, although not shown, an elongated shaft portion was inserted into the fluid-dynamic bearing according to the present invention. A motor was coaxially coupled to one axial end of the shaft portion, and the fluid-dynamic bearing was fixed to an inner periphery of a housing. Then, a gap between the shaft portion and the fluid-dynamic bearing was sealed with a sealing member. After that, the shaft portion was driven at a predetermined rpm by the motor under a state in which a predetermined load, in this case, a radial load (two kinds of loads of 3N and 6N) was being applied to the fluid-dynamic bearing via the housing. The rpm was gradually decreased from 2,000 min.sup.1 by 100 min.sup.1 for each time. Then, the rpm of the shaft portion at the time when conduction (electrical connection) between the shaft portion and the fluid-dynamic bearing was detected was measured with a rotation speed sensor provided to another axial end of the shaft portion. The measured rpm was obtained as the floating minimum rotation speed [min.sup.1]. The floating minimum rotation speeds of Examples 1 to 3 and Comparative Examples 1 to 4 are as shown in Table 1.

[0096] For the eccentricity ratio, a value back-calculated from the load (3N or 6N) used at the time of testing was obtained by using a calculation program that received the eccentricity ratio of the shaft portion at the time of the detection of the conduction between the shaft portion and the fluid-dynamic bearing as described above, a radius value of the radial bearing gap between the fluid-dynamic bearing and the shaft portion, and the rotation speed (floating minimum rotation speed) as input values and output the load as an output value. The eccentricity ratios of Examples 1 to 3 and Comparative Examples 1 to 4 are as shown in Table 2.

TABLE-US-00002 TABLE 2 Example Example Example Comparative Comparative Comparative Comparative 1 2 3 Example 1 Example 2 Example 3 Example 4 Kinematic viscosity of 34 33 46 23 34 30 83 base oil (40 C.), mm.sup.2/s Ratio of non-polar oil 26 26 26 18 18 26 24 to base oil, wt % Kinematic viscosity of 101 104 115 105 115 94 144 lubricating-oil composition (40 C.), mm.sup.2/s Viscosity of lubricating- 19,000 13,000 25,000 13,000 17,000 15,000 57,000 oil composition (40 C.), mPa .Math. s Floating minimum 400 (6N) 200 (3N) 200 (3N) 400 (3N) 600 (6N) 1,000 (6N) 200 (3N) rotation speed, min.sup.1 Eccentricity ratio, % 95 95 95 87 91 83 95

[0097] As shown in Table 2, when Examples 1 to 3 and Comparative Example 1 are compared to each other, Comparative Example 1 and Examples 1 to 3 had equivalent values for the kinematic viscosity of the lubricating-oil composition. Meanwhile, the kinematic viscosity of the base oil in Comparative Example 1 was below the numerical range according to the present invention. In this case, the floating minimum rotation speed of Comparative Example 1 was the same value as the floating minimum rotation speed of Example 1, but under the load different from that of Example 1 (half the load of Example 1). From this result, it is understood that the load-carrying capacity of Comparative Example 1 is smaller than that of Example 1. Further, the floating minimum rotation speed of Comparative Example 1 was significantly higher than the floating minimum rotation speeds of Examples 2 and 3 even under the same level of the load. From this result, it is understood that the load-carrying capacity of Comparative Example 1 is smaller than the load-carrying capacities of Example 2 and Example 3.

[0098] When Example 1 and Comparative Example 2 are compared to each other, Comparative Example 2 and Example 1 had equivalent values for the kinematic viscosity of the base oil and the kinematic viscosity of the lubricating-oil composition. Meanwhile, a content ratio of the non-polar oil to the base oil in Comparative Example 2 was below the numerical range according to the present invention. Thus, there is a high possibility in that the floating minimum rotation speed of Comparative Example 2 may have been measured larger than an actual value, and thus the floating minimum rotation speed of Comparative Example 2 had low reliability as an index indicating the load-carrying capacity.

[0099] When Examples 1 to 3 and Comparative Example 3 are compared to each other, Comparative Example 3 and Examples 1 to 3 had equivalent values for the kinematic viscosity of the lubricating-oil composition. Meanwhile, the kinematic viscosity of the base oil in Comparative Example 3 was below the numerical range (more than 30 mm.sup.2/s and 80 mm.sup.2/s or less) according the present invention. Therefore, the floating minimum rotation speed of Comparative Example 3 was significantly larger than that of Example 1 even under the same level of the load. From this result, it is understood that the load-carrying capacity of Comparative Example 3 is smaller than that of Example 1. Further, when Comparative Example 3 and Examples 2 and 3 are compared to each other, a magnitude of the load on Comparative Example 3 was twice a magnitude of the load on Examples 2 and 3. Meanwhile, the floating minimum rotation speed of Comparative Example 3 was five times larger than those of Examples 2 and 3. From this result, it is understood that the load-carrying capacity of Comparative Example 3 is smaller than the load-carrying capacities of Examples 2 and 3.

[0100] When Examples 1 to 3 and Comparative Example 4 are compared to each other, the kinematic viscosity of the base oil and the kinematic viscosity of the lubricating-oil composition in Comparative Example 4 were both larger than the kinematic viscosities of Examples 1 to 3. Meanwhile, both of the kinematic viscosities of Comparative Example 4 exceeded the numerical ranges according to the present invention (base oil: more than 30 mm.sup.2/s and 80 mm.sup.2/s or less, lubricating-oil composition: 90 mm.sup.2/s or more and 140 mm.sup.2/s or less). Thus, the load-carrying capacity of Comparative Example 4 (floating minimum rotation speed, eccentricity ratio) was at the same level as that of the load-carrying capacities of Example 1 to 3. Meanwhile, the viscosity of the lubricating-oil composition at a low temperature was larger than the viscosities of Examples 1 to 3. Thus, there arises a risk of excessive power consumption (bearing torque) of the motor or a risk of failing to activate the motor.

REFERENCE SIGNS LIST

[0101] 1 fan motor [0102] 2, 12 fluid-dynamic bearing device [0103] 3 rotator [0104] 4 fan [0105] 5 drive portion [0106] 5a coil [0107] 5b magnet [0108] 6 base portion [0109] 7 housing [0110] 7c upper end surface [0111] 7d sealing surface [0112] 8 fluid-dynamic bearing [0113] 8a inner peripheral surface [0114] 8a1 dynamic pressure generating groove [0115] 8a2 inclined ridge portion [0116] 8a3 belt portion [0117] 8b upper end surface [0118] 8b1 dynamic pressure generating groove [0119] 8c lower end surface [0120] 8d outer peripheral surface [0121] 8d1 axial groove [0122] 9 hub portion [0123] 9a disc portion [0124] 9a1 lower end surface [0125] 9b, 9c cylindrical portion [0126] 9d flange portion [0127] 9e mounting hole [0128] 10 shaft portion [0129] 10a outer peripheral surface [0130] 10b lower end portion [0131] 10c lower end surface [0132] 11 thrust receiving portion [0133] A1, A2 dynamic pressure generating groove array region [0134] D1 inner diameter dimension [0135] D2 outer diameter dimension [0136] L axial dimension [0137] R1, R2 radial bearing portion [0138] S sealing space [0139] T1, T2 thrust bearing portion