JP7754975B1 - Sintered bearings - Google Patents

Abstract

[Problem] To provide a sintered bearing that reduces the contact area with the rotating shaft, allows 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.
[Solution] The sintered bearing has bearing surface sections provided at two axially spaced locations on its inner peripheral surface, a relief section between the bearing surface sections whose inner diameter is set larger than that of the bearing surface sections, and contains metal powder and resin powder. An enlarged diameter section that opens axially outward is provided at the axially outer edge of the bearing surface section, and the sintered bearing has a structure in which the metal powder is bound with resin powder interposed between the metal powder sections.
[Selected Figure] Figure 1

Description

The present invention relates to sintered bearings.

Sintered bearings are often used as bearing materials for small motors (small motors primarily 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).

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 at the sliding surface with the shaft as the bearing moves relative to the shaft it supports, and this oil film supports the shaft.

Sintered bearings are inexpensive and highly reliable, and are widely used in home appliance motors, automotive motors, and fan motors for office equipment. Fan motors include cooling fans inside home appliances such as computers and televisions, circulation and cooling fans inside refrigerators, and automotive fans used to cool batteries and draw in temperature sensors inside the vehicle, and demand is growing every year.

However, motors with low drive torque tend to have their characteristics affected by frictional resistance on the sliding surfaces, and oil circulation and supply do not work properly at high speeds or in low-temperature environments, making it difficult to achieve smooth sliding. They also have the disadvantage of being prone to a decrease in motor rotation speed and an increase in current value.

For this reason, conventional bearings, such as those described in Patent Documents 1 and 2, have a recess in the center of the bearing's inner diameter to reduce the rotational load of the motor. In other words, the bearing (sintered bearing) in Patent Document 1 has bearing surfaces at two locations axially spaced apart on the inner circumferential surface (inner diameter surface), and a recess between the bearing surfaces whose inner diameter is set larger than the inner diameter of the bearing surfaces.

In this case, a sizing process was carried out after the sintering process to form the relief. The sizing process involves placing the sintered material back into a mold and compressing it to achieve highly accurate dimensions and shape. When the material is placed in a mold and pressure is applied with the upper and lower punches, it is pressed 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 producing the material larger than the final product dimensions, forcing it into the mold during sizing and rubbing it against the die or core to achieve precision. Negative sizing involves producing the material smaller than the final product dimensions, and compressing it inside the mold to press it against the mold surface to achieve precision.

In addition, the design described in Patent Document 1 has numerous dimples (recessed portions) formed on the bearing surface formed on both axial ends. By providing dimples in this way, it is possible to reduce the sliding area on the bearing surface, thereby reducing frictional resistance. It also shows that oil-impregnated lubricant is stored within each dimple, and when the rotating shaft rotates, the stored lubricant is drawn out between the bearing surface and the rotating shaft, making it possible to reduce the coefficient of friction of the bearing surface.

Patent No. 36253134 JP 2010-31909 A

As mentioned above, conventional sintered bearings with recesses undergo a sizing process after the sintering process, and the recesses are formed by reducing the outer diameter. This results in a smooth boundary between the sliding portion, which has a sliding surface, and the recesses.

Figure 8 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 surface portions 51, 52 and a recess portion 53 between the bearing surface portions 51, 52 are formed on the inner diameter portion of the bearing 50, and a tapered surface 54 is formed between the inner diameter surface 53a of the recess portion 53 and the bearing surface 52a of the bearing surface portion 52.

If the tapered surface 54 is formed in this way, there is a risk that the sliding area will change (increase) compared to the initial state due to wear on the sliding surfaces (bearing surfaces 51a, 52a, 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 will change.

Furthermore, since the outer diameter is narrowed and the inner diameter 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) L7 of the bearing surface 52a. In this case, variations of up to ±0.3 mm may occur.

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

Furthermore, when forming dimples as in Patent Document 1, Patent Document 1 describes the use of plastic processing such as peening, rolling, and coining as methods for forming dimples. Such processing requires additional processing equipment and processing man-hours, and also poses problems in terms of productivity and cost, such as the need to manufacture tools for plastic processing with convex portions.

This application therefore provides a sintered bearing that reduces the contact area with the rotating shaft, ensures stable rotation, keeps the current low, and increases 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.

The sintered bearing of the present invention has bearing surface sections provided at two axially spaced locations on its inner peripheral surface, a relief section between the bearing surface sections whose inner diameter is set larger than the inner diameter of the bearing surface sections, and is a sintered bearing containing metal powder and resin powder, with an expanded diameter section that opens axially outward at the axially outer edge of the bearing surface section, and the sintered bearing has a structure in which the metal powder is bound by the resin powder interposed between the metal powder sections.

The sintered bearing of the present invention is made by compression molding a powder mixture primarily composed of iron powder and resin. The adhesiveness and flexibility of the resin increase the strength of the green compact, while also increasing 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.

The sintered bearing is hardened in an air atmosphere at approximately 200°C (here, approximately 200°C means 150°C to 250°C). This eliminates the need for a sintering furnace to maintain high temperatures of 700°C to 900°C, as is required in conventional sintering processes, and does not require hydrogen or nitrogen gas as a processing gas. Furthermore, the step created in the core pin during molding is transferred directly, forming the step at the boundary between the sliding section and the central relief section. In other words, with this invention, the boundary between the sliding section and the relief section can be formed without sizing, and the tapered surface that occurs between the sliding section and the relief section when sizing is performed is less likely to form. Furthermore, because the bearing surface (the inner diameter surface of the bearing surface section) is formed by in-mold correction during compression molding and sintered at a low temperature, dimensional change is minimized, and variation in the axial length of the sliding surface can be reduced. (For example, this variation can be reduced to approximately ±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, resulting in variation in the axial length of the bearing surface. However, the present invention, which does not involve a sanding process, does not squeeze to form a relief portion, so variation in the axial length of the bearing surface can be reduced. Reducing variation in the axial length of the bearing surface stabilizes the sliding area between the shaft member and bearing surface, resulting in stable bearing characteristics.

In addition, the sintered bearings of the present invention are made by compression molding to physically bring iron powder particles into contact with each other or iron powder and resin powder, and then sintering to thermally harden the resin, forming a cross-sectional structure in which the iron powder is 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 them, achieving a material strength at a level that is acceptable for use in bearings with relatively low loads.

In addition, an expanded diameter section that opens axially outward is provided on the axially outer edge of the bearing surface section, allowing lubricating oil to flow into the sliding area between the bearing surface (the inner diameter surface of the bearing surface section) and the rotating shaft, ensuring a stable supply of oil to the sliding area.

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 expanded diameter portion are approximately equal. By setting them in this way, well-balanced rotation can be achieved. Here, "approximately equal" means that there may be some deviations due to design errors, processing errors, assembly errors, etc. This includes some deviations.

To increase the springback rate, it is preferable for the mixed powder of metal powder and resin powder to be 95 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 a coarse powder 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 will be too coarse, resulting in large voids, oil leaks, and increased frequency of metal contact between the bearing surface and the rotating shaft, deteriorating motor performance.

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 sintering occurring due to interdiffusion between the iron powder particles.

It is preferable that the inner diameter of the recess and the enlarged diameter portion be enlarged by 0.1% to 0.3% compared to the inner diameter of the bearing surface portion, and that the cross-sectional shape of the recess be rectangular. By enlarging the diameter by 0.1% or more in this way, the function of the recess, which reduces the contact area with the rotating shaft, can be effectively fulfilled. By having a rectangular cross-sectional shape of the recess, even if the bearing surface portion wears, the area of the bearing surface does not change, making it less likely for the bearing characteristics to change. Furthermore, by enlarging the diameter by 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, effectively preventing damage to the bearing surface, which is the inner diameter surface of the bearing surface portion, when the core pin is pulled out.

The axial length of each enlarged diameter section 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 to function as an oil retaining section, and if it exceeds 2.0 mm, the bearing surface of the bearing surface section will be too small, making it difficult to stably support the rotating shaft.

Preferably, the sintered bearings are used as bearing components for motors. By using them as motor bearing components in this way, it is possible to provide high-quality motors (small motors) that can achieve stable rotation over the long term.

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 to maintain high temperatures, and does not require hydrogen gas, nitrogen gas, or a mixture of these gases as processing gases. Furthermore, by reducing the frequency of oil shortage and metal contact with the rotating shaft, the sintered bearing has excellent lubrication and sliding properties. In particular, even in motor bearings used at high speeds or in low-temperature environments, oil circulation and supply work smoothly, improving oil shortage and ensuring good sliding. As a result, motor speed reductions and instability can be alleviated, improving motor performance. It also prevents increases in current and reduces power consumption.

1 is a simplified cross-sectional view of a sintered bearing of the present invention. 1 shows a manufacturing process for a sintered bearing of the present invention. 1A shows the compression-forming process for the sintered bearing of the present invention, in which (a) is a front view showing the state in which the concave and convex portions of the core pin are transferred to the green compact, (b) is a simplified diagram showing the relationship between the compressed body and the core pin in the spring-back state, and (c) is a simplified cross-sectional view of the compressed body with the core pin removed. FIG. 2 is a simplified cross-sectional view of the essential parts of the compressed body with the core pin removed. FIG. 1 is a cross-sectional structure image of a material. FIG. 1 is a cross-sectional view of a fan motor using a sintered bearing of the present invention. FIG. 6 is an enlarged cross-sectional view of a main part of the fan motor shown in FIG. 5 . FIG. 2 is a simplified cross-sectional view of a main part of a sintered bearing after a sizing process.

Figure 6 shows a fan motor. This fan motor comprises a fluid dynamic bearing device 1, a motor base 5 forming 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, causing 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.

As shown in Figure 7, the fluid dynamic bearing device 1 is a so-called rotating shaft type bearing device primarily comprising 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) filled in the internal space of the housing 8. The bearing member 10 employs a sintered bearing according to an embodiment of the present invention. For ease of explanation, the upper side of Figure 5 (the side where the seal member 9 is located) will be referred to as the "upper side," and the lower side of Figure 2 will be referred to as the "lower side." However, this is not intended to limit the orientation of the fluid dynamic bearing device 1 during use.

The shaft member 2 is made of a highly rigid metal material such as stainless steel, and its outer surface 2a is a smooth cylindrical surface without any irregularities, while its lower end surface 2b is a convex spherical surface. A rotor 3, to which blades 4 and a rotor magnet 7 (see Figure 1) are attached, is fixed to the upper end of the shaft member 2.

The housing 8 is a cylindrical body with a bottom, having 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 bottom portion 8b are integrally formed from resin or metal. The inner peripheral surface 8a1 of the cylindrical portion 8a is formed into a cylindrical surface of a constant diameter, and its lower end is connected to the outer diameter end of an annular shoulder surface 8b2 that is formed into a flat surface perpendicular to the axial direction. The stator coil 6 and motor base 5 are fixed to the outer peripheral surface 8a2 of the cylindrical portion 8a with a gap between them.

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.

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

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.

The sintered bearing 10 of the present invention, which is the bearing member 10 used in the fan motor of Figures 6 and 7, is made by sintering a compacted powder body and impregnating it with lubricating oil. It has bearing surface sections 21 and 22 at two axially spaced locations on the inner peripheral surface (inner diameter surface) 10d. Between the bearing surface sections 21 and 22, there is a relief section 23 whose inner diameter is set larger than that of the bearing surface sections. Bearing surface section 21 is sometimes referred to as the first bearing surface section, and bearing surface section 22 is sometimes referred to as the second bearing surface section. 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, in which heating bonds the powder particles and induces shrinkage. On the other hand, sintering is a process that often uses ceramic materials, in which a chemical reaction is induced at high temperatures by heating to increase mechanical strength. For this reason, this process is sometimes referred to as sintering or firing in this specification.

In this case, the outer diameter surface 10a of the bearing member 10 is a straight cylindrical surface without any steps. The inner diameter dimensions of the first bearing surface portion 21 and the second bearing surface portion 22 are set to the same dimensions, and the inner diameter dimension of the relief portion 23 is 0.1% to 0.3% larger than the inner diameter dimensions of both bearing surface portions 21 and 22. That is, as shown in FIG. 1, 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, 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 refer to within 0.001 mm.

Enlarged diameter sections 25, 26, each with an inner diameter dimension equivalent to that of the recess 23, are provided at both axially outer ends of the inner diameter surface 10d of the bearing member 10. In other words, if the inner diameter dimension of enlarged diameter section 25 is D5 and the inner diameter dimension of enlarged diameter section 26 is D6, then in this embodiment, D3 = D5 and D3 = D6. Note that "equivalent" means that there may be slight deviations due to design, processing, assembly, and other factors. This slight deviation is included. Therefore, D5 (D6) = (D1 + D1/1000) to (D1 + 3D1/1000), and D5 (D6) = (D2 + D2/1000) to (D2 + 3D2/1000).

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 recess 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). 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 L1 > L5, L1 > L6, L2 > L5, and L2 > L6 hold. Specifically, it is preferable to set L5 (L6) between 0.2 mm and 2.0 mm.

As shown in Figure 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 the 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, gluing, or press-fitting and gluing (a combination of press-fitting and gluing), or it can 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 axially between the seal member 9 and the shoulder surface 8b2 of the housing 8. The latter fixing method in particular 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.

As shown in Figure 2, this bearing member (sintered bearing) 10 is formed by performing a powder-making (mixing) process S1, a compression molding process S2, a sintering process S3, and an oil-impregnating process S4.

In the powder-making step S1, metal powder and resin powder are mixed to produce raw material powder. While the metal powder is not particularly limited, this embodiment uses iron powder, which is commonly used in powder metallurgy applications, has low raw material costs, and is easily available. In other words, it is not limited to iron powder; stainless steel powder, copper powder, tin powder, etc. may also be used. Furthermore, since the resin powder must be thermally cured during the firing process, 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.

It is preferable that the mixed powder of metal powder and resin powder contains 95 to 99 wt% metal powder, with the remainder being resin powder.

Next, in the compression molding process S2, the powder 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. 3(a)) 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 (inner diameter surface 21a of the first bearing surface portion 21), a second molding portion 30b for molding the other bearing surface (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 that forms one enlarged diameter portion 25, and a fifth molding portion 30e that forms the other enlarged diameter portion 26. The third molding portion 30c forms the recess portion 23, the fourth molding portion 30d forms the expanded diameter portion 25, and the fifth molding portion 30e forms the expanded diameter portion 26, and they are set to have larger outer diameters than the first and second molding portions 30a and 30b.

After compact formation, the compact can be removed from the die device by opening the die device, which releases the axial pressure applied to the compact 31. When this pressure is released, the elastic restoring force accumulated inside the compact is released, causing springback in the compact 31, and the inner circumferential surface (inner diameter surface) of the compact 31 expands in diameter, as shown in Figure 3(b). This expansion allows the core pin 30 to be pulled out of the compact 31. This allows the compact 31 to be formed before firing, as shown in Figure 3(c).

By compressing and molding a mixed powder primarily composed of metal powder and resin powder, the adhesiveness and flexibility of the resin can be used to increase the strength of the green compact 31, while also increasing the springback (internal diameter springback rate). Increasing the internal diameter springback rate in this way makes it possible to pull the core pin 30 out of the internal diameter surface of the green compact 31, onto which the irregularities 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.

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 these steps 30f and 30g are perpendicular to the axial direction. 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 and the small-diameter second molded portion 30b, and these steps 30h and 30i are perpendicular to the axial direction. That is, step 30f is perpendicular to the outer diameter surface of first molded portion 30a, step 30g is perpendicular to the outer diameter surface of second molded portion 30b, step 30h is perpendicular to the outer diameter surface of first molded portion 30a, and step 30i is perpendicular to the outer diameter surface of second molded portion 30b.

In this case, the steps 30f, 30e, 30h, and 30f formed on the core pin 30 during compression molding step S2 are transferred directly to the green 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 Figure 4, step 24C between the bearing surface portion 21 and the expanded diameter portion 25, and step 24D between the bearing surface portion 22 and the expanded diameter portion 26. The green compact 31 is then fired in step S3 to harden the resin powder, resulting in a fired body. The product (fired bearing) is then formed by impregnating it with oil in step S4. As a result, the bearing surface portions 21 and 22, relief portion 23, and steps 24A, 24B, 24C, and 24D of the powder compact 31 shown in Figure 3(a) become the bearing surface portions 21 and 22, relief portion 23, and steps 24A, 24B, 24C, and 24D of the sintered bearing (bearing member) 10, as shown in Figure 3. Therefore, no tapered surfaces are formed between the bearing surface portions 21 and 22 and the relief portion 23 in the sintered bearing 10.

In a typical sintered bearing, the sintering step S3 involves heating the green compact 31 obtained in the powder compacting step (compression molding step) to the sintering temperature of the metal powder used to obtain a sintered body. In other words, sintering is carried out in a predetermined atmosphere and at 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, the bearing member 10, which is a sintered bearing according to the present invention, is produced by compression molding a mixture of metal powder and resin powder, so sintering can be carried out in an atmospheric pressure atmosphere at around 200°C. Note that around 200°C means 150°C to 250°C.

In the oil impregnation step S4, the sintered body is impregnated with lubricating oil, thereby completing the sintered bearing 10, a bearing component with its internal pores impregnated with lubricating oil. Impregnation of the internal pores of the sintered bearing 10 with lubricating oil is carried out, for example, by immersing the sintered bearing 10 in a lubricating oil bath filled with lubricating oil for a certain period of time under a specified reduced pressure. To ensure reliable impregnation in a short period of time, the lubricating oil may be heated during the impregnation process.

Figure 5 is an image of the cross-sectional structure of sintered bearing 10. Sintered bearing 10 is made by compression molding, which physically pressurizes and contacts iron powder with other iron powder or iron powder with resin powder, and then sintering to thermally harden the resin, forming a cross-sectional structure in which the iron powder is adhesively bonded. Furthermore, as the resin hardens, it melts and softens, 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.

In this 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. Furthermore, 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.

In other words, 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, micropores (fine voids) are formed. Furthermore, spongy iron powder (reduced iron powder) has micropores that extend to the interior of the powder. The coarse pores (fine voids), which are internal cavities created by the coarse powder, facilitate oil supply and reduce oil shortages, 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 leakage.

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 micropores (average diameter: less than 10 μm) surrounded by resin is set as follows: The area ratio of coarse pores to micropores is set to 1:1 to 9:1 (area ratio of coarse pores = 50% to 90%).

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, especially in low-temperature environments. If the area ratio of coarse pores is 90% or more, oil leaks will occur, and in high-temperature environments, the frequency of metal contact with the rotating shaft will increase, making it more likely that current will increase and bearing wear will occur.

The sintered bearing of the present invention is made by compression molding a composite powder primarily composed of iron powder and resin. The adhesiveness and flexibility of the resin increase the strength of the green compact 31, while also increasing 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 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 30. This makes it easy to form a rectangular relief portion 23 in the center of the inner diameter of the bearing.

Because the sintered bearing is sintered in an air atmosphere at approximately 200°C, it does not require a sintering furnace to maintain high temperatures of 700°C to 900°C, as is required in conventional sintering processes, nor does it require hydrogen gas or nitrogen gas as a processing gas. Furthermore, the step created in the mold core pin during molding is transferred directly, 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, with this 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 less likely to form. Furthermore, because the bearing surface (the inner diameter surface of the bearing surface portions 21, 22) is formed by in-mold correction during compression molding and sintered at a low temperature, dimensional change is 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 surface will be squeezed to form a recess, resulting in variation in the axial length of the bearing surface. However, the present invention, which does not perform a sanding process, does not squeeze to form a recess, so variation in the axial length of bearing surfaces 21a, 22a can be reduced. Reducing variation in the axial length L4 (see Figure 4) of bearing surfaces 21a, 22a (i.e., L1 and L2 in Figure 1) stabilizes the sliding area between shaft member 1 and bearing surfaces 21a, 22a, resulting in stable bearing characteristics.

The sintered bearings of the present invention are produced by compression molding, whereby iron powder particles or iron powder and resin powder are physically pressed into contact with each other, and then sintering, which thermally hardens the resin and forms a cross-sectional structure in which the iron powder is 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. In particular, the sintered bearings of the present invention have 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.

In addition, the bearing surface portions 21, 22 are provided with enlarged diameter portions 25, 26 for retaining oil at their axially outer edge portions, allowing lubricating oil to flow into the sliding areas between the rotating shaft and the bearing surfaces 21a, 22a, which are the inner diameter surfaces of the bearing surface portions 21, 22, ensuring a stable oil supply at the sliding areas. Note that "axially outward" refers to the direction from the bearing surface portion 21 toward the upper end surface 10b and the direction from the bearing surface portion 22 toward the lower end surface 10c.

As a result, 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 to maintain high temperatures, and does not require hydrogen gas, nitrogen gas, or a mixture of these gases as processing gases. In this way, by reducing the frequency of oil shortage and metal contact with the rotating shaft, the sintered bearing has excellent lubrication and sliding properties. In particular, even in motor bearings used at high speeds or in low-temperature environments, oil circulation and supply work smoothly, reducing oil shortage and achieving good sliding. As a result, motor speed reductions and instability can be alleviated, improving motor performance. It also prevents increases in current and reduces power consumption.

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. Setting them in this way allows for balanced rotation. Here, "approximately equal" may mean that there is some deviation due to design errors, processing errors, assembly errors, etc. This slight deviation is included.

To increase the springback rate, it is preferable for the mixed powder of metal powder and resin powder to be 95 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 a coarse powder 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, reducing the material strength. If the average particle size exceeds 200 μm, the powder particles will be too coarse, resulting in large voids, oil leaks, and increased frequency of metal contact between the bearing surface and the rotating shaft, deteriorating motor performance.

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" refers to pores with an average diameter of 10 μm or more. Furthermore, the metal powder (iron powder) is surrounded by resin powder, and micropores (with an average diameter of less than 10 μm) are formed in this surrounded area.

The iron powder is preferably spongy iron powder, which has cavities inside the powder and is capable of holding lubricating oil in those cavities. Here, spongy iron powder refers generally to porous iron powder (such as 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. For this reason, this spongy iron powder has tiny pores that extend to the interior of the powder.

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

The resin powder can 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.

The mixed powder is compression molded at a molding pressure of 98 MPa to 490 MPa, and the springback rate of the compacted inner diameter that occurs during this process can be set to 0.2% or more. By setting it in this way, the core pin can be reliably removed after compression molding.

The inner diameter of the relief portion 23 and the inner diameter of the enlarged diameter portions 25, 26 are preferably enlarged by 0.1% to 0.3% compared to the inner diameter of the bearing surface portions 21, 22, and the cross-sectional shape of the relief portion 23 is preferably rectangular. By enlarging the diameter by 0.1% or more, the relief portion effectively functions to reduce the contact area with the rotating shaft. By having a rectangular cross-sectional shape of the relief portion, 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 enlarging 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.

The axial length of each enlarged diameter section 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 to function as an oil retaining section, and if it exceeds 2.0 mm, the bearing surface of the bearing surface section will be too small, making it difficult to stably support the rotating shaft.

Preferably, sintered bearings are used as bearing components for motors. By using them as motor bearing components, it is possible to provide high-quality motors (small motors) that can achieve stable rotation over the long term.

The manufacturing method for sintered bearings according to the present invention allows for easy formation of a rectangular relief portion in the center of the bearing inner diameter, without requiring hydrogen gas, nitrogen gas, or other processing gases. Furthermore, low-temperature sintering minimizes dimensional change, enabling reduced variation in the axial length of the bearing surfaces 21a, 22a. Therefore, reducing variation in the axial length of the bearing surfaces 21a, 22a stabilizes the sliding area between the shaft member (rotating shaft) 2 and the bearing surfaces 21a, 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 reinforcement and strength sufficient for use in bearings with relatively low loads. Furthermore, the bearing surfaces, relief portions, and enlarged diameter portions can be formed using a core pin during compression molding, resulting in excellent productivity.

The above describes an embodiment of the present invention, but the present invention is not limited to the above embodiment and various modifications are possible. For example, while the axial length L1 of the first bearing surface portion 21 and the axial length L2 of the second bearing surface portion 22 were set to L1≦L2, they may be different. That is, they may be L1≧L2, L1<L2, or L1>L2. Furthermore, while the axial length of the expanded diameter portion 25 and the axial length of the expanded diameter portion 26 were the same in the embodiment, they may be different. That is, when the axial length of the expanded diameter portion 25 is L5 and the axial length of the expanded diameter portion 26 is L6, they may be L5>L6 or L5<L6.

Furthermore, the sintered bearings of the present invention can be used as bearings for fan motors used in home appliances, automobiles, office automation equipment, etc.

As shown in Table 1, sintered bearings 1 to 3 (Examples) and comparative products 1 and 2 were fabricated and subjected to motor characteristic evaluation. Each example and comparative product (sintered bearing) was a fan motor with a side length of 120 mm, an axial length of 25 mm, and a rated current of 12V DC. The bearing specifications were an inner diameter of φ3.0, an outer diameter of φ8.0, a width (axial length) of 14 mm, a sliding length of bearing surface portions 21 and 22 of 3.0 mm each, and a clearance of 4 μm between the bearing surface (inner diameter surface) of the bearing surface portion and the shaft member. The relief and enlarged diameter dimensions of Examples 1 to 3 were 6 μm, while comparative products 1 and 2 did not have enlarged diameter portions. The relief and enlarged diameter dimensions were 6 μm for Comparative Product 1 and 100 μm for the comparative products.

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 1 to 3 and Comparative Example 1, while Comparative Example 2 used an Fe-Cu-Sn sintered material. The resin wt% was the remainder for Examples 1 to 3 and Comparative Example 1. Comparative Example 2 used an Fe-Cu-Sn sintered material and no resin powder. The enlarged diameter length was 0.2 mm for Example 1, 1.0 mm for Example 2, and 2.0 mm for Example 3. Comparative Examples 1 and 2 did not have enlarged diameter sections at either end, so the enlarged diameter length was 0 mm. In terms of price, Example Products 1 to 3, which do not use Fe-Cu-Sn sintered material, and Comparative Product 1 are inexpensive, while Comparative Product 2, which uses Fe-Cu-Sn sintered material, is expensive. Incidentally, the expansion length in Table 1 refers to the dimensions L5 and L6, with L5 = L6.

Motor characteristics at -30°C were rotation speed, current value, and starting voltage, and were evaluated using double circle, circle (single circle), triangle, and cross (x). The rotation speed was 850 rpm for Example 1, 880 rpm for Example 2, 900 rpm for Example 3, 820 rpm for Comparison 1, and 900 rpm for Comparison 2. The current value was 63 mA for Example 1, 61 mA for Example 2, 60 mA for Example 3, 65 mA for Comparison 1, and 60 mA for Comparison 2. The starting voltage was 7.7 V for Example 1, 7.6 V for Example 2, 7.5 V for Example 3, 7.8 V for Comparison 1, and 7.5 V for Comparison 2.

In Table 1, double circles indicate rotation speeds of 880 rpm or higher, currents of 61 mA or lower, and starting voltages of 7.6V or lower, making the product excellent. This applies to Example Products 2 and 3. In Table 1, single circles indicate rotation speeds of 840 rpm or higher, currents of 64 mA or lower, and starting voltages of 7.8V or lower, making the product good. This applies to Example Product 1. In Table 1, triangles indicate rotation speeds of 800 rpm or higher, currents of 67 mA or lower, and starting voltages of 8.0V or lower, making the product acceptable. This applies to Comparison Product 1. Crosses indicate rotation speeds of less than 700 rpm, currents of more than 70 mA, and starting voltages of more than 8.2V, making the product unacceptable. None of the example products or comparison products qualify. In other words, there are no crosses in Table 1.

Product 1 demonstrated improved motor characteristics compared to Comparative Product 1, which does not have enlarged diameter sections 25, 26 at both axial ends, confirming improved oil circulation. It can be seen that by making the axial length of the enlarged diameter sections 25, 26 longer than in Product 1, as in Products 2 and 3, the oil retention effect and oil circulation to the bearing surface are further improved.

Furthermore, Comparative Example 2, which uses an Fe-Cu-Sn sintered material, requires a sizing process. As such, when a sizing process is performed, the pores on the bearing surface tend to become denser, which can hinder smooth oil supply to the bearing's sliding surfaces, particularly in low-temperature environments where the oil's dynamic viscosity is high. However, in the sintered bearings of the present invention, such as Examples 1 to 3, the bearing surface, enlarged diameter portions at both ends, and relief portions are formed during powder compaction. This results in roughly equivalent surface opening rates for both the bearing surface and the enlarged diameter portions. Furthermore, by making the relief portions coarser than conventional sized bearings, it is possible to promote oil circulation from inside the bearing to the sliding surfaces. Therefore, it is possible to achieve roughly equivalent sliding characteristics (motor performance) even when the relief depth is approximately 0.1 to 0.3% of the inner diameter, rather than as large as in Comparative Example 2, which requires a depth of approximately 3% of the inner diameter.

10. Sintered bearing (bearing member)
21, 22 Bearing surface portions 21a, 22a Inner diameter surface 23 Relief portions 24A, 24B, 24C, 24D Steps 25, 26 Expanded diameter portion 30 Core pin 30f Step 30g Step 30h Step 30i Step 31 Powder compact

Claims (7)

A sintered bearing comprising a metal powder and a resin powder, the bearing comprising bearing surface portions provided at two axially spaced locations on an inner peripheral surface, a relief portion having an inner diameter set larger than the inner diameter of the bearing surface portions between the bearing surface portions,
A sintered bearing characterized in that an enlarged diameter portion that opens axially outward is provided at the axially outer edge portion of the bearing surface portion, and the sintered bearing has a structure in which the metal powder is bound by the resin powder interposed between the metal powders. A sintered bearing as described in claim 1, 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. The sintered bearing according to claim 1, characterized in that the mixed powder of metal powder and resin powder is 95 wt% to 99 wt% metal powder, with the remainder being resin powder. The sintered bearing described in claim 1 has a cross-sectional structure in which the iron powder particles are fixed together by the binding force of the resin, without sintering occurring due to interdiffusion between the iron powder particles. A sintered bearing as described in claim 1, characterized in that the inner diameter of the recess and the inner diameter of the enlarged diameter portion are enlarged by 0.1% to 0.3% compared to the inner diameter of the bearing surface portion, and the cross-sectional shape of the recess is rectangular. The sintered bearing described in claim 1, characterized in that the axial length of each enlarged diameter portion is set in the range of 0.2 mm to 2.0 mm. The sintered bearing according to any one of claims 1 to 6, characterized in that it is used as a bearing member for a motor.