Dynamic pressure bearing and fluid dynamic pressure bearing device equipped with same

Abstract

To acquire desired bearing rigidity in a dynamic pressure bearing including a sintered compact having an increased dynamic pressure groove depth.SOLUTION: A bearing sleeve 8 (dynamic pressure bearing) includes a cylindrical sintered compact having an inner peripheral surface 8a formed with a dynamic pressure groove G1, where a ratio D2 / D1 of an internal diameter D2 to an internal diameter D1 of the sintered compact is 2.5 or less and a difference in relative density of three parts 8A, 8B, 8C obtained by dividing the sintered compact into three equal parts in an axial direction is 3% or less.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a dynamic pressure bearing and a fluid dynamic pressure bearing device including the same. [Background technology]

[0002] In a fluid dynamic bearing device, the relative rotation between a bearing member and a shaft member inserted into its inner circumference increases the pressure of a fluid film that forms in the radial bearing gap between the outer surface of the shaft member and the inner surface of the bearing member, and this pressure (dynamic pressure action) supports the shaft member in a non-contact manner, allowing it to rotate freely relative to the shaft.Due to their excellent rotational accuracy and quietness, fluid dynamic bearing devices are ideal for use in supporting rotating shafts such as spindle motors in HDD disk drives, polygon scanner motors in laser beam printers, and fan motors used to cool electronic equipment.

[0003] Dynamic pressure grooves that actively increase the pressure of the lubricating fluid in the radial bearing gap are sometimes formed on the inner peripheral surface of the bearing member of a fluid dynamic bearing device. One known method for forming dynamic pressure grooves is so-called dynamic pressure groove sizing, in which dynamic pressure grooves are molded into the inner peripheral surface of a cylindrical sintered body. In this dynamic pressure groove sizing, a sizing pin is inserted into the inner periphery of the sintered body, and the sintered body is pressed axially by an upper punch and a lower punch while being pressed into the inner periphery of a die, thereby pressing the inner peripheral surface of the sintered body against a molding die formed on the outer peripheral surface of the sizing pin. This transfers the shape of the molding die to the inner peripheral surface of the sintered body, forming dynamic pressure grooves (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 11-182550 Summary of the Invention [Problem to be solved by the invention]

[0005] In a fluid dynamic bearing device having a bearing member with dynamic pressure grooves formed on its inner circumferential surface (hereinafter referred to as a "dynamic pressure bearing"), it is known that dynamic pressure is generated most efficiently when the ratio of the dynamic pressure groove depth to the radial bearing gap width is 1:1. On the other hand, if the width of the radial bearing gap is small, there is a possibility that the shaft member will come into contact with the axial end of the dynamic pressure bearing when tilted relative to the dynamic pressure bearing. Therefore, it is necessary to reduce the risk of contact between the shaft member and the dynamic pressure bearing by increasing the depth of the dynamic pressure groove as much as possible and increasing the width of the radial bearing gap while maintaining a 1:1 ratio between the dynamic pressure groove depth and the radial bearing gap width.

[0006] However, when forming the dynamic pressure grooves by pressing the inner peripheral surface of the sintered compact against the sizing core as described above, if the springback amount of the sintered compact is insufficient, it is impossible to ensure a sufficient dynamic pressure groove depth. Figure 6 shows the relationship between the ratio D2 / D1 of the inner diameter D1 to the outer diameter D2 of the sintered compact and the corresponding dynamic pressure groove depth. As can be seen from this graph, the larger the inner / outer diameter ratio D2 / D1 of the sintered compact, i.e., the larger the radial thickness of the sintered compact, the smaller the dynamic pressure groove depth. Therefore, to increase the dynamic pressure groove depth, it is preferable to decrease the inner / outer diameter ratio D2 / D1 of the sintered compact.

[0007] However, reducing the inner / outer diameter ratio of the sintered compact leads to the following problems with dynamic pressure groove sizing. In dynamic pressure groove sizing, the sintered compact is compressed from both axial and outer diameter sides. When the inner / outer diameter ratio of the sintered compact is large (i.e., the radial thickness is thick), the rigidity of the sintered compact against axial compressive forces is high, so the compressive forces from both axial sides are hardly transmitted to the inner peripheral surface of the sintered compact, and compression from the outer diameter side becomes dominant. On the other hand, when the inner / outer diameter ratio of the sintered compact is small (i.e., the radial thickness is thin), the rigidity of the sintered compact against axial compressive forces is low, so the axial compressive forces are more likely to be transmitted to the inner peripheral surface of the sintered compact, especially near the axial ends. As a result, the density of the sintered compact at the axial ends is higher than that at the axial center. In particular, the density of the sintered compact tends to be high near the upper punch, which presses the sintered compact against the inner peripheral surface of the die (see Figure 7).

[0008] As described above, if the density (i.e., compressibility) of the sintered body varies depending on the axial position, the dynamic pressure groove depth will be deep in areas with high density and shallow in areas with low density. This variation in the dynamic pressure groove depth along the axial direction may result in the failure to obtain the desired bearing rigidity.

[0009] Furthermore, because the density (i.e., compression rate) of the sintered body varies depending on the axial position, the generatrix shape (profile of the axial cross section) of the inner peripheral surface of the sintered body is disrupted, causing the axially central portion, which has low density, to protrude toward the inner diameter side, as shown in Figure 8. When the generatrix shape of the inner peripheral surface is disrupted in this way, the width of the radial bearing gap varies in the axial direction, and there is a risk that the desired bearing rigidity may not be obtained.

[0010] SUMMARY OF THE INVENTION It is therefore an object of the present invention to obtain a desired bearing rigidity in a dynamic pressure bearing in which the dynamic pressure groove depth of the sintered body is increased. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention provides a hydrodynamic bearing having a cylindrical sintered body with hydrodynamic grooves formed on the inner peripheral surface, wherein the ratio D2 / D1 of the inner diameter D1 to the outer diameter D2 of the sintered body is 2.5 or less, and the sintered body is divided into three equal parts in the axial direction. portion The present invention provides a hydrodynamic bearing in which the difference in relative density is within 3%.

[0012] In this way, by reducing the inner / outer diameter ratio D2 / D1 of the hydrodynamic bearing (to 2.5 or less), that is, by reducing the thickness of the hydrodynamic bearing, the hydrodynamic groove depth can be increased. By making the density of such a thin sintered body approximately uniform in the axial direction, specifically, by dividing the sintered body into three equal parts in the axial direction, portion By keeping the difference in relative density within 3%, it is possible to suppress variations in the depth of the dynamic pressure grooves and the collapse of the generatrix shape of the inner peripheral surface caused by density differences in the axial direction of the sintered body.

[0013] By keeping the axial length of the sintered body to 4 mm or less, the axial compressive force applied to the sintered body during molding of the hydrodynamic grooves is easily transmitted to the entire axial area of the inner circumferential surface, thereby reducing variation in the density of the sintered body depending on the axial position.

[0014] By making the density of the sintered body approximately uniform in the axial direction as described above, it is possible to suppress sagging (recession toward the outer diameter) at the axial end of the inner peripheral surface of the sintered body. Specifically, it is possible to suppress the difference in radius between the minimum diameter portion and the maximum diameter portion of the inner diameter surface of the ridge portion that rises toward the inner diameter side from the hydrodynamic grooves on the inner peripheral surface of the hydrodynamic bearing to 2 μm or less.

[0015] Because it is difficult to make the density of a sintered body completely uniform in the axial direction, the density of the sintered body in the axial center is slightly lower than the density at the axial ends. As a result, the depth of the dynamic pressure grooves in the axial center of a hydrodynamic bearing, which has low density, tends to vary from product to product. Therefore, the hydrodynamic pressure grooves in the axial center, where the depth tends to vary, may be omitted. In this case, it is possible to create a cylindrical surface without hydrodynamic pressure grooves in the axial center by, for example, providing herringbone-shaped hydrodynamic pressure grooves in two locations spaced apart in the axial direction. However, in this case, the axial dimension of the hydrodynamic pressure bearing becomes large, making it difficult to uniform the density in the axial direction.

[0016] Therefore, a cylindrical surface may be formed by omitting the axially central groove from the herringbone-shaped dynamic pressure grooves provided at two axial locations. Specifically, the inner peripheral surface of the sintered compact may be formed with a pair of annular hills provided at two axially spaced locations, multiple inclined hills extending axially outward from each annular hill, dynamic pressure grooves provided circumferentially between the multiple inclined hills, and a cylindrical surface extending over the entire axial area between the pair of annular hills and having a diameter larger than the inner diameter of the annular hills. In this way, omitting the inclined hills and dynamic pressure grooves at the axial center of the sintered compact (the axial distance between the pair of annular hills) reduces variation in the dynamic pressure groove depth from product to product, thereby stabilizing the performance of the dynamic pressure bearing.

[0017] A fluid dynamic bearing device comprising the above-mentioned dynamic pressure bearing, a shaft member inserted into the inner circumference of the dynamic pressure bearing, and a radial bearing portion that supports the shaft member in a non-contact manner so that it can rotate freely relative to the shaft member by the dynamic pressure action of a lubricating film generated in the radial bearing gap between the inner surface of the dynamic pressure bearing and the shaft member can stably support the shaft member. [Effects of the Invention]

[0018] As described above, the depth of the dynamic pressure grooves can be increased by reducing the radial thickness of the sintered body, and by making the density of the sintered body approximately uniform in the axial direction, variations in the depth of the dynamic pressure grooves and collapse of the generatrices of the inner peripheral surface can be suppressed, thereby achieving the desired bearing rigidity. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 2 is a cross-sectional view of a fan motor. [Figure 2] FIG. 2 is a cross-sectional view of a fluid dynamic bearing device. [Figure 3] 1 is a cross-sectional view of a hydrodynamic bearing according to an embodiment of the present invention. [Figure 4] 1A and 1B are cross-sectional views showing the sizing process, in which (A) shows the state before the sintered body is pressed into the die, and (B) shows the state after the sintered body has been pressed into the die. [Figure 5] FIG. 10 is a cross-sectional view of a dynamic pressure bearing according to another embodiment. [Figure 6] 1 is a graph showing the relationship between the inner and outer diameter ratio of a dynamic pressure bearing and the depth of a dynamic pressure groove. [Figure 7] 10 is a graph showing a difference in density depending on the axial position of a dynamic pressure bearing. [Figure 8] FIG. 2 is a diagram showing the generatrix shape of the inner peripheral surface of the hydrodynamic bearing. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0021] FIG. 1 conceptually illustrates an example of a fan motor. The fan motor shown in the figure is incorporated into portable information devices such as notebook computers and tablet computers, and generates airflow to cool heat sources such as CPUs. This fan motor includes a fluid dynamic bearing device 1, a motor base 5 constituting the stationary side of the motor, a rotor 3 fixed to a shaft member 2 of the fluid dynamic bearing device 1, impellers 4 attached to the rotor 3, and a stator 6a and magnet 6b arranged opposite each other across a radial gap. The stator 6a is attached to a housing 7 of the fluid dynamic bearing device 1, and the magnet 6b is attached to the rotor 3. When current is applied to the coil of the stator 6a, electromagnetic force between the stator 6a and magnet 6b rotates the shaft member 2 and the rotor 3 fixed to the shaft member 2 as a unit. As the rotor 3 rotates, an airflow is generated in the axial direction or radially outward, depending on the shape of the impellers 4 attached to the rotor 3.

[0022] As shown in Figure 2, the fluid dynamic bearing device 1 comprises a shaft member 2, a housing 7, a bearing sleeve 8 as a dynamic bearing according to one embodiment of the present invention, and a seal member 9. The internal space of the housing 7 is filled with lubricating oil. For ease of explanation, the side of the shaft member 2 that protrudes from the housing 7 in the axial direction (the upper side in Figure 2) will be referred to as the "upper side," and the opposite side (the lower side in Figure 2) will be referred to as the "lower side," but this is not intended to limit the orientation of the fluid dynamic bearing device 1 when in use.

[0023] The shaft member 2 is made of a highly rigid metal material such as stainless steel. The outer peripheral surface 2a of the shaft member 2 is formed as a smooth cylindrical surface without any irregularities. The lower end of the shaft member 2 is formed as a convex spherical surface 2b. The rotor 3 is fixed to the upper end of the shaft member 2.

[0024] The housing 7 has a cylindrical portion 7a and a bottom portion 7b that closes the opening at the lower end of the cylindrical portion 7a. In the illustrated example, the cylindrical portion 7a and the bottom portion 7b are integrally formed from a resin or metal material. A shoulder surface 7b2 is provided on the outer periphery of an upper end surface 7b1 of the bottom portion 7b, and is positioned above the upper end surface 7b1. The stator 6a and the motor base 5 are fixed to the outer circumferential surface 7a2 of the cylindrical portion 7a.

[0025] In the illustrated example, a thrust plate 10 is provided on the inner bottom surface (upper end surface of the bottom portion 7b) 7b1 of the housing 7. The thrust plate 10 is formed in a disk shape from a material that has better sliding properties than the material from which the housing 7 is made. The upper end surface of the thrust plate 10 contacts and supports the convex spherical surface 2b at the lower end of the shaft member 2. Note that the thrust plate 10 may be omitted, in which case the inner bottom surface 7b1 of the housing 7 will contact and support the convex spherical surface 2b of the shaft member 2.

[0026] The seal member 9 is formed in an annular shape from a resin or metal material and is fixed to the upper end of the inner circumferential surface 7a1 of the cylindrical portion 7a of the housing 7. The lower end surface 9b of the seal member 9 abuts against the upper end surface 8b of the bearing sleeve 8. The inner circumferential surface 9a of the seal member 9 forms an annular seal space S between itself and the outer circumferential surface 2a of the shaft member 2 that faces it.

[0027] Note that this fluid dynamic bearing device 1 is a so-called partial fill type in which lubricating oil and air are mixed inside the housing 7, but it is not limited to this and may also be a so-called full fill type in which the entire internal space of the housing 7 is filled with lubricating oil. In the case of a full fill type, the seal space has a wedge-shaped cross section, and the lubricating oil level is always maintained within the axial range of the seal space S.

[0028] The bearing sleeve 8 is formed into a cylindrical shape from a porous sintered body, for example, whose main components are copper and iron. The bearing sleeve 8 is fixed to the inner periphery of the housing 7 in an oil-impregnated state, with the internal pores of the sintered body impregnated with lubricating oil. In the illustrated example, the bearing sleeve 8 is fixed to the inner periphery of the cylindrical portion 7a of the housing 7 with its lower end surface 8c abutting against the shoulder surface 7b2 of the bottom portion 7b of the housing 7. The bearing sleeve 8 is fixed to the inner periphery 7a1 of the cylindrical portion 7a by press-fitting, bonding, or press-fit bonding (a combination of press-fitting and bonding), etc. Alternatively, the bearing sleeve 8 can be loose-fitted to the inner periphery of the housing 7, and then sandwiched from both axial sides between the seal member 9 and the shoulder surface 7b2 of the housing 7 to fix the bearing sleeve 8 to the inner periphery of the cylindrical portion 7a.

[0029] As shown in FIG. 3 , the inner circumferential surface 8a of the bearing sleeve 8 is formed with dynamic pressure grooves G1 and hill portions (cross-hatched regions) that protrude radially inward from the dynamic pressure grooves G1. In this embodiment, herringbone-shaped dynamic pressure grooves G1 are formed at two axially adjacent locations. Specifically, the inner circumferential surface 8a of the sintered compact is formed with two axially spaced annular hill portions G2, multiple inclined hill portions G3 extending axially from each annular hill portion G2, and multiple dynamic pressure grooves G1 circumferentially spaced between the multiple inclined hill portions G3. The annular hill portion G2 and the inclined hill portions G3 on both axial sides thereof are continuous, and their inner diameter surfaces are arranged on the same cylindrical surface. In the illustrated example, the inclined hill portion G3 located axially between a pair of annular hill portions G2 are continuous, and their inner diameter surfaces are arranged on the same cylindrical surface. Furthermore, the dynamic pressure grooves G1 located axially between a pair of annular hill portions G2 are continuous, and their bottom surfaces are arranged on the same cylindrical surface. The inner peripheral surface 8a of the bearing sleeve 8, including the bottom surface of the dynamic pressure groove G1 and the inner diameter surfaces of the annular hill portion G2 and the inclined hill portion G3, is a molded surface formed by pressing a mold against it.

[0030] An axial groove 8d1 is formed in the outer peripheral surface 8d of the bearing sleeve 8. A radial groove 8b1 and an annular groove 8b2 are formed in the upper end surface 8b of the bearing sleeve 8. A radial groove 8c1 is formed in the lower end surface 8c of the bearing sleeve 8. The annular groove 8b2 is provided to identify the up and down directions (i.e., the direction of rotation) when assembling the bearing sleeve 8 to the housing 7. The axial groove 8d1 and the radial grooves 8b1 and 8c1 form a communication passage in the fluid dynamic bearing device 1 that connects the space facing the bottom 7b of the housing 7 with the atmosphere (see FIG. 2). Note that if there is no particular need, any or all of the radial grooves 8b1 and 8c1, the annular groove 8b2, and the axial groove 8d1 may be omitted.

[0031] The inner diameter D1 of the bearing sleeve 8 is, for example, 4 mm or less, preferably 2 mm or less, and more preferably 1.5 mm or less. The outer diameter D2 of the bearing sleeve 8 is, for example, 7 mm or less, and preferably 4 mm or less. The ratio D2 / D1 of the inner diameter D1 to the outer diameter D2 of the bearing sleeve 8 is 2.5 or less, preferably 2.0 or less, and more preferably 1.8 or less. The axial dimension L of the bearing sleeve 8 is 4 mm or less, and preferably 3 mm or less.

[0032] The relative density (density ratio to true density) of the bearing sleeve 8 made of a sintered body is set to 80 to 95%. The density of the bearing sleeve 8 is set to be approximately uniform in the axial direction. Specifically, if the bearing sleeve 8 is divided into three equal parts in the axial direction (see dotted lines in Figure 3), the three divided parts will be (portion) When 8A, 8B, and 8C are formed, the difference in relative density among these segments 8A, 8B, and 8C is within 3%, preferably within 2%. The density of the bearing sleeve 8 is also substantially uniform in the radial direction. Therefore, the porosity of the cross section perpendicular to the axial direction of the bearing sleeve 8 is substantially constant in the radial direction. Specifically, when the porosity (surface opening ratio) is measured at three locations in the cross section perpendicular to the axial direction of the bearing sleeve 8: near the outer surface 8d, the center in the radial direction, and near the inner surface 8a, the difference in porosity among these locations is 1% or less. The relative density of the bearing sleeve 8 is measured in a dry state without oil impregnation, according to the method described in JIS Z 2501. To measure the porosity, a photograph of the area to be measured is taken and binarized by image processing (areas other than the pores are white, and the pores are black). The porosity is calculated as the ratio of the area of the black areas (pores) to the area of the entire field of view. The equipment and photographing conditions used to measure the porosity are as follows: [Equipment used] Microscope: Nikon ECLIPSE ME600 Camera head: Nikon DS-Fi2 -Shooting software: NIS-Elemnets D Analysis software:QuicK Grain Stand Exposure adjustment paper: QP Card 101 [Photography conditions] Exposure time: 10ms Analog gain: 1.0X ·Measurement magnification: 100x

[0033] The generatrix shape (axial cross-sectional shape) of the inner peripheral surface of the bearing sleeve 8 is approximately parallel to the axial direction. Therefore, the inner diameter surfaces of the hill portions (annular hill portion G2 and inclined hill portion G3) of the inner peripheral surface 8a of the bearing sleeve 8 are arranged in the shape of approximately the same cylindrical surface. Specifically, the difference in radius between the minimum diameter portion and the maximum diameter portion of the inner diameter surface of the hill portion is within 2 μm, preferably within 1 μm.

[0034] In the fluid dynamic bearing device 1 having the above configuration, when the shaft member 2 rotates, a radial bearing gap is formed between the outer peripheral surface 2a of the shaft member 2 and the inner peripheral surface 8a of the bearing sleeve 8. Then, the hydrodynamic grooves G1 formed in the inner peripheral surface 8a of the bearing sleeve 8 increase the pressure of the oil film generated in the radial bearing gap, and this pressure (hydrodynamic pressure action) forms a radial bearing portion R that supports the shaft member 2 in the radial direction. Furthermore, the convex spherical surface 2b at the lower end of the shaft member 2 comes into sliding contact with the upper end surface of a thrust plate 10 placed on the bottom portion 7b of the housing 7, thereby forming a thrust bearing portion T that supports (contact support) the shaft member 2 in the thrust direction.

[0035] A method for manufacturing the bearing sleeve 8 will now be described.

[0036] The bearing sleeve 8 is manufactured by undergoing a compression molding step, a sintering step, and a sizing step in that order.

[0037] In the compression molding process, raw material powder, primarily made of metal powder, is compression molded to form a cylindrical green compact having approximately the same shape as the bearing sleeve 8 in FIG. 3. The inner peripheral surface of the green compact is made smooth and cylindrical without any irregularities. Axial grooves 8d1, radial grooves 8b1, annular grooves 8b2, and radial grooves 8c1 are molded on the outer peripheral surface, upper end surface, and lower end surface of the green compact, respectively. The raw material powder is a mixed powder made primarily of metal powder (for example, a mixed powder of copper powder and iron powder, or a copper-iron alloy powder) to which various fillers such as molding aids and solid lubricants are added and mixed.

[0038] In the sintering step, the green compact is heated at a predetermined sintering temperature to obtain a sintered body (not shown) in which adjacent metal powder particles are bonded together by solid phase sintering, liquid phase sintering, or both.

[0039] In the sizing process, dynamic pressure grooves are molded into the inner circumferential surface 28a of the sintered compact 28 using a sizing die 30 shown in FIG. 4. Specifically, as shown in FIG. 4(A), a sizing core 31 is inserted into the inner periphery of the sintered compact 28 via a very small gap, and the axial width of the sintered compact 28 is restricted by upper and lower punches 32 and 33. While maintaining this state, as shown in FIG. 4(B), the sintered compact 28 is pressed into the inner periphery of a die 34, thereby compressing the sintered compact 28 from both axial sides and from the outer periphery. As a result, the inner periphery 28a of the sintered compact 28 is pressed against a molding die 31a formed on the outer periphery of the sizing core 31, and the shape of the molding die 31a is transferred to the inner periphery 28a of the sintered compact 28, thereby forming dynamic pressure grooves G1 and lands G2 and G3 (see FIG. 3).

[0040] Thereafter, the sintered body 28, sizing core 31, and upper and lower punches 32, 33 are raised, and the sintered body 28 and sizing core 31 are removed from the inner periphery of the die 34. At this time, the inner periphery 28a of the sintered body 28 expands in diameter due to spring back, and the sintered body 28 is peeled off from the molding die 31a for the outer periphery of the sintered body 28 (i.e., the bearing sleeve 8) on whose inner periphery the dynamic pressure grooves G1, the annular hill portion G2, and the inclined hill portion G3 have been formed.

[0041] The sintered body 28 of this embodiment has a thin wall thickness in the radial direction. Specifically, the ratio D2 / D1 of the inner diameter D1 to the outer diameter D2 of the sintered body 28 is 2.5 or less. In this case, the compressive force toward the inner diameter by the die 34 is easily transmitted to the inner peripheral surface of the sintered body 28, making it possible to form deep dynamic pressure grooves G1 formed in the inner peripheral surface of the sintered body 28.

[0042] Furthermore, in this embodiment, the axial dimension L of the sintered body 28 is kept to 4 mm or less, so the axial compression force from the upper and lower punches 32, 33 is easily transmitted not only to both axial ends of the sintered body 28 but also to the axial center. This allows the sintered body 28 to be compressed uniformly in the axial direction, making the density of the sintered body 28 uniform in the axial direction. As a result, the depth of the dynamic pressure grooves G1 formed in the inner peripheral surface of the sintered body 28 can be made uniform in the axial direction, and deformation of the generatrix shape of the inner peripheral surface of the sintered body 28 can be suppressed.

[0043] The internal pores of the bearing sleeve 8 manufactured using the procedure described above are impregnated with lubricating oil using a method such as vacuum impregnation. After the bearing sleeve 8 and seal member 9 are fixed to the inner periphery of the housing 7, a predetermined amount of lubricating oil is poured in, and then the shaft member 2 is inserted into the inner periphery of the bearing sleeve 8, completing the fluid dynamic bearing device 1.

[0044] The present invention is not limited to the above-described embodiment. Other embodiments of the present invention will be described below, but explanations of the same points as those in the above-described embodiment will be omitted.

[0045] The bearing sleeve 8 shown in FIG. 5 differs from the above-described embodiment in that it omits the dynamic pressure grooves G1 and the inclined hill portion G3 between the pair of annular hill portions G2 in the axial direction. In this bearing sleeve 8, a cylindrical surface 8a1 is formed on the inner circumferential surface 8a over the entire area between the pair of annular hill portions G2 in the axial direction. The inner diameter of the cylindrical surface 8a1 is larger than the inner diameter of the annular hill portion G2 and, for example, is contiguous with the bottom surface of the dynamic pressure groove G1 on the same cylindrical surface. During the sizing process (see FIG. 4), the axial compression force from the upper and lower punches 32 and 33 is not easily transmitted to the axial center of the sintered compact 28. Therefore, even if the axial dimension of the sintered compact 28 is reduced, the density of the axial center is slightly lower than the density of both axial ends. Therefore, by omitting the dynamic pressure grooves G1 and the inclined hill portion G3 in the axial center, where the density is relatively low, the dynamic pressure effect is reduced, but the variation in the dynamic pressure groove depth, i.e., the variation in bearing rigidity, between products is reduced, thereby improving product reliability.

[0046] The fluid dynamic bearing according to the present invention is not limited to the sintered oil-impregnated bearings with lubricating oil impregnated inside as described above, but can also be used in a dry state without lubricating oil impregnation. Furthermore, the fluid dynamic bearing device 1 described above is not limited to fan motors, but can also be used in spindle motors for HDD disk drives and polygon scanner motors for laser beam printers. [Explanation of symbols]

[0047] 1. Fluid dynamic bearing device 2 Shaft member 7. Housing 8 Bearing sleeve (hydrodynamic bearing) 8A, 8B, 8C division body (3 parts) 8a1 Cylindrical surface 28 Sintered body 30 Sizing mold 31 Sizing Core 32,33 Up and down punch 34 Die G1 hydrodynamic groove G2 Circular Hill G3 Slope section R Radial bearing section T Thrust bearing

Claims

1. A hydrodynamic bearing having a cylindrical sintered body with hydrodynamic grooves formed on its inner circumferential surface, The inner diameter D1 of the sintered body is 1.5 mm or less, the ratio D2 / D1 of the inner diameter D1 to the outer diameter D2 of the sintered body is 2.5 or less, the sintered body is divided into three equal parts in the axial direction, and the density of the central part in the axial direction is lower than the density of the parts at both ends in the axial direction; A hydrodynamic bearing in which the difference in relative density between the three portions is within 3%.

2. 2. A hydrodynamic bearing according to claim 1, wherein the axial length of the sintered body is 4 mm or less.

3. 2. The hydrodynamic bearing according to claim 1, wherein the difference in radius between the minimum diameter portion and the maximum diameter portion of the inner diameter surface of a hill portion that rises radially inward from the hydrodynamic grooves on the inner peripheral surface of the sintered body is 2 μm or less.

4. A hydrodynamic bearing as described in any one of claims 1 to 3, wherein the inner surface of the sintered body is formed with a pair of annular hill portions provided at two locations axially spaced apart, a plurality of inclined hill portions extending axially outward from each annular hill portion, hydrodynamic grooves provided circumferentially between the plurality of inclined hill portions, and a cylindrical surface having a diameter larger than the inner diameter of the annular hill portion and provided over the entire axial area between the pair of annular hill portions.

5. 5. A fluid dynamic bearing device comprising: a hydrodynamic bearing according to any one of claims 1 to 4; a shaft member inserted into the inner periphery of the hydrodynamic bearing; and a radial bearing portion that supports the shaft member in a non-contact manner so as to rotate freely relative to the shaft member by the hydrodynamic action of a lubricating film generated in a radial bearing gap between the inner periphery of the hydrodynamic bearing and the shaft member.

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