Graphite bearing and its manufacturing method

Abstract

A graphite bearing according to the present invention comprises: 78-98 mass% carbon including carbon content in spherical graphite and an un-carbonized resin; 2-12 mass% of volatile content in the resin; and 0-10 mass% components other than the carbon and volatile content. The total of the carbon, the volatile content, and the components other than the carbon and volatile content totals 100 mass%. The graphite bearing can be manufactured to have a net shape or a near-net shape, and a treatment such as resin impregnation or metal impregnation is unnecessary. The graphite bearing exhibits minimal cracking when press-fitted to a housing, and has a high radial crushing strength.

Description

[Technical Field]

[0001] The present invention relates to a graphite bearing and a method for manufacturing the same. [Background technology]

[0002] The general manufacturing method for graphite bearings is to impregnate a molded graphite body with resin under reduced pressure for a long time, then cut the graphite into the required shape and machine it into the shape of the bearing. Resin impregnation is necessary to increase the strength of the graphite bearing; without resin impregnation, for example, the bearing may not be able to support radial loads due to insufficient radial crushing strength. This method is inefficient because it requires time and effort, especially 1) It takes a long time for the resin to be impregnated. 2) The resin does not penetrate the molded body uniformly, resulting in a large amount of material loss, and 3) The bearings must be cut out from the molded body and then machined to the desired bearing shape.

[0003] The graphite bearing is press-fit into the housing hole. The inner diameter of the housing hole is smaller than the outer diameter of the graphite bearing, and the graphite bearing is assembled into the housing while being compressed. If the graphite bearing does not have enough strength at this time, cracks will occur and tend to develop in the radial direction of the graphite bearing.

[0004] Related prior art is shown below. In Patent Document 1 (JPS52-38516A), graphite, carbon black, and binders such as tar and pitch are mixed and kneaded, and then sintered at 1100 to 1200°C into a shape such as a bearing. The strength of the bearing is then increased by impregnating it with furan resin.

[0005] Patent Document 2 (JP4575911B) discloses, as a conventional example, a method of producing an isotropic graphite substrate by cold isostatic pressing (CIP), cutting it out, and machining it into a bearing (

[0052] -

[0054] ). In the examples, it discloses molding a mixture of graphite, coke, and binders such as tar or pitch, sintering it at 1000°C for 0.5 months, and then impregnating it with molten metal in a vacuum furnace (

[0039] -

[0049] ). In the examples, machining is easy, but long periods of sintering and metal impregnation are required. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JPS52-38516A [Patent Document 2] JP4575911B Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention is to 1) It can be manufactured in net shape or near net shape, and does not require processes such as resin impregnation or metal impregnation. 2) When press-fitting into the housing, problems such as cracking are minimized, and 3) The radial crushing strength is close to the axial strength. To provide a graphite bearing and a method for manufacturing the same. [Means for solving the problem]

[0008] The graphite bearing of this invention contains 78 mass% to 98 mass% of carbon, including the carbon content in spherical graphite and uncarbonized resin; 12 mass% to 2 mass% of volatile content in the resin; and 10 mass% to 0 mass% of components other than carbon and volatile content. The total of carbon, volatile content, and carbon and components other than volatile content is 100 mass%. Preferably, the carbon is primarily composed of spherical graphite, for example, 60 mass% or more of the carbon is spherical graphite. The carbon includes spherical graphite and the carbon content in the uncarbonized resin, and may also contain amorphous carbon such as coke. In this specification, "uncarbonized" includes anything from no carbonization to incomplete carbonization. Preferably, the volatile content is 12 mass% to 3 mass% or more, and particularly preferably 10 mass% to 5 mass% or more.

[0009] The method for producing a graphite bearing of this invention is characterized by the steps of: preparing a mixture containing 60 mass% to 90 mass% carbon containing spherical graphite but no resin, 30 mass% to 10 mass% resin, and 10 mass% to 0 mass% components of the graphite bearing other than carbon and volatile components, compression-molding the mixture in a mold to form a bearing body, and sintering the bearing body at a temperature at which the resin is not completely carbonized to a composition of 78 mass% to 98 mass% carbon (including the spherical graphite and the carbon content of the resin), 12 mass% to 2 mass% volatile components of the resin, and 10 mass% to 0 mass% components other than carbon and volatile components. Note that in this specification, descriptions related to graphite bearings also apply directly to the method for producing graphite bearings.

[0010] The graphite bearing of this invention can be manufactured by compression molding, eliminating the need for cutting from a graphite block and machining to the bearing shape. Furthermore, because dimensional change due to sintering is small, a sintered body close to the target size (a sintered body of near net shape to net shape) can be obtained by compression molding and sintering. Note that this invention also encompasses processes such as compression molding slightly larger than the target size, sintering, and grinding to the target size. Furthermore, this invention does not require processes such as resin impregnation or metal impregnation. In other words, this invention allows for efficient manufacturing of graphite bearings.

[0011] In the graphite bearing of this invention, the ratio of the strength in the pressure direction during compression molding (hereinafter "pressure direction strength") to the interlaminar strength during compression molding (hereinafter "interlaminar strength") is relatively close to 1. The pressure direction is the same as the axial direction of the graphite bearing, and the interlaminar direction is perpendicular to the pressure direction and the same as the radial direction of the graphite bearing. The pressure direction strength and interlaminar strength were measured using a plate-shaped sample in a three-point bending test. In contrast, the axial strength and radial crushing strength described below are compressive strengths measured in the shape of the graphite bearing, but the sample shape and measurement method differ. Generally, the pressure direction strength is higher than the interlaminar strength, but in this invention, the ratio can be made closer to 1. Furthermore, high interlaminar strength can reduce cracks and other problems when press-fitted into a housing. As described above, this invention can provide a graphite bearing that has fewer problems when press-fitted into a housing. In this invention, the ratio of the pressure direction strength to the interlaminar strength is, for example, between 1 and 2, preferably between 1 and 1.8, and particularly preferably between 1 and 1.6.

[0012] The graphite bearing of this invention has a ratio of axial strength to radial crushing strength close to 1 (the radial crushing strength is close to the axial strength), so the graphite bearing can support a large radial load.

[0013] Preferably, the uncarbonized resin comprises both a thermoplastic resin and a thermosetting resin, and more preferably, the thermoplastic resin is polyphenylene sulfide (PPS) and the thermosetting resin is a phenolic resin.

[0014] Preferably, the spherical graphite has an average particle size of 10 μm or more and 20 μm or less, and an aspect ratio of 2 or less. Preferably, the carbon content, including the spherical graphite and the carbon content in the uncarbonized resin, is 88 mass% to 98 mass% or less, and the volatile content in the resin is 12 mass% to 2 mass% or more, with the total of carbon and volatile content being 100 mass%. In other words, it contains no components other than carbon and volatile content. More preferably, the spherical graphite content is 60 mass% to 84 mass% or less, the amorphous carbon having an average particle size larger than that of the spherical graphite is 0 mass% to 22 mass% or less, the carbon content in the uncarbonized resin is 8 mass% to 15 mass% or less, and the volatile content is 3 mass% to 12 mass% or less, with the total of the spherical graphite, the amorphous carbon, the carbon content in the uncarbonized resin, and the volatile content being 100 mass%. [Brief explanation of the drawings]

[0015] [Figure 1] Electron microscope image of spherical graphite [Figure 2] Electron microscope image of spherical graphite with an aspect ratio of 1.23 [Figure 3] Electron microscope image of spherical graphite with an aspect ratio of 1.95 [Figure 4] Electron microscope images of the pressed surface of flake graphite; the left is an SEM image and the right is an EDX image. [Figure 5] Electron microscope images of the pressed surface of spherical graphite; the left is an SEM image and the right is an EDX image. [Figure 6] Cross-sectional view of a graphite bearing according to an embodiment [Figure 7] A diagram showing how cracks occur in graphite bearings when they are pressed into a housing. BEST MODE FOR CARRYING OUT THE INVENTION

[0016] The best mode for carrying out the present invention will be described below. The present invention is not limited to the mode, but is defined by the claims, and can be modified by adding matters known to those skilled in the art to the mode. [Example]

[0017] Graphite bearing components The components of graphite bearings are: Carbon (spherical graphite, carbon in uncarbonized resins, and other carbons such as coke) Volatile matter in the resin and Components other than carbon and volatile matter. Components other than carbon and volatile matter include solid lubricants such as tungsten disulfide, metals, etc., and since they are elements other than carbon, their content can be measured by elemental analysis. Other carbon is amorphous carbon such as coke, which is an aggregate with a larger average particle size than spherical graphite. Other carbon may also include flake graphite.

[0018] Measurement method: Volatile content The volatile content in graphite bearings (hereafter referred to as "bearings") is measured as follows: A sample of a specified weight is taken from the bearing, crushed, and then heated from room temperature to 900°C in a nitrogen stream using a thermogravimetric differential thermal analyzer (TG-DTA), and the weights at the start and end of the heating are measured. The difference between these values ​​divided by the weight at the start of the heating is the volatile content of the bearing.

[0019] Measurement method Thermoplastic resins and thermosetting resins When samples taken from the bearings are analyzed using GCMS (gas chromatography mass spectrometry), decomposition products of the resin components can be detected, which allows the type of resin contained in the bearings to be analyzed.

[0020] Spherical graphite Spherical graphite refers to graphite that is nearly spherical in shape, and is a term used in contrast to flake graphite. Figures 1 to 3 show scanning electron microscope (SEM) images of spherical graphite. In this invention, spherical graphite means graphite made up of graphite particles with an aspect ratio (ratio of the particle's major axis diameter to its minor axis diameter) of 2 or less. The thin lines in Figures 2 and 3 indicate the major and minor axes of the graphite particles, and the aspect ratio in Figure 2 is 1.23, and in Figure 3 it is 1.95.

[0021] Figure 4 shows an SEM image (left side) and an EDX image (right side: energy dispersive X-ray analysis) of the pressed surface of flake graphite, and Figure 5 shows an SEM image (left side) and an EDX image (right side) of the pressed surface of spherical graphite. In the images of the pressed surface, flake graphite is rectangular, while spherical graphite is nearly spherical. Flake graphite and spherical graphite can be distinguished from each other by the SEM and EDX images. Representative graphite particles are marked in Figures 4 and 5.

[0022] Graphite shape, strength and dimensional stability of sintered body The strength and dimensional stability of sintered bodies were evaluated for spherical graphite and flake graphite. 75 parts by weight of spherical graphite with an average particle size (D50) of 17 μm (Example) or 75 parts by weight of flake graphite with an average particle size (D50) of 70 μm (Comparative Example) were mixed with 8 parts by weight of amorphous carbon (coke), 12 parts by weight of PPS resin, and 5 parts by weight of phenolic resin, for a total of 100 parts by weight of each. Using each material, rectangular parallelepipeds measuring 20 mm x 10 mm x 5 mm were compression molded to measure the strength in the compression direction and the strength in the interlayer direction. For the specimen for measuring the strength in the compression direction, the pressure direction was the thickness direction. For the specimen for measuring the strength in the interlayer direction, the pressure direction was the long side direction of the 20 mm length.

[0023] The measurement samples for the examples and comparative examples were sintered at 400°C. The sintering temperature is equal to or higher than the melting point of the thermoplastic resin. Note that if sintering is performed at a temperature slightly higher than the melting point, the resin is hardly carbonized. The sintering temperature is, for example, 500°C or lower, a temperature at which neither the thermosetting resin nor the thermoplastic resin is completely carbonized.

[0024] The strength in the pressure direction and the strength in the interlayer direction were measured as three-point bending strength for the measurement samples of the Example and Comparative Example. The dimensional change before and after sintering was also measured. In the Example, the strength in the pressure direction was 57.4 MPa, the strength in the interlayer direction was 40.4 MPa, and the ratio between these was 1.4. In the Comparative Example, the strength in the pressure direction was 82.7 MPa, the strength in the interlayer direction was 31.2 MPa, and the ratio between these was large at 2.7. The dimensional reduction rate before and after sintering in the Example was 0.35% in the pressure direction and 0.3% in the interlayer direction, indicating a small anisotropy of dimensional change. In the Comparative Example, the anisotropy of dimensional change was large at 0.4% in the pressure direction and 0.25% in the interlayer direction.

[0025] Example The raw material powders were mixed to obtain the compositions shown in Table 1 (Examples) and Table 2 (Comparative Examples). The average particle size (D50) of the spherical graphite was 17 μm, and the average particle size (D50) of the flake graphite was 70 μm. The remaining carbon was coke, an amorphous carbon. The mixed powder was compression-molded into a bearing shape and sintered at 400°C. During sintering, the phenolic resin partially decomposed, bonding the graphite particles together, and the thermoplastic resin PPS melted, filling the gaps between the graphite particles. Melting of the PPS resin increased the strength of the sintered body. In the Examples, the dimensional change due to sintering was approximately 0.3% in both the axial and radial directions, and grinding to adjust the bearing dimensions was not required.

[0026] Figure 6 shows the shape of the graphite bearing 2. The bearing 2 is cylindrical, has a hole 4 along the axial direction, and accommodates a shaft (not shown). The bearing 2 has an outer diameter D of 8.9 mm, a diameter r of the hole 4 of 5.1 mm, a length L of 5.0 mm, and the upper and lower edges of the outer periphery are chamfered. The size of the bearing 2 is arbitrary.

[0027] Figure 7 shows a schematic diagram of a graphite bearing 2 being pressed into a housing 6. The housing 6 is made of metal, for example, and houses the bearing 2 in a hole 8, with a shaft (not shown) passing through a hole 10. The bearing 2 is pressed into the hole 8 while being compressed, and at this time, cracks 11 may occur due to stress acting on the inside of the bearing 2. As shown in the upper right of Figure 7, cracks 11 mainly occur in the interlayer direction (radial direction of the bearing) during compression molding.

[0028] For each graphite shaft in the Examples and Comparative Examples, the strength in the compression direction, interlaminar strength, axial strength, radial crushing strength, volatile matter content, and frequency of crack occurrence during press-fitting were measured. These data are shown in Table 1 (Examples) and Table 2 (Comparative Examples). The radial crushing strength of bearing 2 was measured using the radial crushing strength test method specified in (JIS) Z2507. The axial strength is the breaking strength when a compressive load is applied in the axial direction. Furthermore, because the main component is graphite, there were no problems with lubrication in both the Examples and Comparative Examples.

[0029] [Table 1]

[0030] [Table 2]

[0031] In the Examples, the interlaminar strength is high, the ratio of strength in the pressure direction to interlaminar strength is 1.6 or less, close to 1, and there is little cracking during press-fitting. In contrast, in Comparative Examples 1 and 2, the interlaminar strength is low, the ratio of strength in the pressure direction to interlaminar strength is over 2, and there is much cracking during press-fitting. Furthermore, in the Examples, the radial crushing strength is high and the radial load that the bearing can support is large, but in Comparative Examples 1 and 2, the radial crushing strength is low.

[0032] Comparative Example 3 uses spherical graphite, but has low radial crushing strength. The reason for this is presumably that the spherical graphite is not sufficiently bonded by the thermosetting resin due to the low volatile content, or the gaps between the spherical graphite are not sufficiently filled with the thermoplastic resin, making it unable to support a radial load. [Explanation of symbols]

[0033] 2. Graphite bearings 4 holes 6. Housing 8,10 holes 11 Crack

Claims

1. The carbon content, including the carbon content in the spherical graphite and uncarbonized resin, is 78 mass% or more and 98 mass% or less, the volatile content in the resin is 12 mass% or less and 2 mass% or more, and the components other than the carbon and volatile content are 10 mass% or less and 0 mass% or more, and the total of the carbon, volatile content, and the components other than the carbon and volatile content is 100 mass%, The spherical graphite has an average particle size of 10 μm or more and 20 μm or less and an aspect ratio of 2 or less, The ratio of the axial compressive strength to the radial crushing strength is 1 or more and 2 or less, and the shape is cylindrical. A graphite bearing, wherein the uncarbonized resin is made of both a thermoplastic resin and a thermosetting resin.

2. 2. The graphite bearing according to claim 1, wherein said thermoplastic resin is polyphenylene sulfide (PPS) and said thermosetting resin is a phenolic resin.

3. 2. The graphite bearing according to claim 1, wherein the volatile content is 12 mass% or less and 3 mass% or more.

4. The carbon content, including the carbon content in the spherical graphite and uncarbonized resin, is 88 mass% or more and 98 mass% or less. The volatile content is 12 mass% or less and 2 mass% or more, 2. The graphite bearing according to claim 1, wherein the total of carbon and said volatile matter is 100 mass %.

5. The composition contains 60 mass% or more and 84 mass% or less of spherical graphite, 0 mass% or more and 22 mass% or less of amorphous carbon having an average particle diameter larger than that of spherical graphite, 8 mass% or more and 15 mass% or less of carbon in uncarbonized resin, and 3 mass% or more and 12 mass% or less of the volatile matter, 5. The graphite bearing according to claim 4, wherein the sum of the spherical graphite, the amorphous carbon, the carbon content in the uncarbonized resin, and the volatile content is 100 mass %.

6. A step of preparing a mixture containing 60 mass% to 90 mass% of carbon containing spherical graphite with an average particle size of 10 μm to 20 μm and an aspect ratio of 2 or less and not containing resin, 30 mass% to 10 mass% of resin consisting of both thermoplastic resin and thermosetting resin, and 10 mass% to 0 mass% of graphite bearing components other than carbon and volatile components; compression-molding the mixture in a mold to form a cylindrical bearing body; Sintering the bearing matrix at a temperature at which the resin is not completely carbonized so that the composition is 78 mass% or more but 98 mass% or less of carbon, including the spherical graphite and the carbon content in the resin, 12 mass% or less but 2 mass% or more of volatile content in the resin, and 10 mass% or less but 0 mass% or more of graphite bearing components other than carbon and volatile content; a graphite bearing having a ratio of axial compressive strength to radial crushing strength of 1 or more and 2 or less, by carrying out the steps of:

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