Sintered alloy bearing

Abstract

This sintered alloy bearing is obtained by compressing a raw-material powder to form a green compact and sintering the green compact. A sintered alloy contains 1.0-5.0 wt. % of Cu, 0.4-2. 0 wt. % of Sn, and 0 wt. % or 0.3-3.0 wt. % of C as free carbon, with the balance consisting of Fe and unavoidable impurities. An iron structure is formed of an iron powder and is configured so that a ferrite structure has an area ratio of 90% or more.

Description

sintered alloy bearings

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

[0002] Sintered alloy bearings are self-lubricating bearings in which lubricating oil is impregnated into the pores present in the sintered body. Such sintered alloy bearings can be used in transportation equipment such as four-wheeled vehicles and two-wheeled vehicles, industrial machinery such as office machines and general machinery, and electrical machinery such as IT equipment, AV equipment, and home appliances.

[0003] A sintered alloy has been disclosed in the past as "a material suitable for machine parts that require high strength, lubricity, good machinability, relatively high hardness, and wear resistance, in which graphite is not completely dissolved in iron but is partially dispersed in a free state" (Patent Document 1).

[0004] The iron-based sliding member (sintered alloy) described in Patent Document 1 partially contains network-like cementite. Here, "network" refers to a mesh-like structure, which is generally constructed by connecting bases with lines, and is a structure in which bases are bonded like a three-dimensional mesh through chemical bridges. The network also consists of an autonomous whole and autonomous parts. In other words, the network is autonomous as a whole and also autonomous as a part. That is, when iron and graphite are mixed and sintered in the material described in Patent Document 1, the graphite diffuses into the iron, forming a pearlite structure. However, when the graphite content exceeds 0.8%, linear cementite structures precipitate. This results in the formation of network-like cementite. Cementite is a compound of iron and carbon, represented by the chemical formula Fe3C. It is a compound that appears in pearlite structures and tempered martensite structures in steel materials.

[0005] Special Publication No. 56-27591

[0006] Incidentally, cementite in a metal structure tends to be hard, and the greater the cementite content, the greater the aggressiveness toward the mating member (aggressiveness toward the mating member). Aggressiveness toward the mating member refers to the tendency toward wear and damage to the mating member. For this reason, if a sintered alloy bearing is constructed using an iron-based sliding member (sintered alloy) such as that described in Patent Document 1, the bearing will tend to be hard, and when metal-to-metal contact occurs during sliding, the aggressiveness toward the shaft will be high, and the mating member will be more likely to wear out.

[0007] Furthermore, since the alloy contains network-like cementite, the sintering temperature in Patent Document 1 is set at a relatively high temperature of 1000 to 1300°C. However, such a high sintering temperature increases the amount of carbon dioxide (CO2) generated during the manufacturing process, adversely affecting the environment, and also increases the amount of energy used, leading to increased costs. Furthermore, if the sintered alloy contains a large amount of Cu or Sn, this also leads to increased costs.

[0008] In view of the above, the present invention provides a sintered alloy bearing that is less likely to cause wear on a mating member (mating shaft) and that can minimize adverse environmental impacts during manufacturing.

[0009] The sintered alloy bearing of the present invention is obtained by compressing raw material powder to form a green compact, and then sintering this green compact, and the sintered alloy contains, by weight, 1.0 to 5.0 wt% Cu, 0.4 to 2.0 wt% Sn, 0 wt% or 0.3 to 3.0 wt% C as free carbon, the balance being Fe and unavoidable impurities, and the iron structure formed from the iron powder has an area ratio of ferrite structure of 90% or more. The C content is preferably 0.6 to 3.0 wt%.

[0010] If the Cu (copper) content is less than 1.0 wt% or the Sn (tin) content is less than 0.4 wt%, the strength will be low, and if the Cu content is more than 5.0 wt% or the Sn content is more than 2.0 wt%, the price will be high. In addition, the ferrite structure is soft and has excellent ductility, and the iron structure has an area ratio of ferrite structure of 90% or more, so that aggressiveness to the mating member (e.g., mating shaft) can be reduced. Here, the area ratio refers to the ratio of the area of ​​each structure to the observation area.

[0011] Furthermore, if the C content as free carbon is less than 0.3 wt% (particularly 0.6 wt%), the sliding characteristics deteriorate, and if the C content as free carbon is more than 3.0 wt%, the formability of the bearing deteriorates. On the other hand, when used in a linear sliding configuration, if the bearing contains free carbon, the free carbon will adhere to the mating shaft, causing local fluctuations in the friction coefficient and resulting in vibration during sliding. Here, free carbon refers to carbon in an uncompounded state, while combined carbon is called fixed carbon.

[0012] The oil content is preferably 10 to 25 vol%. The oil content is a dimensionless percentage value obtained by dividing the volume of the impregnated oil by the volume of the bearing. If the oil content is less than 10 vol%, there is little lubricating oil, which reduces bearing performance, causes seizure, and shortens the bearing life. Furthermore, if the oil content is more than 25 vol%, it becomes necessary to reduce the density in order to increase the pores inside the bearing, and reducing the density will result in a decrease in the strength of the bearing.

[0013] It is preferable to configure the bearing so that it does not have a network-like cementite structure. The absence of network-like cementite reduces aggressiveness toward the mating side (counter shaft) and does not impair smooth rotational operation as a bearing. Here, "network-like" refers to a mesh-like structure, and cementite is a compound of iron and carbon with the chemical formula Fe3C. It is a compound that appears in pearlite structures and tempered martensite structures in steel materials.

[0014] By setting the sintering temperature to 700°C to 900°C, preferably above 850°C but below 900°C (excluding 850°C and 900°C), a network-like cementite structure is not present and strength can be ensured. If the sintering temperature is below 850°C, sintering does not proceed sufficiently, and sufficient strength cannot be ensured even when the density is reduced to ensure oil content as an oil-impregnated bearing. If the sintering temperature is above 900°C, carbon diffuses into the iron, increasing the pearlite and cementite structures and increasing hardness. Here, the pearlite structure is a eutectoid structure of ferrite (α-iron) and cementite (FeC). The ferrite and cementite are layered. In other words, sintering at a temperature lower than the temperature at which graphite diffuses (above 850°C but below 900°C) can suppress the precipitation of not only cementite but also pearlite, thereby preventing an increase in bearing hardness and reducing aggressiveness to the mating shaft. Furthermore, carbon can be kept in the form of free graphite, ensuring sliding properties. That is, in the present invention, the sintering temperature is set to be higher than 850° C. because the density is low.

[0015] The present invention provides a bearing that is stable in strength and can reduce aggressiveness toward the mating member (e.g., the mating shaft), preventing damage and wear to the mating member and ensuring stable bearing function over a long period of time. Furthermore, since the sintering process is carried out at a relatively low temperature, the energy required during manufacturing can be reduced, and carbon dioxide (CO2) emissions during the manufacturing process can be reduced, minimizing adverse effects on the environment. Furthermore, since the copper (Cu) content is reduced and the sintering process is carried out at a low temperature, costs can be reduced.

[0016] Fig. 1 is a simplified cross-sectional view of a sintered alloy bearing according to the present invention. Fig. 2 is a block diagram showing a manufacturing method of a sintered alloy bearing according to the present invention. Fig. 3 is a block diagram showing a measurement procedure for image analysis. Fig. 4 is a binarized image of a sintered alloy, showing pixels corresponding to pores, copper phases, and pearlite phases. Fig. 5 is a diagram showing a binarized image of a sintered alloy. Fig. 6 is a graph showing measurement results of a friction coefficient.

[0017] An embodiment of the present invention will now be described with reference to FIGS.

[0018] 1 shows a sintered alloy bearing 1, which is composed of a cylindrical body having a bearing surface 1a on its inner circumference. A shaft 2 made of stainless steel or the like is inserted into the inner circumference of the sintered alloy bearing 1, and when the shaft 2 or the sintered alloy bearing 1 is rotated, the outer surface of the shaft 2 is rotatably supported by the bearing surface 1a of the sintered alloy bearing 1.

[0019] The sintered oil-impregnated bearing 1 is manufactured through the steps shown in Figure 2. That is, this manufacturing process involves a powder mixing step S1, a powder compacting step S2, a sintering step S3, a sizing step (dimension sizing step) S4, and an impregnation step S5, in that order. The raw material powders used in the powder mixing step S1 are, for example, copper powder, tin powder, iron powder, etc., which are mixed together to form a mixed powder.

[0020] In this case, the mixture contains, by weight, 1.0 to 5.0 wt% Cu (copper powder), 0.4 to 2.0 wt% Sn (tin powder), 0 wt% or 0.3 to 3.0 wt% (preferably 0.6 to 3.0 wt%) C (carbon powder) as free carbon, and the remainder Fe (iron powder) and unavoidable impurities. Various molding aids, such as lubricants (e.g., metal soap) for improving mold releasability, may be added to the mixed powder as needed. The raw powders used are not limited to these, and powders commonly used for cylindrical sintered alloy bearings can also be used. The maximum particle size of the copper powder is 106 μm, the maximum particle size of the tin powder is 75 μm, the maximum particle size of the iron powder is 212 μm, and the maximum particle size of the carbon powder is 75 μm.

[0021] As the iron powder, reduced iron powder, atomized iron powder, etc. can be used. In this embodiment, it is preferable to use reduced iron powder made from iron ore. Here, reduced iron powder is iron powder produced by reducing iron ore or mill scale (iron oxide) with coke or other carbonizing material and then heat-treating it in a hydrogen atmosphere. It has voids within its particles. Meanwhile, atomized iron powder is iron powder produced by pulverizing and cooling molten steel with high-pressure water and then heat-treating it in a hydrogen atmosphere. It has no voids within its particles and is of higher purity than reduced iron powder. Because atomized iron powder has a spherical powder shape, it tends to be poorly entangled when compacted, resulting in insufficient compact strength and poor oil impregnation. Furthermore, reduced iron powder made from mill scale has inferior compactability compared to iron ore.

[0022] The powder compacting step S2 is a step of compressing and molding the metal powder formed in the powder mixing step S1 into a cylindrical shape. The sintering step S3 is a step of sintering the powder compact obtained in the powder compacting step S2 at a predetermined sintering temperature. The sizing step S4 is a step of applying a compressive force to the sintered body obtained in the sintering step S3 to size it to a predetermined dimension. The impregnation step S5 is a step of impregnating the body with lubricating oil after cleaning or the like as necessary.

[0023] The powder compacting step S2 is a step of forming a powder compact using a press machine, and the press machine is equipped with upper and lower punches, a core rod for forming the inner shape of the powder compact, and a die for forming the outer shape of the powder compact.

[0024] In the sintering step S3, the green compact obtained in the powder compacting step (powder compacting step) S2 is heated to the sintering temperature of the metal powder used to obtain a sintered body. That is, sintering is performed in a predetermined atmosphere and at predetermined temperature conditions. The predetermined atmosphere can be a vacuum, a reducing gas, an inert gas, or the like, and can be selected depending on the metal powder used. In this case, the predetermined temperature condition is greater than 850°C and less than 900°C (excluding 850°C and 900°C, for example, approximately 860°C to 890°C).

[0025] In the sizing step S4, the sintered body, which has been distorted by sintering, is compressed and shaped to a desired size. Specifically, a core rod is inserted into the inner periphery of the sintered body, and the sintered body is pressed into the inner periphery of the die using an upper punch. The two bodies are then pressed together into the inner periphery of the die, and the axial width of the sintered body is compressed to a predetermined size using the upper and lower punches. As a result, the outer periphery of the sintered body is compressed and shaped by the die, and the inner periphery of the sintered body is pressed against the outer periphery of the core rod to shape it (in-mold sizing). This sizing step compresses the surface layer of the sintered body, increasing its density above that of the interior. Furthermore, the outer periphery of the sintered body is compressed more than the inner periphery, resulting in a higher density of the outer periphery than the inner periphery. After the sizing step, the inner periphery of the sintered body may be subjected to further rotational sizing to further reduce the pores open to the inner periphery. In this case, the sizing process includes a first sizing process (in-mold sizing process) and a second sizing process, and the second sizing process includes a rotational sizing process and an in-mold sizing process.

[0026] In the impregnation process S5, the sintered alloy bearing 1, which has been formed into a predetermined shape through the sizing process, is impregnated with lubricating oil, thereby completing the sintered alloy bearing 1 with its internal pores impregnated with lubricating oil. The impregnation of the internal pores of the sintered alloy bearing 1 with lubricating oil is performed, for example, by immersing the sintered alloy bearing 1 in a lubricating oil bath filled with lubricating oil for a certain period of time under a predetermined reduced pressure. To ensure reliable impregnation of the lubricating oil in a short period of time, the impregnation process may be performed with the lubricating oil heated. An oil content of 10 to 25 vol% is preferred. The oil content is a dimensionless percentage obtained by dividing the volume of the impregnated oil by the volume of the bearing. If the oil content is less than 10 vol%, the amount of lubricating oil is insufficient, resulting in reduced bearing performance, seizure, and a shortened bearing life. Furthermore, if the oil content is greater than 25 vol%, the density must be reduced to increase the porosity within the bearing, and reducing the density will result in a decrease in bearing strength.

[0027] Generally, ester-based synthetic oils and PAO-based synthetic oils are widely used as impregnating oils (lubricating oils), but mineral oils and fluorine-based synthetic oils are also used depending on the cost, the temperature of the operating environment, etc. For automotive applications, ester oils, fluorine oils, and mineral oils are used, while for home appliance applications, PAO (polyalphaolefin) oils and mineral oils are used. Therefore, in the present invention, a selection can be made from these impregnating oils depending on the application, etc.

[0028] The iron structure formed from the iron powder has an area ratio of ferrite structure of 90% or more.

[0029] If the Cu content is less than 1.0 wt% or the Sn content is less than 0.4 wt%, the strength will be low, and if the Cu content is more than 5.0 wt% or the Sn content is more than 2.0 wt%, the price will be high. Furthermore, the ferrite structure is soft and has excellent ductility, and the iron structure has an area ratio of ferrite structure of 90% or more, so that aggressiveness to the mating member (e.g., mating shaft 2) can be kept low. Here, the area ratio refers to the ratio of the area of ​​each structure to the observation area.

[0030] For this reason, the hardness of the inner peripheral surface, which is the bearing surface 1a of this sintered alloy bearing 1, is set to, for example, 40 HRH to 80 HRH in Rockwell hardness and 50 to 200 HV in Vickers hardness of the iron powder portion. Furthermore, when the hardness of the inner peripheral surface, which is the bearing surface 1a of the sintered alloy bearing 1, is set to A and the hardness of the outer peripheral surface with the mating member (mating shaft 2) is set to B, the relationship may be A=B, A>B, or A<B.

[0031] If the C content as free carbon is less than 0.6 wt%, the sliding properties will deteriorate, and if the C content as free carbon is more than 3.0 wt%, the formability of the bearing will deteriorate. On the other hand, when used in a linear sliding configuration, if the bearing contains free carbon, the free carbon will adhere to the mating shaft, causing local fluctuations in the friction coefficient and resulting in vibration during sliding. Here, free carbon is carbon in an uncompounded state, while combined carbon is called fixed carbon.

[0032] By setting the sintering temperature to 700°C to 900°C, preferably above 850°C and below 900°C (excluding 850°C and 900°C), a structure can be formed in which no network-like cementite structure exists. If the sintering temperature is 850°C or lower, sintering does not proceed sufficiently, and if the sintering temperature is 900°C or higher, C diffuses into Fe, increasing the pearlite structure and cementite structure, and increasing hardness. Here, the pearlite structure is a eutectoid structure of ferrite (α-iron) and cementite (FeC). The ferrite and cementite are layered.

[0033] Sintered mechanical parts have a density of 6.2 g / cm 3 Low density parts below 6.3 / cm 3 ~6.7g / cm 3 Medium density parts, 6.8 g / cm 3 The sintered alloy bearing 1 in this embodiment has a dry density of 5.4 g / cm 3 to 7.4 g / cm 3 Let's say.

[0034] The density ratio of the sintered alloy bearing 1 can be set to, for example, about 70% to 95%. Here, the density ratio is the relative value of the density of a porous material such as a sintered alloy, and is expressed as a percentage as the ratio of the density of the porous material to the density of a material with the same composition as the porous material but with no voids inside.

[0035] The surface opening ratio (porosity) of the inner peripheral surface, which is the bearing surface 1a of the sintered alloy bearing 1, is set to 5% to 50%. Here, the surface opening ratio refers to the ratio of the sum of the areas of all openings (total area) per unit area. Increasing the surface opening ratio of the bearing 1 promotes oil supply through self-lubrication, preventing oil shortage at high rotational speeds and improving lubrication characteristics. On the other hand, decreasing the surface opening ratio prevents oil loss, enabling the formation of a stable oil film and improving lubrication characteristics at low rotational speeds. For this reason, a surface opening ratio of 5% to 50% is preferable.

[0036] According to the present invention, the bearing has stable strength and is less aggressive to the mating member (e.g., mating shaft), preventing damage and wear to the mating member and ensuring stable bearing function over a long period of time. Moreover, since the sintering process is carried out at a relatively low temperature, the energy required during manufacturing can be reduced, and carbon dioxide (CO2) emissions during the manufacturing process can be reduced, minimizing adverse effects on the environment. Furthermore, since the copper (Cu) content is reduced and the sintering process is carried out at a low temperature, costs can be reduced.

[0037] While the present invention has been described above with reference to an embodiment, it is not limited to the above embodiment and various modifications are possible. While FIG. 2 is a block diagram illustrating a method for manufacturing a sintered alloy bearing, the manufacturing method may include additional steps other than those shown in FIG. For example, a rotary sizing step may be added. In the rotary sizing step, a sizing pin with a generally polygonal cross section is press-fitted into the inner peripheral surface of the sintered body and then rotated (not shown). The corners of the sizing pin that come into contact with the inner peripheral surface of the sintered body are rounded to a generally arc-shaped cross section. This rotary sizing rolls the surface layer material of the inner peripheral surface of the sintered body with the sizing pin, crushing the openings in the inner peripheral surface and reducing the surface opening ratio of the inner peripheral surface. In particular, the sintered body has low hardness, so the load on the sizing pin is also small.

[0038] Furthermore, the sintered alloy bearing according to the present invention can be incorporated into and used not only in spindle motors for disk drive devices such as HDDs, but also in fan motors for cooling fans, polygon scanner motors for laser beam printers, etc. It is also suitable for bearings for ventilation fans and air circulation fan motors used in extremely low temperature ranges such as in household refrigerators and freezers, and commercial low-temperature storage facilities.

[0039] In Example 1, image analysis of the metal structure was performed. The measurement procedure for image analysis includes an image reading step S10, a binarization step S11, and a shape feature measurement step S12, as shown in Figure 3. That is, the photograph (micrograph) to be measured is converted into an electronic file, and then the image is read into publicly known image analysis software and binarized (a process of converting a color image into black and white shades). Necessary measurement values ​​are selected from this binarized image, and shape feature measurement (including size measurement) is performed. Here, the image read in the image reading step S10 was an image of a cross section obtained by cutting the bearing in a direction perpendicular to the axial direction.

[0040] 4 and 5 show binarized images of the sintered alloy formed in the process shown in FIG. 2. The area can be used to evaluate the size of the binarized region, and the area refers to the number of binarized pixels. Counting the number of pixels obtained by binarization is equivalent to measuring the area. Therefore, the area ratio of each structure can be calculated. Furthermore, the process for calculating the area ratio of each structure can be carried out by using monochrome (black and white) photographs (micrographs) and omitting the binarization step S11, and the same results can be obtained.

[0041] 4 and 5 have 47 horizontal and 35 vertical squares, for a total of 1,645 squares, for a total area of ​​1,645. The imaging conditions were a magnification of 500x and a 2 μm grid scale. In addition, in FIG. 4, the gray grids represent pores, with 258 grids, the white grids represent copper phases, with 110 grids, and the black grids represent pearlite structures, with 49 grids. Therefore, if the number of pixels (1,277) obtained by subtracting the number of pore pixels (258) and the number of copper phase images (110) from the total number of pixels (1,645) is taken as the iron structure, and the number of pixels (1,228) obtained by subtracting the number of pearlite structure pixels (49) from this iron structure is taken as the number of ferrite structure pixels, the ferrite structure area ratio is 96%, which can be seen to be 90% or more.

[0042] Next, in Example 2, we compared the rattle values ​​(results of rattle tests) of the green compacts obtained by compacting reduced iron powder made from iron ore and reduced iron powder made from mill scale. The comparative materials contained 1.0 to 5.0 wt% Cu, 0.4 to 2.0 wt% Sn, 0.6 to 3.0 wt% C, and the remainder Fe. Only the iron powder was changed in two types of materials (Material A1 and Material B1). That is, reduced iron powder made from iron ore was used as the iron powder for Material A1, and reduced iron powder made from mill scale was used as the iron powder for Material B1. Here, the rattle test is a method for evaluating the strength of green compacts. Measurements are performed based on the rattle test method for metal green compacts (JSPM Standard 4-69 of the Japan Society of Powder and Powder Metallurgy), and a value calculated from the measurement results is used to determine the ease of collapse of the test piece (green compact) (quantifying the strength of the green compact). The lower the rattle value obtained, the stronger the green compact.

[0043] Table 1 below shows the results of the rattler test (rattle value) when two types of materials (material A1 and material B1) were compacted with different densities. Material A1 had a density (g / cm 3 ) of 5.58, 5.80, and 5.95 were used, and the density (g / cm 3 From Table 1, three types of materials were used: material A1 (density: 5.95 g / cm3) using reduced iron powder made from iron ore as the raw material. 3 The rattler value (0.03%) of the mill scale raw material B1 (density: 5.95 g / cm 3 ) (1.98%), which indicates that the green strength is excellent. In Table 1, "unmeasurable" indicates that the rattle value could not be measured because the sample was cracked during the rattle test.

[0044] Next, in Example 3, the sliding characteristics were compared between the presence and absence of graphite. In this case, the materials contained 1.0 to 5.0 wt% Cu, 0.4 to 2.0 wt% Sn, and the balance Fe, and the C contents of the two types of materials were 0.6 to 3.0 wt% and 0 wt%, respectively.

[0045] A bearing with an inner diameter of 12 mm and a length of 6 mm, manufactured using the process in Figure 2, was subjected to reciprocating sliding for 180 minutes at a surface pressure of 0.6 MPa and a sliding speed of 200 mm / sec, and the results of measuring the friction coefficient are shown in Figure 6. As can be seen from Figure 6, the friction coefficient of the material containing C fluctuates and is unstable, while the friction coefficient of the material not containing C is low and has excellent sliding properties.

[0046] It can be used in transportation equipment such as four-wheeled vehicles and two-wheeled vehicles, industrial machinery such as office machines and general machinery, and electrical machinery such as IT equipment, AV equipment, and home appliances.

[0047] 1. Sintered alloy bearings

Claims

1. A sintered alloy bearing obtained by compressing a raw material powder to form a compact and sintering the compact, wherein the sintered alloy contains, by weight ratio, Cu: 1.0 to 5.0 wt%, Sn: 0.4 to 2.0 wt%, and as free carbon, C: 0 wt% or 0.3 to 3.0 wt%, with the balance being Fe and inevitable impurities, and the iron structure formed of iron powder has a ferrite structure with an area ratio of 90% or more.

2. The sintered alloy bearing according to claim 1, characterized in that it contains C: 0.6 to 3.0 wt%.

3. The sintered alloy bearing according to claim 1, characterized in that the oil content is 10 to 25 vol%.

4. The sintered alloy bearing according to claim 1, characterized in that there is no network-like cementite structure.

5. The sintered alloy bearing according to claim 4, characterized in that the sintering temperature is set to 700°C to 900°C so that no network-like cementite structure exists.

6. The sintered alloy bearing according to claim 4, characterized in that the sintering temperature is set to exceed 850°C and be less than 900°C so that no network-like cementite structure exists.

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