JP2018096420A - Fluid dynamic pressure bearing device, oil-containing porous bearing used in the same, and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce an oil amount charged inside an oil-containing porous bearing without increasing costs.SOLUTION: A fluid dynamic pressure bearing device 1 includes a shaft member 2, an oil-containing porous bearing (bearing sleeve 8), a bottomed-cylindrical housing 7, and a seal part 9. Between an inner peripheral surface 9a of the seal part 9 and an outer peripheral surface 2a1 of the shaft member 2, a seal space S for holding an oil level is formed. The oil-containing porous bearing comprises a green compact where iron particles 11 are connected to each other through an oxide film 12 formed on the surface of the iron particle 11. The oil content of the oil-containing porous bearing is not more than 4 vol%.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a fluid dynamic bearing device, and particularly to a fluid dynamic bearing device having a porous oil-impregnated bearing.

  The fluid dynamic pressure bearing device supports the shaft in a non-contact manner so as to be relatively rotatable by the pressure of the oil film formed in the bearing gap between the shaft and the bearing member in association with the relative rotation. The fluid dynamic pressure bearing device is widely used as a small, high-speed, high-precision bearing device such as an HDD, a polygon mirror motor, and a small cooling fan motor.

  Fluid dynamic pressure bearing devices are roughly classified into a full-fill structure in which oil (or grease, hereinafter the same) is completely filled, and a partial fill structure in which an air layer exists inside.

  Full-fill type fluid dynamic bearing device is less likely to cause oil leakage because the oil level position is less likely to fluctuate compared to partial fill fluid dynamic bearing device, but seals are provided to prevent oil leakage reliably. It is common to provide a structure. For example, in Patent Document 1, a wedge-shaped seal space (so-called taper seal) is formed between an inner peripheral surface of a seal member provided in an opening of a housing and a tapered surface provided on an outer peripheral surface of a shaft member. doing. In the taper seal, the oil surface is maintained in the seal space by a capillary action, and the oil volume is prevented from flowing out by buffering the increase in the oil volume accompanying the temperature rise in the seal space.

JP 2003-065324 A

  Since the seal space as described above needs a volume capable of absorbing the volume change of the oil accompanying the temperature change, the larger the amount of oil filled in the fluid dynamic bearing device, the smaller the volume of the seal space. There is a need to increase it. In particular, when a porous oil-impregnated bearing made of a porous material such as sintered metal is used as the bearing member, oil is also impregnated in the internal pores of the bearing member. This leads to further expansion of the volume of the space.

  On the other hand, with the downsizing and thinning of devices in which the fluid dynamic bearing device is incorporated, the total length (axial length) of the fluid dynamic bearing device is naturally limited. When the seal space and the radial bearing gap are provided side by side in the axial direction as in the fluid dynamic bearing device disclosed in Patent Document 1, the shaft of the seal space is used to secure the volume of the seal space. If the directional dimension is increased, the axial length (bearing span) of the radial bearing gap cannot be sufficiently secured, and the load capacity and rigidity of the bearing may be reduced.

  For example, if the internal pores of the bearing member made of sintered metal are reduced, the amount of oil impregnated in the bearing member, and hence the total amount of oil filled in the fluid dynamic bearing device, is reduced. The volume of the space can be reduced, and the axial length of the seal space can be shortened. As a method of reducing the internal pores of the bearing member, for example, it is conceivable to increase the density of the green compact. However, in this case, a large molding pressure is required when molding the green compact. Incurs cost increase due to size increase.

  In view of the above circumstances, the present invention reduces the axial dimension of the seal space by reducing the amount of oil filled in the fluid dynamic pressure bearing device having a porous oil-impregnated bearing without causing an increase in cost. Therefore, it is an object to improve the load capacity and the bearing rigidity by reducing the axial dimension of the fluid dynamic pressure bearing device or by expanding the bearing span.

  In order to solve the above-described problems, the present invention provides a shaft member, a porous oil-impregnated bearing having the shaft member inserted in an inner periphery, and a bottomed cylindrical housing that holds the porous oil-impregnated bearing in the inner periphery. The shaft member is relatively moved by the dynamic pressure action of the oil film generated in the seal portion provided in the opening of the housing and the radial bearing gap between the inner peripheral surface of the porous oil-impregnated bearing and the outer peripheral surface of the shaft member. A radial bearing portion that is rotatably supported, and a seal space that is formed between the inner peripheral surface of the seal portion and the outer peripheral surface of the shaft member, and holds an interface between oil and the atmosphere filled in the housing. The porous oil-impregnated bearing includes a green compact in which the particles are bonded to each other through an oxide film formed on the surface of the metal powder particles. The oil content of the oil-impregnated bearing is 4 vol% or less. And features.

  The porous oil-impregnated bearing of the fluid dynamic pressure bearing device according to the present invention does not sinter the green compact at a high temperature (for example, 850 ° C.) unlike a general sintered bearing, but relatively low temperature in the green compact. By performing heat treatment at (for example, 500 ° C.), an oxide film is formed on the surface of the metal powder particles constituting the green compact, and the particles are bonded to each other by the oxide film. In this case, at least a part of the internal pores of the green compact is filled with the oxide film and the internal pores of the green compact are reduced, so that the oil content of the porous oil-impregnated bearing can be 4 vol% or less. As described above, the amount of oil impregnated in the porous oil-impregnated bearing is reduced, so that the total amount of oil filled in the fluid dynamic bearing device is reduced. As a result, the volume of the seal space for buffering the volume change of the oil, particularly the axial dimension of the seal space, can be reduced, so the load due to the reduction of the axial dimension of the fluid dynamic bearing device or the expansion of the bearing span. The capacity and bearing rigidity can be improved.

  For example, when a green compact containing multiple types of metal powders is oxidized, the oxide film formation state varies depending on the type of metal, and the thickness of the oxide film and the adhesion between the oxide film and the particles May be non-uniform. Therefore, it is preferable that the metal powder contained in the porous oil-impregnated bearing is substantially composed of only one kind of metal powder. Specifically, it is preferable that 99 wt% or more of the metal powder contained in the green compact constituting the porous oil-impregnated bearing is composed of a single type of metal powder. In particular, considering the durability (wear resistance) and strength of the porous oil-impregnated bearing, the ease of forming an oxide film, etc., it is preferable to use iron powder as the single type of metal powder.

  A motor including the fluid dynamic pressure bearing device described above, a rotor magnet provided on a rotating side of the housing and the shaft member, and a stator coil provided on a fixed side of the housing and the shaft member. Can reduce the size in the axial direction or improve the rotation accuracy.

  The porous oil-impregnated bearing includes a step of compressing metal powder to form a green compact, and heating the green compact to form an oxide film on the surface of the metal powder particles constituting the green compact. Produced through a step of forming and bonding the particles together through the oxide film, and a step of impregnating oil in the internal pores of the green compact to obtain a porous oil-impregnated bearing having an oil content of 4 vol% or less. can do.

By heating the green compact in an air atmosphere, an oxide film is mildly formed on the surface of the green compact as compared with the case of heating in a water vapor atmosphere. Swelling is suppressed. For example, when a green compact consisting essentially of iron powder is heated in an air atmosphere, the oxide film is formed of, for example, Fe ₃ O ₄ or Fe ₂ O ₃ , or a mixture thereof.

  As described above, in the present invention, by reducing the internal pores of the green compact by the oxide film and suppressing the oil content of the porous oil-impregnated bearing, the total amount of oil filled in the fluid dynamic pressure bearing device is reduced. Since it can be reduced, the axial dimension of the seal space can be reduced. Thereby, the load capacity and bearing rigidity can be improved by reducing the axial dimension of the fluid dynamic bearing device or by increasing the bearing span.

It is sectional drawing of a spindle motor. It is sectional drawing of a fluid dynamic pressure bearing apparatus. It is sectional drawing of the porous oil-impregnated bearing (bearing sleeve) which concerns on one Embodiment of this invention. It is a bottom view of the said porous oil-impregnated bearing. It is sectional drawing in the bearing surface vicinity of the said porous oil-impregnated bearing. It is sectional drawing of the forming metal mold | die which performs a compacting process (before shaping | molding). It is sectional drawing of the forming metal mold | die which performs a compacting process (at the time of shaping | molding completion). It is sectional drawing of the forming metal mold | die which performs a compacting process (at the time of mold release). The left figure is a cross-sectional structure diagram of the green compact before heat treatment, the center figure is a cross-sectional structure diagram of the green compact after degreasing, and the right figure is a cross-sectional structure diagram of the green compact after the oxidation treatment.

  Hereinafter, embodiments of the present invention will be described with reference to 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 bearing device 1 that rotatably supports a shaft member 2 in a non-contact manner, and a disk hub 3 mounted on the shaft member 2. And, for example, a stator coil 4 and a rotor magnet 5 that are opposed to each other via a gap in the radial direction. 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 mounted on the inner periphery of the casing 6. The disk hub 3 holds a predetermined number of disks D such as magnetic disks. When the stator coil 4 is energized, the rotor magnet 5 is rotated by the electromagnetic force between the stator coil 4 and the rotor magnet 5, whereby the disk hub 3 and the shaft member 2 are rotated together.

  As shown in FIG. 2, the fluid dynamic pressure bearing device 1 includes a bearing sleeve 8 as a porous oil-impregnated bearing according to an embodiment of the present invention, a shaft member 2 inserted in the inner periphery of the bearing sleeve 8, a bearing It has a bottomed cylindrical housing 7 that holds the sleeve 8 on the inner periphery, and a seal portion 9 provided in an opening at one axial end of the housing 7. In the illustrated example, the side portion 7a and the bottom portion 7b of the housing 7 are formed separately, and the side portion 7a and the seal portion 9 of the housing 7 are integrally formed. In the following description, for the sake of convenience, the closed side of the housing 7 is referred to as the lower side and the open side of the housing 7 is referred to as the upper side in the axial direction, but this is not intended to limit the usage mode of the fluid dynamic bearing device 1.

  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 formed of, for example, metal, and in this embodiment, the entire shaft member 2 including the shaft portion 2a and the flange portion 2b is integrally formed of stainless steel.

  The bearing sleeve 8 has a cylindrical shape, and a radial bearing surface facing the outer peripheral surface 2a1 of the shaft member 2 is provided on the inner peripheral surface 8a. In the illustrated example, radial bearing surfaces A are formed at two locations spaced in the axial direction of the inner peripheral surface 8 a of the bearing sleeve 8. In the present embodiment, the radial bearing surface A has an inner diameter of 3 to 5 mm. In each radial bearing surface A, dynamic pressure grooves are formed, and in this embodiment, as shown in FIG. 3, dynamic pressure grooves G1, G2 arranged in a herringbone shape are provided in each radial bearing surface A. A region indicated by cross-hatching in the figure indicates a hill raised on the inner diameter side (the same applies to FIG. 4). The upper dynamic pressure groove G1 has an asymmetric shape in the axial direction, and the lower dynamic pressure groove G2 has a symmetrical shape in the axial direction. The oil in the radial bearing gap is pushed in the axial direction by the upper dynamic pressure groove G <b> 1 having the axially asymmetric shape, and the oil is forcibly circulated inside the housing 7.

  Note that both the upper and lower dynamic pressure grooves G1, G2 may have an axially symmetrical shape. The upper and lower dynamic pressure grooves G1 and G2 may be continuous in the axial direction, or one or both of the upper and lower dynamic pressure grooves G1 and G2 may be omitted. Further, a spiral dynamic pressure groove or a dynamic pressure groove extending in the axial direction may be formed on the radial bearing surface. Further, the inner circumferential surface 8a (radial bearing surface A) of the bearing sleeve 8 may be a cylindrical surface, and a dynamic pressure groove may be formed on the outer circumferential surface 2a1 of the shaft member 2. Or you may comprise a perfect circle bearing by making both the inner peripheral surface 8a of the bearing sleeve 8 and the outer peripheral surface 2a1 of the shaft member 2 into a cylindrical surface. In this case, a dynamic pressure is generated in the oil film in the bearing gap between the bearing sleeve 8 and the shaft member 2 due to the swing of the shaft member 2, and a fluid dynamic pressure bearing that supports the shaft in a floating manner is configured by this dynamic pressure.

  A thrust bearing surface B facing the upper end surface 2b1 of the flange portion 2b of the shaft member 2 is provided on the lower end surface 8b of the bearing sleeve 8. On the thrust bearing surface B, a pump-in type spiral-shaped dynamic pressure groove G3 as shown in FIG. 4 is formed. In addition, as a shape of the dynamic pressure groove, a herringbone shape, a radiation groove shape, or the like may be adopted. Further, a dynamic pressure groove may be formed on the upper end surface 2b1 of the flange portion 2b of the shaft member 2 with the lower end surface 8b (thrust bearing surface B) of the bearing sleeve 8 as a flat surface.

  An annular groove 8c1 and a plurality of radial grooves 8c2 provided on the inner diameter side of the annular groove 8c1 are formed on the upper end surface 8c of the bearing sleeve 8 (see FIG. 3). A plurality of axial grooves 8d1 are provided on the outer peripheral surface 8d of the bearing sleeve 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 the axial groove 8d1, the annular groove 8c1, the radial groove 8c2, and the like. Occurrence is prevented. The upper end surface 8c of the bearing sleeve 8 may be a flat surface, and a radial groove may be provided on the lower surface of the seal portion 9 that contacts the surface.

  The bearing sleeve 8 is a porous oil-impregnated bearing in which oil is impregnated in the internal pores of a green compact subjected to an oxidation treatment, that is, a green compact in which metal powder particles are bonded together via an oxide film. . The green compact of the present embodiment is composed of a green compact substantially made of a single type of metal powder. Specifically, 99 wt% or more of the metal powder constituting the green compact is a single type of metal powder. (Including oxide coating on the particle surface). In the present embodiment, the bearing sleeve 8 is composed of a green compact made only of iron powder (particularly reduced iron powder) as metal powder. Specifically, as shown in FIG. 5, the bearing sleeve 8 is composed of a green compact composed of iron particles 11 and an oxide film 12 formed on the surface of the iron particles 11. The iron particles 11 are bonded to each other by an oxide coating 12. Specifically, the oxide coating 12 formed on the surface of each iron particle 11 spreads between the iron particles 11 to form a network, thereby ensuring the strength of the bearing sleeve 8.

  The bearing sleeve 8 has an oil content of 4 vol% or less, preferably 2 vol% or less, by reducing gaps (internal pores) between the iron particles 11 by the oxide coating 12. On the surface of the bearing sleeve 8, particularly on the radial bearing surface A and the thrust bearing surface B, a large number of minute recesses 13a that are not in communication with the internal pores 13b and openings 13c that are in communication with the internal pores 13b are formed. The oil content of the bearing sleeve 8 is measured by the open porosity measurement method described in JIS Z 2501: 2000.

  The entire surface of the bearing sleeve 8, including the groove bottom surfaces of the dynamic pressure grooves G1, G2, and G3 and the top and side surfaces of the hills, is a molded surface. The bearing sleeve 8 is not sized after the oxidation treatment, and no sliding trace is formed on the surface.

  The housing 7 includes a cylindrical side portion 7a and a bottom portion 7b that closes the opening at the lower end of the side portion 7a (see FIG. 2). In this embodiment, the side part 7a and the bottom part 7b are formed separately. The side part 7a is formed in a cylindrical shape with resin or metal. The outer peripheral surface 8d of the bearing sleeve 8 is fixed to the inner peripheral surface 7a1 of the side portion 7a by an appropriate means such as adhesion or press fitting. The bottom portion 7b is formed in a disk shape with, for example, resin or metal, and is fixed to the lower end portion of the side portion 7a by appropriate means such as press-fitting and adhesion. A thrust bearing surface C is formed on the upper end surface 7b1 of the bottom 7b. The thrust bearing surface C is formed with, for example, a pump-in type spiral dynamic pressure groove (not shown). In addition, as a shape of the dynamic pressure groove, a herringbone shape, a radiation groove shape, or the like may be adopted. Alternatively, the dynamic pressure groove may be formed in the lower end surface 2b2 of the flange portion 2b of the shaft member 2 with the upper end surface 7b1 (thrust bearing surface C) of the bottom portion 7b as a flat surface. Further, the side portion 7a and the bottom portion 7b of the housing 7 may be integrally formed.

  The seal portion 9 protrudes from the upper end of the side portion 7a of the housing 7 toward the inner diameter side. In the present embodiment, the seal portion 9 is formed integrally with the side portion 7 a of the housing 7. The inner peripheral surface 9a of the seal portion 9 has a tapered shape that is gradually reduced in diameter toward the lower side. Between the inner peripheral surface 9a of the seal portion 9 and the outer peripheral surface 2a1 of the shaft portion 2a, a seal space S having a wedge-shaped cross section is formed in which the radial width is gradually narrowed downward. The axial dimension of the seal space S is, for example, 20% or less of the axial dimension of the bearing sleeve 8. In addition, while the inner peripheral surface of the seal portion 9 is a cylindrical surface, a tapered surface that gradually decreases in diameter is provided on the outer peripheral surface of the shaft portion 2a, or the inner peripheral surface of the seal portion 9 and the shaft portion 2a. A taper surface may be provided on both of the outer peripheral surfaces. Alternatively, the seal portion 9 may be formed separately from the side portion 7a of the housing 7 and fixed to the upper end opening of the side portion 7a.

  Oil is injected into the fluid dynamic bearing device 1 having the above-described configuration. In the present embodiment, the inner circumferential space of the housing 7 is filled with oil including the internal pores of the bearing sleeve 8, and the oil level is held in the seal space S.

  The seal space S has a volume that can absorb the volume change of the oil filled in the fluid dynamic bearing device 1. That is, the volume of the seal space S is set to be larger than the volume change amount of the oil in the assumed operating temperature range of the fluid dynamic bearing device 1. In the present embodiment, part of the internal pores of the bearing sleeve 8 is filled with an oxide film, and the oil content is 4% or less. Therefore, the amount of oil impregnated in the bearing sleeve 8 and thus the fluid dynamic pressure The total amount of oil inside the bearing device 1 is reduced, and the volume (particularly the axial dimension) of the seal space S can be reduced.

  For example, when the bearing sleeve 8 having an inner diameter of 4 mm and an axial dimension of 12.4 mm is constituted by a conventional sintered oil-impregnated bearing, the axial dimension of the seal space S is at least larger than 20% of the axial dimension of the bearing sleeve 8. There was a need to do. On the other hand, when the bearing sleeve 8 having the above size is formed of the above-described green compact subjected to the oxidation treatment, the axial dimension of the seal space S should be 20% or less of the axial dimension of the bearing sleeve 8. Became possible. Thus, by reducing the axial dimension of the seal space S, the axial dimension of the fluid dynamic bearing device 1 can be reduced. Alternatively, while maintaining the axial dimension of the fluid dynamic bearing device 1, the axial distance (bearing span) between the radial bearing portions R1 and R2 is increased by reducing the axial dimension of the seal space S, and the load capacity is increased. In addition, the bearing rigidity can be increased.

  When the shaft member 2 rotates, a radial bearing gap is formed between the radial bearing surface A of the inner peripheral surface 8a of the bearing sleeve 8 and the outer peripheral surface 2a1 of the shaft portion 2a, and the dynamic pressure groove provided on the radial bearing surface A The first radial bearing portion R1 and the second radial bearing portion R2 that support the shaft member 2 in a non-contact manner in the radial direction are configured by increasing the pressure of the oil film in the radial bearing gap by G1 and G2. At the same time, between the lower end surface 8b (thrust bearing surface B) of the bearing sleeve 8 and the upper end surface 2b1 of the flange portion 2b, and the upper end surface 7b1 (thrust bearing surface C) of the bottom portion 7b of the housing 7 and the flange portion. A thrust bearing gap is formed between the lower end face 2b2 of 2b, and the pressure of the oil film in each thrust bearing gap is increased by the dynamic pressure grooves provided on the thrust bearing faces B and C, whereby the shaft member 2 The first thrust bearing portion T1 and the second thrust bearing portion T2 are configured to support in a non-contact manner in both thrust directions.

  In the present embodiment, a large number of minute recesses 13a are provided on the bearing surfaces A and B of the bearing sleeve 8 (see FIG. 5). The minute recesses 13a are blocked from communicating with the internal pores 13b by the oxide coating 12, and serve as an oil reservoir for retaining oil. When the shaft member 2 rotates, the oil retained in the minute recesses 13a is supplied to the bearing gap, thereby preventing the oil film from being cut in the bearing gap and ensuring the slidability between the bearing sleeve 8 and the shaft member 2. be able to.

  Here, a manufacturing method of the bearing sleeve 8 will be described. The bearing sleeve 8 is manufactured through a dusting process, a degreasing process, an oxidation process, and an oil impregnation process. Hereinafter, each process will be described in detail.

(1) Compacting step The compacting step is a step of obtaining a cylindrical compact by supplying raw material powder to a mold and compression molding. The raw material powder includes metal powder (metal having a large ionization tendency) capable of forming an oxide film on the particle surface, and includes, for example, iron powder and copper powder. Iron powder can be used regardless of the production method, and for example, atomized powder or reduced powder can be used. Copper powder can also be used regardless of a manufacturing method, for example, electrolytic powder, atomized powder, and reduced powder can be used. In addition, it is also possible to use an alloy powder whose main component is iron or copper (for example, prealloyed pre-alloy powder, partially diffusion alloyed partial diffusion alloy powder). In order to increase strength and improve lubricity, low melting point metal powders such as Sn and Zn, and carbon-based powders such as graphite and carbon black may be added to the raw material powder.

  In this embodiment, the metal powder contained in the raw material powder is substantially composed of a single type of metal powder. Specifically, 99 wt% or more of the metal powder contained in the raw material powder (or 95 wt% or more of the entire raw material powder) is composed of a single type of metal powder. When raw material powder containing multiple types of metal powder is used, the thickness of the oxide film formed on the surface of the particle and the adhesion between the oxide film and the particle differ depending on the type of metal powder. This is because the bearing characteristics may not be satisfactory. In addition, as long as dimensional accuracy and bearing characteristics are satisfied, a plurality of types of metal powders may be mixed.

  A molding lubricant is added to the raw material powder in order to ensure lubrication between the raw material powder and the mold in the subsequent compacting process or between the raw material powders. As the molding lubricant, metal soap or amide wax can be used. The molding lubricant is mixed with the raw material powder as a powder, and a solution in which the above-mentioned molding lubricant is dispersed in a solvent is sprayed or immersed in a metal powder to volatilize and remove the solvent component. A molding lubricant may be coated on the surface of the metal powder.

  In the present embodiment, the raw material powder consists only of pure iron powder (reduced iron powder) and a molding lubricant. The molding lubricant in the raw material powder is, for example, 0.1 to 1 wt%, preferably 0.3 to 0.6 wt%.

  The compacting process is performed using a forming mold shown in FIG. The forming mold includes a die 21, a core rod 22, an upper punch 23 and a lower punch 24. On the outer peripheral surface of the core rod 22, molding dies 22a and 22b having shapes corresponding to the dynamic pressure grooves G1 and G2 are provided. On the upper surface of the lower punch 24, a molding die 24a having a shape corresponding to the dynamic pressure groove G3 is provided. In addition, although illustration is omitted, a molding die having a shape corresponding to the axial groove 8d1 is provided on the inner peripheral surface of the die 21, and an annular groove 8c1 and a radial groove 8c2 are formed on the lower surface of the upper punch 23. A mold having a corresponding shape is provided.

  First, as shown in FIG. 6, a raw material powder M is filled into a cavity defined by a die 21, a core rod 22, and a lower punch 24. Next, as shown in FIG. 7, the upper punch 23 is lowered to compress the raw material powder M to form a green compact 8 '. At the same time, the dynamic pressure grooves G1 and G2 are formed on the inner peripheral surface of the green compact 8 ′ by the molds 22a and 22b of the core rod 22, and the green compact 8 ′ by the mold 24a of the lower punch 24. A dynamic pressure groove G3 is formed on the lower end surface of the lower surface. The dynamic pressure groove G3 on the lower end face of the green compact 8 'may be formed in a separate process.

  Thereafter, as shown in FIG. 8, the green compact 8 ′ is discharged from the inner periphery of the die 21, so that the force directed to the inner diameter applied to the green compact 8 ′ is released, and a spring is applied to the green compact 8 ′. Back occurs. As a result, the inner peripheral surface of the green compact 8 ′ increases in diameter, and the green compact 8 ′ is released from the molds 22 a and 22 b of the core rod 22.

Usually, in a sintered part, the higher the density, the higher the strength. However, as in this embodiment, in the case of increasing the strength by subjecting the green compact to oxidation treatment, if the density of the green compact is too high, an oxidizing gas such as air cannot penetrate into the green compact. Since the formation of the oxide film is limited to the very surface layer of the green compact, the strength is hardly improved. In view of this point, the green density is 7.2 g / cm ³ or less (true density ratio 91% or less), preferably 7.0 g / cm ³ or less (true density ratio 89% or less).

On the other hand, if the powder density is too low, chipping or cracking may occur during handling (the rattra value is large), and there is a concern that the interparticle distance is too long to form an oxide film between the particles. In view of this point, the green density should be 5.8 g / cm ³ or more (true density ratio 74% or more), preferably 6.0 g / cm ³ or more (true density ratio 76% or more). In particular, in order to reduce the oil content of the bearing sleeve 8 to 4 vol% or less, it is preferable to set the dust density higher. Specifically, the dust density is set to 6.3 g / cm ³ or more (true density ratio). 80% or more), preferably 6.7 g / cm ³ or more (true density ratio 85% or more). In addition, the measurement of a compacting density is based on the dimension measuring method. Further, the density of the green compact remains almost unchanged even after the subsequent degreasing process and oxidation process.

(2) Degreasing process The degreasing process is a process in which the green compact is heated to remove (dewax) the molding lubricant contained in the green compact. The degreasing step is performed at a temperature higher than the decomposition temperature of the molding lubricant and lower than the below-described oxidation step, and is heated, for example, at 300 to 400 ° C. for 60 to 120 minutes. As shown in the left diagram of FIG. 9, the green compact 8 ′ before degreasing is provided with a molding lubricant 14 in the gaps between the iron particles 11, but by performing a degreasing step, FIG. As shown in the center view, the molding lubricant 14 disappears, and a green compact 8 ′ composed only of iron particles 11 is obtained.

  In a conventional sintered bearing manufacturing process, since the green compact is held at a high temperature in the sintering process, the lubricant component contained in the green compact is decomposed and is not included in the sintered product. However, when the present invention is applied, the lubricant component may remain depending on the density of the green compact, the oxidation treatment temperature, and the holding time. Therefore, it is desirable to take a technique in which a degreasing process for decomposing and removing the lubricant component is provided in advance prior to the oxidation process, and the oxidation process is continuously performed in the same atmosphere after the degreasing process. However, it has been confirmed that high strength can be achieved even if an oxidation treatment is carried out while containing a molding lubricant without providing a degreasing step. In addition, the degreasing step may be performed in an atmosphere (for example, an inert gas, a reducing gas, or in a vacuum) different from the oxidation step using a separate heating device.

(3) Oxidation step In the oxidation step, the green compact is heated in an oxidizing atmosphere. Thereby, as shown to the right figure of FIG. 9, the oxide film 12 is produced | generated on the surface of each particle | grain 11 of metal powder (iron powder), and particle | grains 11 are couple | bonded through this oxide film 12, The strength of the green compact 8 ′ is increased. Specifically, the oxide film formed on the surface of each particle of the metal powder by the oxidation process spreads between the iron particles 11 to form a network, thereby bonding by sintering at a high temperature as in the past. Instead of the force, the green compact 8 'is strengthened. Further, in this embodiment, not all particles of iron powder as a main component are bonded via an oxide film, but some particles are directly in contact with each other without an oxide film and fused. doing.

  Due to the formation of the oxide coating 12, the internal pores of the green compact 8 'are reduced (see FIG. 5). Specifically, some of the internal pores of the green compact 8 ′ are filled with the oxide film 12, or the continuous air holes opened on the surface are closed with the oxide film 12 to become independent pores. Thereby, the pores into which oil can enter is reduced, and the oil content is reduced. In the present embodiment, by forming the oxide film 12, the oil content of the green compact 8 'is 4 vol% or less.

  The processing conditions (heating temperature, heating time, heating atmosphere) of the above-described oxidation treatment are such that the strength required for a dynamic pressure bearing is imparted to the green compact 8 ′, and the green compact 8 is formed by the oxide coating 12. The oil content of 'is set to 4 vol% or less. Specifically, the heating temperature in the oxidation step of the present embodiment is set to 350 ° C. or higher, preferably 400 ° C. or higher. Further, if the heating temperature is too high, the dimensional change of the green compact becomes large, so the heating temperature is set to 600 ° C. or lower, preferably 550 ° C. or lower. The heating time is appropriately set in the range of 5 minutes to 2 hours, for example, 10 to 20 minutes. The green compact subjected to the oxidation step has a strength required for the bearing sleeve 8, specifically, a crushing strength of 120 MPa or more, preferably 150 MPa or more.

  The heating atmosphere is an oxidizing atmosphere in order to promote positive oxidation. However, since the generation rate of the oxide film is too high in the water vapor atmosphere, it is preferable to use an oxidizing atmosphere in which the generation rate of the oxide film is lower than that of the water vapor atmosphere. Specifically, heating is preferably performed in an atmosphere of air or oxygen, or an oxidizing gas in which an inert gas such as nitrogen or argon is mixed, and most preferably heating is performed in an air atmosphere. By performing the oxidation treatment in an air atmosphere, the oxide film formed on the surface of the green compact can be suppressed, so that a reduction in the surface roughness of the green compact can be suppressed. Further, in order to obtain a strength that can be used as the bearing sleeve 8 (for example, a crushing strength of 120 MPa or more), the oxygen fraction in the heating atmosphere is preferably 2 vol% or more.

The iron oxide film formed on the surface of the iron powder is made of Fe ₃ O ₄ , Fe ₂ O ₃ , FeO, or the like. The ratio of these oxide coatings varies depending on the material and processing conditions.

  Since the above oxidation process has a lower processing temperature than the conventional high temperature sintering process, the dimensional change of the green compact due to the oxidation process can be suppressed. Further, as described above, the metal powder constituting the green compact is substantially composed of a single type of metal powder (iron powder), thereby forming an oxide film uniformly on the particle surface of the metal powder. Therefore, the dimensional change of the green compact due to the oxidation treatment can be further suppressed. Further, in this embodiment, by performing the oxidation treatment of the green compact in an air atmosphere, an oxide film is mildly formed on the surface of the green compact, thereby preventing the surface properties of the green compact from becoming rough. it can.

  As described above, the deterioration of the dimensional accuracy and surface accuracy of the green compact due to the oxidation treatment is suppressed, so that the dimensional accuracy of the green compact formed with high accuracy by the mold is maintained. In particular, as in this embodiment, when the dynamic pressure groove is formed at the same time as the green compact is formed in the green compacting process, the dimensional change of the green compact due to the subsequent oxidation process is suppressed, so that The dimensional accuracy (groove depth, etc.) of the peripheral surface and the dynamic pressure groove is maintained. Thereby, since the sizing process after the oxidation process can be omitted, the manufacturing process of the bearing can be shortened, the cost can be reduced, and the design of the bearing and the forming mold can be facilitated.

  The above oxidation step can be applied regardless of the shape and size of the green compact. Moreover, since the surface of the green compact which performed the oxidation process is covered with an oxide film, a rust prevention effect is high and a rust prevention process is unnecessary depending on the case. In addition, since the processing temperature of the oxidation process is relatively low, an additive that denatures and decomposes at a temperature exceeding this processing temperature (for example, a material having slidability and lubricity) is added to enhance the functionality of the product. It is also possible to plan.

(4) Oil impregnation step In the oil impregnation step, oil is impregnated in the internal pores of the green compact subjected to the oxidation treatment. Specifically, after the green compact is immersed in oil under a reduced pressure environment, the pressure is returned to atmospheric pressure, so that the oil enters the internal pores from the opening on the surface of the green compact. Thus, the bearing sleeve 8 as a porous oil-impregnated bearing is completed. In addition, you may assemble the fluid dynamic pressure bearing apparatus 1 using the green compact which abbreviate | omitted an oil impregnation process and the oil is not impregnated inside. In this case, when the fluid dynamic pressure bearing device 1 is filled with oil by vacuum impregnation or the like, the oil is impregnated into the internal pores of the green compact in the dry state, whereby a bearing sleeve 8 as a porous oil-impregnated bearing is obtained. Is obtained.

  The embodiment of the present invention is not limited to the above. For example, in the above-described embodiment, the shaft rotation type fluid dynamic pressure bearing device 1 in which the bearing sleeve 8 is fixed and the shaft member 2 is rotated is shown, but on the contrary, the shaft member 2 is fixed, The present invention may be applied to a shaft fixed type fluid dynamic pressure bearing device that rotates the bearing sleeve 8. The fluid dynamic pressure bearing device according to the present invention can be used by being incorporated not only in a spindle motor for a disk drive device such as an HDD but also in a fan motor for a cooling fan or a polygon scanner motor for a laser beam printer. it can.

DESCRIPTION OF SYMBOLS 1 Fluid dynamic pressure bearing apparatus 2 Shaft member 7 Housing 8 Bearing sleeve (porous oil-impregnated bearing)
8 'Compact 9 Seal part 11 Iron particle 12 Oxide coating 14 Molding lubricant A Radial bearing surface B, C Thrust bearing surface G1, G2, G3 Dynamic pressure grooves R1, R2 Radial bearing portion T1, T2 Thrust bearing portion S Seal space

Claims (8)

A shaft member, a porous oil-impregnated bearing in which the shaft member is inserted in the inner periphery, a bottomed cylindrical housing that holds the porous oil-impregnated bearing in the inner periphery, and a seal portion provided in an opening of the housing A radial bearing portion that supports the shaft member in a relatively rotatable manner by a dynamic pressure action of an oil film generated in a radial bearing gap between an inner peripheral surface of the porous oil-impregnated bearing and an outer peripheral surface of the shaft member; and the seal A fluid dynamic bearing device comprising a seal space that is formed between an inner peripheral surface of a portion and an outer peripheral surface of the shaft member and that holds an interface between oil and the atmosphere filled in the housing;
The porous oil-impregnated bearing has a green compact in which the particles are bonded to each other through an oxide film formed on the surface of the metal powder particles,
The fluid dynamic bearing device according to claim 1, wherein an oil content of the porous oil-impregnated bearing is 4 vol% or less. 2. The fluid dynamic bearing device according to claim 1, wherein an inner diameter of the porous oil-impregnated bearing is 3 to 5 mm and an axial dimension of the seal space is 20% or less of an axial dimension of the porous oil-impregnated bearing.   3. The fluid dynamic bearing device according to claim 1, wherein 99 wt% or more of the metal powder contained in the green compact is iron powder. 4. The fluid dynamic bearing device according to claim ₃ , wherein the oxide film is Fe ₃ O _4, Fe ₂ O ₃ , or a mixture thereof.   The fluid dynamic pressure bearing device according to any one of claims 1 to 4, a rotor magnet provided on a rotation side among the housing and the shaft member, and a fixed side among the housing and the shaft member. And a stator coil provided on the motor. A porous oil-impregnated bearing having a green compact in which the particles are bonded to each other through an oxide film formed on the surface of the metal powder particles, and the internal pores are impregnated with oil,
A porous oil-impregnated bearing having an oil content of 4 vol% or less.   Forming a green compact by compressing the metal powder; and heating the green compact to form an oxide film on the surface of the metal powder particles constituting the green compact. A method for producing a porous oil-impregnated bearing comprising: a step of bonding the particles to each other; and a step of impregnating oil in the pores of the green compact to obtain a porous oil-impregnated bearing having an oil content of 4 vol% or less. The method for producing a porous oil-impregnated bearing according to claim 7, wherein the oxide film is formed by heating the green compact in an air atmosphere.

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