Lubricating oil composition for hydrodynamic bearing, hydrodynamic bearing, and hydrodynamic bearing device

Abstract

A lubricating oil composition for fluid dynamic pressure bearings, which is used in a fluid dynamic pressure bearing (8) in which radial dynamic pressure generating sections (A1, A2) are provided on the inner peripheral surface (8a) of a porous body, has the following composition. Namely, a base oil having a kinematic viscosity at 40 DEG C of more than 30 mm2 / s and 80 mm2 / s or less is used as the base oil of the lubricating oil composition, and the composition of the lubricating oil composition is adjusted so that the kinematic viscosity at 40 DEG C of the lubricating oil composition is 90 mm2 / s or more and 140 mm2 / s or less.

Description

Technical Field

[0001] The present invention relates to a lubricating oil composition for a fluid dynamic pressure bearing, a fluid dynamic pressure bearing and a fluid dynamic pressure bearing device, and in particular to a lubricating oil composition used in a fluid dynamic pressure bearing having a radial dynamic pressure generating portion. Background Art

[0002] A fluid dynamic pressure bearing is a bearing used by impregnating lubricating oil (hereinafter referred to as lubricating oil composition in this specification) into the internal pores of a porous body formed of, for example, sintered metal. As the shaft inserted into the inner circumference rotates relative to each other, the lubricating oil composition impregnated into the internal pores seeps out to the sliding part with the shaft to form an oil film, and the shaft is rotatably supported via the oil film. This type of bearing is suitable for use as a bearing device for motors mounted in various electrical equipment represented by information equipment due to its excellent rotation accuracy and quietness. More specifically, it is suitable for use as a spindle motor in an optical drive device for HDD, CD, DVD, and Blu-ray discs, a fan motor installed in these optical drive devices, PCs, etc., or a bearing device for a polygonal scanning motor installed in a laser printer (LBP).

[0003] In addition, in order to further improve the quietness and extend the service life, it is known that a dynamic pressure generating portion such as a dynamic pressure groove is formed on at least one of the inner peripheral surface and the axial end surface of the fluid dynamic pressure bearing. In this case, the lubricating oil composition impregnated in the internal pores of the fluid dynamic pressure bearing is caused to seep into the gap between the area where the dynamic pressure generating portion is formed on the inner peripheral surface of the bearing and the outer peripheral surface of the shaft portion as the shaft portion (rotor) inserted into the inner peripheral surface of the bearing rotates, and a dynamic pressure action is generated in the gap, so that the shaft portion can be rotatably supported by the oil film of the lubricating oil composition in a non-contact manner relative to the bearing.

[0004] Recently, with the miniaturization and thinning of information equipment, various motors installed in information equipment are also required to be miniaturized. For example, the cooling fan motor used in notebook personal computers has been thinned, and the bearing device used in the motor is also required to be thin. On the other hand, the cooling performance is required to be equal to or higher than the previous level. The thinning of the bearing device leads to a reduction in the bearing area. Therefore, when the size of the impeller (blade) is increased, the possibility of reducing the bearing rigidity will increase. Even if the size of the impeller is maintained, the possibility of reducing the bearing rigidity will increase in accordance with the degree of reduction in the bearing area.

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

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent No. 3782889 Summary of the invention

[0009] Problems to be solved by the invention

[0010] However, recently, the cooling fan motors used in notebook personal computers and the like have been further thinned, and the axial dimensions of the bearings used in the motors are also required to be further reduced. On the other hand, if the axial dimension of the bearing is reduced, the area that can form the radial dynamic pressure generating portion is reduced accordingly, and the dynamic pressure that can be generated in the radial bearing gap is also reduced. Therefore, it is difficult to support rotating bodies such as impellers including shafts with high precision, which may lead to a decrease in rotational accuracy (NRRO). As a result of the reduced rotational accuracy, the eccentricity of the shaft (rotating body) increases, thereby reducing the radial bearing gap and increasing the risk of contact between the shaft and the bearing.

[0011] In order to generate sufficient dynamic pressure even when the radial bearing clearance is reduced due to the eccentricity of the shaft, it is conceivable to adjust the composition of the lubricating oil composition to increase the viscosity of the lubricating oil composition, for example, as described in Patent Document 1. However, if the viscosity of the lubricating oil composition is simply increased, although the bearing rigidity is improved, the bearing torque is also increased, which causes a new problem of increasing the power consumption of the motor and thus damaging the benefits.

[0012] In view of the above circumstances, a technical problem to be solved by the present invention is to provide a lubricating oil composition for a fluid dynamic bearing that can exhibit a high load capacity as a bearing for a small motor while avoiding an increase in bearing torque.

[0013] Means for solving problems

[0014] The above-mentioned problems can be solved by the lubricating oil composition for fluid dynamic pressure bearings of the present invention. That is, the lubricating oil composition is characterized in that it is a lubricating oil composition for fluid dynamic pressure bearings used in a fluid dynamic pressure bearing having a radial dynamic pressure generating portion provided on the inner peripheral surface of a porous body having internal pores, and the kinematic viscosity at 40°C is greater than 30 mm 2 / s and 80mm 2 / s or less as the base oil, and the kinematic viscosity of the lubricating oil composition at 40°C is 90 mm 2 / s or more and 140mm 2 / s or less. It should be noted that the fluid dynamic pressure bearing in this specification refers to a dynamic pressure bearing other than a perfect circular bearing. Therefore, the radial dynamic pressure generating portion in this specification refers to a portion made in a shape other than a perfect cylinder that can generate a dynamic pressure effect of the lubricating oil composition in the radial bearing gap between the shaft and a rotating body.

[0015] The present inventors have found that a polymer-based viscosity index improver, which is often added to achieve a high viscosity of a lubricating oil composition, has difficulty entering the radial bearing gap when the eccentricity of the shaft is large and the radial bearing gap is very small. The present invention was made based on the above findings, and a kinematic viscosity at 40°C of more than 30 mm 2 / s and 80mm 2 / s or less as the base oil, and the kinematic viscosity of the lubricating oil composition at 40°C is 90 mm 2 / s or more and 140mm 2 / s or less. It should be noted that the viscosity can be adjusted by using a polymer-based viscosity index improver, but if too much is added, the oil film formation becomes unstable. Therefore, when adding a viscosity index improver, it is desirable to adjust the viscosity index improver in such a way that the amount added is 4wt% or less. By adjusting the kinematic viscosity of the base oil and the lubricating oil composition in this way, an 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. Thus, the fluid dynamic pressure bearing can exert a high load capacity. In addition, by adjusting the composition of the lubricating oil composition as described above without adding a viscosity index improver as much as possible, it is possible to avoid the situation where the viscosity increases to a higher level than necessary at a low temperature as the start of the start. Thus, the required high load capacity can be stably exerted without increasing the bearing torque.

[0016] In the lubricating oil composition for a fluid dynamic bearing of the present invention, the base oil may be a mixture of a nonpolar oil and a polar oil, and the ratio of the nonpolar oil to the entire base oil may be 20 wt % to 30 wt %.

[0017] When evaluating the load capacity (load capacity) of a fluid dynamic pressure bearing, there is a known method using the resistance method. In this method, for example, the rotation speed of the shaft inserted into the inner circumference is changed while a specified load is applied to the fluid dynamic pressure bearing, and the load capacity of the fluid dynamic pressure bearing is evaluated based on the rotation speed at the moment when the contact between the shaft and the fluid dynamic pressure bearing is detected. Here, in the case where the base oil is set as 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 resistivity increases, and as a result, although there is actually no contact, the possibility of misjudging that the shaft and the fluid dynamic pressure bearing are in contact due to the energization of the shaft and the fluid dynamic pressure bearing achieved by the lubricating oil increases. In other words, the possibility of the load capacity being evaluated as lower than the actual value increases. From this point of view, the ratio of non-polar oil with relatively high resistivity is set to 20wt% or more, thereby being able to lower the resistivity of the lubricating oil composition as a whole to the extent that the energization of the shaft and the fluid dynamic pressure bearing achieved by the lubricating oil composition is prevented as much as possible. Therefore, it is possible to prevent misjudgment in the load capacity evaluation process based on the resistance method as much as possible, thereby appropriately evaluating the load capacity of the fluid dynamic pressure bearing. In addition, by suppressing the ratio of non-polar oil to less than 30wt%, it is possible to avoid the situation where the viscosity at low temperature becomes high and the situation where the evaporation amount becomes large.

[0018] In addition, when the base oil is a mixture of a nonpolar oil and a polar oil, the nonpolar oil may be a synthetic hydrocarbon oil containing a poly-α-olefin or a hydrogenated product thereof.

[0019] By using a synthetic hydrocarbon oil containing 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, it is possible to impart excellent lubricity to the fluid dynamic pressure bearing and also impart good initial running-in properties. In addition, it is possible to achieve improved durability.

[0020] Furthermore, when the base oil is a mixture of a nonpolar oil and a polar oil, the polar oil may be an ester oil.

[0021] By using ester oil as a polar oil constituting a part of the base oil, the evaporation characteristics, lubricity and wear resistance can be improved. In particular, when a synthetic hydrocarbon oil containing poly-α-olefin or its hydrogenate is used as a non-polar oil, the solubility which is a disadvantage 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 containing poly-α-olefin or its hydrogenate, the good bearing performance of the fluid dynamic pressure bearing can be maintained stably for a long time. This is considered to be because the generation of bubbles from the poly-α-olefin is suppressed by the mixing of the ester oil, or even if bubbles are generated, they disappear immediately.

[0022] The lubricating oil composition for fluid dynamic bearings described above can be provided in the form of a fluid dynamic bearing having internal pores impregnated with the lubricating oil composition. In this case, in the fluid dynamic bearing of the present invention, a dynamic pressure groove array region as a radial dynamic pressure generating portion can be provided on the inner circumferential surface.

[0023] By providing the dynamic pressure groove array region in this way, the dynamic pressure action of the lubricating oil composition can be effectively generated in the bearing gap. Therefore, in combination with the action of the lubricating oil composition described above, an oil film of the lubricating oil composition can be formed very effectively and continuously in the bearing gap.

[0024] In the fluid dynamic pressure bearing of the present invention, the inner diameter dimension may be 2.0 mm or less and the axial dimension may be 3.5 mm or less.

[0025] The fluid dynamic pressure bearing of the present invention can exert high load capacity as a bearing while avoiding an increase in bearing torque. Therefore, even when the inner diameter and axial dimensions are small as described above and the area of ​​the inner peripheral surface of the bearing forming the radial bearing surface cannot be sufficiently ensured, the required bearing performance can be exerted.

[0026] In addition, the fluid dynamic pressure bearing in the above description can be appropriately provided in the form of, for example, a fluid dynamic pressure bearing device comprising: the fluid dynamic pressure bearing, a housing that fixes the fluid dynamic pressure bearing on the inner circumference, a rotating body having a shaft portion inserted into the inner circumference of the fluid dynamic pressure bearing, and a radial bearing portion that non-contact supports the shaft portion in a radial direction using an oil film of a lubricating oil composition, wherein the lubricating oil composition is formed in a radial bearing gap between the inner circumferential surface of the fluid dynamic pressure bearing and the outer circumferential surface of the shaft portion.

[0027] In addition, the fluid dynamic pressure bearing device of the present invention may be a fluid dynamic pressure bearing device in which the eccentricity of the shaft portion that can rotate in a non-contact manner relative to the fluid dynamic pressure bearing is displayed as a maximum of 98%. It should be noted that the so-called eccentricity mentioned here means the ratio of the eccentricity of the shaft portion to the radius value of the radial bearing clearance.

[0028] As described above, the fluid dynamic pressure bearing of the present invention can exert high load capacity while avoiding an increase in bearing torque. Therefore, even if the eccentricity increases, the risk of contact between the shaft and the fluid dynamic pressure bearing can be minimized by utilizing the stable oil film forming capability, thereby stably exerting high load capacity.

[0029] The fluid dynamic bearing device described above can enable the bearing to exhibit high load capacity while avoiding an increase in bearing torque as described above, and therefore can be preferably provided in the form of a motor including the fluid dynamic bearing device, for example.

[0030] Effects of the Invention

[0031] Based on the above, according to the lubricating oil composition for a fluid dynamic bearing of the present invention, it is possible to provide a fluid dynamic bearing that can exhibit a high load capacity for use in a small motor while avoiding an increase in bearing torque. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a cross-sectional view of a fan motor according to an embodiment of the present invention.

[0033] Figure 2 yes Figure 1 A cross-sectional view of a fluid dynamic pressure bearing device is shown.

[0034] Figure 3 yes Figure 2 A cross-sectional view of a fluid dynamic bearing is shown.

[0035] Figure 4 yes Figure 2 A top view of a fluid dynamic bearing is shown.

[0036] Figure 5 yes Figure 2 A bottom view of a fluid dynamic bearing is shown.

[0037] Figure 6 It is a cross-sectional view of a fluid dynamic bearing device according to another embodiment of the present invention. DETAILED DESCRIPTION

[0038] Hereinafter, an embodiment of the present invention will be described based on the accompanying drawings. It should be noted that in the following description, when viewed from the fluid dynamic pressure bearing, one side of the disk portion of the hub portion is set as the "upper side" and one side of the bottom of the housing is set as the "lower side". Of course, the up and down directions do not limit the actual setting method and usage method of the product.

[0039] Figure 1 1 is a diagram generally showing a configuration example of a fan motor 1 according to the present embodiment. The fan motor 1 includes a fluid dynamic pressure bearing device 2, a plurality of blades 4 provided on a rotating body 3 of the fluid dynamic pressure bearing device 2, and a driving unit 5 for rotating the blades 4 and the rotating body 3 in an integrated manner. The driving unit 5 is formed, for example, by a coil 5a and a magnet 5b facing each other with a gap in a radial direction interposed therebetween. In the present embodiment, the coil 5a is fixed to a base portion 6 serving as a fixed side of the fan motor 1, and the magnet 5b is fixed to the rotating body 3 serving as a rotating side of the fan motor 1.

[0040] In the fan motor 1 having the above structure, when the coil 5a is energized, the magnet 5b rotates due to the excitation force between the coil 5a and the magnet 5b, and due to this rotation, the plurality of blades 4 erected on the outer periphery of the rotating body 3 rotate integrally with the rotating body 3. Due to this rotation, each blade 4 generates an airflow in a direction corresponding to its shape (for example, an airflow toward the radially outer side in this case), and an airflow from the axial upper side toward the lower side of the fan motor 1 is generated secondary in the form of being introduced into the airflow. By generating airflow around the fan motor 1 in this way, it is possible to cool an information device (not shown) on which the fan motor 1 is mounted.

[0041] When airflow is generated 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 in the rotating body 3 of the fluid dynamic pressure bearing device 2. A magnetic force (repulsion force) in the direction of canceling the reaction force acts 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 of the fluid dynamic pressure bearing device 2 (see the following description). Figure 2 ). The magnetic force in the direction of canceling the above-mentioned reaction force can be generated, for example, by arranging the coil 5a and the magnet 5b in an axially staggered manner (detailed illustration is omitted). In addition, when the rotating body 3 rotates, a radial load acts on the shaft portion 10 of the fluid dynamic pressure bearing device 2 described later. This radial load acts on the radial bearing portions R1 and R2 of the fluid dynamic pressure bearing device 2.

[0042] Figure 2 The figure shows a cross-sectional view of a fluid dynamic bearing device 2 incorporating a fan motor 1. The fluid dynamic bearing device 2 includes a housing 7, a fluid dynamic bearing 8 fixed to the inner periphery of the housing 7, and a rotating body 3 that rotates relative to the fluid dynamic bearing 8.

[0043] In the present embodiment, the rotating body 3 includes a boss portion 9 disposed on the upper end opening side of the housing 7 , and a shaft portion 10 inserted into the inner periphery of the fluid dynamic pressure bearing 8 .

[0044] Hub portion 9 Figure 1 and Figure 2 As shown, the housing 7 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 relative to 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 surface (upper end surface 8b) of the fluid dynamic pressure bearing 8 fixed to the inner periphery of the housing 7. In addition, the plurality of blades 4 are integrally provided with the hub portion 9 in the form of being erected from the outer peripheral edge of the flange portion 9d.

[0045] In the present embodiment, the shaft portion 10 is formed separately from the hub portion 9, and its upper end is fixed to the 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, and has a lower end portion 10b, for example, a partially spherical shape, which is continuous with the lower end of the outer peripheral surface 10a. That is, the shaft portion 10 presents a shape that can be inserted into the inner periphery of the fluid dynamic pressure bearing 8 from one side in the axial direction. Of course, the shaft portion 10 and the hub portion 9 can also be formed integrally with the same material. Alternatively, one of the shaft portion 10 and the hub portion 9 formed of different materials can be used as an embedded component and the other can be formed by injection molding of metal or resin.

[0046] The housing 7 is shaped such that its upper end is open and its lower end is closed. In addition, a fluid dynamic pressure bearing 8 is fixed to the 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 Figure 1 ). The axial opposing 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 opposing distance between the upper end surface 8b of the fluid dynamic pressure bearing 8 and the lower end surface 9a1 of the disk portion 9a, and is set to a size that is not considered to have a substantial effect on the increase in loss torque during rotational drive.

[0047] On the upper side of the outer periphery of the housing 7, a tapered sealing surface 7d is formed whose outer diameter dimension increases as it approaches the upper side. The tapered sealing surface 7d forms an annular sealing space S between the tapered sealing surface 7d and the inner peripheral surface 9b1 of the first cylindrical portion 9b, the radial opposing interval of which gradually decreases from the blocking side (lower side) toward the opening side (upper side) of the housing 7. The sealing space S is connected to 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, so that the lubricating oil composition can flow between the bearing internal space including each bearing gap. In addition, in a state where the lubricating oil composition is impregnated into the internal pores of the fluid dynamic pressure bearing 8 and the same lubricating oil composition is filled in the bearing internal space, the filling amount of the lubricating oil composition is adjusted so that the oil surface (gas-liquid interface) of the lubricating oil composition is always maintained in the sealing space S (refer to Figure 2 ).

[0048] In addition, when the housing 7 adopts the sealing structure as described above, the coil 5a is arranged so that the first cylindrical portion 9b of the hub portion 9 is located radially outward and the first cylindrical portion 9b and the coil 5a partially overlap in the axial direction. Thus, the housing 7 and the fluid dynamic pressure bearing device 2 can be made thinner (the axial dimension can be reduced).

[0049] In addition, in the present embodiment, a thrust receiving portion 11 for receiving the lower end portion 10b of the shaft portion 10 presenting a spherical surface is provided at the bottom portion 7e of the housing 7. That is, the thrust receiving portion 11 is always in contact with the lower end portion 10b of the shaft portion 10 in the completed state of the fluid dynamic bearing device 2, and can rotatably support the shaft portion 10. It should be noted that the vertical position of the thrust receiving portion 11 relative to the bearing contact surface 7f of the housing 7 can be set in such a manner that the contact position between the shaft portion 10 and the thrust receiving portion 11 is located, for example, in the vertical direction region of the inner chamfered portion 8e of the fluid dynamic bearing 8.

[0050] The material and composition of the housing 7 are in principle arbitrary, and for example, a known material such as resin or metal may be appropriately adopted according to a fixing mechanism of the fluid dynamic pressure bearing 8 to the housing 7 described later.

[0051] The fluid dynamic pressure bearing 8 is a porous body of sintered metal formed by, for example, compressing and sintering a metal powder having a predetermined composition or a raw material powder having the metal powder as a main component, and has a cylindrical shape. Figure 4 The fluid dynamic pressure bearing 8 has a cylindrical shape as shown in the figure. A plurality of dynamic pressure grooves 8a1 are arranged on the entire or a part of the inner peripheral surface 8a of the fluid dynamic pressure bearing 8 as a region for generating radial dynamic pressure. In other words, the fluid dynamic pressure bearing 8 has a porous body having the above-mentioned structure and a region for arranging dynamic pressure grooves 8a1 provided on the inner peripheral surface 8a of the porous body. In the present embodiment, the region for arranging dynamic pressure grooves 8a1 is as shown in FIG. Figure 3 As shown in the figure, a plurality of dynamic pressure generating grooves 8a1 inclined at a predetermined angle with respect to the circumferential direction, inclined hillocks 8a2 dividing the dynamic pressure generating grooves 8a1 in the circumferential direction, and bands 8a3 extending in the circumferential direction and dividing the dynamic pressure generating grooves 8a1 in the axial direction (the inclined hillocks 8a2 and the bands 8a3 are both in Figure 3 The dynamic pressure generating grooves (cross-hatched portions in the middle) are arranged in a herringbone shape and are continuously formed at two locations in the axial direction. In this case, the upper dynamic pressure generating groove arrangement area A1 and the lower dynamic pressure generating groove arrangement area A2 are both formed axially symmetrically with respect to the axial center line (an imaginary line connecting the axial centers of the belt portions 8a3 in the circumferential direction), and their axial dimensions are equal to each other.

[0052] In this embodiment, a region where a plurality of dynamic pressure grooves 8b1 are arranged as thrust dynamic pressure generating portions is formed on the entire or a portion of the upper end surface 8b of the fluid dynamic pressure bearing 8. The region where the dynamic pressure grooves 8b1 are arranged is, for example, Figure 4 As shown in FIG. 1 , a plurality of dynamic pressure grooves 8b1 extending in a spiral shape are arranged side by side in the circumferential direction. At this time, the spiral direction of the dynamic pressure grooves 8b1 is set to correspond to the rotation direction of the rotating body 3. The arrangement area of ​​the dynamic pressure grooves 8b1 configured as above is Figure 2In the state where the fluid dynamic bearing device 2 shown is driven to rotate, a thrust bearing gap of a first thrust bearing portion T1 described later is formed between the lower end surface 9 a 1 of the disk portion 9 a of the opposed boss portion 9 .

[0053] On the other hand, no thrust dynamic pressure generating portion is formed on the lower end surface 8c of the fluid dynamic pressure bearing 8. Figure 5 As shown, the lower end surface 8c has a flat shape. The lower end surface 8c abuts against the housing 7 at a position located radially outside 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 can be appropriately set within a range where 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.

[0054] One or more (five in this embodiment) axial grooves 8d1 are formed on the outer peripheral surface 8d of the fluid dynamic pressure bearing 8 (see, for example, Figure 4 ). When the fluid dynamic bearing 8 is fixed to the housing 7, the axial groove 8d1 forms a flow path for the lubricating oil composition between the axial groove 8d1 and the inner peripheral surface 7a of the housing 7 (see Figure 2 ).

[0055] Below, refer to Figure 3 , various dimensions of the fluid dynamic pressure bearing 8 are described. The axial dimension L (axial distance between the two end surfaces 8b, 8c) of the fluid dynamic pressure bearing 8 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 pressure bearing device 2 and 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, preferably 1.1 mm or more.

[0056] The inner diameter dimension D1 of the fluid dynamic pressure bearing 8 (to be precise, the inner diameter dimension of the band portion 8a3 that is the smallest diameter portion together with the inclined hill portion 8a2 in the inner peripheral surface 8a) is arbitrary in principle, and is preferably 1.2 mm or more, and more preferably 1.5 mm or more, from the viewpoint of ensuring the biting property of the finishing pin in the dynamic pressure groove finishing described later into the inner peripheral surface 8a. On the other hand, from the viewpoint of avoiding the situation that the pressing force in the dynamic pressure groove finishing is difficult to be reliably transmitted to the surface layer of the inner peripheral surface 8a due to the increase of the final thickness dimension t to a larger extent than necessary, the inner diameter dimension D1 is preferably 2.5 mm or less, and more preferably 2.0 mm or less.

[0057] The outer diameter D2 of the fluid dynamic bearing 8 is also arbitrary in principle, and is preferably 2.5 mm or more, more preferably 3.0 mm or more, for example, in consideration of the necessary inner diameter D1 and thickness t. From the same point of view, the outer diameter D2 is preferably 5.0 mm or less, more preferably 4.5 mm or less.

[0058] The thickness dimension t {= (D2-D1) / 2} of the fluid dynamic pressure bearing 8 is arbitrary in principle, and can be set to 0.5 mm or more, for example, from the viewpoint of ensuring the thrust bearing area required for the upper end surface 8b. On the other hand, from the viewpoint of being able to transmit sufficient compressive force from the die head to the inner peripheral surface layer portion of the sintered body during finishing of the dynamic pressure groove, the thickness dimension t can be set to 1.5 mm or less.

[0059] Next, the composition of the fluid dynamic bearing 8 when it is a porous body of sintered metal is described. The fluid dynamic bearing 8 is obtained, for example, by compressing and molding a raw material powder containing one of the copper-based powder and the iron-based powder in the largest amount and sintering the raw material powder. In other words, the porous body of the fluid dynamic bearing 8 substantially presents a composition in which one of copper and iron is the main component and the other is the second component (the second largest component). It should be noted that the copper-based powder mentioned here includes not only pure copper powder but also copper alloy powder. In addition, pure copper includes not only copper with a purity of 100%, but also copper with a purity of 99.99% or more, which is regarded as pure copper in industry. Similarly, the iron-based powder mentioned here includes not only pure iron powder but also iron alloy powder such as stainless steel. In addition, the pure iron mentioned here includes not only iron with a purity of 100%, but also iron with a purity of 99.99% or more, which is regarded as pure iron in industry. As long as the above composition (powder mixing ratio) is established, the type and mixing ratio of the powders that become the third and subsequent components are arbitrary.

[0060] As the composition of the raw material powder (powder mixing ratio), for example, [copper powder: 50-70 weight %, iron powder: 30-48 weight %, tin powder: 0-5%] can be applied. As a specific example, [pure iron powder below 140 mesh: 38-42 weight %, tin powder below 330 mesh: 1-3%, pure copper powder below 200 mesh: the rest] can be cited.

[0061] Next, the density ratio of the fluid dynamic bearing 8 is described. The density ratio of the entire bearing of the fluid dynamic bearing 8 is set to, for example, 80% or more and 95% or less. The setting of the density ratio can be achieved, for example, by adjusting the material, particle size (distribution), and mixing ratio of the metal powder used as the raw material. It should be noted that the fluctuation of the density ratio at this time can be evaluated using the porosity that is considered to have a certain correlation with the density ratio. The so-called porosity here is expressed as the volume ratio (percentage) of the pores in each unit volume of the bearing, and based on empirical rules, it basically shows a negative correlation with the density ratio (correlation coefficient of -1).

[0062] The surface opening rate of the inner circumferential surface 8a, especially the inner circumferential surface of the inclined protrusion 8a2 and the belt portion 8a3 serving as the radial bearing surface, is adjusted to, for example, 15% or less. The surface opening rate can be adjusted by, for example, rotary finishing described below.

[0063] Of course, the composition, mixing ratio, density ratio and surface porosity of the raw material powders described above are merely examples, and an appropriate form may be adopted according to the application or individual requirements.

[0064] The fluid dynamic pressure bearing 8 of the above structure is manufactured, for example, through a powder forming step S1, a sintering step S2, a dynamic pressure groove finishing step S3, and an oil impregnation step S4. In this case, a dimension finishing step S21 and a rotation finishing step S22 may be provided after the sintering step S2 and before the dynamic pressure groove finishing step S3.

[0065] (S1) Powder pressing process

[0066] First, a raw material powder that will become the material of the final product, i.e., the fluid dynamic pressure bearing 8, is prepared and compressed into a predetermined shape by a die press molding. Specifically, although not shown in the figure, the raw material powder is compressed and molded using a forming die, which is composed of a die head, a core pin inserted into a hole of the die head, a lower punch arranged between the die head and the core pin and capable of being raised and lowered relative to the die head, and an upper punch capable of being relatively displaced (raised and lowered) relative to either the die head or the lower punch. In this case, the raw material powder is filled into a space defined by the inner circumferential surface of the die head, the outer circumferential surface of the core pin, and the upper end surface of the lower punch, and the upper punch is lowered while the lower punch is fixed, and the raw material powder in the filled state is pressurized in the axial direction. Thereafter, the upper punch is lowered to a predetermined position while pressurizing, and the raw material powder is compressed to a predetermined axial dimension, thereby forming a powder compact.

[0067] (S2) Sintering process

[0068] After obtaining the green compact as described above, the green compact is sintered at a temperature corresponding to the type of raw material powder (for example, the melting point of the main component metal) to obtain a sintered body.

[0069] (S21) Dimensional finishing process and (S22) Rotational finishing process

[0070] After that, the sintered body is dimensionally adjusted, and the outer diameter, inner diameter, and axial dimensions of the sintered body are corrected to the dimensions of the final product, and the surface opening ratio of the inner peripheral surface 8a is adjusted to a ratio suitable for a fluid dynamic pressure bearing having a dynamic pressure generating portion on the inner peripheral surface (for example, the above-mentioned numerical range: 15% or less). At this stage, the predetermined dynamic pressure groove 8a1 arrangement area A1, A2 has not yet been formed on the inner peripheral surface 8a of the sintered body. Similarly, although omitted in the figure, the predetermined dynamic pressure groove 8b1 arrangement area has not yet been formed on the upper end surface 8b of the sintered body.

[0071] (S3) Dynamic pressure groove finishing process

[0072] The sintered body obtained through the above series of steps is subjected to a prescribed dynamic pressure groove finishing, thereby forming dynamic pressure groove 8a1 arrangement areas A1 and A2 on the inner peripheral surface 8a of the sintered body. Specifically, although not shown in the figure, a forming device is used to perform prescribed forming on the sintered body, and the forming device comprises: a die head having a press-in hole of the sintered body, a finishing pin arranged to be inserted into the press-in hole of the die head, a lower punch arranged between the die head and the finishing pin and capable of being raised and lowered relative to the die head, and an upper punch capable of being raised and lowered relative to either the die head or the lower punch. In this case, the dynamic pressure groove arrangement areas A1 and A2 ( Figure 3 ) and a forming die having a shape corresponding to that of the upper punch, and a region (arranged with the dynamic pressure groove 8b1 of the upper end face 8b to be formed) is provided on the lower end face of the upper punch. Figure 4 ). Therefore, a sintered body is arranged on the upper end face of the die head, and in a state where a finishing pin is inserted into the inner periphery of the sintered body, the upper punch is lowered to press the upper end face of the sintered body, thereby pressing the sintered body into the press-in hole of the die head, pressing the outer periphery of the sintered body, and the inner periphery of the sintered body bites into the forming die of the finishing pin pre-inserted into the inner periphery. In this way, the shape of the forming die is transferred to the inner periphery of the sintered body, and the dynamic pressure groove 8a1 arrangement area A1, A2 is formed on the inner periphery. In addition, at this time, the forming die provided on the lower end face of the upper punch bites into the upper end face of the sintered body, thereby transferring the shape of the forming die of the upper punch to the upper end face, forming the corresponding arrangement area of ​​the dynamic pressure groove 8b1.

[0073] After the predetermined dynamic pressure grooves 8a1 and 8b1 are formed on the inner circumference and upper end of the sintered body, the die is lowered relative to the lower punch to release the sintered body from the die. As a result, the sintered body rebounds in the outer diameter direction and can be removed from the finishing pin.

[0074] (S4) Oil-containing process

[0075] The fluid dynamic bearing 8 is completed by impregnating the lubricating oil composition into the internal pores of the sintered body (porous body) obtained as described above.

[0076] Here, the lubricating oil composition for the fluid dynamic pressure bearing 8 has the following composition. That is, the lubricating oil composition of the present invention has a kinematic viscosity at 40° C. of more than 30 mm / s. 2 / s and 80mm 2 / s or less, and the kinematic viscosity of the lubricating oil composition at 40°C is 90 mm 2 / s or more and 140mm 2 The lubricating oil composition was adjusted to have a composition of less than 100 wt% and 200 wt% .

[0077] The composition of the base oil is arbitrary as long as it satisfies the above-mentioned kinematic viscosity conditions. However, when the base oil is a mixture of nonpolar oil and polar oil, the ratio of the nonpolar oil to the entire base oil is preferably 20 wt % to 30 wt %.

[0078] Here, the nonpolar oil may be any type of nonpolar oil in principle, and for example, a synthetic hydrocarbon oil containing poly-α-olefin or its hydrogenate which exhibits excellent properties for fluid dynamic bearing applications, such as a wide operating temperature range, excellent lubricity, good initial running-in, and good durability, is suitable. In this case, the ratio of the synthetic hydrocarbon oil containing poly-α-olefin or its hydrogenate to the entire base oil is preferably 20 wt% or more and 30 wt% or less.

[0079] Here, the average molecular weight of the poly-α-olefin is 200 to 1600, preferably 400 to 800, and is preferably a polymer obtained by polymerizing decene-1, isobutylene, etc. using a Lewis acid complex or an alumina catalyst. The hydrogenated product of the poly-α-olefin can be obtained, for example, by hydrogenating the poly-α-olefin in the presence of a hydrogenation catalyst.

[0080] In principle, any nonpolar oil can be used as the polar oil. For example, ester oil having excellent evaporation characteristics, lubricity, and wear resistance can be suitably used. In this case, the ester used can be any of monoester (ester of monohydric alcohol and monohydric fatty acid), diester (ester of monohydric alcohol and dihydric fatty acid), polyol ester (ester of alcohol having a neopentyl skeleton and monohydric fatty acid, etc.), and complex ester (oligomer ester obtained by adding polyhydric fatty acid to polyhydric alcohol ester as raw material and crosslinking polyhydric alcohol).

[0081] Alternatively, the lubricating oil composition of the present 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 detergents, surfactants, friction modifiers, and the like.

[0082] However, among any of the above-mentioned additives, any additive that satisfies the above-mentioned range of kinematic viscosity at 40°C (90 mm 2 / s or more and 140mm 2 It is important to set the type, addition ratio, amount, etc. of the additive within the range of 1:1 to 5:1.

[0083] The fluid dynamic pressure bearing 8 is fixed to the inner periphery of the housing 7 by known means such as press-fitting, press-fitting with bonding, bonding, welding, etc. Then, the shaft portion 10 of the rotating body 3 is inserted into the inner periphery of the fluid dynamic pressure bearing 8. Thereafter, the internal space of the bearing is finally filled (filled) with the lubricating oil composition of the above composition, thereby completing the fluid dynamic pressure bearing device 2 in which the interface of the lubricating oil composition is maintained in the sealed space S (see Figure 2 ).

[0084] It should be noted that, for example, by implementing the above-mentioned oil-containing step S4 after fixing the sintered body that will become the fluid dynamic pressure bearing 8 to the inner periphery of the housing 7, the lubricating oil composition can be simultaneously impregnated into the fluid dynamic pressure bearing 8 and the lubricating oil composition can be filled into the internal space of the fluid dynamic pressure bearing device 2.

[0085] In the fluid dynamic pressure bearing device 2 having the above-mentioned structure, the area (the upper and lower dynamic pressure groove 8a1 arrangement areas A1 and A2) that become the radial bearing surface of the inner peripheral surface 8a of the fluid dynamic pressure bearing 8 faces the outer peripheral surface 10a of the shaft 10 with the radial bearing gap interposed therebetween when the shaft 10 (rotating body 3) rotates. In addition, as the shaft 10 rotates, the lubricating oil composition in the radial bearing gap is pressed into the axial center side of each dynamic pressure groove 8a1 arrangement area A1 and A2, and the pressure of the lubricating oil composition in the area on the axial center side (here, the belt portion 8a3) increases. By the dynamic pressure action of the dynamic pressure groove 8a1, an 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 that support the shaft 10 in a non-contact manner and freely rotatable in the radial direction are formed separately in the axial direction.

[0086] In addition, in the thrust bearing gap between the upper end surface 8b of the fluid dynamic pressure bearing 8 (the area where the dynamic pressure grooves 8b1 are arranged) and the lower end surface 9a1 of the hub portion 9 facing it, an oil film of the lubricating oil composition is formed by the dynamic pressure action of the dynamic pressure grooves 8b1. And, by the pressure of the oil film, a first thrust bearing portion T1 that non-contact-supports the rotating body 3 in the thrust direction is formed. In addition, the lower end portion 10b of the shaft portion 10 is rotatably contact-supported by the thrust receiving portion 11 provided at the bottom 7e of the housing 7, thereby forming a second thrust bearing portion T2 that contact-supports the rotating body 3 in the thrust direction.

[0087] As described above, the lubricating oil composition for the fluid dynamic pressure bearing 8 of the present embodiment has a kinematic viscosity of more than 30 mm / s at 40°C. 2 / s and 80mm 2 / s or less as the base oil, and the kinematic viscosity of the lubricating oil composition at 40°C is 90 mm 2 / s or more and 140mm 2 / s or less. In addition, in the present embodiment, in the adjustment of the viscosity, the amount of the polymer-based viscosity index improver is adjusted so that it is 4wt% or less. By adjusting the kinematic 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. Thus, the fluid dynamic pressure bearing 8 can exert a high load capacity. In addition, even if the shaft 10 is greatly eccentric, resulting in a smaller radial bearing gap, an oil film of the lubricating oil composition can be stably formed in the radial bearing gap. By adjusting the composition of the lubricating oil composition as described above without adding a viscosity index improver as much as possible, it is possible to avoid a situation in which the viscosity increases to a higher level than necessary at a low temperature as the start of the start. Thus, the required high load capacity can be stably exerted without increasing the bearing torque.

[0088] In addition, in the present embodiment, the base oil is a mixture of non-polar oil and polar oil, and the ratio of the non-polar oil to the whole base oil is set to 20 wt% or more and 30 wt% or less. By setting the ratio of the non-polar oil with high relative resistivity to 20 wt% or more in this way, the resistivity of the lubricating oil composition as a whole can be reduced to a level that can prevent the conduction of electricity between the shaft 10 and the fluid dynamic pressure bearing 8 through the lubricating oil composition as much as possible. As a result, it is possible to prevent misjudgment in the evaluation process of the load capacity based on the electrical resistance method (for details, refer to the description below) as much as possible, and thus appropriately evaluate the load capacity of the fluid dynamic pressure bearing 8. In addition, by suppressing the ratio of the non-polar oil to 30 wt% or less, it is possible to avoid the situation where the viscosity of the lubricating oil composition in a low temperature state becomes high, and it is possible to avoid the situation where the evaporation amount becomes large.

[0089] In addition, in this embodiment, since the nonpolar oil uses a synthetic hydrocarbon oil containing poly-α-olefin or its hydrogenated product, the operating temperature range of the lubricating oil composition can be expanded. In addition, it is possible to impart excellent lubricity to the fluid dynamic pressure bearing while also imparting good initial running-in properties. Furthermore, it is possible to achieve improved durability.

[0090] In addition, in this embodiment, since the polar oil uses ester oil, the evaporation characteristics, lubricity, and wear resistance of the lubricating oil composition can be improved. In particular, when a synthetic hydrocarbon oil containing poly-α-olefin or its hydrogenate is used as the non-polar oil, the solubility, which is a disadvantage of polyolefins, can be overcome by using ester oil as the polar oil. In addition, by mixing ester oil with the synthetic hydrocarbon oil containing poly-α-olefin or its hydrogenate, the good bearing performance of the fluid dynamic pressure bearing can be stably maintained for a long time.

[0091] An embodiment of the present invention has been described above; however, the lubricating oil composition for a fluid dynamic bearing, the fluid dynamic bearing impregnated with the lubricating oil composition, and the fluid dynamic bearing device having the bearing are not limited to the above-described exemplary embodiments, and any embodiment may be adopted within the scope of the present invention.

[0092] Figure 6 1 is a cross-sectional view showing a fluid dynamic pressure bearing device 12 according to another embodiment of the present invention. Figure 6 As shown, the fluid dynamic bearing device 12 of this embodiment is different from the fluid dynamic bearing device 12 in that it only has the first thrust bearing portion T1. Figure 2The fluid dynamic pressure bearing device 2 shown in the figure is different. Specifically, in the fluid dynamic pressure bearing device 2 of the present embodiment, there is always a predetermined thrust direction gap 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 opposing 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 opposing distance between the upper end surface 8b of the fluid dynamic pressure bearing 8 and the lower end surface 9a1 of the disc portion 9a, and is set to a size that is considered to have no substantial effect on the increase in loss torque during rotational drive. Regarding the configuration other than this, due to the difference with Figure 2 Since the fluid dynamic bearing device 2 is the same as that shown in the above, the detailed description is omitted.

[0093] The fluid dynamic bearing device 12 of the present embodiment also uses a lubricating oil composition for the fluid dynamic bearing 8 having a kinematic viscosity of more than 30 mm at 40° C. 2 / s and 80mm 2 / s or less as the base oil, and the kinematic viscosity of the lubricating oil composition at 40°C is 90 mm 2 / s or more and 140mm 2 / s or less. As a result, an oil film can be stably formed in the radial bearing gap regardless of the size of the radial bearing gap. As a result, the fluid dynamic pressure bearing 8 can exert a high load capacity. In addition, since the required thrust bearing area can be ensured on the upper end surface 8b of the fluid dynamic pressure bearing 8, even when the thrust bearing part (first thrust bearing part T1) is provided only on the upper end surface 8b side of the fluid dynamic pressure bearing 8, excellent rotation accuracy can be exerted. As a result, Figure 6 As shown, even if the lower end surface 10c of the shaft portion 10 is not supported in the thrust direction, sufficient thrust bearing performance can be exerted, and the shape of the lower end of the shaft portion 10 can be simplified, typified by omitting the thrust receiving portion 11, thereby achieving cost reduction.

[0094] Of course, the radial dynamic pressure generating portion ( Figure 3 Any one or both of the dynamic pressure generating portions (i.e., the arrangement areas A1 and A2 of the dynamic pressure generating grooves 8a1) may be formed into a known radial dynamic pressure generating portion having a shape other than the dynamic pressure generating grooves, such as a so-called multi-arc shape, a step shape, or a wave shape.

[0095] In the above description, the housing 7 of the fluid dynamic pressure bearing device 2 is fixed to the inner periphery of the base portion 6 of the fan motor 1, but the housing 7 may be directly mounted on the base portion (not shown) of the information device to which the fan motor 1 is mounted, for example. Alternatively, the housing 7 may be integrated with the portion corresponding to the base portion.

[0096] In the above description, the external force for pressing the shaft 10 toward the bottom 7e side of the housing 7 is applied to the shaft 10 (rotating body 3) by arranging the magnet 5b and the coil 5a in an axially staggered manner, but the method for applying the external force to the shaft 10 is not limited to the above method. Although omitted in the figure, for example, the magnetic force may be applied to the rotating body 3 by arranging a magnetic member capable of attracting the magnet 5b opposite to the magnet 5b in the axial direction. In addition, in the case where the thrust as the reaction force of the blasting action is large enough and the shaft 10 can be pressed downward only by the thrust, the magnetic force (magnetic attraction) as the external force for pressing the shaft 10 downward may be omitted.

[0097] In the above description, the present invention is applied to the case where the fluid dynamic pressure bearing device 2 is fixed to the shaft 10 with the rotating body 3 having the blades 4, but the present invention can also be preferably applied to the fluid dynamic pressure bearing device 2 in which a disk hub having a disk mounting surface or a polygonal mirror is fixed to the shaft 10 as the rotating body 3. That is, the present invention is not limited to the case where the rotating body 3 is fixed to the shaft 10. Figure 1 The fan motor 1 shown can also be preferably applied to a fluid dynamic pressure bearing device 2 of other electrical equipment such as a spindle motor for a disk loading device and a polygonal scanning motor for a laser printer (LBP).

[0098] Example

[0099] Hereinafter, examples for verifying the effects of the present invention will be described.

[0100] In this example, lubricating oil compositions showing the composition of the present invention (Examples 1 to 3) and lubricating oil compositions showing compositions other than the present invention (Comparative Examples 1 to 4) were prepared, and the kinematic viscosity and viscosity of each lubricating oil composition were evaluated. In addition, the characteristics related to the load capacity of a fluid dynamic bearing device in which each lubricating oil composition was impregnated into a fluid dynamic bearing and the lubricating oil composition was filled in the internal space of the bearing were evaluated.

[0101] Here, the lubricating oil composition used as the evaluation object uses a lubricating oil composition with a mixture of non-polar oil and polar oil as the base oil. Specifically, a synthetic hydrocarbon oil containing poly-α-olefin (PAO) is used as the non-polar oil, a polyol ester oil and a diester oil are used as the polar oil, and a polymethacrylate-based viscosity index improver is used as the viscosity index improver. In addition, any lubricating oil composition uses a lubricating oil composition to which an antioxidant and a rust-proof material are added. Table 1 shows the proportion (compounding ratio) [wt%] of the non-polar oil and polar oil and the viscosity index improver of Examples 1 to 3 and Comparative Examples 1 to 4 in the lubricating oil composition.

[0102]

[0103] In addition, regarding the kinematic viscosity of the base oil at 40°C [mm 2 / s], obtained according to 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, respectively.

[0104] In addition, the kinematic viscosity of each lubricating oil composition at 40°C [mm 2 / s], obtained according to JIS K 2283. The kinematic viscosities at 40°C of the lubricating oil compositions of Examples 1 to 3 and Comparative Examples 1 to 4 are shown in Table 2, respectively.

[0105] The viscosity of each lubricating oil composition at -40°C (mPa·s) was measured using a cone-plate type rotational viscometer (shear rate: 300 s -1 The viscosities of the lubricating oil compositions of Examples 1 to 3 and Comparative Examples 1 to 4 at -40°C are shown in Table 2, respectively.

[0106] In addition, the load-related characteristics of the fluid dynamic bearing device impregnated and filled with each lubricating oil composition were measured by the electrical resistance method for the minimum floating rotation speed. In addition, based on the measurement results, the maximum value of the eccentricity of the shaft portion that can rotate in a non-contact manner with respect to the fluid dynamic bearing was obtained.

[0107] Among them, the minimum rotation speed of suspension is measured as follows. That is, although it is omitted in the figure, a long shaft is inserted into the fluid dynamic pressure bearing of the present invention, a motor is coaxially connected to one axial end thereof, and the fluid dynamic pressure bearing is fixed to the inner periphery of the housing. Moreover, the shaft and the fluid dynamic pressure bearing are sealed with a sealing member. Then, in a state where a prescribed load is applied to the fluid dynamic pressure bearing through the housing, in this case a radial load (two types of 3N and 6N) is applied, the shaft is driven at a prescribed rotation speed by a motor. The rotation speed is increased from 2000min / min to 1000min / min. -1 Start with 100 minutes each time -1 Then, the rotation speed of the shaft is measured by a rotation speed sensor provided at the other axial end of the shaft when the conduction (electrical connection) between the shaft and the fluid dynamic pressure bearing is detected, and the measured rotation speed is set as the minimum suspension rotation speed [min -1 The minimum suspension speeds of Examples 1 to 3 and Comparative Examples 1 to 4 are shown in Table 1, respectively.

[0108] The eccentricity was determined by using a calculation program that uses the eccentricity of the shaft at the time when the conduction between the shaft and the fluid dynamic bearing was detected as described above, the radius of the radial bearing clearance between the fluid dynamic bearing and the shaft, and the rotation speed (leaning minimum rotation speed) as input values ​​and the load as output value, and the value obtained by inverse calculation based on the load during the test (3N or 6N) was obtained as the eccentricity. The eccentricities of Examples 1 to 3 and Comparative Examples 1 to 4 are shown in Table 2.

[0109]

[0110] As shown in Table 2, when comparing Examples 1 to 3 and Comparative Example 1, the kinematic viscosity of the lubricating oil composition shows the same value in Comparative Example 1 and Examples 1 to 3. On the other hand, however, the kinematic viscosity of the base oil of Comparative Example 1 is smaller than the numerical range of the present invention. Here, although the minimum suspension speed of Comparative Example 1 is the same value as the minimum suspension speed of Example 1, the magnitude of the load is different (half that of Example 1). From this result, it can be seen that the load capacity of Comparative Example 1 is smaller than the load capacity of Example 1. In addition, although the load is at the same level, the minimum suspension speed of Comparative Example 1 is also significantly larger than the minimum suspension speeds of Examples 2 and 3. From this result, it can be seen that the load capacity of Comparative Example 1 is smaller than the load capacity of Examples 2 and 3.

[0111] 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 similar in Comparative Example 2 and Example 1. On the other hand, the content of the nonpolar oil relative to the base oil in Comparative Example 2 is smaller than the numerical range of the present invention. Therefore, the minimum suspension speed of Comparative Example 2 is likely to be measured to be larger than the actual value, and lacks reliability as an indicator of load capacity.

[0112] When comparing Examples 1 to 3 and Comparative Example 3, the kinematic viscosity of the lubricating oil composition showed the same value in Comparative Example 3 and Examples 1 to 3. On the other hand, the kinematic viscosity of the base oil of Comparative Example 3 was smaller than the numerical range of the present invention (greater than 30 mm 2 / s and 80mm 2 / s or less). Therefore, although the load is at the same level, the minimum levitation speed of Comparative Example 3 is also significantly greater than the minimum levitation speed of Example 1. From this result, it can be seen that the load capacity of Comparative Example 3 is smaller than that of Example 1. In addition, when comparing Comparative Example 3 with Examples 2 and 3, although the load is twice that of Examples 2 and 3, the minimum levitation speed of Comparative Example 3 is 5 times that of Examples 2 and 3. From this result, it can be seen that the load capacity of Comparative Example 3 is smaller than that of Examples 2 and 3.

[0113] When comparing Examples 1 to 3 and Comparative Example 4, the kinematic viscosity of the base oil and the kinematic viscosity of the lubricating oil composition of Comparative Example 4 are both greater than the kinematic viscosities of Examples 1 to 3, and on the other hand, any kinematic viscosity is greater than the numerical range of the present invention (base oil: greater than 30 mm 2 / s and 80mm 2 / s or less, lubricating oil composition: greater than 90mm 2 / s and 140mm 2 / s or less). Therefore, although the load capacity (lowest suspension speed, eccentricity) of Comparative Example 4 is at the same level as that of Examples 1 to 3, the viscosity of the lubricating oil composition at low temperature is greater than that of Examples 1 to 3. In this state, the power consumption (bearing torque) of the motor is too large, or the motor may not be started.

[0114] Description of Reference Numerals

[0115] 1 Fan motor, 2, 12 Fluid dynamic pressure bearing device, 3 Rotating body, 4 Blade, 5 Driving part, 5a Coil, 5b Magnet, 6 Base part, 7 Housing, 7c Upper end surface, 7d Sealing surface, 8 Fluid dynamic pressure bearing, 8a Inner circumferential surface, 8a1 Dynamic pressure groove, 8a2 Inclined hill, 8a3 Belt part, 8b Upper end surface, 8b1 Dynamic pressure groove, 8c Lower end surface, 8d Outer circumferential surface, 8d1 Axial groove, 9 Hub part, 9a Disc part, 9a1 Lower end surface, 9b, 9c Cylindrical part, 9d Flange part, 9e Mounting hole, 10 Shaft part, 10a Outer circumferential surface, 10b Lower end part, 10c Lower end surface, 11 Thrust receiving part, A1, A2 Dynamic pressure groove arrangement area, D1 Inner diameter dimension, D2 Outer diameter dimension, L Axial dimension, R1, R2 radial bearing part, S sealing space, T1, T2 thrust bearing part.

Claims

1. A lubricating oil composition for a fluid dynamic pressure bearing, which is a lubricating oil composition used in a fluid dynamic pressure bearing having a radial dynamic pressure generating portion provided on the inner peripheral surface of a porous body having internal pores, wherein the lubricating oil composition for a fluid dynamic pressure bearing is characterized in that: The kinematic viscosity at 40°C is greater than 30 mm 2 / s and 80mm 2 / s or less as the base oil, and the kinematic viscosity of the lubricating oil composition at 40°C is 90 mm 2 / s or more and 140mm 2 / s or less.

2. The lubricating oil composition for a fluid dynamic pressure 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 entire base oil is 20 wt % or more and 30 wt % or less.

3. The lubricating oil composition for a fluid dynamic pressure bearing according to claim 2, wherein: The nonpolar oil is a synthetic hydrocarbon oil containing poly-α-olefin or a hydrogenate thereof.

4. The lubricating oil composition for a fluid dynamic pressure bearing according to claim 2, wherein: The polar oil is an ester oil. 5 . A fluid dynamic bearing, wherein the lubricating oil composition according to claim 1 is impregnated into the internal pores.

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

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

8. A fluid dynamic bearing device comprising: The fluid dynamic pressure bearing according to claim 5, A housing having the fluid dynamic pressure bearing fixed on the inner periphery, a rotating body having a shaft portion inserted into the inner periphery of the fluid dynamic pressure bearing, and A radial bearing portion of the shaft portion is non-contact supported in a radial direction by an oil film of the lubricating oil composition, and the lubricating oil composition is formed in a radial bearing gap between the inner peripheral surface of the fluid dynamic pressure bearing and the outer peripheral surface of the shaft portion.

9. The fluid dynamic bearing device according to claim 8, wherein: The eccentricity of the shaft portion that is rotatable in a non-contact manner relative to the fluid dynamic pressure bearing is shown to be 98% at maximum. 10 . A motor comprising the fluid dynamic bearing device according to claim 8 .

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