Sintered oil-impregnated bearing, manufacturing method for sintered oil-impregnated bearing, and motor

Abstract

To improve activation characteristics at a low temperature.SOLUTION: In a method for manufacturing a sintered oil-containing bearing according to the present invention, in a sealing step, surface pores of an end surface s2 of a sintered body are sealed so that a surface porosity of the end surface s2 of the sintered body is lower than that of a bearing surface s1. Consequently, leakage of lubricant from the end surface s2 is suppressed in an outer circumferential side region 12 where density is relatively low, and as a result, it is possible to suppress drop in an oil film pressure, and it is possible to improve activation characteristics at a low temperature.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a sintered oil-impregnated bearing that can improve starting characteristics at low temperatures, a method for manufacturing the sintered oil-impregnated bearing, and a motor that includes the sintered oil-impregnated bearing. [Background technology]

[0002] Conventionally, a sintered oil-impregnated bearing has been known in which the density of the inner peripheral region is made higher than the density of the outer peripheral region by making the compression rate in the thickness direction of the inner peripheral region higher than the compression rate in the thickness direction of the outer peripheral region in the sizing process after powder compaction (see Patent Document 1). In this sintered oil-impregnated bearing, a density gradient is created so that the density increases from the outer peripheral region to the inner peripheral region, allowing the lubricant impregnated in the outer peripheral region to be transported to the inner peripheral region by capillary force.As a result, oil film formation on the bearing surface is promoted, and it is possible to suppress a decrease in starting characteristics even at low temperatures. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 3932394 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in conventional sintered oil-impregnated bearings, the density is relatively low in the outer peripheral region, which makes it easy for the lubricant to leak from the end face of the bearing, resulting in a decrease in oil film pressure and the risk of not being able to sufficiently improve starting characteristics at low temperatures.Furthermore, lubricant leakage leads to a reduction in the bearing life. An object of the present invention is to provide a sintered oil-impregnated bearing that can extend bearing life and improve starting characteristics at low temperatures, a method for manufacturing such a sintered oil-impregnated bearing, and a motor that includes such a sintered oil-impregnated bearing. [Means for solving the problem]

[0005] In order to solve the above problems, a method for manufacturing a sintered oil-impregnated bearing according to a first invention includes a compacting step in which raw materials are compressed to form a green compact, a sintering step in which the green compact is sintered to form a sintered body, a sealing step in which surface pores on the end faces of the sintered body are sealed, and a sizing step in which sizing is applied to the sintered body, wherein in the sealing step, surface pores on the end faces of the sintered body are sealed so that the surface porosity of the end faces of the sintered body is lower than that of the bearing surface, and in the sizing step, the sintered body is compressed so that the density of the inner peripheral region of the sintered body is higher than that of the outer peripheral region. In the manufacturing method of the first invention for an oil-impregnated sintered bearing, the sintered body is compressed in the sizing step so that the density of the inner region of the sintered body is higher than that of the outer region, which allows the lubricant impregnated in the outer region to migrate to the inner region by capillary force, thereby facilitating the formation of an oil film on the bearing surface. In particular, in the manufacturing method of the first invention for a sintered oil-impregnated bearing, in the sealing step, the surface pores on the end faces of the sintered body are sealed so that the surface porosity of the end faces is lower than that of the bearing surface. This suppresses leakage of lubricant from the end faces in the relatively low-density outer peripheral region, thereby suppressing a decrease in oil film pressure and improving starting characteristics at low temperatures. Furthermore, suppressing lubricant leakage makes it possible to extend the bearing life. Here, the raw material corresponds to raw material powder, which will be described later. The green compact corresponds to a green compact, which will be described later. The green compacting process corresponds to a green compacting process, which will be described later. The sintered body corresponds to a sintered body, which will be described later. The sintering process corresponds to a sintering process, which will be described later. The end surface corresponds to an end surface s2, which will be described later. The sealing process corresponds to a sealing process, which will be described later. The sizing process corresponds to a sizing process, which will be described later. The bearing surface corresponds to a bearing surface s1, which will be described later. The inner peripheral region corresponds to an inner peripheral region 11, which will be described later. The outer peripheral region corresponds to an outer peripheral region 12, which will be described later.

[0006] The method for manufacturing a sintered oil-impregnated bearing according to the second invention is characterized in that, in the method for manufacturing a sintered oil-impregnated bearing according to the first invention, the surface porosity of the end face of the sintered body is within the range of 5 to 30%. In the method for manufacturing a sintered oil-impregnated bearing according to the second aspect of the present invention, it is possible to appropriately suppress a decrease in oil film pressure. That is, if the surface porosity of the end face of the sintered body is less than 5%, it becomes difficult for air to flow in from the end face, which hinders the lubricant impregnated in the outer peripheral region from being delivered to the inner peripheral region, resulting in a risk of a decrease in oil film pressure. On the other hand, if the surface porosity of the end face of the sintered body exceeds 30%, the lubricant is more likely to leak from the end face, resulting in a risk of a decrease in oil film pressure. Therefore, by setting the surface porosity of the end face of the sintered body within the range of 5 to 30%, it is possible to suppress the lubricant leaking from the end face while preventing the lubricant impregnated in the outer peripheral region from being hindered from being delivered to the inner peripheral region, thereby appropriately suppressing a decrease in oil film pressure.

[0007] The sintered oil-impregnated bearing of the third invention is characterized in that the density of the inner peripheral region is higher than that of the outer peripheral region, and the surface porosity of the end face is lower than the surface porosity of the bearing surface, being within the range of 5 to 30%. In the oil-impregnated sintered bearing according to the third invention, the density of the inner peripheral region of the sintered body is higher than that of the outer peripheral region, which allows the lubricant impregnated in the outer peripheral region to migrate to the inner peripheral region by capillary force, thereby facilitating the formation of an oil film on the bearing surface. In particular, in the sintered oil-impregnated bearing according to the third aspect of the invention, the surface porosity of the end faces is lower than that of the bearing surface, which prevents lubricant from leaking from the end faces in the relatively low-density outer peripheral region, thereby preventing a decrease in oil film pressure and improving starting characteristics at low temperatures. Furthermore, in the sintered oil-impregnated bearing according to the third aspect of the present invention, the surface porosity of the end face is set to a range of 5 to 30%. This makes it possible to appropriately suppress a decrease in oil film pressure. That is, if the surface porosity of the end face is less than 5%, it becomes difficult for air to flow in from the end face, which hinders the lubricant impregnated in the outer peripheral region from being delivered to the inner peripheral region, resulting in a risk of a decrease in oil film pressure. On the other hand, if the surface porosity of the end face exceeds 30%, the lubricant is more likely to leak from the end face, resulting in a risk of a decrease in oil film pressure. Therefore, by setting the surface porosity of the end face to a range of 5 to 30%, it is possible to suppress the lubricant leaking from the end face while preventing the lubricant impregnated in the outer peripheral region from being hindered from being delivered to the inner peripheral region, thereby making it possible to appropriately suppress a decrease in oil film pressure. Here, the sintered oil-impregnated bearing corresponds to the sintered oil-impregnated bearing 2 described later. The inner peripheral region corresponds to the inner peripheral region 11 described later. The outer peripheral region corresponds to the outer peripheral region 12 described later. The end face corresponds to the end face s2 described later. The bearing surface corresponds to the bearing surface s1 described later.

[0008] A motor according to a fourth aspect of the present invention is characterized in that it includes the oil-impregnated sintered bearing according to the third aspect of the present invention. In the motor according to the fourth aspect of the present invention, it is possible to improve the starting characteristics at low temperatures. Here, the motor corresponds to a motor 1 described later. [Effects of the Invention]

[0009] According to the manufacturing method of a sintered oil-impregnated bearing, the sintered oil-impregnated bearing, and the motor of the present invention, it is possible to improve starting characteristics at low temperatures. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 2 is a partial cross-sectional view of the motor 1. [Figure 2] FIG. 2 is a perspective view of a sintered oil-impregnated bearing 2. [Figure 3]3 is a flowchart of the manufacturing process of the sintered oil-impregnated bearing 2. [Figure 4] FIG. 2 is a diagram illustrating a method for providing a density gradient to a compression molded body according to the first example. [Figure 5] FIG. 10 is a diagram illustrating a method for providing a density gradient to a compression molded body according to a second example. [Figure 6] FIG. 10 is a diagram illustrating a method for providing a density gradient to a compression molded body according to a third example. [Figure 7] FIG. 10 is a diagram illustrating a method for providing a density gradient to a compression molded body according to a fourth example. [Figure 8] FIG. 1 is a diagram showing the composition of raw material powder mp. [Figure 9] 1A and 1B are cross-sectional views of a powder compact and a compression molded body according to an embodiment. [Figure 10] 1 is a diagram comparing the amount of inner diameter wear of a sintered oil-impregnated bearing according to an example with the amount of inner diameter wear of a sintered oil-impregnated bearing according to a comparative example. [Figure 11] 1 is a diagram comparing the inner diameter hardness of a sintered oil-impregnated bearing according to an embodiment with the inner diameter hardness of a sintered oil-impregnated bearing according to a comparative example. [Figure 12] FIG. 10 is a diagram comparing the air permeability of a sintered oil-impregnated bearing according to an embodiment with that of a sintered oil-impregnated bearing according to a comparative example. [Figure 13] FIG. 4 is a diagram showing the surface porosity of the end face of a sintered oil-impregnated bearing according to an example. [Figure 14] FIG. 10 is a diagram showing the surface porosity of the end face of a sintered oil-impregnated bearing according to a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, a sintered oil-impregnated bearing 2 according to an embodiment of the present invention will be described with reference to the drawings. The sintered oil-impregnated bearing 2 can be applied to various motors for home appliances, vehicles, etc. In this embodiment, an example is shown in which the sintered oil-impregnated bearing 2 is applied to a motor 1 for vehicles.

[0012] (Motor 1 configuration) First, the configuration of the motor 1 will be described. FIG. 1 is a partial cross-sectional view of the motor 1. The motor 1 is a DC motor. That is, as shown in Fig. 1, the motor 1 is configured to include a rotor 20 including a coil, a stator 30 including a permanent magnet, and a motor housing 40 that accommodates the rotor 20 and the stator 30. The rotor 20 is provided with a rotating shaft 21. The rotating shaft 21 is made of metal (such as an alloy steel, such as carbon steel or stainless steel) and is formed in a cylindrical shape. A pair of sintered oil-impregnated bearings 2 are disposed in the motor housing 40. The pair of sintered oil-impregnated bearings 2 rotatably support the rotary shaft 21.

[0013] (Configuration of sintered oil-impregnated bearing 2) Next, the configuration of the sintered oil-impregnated bearing 2 will be described. FIG. 2 is a perspective view of the sintered oil-impregnated bearing 2. As shown in FIG. The sintered oil-impregnated bearing 2 is a bearing in which the rotating shaft 21 slides against the bearing surface when the rotating shaft 21 rotates. Therefore, the sintered oil-impregnated bearing 2 does not include a fluid dynamic bearing in which the rotating shaft 21 does not come into contact with the bearing surface when the rotating shaft 21 rotates. The sintered oil-impregnated bearing 2 is made of sintered metal (including sintered alloy) obtained by sintering raw material powder including metal powder. The sintered oil-impregnated bearing 2 has a porous structure, and is impregnated with a lubricant such as lubricating oil or lubricating grease inside. As shown in FIG. 2, the sintered oil-impregnated bearing 2 is formed in a substantially circular ring shape, with a bearing hole h provided along the central axis. The bearing hole h is a through-hole that passes through along the axial direction. Here, the "axial direction" refers to the direction in which the imaginary central axis extends. In the sintered oil-impregnated bearing 2, a rotating shaft 21 is inserted into the bearing hole h, thereby rotatably supporting the rotating shaft 21. As a result, the inner peripheral surface of the bearing hole h becomes a bearing surface s1 that supports the rotating shaft 21. During rotation of the rotating shaft 21, the outer peripheral surface of the rotating shaft 21 slides (contacts / slidingly contacts) against the bearing surface s1. The sintered oil-impregnated bearing 2 comprises an inner peripheral region 11 formed on the inner peripheral side (inner diameter side) and an outer peripheral region 12 formed on the outer peripheral side (outer diameter side). The inner peripheral region 11 is an annular region. A bearing surface s1 (bearing hole h) is formed in the inner peripheral region 11. That is, the inner peripheral surface of the inner peripheral region 11 serves as the bearing surface s1. The outer peripheral region 12 is an annular region provided so as to surround the inner peripheral region 11. An outer peripheral surface s3 is formed in the outer peripheral region 12. That is, the outer peripheral surface of the outer peripheral region 12 serves as the outer peripheral surface s3.

[0014] The density of the inner peripheral region 11 is higher than the density of the outer peripheral region 12. Accordingly, the oil content of the inner peripheral region 11 is lower than the oil content of the outer peripheral region 12. The oil content of the inner peripheral region 11 is within the range of 6 to 20 vol.%. In other words, if the oil content of the inner peripheral region 11 is less than 6 vol.%, the lubricant will not be smoothly drawn from the inside of the inner peripheral region 11 to the bearing surface s1, making it difficult to form an oil film. On the other hand, if the oil content of the inner peripheral region 11 exceeds 20 vol.%, the oil content will be too high, i.e., the density will be too low, reducing the capillary force and making it difficult to draw the lubricant from the outer peripheral side. Therefore, it is preferable that the oil content of the inner peripheral region 11 be within the range of 6 to 20 vol.%. The oil content of the outer peripheral region 12 is within the range of 12 to 26 vol.%. In other words, if the oil content of the outer peripheral region 12 is less than 12 vol.%, the density difference with the inner peripheral region becomes small, making it difficult for the lubricant to move to the inner peripheral region. On the other hand, if the oil content of the outer peripheral region 12 exceeds 26 vol.%, the oil content is too high, i.e., the density is too low, resulting in a decrease in capillary force and increased risk of lubricant leakage from the end face and outer diameter side. Therefore, it is preferable that the oil content of the outer peripheral region 12 be within the range of 12 to 26 vol.%.

[0015] In particular, the end faces s2 and outer peripheral surface s3 of the sintered oil-impregnated bearing 2 are subjected to a sealing treatment to seal (reduce) surface pores. At this time, the sealing treatment is performed on both end faces s2 (one end face s2 and the other end face s2). In addition, the sealing treatment is performed on the entirety of each end face s2 (both the inner peripheral region 11 and the outer peripheral region 12). Furthermore, the sealing treatment is performed on the entirety of the outer peripheral surface s3. On the other hand, the bearing surface s1 is not subjected to the sealing treatment. The surface porosity of each end face s2 is lower than that of the bearing face s1. The surface porosity of the outer peripheral face s3 is also lower than that of the bearing face s1. The surface porosity of each end face s2 and the outer peripheral surface s3 is set to be within a range of 5 to 30%. That is, if the surface porosity of the end face s2 and the outer peripheral surface s3 is less than 5%, it becomes difficult for air to flow in from the end face s2 and the outer peripheral surface s3, which hinders the lubricant impregnated in the outer peripheral region 12 from being delivered to the inner peripheral region 11, resulting in a risk of a decrease in oil film pressure. On the other hand, if the surface porosity of the end face s2 and the outer peripheral surface s3 exceeds 30%, the lubricant is more likely to leak from the end face s2 and the outer peripheral surface s3, resulting in a risk of a decrease in oil film pressure. Therefore, by setting the surface porosity of the end face s2 and the outer peripheral surface s3 within a range of 5 to 30%, it is possible to suppress the lubricant from leaking from the end face s2 and the outer peripheral surface s3 while preventing the lubricant impregnated in the outer peripheral region 12 from being impeded from being delivered to the inner peripheral region 11, thereby appropriately suppressing a decrease in oil film pressure. In particular, the surface porosity of the end surface s2 and the outer peripheral surface s3 is preferably set within a range of 5 to 20%, which can enhance the above-mentioned effects. The surface porosity of the bearing surface s1 is set to be within a range of 5 to 60%. In other words, if the surface porosity of the bearing surface s1 is less than 5%, lubricant may not be smoothly drawn from the inside of the inner peripheral region 11 to the bearing surface s1 during rotation of the rotating shaft 21, which may make it difficult to form an oil film. On the other hand, if the surface porosity of the bearing surface s1 exceeds 60%, oil film pressure leakage at the bearing surface s1 is more likely to occur, resulting in insufficient lubrication. Therefore, by setting the surface porosity of the bearing surface s1 within a range of 5 to 60%, it is possible to suppress oil film pressure leakage at the bearing surface s1 and improve lubrication without impairing the drawing of lubricant from the inside of the inner peripheral region 11 to the bearing surface s1. In particular, it is preferable that the surface porosity of the bearing surface s1 be within a range of 10 to 40%. This makes it possible to enhance the above-mentioned effects.

[0016] (Function of sintered oil-impregnated bearing 2) As a result, in the sintered oil-impregnated bearing 2, a density gradient is provided between the inner peripheral region 11 and the outer peripheral region 12 so that the density of the inner peripheral region 11 is higher than the density of the outer peripheral region 12. As a result, even if the volume of the lubricant shrinks, particularly at low temperatures, the lubricant impregnated in the outer peripheral region 12 can be moved to the inner peripheral region 11 by capillary force, thereby promoting oil film formation on the bearing surface s1. This makes it possible to improve starting characteristics at low temperatures. Furthermore, promoting oil film formation on the bearing surface s1 suppresses wear on the bearing surface s1, thereby enabling the life of the sintered oil-impregnated bearing 2 to be extended. Furthermore, in the sintered oil-impregnated bearing 2, the density of the inner peripheral region 11 is higher than the density of the outer peripheral region 12, so that the occurrence of fatigue wear of the bearing surface s1 due to vibration of the rotating shaft 21 can be suppressed. Furthermore, in the sintered oil-impregnated bearing 2, the oil impregnation rate of the outer peripheral region 12 is higher than that of the inner peripheral region 11, so that when press-fitted into the motor housing 40, it is possible to suppress shrinkage of the bearing hole h. Here, if the density of the outer peripheral region 12 is relatively reduced, the surface porosity of the outer peripheral region 12 increases accordingly. If the surface porosity of the outer peripheral region 12 increases, the lubricant becomes more likely to leak from the surface (end face s2 and outer peripheral surface s3) of the outer peripheral region 12, which may result in a decrease in oil film pressure on the bearing surface s1. Therefore, in the sintered oil-impregnated bearing 2, the density of the outer peripheral region 12 is relatively low, and the surface porosity of the outer peripheral region 12 is made lower than the surface porosity of the bearing surface s1. In other words, a sealing treatment is applied to the end surface s2 and outer peripheral surface s3 of the sintered oil-impregnated bearing 2, and the surface porosity of the end surface s2 and outer peripheral surface s3 is made lower than the surface porosity of the bearing surface s1. This makes it possible to maintain the effect of providing a density gradient between the inner peripheral region 11 and the outer peripheral region 12, while suppressing leakage of lubricant from the surface (end face s2 and outer peripheral surface s3) of the outer peripheral region 12, which has a relatively low density, and as a result, it becomes possible to suppress a decrease in oil film pressure at the bearing surface s1. Furthermore, in the sintered oil-impregnated bearing 2, the end surface s2 is subjected to a sealing treatment, which improves the surface roughness of the end surface s2 and makes it possible to improve the sliding characteristics when supporting a thrust load, particularly when sliding against a thrust washer.

[0017] (Modification of sintered oil-impregnated bearing 2) Here, the sintered oil-impregnated bearing 2 may be configured such that linear grooves extending along the axial direction are provided on the bearing surface s1. In particular, the bearing surface s1 may be configured such that a plurality of linear grooves are provided at predetermined intervals. With this configuration, particularly at low temperatures, the highly viscous lubricant present on the bearing surface s1 can be released into the linear groove, making it possible to stabilize start-up.

[0018] (Manufacturing method of sintered oil-impregnated bearing 2) Next, a method for manufacturing the sintered oil-impregnated bearing 2 will be described. FIG. 3 is a flowchart of the manufacturing process of the sintered oil-impregnated bearing 2. 3, a stirring step is first carried out in the manufacture of the sintered oil-impregnated bearing 2. In the stirring step, a die lubricant is added to metal powder, and the mixture is stirred and mixed to produce a raw material powder. Examples of usable metal powders include copper powder, bronze powder, brass powder, phosphorus copper powder, nickel silver powder, iron powder, tin powder, copper-nickel alloy powder, copper-coated iron powder, stainless steel powder, and mixtures thereof. Examples of usable die lubricants include powders of metal soaps such as zinc stearate and lithium stearate, powders of fatty acid amides such as ethylene bisstearamide, and powders of wax-based lubricants such as polyethylene. Depending on the intended use of the sintered oil-impregnated bearing 2, powders of solid lubricants such as graphite, molybdenum disulfide, and boron nitride may be added in addition to the metal powder. Note that the metal powders, die lubricants, solid lubricants, and antioxidants are not limited to these.

[0019] Next, a powder compacting step is carried out, in which the raw material powder is press-molded in a mold at a pressure of about 100 to 500 MPa to form a green compact. Next, the sintering process is carried out. In this process, the green compact is sintered under predetermined atmosphere and temperature conditions to form a sintered body. By sintering the green compact, adjacent metal particles are diffusion-bonded, bonding the metal particles and forming a porous sintered body. The predetermined atmosphere for sintering is selected appropriately depending on the composition of the raw material powder, such as vacuum, reducing gas (ammonia decomposition gas, hydrogen gas, endothermic gas, etc.), inert gas (nitrogen gas, argon gas, etc.), or a mixture of these reducing gases and inert gases. Furthermore, the predetermined temperature condition for sintering is practically around 600 to 1200°C, and is selected appropriately depending on the composition of the raw material powder.

[0020] Next, a sealing step is carried out. In the sealing step, a sealing treatment is performed on the end surface s2 and the outer peripheral surface s3 of the sintered body to seal (reduce) surface pores. In this embodiment, a co-polishing treatment or a barrel polishing treatment is performed as the sealing treatment. In the co-rubbing process, a container containing a plurality of sintered bodies is rotated, causing the sintered bodies to rub against each other, thereby polishing the surfaces of the sintered bodies. Barrel polishing is a process in which a container containing multiple sintered bodies and multiple polishing media (ceramic chips, etc.) is vibrated, causing the sintered bodies and polishing media to rub against each other, thereby polishing the surface of each sintered body. In the sealing step, the shape and size of the polishing media are adjusted so that the bearing surface s1 of the sintered body is not subjected to the sealing treatment. As a result, in the sealing step, both end faces s2 (one end face s2 and the other end face s2) and the outer peripheral surface s3 of the sintered body are subjected to the sealing treatment, while the bearing surface s1 is not subjected to the sealing treatment. In this case, the entire end faces s2 (both the inner peripheral region 11 and the outer peripheral region 12) and the entire outer peripheral surface s3 are subjected to the sealing treatment. In particular, in the sealing step, sealing treatment is performed on each end face s2 so that the surface porosity of each end face s2 is lower than the surface porosity of the bearing surface s1. Also, sealing treatment is performed on the outer peripheral surface s3 so that the surface porosity of the outer peripheral surface s3 is lower than the surface porosity of the bearing surface s1. In this embodiment, the surface porosity of each end face s2 before the sealing step is performed is within a range of 30 to 60%. Furthermore, the surface porosity of the outer peripheral surface s3 before the sealing step is performed is within a range of 30 to 60%. Then, by performing the sealing step (and the sizing step), the surface porosity of each end face s2 is set within a range of 5 to 30% (preferably within a range of 5 to 20%). Furthermore, by performing the sealing step (and the sizing step), the surface porosity of the outer peripheral surface s3 is set within a range of 5 to 30% (preferably within a range of 5 to 20%). Meanwhile, the surface porosity of the bearing surface s1 after the sealing step is performed (after the sizing step is executed) is set within a range of 5 to 60% (preferably within a range of 10 to 40%).

[0021] Next, the primary oil immersion process is carried out. In this process, the sintered body is immersed for a certain period of time to impregnate the interior of the sintered body with the sizing lubricant. This makes it possible to suppress die wear in the subsequent sizing process and also to suppress the occurrence of rust during the process. In this embodiment, the pore-sealing step is performed before the primary oil-immersion step, but the pore-sealing step may be performed after the primary oil-immersion step. Next, a sizing process is carried out. In the sizing process, the sintered body that has undergone the sealing process is sized (recompressed) in a mold at a predetermined pressure to form a compression-molded body. In the sizing process, the sintered body is compressed so that the density of the inner peripheral region 11 is higher than that of the outer peripheral region 12. That is, the sintered body is compressed so that the compression rate of the inner peripheral region 11 is higher than that of the outer peripheral region 12. As a result, the oil content of the inner peripheral region 11 becomes lower than that of the outer peripheral region 12. In particular, by carrying out the sizing process, the oil content of the inner peripheral region 11 becomes within a range of 6 to 20 [vol.%], and the oil content of the outer peripheral region 12 becomes within a range of 12 to 26 [vol.%]. In particular, by carrying out the sizing process, the oil content of the inner peripheral region 11 is in the range of 6 to 20 [vol.%], the oil content of the outer peripheral region 12 is in the range of 12 to 26 [vol.%], and the oil content of the inner peripheral region 11 is 2 to 10 [vol.%] lower than the oil content of the outer peripheral region 12. Next, a washing step is carried out. In the washing step, the compression-molded body is subjected to a washing treatment to remove metal chips, sizing lubricating oil, and the like generated during processing. Next, a vacuum oil immersion step is carried out, in which the inside of the compression-molded body is impregnated with a lubricant in a vacuum. Next, a centrifugal deoiling step is carried out, in which the container containing the lubricant-impregnated compression-molded body is rotated to remove excess lubricant remaining on the surface of the compression-molded body. In this way, the sintered oil-impregnated bearing 2 is manufactured.

[0022] (Method of creating a density gradient in a compression molded body) Next, a method for providing a density gradient to a compression molded body will be described. Fig. 4 is a diagram illustrating a method for imparting a density gradient to a compression-molded body according to a first example. Fig. 5 is a diagram illustrating a method for imparting a density gradient to a compression-molded body according to a second example. Fig. 6 is a diagram illustrating a method for imparting a density gradient to a compression-molded body according to a third example. Fig. 7 is a diagram illustrating a method for imparting a density gradient to a compression-molded body according to a fourth example.

[0023] (Example 1) First, a first example of a method for providing a compression-molded body with a density gradient will be described. In the first example, in the powder compacting process, a compacted powder A is formed in which the axial thickness of the inner peripheral region 11 is greater than the axial thickness of the outer peripheral region 12, and in the sizing process, the sintered body is compressed so that the axial thicknesses of the inner peripheral region 11 and the outer peripheral region 12 are approximately the same, thereby forming a compressed formed body B in which the density of the inner peripheral region 11 is higher than the density of the outer peripheral region 12. Specifically, in the first example, as shown in Fig. 4(a), in the powder compacting process, a powder compact A is formed using an upper mold 100 and a lower mold 110. The upper mold 100 and the lower mold 110 are independent molds. The upper mold 100 and the lower mold 110 are each formed in an annular shape. A first compression surface 101 corresponding to the inner peripheral region 11 and a second compression surface 102 corresponding to the outer peripheral region 12 are provided in a stepped manner on the lower surface (bottom surface) of the upper mold 100. In this case, the first compression surface 101 is provided at a higher position than the second compression surface 102. The first compression surface 101 and the second compression surface 102 are each a flat horizontal surface extending in an annular shape. The first compression surface 101 and the second compression surface 102 are connected in a stepped manner via a tapered surface 103. A first compression surface 111 corresponding to the inner peripheral region 11 and a second compression surface 112 corresponding to the outer peripheral region 12 are provided in a stepped manner on the upper surface (top surface) of the lower mold 110. In this case, the first compression surface 111 is provided at a lower position than the second compression surface 112. The first compression surface 111 and the second compression surface 112 are each a flat horizontal surface extending in an annular shape. The first compression surface 111 and the second compression surface 112 are connected in a stepped manner via a tapered surface 113. In the powder compacting process, the raw material powder is press-molded using an upper die 100 and a lower die 110. The first compression surface 101 and the first compression surface 111 are arranged to face each other. The second compression surface 102 and the second compression surface 112 are arranged to face each other. The upper die 100 forms the upper end surface s2 of the powder compact A, and the lower die 110 forms the lower end surface s2 of the powder compact A. This results in the powder compact A having an outer peripheral region 12 and an inner peripheral region 11 that is thicker in the axial direction than the outer peripheral region 12. At each end surface s2 of the powder compact A, the inner peripheral region 11 and the outer peripheral region 12 are connected in a stepped manner via a tapered surface. This facilitates flattening each end surface s2 when compressing the sintered body in the sizing process. Here, in the compact A, the difference between the total axial length of the compact A (= the axial thickness of the inner peripheral region 11) and the axial thickness of the outer peripheral region 12 is within the range of 5 to 20% of the axial thickness of the outer peripheral region 12.

[0024] In the first example, as shown in Fig. 4(b), in the sizing step, a compression-molded body B is formed using an upper mold 120 and a lower mold 130. The upper mold 120 and the lower mold 130 are molds independent of each other. The upper mold 120 and the lower mold 130 are each formed in an annular shape. The lower surface (bottom surface) of the upper mold 120 is provided with a compression surface 121 corresponding to the inner peripheral region 11 and the outer peripheral region 12. The compression surface 121 is a flat horizontal surface extending in an annular shape. The upper surface (top surface) of the lower mold 130 is provided with a compression surface 131 corresponding to the inner peripheral region 11 and the outer peripheral region 12. The compression surface 131 is a flat horizontal surface extending in an annular shape. In the sizing process, the sintered body is compressed using an upper mold 120 and a lower mold 130. The compression surfaces 121 and 131 are arranged to face each other. The upper mold 120 compresses the entire upper end surface s2 of the sintered body (the inner peripheral region 11 and the outer peripheral region 12), while the lower mold 130 compresses the entire lower end surface s2 of the sintered body (the inner peripheral region 11 and the outer peripheral region 12). This compresses the sintered body so that the outer peripheral region 12 and the inner peripheral region 11 form a substantially continuous, flat horizontal surface at the upper end surface s2 of the compression-molded body B. The sintered body is also compressed so that the outer peripheral region 12 and the inner peripheral region 11 form a substantially continuous, flat horizontal surface at the lower end surface s2 of the compression-molded body B. As a result, the sintered body can be compressed so that the compression ratio is higher in the inner peripheral region 11 than in the outer peripheral region 12. Therefore, in the compression molded body B, the density of the inner peripheral region 11 can be made higher than the density of the outer peripheral region 12.

[0025] (Example 2) Next, a second example of a method for providing a compression-molded body with a density gradient will be described. In the second example, in the powder compacting process, a powder compact A is formed so that the axial thickness of the inner peripheral region 11 and the outer peripheral region 12 is approximately the same, and in the sizing process, the sintered body is compressed so that the axial thickness of the inner peripheral region 11 is smaller than the axial thickness of the outer peripheral region 12, thereby forming a compressed body B in which the density of the inner peripheral region 11 is higher than the density of the outer peripheral region 12. Specifically, in the second example, as shown in Fig. 5(a), in the powder compacting process, a green compact is formed using an upper mold 200 and a lower mold 210. The upper mold 200 and the lower mold 210 are independent molds. The upper mold 200 and the lower mold 210 are each formed in an annular shape. A compression surface 201 is provided on the lower surface (bottom surface) of the upper mold 200. The compression surface 201 is a flat horizontal surface that extends in an annular shape. Furthermore, a compression surface 211 is provided on the upper surface (top surface) of the lower mold 210. The compression surface 211 is a flat horizontal surface that extends in an annular shape. In the powder compacting step, the raw material powder is press-molded using an upper die 200 and a lower die 210. At this time, the compressed surface 201 and the compressed surface 211 are arranged to face each other. The upper end surface s2 of the powder compact A is molded by the upper die 200, and the lower end surface s2 of the powder compact A is molded by the lower die 210. As a result, each end surface s2 of the powder compact A becomes a flat horizontal surface as a whole.

[0026] In the second example, as shown in Fig. 5(b), in the sizing step, a compression-molded body B is formed using an upper mold 220 and a lower mold 230. The upper mold 220 and the lower mold 230 are molds independent of each other. The upper mold 220 and the lower mold 230 are each formed in an annular shape. A first compression surface 221 corresponding to the inner peripheral region 11 and a second compression surface 222 corresponding to the outer peripheral region 12 are provided in a stepped manner on the lower surface (bottom surface) of the upper mold 220. In this case, the first compression surface 221 is provided at a lower position than the second compression surface 222. The first compression surface 221 and the second compression surface 222 are each a flat horizontal surface extending in an annular shape. The first compression surface 221 and the second compression surface 222 are connected in a stepped manner via a tapered surface 223. A first compression surface 231 corresponding to the inner peripheral region 11 and a second compression surface 232 corresponding to the outer peripheral region 12 are provided in a stepped manner on the upper surface (top surface) of the lower mold 230. In this case, the first compression surface 231 is provided at a higher position than the second compression surface 232. The first compression surface 231 and the second compression surface 232 are each a flat horizontal surface extending in an annular shape. The first compression surface 231 and the second compression surface 232 are connected in a stepped manner via a tapered surface 233. In the sizing process, the sintered body is compressed using an upper mold 220 and a lower mold 230. The first compression surface 221 and the first compression surface 231 are arranged to face each other. The second compression surface 222 and the second compression surface 232 are arranged to face each other. The upper end surface s2 of the sintered body is compressed by the upper mold 220, and the lower end surface s2 of the sintered body is compressed by the lower mold 230. This results in the compression-molded body B having an outer peripheral region 12 and an inner peripheral region 11 whose axial thickness is smaller than that of the outer peripheral region 12. That is, the sintered body is compressed so that the compression rate of the inner peripheral region 11 is higher than that of the outer peripheral region 12. As a result, the density of the inner peripheral region 11 in the compression-molded body B can be made higher than that of the outer peripheral region 12.

[0027] (Example 3) Next, a third example of a method for providing a compression-molded body with a density gradient will be described. In the third example, in the powder compacting process, a compacted powder A is formed in which the axial thickness of the inner peripheral region 11 is greater than the axial thickness of the outer peripheral region 12, and in the sizing process, the sintered body is compressed so that the axial thicknesses of the inner peripheral region 11 and the outer peripheral region 12 are approximately the same, thereby forming a compressed formed body B in which the density of the inner peripheral region 11 is higher than the density of the outer peripheral region 12. Specifically, in the third example, as shown in Fig. 6(a), an upper mold 300 and a lower mold 310 are used in the powder compacting process to form a green compact. The upper mold 300 and the lower mold 310 are independent molds. The upper mold 300 and the lower mold 310 are each formed in an annular shape. The upper mold 300 is composed of a first upper mold 301 corresponding to the inner peripheral region 11 and a second upper mold 302 corresponding to the outer peripheral region 12. The first upper mold 301 and the second upper mold 302 are independent (separate) molds. The first upper mold 301 and the second upper mold 302 are each formed in an annular shape. The first upper mold 301 is disposed inside the second upper mold 302. In particular, the vertical position of the first upper mold 301 relative to the second upper mold 302 can be arbitrarily changed. A first compression surface 301a corresponding to the inner peripheral region 11 is provided on the lower surface (bottom surface) of the first upper mold 301. A second compression surface 302a corresponding to the outer peripheral region 12 is provided on the lower surface (bottom surface) of the second upper mold 302. The first compression surface 301a and the second compression surface 302a are each flat horizontal surfaces extending in an annular shape. The first compression surface 301a and the second compression surface 302a are arranged in a stepped manner. In this case, the first compression surface 301a is arranged at a higher position than the second compression surface 302a. In particular, by adjusting the vertical position of the first upper mold 301 relative to the second upper mold 302, it is possible to adjust the height of the first compression surface 301a relative to the second compression surface 302a, and ultimately, it is possible to arbitrarily control both the compression ratio of the outer peripheral region 12 and the compression ratio of the inner peripheral region 11. The lower mold 310 is composed of a first lower mold 311 corresponding to the inner peripheral region 11 and a second lower mold 312 corresponding to the outer peripheral region 12. The first lower mold 311 and the second lower mold 312 are independent (separate) molds. The first lower mold 311 and the second lower mold 312 are each formed in an annular shape. The first lower mold 311 is disposed inside the second lower mold 312. In particular, the vertical position of the first lower mold 311 relative to the second lower mold 312 can be arbitrarily changed. A first compression surface 311a corresponding to the inner peripheral region 11 is provided on the upper surface (top surface) of the first lower mold 311. A second compression surface 312a corresponding to the outer peripheral region 12 is provided on the upper surface (top surface) of the second lower mold 312. The first compression surface 311a and the second compression surface 312a are each flat horizontal surfaces extending in an annular shape. The first compression surface 311a and the second compression surface 312a are arranged in a stepped manner. In this case, the first compression surface 311a is arranged at a lower position than the second compression surface 312a. In particular, by adjusting the vertical position of the first lower mold 311 relative to the second lower mold 312, it is possible to adjust the height of the first compression surface 311a relative to the second compression surface 312a, and ultimately, it is possible to arbitrarily control both the compression ratio of the outer peripheral region 12 and the compression ratio of the inner peripheral region 11. In the powder compacting process, the raw material powder is press-molded using an upper die 300 and a lower die 310. The first compression surface 301a and the first compression surface 311a are positioned opposite each other. The second compression surface 302a and the second compression surface 312a are positioned opposite each other. The upper die 300 forms the upper end surface s2 of the powder compact A, and the lower die 310 forms the lower end surface s2 of the powder compact A. This results in the powder compact having an outer peripheral region 12 and an inner peripheral region 11 that is thicker in the axial direction than the outer peripheral region 12. At each end surface s2 of the powder compact A, the inner peripheral region 11 and the outer peripheral region 12 are connected in a stepped manner via a tapered surface. This facilitates flattening each end surface s2 when compressing the sintered body in the sizing process. Here, in the compact A, the difference between the total axial length of the compact A (= the axial thickness of the inner peripheral region 11) and the axial thickness of the outer peripheral region 12 is within the range of 5 to 20% of the axial thickness of the outer peripheral region 12.

[0028] In the third example, as shown in Fig. 6(b), a compression-molded body is formed in the sizing step using an upper mold 320 and a lower mold 330. The upper mold 320 and the lower mold 330 are independent molds. The upper mold 320 and the lower mold 330 are each formed in an annular shape. A compression surface 321 corresponding to the inner circumferential region 11 and the outer circumferential region 12 is provided on the lower surface (bottom surface) of the upper mold 320. The compression surface 321 is a flat horizontal surface extending in an annular shape. The upper surface (top surface) of the lower mold 330 is provided with a compression surface 331 corresponding to the inner circumferential region 11 and the outer circumferential region 12. The compression surface 331 is a flat horizontal surface extending in an annular shape. In the sizing process, the sintered body is compressed using an upper mold 320 and a lower mold 330. At this time, the compression surfaces 321 and 331 are arranged so as to face each other. The upper mold 320 compresses the entire upper end surface s2 of the sintered body (the inner peripheral region 11 and the outer peripheral region 12), and the lower mold 330 compresses the entire lower end surface s2 of the sintered body (the inner peripheral region 11 and the outer peripheral region 12). This compresses the sintered body so that the outer peripheral region 12 and the inner peripheral region 11 form a substantially continuous, flat horizontal surface at the upper end surface s2 of the compression-molded body B. Furthermore, the sintered body is compressed so that the outer peripheral region 12 and the inner peripheral region 11 form a substantially continuous, flat horizontal surface at the lower end surface s2 of the compression-molded body B. As a result, the sintered body can be compressed so that the compression ratio is higher in the inner peripheral region 11 than in the outer peripheral region 12. Therefore, in the compression molded body B, the density of the inner peripheral region 11 can be made higher than the density of the outer peripheral region 12.

[0029] (Example 4) Next, a fourth example of the method for providing a compression-molded body with a density gradient will be described. In the fourth example, in the powder compacting process, a compacted powder A is formed in which the axial thickness of the inner peripheral region 11 is greater than the axial thickness of the outer peripheral region 12, and in the sizing process, the sintered body is compressed so that the axial thicknesses of the inner peripheral region 11 and the outer peripheral region 12 are approximately the same, thereby forming a compressed formed body B in which the density of the inner peripheral region 11 is higher than the density of the outer peripheral region 12. Specifically, in the fourth example, as shown in Fig. 7(a), in the powder compacting process, a powder compact A is formed using an upper mold 400 and a lower mold 410. The upper mold 400 and the lower mold 410 are independent molds. The upper mold 400 and the lower mold 410 are each formed in an annular shape. A first compression surface 401 corresponding to the inner peripheral region 11 and a second compression surface 402 corresponding to the outer peripheral region 12 are provided in a stepped manner on the lower surface (bottom surface) of the upper mold 400. In this case, the first compression surface 401 is provided at a higher position than the second compression surface 402. The first compression surface 401 and the second compression surface 402 are each a flat horizontal surface extending in an annular shape. The first compression surface 401 and the second compression surface 402 are connected in a stepped manner via a tapered surface 403. The lower mold 410 is composed of a first lower mold 411 corresponding to the inner peripheral region 11 and a second lower mold 412 corresponding to the outer peripheral region 12. The first lower mold 411 and the second lower mold 412 are molds independent of each other (separate). The first lower mold 411 and the second lower mold 412 are each formed in an annular shape. The first lower mold 411 is disposed inside the second lower mold 412. In particular, the vertical position of the first lower mold 411 relative to the second lower mold 412 can be arbitrarily changed. A first compression surface 411a corresponding to the inner peripheral region 11 is provided on the upper surface (top surface) of the first lower mold 411. A second compression surface 412a corresponding to the outer peripheral region 12 is provided on the upper surface (top surface) of the second lower mold 412. Furthermore, a third compression surface 412b corresponding to the outer peripheral surface s3 is provided on the inner peripheral surface of the second lower mold 412. The first compression surface 411a and the second compression surface 412a are each a flat horizontal surface extending in an annular shape. The third compression surface 412b is a curved surface extending in an annular shape. The first compression surface 411a and the second compression surface 412a are arranged in a stepped manner. In this case, the first compression surface 411a is arranged at a lower position than the second compression surface 412a. In particular, by adjusting the vertical position of the first lower mold 411 relative to the second lower mold 412, it is possible to adjust the height of the first compression surface 411a relative to the second compression surface 412a, and ultimately it is possible to arbitrarily control both the compression rate of the outer peripheral region 12 and the compression rate of the inner peripheral region 11. In the powder compacting process, the raw material powder is press-molded using an upper mold 400 and a lower mold 410. The first compression surface 401 and the first compression surface 411a are arranged to face each other. The second compression surface 402 and the second compression surface 412a are arranged to face each other. The upper mold 400 forms the upper end surface s2 of the powder compact A, and the lower mold 410 forms the lower end surface s2 and the outer peripheral surface s3 of the powder compact A. This results in the powder compact A having an outer peripheral region 12 and an inner peripheral region 11 that is thicker in the axial direction than the outer peripheral region 12. At each end surface s2 of the powder compact A, the inner peripheral region 11 and the outer peripheral region 12 are connected in a stepped manner via a tapered surface. This facilitates flattening each end surface s2 when compressing the sintered body in the sizing process. Here, in the compact A, the difference between the total axial length of the compact A (= the axial thickness of the inner peripheral region 11) and the axial thickness of the outer peripheral region 12 is within the range of 5 to 20% of the axial thickness of the outer peripheral region 12.

[0030] 7(b), in the sizing step, a compression-molded body B is formed using an upper mold 420 and a lower mold 430. The upper mold 420 and the lower mold 430 are independent molds. The upper mold 420 and the lower mold 430 are each formed in an annular shape. A compression surface 421 corresponding to the inner circumferential region 11 and the outer circumferential region 12 is provided on the lower surface (bottom surface) of the upper mold 420. The compression surface 421 is a flat horizontal surface extending in an annular shape. The upper surface (top surface) of the lower mold 430 is provided with a compression surface 431 corresponding to the inner circumferential region 11 and the outer circumferential region 12. The compression surface 431 is a flat horizontal surface extending in an annular shape. In the sizing process, the sintered body is compressed using an upper mold 420 and a lower mold 430. At this time, the compression surfaces 421 and 431 are arranged so as to face each other. The upper mold 420 compresses the entire upper end surface s2 of the sintered body (the inner peripheral region 11 and the outer peripheral region 12), and the lower mold 430 compresses the entire lower end surface s2 of the sintered body (the inner peripheral region 11 and the outer peripheral region 12). This compresses the sintered body so that the outer peripheral region 12 and the inner peripheral region 11 form a substantially continuous, flat horizontal surface at the upper end surface s2 of the compression-molded body B. Furthermore, the sintered body is compressed so that the outer peripheral region 12 and the inner peripheral region 11 form a substantially continuous, flat horizontal surface at the lower end surface s2 of the compression-molded body B. As a result, the sintered body can be compressed so that the compression ratio is higher in the inner peripheral region 11 than in the outer peripheral region 12. Therefore, in the compression molded body B, the density of the inner peripheral region 11 can be made higher than the density of the outer peripheral region 12.

[0031] (Function of manufacturing method of sintered oil-impregnated bearing 2) In the manufacturing method of the sintered oil-impregnated bearing 2, the sintered body is compressed in the sizing step so that the density of the inner peripheral region 11 of the powder compact B is higher than that of the outer peripheral region 12. This allows the lubricant impregnated in the outer peripheral region 12 to move to the inner peripheral region 11 by capillary force, thereby facilitating the formation of an oil film on the bearing surface s1. In particular, in the manufacturing method of the sintered oil-impregnated bearing 2, in the sealing step, the surface pores of the end surface s2 of the sintered body are sealed so that the surface porosity of the end surface s2 of the sintered body is lower than that of the bearing surface s1. This prevents leakage of lubricant from the end surface s2 in the outer peripheral region 12, which has a relatively low density, and as a result, it becomes possible to prevent a decrease in oil film pressure and improve starting characteristics at low temperatures. In addition, in the manufacturing method of the sintered oil-impregnated bearing 2, the surface porosity of the end surface s2 of the sintered body is set to a range of 5 to 30%. This makes it possible to appropriately suppress a decrease in oil film pressure. That is, if the surface porosity of the end surface s2 of the sintered body is less than 5%, it becomes difficult for air to flow in from the end surface s2, which hinders the delivery of the lubricant impregnated in the outer peripheral region 12 to the inner peripheral region 11, resulting in a decrease in oil film pressure. On the other hand, if the surface porosity of the end surface s2 of the sintered body exceeds 30%, the lubricant is more likely to leak from the end surface s2, resulting in a decrease in oil film pressure. Therefore, by setting the surface porosity of the end surface s2 of the sintered body to a range of 5 to 30%, it is possible to suppress the delivery of the lubricant impregnated in the outer peripheral region 12 to the inner peripheral region 11 while also suppressing the leakage of the lubricant from the end surface s2, thereby making it possible to appropriately suppress a decrease in oil film pressure.

[0032] (First Example) Next, a first embodiment of the present invention will be described. Fig. 8 is a diagram showing the composition of raw material powder mp. Fig. 9 is a cross-sectional view of a green compact and a compression molded body according to an example. Fig. 10 is a diagram comparing the amount of inner diameter wear of a sintered oil-impregnated bearing according to an example with the amount of inner diameter wear of a sintered oil-impregnated bearing according to a comparative example. Here, each number in Fig. 8 indicates the weight ratio of each element (metal powder or solid lubricant) to the total weight of the raw material powder mp (unit: wt%). Also, Fig. 9(a) shows a cross section of the green compact A formed by the green compacting process, and Fig. 9(b) shows a cross section of the green compact B formed by the sizing process. Five types of sintered oil-impregnated bearings (Examples a1 to a5) were manufactured as first examples of the present invention, and five types of sintered oil-impregnated bearings (Comparative Examples a1 to a5) were manufactured as comparative examples.

[0033] The sintered oil-impregnated bearings according to Examples a1 to a5 were manufactured by the above-mentioned manufacturing method for sintered oil-impregnated bearing 2. In this case, the raw material powder mp shown in Fig. 8 was used to form a green compact A and a green compact B by a method of imparting a density gradient to the green compact according to the first example. As shown in Figure 8, the composition of the raw material powder mp contains, in weight ratio relative to the total weight of the raw material powder, Fe (iron) in the range of 35 to 45 wt%, Sn (tin) in the range of 2 to 7 wt%, P (phosphorus) not more than 0.6 wt%, C (carbon) in the range of 1 to 3 wt%, and unavoidable impurities not more than 1 wt%, with the remainder being Cu (copper). When manufacturing the sintered oil-impregnated bearings according to Examples a1 to a5, a powder compact A shown in FIG. 9(a) was formed by a powder compacting process. In this powder compact A, if the outer diameter of the inner peripheral region 11 is "e," the outer diameter is "b," and the inner diameter is "a," then e = (ba) × (½). Here, the outer diameter of the inner peripheral region 11 refers to the outer diameter of the inner peripheral region 11 and the portion including the tapered portion connecting the inner peripheral region 11 and the outer peripheral region 12. Furthermore, in this powder compact A, the difference between the total axial length of the powder compact A (= the axial thickness of the inner peripheral region 11) and the axial thickness of the outer peripheral region 12 was 10% of the axial thickness of the outer peripheral region 12. In other words, if the total axial length of the powder compact A (= the axial thickness of the inner peripheral region 11) is "d" and the axial thickness of the outer peripheral region 12 is "c", then (dc) = c × 0.1. Furthermore, when manufacturing the sintered oil-impregnated bearings according to Examples a1 to a5, a compression-molded body B shown in Fig. 9(b) was formed by a sizing step. In this compression-molded body B, the outer peripheral region 12 and the inner peripheral region 11 form a substantially continuous, flat horizontal surface. Here, in the sizing step, the sintered body was compressed so that the overall length of the compression-molded body B was substantially equal to the axial thickness of the outer peripheral region 12 of the green compact A.

[0034] On the other hand, the sintered oil-impregnated bearings of Comparative Examples a1 to a5 are sintered oil-impregnated bearings in which there is no density gradient in the compression-molded body and no sealing treatment is applied to the end faces and outer circumferential surfaces of the sintered body. The sintered oil-impregnated bearings of Comparative Examples a1 to a5 were manufactured using the raw material powder MP shown in Figure 8 by a conventional method for manufacturing sintered oil-impregnated bearings. In this case, the manufacturing conditions, such as the conditions (pressure) for press-molding the raw material powder MP in the powder compacting step, the conditions (atmosphere and temperature) for sintering the green compact in the sintering step, the conditions (pressure) for compressing the sintered body in the sizing step, and the type of lubricant impregnated in the vacuum oil-immersion step, were the same as the manufacturing conditions for the sintered oil-impregnated bearings of Examples a1 to a5.

[0035] For each of the sintered oil-impregnated bearings according to Examples a1 to a5 and Comparative Examples a1 to a5, a motor was manufactured by incorporating the sintered oil-impregnated bearing into a motor housing (each on the housing side and the end bell side), and a vibration and cycle test was carried out by running the motor under a specified environment. During this test, the specified environment consisted of applying vibration to the motor while repeatedly changing the temperature within the range of -40°C to 160°C. Then, for each of the sintered oil-impregnated bearings according to Examples a1 to a5 and Comparative Examples a1 to a5, the amount of inner diameter wear was measured after the vibration and cycle test. Here, the amount of inner diameter wear represents the depth of wear on the bearing surface. As a result, it was confirmed that the sintered oil-impregnated bearings of Examples a1 to a5 (sintered oil-impregnated bearings arranged on the housing side) had a significantly reduced amount of inner diameter wear compared to the sintered oil-impregnated bearings of Comparative Examples a1 to a5 (sintered oil-impregnated bearings arranged on the housing side), as shown in Figure 10. It was also confirmed that the sintered oil-impregnated bearings of Examples a1 to a5 (sintered oil-impregnated bearings arranged on the end bell side) had a reduced amount of inner diameter wear compared to the sintered oil-impregnated bearings of Comparative Examples a1 to a5 (sintered oil-impregnated bearings arranged on the end bell side).

[0036] (Second Example) Next, a second embodiment of the present invention will be described. FIG. 11 is a diagram comparing the inner diameter hardness of the sintered oil-impregnated bearing according to the example with the inner diameter hardness of the sintered oil-impregnated bearing according to the comparative example. As a second example of the present invention, four types of sintered oil-impregnated bearings were manufactured (Example b1 with an oil content of 13.5%, Example b2 with an oil content of 16%, Example b3 with an oil content of 18.5%, and Example b4 with an oil content of 21%).Furthermore, as comparative examples, four types of sintered oil-impregnated bearings were manufactured (Comparative Example b1 with an oil content of 13.5%, Comparative Example b2 with an oil content of 16%, Comparative Example b3 with an oil content of 18.5%, and Comparative Example b4 with an oil content of 21%). The sintered oil-impregnated bearings of Examples b1 to b4 were manufactured by the same manufacturing method as the sintered oil-impregnated bearings of Examples a1 to a5, and the sintered oil-impregnated bearings of Comparative Examples b1 to b4 were manufactured by the same manufacturing method as the sintered oil-impregnated bearings of Comparative Examples a1 to a5. The inner diameter hardness was measured for each of the sintered oil-impregnated bearings according to Examples a1 to a4 and Comparative Examples a1 to a4, where the inner diameter hardness is the hardness of the bearing surface. As a result, as shown in Figure 11, it was confirmed that at the same oil content, the sintered oil-impregnated bearings of Examples a1 to a4 had a higher density of the inner diameter surface layer and therefore a higher inner diameter hardness compared to the sintered oil-impregnated bearings of Comparative Examples a1 to a4.

[0037] (Third Example) Next, a third embodiment of the present invention will be described. FIG. 12 is a diagram comparing the air permeability of the sintered oil-impregnated bearing according to the example with that of the sintered oil-impregnated bearing according to the comparative example. Ten types of sintered oil-impregnated bearings (Examples c1 to c10) were manufactured as a third example of the present invention. Additionally, ten types of sintered oil-impregnated bearings (Comparative Examples c1 to c10) were manufactured as comparative examples. The sintered oil-impregnated bearings of Examples c1 to c10 and Comparative Examples c1 to c10 were manufactured by the same manufacturing method as the sintered oil-impregnated bearings of Examples a1 to a5. However, in the manufacturing method of the sintered oil-impregnated bearings of Comparative Examples c1 to c10, the sealing step was omitted. However, in the manufacturing method of the sintered oil-impregnated bearings of Examples c1 to c10, the sealing step was not omitted. The air permeability was measured for each of the sintered oil-impregnated bearings according to Examples c1 to c10 and Comparative Examples c1 to c10. Here, the air permeability is the amount of air that is ejected from the outer peripheral surface and end face per given time when air is sent into the bearing hole at a given (constant) pressure. As a result, it was confirmed that the sintered oil-impregnated bearings of Examples c1 to c10 had a reduced amount of air permeation compared to the sintered oil-impregnated bearings of Comparative Examples c1 to c10, as shown in Figure 12. This makes it possible to suppress leakage of lubricant from the outer peripheral surface and end face in the sintered oil-impregnated bearings of Examples c1 to c10 compared to the sintered oil-impregnated bearings of Comparative Examples c1 to c10.

[0038] (Fourth Example) Next, a fourth embodiment of the present invention will be described. Fig. 13 is a diagram showing the surface porosity of a sintered oil-impregnated bearing according to an example, and Fig. 14 is a diagram showing the surface porosity of a sintered oil-impregnated bearing according to a comparative example. Here, Figs. 13 and 14 are photographs of the end face of a sintered oil-impregnated bearing observed under a microscope, and the areas shown in grey indicate surface pores. Five types of sintered oil-impregnated bearings (Examples d1 to d5) were manufactured as a fourth example of the present invention. Five types of sintered oil-impregnated bearings (Comparative Examples d1 to d5) were also manufactured as comparative examples. The sintered oil-impregnated bearings of Examples d1 to d5 were manufactured by the same manufacturing method as the sintered oil-impregnated bearings of Examples c1 to c10, while the sintered oil-impregnated bearings of Comparative Examples d1 to d5 were manufactured by the same manufacturing method as the sintered oil-impregnated bearings of Comparative Examples c1 to c10. The end faces of the sintered oil-impregnated bearings according to Examples d1 to d5 and Comparative Examples d1 to d5 were observed under a microscope, and the surface porosity of the end faces was measured. 13 and 14, it was confirmed that the sintered oil-impregnated bearings according to Examples d1 to d5 had reduced surface porosity at the end faces compared to the sintered oil-impregnated bearings according to Comparative Examples d1 to d5. This makes it possible to suppress leakage of lubricant from the end faces in the sintered oil-impregnated bearings according to Examples d1 to d5 compared to the sintered oil-impregnated bearings according to Comparative Examples d1 to d5. [Explanation of symbols]

[0039] 1 モータ 2 Sintered oil bearing 11 Inner circumference area 12 Peripheral area s1 bearing surface s2 end face s3 outer surface

Claims

1. a compacting step of compressing the raw material to form a green compact; a sintering step of sintering the green compact to form a sintered body; a sealing step of sealing surface pores on the end surface of the sintered body; a sizing step of sizing the sintered body, In the pore sealing step, surface pores on the end surface of the sintered body are sealed so that the surface porosity of the end surface of the sintered body is lower than that of the bearing surface, A method for manufacturing a sintered oil-impregnated bearing, characterized in that in the sizing step, the sintered body is compressed so that the density of the inner peripheral region of the sintered body is higher than that of the outer peripheral region.

2. 2. The method for producing a sintered oil-impregnated bearing according to claim 1, wherein the surface porosity of the end face of the sintered body is set within a range of 5 to 30%.

3. The density of the inner peripheral region is higher than that of the outer peripheral region, A sintered oil-impregnated bearing characterized in that the surface porosity of the end face is lower than the surface porosity of the bearing face and is within the range of 5 to 30%.

4. A motor comprising the sintered oil-impregnated bearing according to claim 3.

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