Lubricating oil composition for fluid dynamic bearings, fluid dynamic bearings, and fluid dynamic bearing devices

Abstract

To provide a lubricant composition for a fluid dynamic pressure bearing, that allows the fluid dynamic pressure bearing to exhibit high load capacity as a bearing for a small motor while avoiding an increase in bearing torque.SOLUTION: A lubricant composition for a fluid dynamic pressure bearing used in the fluid dynamic pressure bearing 8 in which radial dynamic pressure generating parts A1, A2 are provided on an inner circumferential surface 8a of a porous body has the following composition. That is, as a base oil of the lubricant composition, the base oil having a kinematic viscosity at 40°C of more than 30 mm2 / s and 80 mm2 / s or less is used, and the composition of the lubricant composition is adjusted so that a kinematic viscosity at 40°C of the lubricant composition is 90 mm2 / s or more and 140 mm2 / s or less.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a lubricating oil composition for a fluid dynamic bearing, a fluid dynamic bearing, and a fluid dynamic bearing device, and more particularly to a lubricating oil composition for use in a fluid dynamic bearing equipped with a radial dynamic pressure generating portion. [Background technology]

[0002] A fluid dynamic bearing is a bearing that is used by impregnating the internal pores of a porous body formed of, for example, sintered metal with a lubricating oil (hereinafter, referred to as a lubricating oil composition in this specification), and the lubricating oil composition impregnated in the internal pores seeps into the sliding part with the shaft part as the shaft part rotates relative to the inner circumference to form an oil film, which supports the rotation of the shaft part. Due to their excellent rotational accuracy and quietness, such bearings are suitably used as bearing devices for motors mounted on various electrical devices including information devices, more specifically, for spindle motors in disk drives for HDDs, CDs, DVDs, and Blu-ray disks, for fan motors mounted in these disk drives and PCs, or for polygon scanner motors mounted in laser beam printers (LBPs).

[0003] It is also known that dynamic pressure generating parts such as dynamic pressure grooves are formed on at least one of the inner peripheral surface and the axial end surface of the fluid dynamic bearing in order to further improve noise reduction and extend the life of the bearing. In this case, the lubricating oil composition impregnated in the internal pores of the fluid dynamic bearing is allowed to seep into the gap between the region of the inner peripheral surface of the bearing where the dynamic pressure generating parts are formed and the outer peripheral surface of the shaft as the shaft (rotor) inserted into the inner circumference of the bearing rotates, thereby generating a dynamic pressure action in the gap, and the shaft can be supported for rotation without contact with the bearing by the oil film of the lubricating oil composition.

[0004] Recently, as information devices become smaller and thinner, there is a demand for miniaturization of various motors mounted on the information devices. For example, cooling fan motors used in notebook computers and the like are becoming thinner, and the bearing devices used in these motors are also required to be thinner. At the same time, cooling performance is required to be at the same level as before or higher. Since the thinner bearing device leads to a reduction in the bearing area, there is a high risk of a decrease in bearing rigidity due to the reduction in bearing area, not only when the size of the impeller (fan) is increased, but also when the size of the impeller is maintained.

[0005] Here, Patent Document 1 proposes a lubricating oil composition suitable for a fluid dynamic bearing having a dynamic pressure generating portion such as a dynamic pressure groove provided on the inner peripheral surface thereof. That is, this patent document proposes a lubricating oil composition containing a base oil selected from a compound of a poly-α-olefin or its hydrogenated product and an ester, and an ester, and a fluid dynamic bearing impregnated with this lubricating oil composition. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 3782889 Summary of the Invention [Problem to be solved by the invention]

[0007] Recently, cooling fan motors used in notebook computers and the like have become thinner, and there is a demand for further reduction in the axial dimension of the bearings used in such motors. On the other hand, as the axial dimension of the bearing becomes smaller, the area in which the radial dynamic pressure generating portion can be formed decreases accordingly, and the dynamic pressure that can be generated in the radial bearing gap also becomes smaller. This makes it difficult to support a rotating body such as an impeller including a shaft with high precision, which may lead to a decrease in rotational precision (NRRO). As a result of this decrease in rotational precision and an increase in the eccentricity of the shaft (rotating body), the radial bearing gap becomes smaller, and the risk of contact between the shaft and the bearing increases.

[0008] In order to generate sufficient dynamic pressure even when the radial bearing clearance is reduced due to eccentricity of the shaft portion, it is conceivable to increase the viscosity of the lubricating oil composition by adjusting the composition of the lubricating oil composition, as described in Patent Document 1, for example. However, simply increasing the viscosity of the lubricating oil composition increases the bearing stiffness while also increasing the bearing torque, resulting in a new problem of increased power consumption by the motor.

[0009] In view of the above circumstances, the technical problem to be solved by the present invention is to provide a lubricating oil composition for a fluid dynamic bearing that can exhibit high load capacity as a bearing for a small motor while avoiding an increase in bearing torque. [Means for solving the problem]

[0010] The above-mentioned problems are solved by the lubricating oil composition for a fluid dynamic bearing according to the present invention. That is, the lubricating oil composition is a porous body having internal pores, and is used in a fluid dynamic bearing having a radial dynamic pressure generating portion provided on the inner peripheral surface of the porous body. The lubricating oil composition contains a base oil having a kinetic viscosity of 30 mm at 40° C. 2 / s and 80mm 2 A base oil having a kinematic viscosity of 90mm / s or less at 40°C is used. 2 / s or more and 140 mm 2 / s or less. In this specification, the term "fluid dynamic bearing" refers to a dynamic pressure bearing other than a perfect circular bearing. Therefore, in this specification, the term "radial dynamic pressure generating portion" refers to a portion having a shape other than a perfect cylinder that can generate the dynamic pressure action of the lubricating oil composition in the radial bearing gap between the rotating body such as a shaft.

[0011] The present inventors have found that, with respect to a polymer-based viscosity index improver, which is often added when increasing the viscosity of a lubricating oil composition, when the eccentricity of the shaft portion is large and the radial bearing gap is very small, this type of viscosity index improver has difficulty entering the radial bearing gap. 2 / s and 80mm 2 The lubricating oil composition has a kinematic viscosity of 90 mm / s or less at 40°C. 2 / s or more and 140 mm 2 The composition of the lubricating oil composition was adjusted so that the viscosity of the lubricating oil composition was 1 / s or less. A polymer-based viscosity index improver can be used to adjust the viscosity, but if the amount added is too large, the oil film formation becomes unstable, so when a viscosity index improver is added, it is preferable to adjust the amount of the viscosity index improver to 4 wt% or less. By adjusting the kinetic viscosity of the base oil and the lubricating oil composition in this way, the oil film of the lubricating oil composition can be stably formed in the radial bearing gap regardless of the size of the radial bearing gap. Therefore, it is possible to make the fluid dynamic bearing exhibit high load capacity. In addition, by adjusting the composition of the lubricating oil composition as described above, such as by not adding viscosity index improvers as much as possible, it is possible to avoid a situation in which the viscosity increases more than necessary at low temperatures, which is the start of startup. Therefore, it is possible to stably exhibit the required high load capacity without increasing the bearing torque.

[0012] In addition, in the lubricating oil composition for a fluid dynamic bearing according to the present invention, the base oil may be a mixture of a non-polar oil and a polar oil, and the ratio of the non-polar oil to the total base oil may be 20 wt % or more and 30 wt % or less.

[0013] A method using an electrical resistance method is known for evaluating the load capacity (load capacity) of a fluid dynamic bearing. In this method, for example, the rotation speed of the shaft part inserted into the inner circumference is changed while a predetermined load is applied to the fluid dynamic bearing, and the load capacity of the fluid dynamic bearing is evaluated based on the rotation speed at the time when contact between the shaft part and the fluid dynamic bearing is detected. Here, when the base oil is a mixture of non-polar oil and polar oil, if the ratio of non-polar oil is too low, the ratio of polar oil with relatively low electrical resistivity increases, and as a result, even if there is no actual contact, the shaft part and the fluid dynamic bearing are erroneously judged to be in contact due to the passage of electricity between them through the lubricating oil. In other words, the load capacity is likely to be evaluated lower than it actually is. In this regard, by making the ratio of non-polar oil with a relatively high electrical resistivity 20wt% or more, the electrical resistivity of the lubricating oil composition as a whole can be kept low to a degree that allows the passage of electricity between the shaft part and the fluid dynamic bearing through the lubricating oil composition to be prevented as much as possible. Therefore, it is possible to prevent erroneous judgments in the load capacity evaluation process based on the electrical resistance method as much as possible and to properly evaluate the load capacity of the fluid dynamic bearing. Also, by keeping the ratio of non-polar oil to 30 wt% or less, it is possible to avoid a situation in which the viscosity becomes high at low temperatures and a situation in which the amount of evaporation becomes large.

[0014] Furthermore, when the base oil is a mixture of a non-polar oil and a polar oil, the non-polar oil may be a synthetic hydrocarbon oil made of poly-α-olefin or a hydrogenated product thereof.

[0015] In this way, by using a synthetic hydrocarbon oil made of poly-α-olefin (also called PAO) or its hydrogenated product (also called PAOH) as a non-polar oil constituting a part of the base oil, the operating temperature range of the lubricating oil composition can be expanded. In addition to excellent lubricity, good initial running-in can be imparted to the fluid dynamic bearing. Furthermore, it is possible to improve durability.

[0016] Also, when the base oil is a mixture of a non-polar oil and a polar oil, the polar oil may be an ester oil.

[0017] In this way, by using ester oil as a polar oil that constitutes a part of the base oil, it is possible to improve evaporation characteristics, lubricity, and even wear resistance. In particular, when a synthetic hydrocarbon oil made of poly-α-olefin or its hydrogenated product is used as a non-polar oil, the solubility, which is a drawback of polyolefins, can be overcome by using ester oil as a polar oil composition. In addition, by mixing ester oil with a synthetic hydrocarbon oil made of poly-α-olefin or its hydrogenated product, it is possible to stably maintain the good bearing performance of the fluid dynamic bearing for a long period of time. This is thought to be because the mixing of ester oil suppresses the generation of bubbles from poly-α-olefin, or even if bubbles are generated, they disappear immediately.

[0018] The lubricating oil composition for a fluid dynamic bearing according to the above description may be provided as a fluid dynamic bearing in which the lubricating oil composition is impregnated into internal pores. In this case, the fluid dynamic bearing according to the present invention may be provided with a dynamic pressure groove arrangement region as a radial dynamic pressure generating portion on its inner circumferential surface.

[0019] By providing the dynamic pressure groove region in this manner, the dynamic pressure action of the lubricating oil composition can be effectively generated in the bearing gap, which, combined with the action of the lubricating oil composition described above, makes it possible to very effectively and sustainably form an oil film of the lubricating oil composition in the bearing gap.

[0020] In the fluid dynamic bearing according to the present invention, the inner diameter may be 2.0 mm or less and the axial dimension may be 3.5 mm or less.

[0021] The fluid dynamic bearing according to the present invention can exhibit high load carrying capacity while avoiding an increase in bearing torque, and therefore can exhibit the required bearing performance even in cases where the inner diameter and axial dimensions are small as described above and the area of ​​the inner circumferential surface of the bearing where the radial bearing surface is formed cannot be sufficiently secured.

[0022] Furthermore, the fluid dynamic bearing described above can be suitably provided as a fluid dynamic bearing device comprising, for example, the fluid dynamic bearing, a housing to whose inner periphery the fluid dynamic bearing is fixed, a rotating body having a shaft portion inserted into the inner periphery of the fluid dynamic bearing, and a radial bearing portion that supports the shaft portion in a non-contact manner in the radial direction by an oil film of a lubricating oil composition formed in a radial bearing gap between the inner periphery of the fluid dynamic bearing and the outer periphery of the shaft portion.

[0023] Furthermore, the fluid dynamic bearing device according to the present invention may be one in which the eccentricity of the shaft portion, which can rotate without contacting the fluid dynamic bearing, is a maximum of 98%. Note that the eccentricity here refers to the ratio of the amount of eccentricity of the shaft portion to the radius of the radial bearing gap.

[0024] As described above, the fluid dynamic bearing according to the present invention can exhibit high load capacity while avoiding an increase in bearing torque. Therefore, even if the eccentricity increases, the stable oil film formation capability keeps the risk of contact between the shaft and the fluid dynamic bearing as low as possible, making it possible to stably exhibit high load capacity.

[0025] As described above, the fluid dynamic bearing device described above is capable of providing a high load capacity while avoiding an increase in bearing torque, and can therefore be suitably used, for example, as a motor equipped with this fluid dynamic bearing device. Effect of the Invention

[0026] As described above, the lubricating oil composition for a fluid dynamic bearing according to the present invention makes it possible to provide a fluid dynamic bearing that is capable of exhibiting high load capacity for use in a small motor while avoiding an increase in bearing torque. [Brief description of the drawings]

[0027] [Figure 1] 1 is a cross-sectional view of a fan motor according to an embodiment of the present invention. [Diagram 2] 2 is a cross-sectional view of the fluid dynamic bearing device shown in FIG. 1. [Diagram 3] FIG. 3 is a cross-sectional view of the fluid dynamic bearing shown in FIG. 2. [Figure 4] FIG. 3 is a plan view of the fluid dynamic bearing shown in FIG. 2. [Diagram 5] FIG. 3 is a bottom view of the fluid dynamic bearing shown in FIG. 2. [Figure 6] FIG. 4 is a cross-sectional view of a fluid dynamic bearing device according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] An embodiment of the present invention will be described below with reference to the drawings. In the following description, the disk side of the hub is treated as the "upper side" and the bottom side of the housing is treated as the "lower side" when viewed from the fluid dynamic bearing. Of course, this up-down direction does not limit the actual installation or usage of the product.

[0029] 1 conceptually shows an example of the configuration of a fan motor 1 according to this embodiment. This fan motor 1 comprises a fluid dynamic bearing device 2, a plurality of fans 4 provided on a rotor 3 of the fluid dynamic bearing device 2, and a drive unit 5 for rotating these fans 4 integrally with the rotor 3. The drive unit 5 is made up of, for example, a coil 5a and a magnet 5b opposed to each other with a radial gap in between, and in this embodiment, the coil 5a is fixed to a base part 6 which is the fixed side of the fan motor 1, and the magnet 5b is fixed to the rotor 3 which is the rotating side of the fan motor 1.

[0030] In the fan motor 1 configured as above, when current is applied to the coil 5a, the magnet 5b rotates due to the excitation force between the coil 5a and the magnet 5b, and this rotation causes the multiple fans 4 erected on the outer periphery of the rotor 3 to rotate integrally with the rotor 3. This rotation causes each fan 4 to generate an airflow in a direction according to its shape (here, for example, an airflow toward the outside in the radial direction), and a secondary airflow is generated that is drawn into this airflow and flows from the upper side to the lower side in the axial direction of the fan motor 1. By generating an airflow around the fan motor 1 in this way, it is possible to cool an information device (not shown) to which the fan motor 1 is attached (not shown).

[0031] Furthermore, when an airflow occurs in the axial direction of the fan motor 1 as described above, a force (reaction force) in the opposite direction to the airflow is generated on the rotor 3 of the fluid dynamic bearing device 2. A magnetic force (repulsive force) in a direction that cancels out the reaction force is applied between the coil 5a and the magnet 5b, and a thrust load generated by the difference in magnitude between the reaction force and the magnetic force acts on the thrust bearing parts T1 and T2 (see FIG. 2 described later) of the fluid dynamic bearing device 2. The magnetic force in the direction that cancels out the reaction force can be generated, for example, by displacing the coil 5a and the magnet 5b in the axial direction (detailed illustration is omitted). Furthermore, when the rotor 3 rotates, a radial load acts on the shaft part 10 of the fluid dynamic bearing device 2 described later. This radial load acts on the radial bearing parts R1 and R2 of the fluid dynamic bearing device 2.

[0032] 2 shows a cross-sectional view of a fluid dynamic bearing device 2 incorporated in a fan motor 1. This fluid dynamic bearing device 2 comprises a housing 7, a fluid dynamic bearing 8 fixed to the inner periphery of the housing 7, and a rotor 3 that rotates relative to the fluid dynamic bearing 8.

[0033] In this embodiment, the rotating body 3 has a hub portion 9 arranged on the upper end opening side of the housing 7 , and a shaft portion 10 inserted into the inner periphery of the fluid dynamic bearing 8 .

[0034] 1 and 2, the hub portion 9 is composed of a disk portion 9a covering the upper end opening side of the housing 7, a first cylindrical portion 9b extending axially downward from the disk portion 9a, a second cylindrical portion 9c located radially outward from the first cylindrical portion 9b and extending axially downward from the disk portion 9a, and a flange portion 9d extending radially outward from the axial lower end of the second cylindrical portion 9c. The disk portion 9a faces one end face (upper end face 8b) of the fluid dynamic bearing 8 fixed to the inner circumference of the housing 7. The fans 4 are provided integrally with the hub portion 9 in a standing manner from the outer circumferential edge of the flange portion 9d.

[0035] In this embodiment, the shaft portion 10 is formed separately from the hub portion 9, and its upper end is fixed to a mounting hole 9e provided in the hub portion 9. Here, the shaft portion 10 has an outer peripheral surface 10a with a constant outer diameter dimension, and a lower end portion 10b, for example, in a partially spherical shape, which is continuous with the lower end of the outer peripheral surface 10a. In other words, the shaft portion 10 has a shape that allows it to be inserted into the inner circumference of the fluid dynamic bearing 8 from one axial side. Of course, the shaft portion 10 and the hub portion 9 may be integrally formed from the same material. Alternatively, the shaft portion 10 and the hub portion 9 may be formed from different materials, with one of them being an insert part and the other being formed by injection molding of metal or resin.

[0036] The housing 7 has an open upper end and a closed lower end. A 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 (see FIG. 1). The axial distance between the upper end surface 7c of the housing 7 and the lower end surface 9a1 of the disk portion 9a of the hub portion 9 is larger than the distance between the upper end surface 8b of the fluid dynamic bearing 8 and the lower end surface 9a1 of the disk portion 9a, and is set to a size that is deemed to have no substantial effect on an increase in loss torque during rotational driving.

[0037] A tapered seal surface 7d is formed on the upper outer periphery of the housing 7, the outer diameter of which increases as it goes upward. This tapered seal surface 7d forms an annular seal space S between the inner periphery 9b1 of the first cylindrical portion 9b, in which the radial opposing distance gradually decreases from the closed side (lower side) of the housing 7 to the open side (upper side). This seal space S communicates with the outer diameter side of the thrust bearing gap of the first thrust bearing portion T1 described later when the shaft portion 10 and the hub portion 9 rotate, and allows the lubricating oil composition to flow between the seal space S and the bearing internal space including each bearing gap. In addition, in a state in which the lubricating oil composition is impregnated into the internal cavity of the fluid dynamic bearing 8 and the same lubricating oil composition is filled into the bearing internal space, the filling amount of the lubricating oil composition is adjusted so that the oil level (gas-liquid interface) of the lubricating oil composition is always maintained in the seal space S (see FIG. 2).

[0038] Furthermore, when the housing 7 has the above-mentioned seal structure, the coil 5a is positioned radially outward of the first cylindrical portion 9b of the hub portion 9, and is disposed so that the first cylindrical portion 9b and the coil 5a partially overlap in the axial direction. This makes it possible to reduce the thickness of the housing 7 and, ultimately, the fluid dynamic bearing device 2 (reduce the axial dimension).

[0039] In this embodiment, a thrust receiving portion 11 that receives the lower end portion 10b of the spherical shaft portion 10 is provided on the bottom portion 7e of the housing 7. That is, when the fluid dynamic bearing device 2 is in a completed state, this thrust receiving portion 11 is always in contact with the lower end portion 10b of the shaft portion 10, and is capable of supporting the rotation of the shaft portion 10. The vertical position of the thrust receiving portion 11 with respect to the bearing abutment surface 7f of the housing 7 should preferably be set so that the abutment position between the shaft portion 10 and the thrust receiving portion 11 is within the vertical region of the inner chamfered portion 8e of the fluid dynamic bearing 8, for example.

[0040] In principle, the material and composition of the housing 7 are arbitrary, and known materials such as resins and metals can be appropriately adopted depending on the fixing means for the fluid dynamic bearing 8 to the housing 7, which will be described later.

[0041] The fluid dynamic bearing 8 is a porous body of sintered metal formed by compressing and sintering, for example, a metal powder having a predetermined composition or a raw powder mainly composed of this metal powder, and has a cylindrical shape. In this embodiment, it has a cylindrical shape as shown in FIG. 4 etc. An area in which a plurality of dynamic pressure grooves 8a1 are arranged is formed as a radial dynamic pressure generating part on the entire or part of the inner circumferential surface 8a of the fluid dynamic bearing 8. In other words, the fluid dynamic bearing 8 includes the porous body having the above configuration and a dynamic pressure groove 8a1 arrangement area provided on the inner circumferential surface 8a of the porous body. In this embodiment, as shown in Fig. 3, the dynamic pressure groove 8a1 arrangement region is formed by arranging a plurality of dynamic pressure grooves 8a1 inclined at a predetermined angle with respect to the circumferential direction, inclined hills 8a2 dividing the dynamic pressure grooves 8a1 in the circumferential direction, and bands 8a3 (both the inclined hills 8a2 and bands 8a3 are cross-hatched in Fig. 3) in a herringbone pattern and extending in the circumferential direction to divide each dynamic pressure groove 8a1 in the axial direction, and is formed in two consecutive locations in the axial direction. In this case, the upper dynamic pressure groove arrangement region A1 and the lower dynamic pressure groove arrangement region A2 are both formed axially symmetrically with respect to the axial center line (a virtual line connecting the axial centers of the bands 8a3 in the circumferential direction), and their axial dimensions are equal to each other.

[0042] In this embodiment, an area in which a plurality of dynamic pressure grooves 8b1 are arranged as a thrust dynamic pressure generating portion is formed on the entire surface or a part of the upper end surface 8b of the fluid dynamic pressure bearing 8. This dynamic pressure groove 8b1 arrangement area has a form in which a plurality of dynamic pressure grooves 8b1 extending in a spiral shape are arranged side by side in the circumferential direction, as shown in Fig. 4, for example. In this case, the spiral direction of the dynamic pressure grooves 8b1 is set to a direction corresponding to the rotation direction of the rotor 3. When the fluid dynamic pressure bearing device 2 shown in Fig. 2 is rotationally driven, the dynamic pressure groove 8b1 arrangement area configured as described above forms a thrust bearing gap of a first thrust bearing portion T1 described later between the facing lower end surface 9a1 of the disk portion 9a of the hub portion 9.

[0043] On the other hand, no thrust dynamic pressure generating portion is formed on the lower end surface 8c of the fluid dynamic bearing 8. That is, in this embodiment, the lower end surface 8c has a flat shape as shown in Fig. 5. The lower end surface 8c abuts on the housing 7 on the radially outer side of the housing 7 relative to the thrust receiving portion 11. In this embodiment, the vertical position of the bearing abutment surface 7f of the housing 7 is appropriately set within a range in which the space between the upper end surface 8b of the fluid dynamic bearing 8 and the lower end surface 9a1 of the hub portion 9 can function as a thrust bearing gap.

[0044] One or more (five in this embodiment) axial grooves 8d1 are formed in the outer peripheral surface 8d of the fluid dynamic bearing 8 (see, for example, FIG. 4). When the fluid dynamic bearing 8 is fixed to the housing 7, the axial grooves 8d1 form a flow path for the lubricating oil composition between the inner peripheral surface 7a of the housing 7 (see FIG. 2).

[0045] Next, various dimensions of the fluid dynamic bearing 8 will be described with reference to Fig. 3. The axial dimension L of the fluid dynamic bearing 8 (the axial distance between both end faces 8b, 8c) is set to 4.8 mm or less, preferably 3.5 mm or less, and more preferably 1.8 mm or less, from the viewpoint of reducing the thickness of the fluid dynamic bearing device 2 and therefore the fan motor 1. On the other hand, from the viewpoint of ensuring the required radial bearing rigidity, the axial dimension L is set to 0.8 mm or more, and preferably 1.1 mm or more.

[0046] In principle, the inner diameter dimension D1 of the fluid dynamic bearing 8 (more precisely, the inner diameter dimension of the band portion 8a3 which, together with the inclined hill portion 8a2, forms the smallest diameter portion of the inner circumferential surface 8a) is arbitrary, and from the viewpoint of ensuring the bite of the sizing pin into the inner circumferential surface 8a during dynamic pressure groove sizing, which will be described later, it is preferable that it is 1.2 mm or more, and more preferable that it is 1.5 mm or more. On the other hand, from the viewpoint of avoiding a situation in which it becomes difficult to reliably transmit the press-fitting force to the surface portion of the inner circumferential surface 8a during dynamic pressure groove sizing due to an unnecessary increase in the thickness dimension t, the inner diameter dimension D1 is preferable to be 2.5 mm or less, and more preferable that it is 2.0 mm or less.

[0047] The outer diameter D2 of the fluid dynamic bearing 8 is also arbitrary in principle, and is preferably 2.5 mm or more, and more preferably 3.0 mm or more, taking into account the required balance between the inner diameter D1 and thickness t. From the same viewpoint, the outer diameter D2 is preferably 5.0 mm or less, and more preferably 4.5 mm or less.

[0048] The thickness dimension t {=(D2-D1) / 2} of the fluid dynamic bearing 8 is in principle arbitrary, and for example, from the viewpoint of ensuring the necessary thrust bearing area for the upper end face 8b, it is preferable to set it to 0.5 mm or more. On the other hand, from the viewpoint of being able to transmit a sufficient compressive force from the die to the inner peripheral surface layer portion of the sintered compact during dynamic pressure groove sizing, the thickness dimension t is preferable to be set to 1.5 mm or less.

[0049] Next, the composition of the fluid dynamic bearing 8 when it is a porous body of sintered metal will be described. The fluid dynamic bearing 8 is obtained by compression molding and sintering raw material powder that contains, for example, one of copper-based powder and iron-based powder in the largest amount and the other in the second largest amount. In other words, the porous body of the fluid dynamic bearing 8 is substantially composed of one of copper and iron as the main component and the other as the second component (the second largest component). The copper-based powder referred to here includes not only pure copper powder but also copper alloy powder. Furthermore, pure copper includes not only 100% pure copper but also 99.99% or more copper that is industrially recognized as pure copper. Similarly, the iron-based powder referred to here includes not only pure iron powder but also iron alloy powder such as stainless steel. Furthermore, pure iron referred to here includes not only 100% pure iron but also 99.99% or more iron that is industrially recognized as pure iron. As long as the above composition (powder blending ratio) is established, the type and blending ratio of the powder that is the third and subsequent components are arbitrary.

[0050] The composition of the raw powder (powder blend ratio) that can be applied is, for example, [copper-based powder: 50-70% by weight, iron-based powder: 30-48% by weight, tin powder: 0-5%], and a specific example can be [pure iron powder of 140 mesh or less: 38-42% by weight, tin powder of 330 mesh or less: 1-3%, pure copper powder of 200 mesh or less: balance].

[0051] Next, the density ratio of the fluid dynamic bearing 8 will be described. The density ratio of the entire fluid dynamic bearing 8 is set, for example, to 80% or more and 95% or less. The density ratio can be set, for example, by adjusting the material, particle size (distribution), and compounding ratio of the raw metal powder. The variation in density ratio at this time can be evaluated using the porosity, which has a certain correlation with the density ratio. Here, the porosity is expressed as the volume ratio (percentage) of pores per unit volume of the bearing, and empirically, it shows an almost negative correlation with the density ratio (correlation coefficient of -1).

[0052] The surface porosity of the inner circumferential surface 8a, particularly the inner circumferential surface of the inclined hill portion 8a2 and band portion 8a3 that form the radial bearing surface, is adjusted to, for example, 15% or less. The surface porosity can be adjusted, for example, by rotational sizing, which will be described later.

[0053] Of course, the composition, blending ratio, density ratio, and surface porosity of the raw material powders described above are merely examples, and appropriate forms can be adopted depending on the application or individual requirements.

[0054] The fluid dynamic bearing 8 having the above configuration is manufactured, for example, through a powder compacting step S1, a sintering step S2, a dynamic pressure groove sizing step S3, and an oil impregnation step S4. In this case, a dimension sizing step S21 and a rotation sizing step S22 may be provided after the sintering step S2 and before the dynamic pressure groove sizing step S3.

[0055] (S1) Powder compaction process First, raw powder that will be the material of the fluid dynamic bearing 8 that will be the final product is prepared, and this is compression molded into a predetermined shape by die press molding. Specifically, although not shown in the figure, the raw powder is compression molded using a molding die that is composed of a die, a core pin inserted and arranged in a hole of the die, a lower punch arranged between the die and the core pin and configured to be able to move up and down with respect to the die, and an upper punch configured to be able to move (up and down) relative to both the die and the lower punch. In this case, the raw powder is filled into a space defined by the inner peripheral surface of the die, the outer peripheral surface of the core pin, and the upper end surface of the lower punch, and the upper punch is lowered with the lower punch fixed, and the filled raw powder is compressed in the axial direction. Then, the upper punch is lowered to a predetermined position while applying pressure, and the raw powder is compressed to a predetermined axial dimension, thereby forming a green compact.

[0056] (S2) Sintering process After the green compact is obtained as described above, the green compact is sintered at a temperature according to the type of raw material powder (for example, the melting point of the main component metal) to obtain a sintered body.

[0057] (S21) Dimensional sizing process, and (S22) Rotational sizing process The sintered body is then sized to correct the outer diameter, inner diameter, and axial dimensions of the sintered body to those of the final product, and the surface porosity of the inner peripheral surface 8a is adjusted to a suitable ratio (for example, the above-mentioned numerical range: 15% or less) for a fluid dynamic pressure bearing having a dynamic pressure generating portion on the inner peripheral surface. At this stage, the predetermined dynamic pressure groove 8a1 arrangement regions A1, A2 have not yet been formed on the inner peripheral surface 8a of the sintered body. Similarly, although not shown, the predetermined dynamic pressure groove 8b1 arrangement region has not yet been formed on the upper end surface 8b of the sintered body.

[0058] (S3) Dynamic groove sizing process The sintered body obtained through the above series of steps is subjected to a predetermined dynamic pressure groove sizing to form the dynamic pressure groove 8a1 arrangement areas A1, A2 on the inner peripheral surface 8a of the sintered body. Specifically, although not shown in the figure, the sintered body is formed in a predetermined manner using a molding device having a die having a press-fit hole for the sintered body, a sizing pin arranged so as to be inserted into the press-fit hole of the die, a lower punch arranged between the die and the sizing pin and configured so as to be able to rise and fall relative to the die, and an upper punch configured so as to be able to rise and fall relative to both the die and the lower punch. In this case, a molding die having a shape corresponding to the dynamic pressure groove arrangement areas A1, A2 (FIG. 3) of the inner peripheral surface 8a to be molded is provided on the outer peripheral surface of the sizing pin, and a molding die having a shape corresponding to the dynamic pressure groove 8b1 arrangement area (FIG. 4) of the upper end surface 8b to be molded is provided on the lower end surface of the upper punch. Therefore, the sintered body is placed on the upper end surface of the die, and the upper punch is lowered to press the upper end surface of the sintered body with the sizing pin inserted into the inner circumference of the sintered body, so that the sintered body is pressed into the press-fit hole of the die, the outer circumferential surface of the sintered body is compressed, and the inner circumferential surface of the sintered body is bitten into the molding die of the sizing pin previously inserted into the inner circumference. In this way, the shape of the molding die is transferred to the inner circumferential surface of the sintered body, and the dynamic pressure groove 8a1 arrangement areas A1 and A2 are formed on this inner circumferential surface. At this time, the molding die provided on the lower end surface of the upper punch bites into the upper end surface of the sintered body, so that the shape of the molding die of the upper punch is transferred to the upper end surface, and the corresponding arrangement area of ​​the dynamic pressure groove 8b1 is formed.

[0059] After forming the predetermined arrangement areas of the dynamic pressure grooves 8a1, 8b1 on the inner peripheral surface and the upper end surface of the sintered body in this way, the die is lowered relative to the lower punch to release the restraint of the sintered body by the die. This causes the sintered body to spring back in the outer diameter direction, making it possible to remove the sintered body from the sizing pin.

[0060] (S4) Oil impregnation process The fluid dynamic bearing 8 is completed by impregnating the internal pores of the sintered body (porous body) obtained as described above with a lubricating oil composition.

[0061] The lubricating oil composition for the fluid dynamic bearing 8 has the following composition: 2 / s and 80 mm 2 The lubricating oil composition has a base oil having a kinematic viscosity of 90 mm / s or less at 40°C. 2 / s or more and 140 mm 2 A lubricating oil composition having a composition adjusted so that the viscosity is less than or equal to 1 / s is used.

[0062] The composition of the base oil is arbitrary as long as it satisfies the above-mentioned kinematic viscosity conditions, but when the base oil is a mixture of a non-polar oil and a polar oil, it is preferable that the ratio of the non-polar oil to the total base oil is 20 wt % or more and 30 wt % or less.

[0063] Here, in principle, any type of non-polar oil can be used as the non-polar oil, and synthetic hydrocarbon oils made of poly-α-olefins or hydrogenated products thereof that can exhibit excellent properties for use in fluid dynamic bearings, such as a wide operating temperature range, excellent lubricity, good initial break-in, and good durability, are suitable. In this case, the ratio of synthetic hydrocarbon oils made of poly-α-olefins or hydrogenated products thereof to the entire base oil is preferably 20 wt% or more and 30 wt% or less.

[0064] Here, the poly-α-olefin has an average molecular weight of 200 to 1600, preferably 400 to 800, and is suitably one obtained by polymerizing decene-1, isobutene, or the like with a Lewis acid complex or an aluminum oxide catalyst, etc. The hydrogenated poly-α-olefin can be obtained, for example, by hydrogenating the poly-α-olefin in the presence of a hydrogenation catalyst.

[0065] In principle, any non-polar oil can be used as the polar oil, and for example, an ester oil having excellent evaporation characteristics, lubricity, and wear resistance is preferably used. In this case, the ester used may be any of monoesters (esters of monohydric alcohol and monohydric fatty acid), diesters (esters of monohydric alcohol and dihydric fatty acid), polyol esters (esters of alcohol having a neopentyl skeleton and monohydric fatty acid, etc.), and complex esters (oligomer esters obtained by adding polyhydric fatty acid to polyol ester as a raw material and crosslinking the polyol).

[0066] Alternatively, the lubricating oil composition according to this embodiment may contain, as components other than the base oil, known viscosity index improvers, metal deactivators, antioxidants, rust inhibitors, pour point depressants, ashless dispersants, metal-based detergents, surfactants, friction modifiers, and the like.

[0067] However, in any of the above types of additives, the lubricating oil composition is required to have a kinematic viscosity at 40°C within the above-mentioned range (90 mm 2 / s or more and 140 mm 2 It is important to set the type, addition ratio, number of additives, etc. within the range that satisfies the above requirement (≦1 / s).

[0068] The fluid dynamic bearing 8 is fixed to the inner periphery of the housing 7 by known means such as press-fitting, press-fitting accompanied by adhesion, adhesion, welding, etc. Thereafter, the shaft portion 10 of the rotating body 3 is inserted into the inner periphery of the fluid dynamic bearing 8. Finally, the internal space of the bearing is filled (packed) with the lubricating oil composition having the above composition, thereby completing the fluid dynamic bearing device 2 with the interface of the lubricating oil composition held in the seal space S (see FIG. 2).

[0069] Incidentally, by carrying out the above-mentioned oil impregnation step S4, for example, after fixing the sintered body that will become the fluid dynamic bearing 8 to the inner periphery of the housing 7, the operation of impregnating the fluid dynamic bearing 8 with the lubricating oil composition and the operation of filling the internal space of the fluid dynamic bearing device 2 with the lubricating oil composition can be carried out simultaneously.

[0070] In the fluid dynamic bearing device 2 having the above-mentioned configuration, when the shaft portion 10 (rotor 3) rotates, the regions (two upper and lower dynamic pressure groove 8a1 arrangement regions A1, A2) of the inner peripheral surface 8a of the fluid dynamic pressure bearing 8 that become the radial bearing surface face the outer peripheral surface 10a of the shaft portion 10 via a radial bearing gap. Then, as the shaft portion 10 rotates, the lubricating oil composition in the radial bearing gap is pushed toward the axial center of each of the dynamic pressure groove 8a1 arrangement regions A1, A2, and the pressure of the lubricating oil composition increases in the region on the axial center side (here, the band portion 8a3). Due to the dynamic pressure action of the dynamic pressure grooves 8a1, an oil film of the lubricating oil composition is formed in the radial bearing gap, and a first radial bearing portion R1 and a second radial bearing portion R2 that support the shaft portion 10 in a non-contact manner so as to be rotatable in the radial direction are respectively configured to be separated in the axial direction.

[0071] In addition, an oil film of the lubricating oil composition is formed in a thrust bearing gap between the upper end surface 8b (the region where the dynamic pressure grooves 8b1 are arranged) of the fluid dynamic pressure bearing 8 and the lower end surface 9a1 of the hub portion 9 facing the upper end surface 8b (the region where the dynamic pressure grooves 8b1 are arranged) by the dynamic pressure action of the dynamic pressure grooves 8b1. Then, a first thrust bearing portion T1 that supports the rotor 3 in the thrust direction without contact is formed by the pressure of this oil film. In addition, the lower end portion 10b of the shaft portion 10 is rotatably contact-supported by a thrust receiving portion 11 provided on the bottom portion 7e of the housing 7, thereby forming a second thrust bearing portion T2 that supports the rotor 3 in the thrust direction by contact.

[0072] As described above, in the lubricating oil composition for the fluid dynamic bearing 8 according to this embodiment, the base oil is a kinetic oil having a kinetic viscosity of 30 mm at 40° C. 2 / s and 80mm 2 The lubricating oil composition has a kinematic viscosity of 90 mm / s or less at 40°C. 2 / s or more and 140 mm 2The composition of the lubricating oil composition was adjusted so that the viscosity was 4 wt% or less. In this embodiment, the amount of polymer-based viscosity index improver added was adjusted to 4 wt% or less to adjust the viscosity. By adjusting the kinetic viscosity of the base oil and the lubricating oil composition in this way, an oil film can be stably formed in the radial bearing gap regardless of the size of the radial bearing gap. Therefore, it is possible to make the fluid dynamic bearing 8 exhibit high load capacity. In addition, even if the radial bearing gap becomes small as a result of the shaft portion 10 being largely eccentric, it is possible to stably form an oil film of the lubricating oil composition in the radial bearing gap. By adjusting the composition of the lubricating oil composition as described above, such as by not adding viscosity index improver as much as possible, it is possible to avoid a situation in which the viscosity increases more than necessary at low temperatures, which is the start of startup. Therefore, it is possible to stably exhibit the required high load capacity without increasing the bearing torque.

[0073] In addition, in this embodiment, the base oil is a mixture of a non-polar oil and a polar oil, and the ratio of the non-polar oil to the entire base oil is 20 wt% or more and 30 wt% or less. In this way, by making the ratio of the non-polar oil having a relatively high electrical resistivity 20 wt% or more, the electrical resistivity of the entire lubricating oil composition can be kept low to a degree that can prevent as much as possible the passage of electricity between the shaft part 10 and the fluid dynamic bearing 8 through the lubricating oil composition. Therefore, it is possible to prevent as much as possible erroneous judgment in the load capacity evaluation process based on the electrical resistance method (see the explanation below for details) and properly evaluate the load capacity of the fluid dynamic bearing 8. In addition, by keeping the ratio of the non-polar oil to 30 wt% or less, it is possible to avoid a situation in which the viscosity of the lubricating oil composition becomes high at low temperatures and a situation in which the amount of evaporation becomes large.

[0074] In addition, in this embodiment, since a synthetic hydrocarbon oil made of poly-α-olefin or its hydrogenated product is used as the non-polar oil, the temperature range in which the lubricating oil composition can be used can be expanded. In addition to excellent lubricity, good initial running-in can be imparted to the fluid dynamic bearing. Furthermore, it is possible to improve durability.

[0075] In addition, in this embodiment, since an ester oil is used as the polar oil, the evaporation characteristics, lubricity, and even wear resistance of the lubricating oil composition can be improved. In particular, when a synthetic hydrocarbon oil made of poly-α-olefin or its hydrogenated product is used as the non-polar oil, the solubility, which is a drawback of polyolefins, can be overcome by using an ester oil as the polar oil. In addition, by mixing an ester oil with a synthetic hydrocarbon oil made of poly-α-olefin or its hydrogenated product, it becomes possible to stably maintain the good bearing performance of the fluid dynamic bearing for a long period of time.

[0076] While 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 impregnated with this lubricating oil composition, and the fluid dynamic bearing device equipped with this bearing are not limited to the above-mentioned exemplary forms and may take any form within the scope of the present invention.

[0077] FIG. 6 shows a cross-sectional view of a fluid dynamic bearing device 12 according to another embodiment of the present invention. As shown in FIG. 6, the fluid dynamic bearing device 12 according to this embodiment differs from the fluid dynamic bearing device 2 shown in FIG. 2 in that it has only a first thrust bearing portion T1. To be more specific, in the fluid dynamic bearing device 2 according to this embodiment, a predetermined thrust direction gap always exists between the lower end surface 10c of the shaft portion 10 and the upper end surface 7e1 of the bottom portion 7e of the housing 7. Here, the size of the facing distance (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 the facing distance between the upper end surface 8b of the fluid dynamic bearing 8 and the lower end surface 9a1 of the disk portion 9a, and is set to a size that is considered to have no substantial effect on the increase in loss torque during rotational driving. Other configurations are the same as those of the fluid dynamic bearing device 2 shown in FIG. 2 and the like, so detailed description will be omitted.

[0078] In the fluid dynamic bearing device 12 according to this embodiment, the base oil is a viscous liquid having a dynamic viscosity of 30 mm at 40° C. 2 / s and 80mm 2 The lubricating oil composition has a kinematic viscosity of 90 mm / s or less at 40°C. 2 / s or more and 140 mm 2 A lubricating oil composition for a fluid dynamic bearing 8, the composition of which is adjusted so that the viscosity of the lubricating oil is 1 / s or less, is used. This allows an oil film to be stably formed in the radial bearing gap regardless of the size of the radial bearing gap. This allows the fluid dynamic bearing 8 to exhibit high load capacity. In addition, since the necessary thrust bearing area can be secured on the upper end surface 8b of the fluid dynamic bearing 8, excellent rotation accuracy can be exhibited even when a thrust bearing portion (first thrust bearing portion T1) is provided only on the upper end surface 8b side of the fluid dynamic bearing 8. Therefore, as shown in FIG. 6, sufficient thrust bearing performance can be exhibited even if the lower end surface 10c of the shaft portion 10 is not supported in the thrust direction, and costs can be reduced by the amount of omission of the thrust receiving portion 11 and the simplification of the lower end shape of the shaft portion 10.

[0079] Of course, for either or both of the radial dynamic pressure generating portions (dynamic pressure groove 8a1 arrangement areas A1, A2 in Figure 3) formed between the radial bearing portions R1, R2 and the outer peripheral surface 10a of the shaft portion 10, it is possible to adopt known radial dynamic pressure generating portions having shapes other than dynamic pressure grooves, such as so-called multiple arc shapes, step shapes, and wave shapes.

[0080] In the above explanation, the housing 7 of the fluid dynamic bearing device 2 is fixed to the inner periphery of the base 6 of the fan motor 1, but the housing 7 may also be attached directly to, for example, the base (not shown) of an information device to which the fan motor 1 is attached. Alternatively, the parts equivalent to these base parts may be provided integrally with the housing 7.

[0081] In the above description, the magnet 5b and the coil 5a are arranged offset in the axial direction to apply an external force to the shaft portion 10 (rotating body 3) to press the shaft portion 10 against the bottom portion 7e of the housing 7, but the means for applying such an external force to the shaft portion 10 is not limited to the above. Although not shown in the drawings, for example, the magnetic force can be applied to the rotating body 3 by arranging a magnetic member capable of attracting the magnet 5b opposite the magnet 5b in the axial direction. In addition, if the thrust as a reaction force of the air blowing action is sufficiently large and the shaft portion 10 can be pressed downward by this thrust alone, the magnetic force (magnetic attraction force) as an external force for pressing the shaft portion 10 downward may be omitted.

[0082] Furthermore, in the above explanation, the present invention has been described as being applied to a fluid dynamic bearing device 2 in which a rotor 3 with a fan 4 is fixed to the shaft section 10, but the present invention can also be preferably applied to fluid dynamic bearing devices 2 in which a disk hub having a disk mounting surface, or a polygon mirror, is fixed to the shaft section 10 as the rotor 3. That is, the present invention can be preferably applied not only to a fan motor 1 as shown in Fig. 1, but also to fluid dynamic bearing devices 2 incorporated in other electrical equipment, such as spindle motors for disk devices and polygon scanner motors for laser beam printers (LBPs). EXAMPLES

[0083] Examples for demonstrating the effects of the present invention will now be described.

[0084] In the present examples, lubricating oil compositions having the composition according to the present invention (Examples 1 to 3) and lubricating oil compositions having compositions other than those according to the present invention (Comparative Examples 1 to 4) were prepared, and the kinetic viscosity and viscosity of each lubricating oil composition were evaluated. In addition, each lubricating oil composition was impregnated into a fluid dynamic bearing, and the load capacity characteristics of a fluid dynamic bearing device in which the lubricating oil composition was filled into the internal space of the bearing were evaluated.

[0085] The lubricating oil compositions to be evaluated here were lubricating oil compositions based on a mixture of non-polar oil and polar oil. Specifically, a synthetic hydrocarbon oil made of poly-α-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 the viscosity index improver. In addition, an antioxidant and a rust inhibitor were added to each of the lubricating oil compositions. Table 1 shows the proportions (mixing ratios) [wt%] of the non-polar oil, polar oil, and viscosity index improver in the lubricating oil compositions according to Examples 1 to 3 and Comparative Examples 1 to 4.

[0086] [Table 1]

[0087] In addition, the kinematic viscosity of the base oil at 40°C [mm 2 / s] was obtained in accordance with JIS K 2283. The kinematic viscosities at 40° C. of the base oils of Examples 1 to 3 and Comparative Examples 1 to 4 are shown in Table 2.

[0088] In addition, the kinematic viscosity [mm 2 / s] was obtained in accordance with JIS K 2283. The kinetic viscosities at 40° C. of the lubricating oil compositions of Examples 1 to 3 and Comparative Examples 1 to 4 are respectively as shown in Table 2.

[0089] The viscosity [mPa·s] of each lubricating oil composition at -40°C was measured using a cone-plate type rotational viscometer (shear rate: 300 s -1 The viscosities at −40° C. of the lubricating oil compositions of Examples 1 to 3 and Comparative Examples 1 to 4 are as shown in Table 2.

[0090] The load-carrying capacity of the fluid dynamic bearing device impregnated or filled with each lubricating oil composition was measured by measuring the minimum floating rotation speed based on the electrical resistance method, and the maximum eccentricity of the shaft portion that could rotate without contacting the fluid dynamic bearing was determined based on the measurement results.

[0091] Of these, the minimum floating rotation speed was measured as follows. That is, although not shown in the drawings, a long shaft portion was inserted into the fluid dynamic bearing according to the present invention, and a motor was coaxially connected to one axial end of the shaft portion, while the fluid dynamic bearing was fixed to the inner circumference of a housing. Then, a seal member was used to seal the gap between the shaft portion and the fluid dynamic bearing. Thereafter, the shaft portion was driven at a predetermined rotation speed by the motor while a predetermined load, here a radial load (two types of loads, 3N and 6N), was applied to the fluid dynamic bearing via the housing. The rotation speed was measured at 2000 min -1 Starting from 100min -1 The rotation speed of the shaft is then gradually decreased. The rotation speed of the shaft at the time when electrical connection between the shaft and the fluid dynamic bearing is detected is measured by a rotation speed sensor provided at the other axial end of the shaft, and the measured rotation speed is set as the minimum floating rotation speed [min -1 The minimum floating rotation speeds of Examples 1 to 3 and Comparative Examples 1 to 4 are as shown in Table 1.

[0092] The eccentricity was calculated by back-calculating from the load (3N or 6N) during the test using a calculation program that took the eccentricity of the shaft at the time when electrical continuity between the shaft and the fluid dynamic bearing as detected, the radius of the radial bearing gap between the fluid dynamic bearing and the shaft, and the rotation speed (minimum floating rotation speed) as input values ​​and the load as output value. The eccentricities of Examples 1 to 3 and Comparative Examples 1 to 4 are shown in Table 2.

[0093] [Table 2]

[0094] As shown in Table 2, when Examples 1 to 3 are compared with Comparative Example 1, the kinematic viscosity of the lubricating oil composition is equivalent in Comparative Example 1 and Examples 1 to 3. On the other hand, the kinematic viscosity of the base oil in Comparative Example 1 is below the numerical range according to the present invention. Here, the minimum floating rotation speed of Comparative Example 1 is the same value as that of Example 1, but the magnitude of the load is different (half that of Example 1). This shows that the load capacity of Comparative Example 1 is smaller than that of Example 1. In addition, the minimum floating rotation speed of Comparative Example 1 is significantly larger than the minimum floating rotation speeds of Examples 2 and 3, despite the load being at the same level. This shows that the load capacity of Comparative Example 1 is smaller than the load capacity of Examples 2 and 3.

[0095] When comparing Example 1 and Comparative Example 2, the kinematic viscosity of the base oil and the kinematic viscosity of the lubricating oil composition are comparable between Comparative Example 2 and Example 1. On the other hand, the content of non-polar oil relative to the base oil in Comparative Example 2 is below the numerical range according to the present invention. Therefore, it is highly likely that the minimum floating rotation speed in Comparative Example 2 is measured to be higher than the actual value, and is therefore unreliable as an index of load capacity.

[0096] When comparing Examples 1 to 3 with Comparative Example 3, the kinematic viscosity of the lubricating oil composition is comparable between Comparative Example 3 and Examples 1 to 3. On the other hand, the kinematic viscosity of the base oil in Comparative Example 3 is within the numerical range (30 mm 2 / s and 80mm 2 / s or less). Therefore, the minimum rotational speed for levitation in Comparative Example 3 is significantly higher than that in Example 1, even though the load is at the same level. This shows that the load capacity of Comparative Example 3 is lower than that of Example 1. Furthermore, when Comparative Example 3 is compared with Examples 2 and 3, the load is twice as large as that in Examples 2 and 3, while the minimum rotational speed for levitation in Comparative Example 3 is five times as large as that in Examples 2 and 3. This shows that the load capacity of Comparative Example 3 is lower than that of Examples 2 and 3.

[0097] When comparing Examples 1 to 3 with Comparative Example 4, the kinematic viscosity of the base oil and the kinematic viscosity of the lubricating oil composition in Comparative Example 4 were both higher than those in Examples 1 to 3, while all of the kinematic viscosities were within the numerical range according to the present invention (base oil: 30 mm 2 / s and 80mm 2 / s or less, lubricating oil composition: 90mm 2 / s and 140mm 2 / s or less). Therefore, while the load capacity (minimum floating rotation speed, eccentricity) of Comparative Example 4 is at the same level as the load capacity of Examples 1 to 3, the viscosity of the lubricating oil composition at low temperatures is higher than the viscosities of Examples 1 to 3. This causes the power consumption (bearing torque) of the motor to become excessive, or there is a risk that the motor cannot be started. [Explanation of symbols]

[0098] 1 Fan motor 2,12 Fluid dynamic bearing device 3 Rotating body 4 Fans 5 Drive unit 5a coil 5b Magnet 6 Base 7. Housing 7c Top surface 7d Sealing surface 8. Fluid dynamic bearing 8a Inner surface 8a1 Dynamic pressure groove 8a2 Slope section 8a3 Belt 8b Top surface 8b1 Dynamic pressure groove 8c Lower end surface 8d Outer surface 8d1 Axial groove 9 Hub section 9a Disc Section 9a1 Lower end surface 9b, 9c Cylindrical part 9d Tsubabe 9e Mounting hole 10 Shaft 10a Outer surface 10b Bottom end 10c Lower end surface 11 Thrust receiving part A1, A2 dynamic pressure groove arrangement area D1 Inner diameter D2 Outer diameter L Axial dimension R1, R2 radial bearing section S Sealing space T1, T2 Thrust bearing section

Claims

1. 1. A lubricating oil composition for use in a fluid dynamic bearing comprising a porous body having internal pores and a radial dynamic pressure generating portion provided on an inner circumferential surface of the porous body, comprising: As the base oil, a kinetic viscosity of 30 mm at 40°C is used. ２ / s and 80 mm ２ / s or less is used, The lubricating oil composition has a kinetic viscosity of 90 mm at 40°C. ２ / s or more and 140 mm ２ / s or less.

2. 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 the ratio of the non-polar oil to the total amount of the base oil is 20 wt % or more and 30 wt % or less.

3. 3. The lubricating oil composition for a fluid dynamic bearing according to claim 2, wherein the non-polar oil is a synthetic hydrocarbon oil comprising a poly-α-olefin or a hydrogenated product thereof.

4. 3. 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, the internal pores of which are impregnated with the lubricating oil composition according to any one of claims 1 to 4.

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

7. 6. A fluid dynamic bearing according to claim 5, wherein the inner diameter is 2.0 mm or less and the axial dimension is 3.5 mm or less.

8. 6. A fluid dynamic bearing device comprising: the fluid dynamic bearing according to claim 5; a housing to whose inner periphery the fluid dynamic bearing is fixed; a rotating body having a shaft portion to be inserted into the inner periphery of the fluid dynamic bearing; and a radial bearing portion which supports the shaft portion in a non-contact manner in the radial direction by an oil film of the lubricating oil composition which is formed in a radial bearing gap between the inner periphery of the fluid dynamic bearing and the outer periphery of the shaft portion.

9. 9. A fluid dynamic bearing device according to claim 8, wherein the eccentricity of said shaft portion which can rotate without contacting said fluid dynamic bearing is a maximum of 98%.

10. A motor comprising the fluid dynamic bearing device according to claim 8.

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