Sintered bearing

Abstract

Provided is a sintered bearing that is provided with bearing surface parts at two locations on an inner circumferential surface which are separated from each other in the axial direction, that has a relief part between said bearing surface parts whose inner diameter is set to be greater than the inner diameters of the bearing surface parts, and that includes a metal powder and a resin powder. The sintered bearing has a structure in which the metal powder is bonded via the resin powder interposed therebetween. The resin powder is an epoxy resin powder including a thermosetting latent curing agent.

Description

Sintered bearings

[0001] The present invention relates to a sintered bearing.

[0002] Sintered bearings are often used as bearing materials for small motors (small motors mainly refer to motors with low output, and are incorporated into a variety of products, including general home appliances such as air conditioners and microwave ovens, as well as computers, audio equipment, industrial equipment, and automobiles).

[0003] This type of sintered bearing is generally made of a porous material. Sintered bearings are used with their internal pores impregnated with lubricating oil. In this case, an oil film is formed on the sliding surface of the bearing as it moves relative to the shaft it supports, and this oil film supports the shaft.

[0004] As inexpensive and highly reliable bearings, sintered bearings are widely used as fan motors for home appliances, automotive motors, office automation equipment, etc. That is, fan motors include cooling fans inside home appliances such as computers and televisions, fans for circulation and cooling inside refrigerators, and automotive fans used to cool batteries and for suction of interior temperature sensors, and demand for these motors is increasing year by year.

[0005] However, motors with low drive torque have drawbacks such as the frictional resistance of the sliding surfaces that can easily affect motor characteristics, and the insufficient circulation and supply of oil at high speeds or in low temperatures makes it difficult to achieve smooth sliding. Furthermore, motors have the disadvantage of being prone to a decrease in rotation speed and an increase in current.

[0006] For this reason, there have been conventional bearings in which a relief portion is provided in the center of the inner diameter of the bearing to reduce the rotational load of the motor, as described in Patent Documents 1 and 2. That is, the bearing (sintered bearing) in Patent Document 1 has bearing surface portions provided at two locations axially spaced apart on the inner circumferential surface (inner diameter surface), and has a relief portion between the bearing surface portions whose inner diameter is set larger than the inner diameter of the bearing surface portions.

[0007] In this case, a sizing process was performed after the sintering process to form a relief. The sizing process involves placing the sintered material back into a mold and compressing it to obtain highly accurate dimensions and shapes. Specifically, the material is placed in a mold and pressure is applied with upper and lower punches, forcing it against the die and upper and lower punches, correcting any deformation or dimensions. There are two types of sizing methods: positive sizing and negative sizing. Positive sizing involves preparing the material larger than the final product dimensions, forcing it into a mold during sizing, and rubbing it against the die or core to achieve precision. Negative sizing involves preparing the material smaller than the final product dimensions, forcing it against the mold surface by compressing it in the mold.

[0008] In addition, in the device described in Patent Document 1, a large number of dimples (recessed portions) are provided on the bearing surface formed on both axial end sides. By providing dimples in this way, it is possible to reduce the sliding area on the bearing surface and reduce frictional resistance. It is also shown that oil-impregnated lubricant is stored in each dimple, and when the rotating shaft rotates, the stored lubricant is drawn between the bearing surface and the rotating shaft, making it possible to reduce the friction coefficient of the bearing surface.

[0009] Japanese Patent No. 6253134 JP 2010-31909 A

[0010] As described above, in conventional sintered bearings with recesses, a sizing process is carried out after the sintering process, and the recesses are formed by reducing the outer diameter, resulting in a smooth boundary between the sliding portion having the sliding surface and the recesses.

[0011] 12 shows a sintered bearing that has undergone a sizing process after the sintering process. By performing the sizing process in this way, a pair of bearing surfaces 51, 52 and a recess 53 between the bearing surfaces 51, 52 are formed in the inner diameter portion of the bearing 50, and a tapered surface 54 is formed between the inner diameter surface 53a of the recess 53 and the bearing surface 52a of the bearing surface 52.

[0012] If the tapered surface 54 is formed in this manner, the sliding area may change (become larger) compared to the initial state due to wear of the sliding surfaces (bearing surfaces 51 a, 52 a, which are the inner diameter surfaces of the bearing surface portions 51, 52). If the sliding area changes in this way, there is a risk that the bearing characteristics may change.

[0013] Furthermore, since the outer diameter is narrowed and the inner diameter is reduced in the sizing process after molding and sintering, the overall length and outer diameter of the sintered body are easily affected. This causes variations in the dimension (axial dimension) L5 of the bearing surface 53a. In this case, variations of up to ±0.3 mm may occur.

[0014] Generally, the sintering process requires high temperatures (approximately 700°C to 900°C) and hydrogen gas, nitrogen gas, or a mixture of these gases as heat treatment gases. Therefore, improvements are desired from the viewpoint of energy consumption.

[0015] Furthermore, when forming dimples as in Patent Document 1, Patent Document 1 discloses that the dimples are formed using plastic processing such as peening, rolling, coining, etc. Such processing requires additional processing equipment and processing steps, and also poses problems in terms of productivity and cost, such as the need to manufacture tools for the plastic processing that have protrusions.

[0016] Therefore, the present application provides a sintered bearing that reduces the contact area with the rotating shaft, allows for stable rotation, keeps the current low, and increases the rotation speed, and does not require a sintering furnace or a large amount of electrical energy to maintain high temperatures, and does not require hydrogen gas, nitrogen gas, or a mixture of these gases as a processing gas.

[0017] The sintered bearing of the present invention has bearing surface portions provided at two locations axially spaced apart on its inner peripheral surface, and has a relief portion between the bearing surface portions whose inner diameter is set to be larger than the inner diameter of the bearing surface portions, and is a sintered bearing containing metal powder and resin powder, wherein the sintered bearing has a structure in which the metal powder is bound by the resin powder interposed between the metal powders, and the resin powder is an epoxy resin powder containing a thermosetting latent curing agent.

[0018] According to the sintered bearing of the present invention, by compression molding a mixed powder composed primarily of iron powder and resin, the adhesiveness and flexibility of the resin can be used to increase the strength of the green compact and increase the inner diameter springback rate. Furthermore, by increasing the springback rate during powder molding, when removing the core pin from the green compact, it becomes possible to forcibly remove the undercut portion, which is made up of the irregularities on the inner diameter surface of the bearing that are transferred from the irregularities formed on the core pin. This makes it easy to form a rectangular relief portion in the center of the inner diameter of the bearing.

[0019] The sintered bearing is hardened in an air atmosphere at approximately 200°C (here, approximately 200°C means 150°C to 250°C). Therefore, it does not require a sintering furnace for maintaining high temperatures of 700°C to 900°C, as is required in a typical sintering process, nor does it require hydrogen gas or nitrogen gas as a processing gas. Furthermore, the step formed in the core pin during molding is transferred as is, forming the step at the boundary between the sliding portion and the central relief portion. In other words, in this invention, the boundary between the sliding portion and the relief portion can be formed without sizing, and the tapered surface that occurs between the sliding portion and the relief portion when sizing is performed is unlikely to be formed. Furthermore, because the bearing surface (the inner diameter surface of the bearing surface portion) is formed by in-mold correction during compression molding and sintered at a low temperature, dimensional change can be minimized, and variation in the axial length of the sliding surface can be reduced. (For example, this variation can be reduced to about ±0.1 mm.) Furthermore, if a sizing process is carried out after the sintering process, the bearing surface will be squeezed to form a relief portion, which will result in variation in the axial length of the bearing surface, but the present invention, which does not carry out a sizing process, does not squeeze to form a relief portion, so variation in the axial length of the bearing surface can be reduced. If variation in the axial length of the bearing surface can be reduced, the sliding area between the shaft member and the bearing surface will be stable, and the bearing characteristics will be stable.

[0020] Furthermore, the sintered bearing of the present invention is produced by compression molding to physically bring iron powder particles into contact with each other or iron powder and resin powder, followed by sintering to thermally harden the resin, forming a cross-sectional structure in which the iron powder particles are adhesively bonded. As a result, the resin melts and softens as it hardens, and the resin intervenes and adheres to the contact points (necks) between the powder particles, reinforcing and strengthening the material, achieving a level of material strength that is acceptable for use in bearings with relatively low loads.

[0021] In order to increase the springback rate, it is preferable that the mixed powder of metal powder and resin powder be 95 wt% to 99 wt% metal powder, with the remainder being resin powder. The metal powder in the mixed powder is iron powder, and the iron powder can be set to be coarse powder with an average particle size of 50 μm to 200 μm. If the average particle size is less than 50 μm, it becomes difficult to form a resin film on the iron powder surface, and the material strength decreases. If the average particle size exceeds 200 μm, the powder particles are coarse, resulting in large voids, which can cause oil leaks and increase the frequency of metal contact between the bearing surface and the rotating shaft, deteriorating motor characteristics.

[0022] By using coarse iron powder with an average particle size of 50 μm to 200 μm, coarse pores can be formed throughout the material. Here, coarse pores refer to pores with an average diameter of 10 μm or more. Furthermore, the metal powder (iron powder) is surrounded by the resin powder, and micropores (with an average diameter of less than 10 μm) are formed in this surrounded area.

[0023] The iron powder is preferably spongy iron powder, which has cavities inside the powder and can hold lubricating oil in those cavities. Here, spongy iron powder generally refers to porous iron powder (e.g., sponge iron powder) containing many pores, which is produced by reducing oxidized iron powder such as iron oxide with a gaseous or solid reducing agent. Therefore, this spongy iron powder has micropores that extend to the interior of the powder.

[0024] By setting it in this way, the large pores (micropores) that are internal cavities caused by the coarse powder facilitate the supply of oil and reduce oil shortages, the micropores formed by the resin coating have the effect of suppressing oil leaks and acting as an oil reservoir, and the micropores inside the iron powder have the effect of retaining oil and preventing it from leaking out.

[0025] The resin powder may be an epoxy resin powder containing a thermosetting latent curing agent. Since the resin powder needs to be thermally cured during the baking process, it is preferable to use a thermosetting epoxy resin.

[0026] The sintered bearing preferably has a cross-sectional structure in which the iron powder particles are fixed together by the binding force of the resin without the progression of sintering due to mutual diffusion between the iron powder particles.

[0027] The mixed powder is compression molded in a die, and the springback rate of the compacted powder body that occurs during this process can be set to 0.2% or more. By setting this, the core pin can be stably removed after compression molding.

[0028] It is preferable that the inner diameter of the recess is 0.1% to 0.3% larger than the inner diameter of the bearing surface portion, and that the cross-sectional shape of the recess is rectangular. By setting the difference to 0.1% or more, the function of the recess, which is to reduce the contact area with the rotating shaft, can be effectively exhibited. Furthermore, by setting the cross-sectional shape of the recess to a rectangular shape, even if the bearing surface portion wears, the area of ​​the bearing surface does not change, and the bearing characteristics are less likely to change. By setting the difference to less than 0.3%, the inner diameter dimension of the recess does not become too large compared to the inner diameter dimension of the bearing surface portion, and damage to the bearing surface, which is the inner diameter surface of the bearing surface portion, can be effectively prevented when the core pin is pulled out.

[0029] The inner diameter surface opening ratio of the bearing surface portion (the surface opening ratio of the bearing surface) is preferably 40% to 80%. The surface opening ratio is the area ratio of all openings in the bearing surface, including not only openings that do not communicate with the internal pores as described above, but also openings that communicate with the internal pores. Increasing the surface opening ratio of the bearing surface in this way reduces the contact area, resulting in more stable rotation.

[0030] An enlarged diameter portion that opens axially outward may be provided on an axially outer edge of the bearing surface portion. By providing the enlarged diameter portion in this manner, lubricating oil flows into a sliding portion between the bearing surface, which is the inner diameter surface of the bearing surface portion, and the rotating shaft, thereby stably supplying oil to the sliding portion.

[0031] It is preferable that the surface opening ratios of the inner surface of the bearing surface portion, the inner surface of the recess portion, and the inner surface of the enlarged diameter portion are approximately equal. By setting them in this manner, well-balanced rotation can be obtained. Here, "approximately equal" may not match but may deviate slightly due to design errors, processing errors, assembly errors, etc. This includes this slight deviation.

[0032] The axial length of each enlarged diameter portion is preferably set in the range of 0.2 mm to 2.0 mm. If it is less than 0.2 mm, it will be difficult for it to function as an oil retaining portion, and if it exceeds 2.0 mm, the bearing surface of the bearing surface portion will be too small to stably support the rotating shaft.

[0033] The sintered bearing is preferably used as a bearing member for a motor. By using it as a bearing member for a motor, a high-quality motor (small motor) can be provided that can achieve stable rotation for a long period of time.

[0034] The present invention provides a sintered bearing that reduces the contact area with the rotating shaft, ensuring stable rotation, keeping current low, and increasing rotational speed. It does not require a sintering furnace or significant electrical energy for maintaining high temperatures, and does not require hydrogen gas, nitrogen gas, or a mixture of these gases as a processing gas. Furthermore, by reducing the frequency of oil shortage and metal contact with the rotating shaft, the sintered bearing exhibits excellent lubrication and sliding properties. In particular, bearings with an enlarged diameter section ensure smooth oil circulation and supply, reducing oil shortage and achieving good sliding, even in motor bearings used at high speeds or in low-temperature environments. As a result, motor speed reductions and instability can be alleviated, improving motor performance. Furthermore, the increase in current can be reduced, resulting in reduced power consumption.

[0035] 1 is a simplified cross-sectional view of a sintered bearing of the present invention;

[0033] FIG. 1 is a manufacturing process for a sintered bearing of the present invention;

[0034] FIG. 2 is a front view showing the compression-forming process for a sintered bearing of the present invention, illustrating the state in which the concave and convex portions of the core pin are being transferred to a green compact;

[0035] FIG. 3 is a simplified diagram showing the compression-forming process for a sintered bearing of the present invention, illustrating the relationship between the core pin and the compressed body in a spring-back state;

[0036] FIG. 4 is a simplified cross-sectional view of the compressed body with the core pin removed, illustrating the compression-forming process for a sintered bearing of the present invention;

[0037] FIG. 5 is a simplified cross-sectional view of a main portion of the compressed body with the core pin removed;

[0038] FIG. 6 is an image of the cross-sectional structure of a material;

[0039] FIG. 7 is a cross-sectional view of a fan motor using a sintered bearing of the present invention;

[0039] FIG. 8 is an enlarged cross-sectional view of a main portion of the fan motor using the sintered bearing shown in FIG. 8;

[0039] FIG. 9 is a front view showing the state in which the concave and convex portions of the core pin are being transferred to a green compact;

[0039] FIG. 10 is a simplified diagram showing the compression-forming process for a sintered bearing of the present invention, illustrating the relationship between the core pin and the compressed body in a spring-back state;

[0039] FIG. 11 is a simplified cross-sectional view of the compressed body with the core pin removed, illustrating the compression-forming process for a sintered bearing of the present invention; Fig. 10 is a simplified cross-sectional view of the essential parts of the compressed body with the core pin removed Fig. 11 is a simplified cross-sectional view of the essential parts of the sintered bearing after a sizing step.

[0036] 6 shows a fan motor. This fan motor comprises 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 6 and rotor magnet 7 arranged opposite each other with a radial gap between them. The stator coil 6 is attached to a housing 8 of the fluid dynamic bearing device 1, and the rotor magnet 7 is attached to the rotor 3. In a fan motor configured in this manner, when current is applied to the stator coil 6, the rotor magnet 7 rotates due to the electromagnetic force between the stator coil 6 and the rotor magnet 7, and this causes the shaft member 2 and the rotor 3 fixed to the shaft member 2 to rotate together. As the rotor 3 rotates, an airflow is generated in the axial direction or radially outward, depending on the shape of the blades 4 attached to the rotor 3, etc.

[0037] As shown in Figure 7, the fluid dynamic bearing device 1 is a so-called rotating shaft type bearing device whose main components are a shaft member 2 that forms the rotating side, a housing 8, bearing member 10, and seal member 9 that form the stationary side, and lubricating oil (not shown) that fills the internal space of the housing 8, and a sintered bearing according to an embodiment of the present invention is used for the bearing member 10. Note that, for ease of explanation, the upper side of the page in Figure 5 (the side where the seal member 9 is located) will be referred to as the "upper side," and the lower side of the page in Figure 2 will be referred to as the "lower side," but this is not intended to limit the orientation of the fluid dynamic bearing device 1 when in use.

[0038] 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 irregularities, and its lower end surface 2b is a convex spherical surface. The rotor 3, to which the blades 4 and the rotor magnet 7 (see Figure 1) are attached, is fixed to the upper end of the shaft member 2.

[0039] The housing 8 is cylindrical and has a cylindrical portion 8a and a bottom portion 8b that closes the opening at the lower end of the cylindrical portion 8a. In the illustrated example, the cylindrical portion 8a and the bottom portion 8b are integrally formed from a resin or metal material. The inner peripheral surface 8a1 of the cylindrical portion 8a is formed into a cylindrical surface with a constant diameter, and the outer diameter end of the annular shoulder surface 8b2, which is formed into a flat surface perpendicular to the axial direction, is connected to the lower end. The stator coil 6 and the motor base 5 are fixed to the outer peripheral surface 8a2 of the cylindrical portion 8a with a gap between them.

[0040] In the illustrated example, a thrust plate 11 formed in a disk shape from a material with better sliding properties than the material from which the housing 8 is made is placed on the inner bottom surface 8b1 of the housing 8 (the upper end surface of the bottom 8b), and the upper end surface of the thrust plate 11 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 11 is not necessarily required and may be omitted. If the thrust plate 11 is omitted, the lower end surface 2b of the shaft member 2 is contact-supported by the inner bottom surface 8b1 of the housing 8.

[0041] The seal member 9 is formed in an annular shape from a resin or metal material and is fixed to the upper end of the inner circumferential surface 8a1 of the cylindrical portion 8a of the housing 8 with its lower end surface 9b abutting against the upper end surface 10b of the bearing member 10. 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 8 from leaking out.

[0042] The fluid dynamic bearing device 1 may be used in a so-called fully filled state, where the entire internal space of the housing 8 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 8 (lubricating oil and air are mixed in the internal space of the housing 8). When the fluid dynamic bearing device 1 is used in a fully filled state, the volume of the seal space S is determined so that the lubricating oil level is always maintained within the axial range of the seal space S, even if the lubricating oil level position fluctuates in the axial direction due to temperature changes.

[0043] The bearing member 10 used in the fan motor shown in Figures 6 and 7 is a sintered bearing 10 according to the present invention. The sintered bearing 10 is formed by sintering a compacted powder body and impregnating it with lubricating oil. Two bearing surface portions 21 and 22 are provided axially on the inner peripheral surface (inner diameter surface) 10d. Between the bearing surface portions 21 and 22, a relief portion 23 is provided, the inner diameter of which is larger than the inner diameter of the bearing surface portion. The bearing surface portion 21 is sometimes referred to as the first bearing surface portion, and the bearing surface portion 22 is sometimes referred to as the second bearing surface portion. Both sintering and sintering are processing methods for creating products by applying heat to materials such as metals and ceramics. Heating a material promotes bonding between raw material particles. Sintering is a process that primarily uses metal-based powder materials, where heating bonds the powder particles and induces shrinkage. On the other hand, sintering is a process that often uses ceramic materials, where heating induces a chemical reaction at high temperatures to increase mechanical strength. For this reason, in this specification, this process is sometimes called sintering or firing.

[0044] In this case, the outer diameter surface 10a of the bearing member 10 is a straight cylindrical surface without any steps, and the inner diameter dimensions of the first bearing surface portion 21 and the second bearing surface portion 22 are set to the same dimensions, with the inner diameter dimension of the relief portion 23 being 0.1% to 0.3% larger than the inner diameter dimensions of both bearing surface portions 21, 22. That is, when the inner diameter dimension of the first bearing surface portion 21 is D1, the inner diameter dimension of the second bearing surface portion 22 is D2, and the inner diameter dimension of the relief portion 23 is D3, then D1 = D2 < D3, and D3 = (D1 + D1 / 1000) to (D1 + 3D1 / 1000) or D3 = (D2 + D2 / 1000) to (D2 + 3D2 / 1000). Here, the same dimensions are within 0.001 mm.

[0045] Furthermore, when the axial length of the bearing member 10 is L, the axial length of the first bearing surface portion 21 is L1, the axial length of the second bearing surface portion 22 is L2, and the axial length of the relief portion 23 is L3, the relationship L1≦L2<L3 holds. Furthermore, L3 = 1.0 × (L1 + L2) to 4.0 × (L1 + L2). When the axial length of the bearing member 10 is L, the relationship L = 2.0 × (L1 + L2) to 5.0 × (L1 + L2).

[0046] 7, the bearing member 10 is fixed to the inner periphery of the cylindrical portion 8a of the housing 8 with its lower end surface 10c abutting against a shoulder surface 8b2 of the bottom portion 8b of the housing 8. The bearing member 10 can be fixed to the inner periphery 8a1 of the cylindrical portion 8a by press-fitting, bonding, or press-fit bonding (a combination of press-fitting and bonding), or it can also be fixed to the inner periphery of the cylindrical portion 8a by a clearance fit (see JIS B 0401-1) to the inner periphery of the housing 8 and then sandwiched from both axial sides between the seal member 9 and the shoulder surface 8b2 of the housing 8. In particular, the latter fixing method allows the bearing member 10 to be fixed to the housing 8 at the same time as the seal member 9 is fixed to the housing 8, thereby reducing the effort required to assemble the components.

[0047] As shown in FIG. 2, this bearing member (sintered bearing) 10 is formed by carrying out a powder-making (mixing) step S1, a compression molding step S2, a sintering step S3, and an oil-impregnating step S4.

[0048] In the powder-making step S1, a metal powder and a resin powder are kneaded and mixed to produce a raw material powder. While the metal powder is not particularly limited, in this embodiment, iron powder, which is commonly used in powder metallurgy applications, has low raw material costs, and is easily supplied, is used. In other words, it is not limited to iron powder, and stainless steel powder, copper powder, tin powder, etc. may also be used. Furthermore, since the resin powder must be thermally cured during the firing step, epoxy resin (EP) powder containing a latent curing agent is used. In addition to epoxy resin, phenolic resin (PF), polyurethane (PUR), melamine resin (MF), etc. can also be used.

[0049] The mixed powder of metal powder and resin powder preferably contains 95 wt % to 99 wt % metal powder, with the remainder being resin powder.

[0050] Next, in the compression molding step S2, a green compact 31 is formed by press molding using a mold device. This mold device includes cylindrical upper and lower punches (not shown), a core pin 30 (see FIG. 3A ) that forms the inner shape of the green compact 31, and a die (not shown) that forms the outer shape of the green compact. The core pin 30 has a first molding portion 30a for molding one bearing surface (the inner diameter surface 21a of the first bearing portion 21), a second molding portion 30b for molding the other bearing surface (the inner diameter surface 22a of the second bearing portion 22), and a third molding portion 30c between the first molding portion 30a and the second molding portion 30b. The third molding portion 30c forms the relief portion 23 and is set to be larger than the outer diameters of the first and second molding portions 30a, 30b.

[0051] After compacting, the powder compact is removed from the die device by opening the die device, which releases the axial pressure applied to the compacted powder compact 31. When this pressure is released, the elastic restoring force accumulated inside the powder compact is released, causing springback in the powder compact 31, and the inner circumferential surface (inner diameter surface) of the powder compact 31 expands in diameter, as shown in Fig. 3B. This expansion allows the core pin 30 to be pulled out of the powder compact 31. This allows the pre-sintered powder compact 31 to be formed, as shown in Fig. 3C.

[0052] By compression molding a mixed powder containing metal powder and resin powder as its main components, the strength of the green compact 31 can be increased due to the adhesiveness and flexibility of the resin, and the springback (inner diameter springback rate) can be increased. In this way, by increasing the inner diameter springback rate, the core pin 30 can be pulled out from the inner diameter surface of the green compact 31 to which the concave and convex portions of the core pin 30 have been transferred. This allows the cross-sectional shape of the relief portion 23 to be formed into a rectangle.

[0053] That is, steps 30d, 30e are formed at the boundary between the large-diameter third molded portion 30c and the small-diameter first molded portion 30a, and at the boundary between the large-diameter third molded portion 30c and the small-diameter second molded portion 30b, and these steps 30d, 30e are perpendicular to the axial direction. That is, step 30d is perpendicular to the outer diameter surface of first molded portion 30a, and step 30e is perpendicular to the outer diameter surface of second molded portion 30b.

[0054] In this case, the steps 30d and 30e formed on the core pin 30 during the compression molding step S2 are transferred directly to the powder compact 31, forming steps 24A and 24B between the bearing surface portion 21 and the relief portion 23, and between the bearing surface portion 22 and the relief portion 23, as shown in Fig. 4. The powder compact 31 is then sintered in a firing step S3 to harden the resin powder, and the resulting product (sintered bearing) is then impregnated with oil in an oil-impregnation step S4. Therefore, the bearing surface portions 21 and 22, the relief portion 23, and the steps 24A and 24B of the powder compact 31 shown in Fig. 3A become the bearing surface portions 21 and 22, the relief portion 23, and the steps 24 and 24 of the sintered bearing (bearing member) 10, as shown in Fig. 1. Therefore, no tapered surfaces are formed between the bearing surface portions 21 and 22 and the relief portion 23 in the sintered bearing 10.

[0055] In a typical sintered bearing, the sintering process involves heating the green compact obtained in the powder compacting process (compression molding process) to the sintering temperature of the metal powder used to obtain a sintered body. That is, sintering is performed in a predetermined atmosphere and under predetermined temperature conditions. The predetermined atmosphere is generally a vacuum, a reducing gas, an inert gas, or the like, and can be selected depending on the metal powder used. However, in the case of the bearing member 10, which is a sintered bearing according to the present invention, a mixed powder of metal powder and resin powder is compression molded, so sintering can be performed in an atmospheric pressure atmosphere at about 200°C. Note that about 200°C means 150°C to 250°C.

[0056] In the oil impregnation step S4, the sintered body is impregnated with lubricating oil, thereby completing the bearing member 10 whose internal pores are impregnated with lubricating oil. The impregnation of the internal pores of the bearing member 10 with lubricating oil is carried out, for example, by immersing the bearing member 10 in a lubricating oil bath filled with lubricating oil for a certain period of time under a predetermined reduced pressure environment. At this time, the impregnation process may be carried out while the lubricating oil is heated, in order to ensure that the lubricating oil is impregnated reliably and in a short period of time.

[0057] Figure 5 is an image of the cross-sectional structure of a sintered bearing 10. In sintered bearing 10, iron powder particles or iron powder and resin powder are physically pressed into contact with each other through compression molding, and then the resin is thermally hardened by sintering to form a cross-sectional structure in which the iron powder particles are adhesively bonded. In addition, the resin melts and softens as it hardens, and the resin intervenes and adheres to the contact points (necks) between the powder particles, reinforcing and strengthening the material, achieving a level of material strength that is acceptable for use in bearings with relatively low loads.

[0058] In the present invention, the average particle size of the iron powder is set to 50 μm to 200 μm. If the average particle size is less than 50 μm, it becomes difficult to form a resin film on the iron powder surface, resulting in a decrease in material strength. On the other hand, if the average particle size exceeds 200 μm, the powder particles become coarse, resulting in large voids, which can cause oil leaks and increase the frequency of metal contact between the bearing surface and the rotating shaft, thereby deteriorating motor performance.

[0059] That is, by using coarse metal powder (iron powder) with an average particle size of 50 to 200 μm, coarse pores are formed throughout. Also, in some areas surrounded by a resin coating, minute pores (micropores) are formed. Furthermore, spongy iron powder (reduced iron powder) has micropores that extend to the interior of the powder. The coarse pores (micropores), which are internal cavities created by the coarse powder, facilitate oil supply and reduce oil shortage, while the micropores formed by the resin coating act as an oil reservoir by suppressing oil leaks, and the micropores inside the iron powder have the effect of retaining oil and preventing it from leaking.

[0060] Therefore, in the present invention, the ratio of coarse pores (average diameter: 10 μm or more) on the sliding surface (inner diameter surface) of the sliding surface portion to minute pores (average diameter: less than 10 μm) surrounded by resin is set as follows: the area ratio of coarse pores to minute pores is set to 1:1 to 9:1 (area ratio of coarse pores = 50% to 90%).

[0061] If the area ratio of coarse pores is 50% or less, oil supply will not function smoothly, and there is a risk of oil shortage, particularly in low-temperature environments. If the area ratio of coarse pores is 90% or more, oil leaks will occur, and the frequency of metal contact with the rotating shaft will increase in high-temperature environments, making it more likely that an increase in current value and bearing wear will occur.

[0062] According to the sintered bearing of the present invention, by compression molding a composite powder whose main components are iron powder and resin, the adhesiveness and flexibility of the resin can be used to increase the strength of the green compact 31 and increase the inner diameter springback rate. Furthermore, by increasing the springback rate during powder molding, when removing the core from the green compact, it becomes possible to forcefully remove the undercut portion, which is made up of the irregularities on the inner diameter surface of the bearing that are transferred from the irregularities formed on the core pin 30. This makes it easy to form a rectangular relief portion 23 in the center of the inner diameter of the bearing.

[0063] Because the sintered body is sintered in an air atmosphere at approximately 200°C, it does not require a sintering furnace for maintaining high temperatures of 700°C to 900°C, as is required in a typical sintering process, nor does it require hydrogen gas or nitrogen gas as a processing gas. Furthermore, the step formed on the mold core pin during molding is transferred as is, forming the step 24 at the boundary between the sliding portion (bearing surface portions 21, 22) and the central relief portion 23. In other words, in the present invention, the boundary between the sliding portion (bearing surface portions 21, 22) and the relief portion 23 can be formed without sizing, and the tapered surface that occurs between the sliding portion (bearing surface portions 21, 22) and the relief portion 23 when sizing is performed is unlikely to be formed. Furthermore, because the bearing surfaces (inner diameter surfaces of the bearing surface portions 21, 22) are formed by in-mold correction during compression molding and sintered at a low temperature, dimensional change can be minimized, and variation in the axial length of the sliding surface can be reduced. (For example, this variation can be approximately ±0.1 mm.) Furthermore, if a sizing process is performed after the sintering process, the bearing surfaces will be squeezed to form a relief portion, resulting in variation in the axial length of the bearing surfaces, but the present invention, which does not perform a sizing process, does not squeeze to form a relief portion, so variation in the axial length of the bearing surfaces 21 a, 22 a can be reduced. If variation in the axial length L4 (see Figure 4) of the bearing surfaces 21 a, 22 a (i.e., L1 and L2 in Figure 1) can be reduced, the sliding area between the shaft member 2 and the bearing surfaces 21 a, 22 a will be stable, and the bearing characteristics will be stable.

[0064] Furthermore, the sintered bearing of the present invention is produced by compression molding to physically pressurize and contact iron powder particles together or iron powder and resin powder, followed by sintering to thermally harden the resin, forming a cross-sectional structure in which the iron powder particles are adhesively bonded. As a result, the resin melts and softens as it hardens, and the resin intervenes and adheres to the contact points (necks) between the powder particles, reinforcing and strengthening the material, achieving a level of strength that is acceptable for use in bearings under relatively low loads. In particular, the sintered bearing of the present invention has a cross-sectional structure in which the iron powder particles are bonded together by the binding force of the resin, without the progression of sintering due to interdiffusion between the iron powder particles.

[0065] As a result, the present invention makes it possible to obtain a sintered bearing that reduces the contact area with the rotating shaft, achieves stable rotation, keeps the current low, and increases the rotational speed, without requiring a sintering furnace to maintain high temperatures or a large amount of electrical energy, and without requiring hydrogen gas, nitrogen gas, or a mixture of these gases as a processing gas.

[0066] To increase the springback rate, the mixed powder preferably contains 95 to 99 wt% metal powder and the remainder resin powder. The mixed powder can be iron powder, and the iron powder can be coarse with an average particle size of 50 to 200 μm. If the average particle size is less than 50 μm, it becomes difficult to form a resin film on the iron powder surface, resulting in a decrease in material strength. If the average particle size exceeds 200 μm, the powder particles become coarse, resulting in large voids, which can cause oil leaks and increase the frequency of metal contact between the bearing surface and the rotating shaft, thereby deteriorating motor characteristics.

[0067] By using coarse iron powder with an average particle size of 50 μm to 200 μm, coarse pores can be formed throughout the material. Here, coarse pores refer to pores with an average diameter of 10 μm or more. Furthermore, the metal powder (iron powder) is surrounded by the resin powder, and micropores (with an average diameter of less than 10 μm) are formed in this surrounded area.

[0068] The iron powder is preferably spongy iron powder, which has cavities inside the powder and can hold lubricating oil in the cavities. Here, spongy iron powder generally refers to porous iron powder (e.g., sponge iron powder) containing many pores, which is produced by reducing oxidized iron powder such as iron oxide with a gaseous or solid reducing agent. Therefore, this spongy iron powder has micropores that extend to the interior of the powder.

[0069] By setting it in this way, the large pores (micropores) that are internal cavities caused by the coarse powder facilitate the supply of oil and reduce oil shortages, the micropores formed by the resin coating have the effect of suppressing oil leaks and acting as an oil reservoir, and the micropores inside the iron powder have the effect of retaining oil and preventing it from leaking out.

[0070] The resin powder may be an epoxy resin powder containing a thermosetting latent curing agent. Since the resin powder needs to be thermally cured during the baking process, it is preferable to use a thermosetting epoxy resin.

[0071] The mixed powder is compression molded at a molding pressure of 98 MPa to 490 MPa, and the springback rate of the compacted powder body that occurs during this process can be set to 0.2% or more. By setting this, the core pin can be stably removed after compression molding.

[0072] Preferably, the inner diameter of the relief portion 23 is larger than the inner diameter of the bearing surface portions 21, 22 by 0.1% to 0.3%, and the cross-sectional shape of the relief portion 23 is rectangular. By increasing the diameter by 0.1% or more, the relief portion can effectively function to reduce the contact area with the rotating shaft. By having the cross-sectional shape of the relief portion be rectangular, even if the bearing surface portions 21, 22 wear, the area of ​​the bearing surfaces 21a, 22a does not change, making it less likely for the bearing characteristics to change. Furthermore, by increasing the diameter by less than 0.3%, the inner diameter dimension of the relief portion 23 does not become too large compared to the inner diameter dimension of the bearing surface portions 21, 22, effectively preventing damage to the bearing surfaces 21a, 22a, which are the inner diameter surfaces of the bearing surface portions, when the core pin 30 is pulled out.

[0073] The inner diameter surface opening ratio of the bearing surface portion (the surface opening ratio of the bearing surface) is preferably 40% to 80%. The surface opening ratio is the area ratio of all openings in the bearing surface, including not only openings that do not communicate with the internal pores as described above, but also openings that communicate with the internal pores. Increasing the surface opening ratio of the bearing surface in this way reduces the contact area, resulting in more stable rotation.

[0074] The sintered bearing is preferably used as a bearing member for a motor. By using it as a bearing member for a motor, a high-quality motor (small motor) can be provided that can achieve stable rotation for a long period of time.

[0075] The manufacturing method for a sintered bearing according to the present invention allows for easy formation of a rectangular relief portion in the center of the bearing bore without requiring hydrogen gas or nitrogen gas as a processing gas. Furthermore, low-temperature sintering minimizes dimensional change, reducing axial length variation of the bearing surfaces 21a and 22a. Therefore, reducing axial length variation of the bearing surfaces 21a and 22a stabilizes the sliding area between the shaft member 2 and the bearing surfaces 21a and 22a, resulting in stable bearing characteristics. The resin is melted and softened during hardening, and the resin intervenes and adheres to the contact points (neck portions) between the powders, providing sufficient material strength for use in bearings with relatively low loads. Furthermore, the bearing surfaces 21 and 22 and relief portion 23 can be formed using a core pin during compression molding, resulting in excellent productivity.

[0076] Next, FIG. 8 shows another sintered bearing 10, and FIG. 9 is an enlarged cross-sectional view of a main portion of a fan motor using the sintered bearing 10 shown in FIG. 8. This sintered bearing 10 has enlarged diameter sections 25, 26 at both axially outer ends of the inner diameter surface 10d, each having an inner diameter dimension equivalent to that of the relief section 23. That is, when the inner diameter dimension of the enlarged diameter section 25 is D5 and the inner diameter dimension of the enlarged diameter section 26 is D6, in this embodiment, D3 = D5 and D3 = D6. Note that the other components of the sintered bearing 10 shown in FIG. 8 are the same as those of the sintered bearing shown in FIG. 1, and similar components are designated with the same reference numerals as those shown in FIG. 1 and will not be described again. The fan motor shown in FIG. 9 differs from the fan motor shown in FIG. 7 in that the sintered bearing 10 is different from that shown in FIG. 7, and similar components are designated with the same reference numerals as those shown in FIG. 7 and will not be described again.

[0077] The dimensional relationships shown in Figure 8 are also the same as those shown in Figure 1. That is, when the axial length of the bearing member 10 is L, the axial length of the first bearing surface portion 21 is L1, the axial length of the second bearing surface portion 22 is L2, and the axial length of the relief portion 23 is L3, the relationship L1 ≤ L2 < L3 holds. Furthermore, L3 = 1.0 x (L1 + L2) to 4.0 x (L1 + L2). When the axial length of the bearing member 10 is L, the relationship L = 2.0 x (L1 + L2) to 5.0 x (L1 + L2).

[0078] When the axial length of the expanded diameter portion 25 is L5 and the axial length of the expanded diameter portion 26 is L6, the relationships are L1 > L5, L1 > L6, L2 > L5, and L2 > L6. Specifically, it is preferable to set L5 (L6) to 0.2 mm to 2.0 mm.

[0079] As shown in Fig. 9, in the case of a bearing member 10 having enlarged diameter portions 25, 26, the powder-making (mixing) step S1, compression molding step S2, firing step S3, and oil-impregnating step S4 are also performed as shown in Fig. 2. The powder-making (mixing) step S1, firing step S3, and oil-impregnating step S4 are the same as those used in the molding of the bearing member 10 shown in Fig. 1, and therefore a description of these steps will be omitted.

[0080] The compression molding step S2 uses a die apparatus having a core pin 30 as shown in Figures 10A and 10B. This die apparatus includes cylindrical upper and lower punches (not shown), a core pin 30 (see Figure 10A) that forms the inner shape of the powder compact 31, and a die (not shown) that forms the outer shape of the powder compact. The core pin 30 has a first molding portion 30a for molding one bearing surface (the inner diameter surface 21a of the first bearing surface portion 21), a second molding portion 30b for molding the other bearing surface (the inner diameter surface 22a of the second bearing surface portion 22), a third molding portion 30c between the first molding portion 30a and the second molding portion 30b, a fourth molding portion 30d for molding one enlarged diameter portion 25, and a fifth molding portion 30e for forming the other enlarged diameter portion 26. The third molding portion 30c forms the escape portion 23, the fourth molding portion 30d forms the enlarged diameter portion 25, and the fifth molding portion 30e forms the enlarged diameter portion 26, and they are set to have larger outer diameters than the first and second molding portions 30a and 30b.

[0081] After compacting, the powder compact is removed from the die apparatus by opening the die apparatus, which releases the axial pressure applied to the compact 31. When this pressure is released, the elastic restoring force accumulated inside the powder compact is released, causing springback in the powder compact 31, and the inner circumferential surface (inner diameter surface) of the powder compact 31 expands in diameter, as shown in Fig. 10B. This expansion allows the core pin 30 to be pulled out of the powder compact 31. This allows the pre-sintered powder compact 31 to be formed, as shown in Fig. 10C.

[0082] By compression molding a mixed powder containing metal powder and resin powder as its main components, the strength of the green compact 31 can be increased due to the adhesiveness and flexibility of the resin, and the springback (inner diameter springback rate) can be increased. In this way, by increasing the inner diameter springback rate, the core pin 30 can be pulled out from the inner diameter surface of the green compact 31 to which the concave and convex portions of the core pin 30 have been transferred. This allows the cross-sectional shape of the relief portion 23 to be formed into a rectangle.

[0083] That is, steps 30f and 30g are formed at the boundary between the large-diameter third molded portion 30c and the small-diameter first molded portion 30a and at the boundary between the large-diameter third molded portion 30c and the small-diameter second molded portion 30b, and steps 30h and 30i are formed at the boundary between the large-diameter fourth molded portion 30d and the small-diameter first molded portion 30a and at the boundary between the large-diameter fifth molded portion 30e ... That is, step 30f is perpendicular to the outer diameter surface of the first molding portion 30a, step 30g is perpendicular to the outer diameter surface of the second molding portion 30b, step 30h is perpendicular to the outer diameter surface of the first molding portion 30a, and step 30i is perpendicular to the outer diameter surface of the second molding portion 30b.

[0084] In this case, the steps 30f, 30e, 30h, and 30e formed on the core pin 30 during molding in the compression molding step S2 are transferred directly to the green compact 31, and as shown in Figure 11, steps 24A and 24B are formed between the bearing surface portion 21 and the relief portion 23, and between the bearing surface portion 22 and the relief portion 23, respectively, a step 24C is formed between the bearing surface portion 21 and the expanded diameter portion 25, and a step 24D is formed between the bearing surface portion 22 and the expanded diameter portion 26. The green compact 31 is then fired in a firing step S3 to harden the resin powder, and is then impregnated with oil in an oil impregnation step S4 to form a finished product (fired bearing). 10A become the bearing surface portions 21, 22, relief portion 23, and steps 24A, 24B, 24C, and 24D of the sintered bearing (bearing member) 10, as shown in Figures 10A to 10C. Therefore, no tapered surfaces are formed between the bearing surface portions 21, 22 and relief portion 23 in the sintered bearing 10.

[0085] 8, the bearing member 10 has enlarged diameter portions 25, 26 for retaining oil provided at the axially outer edge portions of the bearing surface portions 21, 22. This allows lubricating oil to flow into the sliding portions between the rotating shaft and the bearing surfaces 21 a, 22 a, which are the inner diameter surfaces of the bearing surface portions 21, 22, thereby ensuring a stable supply of oil to the sliding portions. Note that "outward in the axial direction" refers to the direction from the bearing surface portion 21 toward the upper end surface 10 b and the direction from the bearing surface portion 22 toward the lower end surface 10 c.

[0086] As a result, this bearing member 10 reduces the contact area with the rotating shaft, ensuring stable rotation, keeping current low and increasing rotational speed. Furthermore, it is possible to obtain a sintered bearing that does not require a sintering furnace for maintaining high temperatures, or significant electrical energy, and does not require hydrogen gas, nitrogen gas, or a mixture of these gases as a processing gas. In this way, by reducing the frequency of oil shortage and metal contact with the rotating shaft, the sintered bearing exhibits excellent lubrication and sliding characteristics. In particular, even in motor bearings used at high speeds or in low-temperature environments, smooth oil circulation and supply improves oil shortage, resulting in good sliding. As a result, motor speed reduction and instability are alleviated, improving motor performance. Furthermore, it is possible to prevent increases in current and reduce power consumption.

[0087] It is preferable that the surface opening ratios of the inner surfaces of the bearing surface portions 21 and 22, the inner surface of the recess portion 23, and the inner surface of the enlarged diameter portions 25 and 26 are approximately equal. By setting them in this manner, it is possible to obtain balanced rotation. Here, "approximately equal" may not match but may deviate slightly due to design errors, processing errors, assembly errors, etc. This includes this slight deviation.

[0088] The axial lengths L5 and L6 of the enlarged diameter portions 25 and 26 are preferably set in the range of 0.2 mm to 2.0 mm. If they are less than 0.2 mm, they will have difficulty functioning as oil retaining portions, and if they exceed 2.0 mm, the bearing surface of the bearing surface portion will become too small to stably pivotally support the rotating shaft.

[0089] The bearing member 10 having the enlarged diameter portions 25, 26 shown in FIG. 8 also provides the same effects as the bearing member 10 shown in FIG.

[0090] Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment and various modifications are possible. For example, although the axial length L1 of the first bearing surface portion 21 and the axial length L2 of the second bearing surface portion 22 are set to L1≦L2, they may be different. That is, L1≧L2, L1<L2, or L1>L2 may be satisfied. Furthermore, although the axial length of the enlarged diameter portion 25 and the axial length of the enlarged diameter portion 26 are the same in the embodiment, they may be different. That is, when the axial length of the enlarged diameter portion 25 is L5 and the axial length of the enlarged diameter portion 26 is L6, L5>L6 or L5<L6 may be satisfied.

[0091] Furthermore, the sintered bearing according to the present invention can be applied to bearings for fan motors used in home appliances, automobiles, office automation equipment, and the like.

[0092] As shown in Table 1, Example 1 to Example 6 (sintered bearings) and Comparative 1 to Comparative 3 (sintered bearings) were fabricated and their motor characteristics were evaluated. Example 1 to Example 6 and Comparative 1 and Comparative 3 were fan motors with a side length of 120 mm and an axial length of 25 mm, and a rated current of DC 12 V. The bearing structure, except for Comparative 3, consisted of 97 wt% iron powder and 3 wt% epoxy resin powder. Comparative 3 used a mixed powder consisting of copper powder, iron powder, and tin powder. The bearing specifications were an inner diameter of φ3.0, an outer diameter of φ8.0, a width (axial length) of 12 mm, and a clearance of 4 μm between the bearing surface (inner diameter surface) of the bearing surface and the shaft member.

[0093] The bearing specifications were the surface open area ratio, the relief expansion diameter, and the average iron powder particle size. The surface open area ratio was 40% for Example 1, 50% for Example 2, 70% for Example 3, 80% for Example 4, 60% for Example 5, 60% for Example 6, 60% for Comparative Example 1, 60% for Comparative Example 2, and 40% for Comparative Example 3. The relief expansion diameter was 6 μm for Example 1 to Example 4, 3 μm for Example 5, 9 μm for Example 6, 0 μm for Comparative Example 1, 6 μm for Comparative Example 2, and 100 μm for Comparative Example 3. That is, Comparative Example 1 had no relief area, and the average iron powder particle size was 90 μm for Example 1 to Example 6 and Comparative Example 1, and 60 μm for Comparative Example 2. It should be noted that the average particle size of the iron powder cannot be limited for Comparative Product 3, since it uses a mixed powder of copper powder, iron powder, and tin powder, rather than just iron powder. The relief diameter expansion in Table 1 is expressed as t = D3 - D1 (D2), where t is the relief diameter expansion, D1 (D2) is the diameter of the bearing surface, and D3 is the diameter of relief 23.

[0094] The motor characteristics at -30°C were the rotation speed, current value, and starting voltage, and the evaluations for these were indicated by double circle, circle (single circle), triangle, and cross (x). The rotation speed was 820 rpm for Example 1, 870 rpm for Example 2, 890 rpm for Example 3, 850 rpm for Example 4, 800 rpm for Example 5, 850 rpm for Example 6, 650 rpm for Comparison 1, 720 rpm for Comparison 2, and 830 rpm for Comparison 3. The current values ​​were 65 mA for Example 1, 62 mA for Example 2, 61 mA for Example 3, 64 mA for Example 4, 66 mA for Example 5, 63 mA for Example 6, 72 mA for Comparison Product 1, 69 mA for Comparison Product 2, and 65 mA for Comparison Product 3. The starting voltage was 7.9 V for Example 1, 7.6 V for Example 2, 7.5 V for Example 3, 7.7 V for Example 4, 8.0 V for Example 5, 7.7 V for Example 6, 8.5 V for Comparison Product 1, 8.2 V for Comparison Product 2, and 7.8 V for Comparison Product 3.

[0095] In Table 1, double circles indicate rotation speeds of 860 rpm or more, current values ​​of 62 mA or less, and starting voltages of 7.6 V or less, making the product excellent, and examples 2 and 3 are included. In Table 1, single circles indicate rotation speeds of 800 rpm or more, current values ​​of 66 mA or less, and starting voltages of 8.0 V or less, making the product good, and examples 1, 4, 5, 6, and comparative example 3 are included. In Table 1, triangles indicate rotation speeds of 700 rpm or more, current values ​​of 70 mA or less, and starting voltages of 8.6 V or less, making the product acceptable, and comparative example 2 is included. In Table 1, crosses indicate rotation speeds of less than 700 rpm, current values ​​of more than 70 mA, and starting voltages of more than 8.5 V, making the product unacceptable, and examples 1 are included.

[0096] As can be seen from Example 5, if the expansion diameter of the relief portion is less than 3 μm (when the expansion diameter is 0.1% relative to the inner diameter dimension of the bearing surface portion), the characteristic improvement effect of providing the relief portion is diminished. Also, as can be seen from Example 6, if the expansion diameter of the relief portion exceeds 9 μm (when the expansion diameter is 0.3% relative to the inner diameter dimension of the bearing surface portion), the large diameter portion 30 c of the core pin 30 becomes too large compared to the small diameter portion 30 a (30 b), and the inner diameter surface (bearing surface) of the bearing surface portion is damaged when the core pin 30 is pulled out. Comparative Example 1 does not have a relief portion, and does not have any characteristic improvement effect due to the provision of a relief portion.

[0097] Thus, products with a rotation speed of 800 rpm or more, a current of 66 mA or less, and a starting voltage of 8.0 V or less, such as Examples 1, 4, 5, and 6, are preferable as products, while products with a rotation speed of 860 rpm or more, a current of 62 mA or less, and a starting voltage of 7.6 V or less, such as Examples 2 and 3, are more preferable as products. Products with a rotation speed of 700 rpm or more, a current of 70 mA or less, and a starting voltage of 8.6 V or less, such as Comparative Example 2, are unpreferable as products, while products with a rotation speed of less than 700 rpm, a current of more than 70 mA, and a starting voltage of more than 8.5 V, such as Comparative Example 1, are even less preferable as products. Although Comparative Example 3 is preferable as a product, the green compact is not one made by compression molding a mixed powder of metal powder and resin powder, but one made by compression molding a copper-iron-tin mixed powder, and requires subsequent high-temperature sintering and sizing processes.

[0098] As shown in Table 2, sintered bearings 7 to 9 (Examples) and comparative samples 4 to 5 were fabricated and subjected to motor characteristic evaluation. Each example and comparative sample (sintered bearing) was a fan motor with a side length of 120 mm, an axial length of 25 mm, and a rated current of 12 V DC. The bearing specifications were an inner diameter of 3.0 mm, an outer diameter of 8.0 mm, a width (axial length) of 14 mm, a sliding length of each of bearing surface portions 21 and 22 of 3.0 mm, and a clearance of 4 μm between the bearing surface (inner diameter surface) of the bearing surface portion and the shaft member. The dimensions of the relief and enlarged diameter portions of the enlarged diameter portions were 6 μm for Examples 7 to 9, while Comparative Samples 4 and 5 did not have enlarged diameter portions. The relief and enlarged diameter portions were 6 μm for Comparative Sample 4 and 100 μm for Comparative Sample 5.

[0099] The bearing specifications included the amount of metal powder (metal wt%), the amount of resin (epoxy resin) powder (resin wt%), the axial length of both enlarged diameter sections (enlarged diameter length), and price. The metal wt% was 97% Fe for Examples 7 to 9 and Comparative Example 4, while Comparative Example 5 used an Fe-Cu-Sn sintered material. The resin wt% was the remainder for Examples 7 to 9 and Comparative Example 4. Comparative Example 5 used an Fe-Cu-Sn sintered material and no resin powder. The enlarged diameter length was 0.2 mm for Example 7, 1.0 mm for Example 8, and 2.0 mm for Example 9. Comparative Examples 4 and 5 did not have enlarged diameter sections at either end, so the enlarged diameter length was 0 mm. In terms of price, Example Products 7 to 9, which do not use Fe-Cu-Sn sintered material, and Comparative Product 4 are cheap, while Comparative Product 2, which uses Fe-Cu-Sn sintered material, is expensive. Incidentally, the expansion length in Table 2 refers to the dimensions L5 and L6, where L5 = L6.

[0100] The motor characteristics at -30°C were the rotation speed, current value, and starting voltage, and the evaluations for these were indicated by double circles, circles (single circles), triangles, and crosses (x). The rotation speed was 850 rpm for Example 7, 880 rpm for Example 8, 900 rpm for Example 9, 820 rpm for Comparison Example 4, and 900 rpm for Comparison Example 5. The current value was 63 mA for Example 7, 61 mA for Example 8, 60 mA for Example 9, 65 mA for Comparison Example 4, and 60 mA for Comparison Example 5. The starting voltage was 7.7 V for Example 7, 7.6 V for Example 8, 7.5 V for Example 9, 7.8 V for Comparison Example 4, and 7.5 V for Comparison Example 5.

[0101] In Table 2, double circles indicate rotation speeds of 880 rpm or more, current values ​​of 61 mA or less, and starting voltages of 7.6 V or less, making the product excellent. Examples 8 and 9 are examples. In Table 2, single circles indicate rotation speeds of 840 rpm or more, current values ​​of 64 mA or less, and starting voltages of 7.8 V or less, making the product good. Examples 7 and 9 are examples. In Table 2, triangles indicate rotation speeds of 800 rpm or more, current values ​​of 67 mA or less, and starting voltages of 8.0 V or less, making the product acceptable. Examples 4 and 5 are examples. Crosses indicate rotation speeds of less than 700 rpm, current values ​​of more than 70 mA, and starting voltages of more than 8.2 V, making the product unacceptable. Examples 7 to 9 and comparative examples 4 and 5 were not examples. In other words, there are no crosses in Table 2.

[0102] The motor characteristics of Example 7 were improved compared to Comparative Example 4, which did not have the enlarged diameter portions 25, 26 at both axial ends, and it was confirmed that the oil circulation was improved. It can be seen that by making the axial length of the enlarged diameter portions 25, 26 longer than that of Example 7, as in Example 8 and Example 9, the oil retention effect and the oil circulation to the bearing surface were further improved.

[0103] Furthermore, Comparative Example 5, which uses an Fe—Cu—Sn sintered material, requires a sizing process. Thus, when a sizing process is performed, the pores on the bearing surface tend to become dense, which hinders smooth oil supply to the bearing sliding portion, especially in low-temperature environments where the oil's dynamic viscosity is high. However, in the sintered bearings according to the present invention, such as Examples 7 to 9, the bearing surface, enlarged diameter portions at both ends, and relief portions are formed during powder compaction. This results in the bearing surface and enlarged diameter portions having approximately the same surface opening ratio. Furthermore, by making the relief portions coarser than conventional sized bearings, oil circulation from the inside of the bearing to the sliding surface can be promoted. Therefore, it is possible to obtain approximately the same sliding characteristics (motor characteristics) even when the relief depth is approximately 0.1 to 0.3% of the inner diameter, rather than as large as in Comparative Example 5, which requires a relief depth of approximately 3% of the inner diameter.

[0104] The sintered bearing of the present invention is a sintered bearing that can keep flow values ​​low, increase rotational speeds, and does not require hydrogen gas, nitrogen gas, or a mixture of these gases as a treatment gas, and can be used as a bearing for fan motors for home appliances, automobiles, office automation equipment, etc.

[0105] 10 sintered bearing (bearing member) 21, 22 bearing surface portion 21a, 21a inner diameter surface 23 relief portion 24A, 24B, 24C, 24D step 25, 26 enlarged diameter portion 30 core pin 30f step 30g step 30h step 30i step 31 powder compact

Claims

1. A sintered bearing comprising metal powder and resin powder, having bearing surface sections provided at two locations axially spaced apart on its inner peripheral surface, with a relief section between the bearing surface sections whose inner diameter is set to be larger than that of the bearing surface sections, wherein the sintered bearing has a structure in which the metal powder is bound by the resin powder interposed between the metal powders, and the resin powder is an epoxy resin powder containing a thermosetting latent curing agent.

2. The sintered bearing according to claim 1, characterized in that the mixed powder of metal powder and resin powder is 95 to 99 wt % metal powder, with the remainder being resin powder.

3. A sintered bearing according to claim 1, characterized in that the metal powder in the mixed powder of metal powder and resin powder is iron powder, and the iron powder is a coarse powder with an average particle size of 50 μm to 200 μm.

4. A sintered bearing according to claim 3, characterized in that the iron powder is spongy iron powder having cavities inside the powder, capable of holding lubricating oil in the cavities.

5. The sintered bearing according to claim 1, characterized in that the sintered bearing has a cross-sectional structure in which the iron powder particles are fixed together by the binding force of the resin, without the progression of sintering due to interdiffusion between the iron powder particles.

6. A sintered bearing according to claim 1, characterized in that the mixed powder of the metal powder and the resin powder is compressed in a mold, and the springback rate of the compacted powder body diameter generated during this process is 0.2% or more.

7. A sintered bearing as described in claim 1, characterized in that the inner diameter of the recess is 0.1% to 0.3% larger than the inner diameter of the bearing surface portion, and the cross-sectional shape of the recess is rectangular.

8. A sintered bearing according to claim 1, characterized in that the inner diameter surface opening ratio of the bearing surface portion is 40% to 80%.

9. A sintered bearing according to claim 1, characterized in that an enlarged diameter portion opening axially outward is provided on the axially outer edge of said bearing surface portion.

10. A sintered bearing according to claim 9, characterized in that the surface opening ratios of the inner surface of the bearing surface portion, the inner surface of the relief portion, and the inner surface of the enlarged diameter portion are approximately equal.

11. A sintered bearing according to claim 9, characterized in that the axial length of each enlarged diameter portion is set in the range of 0.2 mm to 2.0 mm.

12. A sintered bearing according to any one of claims 1 to 11, which is used as a bearing member for a motor.

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