JP2024034792A - Sintered oil-impregnated bearing - Google Patents

Abstract

[Problem] To suppress the decline in oil film strength due to dynamic pressure release over a long period of time. SOLUTION: A sintered oil-impregnated bearing 8 is formed with a bearing surface 8a1 that forms a radial bearing gap with the shaft member 2. A large number of pores are opened in the bearing surface 8a1. Among the pores, those having a pore volume exceeding 0.0005 mm 3 are defined as coarse pores 40, and no coarse pores 40 are present in a region from the bearing surface 8a1 to a depth of 50 μm. [Selection diagram] Figure 6

Description

The present invention relates to a sintered oil-impregnated bearing.

A sintered oil-impregnated bearing is a bearing formed of porous sintered metal, and is used with the internal pores of the sintered body impregnated with lubricating oil. As the shaft inserted into the inner circumference of the sintered oil-impregnated bearing rotates relative to each other, the lubricating oil impregnated into the internal pores from the inner circumferential surface (bearing surface) of the sintered bearing seeps into the bearing gap, forming an oil film in the bearing gap. is formed, and the shaft is supported by this oil film.

Due to its excellent rotational accuracy and quietness, sintered oil-impregnated bearings are used as bearing devices for motors installed in various electrical devices including information devices.More specifically, they are used in HDDs, CDs, DVDs, and Blu-ray discs It is used as a bearing device for a spindle motor in a disk drive device, a fan motor built into these disk drive devices, a PC, etc., or a polygon scanner motor built into a laser beam printer (LBP).

As an example of a sintered oil-impregnated bearing, a dynamic pressure generating part such as a dynamic pressure generating groove is formed on the bearing surface or the outer peripheral surface of the shaft part, and when the shaft part rotates relative to each other, the dynamic pressure generated by the dynamic pressure generating part causes the bearing gap to Fluid dynamic pressure bearings that increase the pressure (oil film strength) of filled lubricating oil are known.

In this fluid dynamic pressure bearing, when the pressure of the lubricating oil filled in the bearing gap increases, the lubricating oil infiltrates into the internal pores through the countless pores opened on the bearing surface, and the lubricating oil pressure (oil film) in the bearing gap increases. This causes the problem of so-called "dynamic pressure release," which causes a decrease in strength. A similar problem occurs in the form of a drop in the pressure of lubricating oil in the bearing gap even in sintered oil-impregnated bearings (perfect circular bearings) that do not have a dynamic pressure generating portion formed on the bearing surface.

As a countermeasure against dynamic pressure release, in Patent Document 1 listed below, in view of the fact that coarse pores on the bearing surface mainly occur around iron powder having a distorted shape, fine copper powder is diffused and bonded to the surface of iron powder. It is disclosed that partially diffused alloy powder is used as raw material powder. It is stated that because the fine copper powder enters the recesses of the distorted iron powder, the distorted shape of the partially diffused alloy powder as a whole is relaxed.

Further, in Patent Document 2 listed below, before the step of forming dynamic pressure generating grooves on the inner peripheral surface of the sintered body, the outer peripheral surface of the sintered body is compressed with a minute compression amount of 50 μm or less (light sizing). It is disclosed that the following is provided. It is stated that by this light sizing, the copper exposed on the outer peripheral surface of the sintered body is plastically deformed and enters the coarse pores on the outer peripheral surface of the sintered body, so that the rough pores on the outer peripheral surface can be reduced.

Japanese Patent Application Publication No. 2017-150596 JP2019-183868A

In this way, Patent Document 1 reduces the rough pores on the inner circumferential surface (bearing surface) of the sintered body, and Patent Document 2 reduces the rough pores on the outer circumferential surface of the sintered body, thereby improving oil film strength due to dynamic pressure release. is trying to suppress the decline in In both cases, attention is paid to the fact that coarse pores opened on the surface of the sintered body become a cause of dynamic pressure release.

However, even if the rough pores on the surface are reduced, if the bearing surface wears out due to sliding with the shaft, the rough pores that existed inside the bearing will appear on the surface, causing the problem of dynamic pressure release again. It becomes like this. Thus, when long-term use is considered, simply reducing the number of coarse pores opened on the surface is insufficient as a measure against dynamic pressure release.

Therefore, an object of the present invention is to provide a sintered oil-impregnated bearing that can suppress a decrease in oil film strength due to dynamic pressure release over a long period of time.

In order to achieve the above objects, the present invention comprises a cylindrical sintered body impregnated with lubricating oil, and has a bearing surface that forms a radial bearing gap with a shaft member to be supported. In the bearing, a large number of pores are opened on the bearing surface, and among the pores, those with a pore volume exceeding 0.0005 ^mm are defined as coarse pores, and the coarse pores are present in an area up to a depth of 50 μm from the bearing surface. It is characterized by not

With such a sintered oil-impregnated bearing, even if the bearing surface is worn out due to long-term use, it is possible to prevent the formation of rough pores on the bearing surface. Therefore, dynamic pressure loss can be suppressed and stable oil film strength can be ensured for a long period of time.

In addition, since there are many pores of 0.0005 mm ³ or less in the region up to a depth of 50 μm from the bearing surface, including the bearing surface, lubricating oil can actively seep out from the bearing surface. This allows ample lubricating oil to be supplied to the bearing gap and prevents air from entering. In addition, the lubricating oil in the radial bearing gap returns to the inside of the sintered oil-impregnated bearing, and as the lubricating oil passes through the narrow pores, foreign matter is sufficiently removed (filtered), improving the durability of the lubricating oil. . Therefore, it is possible to improve the durability life of the sintered oil-impregnated bearing.

This sintered oil-impregnated bearing preferably has a large number of pores on the surface located on the opposite side in the radial direction from the bearing surface, and has no coarse pores in a region up to a depth of 100 μm from the surface. This makes it difficult for the lubricating oil inside the sintered oil-impregnated bearing to flow out from the surface located on the opposite side in the radial direction from the bearing surface, so that dynamic pressure loss can be suppressed even more effectively.

In addition, in this sintered oil-impregnated bearing, in a radial cross section passing through the bearing surface, the number of rough holes is on the bearing surface side from a position 50% away from the bearing surface with respect to the wall thickness in the radial direction. It is preferable to provide an annular region where the maximum value is obtained.

With such a configuration, a large amount of lubricating oil is retained in a region inside the sintered oil-impregnated bearing close to the bearing surface, so that the amount of lubricating oil that flows back into the radial bearing gap via the bearing surface increases. Furthermore, the flow of lubricating oil in this area becomes more active. Therefore, the circulation of lubricating oil between the radial bearing gap and the sintered oil-impregnated bearing can be activated. This makes it possible to increase the strength of the oil film and to enhance the filtering effect.

This sintered oil-impregnated bearing preferably has an oil permeability of 0.004 g/20 min or less. If the oil permeability is 0.004 g/20 min or less, a high dynamic pressure release suppressing effect can be obtained.

The sintered oil-impregnated bearing preferably contains 0.8 wt% or more of graphite structure. By increasing the amount of graphite structure contained in the sintered oil-impregnated bearing in this way, the size of the coarse pores can be reduced, making it possible to lower the oil permeability, which is less than 0.004 g/20 min. It is also easy to achieve the oil level.

In the sintered oil-impregnated bearing described above, a dynamic pressure generating section can be provided on the bearing surface. When the dynamic pressure generating portion is provided on the bearing surface in this way, the strength of the oil film in the radial bearing gap can be increased by the dynamic pressure action generated during relative rotation between the shaft member and the sintered oil-impregnated bearing.

a sintered oil-impregnated bearing having the bearing surface formed on its inner peripheral surface; a housing having an open end at one axial end and a closed end at the other end to which the sintered oil-impregnated bearing is fixed; and the shaft member inserted into the inner periphery of the oil-impregnated bearing, and the dynamic pressure generating section forms an oil film in the radial bearing gap between the bearing surface of the sintered oil-impregnated bearing and the outer peripheral surface of the shaft. A fluid dynamic bearing device that supports a shaft member in a radial direction in a non-contact manner has excellent bearing rigidity and rotation accuracy because dynamic pressure loss is suppressed.

In this fluid dynamic bearing device, a region with many coarse pores is provided on one axial side of the sintered oil-impregnated bearing, and a region with few coarse pores is provided on the other axial side, and the region with many coarse pores is provided on the other axial side. By arranging it on the high load side of the fluid dynamic bearing device, it is possible to suppress oil film shortage or deterioration of the lubricating oil on the high load side of the radial bearing gap.

This fluid dynamic bearing device can be used in a motor.

As described above, according to the present invention, it is possible to ensure stable oil film strength even when a sintered oil-impregnated bearing is used for a long period of time.

FIG. 3 is a cross-sectional view of a spindle motor. FIG. 2 is a cross-sectional view of a fluid dynamic bearing device. 1 is a cross-sectional view of a sintered oil-impregnated bearing according to an embodiment of the present invention. FIG. 3 is a bottom view of the sintered oil-impregnated bearing. (A) and (B) are cross-sectional views showing a dynamic pressure groove sizing process. 1 is a cross-sectional view schematically showing the distribution of pores in a sintered oil-impregnated bearing according to an embodiment of the present invention. It is a graph showing the frequency distribution of coarse pores actually measured in the radial direction passing through the bearing surface of a sintered oil-impregnated bearing. It is a table showing the measured values of pore volume and oil permeability when the blending amount of graphite powder contained in the raw material powder is changed. It is a graph showing the frequency distribution of coarse pores actually measured in the axial direction of a sintered oil-impregnated bearing.

Embodiments of the present invention will be described below based on the drawings.

The spindle motor shown in FIG. 1 is used in a disk drive device such as an HDD, and includes a fluid dynamic pressure bearing device 1 that rotatably supports a shaft member 2 in a non-contact manner, and a disk hub 3 attached to the shaft member 2. , a stator coil 4 and a rotor magnet 5 which are opposed to each other with a radial gap therebetween. The stator coil 4 is attached to the casing 6 and the rotor magnet 5 is attached to the disk hub 3. The housing 7 of the fluid dynamic bearing device 1 is attached to the inner periphery of the casing 6. The disk hub 3 holds a predetermined number (two in the illustrated example) of disks D such as magnetic disks. When the stator coil 4 is energized, an electromagnetic force is generated between the stator coil 4 and the rotor magnet 5, and this electromagnetic force causes the disk hub 3 and the shaft member 2 to rotate together.

As shown in FIG. 2, the fluid dynamic bearing device 1 includes a shaft member 2, a bearing member 8 as a sintered oil-impregnated bearing according to the present embodiment, a housing 7 that holds the bearing member 8 on its inner periphery, and a housing. The housing 7 has a seal portion 9 provided at an opening at one axial end of the housing 7, and a lid portion 10 that closes the other axial end of the housing 7. In the following description, for convenience, the closed side of the housing 7 in the axial direction will be referred to as the lower side, and the open side of the housing 7 will be referred to as the upper side, but this is not intended to limit the manner in which the fluid dynamic bearing device 1 is used.

The shaft member 2 includes a shaft portion 2a and a flange portion 2b provided at the lower end of the shaft portion 2a. The shaft member 2 is made of a metal material such as stainless steel, and in this embodiment, the entire shaft member 2 including the shaft portion 2a and the flange portion 2b is integrally formed. Note that the shaft portion 2a and the flange portion 2b may be formed separately.

The outer peripheral surface of the shaft portion 2a is provided with cylindrical surfaces 2a1 formed at two locations separated in the axial direction, and an annular recess 2a2 provided between the two cylindrical surfaces 2a1 and having a smaller diameter than the cylindrical surface 2a1. It will be done. The cylindrical surface 2a1 functions as a bearing facing surface that faces the bearing surface 8a1 of the inner circumferential surface 8a of the bearing member 8 in the radial direction.

The housing 7 is made of resin or metal and has a cylindrical shape. The outer circumferential surface 8d of the bearing member 8 is fixed to the inner circumferential surface 7a of the housing 7 by appropriate means such as adhesion or press fitting.

The bearing member 8 has a cylindrical shape, and a radial bearing surface is provided on the inner peripheral surface 8a. In the illustrated example, radial bearing surfaces 8a1 are formed on the inner peripheral surface 8a of the bearing member 8 at two locations spaced apart in the axial direction. A dynamic pressure generating portion is formed in each radial bearing surface 8a1, and in this embodiment, as shown in FIG. Pressure grooves G1 and G2 are provided. The cross-hatched area in the figure indicates a hill that is higher than the surrounding area (the same applies to FIG. 4). The upper dynamic pressure groove G1 has an axially asymmetrical shape, and the lower dynamic pressure groove G2 has an axially symmetrical shape. A cylindrical surface 8a2 that is continuous with the groove bottom surfaces of the dynamic pressure grooves G1 and G2 is provided in the axially interspaced region of the radial bearing surface 8a1. The depth of the dynamic pressure grooves G1 and G2 is several μm to several tens of μm.

Note that both the upper and lower dynamic pressure grooves G1 and G2 may have an axially symmetrical shape. Furthermore, the upper and lower dynamic pressure grooves G1 and G2 may be continuous in the axial direction, or one of the upper and lower dynamic pressure grooves G1 and G2 may be omitted. In addition, as a dynamic pressure generating part, dynamic pressure grooves of other shapes such as a spiral shape, multi-arc bearings that combine multiple cylindrical surfaces, step bearings that have multiple axial grooves arranged at equal intervals in the circumferential direction, etc. may be formed.

A thrust bearing surface is provided on the lower end surface 8b of the bearing member 8. A pump-in type spiral dynamic pressure groove G3 as shown in FIG. 4 is formed on the thrust bearing surface. Note that a herringbone shape, a radial groove shape, or the like may be adopted as the shape of the dynamic pressure groove G3. Alternatively, the lower end surface 8b of the bearing member 8 may be a flat surface, and a dynamic pressure groove may be formed on the upper end surface 2b1 of the flange portion 2b of the shaft member 2.

As shown in FIG. 3, the upper end surface 8c of the bearing member 8 is formed with an annular groove 8c1 and a plurality of radial grooves 8c2 provided on the inner diameter side of the annular groove 8c1. A plurality of axial grooves 8d1 are provided on the outer peripheral surface 8d of the bearing member 8 at equal intervals in the circumferential direction. The space on the outer diameter side of the flange portion 2b of the shaft member 2 communicates with the seal space S through these axial grooves 8d1, annular grooves 8c1, radial grooves 8c2, etc., thereby reducing negative pressure in this space. Occurrence is prevented. Incidentally, if there is no particular need, the annular groove 8c1 and the radial groove 8c2 may be omitted, and the upper end surface 8c of the bearing member 8 may be made a flat surface.

The bearing member 8 is formed of a sintered body containing 25% by mass or more of copper, and in this embodiment, is formed of a sintered body containing 25% by mass or more of copper and iron, respectively. The true density ratio of the bearing member 8 is 85 to 95%. Note that the true density ratio is defined by the following formula. ρ1 is the density of the bearing member, and ρ0 is the density (true density) assuming that the bearing member has no pores.
True density ratio [%] = (ρ1/ρ0) x 100

As shown in FIG. 2, the seal portion 9 protrudes inward from the upper end of the housing 7. In this embodiment, the seal portion 9 is formed integrally with the housing 7, but the seal portion 9 may be formed separately from the housing 7. The inner circumferential surface 9a of the seal portion 9 has a tapered shape whose diameter gradually decreases downward. A seal space S is formed between the inner peripheral surface 9a of the seal portion 9 and the outer peripheral surface of the shaft portion 2a, and the seal space S has a wedge-shaped cross section and whose radial width gradually decreases downward. In addition, while the inner circumferential surface of the seal portion 9 is a cylindrical surface, a tapered surface whose diameter gradually decreases upward is provided on the outer circumferential surface of the shaft portion 2a, and a seal space S having a wedge-shaped cross section is formed between them. You may. The lower end surface 9b of the seal portion 9 is in contact with the upper end surface 8c of the bearing member 8.

The lid portion 10 is made of metal such as brass or resin, and is fixed to the lower end of the inner circumferential surface 7a of the housing 7 by press fitting, adhesion, or other suitable means. As a result, the space inside the housing 7 becomes a sealed space where only the sealed space S is open to the atmosphere. The lid part 10 can also be formed integrally with the housing 7.

A thrust bearing surface is formed on the end surface 10a of the lid portion 10. For example, a pump-in type spiral dynamic pressure groove is formed on this thrust bearing surface (not shown). Note that the shape of the dynamic pressure groove may be a herringbone shape, a radial groove shape, or the like. Alternatively, the end surface 10a of the lid portion 10 may be a flat surface, and a dynamic pressure groove may be formed on the lower end surface 2b2 of the flange portion 2b of the shaft member 2.

Oil is injected into the fluid dynamic bearing device 1 configured as described above, and an oil level is formed in the seal space S (see FIG. 2). The fluid dynamic pressure bearing device 1 of this embodiment is of a so-called full-fill type in which the space around the inner circumference of the housing 7 (the space inside the seal space S) is filled with oil, including the internal pores of the bearing member 8. It is.

When the shaft member 2 rotates, a radial bearing gap is formed between the radial bearing surface 8a1 of the inner peripheral surface 8a of the bearing member 8 and the outer peripheral surface (cylindrical surface 2a1) of the shaft portion 2a, and the dynamic pressure grooves G1 and G2 By increasing the pressure of the oil film in the radial bearing gap, the first radial bearing part R1 and the second radial bearing part R2 are formed which are supported by the shaft member 2 in a non-contact manner in the radial direction. At the same time, thrust bearing gaps are created between the lower end surface 8b of the bearing member 8 and the upper end surface 2b1 of the flange portion 2b, and between the end surface 10a of the lid portion 10 and the lower end surface 2b2 of the flange portion 2b. is formed. Then, the pressure of the oil film formed in each thrust bearing gap is increased by the dynamic pressure groove G3 on the lower end surface 8b of the bearing member 8 and the dynamic pressure groove on the end surface 10a of the lid part 10. A first thrust bearing part T1 and a second thrust bearing part T2 are configured for non-contact support in the direction.

The bearing member 8 described above is manufactured mainly through a raw material powder mixing process, a forming process, a sintering process, a rotational sizing process, and a dynamic pressure groove sizing process in this order.

In the raw material powder mixing step, raw material powder for the bearing member 8 is produced by mixing a plurality of types of powder. The raw material powder includes metal powders such as iron-based powder, copper-based powder, and powder of a low melting point element. If necessary, various molding lubricants (for example, a lubricant for improving mold releasability), solid lubricants (for example, graphite powder), etc. may be added to this raw material powder.

As the iron-based powder, in addition to iron powder (pure iron powder), iron alloy powder (for example, stainless steel powder) can be used. As the iron-based powder, reduced powder or atomized powder can be used. As the copper-based powder, copper alloy powder can be used in addition to copper powder (pure copper powder). As the copper-based powder, electrolytic powder or atomized powder can be used. As the low melting point element powder, a powder containing an element having a lower melting point than copper, such as tin, zinc, or phosphorus, can be used. In this embodiment, tin powder is used.

The raw material powder contains 25% by mass or more of copper as a metal powder, for example, contains 25% by mass or more of iron and copper each. The metal powder in the raw material powder of this embodiment contains 25 to 70% by mass of copper powder, 1 to 3% by mass of tin powder, and the remainder is iron powder (or iron alloy powder) and inevitable impurities.

In the forming step, the raw material powder is introduced into a cavity of a forming mold (not shown) and compressed to obtain a cylindrical green compact resembling the bearing member 8 shown in FIG. 3. In the forming process, an axial groove 8d1 (see FIG. 3) is formed on the outer peripheral surface of the green compact.

In the sintering step, the green compact is sintered at a sintering temperature (for example, 700° C. to 900° C.) that does not exceed the melting point of copper (1086° C.) to obtain a sintered body. When various molding lubricants such as fluid lubricants are added to the raw material powder, the molding lubricants volatilize during sintering.

In the rotational sizing process, a jig (sizing pin) is pressed against the inner peripheral surface of the sintered body with a tightening margin, and in this state, the jig is rotated along the circumferential direction of the inner peripheral surface of the sintered body (not shown). ). As a result, the material on the surface layer of the inner circumferential surface of the sintered body is rolled with a jig, the openings on the inner circumferential surface are crushed, and the surface porosity of the inner circumferential surface (the area of each pore opened on the inner circumferential surface) is ratio) is reduced.

In the dynamic pressure groove sizing process, dynamic pressure grooves are formed on the inner peripheral surface 28a of the sintered body 28 using a dynamic pressure groove sizing mold 30 shown in FIGS. 5(A) and 5(B). Specifically, as shown in FIG. 5(A), the core rod 31 is inserted into the inner circumference of the sintered body 28 through a very small gap, and the axial width of the sintered body 28 is adjusted by the upper and lower punches 32, Restraint at 33. At this time, the inner diameter of the die 34 is determined so that an interference margin is generated between the die 34 and the outer circumferential surface 28d of the sintered body 28. While maintaining this state, the sintered body 28 is press-fitted into the inner periphery of the die 34, as shown in FIG. 5(B). As a result, the sintered body 28 is pressed from the outer periphery while being restrained on both sides in the axial direction, and the inner circumferential surface 28a of the sintered body 28 is pressed against the mold 31a formed on the outer circumferential surface of the core rod 31. As a result, the shape of the mold 31a is transferred to the inner circumferential surface 28a of the sintered body 28, and the dynamic pressure grooves G1 and G2 (see FIG. 3) are formed.

After that, the sintered body 28, the core rod 31, and the upper and lower punches 32, 33 are raised, and the sintered body 28 and the core rod 31 are taken out from the inner periphery of the die 34. At this time, the inner circumferential surface 28a of the sintered body 28 expands in diameter due to springback, and is separated from the mold 31a of the outer circumferential surface of the core rod 31. Then, the core rod 31 is pulled out from the inner periphery of the sintered body 28.

When the internal pores of the sintered body 28 thus formed are impregnated with lubricating oil by a method such as vacuum impregnation, the bearing member 8 shown in FIG. 1 is completed.

The inventors of the present application proceeded to verify the wear of the bearing surface 8a1 of the bearing member 8 described above, and found that if coarse pores can be reduced in a region up to a certain depth from the bearing surface 8a1, the wear of the bearing surface 8a1 due to long-term use It has been found that the strength of the oil film can be maintained by suppressing the generation of coarse pores on the bearing surface 8a1.

In this way, it is sufficient to suppress the generation of coarse pores in an area at a certain depth from the surface, because in a sintered oil-impregnated bearing immediately after manufacture, the wear progresses quickly on the bearing surface 8a1 as the shaft member 2 rotates. However, as the bearing surface 8a1 gradually adapts (smoothes) as the bearing operating time elapses, the wear progresses more slowly as the bearing operating time elapses. As a result of experiments, it has become clear that even during long-term use, it is rare for the bearing surface 8a1 to wear to a depth of more than 50 μm immediately after manufacture.

Based on the above findings, in the bearing ^member 8 according to the present embodiment, as schematically illustrated in FIG. No coarse pores 40 exist in the region M up to a depth of 50 μm. The rough holes 40 are not opened in the bearing surface 8a1 either. In addition, in FIG. 6, only the coarse pores 40 are illustrated as the pores included in the bearing member 8, and the illustration of fine pores having a volume of 0.0005 mm ³ or less is omitted.

The volume of each pore in the region can be measured by CT scanning using X-rays. Measurement using the CT scan method is performed by taking, for example, 4,500 images of the object to be measured, constructing 3D data from the image data, and calculating the volume of internal pores. As a measuring device, for example, GE phoenix v|tome|x m300 manufactured by Waygate Technologies can be used. The measurement can be performed, for example, under conditions of a voltage of 250 kV and a current of 300 mA.

On the other hand, the inner circumferential surface 8a of the bearing member 8 including the bearing surface 8a1 is not completely sealed, and the region M from the bearing surface 8a1 to a depth of 50 μm has many holes with a volume of 0.0005 mm ³ or less. Pores are present. As a result, many pores with a volume of 0.0005 mm ³ or less are opened also in the bearing surface 8a1. The surface porosity ratio of the bearing surface 8a1 is preferably in the range of 2% or more and 15% or less in order to ensure sufficient seepage of lubricating oil from the bearing surface 8a1 while suppressing dynamic pressure release.

In this way, by opening a large number of pores in the bearing surface 8a1 and setting the pore volume in the region from the bearing surface 8a1 to a depth of 50 μm to 0.0005 mm ³ or less, when the bearing surface 8a1 is worn out due to long-term use, Also, generation of coarse pores on the bearing surface 8a1 can be prevented. Therefore, dynamic pressure loss can be suppressed and stable oil film strength can be ensured for a long period of time. In addition, since the bearing surface 8a1 has many pores with a volume of 0.0005 mm ³ or less, including after wear, the lubricating oil oozes out from the bearing surface 8a1, and the bearing gap is filled with water. It is possible to prevent air from entering by supplying lubricating oil. In addition, when the lubricating oil passes through the narrow pores, foreign matter is removed (filtered) sufficiently, and the durability of the lubricating oil is improved. Therefore, it is possible to improve the durability life of the fluid dynamic bearing device 1.

In addition, in the bearing member 8 according to the present embodiment, as shown in FIG. In region N, the coarse pores 40 described above do not exist. Note that there are many fine pores with a volume of 0.0005 mm ³ or less in the region N from the outer circumferential surface 8d to 100 μm, and many fine pores are open in the outer circumferential surface 8d, so the lubricating oil is absorbed into the bearing member 8. It also oozes out from the outer peripheral surface 8d.

In this way, by preventing the coarse pores 40 from existing in the region N from the outer circumferential surface 8d of the bearing member 8 to a depth of 100 μm, the amount of lubricating oil seeping out from the outer circumferential surface 8d of the bearing member 8 is reduced. Therefore, the effect of suppressing dynamic pressure release is further enhanced. Even if the bearing surface 8a1 were to wear excessively and coarse pores 40 were opened in the bearing surface 8a1, the effect of suppressing dynamic pressure release could be maintained because the surface porosity of the outer circumferential surface 8d is low.

Note that, as shown in FIG. 6, a large number of coarse pores 40 having a volume exceeding 0.0005 mm ³ are dispersed in a portion of the bearing member 8 excluding areas M and N.

FIG. 7 is a graph showing the frequency distribution of the coarse pores 40 actually measured in the radial direction passing through the bearing surface 8a1 of the bearing member 8 manufactured by the above procedure. This frequency distribution is determined by equally dividing the radial cross section passing through the bearing surface 8a1 of the bearing member 8 at multiple points in the radial direction to set a plurality of annular regions (for example, an annular region with a radial width of 0.1 mm). It is obtained by counting the number of coarse pores 40 existing in the area using a CT scan method. The radial position represented on the horizontal axis in FIG. 7 is based on the axis of the bearing member 8 being zero. Moreover, the curve in FIG. 7 is a curve approximated to a frequency distribution. The sample used for this measurement had a cylindrical shape with an inner diameter of 4.0 mm, an outer diameter of 7.5 mm, and an axial length of 12.47 mm.

As is clear from FIG. 7, this bearing member 8 has a region from the inner peripheral surface 8a (bearing surface 8a1) to a depth of 50 μm (radius 2.05 mm position) and from the outer peripheral surface 8d to a depth of 100 μm (radius 3 It can be seen that there are no coarse pores 40 with a volume exceeding 0.0005 mm ³ in the region up to 0.65 mm. Therefore, a high dynamic pressure drop suppression effect can be obtained.

On the other hand, the frequency distribution of the coarse air holes 40 is larger than that at a position (near the radial position 2.9 mm) that is 50% away from the bearing surface 8a1 (radial position 2.0 mm) with respect to the radial wall thickness of the bearing member 8. An annular region on the bearing surface 8a1 side, more specifically, an annular region on the bearing surface 8a1 side, more specifically, 35% away from the bearing surface 8a1 than a position 40% away from the bearing surface 8a1 (near a radius position of 2.7 mm). It is maximum in the annular region closer to the bearing surface 8a1 than the position (near the radius position 2.6 mm). Specifically, it is maximum in the annular region at a position 30% away from the bearing surface (around a radius of 2.5 mm).

As a result, a large amount of lubricating oil is retained in a region inside the bearing member 8 close to the bearing surface 8a1, so that the amount of lubricating oil flowing back into the radial bearing gap via the bearing surface 8a1 increases. Additionally, the flow of lubricating oil becomes active in this area. Therefore, the circulation of lubricating oil between the radial bearing gap and the bearing member 8 can be activated. This makes it possible to increase the strength of the oil film and to enhance the filtering effect.

From this point of view, in the bearing member 8, an annular region on the bearing surface 8a1 side from a position 50% away from the bearing surface 8a1 (radial position 2.0 mm) with respect to the radial wall thickness of the bearing member 8, preferably The number of coarse pores is maximum in an annular region closer to the bearing surface 8a1 than a position 40% away from the bearing surface 8a1, more preferably in an annular region closer to the bearing surface 8a1 than a position 35% away from the bearing surface 8a1. It is hoped that

The above-described bearing member 8 in which the coarse holes 40 are excluded in the regions M and N can be obtained, for example, by adjusting the interference in the rotational sizing process and the dynamic pressure groove sizing process. The interference in the rotational sizing process mainly affects the frequency of occurrence of rough holes 40 in the region M on the inner diameter side, and the interference in the dynamic pressure groove sizing process mainly affects the frequency of coarse holes 40 in the region N on the outer diameter side. affect the frequency of occurrence. For the sample size used in the measurement test shown in Figure 7 (inner diameter φ4.0 mm, outer diameter φ7.5 mm, axial length 12.47 mm), the interference in the rotational sizing process is approximately φ50 μm, and the dynamic pressure groove Appropriate tightening margin in the sizing process is approximately φ200 μm.

The size of the pores in the bearing member 8 can also be controlled by adjusting the amount of graphite powder added to the raw material powder. Generally, the larger the amount of graphite powder blended, the smaller the pore size.

FIG. 8 shows measured values of pore volume and oil permeability when the amount of graphite powder contained in the raw material powder of the bearing member 8 was changed. As shown in Figure 8, even if the ingredients other than graphite powder are the same and the same amount of ingredients, when the blending amount of graphite powder is 0.5 wt%, the maximum pore volume is 0.0335 mm ³ and the average volume is However, when the blending amount of graphite powder is 0.8 wt ^% , the maximum pore volume is 0.0115 mm ³ and the average volume is 0.0017 mm ³ . Therefore, it can be understood that the larger the amount of graphite powder blended, the smaller the pore volume becomes. Correspondingly, the oil permeability becomes smaller as the amount of graphite powder is increased, and by setting the amount of graphite powder to 0.8 wt%, an oil permeability of 0.004 g/20 min can be obtained. I can also understand that. Note that the blending ratio of graphite powder in the raw material powder becomes the content of graphite structure after sintering.

From the above knowledge, it is preferable to set the blending amount of graphite powder to 0.8 wt% or more (the content of graphite structure contained in the bearing member 8 is 0.8 wt% or more), thereby increasing the oil permeability to 0.004 g. /20min or less. Therefore, the effect of suppressing dynamic pressure release is further enhanced. Note that "oil permeability" here refers to the pressure applied to the lubricating oil filled in the inner circumference of the sintered body at a predetermined pressure (here, 0.4 MPa) with the pores opened on both axial end faces of the sintered body sealed. ) is added and this state is maintained for 20 minutes, it means the total weight of lubricating oil that seeped out from the pores opened on the outer peripheral surface of the sintered body.

FIG. 9 is a graph showing the frequency distribution of the coarse pores 40 actually measured in the axial direction of the bearing member 8. As shown in FIG. This frequency distribution is determined by equally dividing the axial cross section of the bearing member 8 at multiple points in the axial direction to set a plurality of band-shaped areas (for example, a band-shaped area with an axial width of 0.1 mm), and This can be obtained by counting the number of pores 40 using a CT scan method. The axial position shown on the horizontal axis in FIG. 9 is zero at one end surface of the bearing member 8. Moreover, the curve in FIG. 9 is a curve approximated to a frequency distribution. The sample used for this measurement had a cylindrical shape with an inner diameter of 4.0 mm, an outer diameter of 7.5 mm, and an axial length of 12.4 mm.

As shown in FIG. 9, a region A with many coarse pores 40 is provided on one axial side of the bearing member 8, and a region B with few coarse pores 40 is provided on the other axial side. In the case of an HDD or a fan motor, the load applied to the rotor side (upper side in Figure 1) is generally large, so it is recommended that the bearing member 8 in the housing 7 be arranged with the area A where there are many coarse air holes 40 facing the rotor side. is preferred. As a result, in the region A where there are many coarse pores 40, lubricating oil actively circulates between the radial bearing gap and the inside of the bearing member 8, resulting in lack of oil film and deterioration of the lubricating oil in the radial bearing gap on the high load side. This makes it possible to prevent

The present invention is not limited to the above embodiments. For example, in the above embodiment, the fluid dynamic pressure bearing device 1 has a so-called full-fill structure in which the internal space of the housing 7 and the internal pores of the bearing member 8 are filled with lubricating oil, but the present invention is not limited to this. The present invention may be applied to a fluid dynamic pressure bearing device having a partial fill structure in which a cavity not filled with lubricating oil is provided in the internal space of the housing 7 (not shown). Furthermore, the present invention can also be applied to so-called perfect circular bearings that do not have a dynamic pressure generating section.

Further, in the above embodiment, the shaft member 2 is on the rotating side, and the housing 7 and the bearing member 8 are on the stationary side. 8 may be on the rotating side.

Further, in the above embodiment, a case has been shown in which the fluid dynamic pressure bearing device 1 is applied to a spindle motor of a disk drive device such as an HDD. The fluid dynamic pressure bearing device according to the present invention can also be applied to a cooling fan motor or the like.

1 Fluid dynamic bearing device 2 Shaft member 7 Housing 8 Bearing member (sintered oil-impregnated bearing)
8a Inner peripheral surface 8a1 Bearing surface (radial bearing surface)
8d Outer peripheral surface 9 Seal member 40 Coarse hole M Area N from the bearing surface to a depth of 50 μm Area from the surface to a depth of 100 μm

Claims (9)

In a sintered oil-impregnated bearing that is made of a cylindrical sintered body impregnated with lubricating oil and has a bearing surface that forms a radial bearing gap with the shaft member to be supported,
A large number of pores are opened on the bearing surface,
A sintered oil-impregnated bearing characterized in that among the pores, those having a pore volume exceeding 0.0005 mm ³ are defined as coarse pores, and the coarse pores do not exist in a region up to a depth of 50 μm from the bearing surface. A large number of pores are opened on the surface located on the opposite side in the radial direction from the bearing surface,
The sintered oil-impregnated bearing according to claim 1, wherein the coarse pores are not present in a region up to a depth of 100 μm from the surface. In a radial cross section passing through the bearing surface, an annular region in which the number of coarse pores is maximized is provided closer to the bearing surface than a position 50% away from the bearing surface with respect to the wall thickness in the radial direction. The sintered oil-impregnated bearing according to item 2. The sintered oil-impregnated bearing according to claim 1, having an oil permeability of 0.004 g/20 min or less. The sintered oil-impregnated bearing according to claim 4, which contains a graphite structure of 0.8 wt% or more. The sintered oil-impregnated bearing according to claim 1, wherein a dynamic pressure generating section is provided on the bearing surface. The sintered oil-impregnated bearing according to claim 6, wherein the bearing surface is formed on the inner peripheral surface, and the sintered oil-impregnated bearing is fixed to the inner periphery, and the sintered oil-impregnated bearing is opened at one end in the axial direction and closed at the other end. a housing; and the shaft member inserted into the inner periphery of the sintered oil-impregnated bearing; A fluid dynamic bearing device that supports the shaft member in a radial direction in a non-contact manner by forming an oil film. A region with many coarse pores is provided on one axial side of the sintered oil-impregnated bearing, a region with few coarse pores is provided on the other axial side, and the region with many coarse pores is arranged on a high load side. 7. The fluid dynamic bearing device according to 7. A motor comprising the fluid dynamic bearing device according to claim 7 or 8.

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