Sintered bearing and fluid dynamic bearing device equipped with same

Abstract

To provide a sintered bearing which can accurately support a shaft inserted into an internal periphery in a radial direction.SOLUTION: In a sintered bearing (bearing member 8) which is composed of a cylindrical metallic sintered body which is formed by sintering green compacts of raw material powder M, and in which a radial bearing face forming a radial bearing clearance between an external peripheral face 2a of a supported shaft (shaft member 2) and itself is formed at an internal peripheral face 8a, and an outer surface and the internal peripheral face 8a are molded faces which are molded following sizing metal molds, a trace of one burr out of four annular burrs in total which are formed accompanied by the compression-molding of the green compacts 18 exists within a range of one end face (upper end face 8b), and a burr and its trace do not exist at a one-end internal peripheral edge part (upper-end internal peripheral edge part).SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a sintered bearing and a fluid dynamic bearing device equipped with the same. [Background technology]

[0002] For example, in fluid dynamic bearing devices incorporated in spindle motors for HDDs, fan motors for PCs, etc., a cylindrical bearing member that radially supports a shaft inserted into the inner circumference is preferably a sintered metal bearing (sintered bearing), which has excellent mechanical strength, wear resistance, mass producibility, etc. As described in Patent Document 1 listed below, this sintered bearing is manufactured through, in order, a compression molding process in which a green compact is obtained from raw material powder, the main raw material of which is metal powder, a sintering process in which the green compact is heated at a temperature equal to or higher than the sintering temperature of the metal powder to obtain a sintered body, and a sizing process in which the sintered body is plastically deformed according to a sizing die to finish the sintered body into the shape of a finished product.

[0003] In the compression molding process, a cylindrical green compact is molded using a molding die device in which multiple dies (usually an axial core, a cylindrical die, a lower punch, and an upper punch) are combined, and in this case, annular protrusions (burrs) resulting from powder that has entered the mating portion between the dies are inevitably formed on the outer surface of the green compact. For this reason, as shown in Fig. 1 of Patent Document 1, when the inner peripheral surface, outer peripheral surface, lower end surface (one end surface), and upper end surface (the other end surface) of the green compact are molded by the outer peripheral surface of the core, the inner peripheral surface of the die, the upper end surface of the lower punch, and the lower end surface of the upper punch, respectively, annular burrs are formed at a total of four locations: the inner peripheral edge portions at one end and the other end, and the outer peripheral edge portions at one end and the other end of the green compact.

[0004] In the sintered bearing obtained by sintering the above-mentioned green compact, if annular burrs remain on the inner peripheral edge portions at one end and the other end, When inserting a shaft into the inner circumference of a sintered bearing, the shaft comes into contact with the burrs, causing some or all of them to fall off (entering the internal space of the bearing device). When the sintered bearing rotates relative to the shaft inserted inside it, the shaft comes into sliding contact with the burrs, causing abnormal noise and even causing some or all of the burrs to fall off. For this reason, the annular burrs formed on the green compact are generally removed by subjecting the sintered compact to a removal process before sizing.

[0005] Methods for removing burrs that can be used include, for example, barrel processing, in which the sintered body and media as an abrasive are mixed in a chamber, mechanical processing, in which the burrs are removed using a cutting tool, and a method in which the burrs are crushed by pressing a die against the burrs. Of these methods, barrel processing is advantageous in terms of reducing the manufacturing costs of mass-produced sintered bearings, since it allows removal processing to be performed on multiple sintered bodies at once. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 3954695 Summary of the Invention [Problem to be solved by the invention]

[0007] Incidentally, ultra-small sintered bearings with an inner diameter of φ2 mm or less may be used in fluid dynamic bearing devices incorporated in fan motors for portable information devices (mobile devices) such as notebook computers and tablet terminals. It has been found that when annular burrs are formed during compression molding of green compacts for such ultra-small sintered bearings, particularly on the inner peripheral edges at one and the other ends, these burrs may not be sufficiently removed by barrel processing.

[0008] This is thought to be largely due to the fact that, in order to thoroughly remove the annular burrs remaining on the inner periphery of one end and the other end of the sintered body by barrel processing, it is necessary to penetrate the media (e.g., steel balls) into the inner periphery of the sintered body as well, and therefore media with a diameter smaller than the inner diameter of the sintered body are selected and used. In other words, the smaller the grain size of the media, the weaker the processing force (burr removal force), so it becomes difficult to thoroughly remove the burrs remaining on the inner periphery of one end and the other end of the sintered body with fine media selected and used to perform barrel processing on ultra-small sintered bodies with an inner diameter of φ2 mm or less. If the burrs remaining on the inner periphery of one end and the other end of the sintered body cannot be sufficiently removed by barrel processing, it becomes necessary to take measures such as removing the burrs by a method other than barrel processing (e.g., mechanical processing) or extending the processing time of barrel processing, but this leads to an increase in the manufacturing cost of sintered bearings.

[0009] In view of the above circumstances, the present invention has an object to provide a sintered bearing that is capable of supporting a shaft with high precision in the radial direction even when the process of removing annular burrs that is inevitably formed when compressing a green compact is simplified or omitted, thereby ensuring the bearing performance required for a fluid dynamic bearing device in which this sintered bearing is used as a bearing member, while contributing to reducing the cost thereof. [Means for solving the problem]

[0010] As a result of extensive investigations, the inventors came to the idea that the above-mentioned object could be achieved by changing the position of the four annular burrs that are inevitably formed during compression molding of the cylindrical green compact that serves as the base material for a sintered bearing, but which are difficult to fully remove by simple and efficient processing methods such as barrel processing. This idea led to the invention.

[0011] That is, the present invention, which was devised to achieve the above-mentioned object, is a sintered bearing which consists of a cylindrical sintered body obtained by sintering a compact of raw material powder, the main raw material of which is metal powder, and which has a radial bearing surface on its inner surface which forms a radial bearing gap between it and the outer peripheral surface of the shaft to be supported, and whose outer and inner surfaces are molding surfaces formed in imitation of a sizing die, and which is characterized in that of a total of four annular burrs formed in conjunction with the compression molding of the compact, the trace of one burr is present within the range of one end face, and no annular burr or its trace is present on the inner peripheral edge of one end.

[0012] In the sintered bearing according to the present invention, there is no annular burr or trace thereof on the inner peripheral edge of one end, but instead there is one trace of a burr within the range (radial range) of one end face. This means that, of the total of four annular burrs that are inevitably formed when a cylindrical green compact is compressed, the position of the burr that was conventionally formed on the inner peripheral edge of one end (the end on one axial side of the inner peripheral surface) has been changed to within the range of the one end face. In this case, if the sintered bearing is arranged so that the one end face is located on the side where the shaft insertion starts, it is possible to prevent problems such as the shaft coming into contact with the burr and even the burr falling off as the shaft is inserted into the inner circumference of the sintered bearing.

[0013] In the sintered bearing of the present invention, whose outer surface is a molding surface formed following a sizing die, the trace of one burr present within the range of one end face can be an annular burr formed within the range of one end face of the green compact, for example, crushed by a sizing die (punch). In this case, there is no need to perform a separate removal process to remove the burr formed within the range of the one end face. Therefore, according to the present invention, even if the removal process of the annular burr that is inevitably formed when the green compact is compression molded is simplified or omitted, it is possible to realize a sintered bearing that can accurately support the shaft to be supported in the radial direction.

[0014] The sintered bearing according to the present invention may be configured such that, among the three burrs different from the one burr (burr formed within the range of one end face of the green compact), the trace of one burr is present within the range of the inner circumferential surface (axial range), and no annular burr or its trace is present on the inner circumferential edge of the other end. Such a configuration can be obtained by changing the formation position of the annular burr, which was conventionally formed on the inner circumferential edge of the other end of the green compact (the end of the inner circumferential surface on the other side in the axial direction), to within the range of the inner circumferential surface of the green compact. In this case, the trace of the burr present within the range of the inner circumferential surface can be formed by crushing the burr formed on the inner circumferential surface of the green compact with a sizing die (sizing core), so that a removal process for removing the burr formed on the inner circumferential surface of the green compact is not required. In this case, even if a shaft is inserted into the inner circumference of the sintered bearing from either one end side or the other end side of the sintered bearing, there is no possibility of the shaft and the burr coming into contact with each other, and further there is no possibility of the shaft and the burr coming into contact with each other during relative rotation of the sintered bearing and the shaft. As a result, there is no need to consider the position (up or down) of the sintered bearing when inserting the shaft, which not only improves the ease of inserting the shaft (assembly of the bearing device), but also stabilizes the bearing performance.

[0015] If the four burrs in total include burrs formed on the outer periphery of one end and the outer periphery of the other end, it is preferable to remove at least these two burrs. This makes it possible to minimize the possibility that the two burrs will fall off when, for example, the sintered bearing is incorporated into a fluid dynamic bearing device (when it is inserted into the inner circumference of a housing). When removing the two burrs formed on the outer periphery of one end and the other end by barrel processing, large grain media can be used, which can remove the burrs appropriately and efficiently.

[0016] The present invention is particularly suitable for use in sintered bearings with an inner diameter of φ2 mm or less, i.e., small sintered bearings in which annular burrs are formed on the inner peripheral edges at one end and the other end as a result of molding a powder compact, and which are difficult to sufficiently remove by barrel processing.

[0017] A fluid dynamic bearing device comprising a bearing member consisting of the sintered bearing according to the present invention as described above, a bottomed cylindrical housing that accommodates this on its inner circumference, a radial bearing portion that supports the shaft in the radial direction in a non-contact manner by an oil film formed in the radial bearing gap, and a thrust bearing portion that supports one end of the shaft in the thrust direction on the bottom side of the housing, has the above-mentioned characteristics of the sintered bearing according to the present invention, and therefore has the characteristics of being inexpensive and being able to support the shaft to be supported with high precision.

[0018] Furthermore, in order to achieve the above object, the present invention provides a method for producing a sintered bearing, which includes a compression molding step in which a cavity defined by an axial core, a cylindrical die arranged radially outward of the core, and a lower punch arranged between the core and the die is filled with raw material powder, the main ingredient of which is a metal powder, and then the upper punch is moved closer to the lower punch relative to the lower punch to compress the raw material powder to obtain a cylindrical green compact, a sintering step in which the green compact is heated to obtain a sintered body, and a sizing step in which the outer and inner surfaces of the sintered body are plastically deformed according to a sizing die, thereby finishing the outer and inner surfaces of the metal sintered body into a predetermined shape, wherein the compression molding step is characterized in that the inner surface of the green compact is molded by the outer surface of the core, and one end surface (lower end surface) of the green compact is molded by the upper end surface of the lower punch and a shoulder surface of the core arranged on the same plane as the lower punch.

[0019] According to this manufacturing method, it is possible to obtain a green compact in which one of the four annular burrs formed during compression molding of the green compact is formed within the range of one end face, and no annular burr is formed on the inner peripheral edge of one end, thereby providing the same effects as those of the sintered bearing according to the present invention.

[0020] In the above manufacturing method, in the compression molding step, the shaft portion and the core integrally provided with the upper punch may be vertically connected and coaxially arranged, and the entire inner peripheral surface of the green compact may be molded by the outer peripheral surface of the shaft portion and the outer peripheral surface of the core. In this way, a green compact can be obtained in which one of three burrs different from the annular burr formed within the range of one end face is formed within the range of the inner peripheral surface, and no annular burr is formed on the inner peripheral edge of the other end. Effect of the Invention

[0021] As described above, according to the present invention, it is possible to realize a sintered bearing that can support a shaft with high precision in the radial direction even if the process of removing burrs that is inevitably formed when compressing a green compact is simplified or omitted. Therefore, it is possible to ensure the bearing performance required for a fluid dynamic bearing device in which this sintered bearing is used as a bearing member, and to contribute to reducing the costs thereof. [Brief description of the drawings]

[0022] [Figure 1] FIG. 1 is a cross-sectional view conceptually illustrating an example of a fan motor. [Diagram 2] 1 is a schematic vertical cross-sectional view of a fluid dynamic bearing device equipped with a sintered bearing according to an embodiment of the present invention. [Diagram 3] FIG. 2 is a flow chart showing a manufacturing procedure of a sintered bearing according to an embodiment of the present invention. [Figure 4] 1 is a vertical cross-sectional view of a green compact for a sintered bearing according to an embodiment of the present invention. FIG. [Diagram 5] 5A and 5B are longitudinal sectional views conceptually showing the compression molding process for the powder compact shown in FIG. 4, in which FIG. 5A is a longitudinal sectional view showing an initial stage of the process, and FIG. 5B is a longitudinal sectional view showing an intermediate stage of the process. [Figure 6] FIG. 4 is a vertical sectional view of a green compact for a sintered bearing according to another embodiment of the present invention. [Figure 7] 7A to 7C are longitudinal sectional views conceptually showing the compression molding process of the powder compact shown in FIG. 6, in which (a) shows an initial stage of the process, (b) shows an intermediate stage of the process, and (c) shows the final stage of the process. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0024] FIG. 1 conceptually illustrates an example of a fan motor. The fan motor illustrated in the figure is incorporated in a portable information device (mobile device) such as a notebook computer or tablet terminal, and generates wind (airflow) to cool a heat source such as a CPU. The 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, blades 4 attached to the rotor 3, and a stator coil 6a and a rotor magnet 6b arranged opposite each other with a radial gap therebetween. The stator coil 6a is attached to a housing 7 of the fluid dynamic bearing device 1, and the rotor magnet 6b is attached to the rotor 3. In the fan motor having such a configuration, when the stator coil 6a is energized, the rotor magnet 6b rotates due to the electromagnetic force between the stator coil 6a and the rotor magnet 6b, and the shaft member 2 and the rotor 3 fixed to the shaft member 2 rotate together. As the rotor 3 rotates, an airflow is generated in the axial direction or the radial outward direction depending on the shape of the blades 4 attached to the rotor 3.

[0025] As shown in Fig. 2, the fluid dynamic bearing device 1 is a so-called rotating shaft type bearing device mainly comprising a shaft member 2 which constitutes the rotating side, a housing 7, bearing member 8 and seal member 9 which constitute the stationary side, and lubricating oil (not shown) which is filled in the internal space of the housing 7, and for the bearing member 8, a sintered bearing according to an embodiment of the present invention is used. Below, we will first briefly explain the overall configuration of the fluid dynamic bearing device 1, and then explain the bearing member 8 which is a sintered bearing. Also, for convenience of explanation, below, the upper side of the paper in Fig. 2 (the side where the seal member 9 is arranged) will be referred to as the "upper side" and the lower side of the paper in Fig. 2 will be referred to as the "lower side", but this is not intended to limit the position of the fluid dynamic bearing device 1 when in use.

[0026] The shaft member 2 is made of a highly rigid metal material such as stainless steel, and its outer peripheral surface 2a is a smooth cylindrical surface without any irregularities, and its lower end surface 2b is a convex spherical surface. The rotor 3, to which the blades 4 and the rotor magnet 6b (see Figure 1) are attached, is fixed to the upper end of the shaft member 2.

[0027] The housing 7 is a bottomed cylinder having a cylinder portion 7a and a bottom portion 7b that closes the lower end opening of the cylinder portion 7a, and in the illustrated example, the cylinder portion 7a and the bottom portion 7b are integrally formed from a resin or metal material. The inner peripheral surface 7a1 of the cylinder portion 7a is formed into a cylindrical surface of a constant diameter, and the outer diameter end of the annular shoulder surface 7b2 formed into a flat surface in a direction perpendicular to the axial direction is connected to the inner peripheral surface 7a2 of the cylinder portion 7a with a gap therebetween.

[0028] In the illustrated example, a thrust plate 10 formed in a disk shape from a material with better sliding properties than the material from which the housing 7 is made is placed on the inner bottom surface (upper end surface of the bottom part 7b) 7b1 of the housing 7, and the upper end surface of the thrust plate 10 contacts and supports the lower end surface 2b of the shaft member 2 (contact supports the shaft member 2 in the thrust direction). However, the thrust plate 10 is not necessarily required and may be omitted. When the thrust plate 10 is omitted, the lower end surface 2b of the shaft member 2 is contact-supported by the inner bottom surface 7b1 of the housing 7.

[0029] The seal member 9 is formed in an annular shape from 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 with the lower end surface 9b abutting against the upper end surface 8b of the bearing member 8. The inner circumferential surface 9a of the seal member 9 forms an annular seal space S between itself and the opposing outer circumferential surface 2a of the shaft member 2. This seal space S prevents the lubricating oil filled in the internal space of the housing 7 from leaking out to the outside.

[0030] The fluid dynamic bearing device 1 may be used in a so-called fully filled state, where the entire internal space of the housing 7 is filled with lubricating oil, or in a so-called partially filled state, where lubricating oil is present in only a portion of the internal space of the housing 7 (lubricating oil and air are mixed in the internal space of the housing 7).When the fluid dynamic bearing device 1 is used in a fully filled state, the volume of the sealed space S is determined so that the lubricating oil level is always maintained within the axial range of the sealed space S, even if the lubricating oil level position fluctuates in the axial direction due to temperature changes.

[0031] The bearing member 8 is fixed to the inner circumference of the cylindrical portion 7a of the housing 7 with the lower end surface 8c in contact with the shoulder surface 7b2 of the bottom portion 7b of the housing 7. The bearing member 8 can be fixed to the inner circumference surface 7a1 of the cylindrical portion 7a by press-fitting, gluing, or press-fitting and gluing (a combination of press-fitting and gluing), or can be fixed to the inner circumference of the cylindrical portion 7a by sandwiching the bearing member 8 from both axial sides between the seal member 9 and the shoulder surface 7b2 of the housing 7 after a clearance fit (see JIS B 0401-1) to the inner circumference of the housing 7. In particular, the latter fixing method allows the bearing member 8 to be fixed to the housing 7 at the same time as the seal member 9 is fixed to the housing 7, thereby reducing the effort required for assembling the members together. Furthermore, for example, if the bearing member 8 is press-fitted into the inner periphery of the cylindrical portion 7a of the housing 7 with a large interference, deformation of the bearing member 8 caused by the press-fitting will spread to the inner periphery 8a of the bearing member 8, which may adversely affect the width accuracy of the radial bearing gap and, ultimately, the bearing performance of the radial bearing portion R. In contrast, the method in which the bearing member 8 is sandwiched between the seal member 9 and the shoulder surface 7b2 of the housing 7 from both axial sides can effectively prevent the occurrence of such problems.

[0032] The bearing member 8 is formed into a cylindrical shape from a porous sintered metal body whose main components are, for example, copper and iron, and is fixed to the inner circumference of the housing 7 in an oil-impregnated state with its internal pores impregnated with lubricating oil. In this embodiment, from the viewpoint of realizing a compact fan motor (see FIG. 1) that can be easily incorporated into the above-mentioned mobile device, for example, a sintered bearing having an inner diameter of φ2 mm or less (preferably less than φ1.5 mm), an outer diameter of φ4 mm or less, and an axial dimension of 10 mm or less is used.

[0033] The inner peripheral surface 8a of the bearing member 8 is provided with a cylindrical radial bearing surface that forms a radial bearing gap between itself and the outer peripheral surface 2a of the shaft member 2. Although detailed illustration is omitted, the radial bearing surface of this embodiment is formed as a smooth cylindrical surface on which no dynamic pressure generating portion (radial dynamic pressure generating portion) such as a dynamic pressure groove is formed. The inner peripheral surface 8a of the bearing member 8 including the radial bearing surface, and the outer surface of the bearing member 8 (both end surfaces 8b, 8c and the outer peripheral surface 8d, etc.) are formed as molded surfaces molded following a sizing die.

[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 (radial bearing surface) of the bearing member 8, which face each other, and the lubricating oil impregnated in the internal pores of the bearing member 8 gradually seeps out of the bearing member 8 through the surface openings of the bearing member 8 due to the generation of pressure (negative pressure) accompanying the rotation of the shaft member 2 and thermal expansion of the lubricating oil, forming an oil film in the radial bearing gap. This forms a radial bearing portion R that supports the shaft member 2 in the radial direction. In addition, when the shaft member 2 rotates, the lower end surface 2b of the shaft member 2 comes into sliding contact with the upper end surface of the thrust plate 10 placed on the bottom portion 7b of the housing 7. This forms a thrust bearing portion T that supports (contact supports) the shaft member 2 in the thrust direction.

[0035] In this embodiment, the outer peripheral surface 2a of the shaft member 2 and the inner peripheral surface 8a of the bearing member 8, which face each other via a radial bearing gap, are both formed as smooth cylindrical surfaces, so that the radial bearing portion R is configured as a so-called perfect circular bearing. In the fluid dynamic bearing device 1 of this embodiment, which is formed with such a radial bearing portion R and a thrust bearing portion T consisting of a so-called pivot bearing that contacts and supports the lower end of the shaft member 2, when the shaft member 2 rotates, the shaft member 2 performs a pestle motion with the lower end portion (contact portion with the thrust plate 10) as a reference. In other words, the shaft member 2 rotates while swinging in the radial direction with the lower end portion as a fulcrum, and as a result of this swinging, the pressure of the oil film is increased in a partial circumferential region of the radial bearing gap (a region where the gap width of the radial bearing gap is smaller than in other regions), so that the shaft member 2 is supported in a non-contact manner in the radial direction.

[0036] Hereinafter, a bearing member 8 made of a sintered bearing according to one embodiment of the present invention and a method for producing the same will be described in detail.

[0037] The bearing member 8 is manufactured by sequentially performing a compression molding step, a sintering step, a removing step, a sizing step, and an oil impregnation step, as shown in Fig. 3. Each step will be described below.

[0038] [Compression molding process] In the compression molding process, the raw material powder M, which is mainly made of metal powder, is compression molded to produce a cylindrical green compact 18 shown in FIG. Upper end inner peripheral edge portion (connection portion between inner peripheral surface 18a and upper end inner peripheral chamfer) P1, Upper outer peripheral edge portion (connection portion between the outer peripheral surface 18d and the upper outer peripheral chamfer) P2, A predetermined radial position P3 of the lower end surface 18c (here, approximately the radial center of the lower end surface 18c), and ·Lower end outer periphery edge (connection between outer periphery surface 18d and lower end outer periphery chamfer) P4 A green compact 18 having an annular protrusion (burr) formed thereon is obtained.

[0039] The green compact 18 shown in Fig. 4 can be formed by compressing the raw material powder M using a molding die apparatus 20 shown in Fig. 5(a) and Fig. 5(b). The molding die apparatus 20 includes a core 21, a cylindrical die 22, a lower punch 23, and an upper punch 24 that are coaxially arranged. The core 21, the lower punch 23, and the upper punch 24 are provided so as to be movable up and down relative to the die 22.

[0040] The core 21 is in the form of a stepped shaft integrally having a shaft portion 21a and a base portion 21b disposed below the shaft portion 21a and having an outer diameter larger than that of the shaft portion 21a, and the outer peripheral surface 21c of the shaft portion 21a which forms the inner peripheral surface 18a of the powder compact 18 is formed into a smooth cylindrical surface. The inner peripheral surface 22a of the die 22 which forms the outer peripheral surface 18d of the powder compact 18 is formed into a smooth cylindrical surface. In addition, the shoulder surface 21d of the core 21 which forms the inner diameter side region of the lower end surface 18c of the powder compact 18, and the upper end surface 23a of the lower punch 23 which forms the outer diameter side region of the lower end surface 18c of the powder compact 18 are formed into smooth flat surfaces. The upper punch 24 is integrally provided with a molding portion for molding the upper end inner chamfer and upper end outer chamfer of the powder compact 18, the lower punch 23 is integrally provided with a molding portion for molding the lower end outer chamfer of the powder compact 18, and the core 21 is integrally provided with a molding portion for molding the lower end inner chamfer of the powder compact 18.

[0041] 5(a), in the molding die device 20 having the above-mentioned configuration, the shoulder surface 21d of the core 21 and the upper end surface 23a of the lower punch 23 are arranged on the same plane on the inner circumference of the die 22, and a cylindrical powder filling section 25 into which the raw material powder M is filled is defined by the outer circumference surface 21c of the core 21, the inner circumference surface 22a of the die 22, the upper end surface 23a of the lower punch 23, and the shoulder surface 21d of the core 21, and the raw material powder M is filled into this powder filling section 25. As the raw material powder M, a mixed powder is used in which a metal powder (for example, a mixed powder of copper powder and iron powder, or a copper-iron alloy powder) is used as the main raw material, and various fillers such as a molding assistant and a solid lubricant are added and mixed therein.

[0042] After the raw material powder M is filled into the powder filling section 25, as shown in Fig. 5(b), the upper punch 24 moves relatively closer to the lower punch 23 (the upper punch 24 is moved downward) to compress the raw material powder M in the axial direction, whereby the raw material powder M is pressed against the outer peripheral surface 21c of the core 21, the inner peripheral surface 22a of the die 22, the upper end surface 23a of the lower punch 23, the shoulder surface 21d of the core 21, and the lower end surface 24a of the upper punch 24, thereby forming a cylindrical green compact 18. Thereafter, for example, the core 21, the lower punch 23, and the upper punch 24 are moved upward together to discharge the green compact 18 to the upper side of the die 22, and then the upper punch 24 is further moved upward and the core 21 is moved downward to release the green compact 18.

[0043] Since the four dies (core 21, die 22, lower punch 23, and upper punch 24) constituting the molding die device 20 are provided so as to be movable relative to one another, minute gaps are present at the mating portions between the dies, i.e., the mating portion between the shaft portion 21a of the core 21 and the upper punch 24, the mating portion between the die 22 and the upper punch 24, the mating portion between the base portion 21b of the core 21 and the lower punch 23, and the mating portion between the die 22 and the lower punch 23, and a part of the raw material powder M enters each minute gap when the raw material powder M is filled into the powder filling portion 25 and when the raw material powder M filled into the powder filling portion 25 is compressed. Therefore, when the green compact 18 is released from the molding die device 20, the green compact 18 (FIG. 4) is obtained in which annular burrs are formed at the upper end inner peripheral edge portion P1, the upper end outer peripheral edge portion P2, the radially predetermined position P3 of the lower end surface 18c, and the lower end outer peripheral edge portion P4, as described above.

[0044] [Sintering process] In the sintering step, the powder compact 18 is heated and sintered at a temperature equal to or higher than the sintering temperature of the metal powder constituting the powder compact 18, thereby obtaining a sintered body (not shown) in which adjacent metal powder particles are neck-bonded together.

[0045] [Removal process] In the removal step, the sintered body is subjected to a removal process to remove burrs that have hardened through the sintering process. The removal process may be any process that can properly remove at least the burrs formed on the upper outer peripheral edge P2 and the lower outer peripheral edge P4 of the total of four annular burrs formed on the powder compact 18. In this case, a so-called barrel process is used in which the sintered body and abrasive media (e.g., steel balls) are mixed in a chamber and the media is brought into contact with and collided with the burrs to remove the burrs. The barrel process is advantageous in reducing the manufacturing cost of the sintered bearing because it allows the burr removal process to be performed on multiple (many) sintered bodies at once. In addition, if the burrs formed on the upper outer peripheral edge P2 and the lower outer peripheral edge P4 of the powder compact 18 are properly removed, it is possible to prevent the burrs from falling into the internal space of the housing 7 when the bearing member 8 made of a sintered body obtained by sintering the powder compact 18 is assembled into the inner periphery of the housing 7.

[0046] The larger the diameter of the steel balls used as media in the barrel processing, the higher the burr removal capability, so in order to sufficiently remove the burrs, it is preferable to use steel balls having a diameter larger than the inner diameter of the sintered body. In this case, among the four annular burrs formed on the powder compact 18, at least the burrs formed on the upper end outer peripheral edge P2 and the lower end outer peripheral edge P4, and the burrs formed on the radially specified position P3 of the lower end face 18c can be sufficiently removed, while the burrs formed on the upper end inner peripheral edge P1 may not be sufficiently removed. This is because the steel balls used as media have a diameter larger than the inner diameter of the sintered body, so the media cannot penetrate into the inner circumference of the sintered body. Therefore, when the barrel processing is completed, there are traces (traces) of the burrs removed at the parts of the sintered body corresponding to the upper end outer peripheral edge P2, the lower end outer peripheral edge P4, and the radially specified position P3 of the lower end face 18c of the powder compact 18, while burrs may remain in the part corresponding to the upper end inner peripheral edge P1 of the powder compact 18.

[0047] [Sizing process] Although not shown in the figure, in the sizing process, the sintered body from which the burrs have been removed is sized using a sizing die having an axial sizing core, a cylindrical die, and a lower punch and an upper punch. Briefly, the sizing process is performed by compressing the sintered body in the axial direction with the upper punch and the lower punch, and pressing the inner and outer peripheral surfaces of the sintered body against the outer peripheral surface of the sizing core and the inner peripheral surface of the die, respectively, to cause plastic deformation. As a result, the inner and outer surfaces of the sintered body (the inner peripheral surface, the outer peripheral surface, both end faces, etc.) are finished into the shape of the finished product.

[0048] Even if a burr remains in the sintered body from which the burr removal process has been performed at a portion corresponding to a predetermined radial position P3 on the lower end surface 18c of the powder compact 18, the burr is crushed by the axial compression of the sintered body with the upper and lower punches of the sizing die. In contrast, a burr remaining in the sintered body at a portion corresponding to the upper inner peripheral edge portion P1 of the powder compact 18 may enter a minute gap that exists in the joint between the sizing core and the punch (upper punch or lower punch), and may not be crushed by the sizing process and may still remain.

[0049] [Oil impregnation process] In this oil impregnation process, the internal pores of the sintered body that has been finished into the shape of a finished product are impregnated with lubricating oil, for example, by vacuum impregnation or other methods. This results in a sintered bearing (oil-impregnated sintered bearing) that can be used as the bearing member 8 shown in Fig. 2. The sintered bearing obtained through the above processes has a trace of one of the annular burrs (the burr formed in the portion shown by reference symbol P3 in Fig. 4) among the annular burrs formed in the portions shown by reference symbols P1 to P4 of the green compact 18 shown in Fig. 4, present within the range of one end face, but has no annular burr or its trace on the inner peripheral edge of one end (the portion corresponding to the portion shown by reference symbol Q in Fig. 4).

[0050] The sintered bearing is fixed to the inner periphery of the housing 7 so that the bottom end surface 18c of the powder compact 18 shown in Fig. 4 is disposed on the side opposite the bottom 7b of the housing 7 (the bottom end surface 18c of the powder compact 18 forms the upper end surface 8b of the bearing member 8). Then, the shaft member 2 is inserted into the inner periphery of the bearing member 8 from the opening on the side of the upper end surface 8b of the bearing member 8, and the internal space of the housing 7 is filled with lubricating oil, thereby completing the fluid dynamic bearing device 1 shown in Fig. 2.

[0051] As described above, in the sintered bearing (bearing member 8 made of the sintered bearing) according to one embodiment of the present invention, there is no annular burr or traces thereof on the inner periphery of one end (the inner periphery of the upper end in FIG. 2), but instead there is a trace of a single burr within the range of one end face (upper end face 8b in FIG. 2). This means that, of the total of four annular burrs that are inevitably formed when the cylindrical green compact 18 is compression molded, the position of the burr that was conventionally formed on the inner periphery of one end has been changed to within the range of one end face.

[0052] In this case, it is possible to prevent problems such as the shaft member 2 coming into contact with burrs and falling off as the shaft member 2 is inserted into the inner periphery of the bearing member 8. In the fluid dynamic bearing device 1 shown in Fig. 2, the shaft member 2 inserted into the inner periphery of the bearing member 8 rotates in a precessing motion with the lower end (contact portion with the thrust plate 10) as a reference. Therefore, even if burrs remain on the inner periphery of the lower end of the bearing member 8 (the portion corresponding to the inner periphery P1 of the upper end of the powder compact 18 shown in Fig. 4), there is a low possibility that the burrs will slide against the shaft member 2. Therefore, the generation of abnormal noise and the falling off of burrs during operation of the fluid dynamic bearing device 1 are prevented as much as possible.

[0053] In the bearing member 8 made of the sintered bearing of this embodiment, whose outer surface is a molding surface formed following the sizing die, the trace of one burr present within the range of one end face (upper end face 8b in FIG. 2) can be considered to be an annular burr formed within the range of the lower end face 18c of the powder compact 18 that has been crushed by the sizing die. Therefore, according to the present invention, even if the process of removing the annular burr that is inevitably formed when the powder compact 18 is compression molded is simplified or omitted, it is possible to realize a sintered bearing that can accurately support the shaft member 2 inserted into the inner circumference in the radial direction.

[0054] Fig. 6 shows a longitudinal cross-sectional view of a powder compact 28 which is the base material of a sintered bearing according to another embodiment of the present invention. That is, a sintered bearing (sintered oil-impregnated bearing) according to another embodiment of the present invention is obtained by subjecting the powder compact 28 shown in Fig. 6 to the sintering step, removing step, sizing step and oil-impregnation step in that order, similar to those described above. The powder compact 28 shown in Fig. 6 and a method for forming it will be described in detail below.

[0055] The powder compact 28 shown in FIG. A predetermined axial position P1 on the inner circumferential surface 28a, Upper outer peripheral edge portion (connection portion between the outer peripheral surface 28d and the upper outer peripheral chamfer) P2, A predetermined radial position P3 of the lower end surface 28c (here, approximately the radial center of the lower end surface 28c), and Lower end outer peripheral edge (connection between outer peripheral surface 28d and lower end outer peripheral chamfer) P4 That is, the powder compact 28 is compression molded so that one of three annular burrs different from the annular burr formed within the range of the lower end surface 28c is formed within the range of the inner circumferential surface 28a.

[0056] The green compact 28 shown in Fig. 6 can be compression molded using a molding die apparatus 30 conceptually shown in Fig. 7(a) to (c). Similar to the molding die apparatus 20 shown in Fig. 5, the molding die apparatus 30 includes four coaxially arranged dies (a core 31, a die 32, a lower punch 33, and an upper punch 34), and the core 31, the lower punch 33, and the upper punch 34 are provided so as to be movable up and down relative to the die 32.

[0057] The molding die device 30 has a different configuration from the molding die device 20 shown in Fig. 5(a)(b) in that the entire inner peripheral surface 28a of the powder compact 28 is molded by cooperation between a shaft portion 34a of an upper punch 34 and (shaft portion 31a) of a core 31, which are arranged coaxially in a vertical line. That is, the upper punch 34 integrally includes a shaft portion 34a, the outer peripheral surface of which molds a partial axial region (upper region) of the inner peripheral surface 28a of the powder compact 28, and a base portion 34b, the lower end surface of which molds the upper end surface 28b of the powder compact 28. The core 31 integrally includes a shaft portion 31a, which molds the remaining axial region (lower region) of the inner peripheral surface 28a of the powder compact 28, and a base portion 31b having a shoulder surface 31d which molds an inner diameter side region of the lower end surface 28c of the powder compact 28. The outer circumferential surface of the shaft portion 34a of the upper punch 34 which forms the inner circumferential surface 28a of the powder compact 28, and the outer circumferential surface 31c of the shaft portion 31a of the core 31 are both formed into smooth cylindrical surfaces.

[0058] The inner peripheral surface 32a of the die 32 which forms the outer peripheral surface 28d of the powder compact 28 is formed into a smooth cylindrical surface. In addition, the shoulder surface 31d of the core 31 which forms the inner diameter side region of the lower end surface 28c of the powder compact 28, and the upper end surface 33a of the lower punch 33 which forms the outer diameter side region of the lower end surface 28c of the powder compact 28 are formed into smooth flat surfaces. The upper punch 34 is integrally provided with a molding portion for molding the upper end inner peripheral chamfer and the upper end outer peripheral chamfer of the powder compact 28, the lower punch 33 is integrally provided with a molding portion for molding the lower end outer peripheral chamfer of the powder compact 28, and the core 31 is integrally provided with a molding portion for molding the lower end inner peripheral chamfer of the powder compact 28.

[0059] In the molding die apparatus 30 having the above-mentioned configuration, first, as shown in FIG. 7(a), the upper end surface of the shaft portion 31a of the core 31 is placed on the same plane as the upper end surface of the die 32, and a powder filling section 35 is defined by the outer peripheral surface 31c of the shaft portion 31a of the core 31, the shoulder surface 31d and outer peripheral surface of the base portion 31b of the core 31, the inner peripheral surface 32a of the die 32, and the upper end surface 33a of the lower punch 33, and this powder filling section 35 is filled with the same raw material powder M as described above.

[0060] After the raw material powder M is filled in the powder filling section 35, as shown in FIG. 7(b), the upper punch 34 is moved relatively close to the core 31 and the like (the upper punch 34 is moved downward), and the lower end surface of the shaft portion 34a of the upper punch 34 is brought into contact with the upper end surface of the shaft portion 31a of the core 31. Then, while maintaining this contact state, the core 31 and the upper punch 34 are moved downward together until the shoulder surface 31d of the core 31 is positioned on the same plane as the upper end surface 33a of the lower punch 33, as shown in FIG. 7(c). Accordingly, the raw material powder M filled in the powder filling section 35 is compressed, and a cylindrical green compact 28 is formed. Thereafter, for example, the core 31, the lower punch 33, and the upper punch 34 are moved upward together to discharge the green compact 28 to the upper side of the die 32, and then the upper punch 34 is further moved upward and the core 31 is moved downward to release the green compact 28.

[0061] This results in a green compact 28 as shown in Figure 6, in which annular burrs are formed at locations corresponding to the mating portions between the dies (more specifically, the mating portion between the shaft portion 31a of the core 31 and the shaft portion 34a of the upper punch 34, the mating portion between the die 32 and the upper punch 34, the mating portion between the base portion 31b of the core 31 and the lower punch 33, and the mating portion between the die 32 and the lower punch 33), i.e., at a predetermined axial position P1 on the inner circumferential surface 28a, the upper outer peripheral edge portion P2, a predetermined radial position P3 on the lower end face 18c, and the lower outer peripheral edge portion P4.

[0062] The green compact 28 is then put into a sintering process to become a sintered body, similar to the green compact 18 shown in FIG. 4, and the sintered body is subjected to barrel processing to remove burrs in a removal process. The barrel processing is performed using media having a particle size (diameter) larger than the inner diameter of the sintered body, similar to the above. In this case, among the four annular burrs formed in the green compact 28, the burrs formed on the upper outer peripheral edge P2 and the lower outer peripheral edge P4, and the burrs formed at the radially specific position P3 of the lower end face 28c can be sufficiently removed, while the burrs formed at the axially specific position P1 of the inner peripheral surface 28a cannot be basically removed. Therefore, when the barrel processing is completed, in the sintered body obtained by sintering the green compact 28, there are traces (traces) of the burrs removed at the portions corresponding to the upper outer peripheral edge P2, the lower outer peripheral edge P4, and the radially specific position P3 of the lower end face 28c of the green compact 28, while the burrs remain on the inner peripheral surface. However, burrs remaining on the inner peripheral surface are crushed by being pressed against the outer peripheral surface of the sizing core when the sintered compact is subjected to the sizing process similar to that described above.

[0063] Therefore, when a sintered bearing (oil-impregnated sintered bearing) that can be used as the bearing member 8 shown in Figure 2 is obtained by impregnating the internal pores of the sintered body that has been finished into the finished shape in the sizing process with lubricating oil, this sintered bearing will have a trace of one burr (formed in the part indicated by symbol P3) of the total of four annular burrs formed in the parts indicated by symbols P1 to P4 of the compact 28 shown in Figure 6, present within the range of one end face, and a trace of one burr (formed in the part indicated by symbol P1) of the three other burrs (formed in the parts indicated by symbols P1, P2, and P4) present within the range of the inner surface, and there are no annular burrs or their traces on the inner peripheral edge of one end or the other end.

[0064] In this case, the traces of the burrs present within the range of the inner peripheral surface can be formed by crushing the burrs formed on the inner peripheral surface of the powder compact with a sizing die (sizing core) as described above, so that a removal process for removing the burrs formed on the inner peripheral surface of the powder compact is not required. Furthermore, when this sintered bearing is used as the bearing member 8 of the fluid dynamic bearing device 1 shown in FIG. 2, there is no possibility that the shaft member 2 will come into contact with the burrs even if the shaft member 2 is inserted into the inner peripheral surface of the bearing member 8 from either one end side or the other end side, and furthermore, there is no possibility that the shaft member 2 will come into contact with the burrs when the shaft member 2 rotates. Therefore, there is no need to consider the attitude (up and down) of the bearing member 8 when assembling the bearing member 8 into the fluid dynamic bearing device 1, which not only improves the ease of assembly of the fluid dynamic bearing device 1 but also stabilizes the bearing performance.

[0065] Although the sintered bearing according to the embodiment of the present invention and the fluid dynamic bearing device 1 using the sintered bearing as the bearing member 8 have been described above, the embodiment of the present invention is not limited to this.

[0066] For example, in the embodiment described above, the present invention is applied to the bearing member 8 consisting of a sintered bearing whose inner surface 8a (radial bearing surface) is formed into a smooth cylindrical surface, but the present invention can also be applied without problem to a sintered bearing in which a dynamic pressure generating portion such as a dynamic pressure groove is provided on the radial bearing surface.

[0067] Furthermore, in a fluid dynamic bearing device 1 that uses the sintered bearing according to an embodiment of the present invention as the bearing member 8, the thrust bearing portion T that supports the shaft member 2 in the thrust direction can also be configured with a so-called dynamic pressure bearing. Such a configuration can be realized, for example, by forming the lower end face of the shaft member 2 into a flat surface in a direction perpendicular to the axial direction, and providing a dynamic pressure generating portion such as a dynamic pressure groove on the lower end face of the shaft member 2 or on the inner bottom surface 7b1 of the housing 7 that faces said lower end face. [Explanation of symbols]

[0068] 1. Fluid dynamic bearing device 2 Shaft member 8 Bearing materials 18 Powder Compacts 20 Compression molding die 21 cores 21c Outer surface 21d shoulder surface 22 Die 23 Lower Punch 24 Upper Punch 28 Powder Compacts 30 Compression molding die 31 cores 31c Outer surface 31d shoulder surface 32 Die 33 Lower Punch 34 Upper Punch 34a Shaft

Claims

1. A sintered bearing is made of a cylindrical sintered body obtained by sintering a compact of raw material powder made mainly of metal powder, the inner circumferential surface being provided with a radial bearing surface which forms a radial bearing gap between the inner circumferential surface of the shaft to be supported, and the outer surface and the inner circumferential surface are formed as molded surfaces following a sizing die, Of the four annular burrs formed in the course of compression molding the green compact, the trace of one burr is present within the range of one end face, and no annular burr or its trace is present on the inner peripheral edge of one end, A sintered bearing characterized in that, of the three burrs different from the one burr, a trace of one burr is present within the range of the inner circumferential surface, and no annular burr or trace thereof is present on the inner circumferential edge portion of the other end.

2. A sintered bearing comprising a cylindrical sintered body obtained by sintering a compact of raw powder made mainly of metal powder, a radial bearing surface is provided on the inner peripheral surface thereof which forms a radial bearing gap between the inner peripheral surface of the shaft to be supported, and the outer surface and the inner peripheral surface are formed as molded surfaces following a sizing die, Of the four annular burrs formed in the course of compression molding the green compact, the trace of one burr is present within the range of one end face, and no annular burr or its trace is present on the inner peripheral edge of one end, A sintered bearing characterized in that the total of four burrs include a burr formed on one outer peripheral edge portion and a burr formed on the other outer peripheral edge portion, and at least these two burrs have been subjected to removal processing.

3. 3. The sintered bearing according to claim 1, wherein the inner diameter is φ2 mm or less.

4. A bearing member comprising the sintered bearing according to any one of claims 1 to 3; a cylindrical housing having a bottom and accommodating the bearing member in its inner periphery; a radial bearing portion that supports the shaft in a non-contact manner in a radial direction by an oil film formed in the radial bearing gap; a thrust bearing portion that supports one end of the shaft in a thrust direction on a bottom side of the housing; A fluid dynamic bearing device comprising:

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