Sintered oil-impregnated bearings, hydrodynamic bearing assemblies incorporating such bearings, and motors

Abstract

In this invention, a lubricating fluid is impregnated in the internal cavity of a sintered metal bearing 8, and a radial dynamic pressure generating part 8a1 is formed on the inner circumferential surface 8a. The axial dimension L is 4.8 mm or less, the thickness t is 0.5 mm or more and 1.5 mm or less, and the ratio of the axial dimension L to the outer diameter D2, L / D2, is 0.24 or more and 0.6 or less.

Description

Technical Field

[0001] The present invention relates to a sintered oil-impregnated bearing and a hydrodynamic bearing device having the bearing, and more particularly to a sintered oil-impregnated bearing including a radial hydrodynamic generating section and a hydrodynamic bearing device having the bearing. Prior Technology

[0002] Sintered oil-impregnated bearings are bearings in which lubricating oil is impregnated within the internal cavities of a porous body formed by sintered metal. As the shaft portion inserted into the inner circumference rotates relative to the bearing, the lubricating oil impregnated within the internal cavities seeps out to the sliding portion with the shaft portion, forming an oil film. This oil film then rotates to support the shaft portion. Due to their excellent rotational accuracy and quiet operation, these bearings are suitable for use as bearing assemblies for motors in various electrical machines, including information machines. More specifically, they are suitable for use as shaft motors in disc drives for hard disk drives (HDDs), compact discs (CDs), digital versatile discs (DVDs), and Blu-ray discs; fan motors in these disc drives or personal computers (PCs); or polygon scanner motors in laser beam printers (LBPs).

[0003] Furthermore, to further improve quietness and extend lifespan, dynamic pressure generating portions, such as dynamic pressure grooves, are sometimes formed on at least one of the inner circumferential surface and the axial end face of the sintered oil-impregnated bearing. Here, as a method for forming dynamic pressure grooves, so-called dynamic pressure groove finishing is known. This finishing, for example, utilizes an upper punch and a lower punch to press the sintered body of the bearing in the axial direction, simultaneously pressing the sintered body into the inner circumference of a die head, thereby causing a forming die with a finishing pin pre-inserted into the inner circumference of the sintered body to grip the sintered body. In this way, the shape of the forming die, i.e., the shape corresponding to the dynamic pressure groove, is transferred onto the inner circumferential surface of the sintered body, thereby forming a dynamic pressure groove as a radial dynamic pressure generating portion (see, for example, Patent Document 1).

[0004] Furthermore, recently, with the miniaturization and thinning of information machines, there is also a demand for miniaturization of various motors mounted on these machines. For example, the cooling fan motors used in laptops are becoming thinner, and the bearing assemblies used in these motors are also required to be thinner. On the other hand, cooling performance is required to be at the same or higher level as before. The thinning of the bearing assembly results in a reduction in the bearing area. Therefore, when the size of the impeller (fan) is increased, and even when the impeller size is maintained, the concern that the reduced bearing area will lead to a decrease in bearing rigidity also increases.

[0005] Here, Patent Document 2 describes a sintered oil-impregnated bearing, wherein the sintered oil-impregnated bearing has a hydrodynamic groove arrangement area on its inner circumferential surface, the axial dimension of which is 6 mm or less, and the overall density ratio of the bearing is 80% or more and 95% or less. Furthermore, it states that, according to the bearing with this structure, compared to conventional bearings, the overall density ratio of the bearing is made more uniform; therefore, even if the lubricating fluid storage portion (recess) between the hydrodynamic groove arrangement areas that existed in the past is omitted and the axial dimension is reduced, sufficient bearing rigidity can be expected. [Existing technical documents] [Patent Literature]

[0006] Patent Document 1: Japanese Patent No. 3607492 Patent Document 2: Japanese Patent Application Publication No. 2015-64019 Summary of the Invention

[0007] [The problem that the invention aims to solve]

[0008] Furthermore, recent advancements in the miniaturization of cooling fan motors used in laptops and similar devices necessitate further reductions in the axial dimensions of the bearings used in these motors. However, a smaller axial dimension reduces the area available for generating radial dynamic pressure, consequently decreasing the dynamic pressure generated by the radial bearing clearance. This makes it difficult to precisely support the shaft (such as an impeller), potentially leading to reduced rotational accuracy (Non-Repeatable Run-Out, NRRO). For example, as described in Patent Document 2, while adding thrust dynamic pressure grooves to both ends of the bearing can improve bearing rigidity and thus rotational accuracy, it prevents the assembly of the hydrodynamic bearing device from being completed simply by inserting the shaft of the rotating body into the bearing's inner circumference. Additional fixtures or steps are required, potentially increasing assembly costs.

[0009] In view of the above actual situation, the technical problem to be solved by the present invention is to achieve thin-walled and high-rotational-precision fluid dynamic bearing devices without increasing assembly costs. [Methods for solving problems]

[0010] The aforementioned problem is solved by the sintered oil-impregnated bearing of the present invention. Specifically, this bearing is a sintered metal bearing obtained by compressing metal powder into a cylindrical shape to form a pressurized powder, and then sintering the formed pressurized powder. It is a sintered oil-impregnated bearing with lubricating fluid impregnated in its internal pores and a radial dynamic pressure generating portion formed on its inner circumferential surface. Its characteristics include: an axial dimension L of 4.8 mm or less, a thickness t of 0.5 mm or more and 1.5 mm or less, and a ratio of axial dimension L to outer diameter D2, L / D2, of 0.24 or more and 0.6 or less.

[0011] The inventors have discovered that, during the finishing of the aforementioned dynamic pressure groove, even when the axial dimension L of the sintered metal bearing is smaller than before (set to 4.8 mm or less), by setting the thickness t of the bearing and the ratio L / D2 of the axial dimension L to the outer diameter D2 within specific numerical ranges, the pressing force for forming the cylindrical sintered body can be sufficiently transmitted to the surface of the inner circumferential surface of the bearing in the form of a radially directed pressure force. The present invention was made in view of the above insights, whereby, when the axial dimension L of the sintered oil-impregnated bearing is reduced compared to the past (set to 4.8 mm or less), the thickness t is set to a range of 0.5 mm to 1.5 mm, and compared to the past, the bearing is made into a significantly flatter shape (i.e., the ratio L / D2 of the axial dimension L to the outer diameter D2 is set to a value significantly less than 1: 0.24 to 0.6). By using a bearing of this shape and size, for example during the finishing of the aforementioned hydrodynamic groove, regardless of the size of the axial dimension L, the radial pressure generated by pressing into the die head can be sufficiently transmitted to the inner circumferential surface of the sintered body that forms the bearing. Therefore, even when the axial dimension L is set smaller than before, the radial hydrodynamic generation part (typically a groove shape) can be accurately and stably formed, and the required radial bearing performance can be stably achieved. Furthermore, by setting the thickness t to 0.5 mm or more, the required thrust bearing area on the axial side of the bearing can be ensured. Therefore, even when the thrust bearing part is only provided on the axial side of the bearing, excellent rotational accuracy can be achieved. Thus, for example, by simply inserting the shaft of the rotating body into the inner circumference of the sintered oil-impregnated bearing, the assembly of a hydrodynamic bearing device equipped with this bearing can be carried out at low cost.

[0012] Furthermore, in the sintered oil-impregnated bearing of the present invention, the density ratio can be 86% or more and 92% or less. Moreover, the term "density ratio" here refers to the percentage obtained by dividing the density of the porous material forming the sintered oil-impregnated bearing by the density assuming that there are no pores in the porous material.

[0013] As described above, by setting the density ratio of the sintered oil-impregnated bearing to 86% or more, especially during dynamic pressure groove finishing, the pressing force toward the die head is sufficiently transmitted to the surface of the bearing's inner circumference as a radial pressure force. Therefore, a dynamic pressure groove of sufficient depth can be transferred onto the bearing's inner circumference. Furthermore, by suppressing the density ratio to 92% or less, excessive load on the finishing die can be suppressed, thus enabling stable and continuous good dynamic pressure groove finishing. In particular, by setting the bearing to the aforementioned axial dimension L and flat shape, and setting the density ratio within the aforementioned range, the unevenness of the density ratio within the sintered body of the bearing can be further reduced (making it 1% or less), thus enabling stable high rotational performance. Specifically, the density ratio of the surface layer can be set to a degree that prevents dynamic pressure from shifting from the bearing surface and allows lubricating oil and other substances to seep into the bearing clearance appropriately and evenly. Furthermore, the density ratio of the area excluding the surface layer (the area further inside the surface layer) can be set in such a way that the amount of lubricating oil or the like in the inner region is appropriate and uniform.

[0014] Furthermore, in the sintered oil-impregnated bearing of the present invention, the ratio of the outer diameter dimension D2 to the inner diameter dimension D1, D2 / D1, can be 1.5 or more and 3.0 or less.

[0015] As described above, by setting the ratio of the outer diameter dimension D2 to the inner diameter dimension D1, D2 / D1, to be 1.5 or more and 3.0 or less, the gripping ability of the finishing pin on the inner circumferential surface can be ensured, thereby ensuring the formability of the dynamic pressure groove, etc. Furthermore, by determining the ratio of the inner diameter dimension D1 to the outer diameter dimension D2, D2 / D1, according to the appropriate numerical range of the thickness dimension t, the radial pressure force generated by the die pressing can be applied to the inner circumferential surface layer of the sintered body more effectively.

[0016] Furthermore, in the sintered oil-impregnated bearing of the present invention, the inner diameter dimension D1 can be 1.5 mm or more and 2.0 mm or less.

[0017] As described above, by setting the inner diameter dimension D1 to 1.5 mm or more, the gripping ability of the finishing pin on the inner circumferential surface can be ensured, thereby ensuring the formability of the dynamic pressure groove, etc. Furthermore, by determining the appropriate range of the inner diameter dimension D1 based on the appropriate range of the thickness dimension t, sufficient springback can be generated in the sintered body (porous body). Therefore, even dynamic pressure grooves of a corresponding depth (approximately 1 mm to 4 mm) can be reliably formed on the inner circumferential surface. Moreover, by suppressing the inner diameter dimension D1 to 2.0 mm or less, the situation where the outer diameter dimension D2 and thus the thickness dimension t increase to a necessary level can be avoided, and the radial pressure force generated by pressing into the die head can be reliably transmitted to the surface layer of the inner circumferential surface.

[0018] Furthermore, in the sintered oil-impregnated bearing of the present invention, a thrust dynamic pressure generating part can be formed on one end face in the axial direction, and the end face on the other end face in the axial direction can be formed into a flat shape.

[0019] As described above, the sintered oil-impregnated bearing according to the present invention can simultaneously achieve thin-walled design and high rotational precision in hydrodynamic bearing devices. Therefore, even when the thrust dynamic pressure generating portion is formed only on one end face in the axial direction, sufficient bearing rigidity can be achieved. Thus, for example, the assembly of the shaft portion (rotating body) to the bearing can be completed simply by inserting the shaft portion of the rotating body into the inner circumference of the bearing, enabling the assembly of hydrodynamic bearing devices at low cost.

[0020] Furthermore, the sintered oil-impregnated bearing described above can be suitably provided as a hydrodynamic bearing device, which for example includes: the sintered oil-impregnated bearing; a housing for fixing the sintered oil-impregnated bearing on its inner circumference; a rotating body including a shaft portion inserted into the inner circumference of the sintered oil-impregnated bearing; and a radial bearing portion for non-contact support of the shaft portion by a film of lubricating fluid along the radial direction, wherein the film of lubricating fluid is formed in the radial bearing clearance between the inner circumferential surface of the sintered oil-impregnated bearing and the outer circumferential surface of the shaft portion by the hydrodynamic action of the radial hydrodynamic generating portion.

[0021] Furthermore, the hydrodynamic bearing device of the present invention may further include: a first thrust bearing portion, which non-contactly supports the shaft portion with a film of lubricating fluid along the thrust direction, the film of lubricating fluid being formed in the thrust bearing gap between the end face of the rotating body on the other side of the axial direction opposite to the thrust direction and the end face of the sintered oil-impregnated bearing on one side of the axial direction.

[0022] Furthermore, in this case, the hydrodynamic bearing device of the present invention may provide a thrust dynamic pressure generating part on the end face of the sintered oil-impregnated bearing on the axial direction side, which is opposite to the disc portion of the hub portion, which is a rotating body, in the thrust direction.

[0023] Furthermore, the hydrodynamic bearing device of the present invention may further include: a second thrust bearing portion, wherein one end face of the housing in the axial direction contacts the end face of the support shaft portion in the axial direction on the other side along the thrust direction.

[0024] As described above, by providing a first thrust bearing portion between the rotating body and the sintered oil-impregnated bearing, which are opposite each other in the thrust direction, and a second thrust bearing portion between the shaft portion and the housing, which are also opposite each other in the thrust direction, the sintered oil-impregnated bearing can be made into the simplest possible shape, and sufficient bearing rigidity in the thrust direction can be achieved. Furthermore, by providing a thrust dynamic pressure generating portion on the axial side end face of the sintered oil-impregnated bearing, which is opposite the disc portion of the hub portion (which is the rotating body) in the thrust direction, the thrust dynamic pressure generating portion can be formed with high precision. This allows for the achievement of very excellent thrust bearing performance.

[0025] As described above, the hydrodynamic bearing device described above can achieve thin-walled design and high rotational accuracy without increasing assembly costs, and therefore can be suitably provided, for example, as a motor equipped with the hydrodynamic bearing device. [The effects of the invention]

[0026] Based on the above, the present invention enables the thinning of the wall and the high rotational precision of the hydrodynamic bearing device without increasing assembly costs. Simple Explanation of the Diagram

[0027] Figure 1 is a cross-sectional view of a fan motor according to an embodiment of the present invention. Figure 2 is a cross-sectional view of the hydrodynamic bearing device shown in Figure 1. Figure 3 is a cross-sectional view of the sintered oil-impregnated bearing shown in Figure 2. Figure 4 is a plan view of the sintered oil-impregnated bearing shown in Figure 2. Figure 5 is a bottom view of the sintered oil-impregnated bearing shown in Figure 2. Figure 6 is a cross-sectional view of a hydrodynamic bearing device according to another embodiment of the present invention. Implementation

[0028] Hereinafter, one embodiment of the present invention will be described based on the drawings. Furthermore, in the following description, viewed from the perspective of the sintered oil-impregnated bearing, the disc portion side of the hub is designated as the "upper side," and the bottom side of the outer casing is designated as the "lower side." Of course, this vertical direction does not limit the actual arrangement or usage of the product.

[0029] Figure 1 conceptually illustrates a structural example of a fan motor 1 according to this embodiment. The fan motor 1 includes a hydrodynamic bearing assembly 2, a plurality of fans 4 disposed on a rotating body 3 of the hydrodynamic bearing assembly 2, and a drive unit 5 for rotating the fans 4 integrally with the rotating body 3. The drive unit 5 includes, for example, a coil 5a and a magnet 5b facing each other with a radial gap. In this embodiment, the coil 5a is fixed to a base 6, which serves as the fixed side of the fan motor 1, and the magnet 5b is fixed to the rotating body 3, which serves as the rotating side of the fan motor 1.

[0030] In the fan motor 1 of the aforementioned structure, when coil 5a is energized, the magnetic force between coil 5a and magnet 5b causes magnet 5b to rotate. This rotation causes multiple fans 4, erected on the outer periphery of the rotating body 3, to rotate integrally with the rotating body 3. Through this rotation, each fan 4 generates an airflow in a direction corresponding to its shape (here, for example, an airflow directed radially outward), and a secondary airflow is generated from the upper axial direction of the fan motor 1 downward in the form of being drawn into this airflow. By generating airflow around the fan motor 1 in the above manner, the information machine (not shown) on which the fan motor 1 is mounted can be cooled.

[0031] Furthermore, as described above, if airflow is generated along the axial direction of the fan motor 1, the rotating body 3 of the hydrodynamic bearing device 2 generates a force (reaction force) in the opposite direction to the airflow. A magnetic force (repulsive force) in the direction that counteracts this reaction force acts between the coil 5a and the magnet 5b, and the thrust load generated by the difference in magnitude between the reaction force and the magnetic force acts on the thrust bearing section T1 and thrust bearing section T2 of the hydrodynamic bearing device 2 (see Figure 2 below). The magnetic force in the direction that counteracts the reaction force can be generated, for example, by offsetting the coil 5a and the magnet 5b along the axial direction (detailed illustration omitted). Also, when the rotating body 3 rotates, a radial load acts on the shaft section 10 of the hydrodynamic bearing device 2 described below. This radial load acts on the radial bearing section R1 and radial bearing section R2 of the hydrodynamic bearing device 2.

[0032] Figure 2 shows a cross-sectional view of the hydrodynamic bearing assembly 2 assembled in the fan motor 1. The hydrodynamic bearing assembly 2 includes a housing 7, a sintered oil-impregnated bearing 8 fixed to the inner circumference of the housing 7, and a rotating body 3 that rotates relative to the sintered oil-impregnated bearing 8.

[0033] The rotating body 3 includes a hub portion 9 disposed on the upper opening side of the housing 7 and a shaft portion 10 inserted into the inner circumference of the sintered oil-impregnated bearing 8.

[0034] As shown in Figures 1 and 2, the hub portion 9 includes: a disc portion 9a covering the upper opening side of the outer casing 7; a first cylindrical portion 9b extending downward in the axial direction from the disc portion 9a; a second cylindrical portion 9c located radially outward from the first cylindrical portion 9b, extending downward in the axial direction from the disc portion 9a; and a flange portion 9d extending further radially outward from the lower axial end of the second cylindrical portion 9c. The disc portion 9a faces one end face (upper end face 8b) of the sintered oil-impregnated bearing 8 fixed to the inner circumference of the outer casing 7. Furthermore, a plurality of fans 4 are integrally provided with the hub portion 9 in a form that stands upright from the outer periphery of the flange portion 9d.

[0035] In this embodiment, the shaft portion 10 and the hub portion 9 are formed separately, and the upper end of the shaft portion 10 is fixed to the mounting hole 9e provided in the hub portion 9. Here, the shaft portion 10 includes an outer peripheral surface 10a with a fixed outer diameter, and a lower end portion 10b that is continuous with the lower end of the outer peripheral surface 10a, for example, partially spherical. That is, the shaft portion 10 is shaped to allow insertion into the inner circumference of the sintered oil-impregnated bearing 8 from one side in the axial direction. Of course, the shaft portion 10 and the hub portion 9 can also be integrally formed from the same material. Alternatively, one of the shaft portion 10 and the hub portion 9, which are formed from different materials, can be used as an insertion assembly, while the other can be formed by injection molding of metal or resin.

[0036] The outer casing 7 is shaped to have an open upper end and a closed lower end. Furthermore, a sintered oil-impregnated bearing 8 is fixed to the inner circumferential surface 7a of the outer casing 7, and the outer circumferential surface 7b of the outer casing 7 is fixed to the base 6 (see Figure 1). The axial distance between the upper end surface 7c of the outer casing 7 and the lower end surface 9a1 of the disc portion 9a of the hub portion 9 is greater than the axial distance between the upper end surface 8b of the sintered oil-impregnated bearing 8 and the lower end surface 9a1 of the disc portion 9a; this distance is set to a level that is considered not to substantially affect the increase in loss torque during rotary drive.

[0037] A tapered sealing surface 7d is formed on the upper outer periphery of the outer casing 7, with its outer diameter increasing upwards. This tapered sealing surface 7d forms an annular sealing space S between itself and the inner peripheral surface 9b1 of the first cylindrical portion 9b, with the space gradually decreasing in diameter from the closed side (lower side) to the open side (upper side) of the outer casing 7. When the shaft portion 10 and the hub portion 9 rotate, this sealing space S communicates with the outer diameter side of the thrust bearing clearance of the first thrust bearing portion T1 (described below), allowing lubricating oil to circulate between it and the internal space of the bearing, including each bearing clearance. Furthermore, while lubricating oil is filled into the internal cavities of the sintered oil-impregnated bearing 8 and the internal space of the bearing, the amount of lubricating oil is adjusted (see Figure 2) to maintain the oil level (gas-liquid interface) of the lubricating oil within the sealing space S.

[0038] Furthermore, when the housing 7 employs the sealing structure described above, the coil 5a is located radially outside the first cylindrical portion 9b of the hub portion 9, and the first cylindrical portion 9b and the coil 5a are arranged in a manner that partially repeats each other in the axial direction. This achieves a thinner wall (reduction in axial dimension) for the housing 7 and consequently the hydrodynamic bearing device 2.

[0039] Furthermore, in this embodiment, a thrust receiving portion 11 is provided at the bottom 7e of the outer casing 7 to receive the lower end 10b of the spherical shaft portion 10. That is, the thrust receiving portion 11 is always in contact with the lower end 10b of the shaft portion 10 when the hydrodynamic bearing device 2 is in its completed state, and can rotatably support the shaft portion 10. Moreover, the vertical position of the thrust receiving portion 11 relative to the bearing contact surface 7f of the outer casing 7 can be set such that the contact position between the shaft portion 10 and the thrust receiving portion 11 is located in the vertical direction region of, for example, the inner chamfered portion 8e of the sintered oil-impregnated bearing 8.

[0040] The material and composition of the outer casing 7 are arbitrary in principle. For example, known materials such as resin and metal can be appropriately used according to the method of fixing the sintered oil-impregnated bearing 8 to the outer casing 7 as described below.

[0041] The sintered oil-impregnated bearing 8 is a porous body of sintered metal formed by compressing and sintering specific raw material powder, and is formed into a cylindrical shape. In this embodiment, as shown in FIG4, a cylindrical shape is formed. On the entire surface or part of the inner circumferential surface 8a of the sintered oil-impregnated bearing 8, a region is formed with multiple dynamic pressure grooves 8a1 arranged as radial dynamic pressure generating parts. In this embodiment, as shown in FIG3, the arrangement region of dynamic pressure grooves 8a1 is formed by arranging multiple dynamic pressure grooves 8a1 inclined at a specific angle relative to the circumferential direction, inclined mounds 8a2 dividing these dynamic pressure grooves 8a1 along the circumferential direction, and bands 8a3 extending along the circumferential direction and dividing each dynamic pressure groove 8a1 along the axial direction (the inclined mounds 8a2 and bands 8a3 are the parts marked with crosshairs in FIG3) in a herringbone shape, and is continuously formed in two places along the axial direction. In this case, the upper dynamic pressure groove arrangement area A1 and the lower dynamic pressure groove arrangement area A2 are both formed symmetrically along the axial direction with respect to the axial direction center line (an imaginary line connecting the axial center of the belt portion 8a3 along the circumferential direction), and their axial direction dimensions are equal to each other.

[0042] A region is formed on the entire surface or a portion of the upper end face 8b of the sintered oil-impregnated bearing 8, in which multiple dynamic pressure grooves 8b1, serving as thrust dynamic pressure generating parts, are arranged. In this embodiment, for example, as shown in FIG4, a region is formed in which multiple dynamic pressure grooves 8b1 extending in a spiral shape are arranged along the circumferential direction. At this time, the spiral orientation of the dynamic pressure grooves 8b1 is set to the orientation corresponding to the rotation direction of the rotating body 3. When the dynamic pressure groove 8b1 arrangement region of the structure is driven to rotate the fluid dynamic pressure bearing device 2 shown in FIG2, a thrust bearing clearance of the first thrust bearing part T1, as described below, is formed between the lower end face 9a1 of the disc part 9a of the opposing hub part 9.

[0043] No thrust dynamic pressure generating portion is formed on the lower end face 8c of the sintered oil-impregnated bearing 8. That is, in this embodiment, as shown in FIG5, the lower end face 8c has a flat shape. The lower end face 8c abuts against the housing 7 at a position further outward in the radial direction than the thrust receiving portion 11. In this case, the vertical position of the bearing abutment surface 7f of the housing 7 can be appropriately set within a range that allows it to function as a thrust bearing clearance between the upper end face 8b of the sintered oil-impregnated bearing 8 and the lower end face 9a1 of the hub portion 9.

[0044] One or more axial grooves 8d1 (five in this embodiment) are formed on the outer peripheral surface 8d of the sintered oil-impregnated bearing 8 (see Figure 4 for example). When the sintered oil-impregnated bearing 8 is fixed to the housing 7, the axial groove 8d1 forms a flow path for lubricating oil between itself and the inner peripheral surface 7a of the housing 7 (see Figure 2).

[0045] Next, various dimensions of the sintered oil-impregnated bearing 8 will be explained with reference to FIG3. From the viewpoint of reducing the thickness of the hydrodynamic bearing device 2 and thus the fan motor 1, the axial dimension L (the axial distance between the two end faces 8b and 8c) of the sintered oil-impregnated bearing 8 is set to 4.8 mm or less, preferably 3.0 mm or less, and more preferably 1.8 mm or less. On the other hand, from the viewpoint of ensuring the required radial bearing rigidity, the axial dimension L is set to 0.8 mm or more, preferably 1.1 mm or more.

[0046] As long as the thickness t of the sintered oil-impregnated bearing 8 is set within the range described below, the inner diameter D1 of the sintered oil-impregnated bearing 8 (more precisely, the inner diameter of the strip portion 8a3, which together with the inclined mound portion 8a2 forms the smallest diameter portion in the inner circumferential surface 8a) can be arbitrary. From the viewpoint of ensuring the gripping property of the finishing pin on the inner circumferential surface 8a during dynamic pressure groove finishing, it is ideally 1.2 mm or more, and more ideally 1.5 mm or more. On the other hand, from the viewpoint of avoiding the situation where it becomes difficult to reliably transmit the pressing force during dynamic pressure groove finishing to the surface portion of the inner circumferential surface 8a due to the thickness t increasing to a necessary level, the inner diameter D1 is ideally 2.5 mm or less, and more ideally 2.0 mm or less.

[0047] As long as the ratio of the thickness dimension t and the axial dimension L of the sintered oil-impregnated bearing 8 to the outer diameter dimension D2 is set within the range described below, the outer diameter dimension D2 of the sintered oil-impregnated bearing 8 can be arbitrary. Ideally, it should be 2.5 mm or more, and more ideally, 3.0 mm or more, while also considering the required inner diameter dimension D1 and thickness dimension t. Similarly, ideally, the outer diameter dimension D2 should be 5.0 mm or less, and more ideally, 4.5 mm or less.

[0048] From the viewpoint of ensuring the gripping property of the finishing pin on the inner circumferential surface 8a, and thus ensuring the formability of the dynamic pressure groove 8a1, the ratio of the outer diameter dimension D2 to the inner diameter dimension D1, D2 / D1, should preferably be set to 1.5 or more and 3.0 or less.

[0049] For example, from the viewpoint of ensuring the required thrust bearing area on the upper end face 8b, the thickness t{=(D2-D1) / 2} of the sintered oil-impregnated bearing 8 should preferably be set to 0.5 mm or more. On the other hand, from the viewpoint of being able to transmit sufficient pressure from the die head to the inner circumferential surface of the sintered body during dynamic pressure groove finishing, the thickness t should preferably be set to 1.5 mm or less.

[0050] Furthermore, from the viewpoint of the efficiency of the transmission of radial pressure force to the inner circumferential surface of the sintered body during dynamic pressure groove finishing, the ratio of axial dimension L to outer diameter dimension D2, L / D2, is set to be 0.24 or more and 0.6 or less.

[0051] Next, the composition of the sintered oil-impregnated bearing 8 will be explained. This sintered oil-impregnated bearing 8 is obtained by compressing and sintering a raw material powder containing, but not most, one of copper-based powder and one of iron-based powder, and second most of the other. In other words, the porous body of the sintered oil-impregnated bearing 8 essentially forms a composition with one of copper and one of iron as the main component, and the other as the second component (the second most abundant component). Furthermore, the copper-based powder referred to here includes not only pure copper powder but also copper alloy powder. Also, pure copper includes not only copper with 100% purity but also copper that is industrially considered to be 99.99% or more pure. Similarly, the iron-based powder referred to here includes not only pure iron powder but also iron alloy powders such as stainless steel. Also, pure iron includes not only iron with 100% purity but also iron that is industrially considered to be 99.99% or more pure. As long as the aforementioned composition (powder mixing ratio) is met, the type and mixing ratio of the powder that becomes the third most abundant component or less are arbitrary.

[0052] As a composition of raw material powder (powder mixing ratio), for example, [copper-based powder: 50%~70% by weight, iron-based powder: 30%~48% by weight, tin powder: 0%~5%] can be used. As a specific example, [pure iron powder below 140 mesh: 38%~42% by weight, tin powder below 330 mesh: 1%~3%, pure copper powder below 200 mesh: the remainder].

[0053] Next, the density ratio of the sintered oil-impregnated bearing 8 will be explained. The overall density ratio of the sintered oil-impregnated bearing 8 is set to be 80% or more and 95% or less, preferably 86% or more and 92% or less. The density ratio can be set by, for example, adjusting the material, particle size (distribution), and mixing ratio of the metal powder used as raw material. Furthermore, the inconsistency of the density ratio can be evaluated using porosity, which is considered to have a certain correlation with the density ratio. Here, porosity is expressed as the volume ratio (percentage) of pores per unit volume of the bearing, and empirically, it is expressed as a correlation coefficient that is approximately negatively correlated with the density ratio (-1).

[0054] Furthermore, the surface aperture ratio of the inner circumferential surface 8a, especially the inclined mound 8a2 and the belt portion 8a3 that serve as radial bearing surfaces, is adjusted to, for example, 2% or more and 15% or less. The adjustment of the surface aperture ratio can be achieved, for example, by rotational finishing as described below.

[0055] The sintered oil-impregnated bearing 8 of the structure is manufactured, for example, via a powder forming step S1, a sintering step S2, a dynamic pressure groove finishing step S3, and an oil-impregnating step S4. Hereinafter, each step will be described in detail as an example where a dimensional finishing step S21 and a rotation finishing step S22 are set after the sintering step S2 and before the dynamic pressure groove finishing step S3.

[0056] (S1) Powder pressing and forming steps First, raw material powder, which will become the material for the sintered oil-impregnated bearing 8, is prepared and compressed into a specific shape using a mold. Specifically, although not shown in the figure, the raw material powder is compressed using a forming mold, which includes: a die head; a center pin inserted into a hole in the die head; a lower punch disposed between the die head and the center pin, configured to be able to move up and down relative to the die head; and an upper punch configured to be able to move (move up and down) relative to either the die head or the lower punch. In this case, the raw material powder is filled into the space formed by the inner circumferential surface of the die head, the outer circumferential surface of the center pin, and the upper end surface of the lower punch. With the lower punch fixed, the upper punch is lowered to pressurize the filled raw material powder along the axial direction. Then, while pressing, the upper punch is lowered to a specific position to compress the raw material powder to a specific axial dimension, thereby forming a pressurized powder. At this point, the axial dimension of the pressurized powder can be set to an appropriate range by controlling the distance between the lower end face of the upper punch and the upper end face of the lower punch, or more specifically, the bottom dead center of the upper punch, based on the axial dimension that should be set as the target (more precisely, considering the dimensional changes caused by subsequent sintering and various finishing processes).

[0057] (S2) Sintering Step After obtaining the pressurized powder in the above manner, the pressurized powder is sintered at a temperature corresponding to the type of raw material powder (the melting point of the main component metal) to obtain a sintered body.

[0058] (S21) Dimensional finishing step and (S22) Rotation finishing step Then, the sintered body undergoes dimensional finishing, correcting its outer or inner diameter and axial dimensions to match the dimensions of the final product. The surface porosity of the inner circumferential surface 8a is adjusted to a suitable ratio for dynamic pressure bearings (e.g., within the range of 2% to 15%). At this stage, specific dynamic pressure groove arrangement regions A1 and A2 have not yet been formed on the inner circumferential surface 8a of the sintered body. Similarly, although not shown in the figure, specific dynamic pressure groove arrangement regions 8b1 have not yet been formed on the upper end surface 8b of the sintered body.

[0059] (S3) Finishing steps for dynamic pressure groove The sintered body obtained through the aforementioned series of steps undergoes specific dynamic pressure groove finishing, thereby forming dynamic pressure groove arrangement regions A1 and A2 on the inner circumferential surface 8a of the sintered body. Although not shown in the figure, the forming apparatus used here includes: a die head including a pressing hole for the sintered body; a finishing pin configured to be inserted into the pressing hole of the die head; a lower punch disposed between the die head and the finishing pin, configured to be able to move relative to the die head; and an upper punch configured to be able to move relative to either the die head or the lower punch. In this case, the inner diameter of the pressing hole of the die head is appropriately set according to the pressing allowance of the sintered body to be finished. Furthermore, a forming die with a shape corresponding to the dynamic pressure groove arrangement area A1 and dynamic pressure groove arrangement area A2 (Fig. 3) of the inner peripheral surface 8a to be formed is provided on the outer peripheral surface of the finishing pin, and a forming die with a shape corresponding to the dynamic pressure groove 8b1 arrangement area (Fig. 4) of the upper end surface 8b to be formed is provided on the lower end surface of the upper punch.

[0060] Next, an example of dynamic pressure groove finishing using the forming apparatus will be described. First, with the sintered body positioned on the upper end face of the die head, the upper punch and finishing pin are lowered from above. This inserts the finishing pin into the inner circumference of the sintered body, so that the forming die, located on the outer circumference of the finishing pin, faces the inner circumferential surface of the sintered body radially. Then, after the forming die reaches a specific position in the axial direction of the inner circumferential surface, only the upper punch continues to descend, pushing against the upper end face of the sintered body. This presses the sintered body into the pressing hole of the die head, compressing the outer circumferential surface of the sintered body, and the inner circumferential surface of the sintered body grips the forming die with the finishing pin pre-inserted into the inner circumference. Furthermore, from this state, the upper punch is lowered further, clamping the sintered body between the upper and lower punches, pressing along the axial direction against the sintered body, which is in a state where deformation in the outer diameter direction is restricted by the die head, thereby further gripping the forming die with the inner circumferential surface. As described above, the shape of the forming mold is transferred onto the inner circumferential surface of the sintered body, forming dynamic pressure groove 8a1 arrangement area A1 and dynamic pressure groove 8a1 arrangement area A2 on the inner circumferential surface. At this time, the forming mold provided on the lower end face of the upper punch is embedded into the upper end face of the sintered body, thereby transferring the shape of the forming mold of the upper punch to the upper end face, and forming a corresponding dynamic pressure groove 8b1 arrangement area.

[0061] After forming specific dynamic pressure grooves 8a1 and 8b1 arrangement areas on the inner circumferential surface and upper end surface of the sintered body in the above manner, the die head is lowered relative to the lower punch, releasing the restriction of the die head on the sintered body. This causes the sintered body to spring back in the outer diameter direction, allowing it to be removed from the finishing pin. The required springback amount is the degree to which the forming die provided on the finishing pin does not hook onto the inner circumferential surface of the finished sintered body (especially the dynamic pressure groove 8a1 arrangement areas A1 and A2) in the axial direction. Therefore, the thickness dimension t of the sintered body is set considering the groove depth (a few μm) of the dynamic pressure grooves 8a1 to be formed. In this embodiment, by setting the thickness dimension t as described above to 0.5 mm or more and 1.5 mm or less, the required depth of the dynamic pressure grooves 8a1 can be formed, and the finishing pin supplied for forming can be pulled out from the finished sintered body without hooking.

[0062] (S4) Oil-containing steps The lubricating oil, which serves as a lubricating fluid, is impregnated in the internal pores of the sintered body obtained in the above manner, thereby completing the sintering of the oil-impregnated bearing 8.

[0063] Here, various lubricating oils can be used, such as ester-based lubricating oils with low evaporation rates and minimal viscosity reduction at low temperatures, or fluorinated lubricating oils with superior resistance compared to ester-based oils. Furthermore, from the viewpoint of kinematic viscosity, lubricating oils exhibiting a kinematic viscosity of 20 cSt or higher and 170 cSt or lower at 40°C, and a kinematic viscosity of 2 cSt or higher and 50 cSt or lower at 100°C, can be used.

[0064] The sintered oil-impregnated bearing 8 is fixed to the inner circumference of the housing 7 by known methods such as press-fitting, subsequent press-fitting, bonding, and welding. Then, the shaft portion 10 of the rotating body 3 is inserted into the inner circumference of the sintered oil-impregnated bearing 8, thereby completing the hydrodynamic bearing device 2. In this case, the lubricating oil interface is maintained in the sealed space S (see Figure 2).

[0065] Furthermore, the aforementioned oil-impregnation step S4 can also be performed, for example, after the sintered body that will become the sintered oil-impregnated bearing 8 is fixed to the inner circumference of the outer casing 7.

[0066] In the hydrodynamic bearing device 2 of the aforementioned structure, when the shaft 10 (rotating body 3) rotates, the region of the inner circumferential surface 8a of the sintered oil-impregnated bearing 8 that becomes the radial bearing surface (the upper and lower hydrodynamic groove 8a1 arrangement region A1 and the hydrodynamic groove 8a1 arrangement region A2) faces the outer circumferential surface 10a of the shaft 10 across the radial bearing clearance. Furthermore, as the shaft 10 rotates, lubricating oil from the radial bearing clearance is forced into the axial-direction center side of each hydrodynamic groove 8a1 arrangement region A1 and the hydrodynamic groove 8a1 arrangement region A2, and the pressure of the lubricating oil rises in the region on the axial-direction center side (here, the belt 8a3). Through the hydrodynamic action of the hydrodynamic grooves 8a1, a first radial bearing portion R1 and a second radial bearing portion R2 are respectively formed in a manner that separates along the axial direction, allowing for free rotation in the radial direction and non-contact support of the shaft 10.

[0067] Furthermore, the dynamic pressure of the dynamic pressure groove 8b1 acts on the thrust bearing clearance between the upper end face 8b of the sintered oil-impregnated bearing 8 (the area where the dynamic pressure groove 8b1 is arranged) and the lower end face 9a1 of the hub portion 9 facing it, forming a lubricating oil film. Then, by the pressure of this oil film, a first thrust bearing portion T1 is formed to non-contactly support the rotating body 3 along the thrust direction. Furthermore, the lower end portion 10b of the shaft portion 10 is rotatably supported by the thrust receiving portion 11 provided at the bottom 7e of the housing 7, thereby forming a second thrust bearing portion T2 that contacts and supports the rotating body 3 along the thrust direction.

[0068] As described above, in the sintered oil-impregnated bearing 8 of this embodiment, when its axial dimension L is reduced compared to the conventional design (set to 4.8 mm or less), its thickness t is set to 0.5 mm or more and 1.5 mm or less, and the sintered oil-impregnated bearing 8 is made into a significantly flatter shape compared to the conventional design. That is, the ratio L / D2 of the axial dimension L of the sintered oil-impregnated bearing 8 to its outer diameter D2 is set to a value significantly less than 1: 0.24 or more and 0.6 or less. With a sintered oil-impregnated bearing 8 of this shape and size, for example, during the above-mentioned dynamic pressure groove finishing, regardless of the size of the axial dimension L, the radial pressure force generated by pressing into the die head can be sufficiently transmitted to the inner circumferential surface of the sintered body that becomes the sintered oil-impregnated bearing 8. Therefore, even when the axial dimension L is set smaller than before, the dynamic pressure groove arrangement area A1 and the dynamic pressure groove arrangement area A2, which serve as the radial dynamic pressure generating part, can be accurately and stably formed, and the required radial bearing performance can be stably achieved. Furthermore, by setting the thickness dimension t to 0.5 mm or more, the required thrust bearing area of ​​the upper end face 8b of the sintered oil-impregnated bearing 8 can be ensured. Therefore, even when the thrust bearing part (first thrust bearing part T1) is provided only on the upper end face 8b side of the sintered oil-impregnated bearing 8, excellent rotational accuracy can be achieved. Therefore, for example, by simply inserting the shaft part 10 of the rotating body 3 into the inner circumference of the sintered oil-impregnated bearing 8, the assembly of the hydrodynamic bearing device 2 equipped with the sintered oil-impregnated bearing 8 can be carried out at low cost.

[0069] Furthermore, as in this embodiment, by setting the density ratio of the porous material forming the sintered oil-impregnated bearing 8 to 86% or more, especially during dynamic pressure groove finishing, the pressing force toward the die head is sufficiently transmitted to the surface of the inner circumferential surface of the porous material (sintered body) in the form of radial pressure. Therefore, a dynamic pressure groove 8a1 of sufficient depth can be transferred and formed on the inner circumferential surface of the sintered body. Moreover, by suppressing the density ratio to 92% or less, excessive load on the finishing die can be suppressed, thus enabling stable and continuous performance of good dynamic pressure groove finishing. In particular, by setting the axial dimension L of the sintered oil-impregnated bearing 8 to 4.8 mm or less, the thickness t to 0.5 mm or more and 1.5 mm or less, the ratio of the axial dimension L to the outer diameter D2 (L / D2) to 0.24 or more and 0.6 or less, and the density ratio to 86% or more and 92% or less, the non-uniformity of the density ratio of the porous material in the sintered oil-impregnated bearing 8 can be further reduced (to 1% or less). This allows for stable high rotational performance. Specifically, the rotational accuracy (NRRO) of the bearing during use can be set to 50 nm or less.

[0070] The above has described one embodiment of the present invention, but the sintered oil-impregnated bearing and the hydrodynamic bearing device having the bearing are not limited to the illustrated embodiment, and any embodiment can be adopted within the scope of the present invention.

[0071] Figure 6 shows a cross-sectional view of a hydrodynamic bearing device 12 according to another embodiment of the present invention. As shown in Figure 6, the hydrodynamic bearing device 12 in this embodiment only includes a first thrust bearing portion T1, which differs from the hydrodynamic bearing device 2 shown in Figure 2. Specifically, in the hydrodynamic bearing device 2 of this embodiment, a specific thrust direction clearance always exists between the lower end face 10c of the shaft portion 10 and the upper end face 7e1 of the bottom 7e of the housing 7. Here, the size of the opposing distance (thrust direction clearance) between the upper end face 7e1 of the bottom 7e and the lower end face 10c of the shaft portion 10 is greater than the opposing distance between the upper end face 8b of the sintered oil-impregnated bearing 8 and the lower end face 9a1 of the disk portion 9a, which is set to a size that is considered not to have a substantial impact on the increase in loss torque during rotary drive. Other than this, the structure is the same as that of the hydrodynamic bearing device 2 shown in Figure 2, etc., so detailed description is omitted.

[0072] In the hydrodynamic bearing device 12 of this embodiment, when the axial dimension L is set to 4.8 mm or less, its thickness t is set to 0.5 mm or more and 1.5 mm or less, and the ratio of the axial dimension L to the outer diameter dimension D2, L / D2, is set to a value significantly less than 1: 0.24 or more and 0.6 or less. Therefore, the hydrodynamic groove 8a1 arrangement area A1 and the hydrodynamic groove 8a1 arrangement area A2, which serve as the radial hydrodynamic generation part, can be accurately and stably formed, and the required radial bearing performance can be stably achieved. Furthermore, the thrust bearing area required for the upper end face 8b of the sintered oil-impregnated bearing 8 can be ensured, so even when the thrust bearing part (first thrust bearing part T1) is only provided on the upper end face 8b side of the sintered oil-impregnated bearing 8, excellent rotational accuracy can be achieved. Therefore, as shown in Figure 6, even without supporting the lower end face 10c of the shaft portion 10 along the thrust direction, the thrust bearing performance can be fully utilized. The shape of the lower end of the shaft portion 10 is simplified by omitting the thrust receiving portion 11, thereby reducing costs accordingly.

[0073] Of course, either or both of the radial dynamic pressure generating portions (regions A1 and A2 of dynamic pressure grooves 8a1 arranged in FIG3) formed between the radial bearing portion R1 and the outer peripheral surface 10a of the shaft portion 10 can be used, or known radial dynamic pressure generating portions with shapes other than so-called multi-arc, stepped, and wave-shaped dynamic pressure grooves can be used.

[0074] Furthermore, in the above description, the housing 7 of the hydrodynamic bearing device 2 is fixed to the inner periphery of the base 6 of the fan motor 1. However, for example, the housing 7 can also be directly mounted on the base of the information machine on which the fan motor 1 is mounted (illustration omitted). Alternatively, the part corresponding to these bases can be integrally provided with the housing 7.

[0075] Furthermore, in the above description, by offsetting the magnet 5b and coil 5a along the axial direction, an external force is applied to the shaft 10 (rotating body 3) to press it against the bottom 7e side of the outer casing 7. However, the method for applying this external force to the shaft 10 is not limited to the method described above. Although the illustration is omitted, for example, the magnetic force can also be applied to the rotating body 3 by arranging a magnetic member capable of attracting the magnet 5b facing the magnet 5b along the axial direction. Furthermore, when the thrust of the reaction force acting as airflow is sufficiently large, and the shaft 10 can be pressed downwards by the thrust alone, the magnetic force (magnetic attraction force) used as the external force to press the shaft 10 downwards can also be omitted.

[0076] Furthermore, the above description has explained the application of the present invention to a hydrodynamic bearing device 2 in which a rotating body 3, including a fan 4, is fixed to a shaft portion 10. However, the present invention can also be preferably applied to a hydrodynamic bearing device 2 in which a disc hub, including a disc mounting surface, or a polygonal mirror is fixed to a shaft portion 10 as a rotating body 3. That is, the present invention can be applied not only to the fan motor 1 shown in FIG. 1, but also preferably to other hydrodynamic bearing devices 2 assembled in electrical machinery, such as a shaft motor for a disc device or a polygonal scanner motor for a laser beam printer (LBP).

[0077] 1: Fan motor 2, 12: Hydrodynamic bearing assembly 3: Rotational body 4: Fan 5: Drive Unit 5a: Coil 5b: Magnet 6: Base 7: Outer shell 7a, 8a, 9b1: Inner circumferential surface 7b, 8d, 10a: outer peripheral surface 7c, 7e1, 8b: Upper surface 7d: Sealing surface 7e: Bottom 7f: Bearing contact surface 8: Sintered oil-impregnated bearings 8a1, 8b1: ​​Dynamic pressure groove 8a2: Sloping hill 8a3: Belt section 8c, 9a1, 10c: Lower end face 8d1: Axial groove 8e: Inner chamfer 9: Wheel hub 9a: Disc section 9b, 9c: cylindrical part 9d: Flange portion 9e: Mounting hole 10: Shaft 10b: Lower end 11: Thrust Receiving Section A1, A2: Dynamic pressure groove arrangement area D1: Inner diameter D2: Outer diameter L: Axial dimension R1, R2: Radial bearing section S: Sealed space t: thickness dimension T1, T2: Thrust bearing section

Claims

1. A sintered oil-impregnated bearing, comprising a bearing made of sintered metal obtained by compressing metal powder into a cylindrical shape to form a pressurized powder, and sintering the pressurized powder, wherein the sintered oil-impregnated bearing contains a lubricating fluid in its internal pores and has a radial dynamic pressure generating portion formed on its inner circumferential surface, characterized in that: the density ratio is 86% or more and 92% or less; the axial dimension L is 0.8 mm or more and 1.8 mm or less; the inner diameter D1 is 1.2 mm or more and 2.5 mm or less; the outer diameter D2 is 2.5 mm or more and 5.0 mm or less; the thickness t is 0.5 mm or more and 1.5 mm or less; the ratio of the axial dimension L to the outer diameter D2, L / D2, is 0.24 or more and 0.6 or less; and a region with a plurality of dynamic pressure grooves arranged as the radial dynamic pressure generating portion is formed on the inner circumferential surface. A thrust dynamic pressure generating part is formed on one end face in the axial direction, and the end face on the other end face in the axial direction is formed into a flat shape.

2. The sintered oil-impregnated bearing as claimed in claim 1, wherein the ratio of the outer diameter dimension D2 to the inner diameter dimension D1, D2 / D1, is 1.5 or more and 3.0 or less.

3. The sintered oil-impregnated bearing as claimed in claim 1 or claim 2, wherein the inner diameter dimension D1 is 1.5 mm or more and 2.0 mm or less.

4. A hydrodynamic bearing device, comprising: Sintered oil-impregnated bearings as described in any one of claims 1 to 3; The outer casing secures the sintered oil-impregnated bearing to its inner circumference; A rotating body having a shaft portion inserted into the inner circumference of the sintered oil-impregnated bearing; The radial bearing portion supports the shaft portion non-contactly in the radial direction with a film of the lubricating fluid. The film of the lubricating fluid is formed in the radial bearing clearance between the inner circumferential surface of the sintered oil-impregnated bearing and the outer circumferential surface of the shaft portion by the dynamic pressure action of the radial dynamic pressure generating portion.

5. The hydrodynamic bearing device as described in claim 4, further comprising: The first thrust bearing portion supports the shaft portion non-contactly along the thrust direction with a film of the lubricating fluid, the film of the lubricating fluid being formed in the thrust bearing gap between the end face of the rotating body on the other side of the axial direction opposite to it in the thrust direction and the end face of the sintered oil-impregnated bearing on one side of the axial direction.

6. The hydrodynamic bearing device as claimed in claim 5, wherein a thrust dynamic pressure generating part is provided on the end face of the sintered oil-impregnated bearing on the axial direction side opposite to the disc portion of the hub portion which is the rotating body in the thrust direction.

7. The hydrodynamic bearing device as described in claim 5, further comprising: The second thrust bearing portion has its end face on one side of the housing in the axial direction contacting the end face on the other side of the shaft portion in the axial direction along the thrust direction.

8. A motor comprising a hydrodynamic bearing assembly as described in any one of claims 4 to 7.

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