Sintered alloy bearing

Abstract

To provide a sintered alloy bearing that is less prone to wear on the mating component (matting shaft) and can minimize adverse environmental impacts during manufacturing. [Solution] This is a sintered alloy bearing obtained by compressing raw material powder to form a compact, and then sintering this compact. The sintered alloy contains, by weight ratio, Cu: 1.0~5.0 wt%, Sn: 0.4~2.0 wt%, and free carbon C: 0 wt% or 0.6~3.0 wt%, with the remainder being Fe and unavoidable impurities. The iron structure formed from the iron powder has a ferrite structure with an area ratio of 90% or more.

Description

[Technical Field]

[0001] This invention relates to a sintered alloy bearing. [Background technology]

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

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

[0004] The iron-based sliding member (sintered alloy) described in Patent Document 1 has a network-like cementite structure in part. Here, "network-like" refers to a mesh-like structure, which is generally constructed by connecting bases with lines, and is a structure that is bonded like a three-dimensional mesh by chemical bridging. Furthermore, the network consists of an autonomous whole and autonomous parts. In other words, the network is autonomous both as a whole and as a part. Specifically, in the case of the material described in Patent Document 1, when iron and graphite are mixed and sintered, the graphite diffuses into the iron and a pearlite structure is formed. However, when the graphite content exceeds 0.8%, a linear cementite structure precipitates. This forms a network-like cementite structure. Cementite is a compound of iron and carbon, and its chemical formula is Fe3C. It is a compound that appears in the pearlite structure and tempered martensitic structure in steel materials. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Special Publication No. 56-27591 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Incidentally, cementite is a hard component in metal structures, and a higher cementite content increases aggressiveness towards the mating material. Aggressiveness towards the mating material refers to wear and damage to the mating material. Therefore, if a sintered alloy bearing is constructed using an iron-based sliding member (sintered alloy) as described in Patent Document 1, the bearing will tend to be hard, and when metal-to-metal contact occurs during sliding, it will be highly aggressive towards the shaft, making the mating material more prone to wear.

[0007] Furthermore, because it has a network-like cementite structure, Patent Document 1 sets the sintering temperature to 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, negatively impacting the environment, and also increases the amount of energy used, leading to higher costs. Moreover, if the sintered alloy contains a large amount of Cu or Sn, it will also lead to higher costs.

[0008] Therefore, in view of the above problems, the present invention provides a sintered alloy bearing that is less prone to wear against the mating member (matting shaft) and can minimize adverse environmental impacts during manufacturing. [Means for solving the problem]

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

[0010] If the amount of Cu (copper) is less than 1.0 wt% or the amount of Sn (tin) is less than 0.4 wt%, the strength will be low, and if the amount of Cu is more than 5.0 wt% or the amount of Sn is more than 2.0 wt%, the price will be high. Furthermore, the ferrite structure is soft and has excellent ductility, and since the iron structure has an area ratio of 90% or more of the ferrite structure, it can keep the aggressiveness towards the mating member (for example, the mating shaft) low. Here, the area ratio refers to the proportion of the area of ​​each structure to the observed area.

[0011] Furthermore, if the amount of free carbon (C) falls below 0.6 wt%, the sliding properties deteriorate, and if the amount of free carbon (C) exceeds 3.0 wt%, the formability of the bearing deteriorates. On the other hand, when used in linear sliding applications, the presence of free carbon in the bearing causes it to adhere to the mating shaft, resulting in localized fluctuations in the coefficient of friction and generating vibration during sliding. Here, free carbon refers to carbon in an uncompounded state, while compounded carbon is called fixed carbon.

[0012] A preferred oil content is 10 to 25 vol%. The oil content is a dimensionless percentage obtained by dividing the volume of impregnated oil by the volume of the bearing. If the oil content is less than 10 vol%, there is insufficient lubrication, which reduces bearing performance, can lead to seizing, and shortens the bearing life. Conversely, 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 leads to a decrease in bearing strength.

[0013] It is preferable to construct the bearing in such a way that a network-like cementite structure is absent. By eliminating the network-like cementite, the aggressiveness towards the mating part (the mating shaft) can be reduced, and smooth rotational operation as a bearing is not impaired. 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 the pearlite structure and tempered martensitic structure of steel materials.

[0014] By setting the sintering temperature between 850°C and 900°C (excluding 850°C and 900°C), a structure can be created that does not have a network-like cementite structure while ensuring sufficient strength. If the sintering temperature is below 850°C, sintering does not proceed sufficiently, and sufficient strength cannot be ensured in a state where the density is low in order to ensure the oil content as an oil-impregnated bearing. If the sintering temperature is above 900°C, carbon diffuses into the Fe, increasing the pearlite and cementite structures and increasing the hardness. Here, the pearlite structure is a eutectoid structure of ferrite (α-iron) and cementite (Fe3C). The ferrite and cementite are layered. In other words, by sintering at a temperature lower than the temperature at which graphite diffuses (between 850°C and below 900°C), the precipitation of not only cementite but also pearlite can be suppressed, thus reducing the hardness of the bearing and reducing its aggressiveness towards the mating shaft. Furthermore, carbon can be in the state of free graphite, ensuring sliding properties. In other words, in this invention, because the density is low, the sintering temperature is raised to over 850°C. [Effects of the Invention]

[0015] This invention provides structural stability while minimizing aggressiveness towards mating components (e.g., mating shafts), preventing damage and wear to the mating components and ensuring stable bearing function over the long term. Furthermore, since the sintering process is performed at a relatively low temperature, the energy required during manufacturing can be reduced, and the generation of carbon dioxide (CO2) during the manufacturing process can be minimized, thus reducing the negative impact on the environment. In addition, by reducing the copper (Cu) content and performing the sintering process at a low temperature, costs can be reduced. [Brief explanation of the drawing]

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

Embodiments for Carrying Out the Invention

[0017] Hereinafter, embodiments of the present invention will be described based on FIGS. 1 to 5.

[0018] FIG. 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 (shaft) 2 made of stainless steel or the like is inserted into the inner circumference of the sintered alloy bearing 1, and the outer peripheral surface of the shaft 2 is rotatably supported by the bearing surface 1a of the sintered alloy bearing 1 when the shaft 2 is rotated or the sintered alloy bearing 1 is rotated.

[0019] The sintered oil-impregnated bearing 1 is manufactured by the process shown in FIG. 2. That is, this manufacturing process is manufactured through a powder mixing process S1, a powder compacting process S2, a sintering process S3, a sizing process (dimension sizing process) S4, and an impregnation process S5 in sequence. As the raw material powder in the powder mixing process S1, for example, copper powder, tin powder, iron powder, etc. are mixed to form a mixed powder.

[0020] In this case, it contains Cu (copper powder): 1.0 to 5.0 wt%, Sn (tin powder): 0.4 to 2.0 wt% in terms of weight ratio, and as free carbon, C (carbon powder): 0 wt% or 0.6 to 3.0 wt%, and the balance: Fe (iron powder) and unavoidable impurities. Various molding aids, for example, lubricants (for example, metal soaps, etc.) for improving mold release properties, are added to the mixed powder as necessary. Note that the raw material powder used is not limited to these, and those generally used for cylindrical sintered alloy bearings can 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 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 the like (carbonizing agent), and then heat-treating it in a hydrogen atmosphere, and has voids within the particles. Atomized iron powder, on the other hand, is iron powder produced by pulverizing and cooling molten steel with high-pressure water, and then heat-treating it in a hydrogen atmosphere, and has no voids within the particles and is of higher purity than reduced iron powder. For this reason, because atomized iron powder has a spherical powder shape, the entanglement between iron powder particles is weak when compacted, so the strength of the compact cannot be ensured, and it also has poor oil content. Furthermore, reduced iron powder made from mill scale has inferior moldability compared to iron ore.

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

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

[0024] In the sintering process S3, the compacted body obtained in the compaction process (compacting process) 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 under predetermined temperature conditions. The predetermined atmosphere can be a vacuum, a reducing gas, or an inert gas, and can be selected in various ways depending on the metal powder used. In this case, the predetermined temperature conditions are greater than 850°C and less than 900°C (excluding 850°C and 900°C, for example, around 860°C to 890°C).

[0025] In the sizing process S4, the sintered body, which has been distorted by sintering, is compressed to shape its dimensions. Specifically, a core rod is inserted into the inner circumference of the sintered body, and the sintered body is pressed in with an upper punch, and these are pressed together into the inner circumference of the die, while the axial width of the sintered body is compressed to a predetermined dimension by the upper and lower punches. As a result, the outer surface of the sintered body is compressed and shaped by the die, and the inner surface of the sintered body is pressed against the outer surface of the core rod and shaped (in-mold sizing). This sizing process compresses the surface layer of the sintered body, and the density of the surface layer becomes greater than the density of the interior. In addition, the outer surface of the sintered body is compressed more than the inner surface, so the density of the outer surface is greater than that of the inner surface. After the sizing process, the inner surface of the sintered body may be further rotationally sized to make the voids that have opened on the inner surface even smaller. In this case, the sizing process consists of 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. This completes the sintered alloy bearing 1, in which the internal pores are 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 environment. In order to ensure that the impregnation of the lubricating oil is carried out reliably and quickly, the impregnation work may be performed with the lubricating oil heated. The oil content is preferably 10 to 25 vol%. The oil content is a dimensionless percentage value obtained by dividing the volume of impregnated oil by the volume of the bearing. If the oil content is less than 10 vol%, there is insufficient lubricating oil, which reduces bearing performance, can cause seizure, and shortens the bearing life. If the oil content is more than 25 vol%, it becomes necessary to reduce the density in order to increase the number of pores inside the bearing, and reducing the density leads to a decrease in the strength of the bearing.

[0027] Incidentally, while ester-based synthetic oils and PAO-based synthetic oils are commonly used as impregnation oils (lubricants), mineral oils and fluorine-based synthetic oils are also used depending on cost, operating temperature, 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 this invention, an impregnation oil can be selected from these oils according to the application and other factors.

[0028] The iron structure formed from iron powder is one in which ferrite structures account for 90% or more of the surface area.

[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 since the iron structure has a ferrite area ratio of 90% or more, it can keep the aggressiveness towards the mating member (for example, the mating shaft 2) low. Here, the area ratio refers to the proportion of the area of ​​each structure to the observed area.

[0030] Therefore, the hardness of the inner circumferential surface, which is the bearing surface 1a of this sintered alloy bearing 1, is set to, for example, 40 HRH to 80 HRH on the Rockwell hardness scale, and 50 to 200 HV on the Vickers hardness scale of the iron powder portion. Furthermore, if the hardness of the inner circumferential surface, which is the bearing surface 1a of the sintered alloy bearing 1, is A, and the hardness of the outer circumferential surface of the mating member (mating shaft 2) is B, then whether A=B or A>B, A <Bであってもよい。

[0031] When the amount of free carbon (C) falls below 0.6 wt%, the sliding properties deteriorate, and when the amount of free carbon (C) exceeds 3.0 wt%, the formability of the bearing deteriorates. On the other hand, when used in linear sliding applications, the presence of free carbon in the bearing causes it to adhere to the mating shaft, resulting in localized fluctuations in the coefficient of friction and generating vibration during sliding. Here, free carbon refers to carbon in an uncompounded state, while compounded carbon is called fixed carbon.

[0032] By setting the sintering temperature between 850°C and 900°C, a structure can be created that does not have a network-like cementite structure. If the sintering temperature is below 850°C, sintering does not proceed sufficiently, and if the sintering temperature is above 900°C, carbon diffuses into the Fe, increasing the pearlite and cementite structures and resulting in higher hardness. Here, the pearlite structure is a eutectoid structure of ferrite (α-iron) and cementite (Fe3C). The ferrite and cementite are layered.

[0033] Sintered mechanical parts have a density of 6.2 g / cm³. 3 The following low-density components, 6.3 / cm² 3 ~6.7g / cm 3 Medium-density components, 6.8 g / cm³ 3 The above high-density components can be separated, but in this embodiment, the density of the sintered alloy bearing 1 is 5.4 g / cm³ in dry condition. 3 From 7.4 g / cm³ 3 Let's assume that.

[0034] The density ratio of the sintered alloy bearing 1 can be set to, for example, approximately 70% to 95%. Here, the density ratio is the relative value of the density of porous materials such as sintered alloys, 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 without voids inside the solid.

[0035] The surface opening ratio (porosity) of the inner circumferential 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 each opening (total area) to the area per unit area. Increasing the surface opening ratio of the bearing 1 promotes oil supply through self-lubrication, preventing oil depletion in the high rotational speed range and improving lubrication characteristics. On the other hand, decreasing the surface opening ratio prevents oil escape, enabling the formation of a stable oil film and improving lubrication characteristics in the low rotational speed range. For this reason, it is preferable to set the surface opening ratio to 5% to 50%.

[0036] According to the present invention, the bearing is structurally stable and less aggressive towards the mating component (e.g., the mating shaft), preventing damage and wear to the mating component and ensuring stable bearing function over a long period. Furthermore, since the sintering process is performed at a relatively low temperature, the energy required during manufacturing can be reduced, and the generation of carbon dioxide (CO2) during the manufacturing process can be minimized, thus reducing the negative impact on the environment. In addition, by reducing the copper (Cu) content and performing the sintering process at a low temperature, costs can be reduced.

[0037] Although embodiments of the present invention have been described above, the present invention is not limited to the above embodiments and can be modified in various ways. Figure 2 is a block diagram illustrating a method for manufacturing a sintered alloy bearing, but other steps besides those shown in Figure 2 may be added to the manufacturing method. For example, a rotational sizing step may be added. In the rotational sizing step, a sizing pin with a substantially polygonal cross-section is pressed into the inner circumferential surface of the sintered body, and the sizing pin is rotated in this state (not shown). The corners of the sizing pin that come into contact with the inner circumferential surface of the sintered body are rounded into a substantially arc shape in cross-section. Due to this rotational sizing, the surface material of the inner circumferential surface of the sintered body is rolled by the sizing pin, the openings of the inner circumferential surface are crushed, and the surface opening ratio of the inner circumferential surface is reduced. In particular, because the hardness of the sintered body is low, the load on the sizing pin is also small.

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

[0039] As Example 1, image analysis of the metal structure was performed. The measurement procedure for image analysis, as shown in Figure 3, comprises an image loading step S10, a binarization step S11, and a shape feature measurement step S12. Specifically, the photograph to be measured (microscope image) is saved as an electronic file, then the image is loaded into publicly available image analysis software to perform binarization (a process that converts a color image into grayscale), and then the necessary measurement values ​​are selected from this binarized image to perform shape feature measurement (including size measurement). Here, the image loaded in the image loading step S10 was an image of a cross-section obtained by cutting the bearing in a direction perpendicular to the axial direction.

[0040] Figures 4 and 5 show binarized images of the sintered alloy formed in the process shown in Figure 2. Area can be used to evaluate the size of the binarized region. Area is the number of binarized pixels; therefore, the area is measured by counting the number of pixels obtained through binarization. Thus, the area ratio of each tissue can be determined. Furthermore, the process for determining the area ratio of each tissue can be performed by using a monochrome (black and white) photograph (microscope image) and omitting the binarization step S11, yielding the same results.

[0041] Figures 4 and 5 have 47 squares horizontally and 35 squares vertically, totaling 1645 squares, for a total area of ​​1645. The imaging conditions were a magnification of ×500 and a grid scale of 2 μm. In Figure 4, the gray grid represents pores and there are 258 squares, the white grid represents the copper phase and there are 110 squares, and the black grid represents the pearlite structure and there are 49 squares. Therefore, if we subtract the number of pore pixels (258) and the number of images of the copper phase (110) from the total number of pixels (1645) to get the number of pixels of the iron structure (1277), and then subtract the number of pixels of the pearlite structure (49) from this iron structure to get the number of pixels of the ferrite structure (1228), we can see that the ferrite structure accounts for 96% of the area, which is more than 90%. [Examples]

[0042] Next, as Example 2, the Rattr values (results of the Rattr test) of the green compacts when powder compaction was performed using reduced iron powder made from iron ore as a raw material and reduced iron powder made from mill scale as a raw material were compared. The materials for comparison had Cu at 1.0 to 5.0 wt%, Sn at 0.4 to 2.0 wt%, C at 0.6 to 3.0 wt%, and the balance being Fe, and only the iron powder was changed in two types of materials (Material A1 and Material B1). That is, in Material A1, reduced iron powder made from iron ore was used as the iron powder, and in Material B1, reduced iron powder from mill scale raw material was used as the iron powder. Here, the Rattr test is for evaluating the strength of the green compact. Measurement is carried out based on the Rattr test method for metal green compacts (JSPM Standard 4-69 of the Powder Metallurgy Society of Japan), and a value calculated from the obtained measurement results is used to determine the ease of disintegration (quantifying the strength of the green compact) of the test piece (green compact). The lower the obtained Rattr value, the stronger the green compact.

[0043] The following Table 1 shows the results (Rattr values) of the Rattr test when powder compaction was performed with two types of materials (Material A1 and Material B1) at different densities. For Material A1, three types with densities (g / cm 3 ) of 5.58, 5.80, and 5.95 were used, and for Material B1, three types with densities (g / cm 3 ) of 5.57, 5.76, and 5.95 were used. From this Table 1, it can be seen that the Rattr value (0.03%) of Material A1 using reduced iron powder made from iron ore (density: 5.95 g / cm 3 ) is lower than the Rattr value (1.98%) of Material B1 using reduced iron powder from mill scale raw material (density: 5.95 g / cm 3 ), indicating that it has better green compact strength. In Table 1, "unmeasurable" indicates that the Rattr value could not be measured because the sample cracked during the Rattr test.

Table 1

Example

[0044] Next, in Example 3, the sliding properties with and without graphite were compared. In this case, the material consisted of 1.0-5.0 wt% Cu, 0.4-2.0 wt% Sn, and the remainder Fe, with C content set to 0.6-3.0 wt% and 0 wt% in the two types of materials, respectively.

[0045] Figure 6 shows the results of measuring the coefficient of friction when a bearing with an inner diameter of φ12 mm and a length of 6 mm, manufactured using the process shown in Figure 2, was subjected to reciprocating sliding at a surface pressure of 0.6 MPa and a sliding speed of 200 mm / sec for 180 minutes. As can be seen from Figure 6, the coefficient of friction fluctuates and is unstable in materials containing carbon, while the coefficient of friction of materials without carbon is low and exhibits superior sliding characteristics. [Explanation of Symbols]

[0046] 1. Sintered alloy bearing

Claims

1. A sintered alloy bearing is obtained by compressing raw material powder to form a compact, and then sintering this compact. The sintered alloy contains, by weight, Cu: 1.0-5.0 wt%, Sn: 0.4-2.0 wt%, and free carbon C: 0 wt% or 0.6-3.0 wt%, with the remainder being Fe and unavoidable impurities. A sintered alloy bearing characterized in that the iron structure formed from 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 the oil content is 10 to 25 vol%.

3. The sintered alloy bearing according to claim 1, characterized in that a network-like cementite structure is absent.

4. The sintered alloy bearing according to claim 3, characterized in that the sintering temperature is set to be above 850°C and below 900°C so that a network-like cementite structure does not exist.

Images