Sintered bearing and manufacturing method for the same

Abstract

To provide a sintered bearing.SOLUTION: A sintered bearing 1 formed into a cylindrical shape includes: an annular first end surface part 15A in which a first bearing part and a second bearing part differ in shape or a first connection part connected to the first bearing part of a center void part and a second connection part connected to the second bearing part of the center void part differ in shape, and whose outer surface side extends from an edge opened outside the first bearing part outward in a radial direction; an annular second end surface part 15B which extends outward in the radial direction from an edge opened outside the second bearing part and is positioned opposite the first end surface part 15A; a first curve surface part 16A extending from an outer peripheral edge of the first end surface part 15A; a second curve surface part 16B extending from an outer peripheral edge of the second end surface part 15B; and an annular groove part 17 formed between the first curve surface part 16A and the second curve surface part 16B and extending in a circumferential direction. Moreover, the annular groove part 17 is provided on the side of the second end surface part 15B.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a sintered bearing for supporting a rotating shaft, and a method for manufacturing the same. [Background technology]

[0002] A known cylindrical sintered bearing has an inner surface that includes bearing sections formed at both ends to support a shaft, and a recessed central relief section located between these bearing sections and formed with a larger diameter than the bearing sections. Sintered bearings with such a central relief shape have been widely used in the past, and a method for manufacturing a sintered bearing can involve, for example, producing a cylindrical sintered body in which the inside of one end is formed as a reduced diameter section with a smaller diameter than the inside of the other end, and then coining the other end of this sintered body to form the inside of the other end with a smaller diameter, thereby giving the sintered body a central relief shape.

[0003] Patent Document 1 discloses a manufacturing method in which the outer peripheral surface of one end of a cylindrical sintered body is strongly squeezed and narrowed by coining to form a central relief shape.

[0004] Patent Document 2 discloses a method for manufacturing a sintered bearing with a central relief. In this manufacturing method, a cylindrical sintered body is produced in which the inside of one end is a reduced diameter portion with a smaller diameter than the inside of the other end, and the outside of the other end is an expanded diameter portion formed with a larger diameter than the outside of the one end, and the expanded diameter portion is then squeezed by coining to form a smaller diameter on the inside. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 200151245 Figure 1 [Patent Document 2] Japanese Patent Application Laid-Open No. 9-68225 Figure 5 Summary of the Invention [Problem to be solved by the invention]

[0006] However, when the sintered bearing with a narrow end in Patent Document 1 is press-fitted into a cylindrical housing, the outer peripheral surface of the narrow part of the sintered bearing does not come into close contact with the housing, resulting in a short press-fit length into the housing. This type of non-press-fitted part not only makes it difficult to control the inner diameter, but also has other disadvantages, such as the possibility of lubricating oil generated by shaft rotation leaking onto the outer peripheral surface of the narrow part.

[0007] The sintered bearing of Patent Document 2 has an outer circumferential surface with a constant diameter due to coining, so it does not suffer from the problem of insufficient press-fit length that occurs with the sintered bearing of Patent Document 1. However, there are cases where the dimension along the central axis of one bearing portion differs from the dimension along the central axis of the other bearing portion. Furthermore, not only are there differences in the dimensions of one bearing portion and the other, but there are also cases where the shape of the portion of the center flank where it connects to one bearing portion differs from the shape of the portion where it connects to the other bearing portion. This type of directionality in sintered bearings often raises concerns about differences in the characteristics of sintered bearings.

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a sintered bearing and a method for manufacturing the same. [Means for solving the problem]

[0009] The present invention is a cylindrically formed sintered bearing, the inner surface of which comprises an annular first bearing portion that supports a rotating shaft, an annular second bearing portion that supports the shaft, and an annular central relief portion that is formed between the first bearing portion and the second bearing portion and is recessed radially outward from the first bearing portion and extends circumferentially, wherein the first bearing portion and the second bearing portion have different shapes, or a first connection portion of the central relief portion that connects to the first bearing portion and a second connection portion of the central relief portion that connects to the second bearing portion have different shapes. The outer surface side of the sintered bearing comprises: an annular first end surface portion extending radially outward from the outwardly opening edge of the first bearing portion; an annular second end surface portion extending radially outward from the outwardly opening edge of the second bearing portion and positioned opposite the first end surface portion; a first curved surface portion extending from the outer peripheral edge of the first end surface portion; a second curved surface portion extending from the outer peripheral edge of the second end surface portion; and an annular groove portion formed between the first curved surface portion and the second curved surface portion and extending in the circumferential direction, and further the annular groove portion is provided on the first end surface portion side or the second end surface portion side.

[0010] In the sintered bearing, the annular groove is provided on the first end surface side or the second end surface side, and this annular groove can be used as a mark for identifying the directionality from the appearance.

[0011] The sintered bearing of the present invention may include vertical grooves instead of annular grooves. In this case, the outer surface side of the sintered bearing includes: an annular first end surface portion extending radially outward from the outwardly opening edge of the first bearing portion; an annular second end surface portion extending radially outward from the outwardly opening edge of the second bearing portion and positioned opposite the first end surface portion; a plurality of vertical grooves extending parallel to the central axis from the outer peripheral edge of one of the first end surface portion and the second end surface portion and spaced apart from one another around the central axis; a first curved surface portion extending from the outer peripheral edge of the other of the first end surface portion and the second end surface portion to the plurality of vertical grooves; and a plurality of second curved surface portions extending between the vertical grooves from one of the vertical grooves to the other of the vertical grooves and continuing to the first curved surface portion to the other outer peripheral edge, wherein the angle of the vertical grooves about the central axis is smaller than the angle of the second curved surface portions about the central axis.

[0012] In the sintered bearing of the present invention, instead of the annular groove portion, the first end surface portion and the second end surface portion may have different shapes. In this case, the outer surface side of the sintered bearing includes an annular first end surface portion extending radially outward from the outwardly opening edge of the first bearing portion, an annular second end surface portion extending radially outward from the outwardly opening edge of the second bearing portion and positioned opposite the first end surface portion, and a curved surface portion extending from the outer peripheral edge of the first end surface portion to the outer peripheral edge of the second end surface portion.

[0013] The method for producing a sintered bearing of the present invention includes a recompressing step in which a cylindrical sintered body is recompressed in a recompressing die, the sintered body having an inner reduced diameter portion formed adjacent to one annular sintered end face portion and having a smaller inner diameter, and an outer expanded diameter portion formed adjacent to the other annular sintered end face portion opposite the one sintered end face portion. The recompressing step is characterized in that the sintered body is placed in a large recompression hole formed in a through-hole formed in a recompression die so that the sintered expanded diameter portion is lower than the sintered reduced diameter portion, and then the sintered body is forced into the small recompression hole of the through-hole portion with a cylindrical upper recompression punch, and a recompression core rod inserted in the upper recompression punch is forced into the sintered body and the cylindrical lower recompression punch.

[0014] The sintered reduced diameter portion of the sintered body is compressed radially outward by sizing using the recompression core rod, forming one of the first bearing portion and the second bearing portion that supports the shaft, and the area inside the sintered expanded diameter portion is formed into the other of the first bearing portion and the second bearing portion that supports the shaft in close contact with the recompression core rod by coining the sintered expanded diameter portion using the recompression die, and the one sintered end surface portion is formed into an annular first end surface portion by the recompression upper punch and the recompression lower punch, and the other sintered end surface portion is formed into an annular second end surface portion located opposite the first end surface portion, and further, a first curved surface portion extending from the first end surface portion, a second curved surface portion extending from the second end surface portion, and an annular groove portion extending in the circumferential direction and provided between the first curved surface portion and the second curved surface portion are formed by sizing a first sintered outer peripheral portion that is adjacent to the sintered expanded diameter portion and extends from the one sintered end surface portion, and the coining of the sintered expanded diameter portion. In this way, an annular groove for visually identifying the directionality can be provided between the first curved surface and the second curved surface by coining the sintered enlarged diameter portion. Furthermore, if the sintered body has a groove-like portion extending parallel to the central axis in the sintered enlarged diameter portion, a vertical groove can be formed by coining the sintered enlarged diameter portion.

[0015] In the method for producing a sintered bearing of the present invention, the sintered body has, in a cross section passing through the central axis, a radially outward step of the sintered expansion portion of 30 μm or more, and at the point where the sintered expansion portion connects to a first sintered expansion portion extending from the outer periphery of the one sintered end face portion, an angle formed by the sintered expansion portion and an extension of the first sintered expansion portion is 30 degrees or more and 90 degrees or less, with the step being preferably 35 μm or more. Alternatively, the sintered body has, in a cross section passing through the central axis, a radially outward step of the sintered expansion portion of 55 μm or more, and at the point where the sintered expansion portion connects to the first sintered expansion portion, an angle formed by the sintered expansion portion and an extension of the first sintered expansion portion is 10 degrees or more and 90 degrees or less, with the step being preferably 85 μm or more.

[0016] By setting the step and angle of the sintered expanded diameter portion of the sintered body, it is possible to provide the sintered bearing with a visible annular groove portion.

[0017] The method for producing a sintered bearing of the present invention includes the following steps, prior to the re-compressing step: a green compact forming step in which, using a molding die, a raw material powder is compressed to form a cylindrical green compact having an inner reduced diameter portion formed adjacent to one annular end face and an outer expanded diameter portion formed adjacent to the other annular end face opposite the one end face; and a sintering step in which the green compact is sintered to form the sintered body. The molding die has a stepped outer surface portion for forming the inner periphery of the green compact and a stepped inner surface portion disposed around the stepped outer surface portion for forming the outer periphery of the green compact, the stepped outer surface portion having a large outer diameter portion for forming a green compact, a small outer diameter portion for forming a green compact whose outer diameter is smaller than the large outer diameter portion, and a first forming step formed between the large outer diameter portion and the small outer diameter portion. The stepped inner surface portion has a large forming hole portion and a larger diameter portion than the large forming hole portion. The molded article is provided with a molding small hole portion formed with a small inner diameter, and a molding second step portion formed between the molding large hole portion and the molding small hole portion, and the stepped inner surface portion has a step difference in the radial outward direction between the molding large hole portion and the molding small hole portion of 30 μm or more in a cross section passing through the central axis, and an angle formed by the molding second step portion and an extension line of the molding small hole portion at a location where the molding second step portion connects to the molding small hole is 30 degrees or more and 90 degrees or less. or the stepped inner surface portion has a step between the large forming hole portion and the small forming hole portion in a radially outward direction of 55 μm or more in a cross section passing through the central axis, and an angle formed by the second forming step portion and an extension line of the small forming hole portion at a point where the second forming step portion is connected to the small forming hole portion is 10 degrees or more and 90 degrees or less, and the large forming outer diameter portion, the first forming step portion, and the small forming outer diameter portion are arranged in this order upward or downward, and The large forming hole portion, the second forming stage portion, and the small forming hole portion are arranged in this order, and the powder compact molding process involves moving at least one of the stepped outer surface portion and the stepped inner surface portion downward until the small forming hole portion is positioned radially outward from the first forming stage portion, and compressing the raw material powder with the stepped outer surface portion and the stepped inner surface portion, thereby molding the reduced diameter portion inside the powder compact and molding the expanded diameter portion outside the powder compact.

[0018] For example, the large outer diameter portion for forming, the first stage for forming, and the small outer diameter portion for forming are arranged in this order from top to bottom, and the large hole portion for forming, the second stage for forming, and the small hole portion are arranged in this order, and the powder compact forming process includes the steps of: introducing the raw material powder into a space defined by a cylindrical lower outer punch, a cylindrical lower inner punch inserted into the lower outer punch and having a punch surface located below an upper end of the lower outer punch, and a forming core rod inserted into the lower inner punch and having the stepped outer surface portion protruding above the punch surface of the lower inner punch; and lowering a cylindrical upper outer punch whose lower end abuts on the upper end of the lower outer punch, and an upper inner punch inserted into the upper outer punch and having a punch surface held at a position higher than the stepped inner surface of the upper outer punch, to compress the raw material powder between the stepped outer surface portion and the stepped inner surface, and between the punch surfaces of the lower inner punch and the upper inner punch.

[0019] The powder compacting step makes it possible to provide an undercut shape to the reduced diameter portion, thereby reducing processing costs.

[0020] The method for producing a sintered bearing of the present invention may use a cylindrical sintered body without a sintered reduced diameter portion or a sintered expanded diameter portion, instead of a cylindrical sintered body having an inner sintered reduced diameter portion formed adjacent to one annular sintered end face portion and an outer sintered expanded diameter portion formed adjacent to the other annular sintered end face portion opposite the one sintered end face portion. In this case, the method for producing a sintered bearing of the present invention includes a first compression step of re-compressing the cylindrical sintered body in a first re-compressing die to form a cylindrical re-compressed body having an inner sintered reduced diameter portion formed adjacent to one annular sintered end face portion and an outer sintered expanded diameter portion formed adjacent to the other annular sintered end face portion opposite the one sintered end face portion, and a second re-compressing step of re-compressing the re-compressed body in a second re-compressing die. The first recompression mold has a stepped outer surface portion for recompressing the inner periphery of the sintered body, and a stepped inner surface portion disposed around the stepped outer surface portion for molding the outer periphery of the sintered body. The first re-compressing step is performed by inserting one annular end surface of the sintered body into the first large hole portion in a state where a cylindrical first lower punch is placed in the first small hole portion of a first die having a stepped inner surface portion formed therein and a first large hole portion having a larger inner diameter, and then inserting a cylindrical first upper punch into the first large hole portion with a punch surface at its lower end into the first large hole portion, and inserting a re-compression large outer diameter portion into the first upper punch and a re-compression small outer diameter portion having a larger outer diameter into the first upper punch with a punch surface at its lower end into the first small hole portion. A first core rod, with its stepped outer surface portion protruding downward below the first upper punch, is lowered to push one end surface side of the sintered body into the first small hole portion and the other annular end surface side of the sintered body into the first large hole portion, and the first core rod is inserted into the sintered body and the first lower punch to move the first recompression step portion of the stepped outer surface portion to a position lower than the second recompression step portion of the stepped inner surface portion, thereby compressing the sintered body between the stepped inner surface portion and the stepped outer surface portion. In the first re-compression step, the stepped inner surface portion has a radially outward step between the first large hole portion and the first small hole portion of 30 μm or more in a cross section passing through the central axis, and the angle between the second re-compression step portion and an extension of the first small hole portion at the point where the second re-compression step portion connects to the first small hole portion is 30 degrees or more and 90 degrees or less, or the stepped inner surface portion has a radially outward step between the first large hole portion and the first small hole portion of 55 μm or more in a cross section passing through the central axis, and the angle between the second re-compression step portion and an extension of the first small hole portion at the point where the second re-compression step portion connects to the first small hole portion is 10 degrees or more and 90 degrees or less. This forms the sintered reduced diameter portion on the inside of the sintered compact and the sintered expanded diameter portion on the outside of the sintered compact. In the second re-compression process, the re-pressed body is placed in a second large hole portion of the through-hole formed in the second die so that the sintered expanded diameter portion is positioned lower than the sintered reduced diameter portion, and then the re-pressed body is forced into a second small hole portion of the through-hole portion with a cylindrical second upper punch, and a second core rod inserted in the second upper punch is forced into the re-pressed body and a cylindrical second lower punch. As a result, the sintered reduced diameter portion of the re-pressed body is compressed radially outward by the second core rod and formed into one of the first bearing portion and the second bearing portion that supports the shaft, and the area inside the sintered expanded diameter portion is formed into the other of the first bearing portion and the second bearing portion that supports the shaft in close contact with the second core rod by coining with the second die, and the one sintered end surface portion is formed into an annular first end surface portion by the second upper punch and the second lower punch, and the other sintered end surface portion is formed into a second end surface portion located opposite to the first end surface portion, and further, a first curved surface portion extending from the first end surface portion, a second curved surface portion extending from the second end surface portion, and an annular groove portion extending in the circumferential direction and provided between the first curved surface portion and the second curved surface portion are formed by sizing the first sintered outer peripheral portion that is adjacent to the sintered expanded diameter portion and extends from the one sintered end surface portion, and the coining of the sintered expanded diameter portion.

[0021] In the repressed body, in a cross section passing through the central axis, the sintered conical portion has a step of 30 μm or more in the radial outward direction, and the angle between the sintered conical portion and an extension of the first sintered outer peripheral portion at the point where the sintered conical portion connects to the first sintered outer peripheral portion is 30 degrees or more and 90 degrees or less, and the step is preferably 35 μm or more. Alternatively, in the sintered body, in a cross section passing through the central axis, the sintered conical portion has a step of 55 μm or more in the radial outward direction, and the angle between the sintered conical portion and an extension of the first sintered outer peripheral portion at the point where the sintered conical portion connects to the first sintered outer peripheral portion is 10 degrees or more and 90 degrees or less, and the step is preferably 85 μm or more.

[0022] By setting the step and angle of the sintered expanded diameter portion of the repressed body, a visible annular groove can be imparted to the sintered bearing. Furthermore, the first compression step makes it possible to impart a sintered expanded diameter portion and a sintered reduced diameter portion to the cylindrical sintered body, thereby reducing processing costs. It is possible. [Effects of the Invention]

[0023] According to the sintered bearing of the present invention, the directionality can be easily identified from the appearance. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 is a front view showing a sintered bearing according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view of the sintered bearing taken along line A1-A1 in FIG. [Figure 3] FIG. 2 is a cross-sectional view of an annular groove portion of the sintered bearing of FIG. 1. [Figure 4] FIG. 2 is a front view showing a green compact used in manufacturing the sintered bearing of FIG. [Figure 5] FIG. 5 is a cross-sectional view of the powder compact taken along the line A2-A2 in FIG. 4. [Figure 6] FIG. 6 is a cross-sectional view of a molding die used to manufacture the green compact of FIG. 5. [Figure 7] FIG. 7 is an enlarged cross-sectional view of the area circled by the dashed line in FIG. 6. [Figure 8] FIG. 7 is a cross-sectional view showing a state in which raw material powder is compressed by the molding die of FIG. 6. [Figure 9] FIG. 2 is a cross-sectional view of a recompression mold used to recompress a sintered body. [Figure 10] FIG. 10 is a cross-sectional view of a state in which the sintered body is compressed by the recompression die of FIG. 9. [Figure 11] FIG. 2 is a view showing a sintered bearing according to a first modified example of the first embodiment of the present invention. [Figure 12] FIG. 10 is a view showing a sintered bearing according to a second modified example of the first embodiment of the present invention. [Figure 13] FIG. 4 is a front view showing a sintered bearing according to a second embodiment of the present invention. [Figure 14] FIG. 14 is a cross-sectional view of the sintered bearing taken along line A3-A3 in FIG. [Figure 15] FIG. 14 is a cross-sectional view of the sintered bearing taken along line A4-A4 in FIG. [Figure 16]FIG. 14 is a front view showing a sintered body used in manufacturing the sintered bearing of FIG. [Figure 17] FIG. 4 is a view showing a sintered bearing according to a third embodiment of the present invention. [Figure 18] FIG. 18 is a view showing a second end surface portion of the sintered bearing of FIG. [Figure 19] FIG. 10 is a cross-sectional view showing another example of the configuration of a molding die used to manufacture a powder compact. [Figure 20] FIG. 20 is a cross-sectional view showing a state in which raw material powder is compressed by the molding die of FIG. 19. [Figure 21] FIG. 20 is an enlarged cross-sectional view of the area circled by the dashed line in FIG. 19. [Figure 22] 3 is a cross-sectional view showing another example of the configuration of a sintered body used in the production of the sintered bearing of FIG. 1 and another example of the configuration of a re-compression mold. [Figure 23] FIG. 23 is a cross-sectional view of a state in which a sintered body is compressed by the recompression die of FIG. 22. DETAILED DESCRIPTION OF THE INVENTION

[0025] As shown in FIGS. 1 and 2, a sintered bearing 1 according to a first embodiment of the present invention is formed in a cylindrical shape.

[0026] The inner surface of the sintered bearing 1 is provided with an annular first bearing portion 11 that supports a rotating shaft 90, an annular second bearing portion 12 that supports the shaft 90, and an annular center relief portion 13 that is formed between the first bearing portion 11 and the second bearing portion 12 and that extends circumferentially and has an inner side recessed radially outward from the first bearing portion 11 and the second bearing portion 12. In the drawings, the central axis is given the symbol C, and the radially outward direction is the direction extending outward at a right angle from the central axis C.

[0027] The central relief portion 13 is made up of an annular first connecting portion 13A that is connected to the first bearing portion 11 and extends in the circumferential direction, an annular second connecting portion 13B that is connected to the second bearing portion 12 and extends in the circumferential direction, and an annular bottom portion 13C that is formed between the first connecting portion 13A and the second connecting portion 13B and extends in the circumferential direction. For example, the bottom portion 13C is formed to have a constant inner diameter, and the inner diameter of the first connecting portion 13A gradually increases from the first bearing portion 11 to the bottom portion 13C, and the inner diameter of the second connecting portion 13B also gradually increases from the second bearing portion 12 to the bottom portion 13C.

[0028] The first bearing portion 11 and the second bearing portion 12 have different shapes. Specifically, the dimension L1 of the first bearing portion 11 in the direction in which the central axis C of the sintered bearing 1 extends is different from the dimension L2 of the second bearing portion 12, and in the illustrated example, the dimension L1 of the first bearing portion 11 is set larger than that of the second bearing portion 12 (L1>L2).

[0029] Furthermore, the outer surface side of the sintered bearing 1 is provided with an annular first end surface portion 15A extending radially outward from the edge that opens outward of the first bearing portion 11, an annular second end surface portion 15B extending radially outward from the edge that opens outward of the second bearing portion 12 and positioned opposite the first end surface portion 15A, a first curved surface portion 16A extending from the outer peripheral edge of the first end surface portion 15A, a second curved surface portion 16B extending from the outer peripheral edge of the second end surface portion 15B, and an annular groove portion 17 formed between the first curved surface portion 16A and the second curved surface portion 16B and extending circumferentially.

[0030] First end surface portion 15A and second end surface portion 15B are formed to have the same shape. In first end surface portion 15A and second end surface portion 15B, the outer peripheral edge and inner peripheral edge may be chamfered to have an inclined cross section, for example. In addition, the outer diameter of first curved surface portion 16A and the outer diameter of second curved surface portion 16B are set to be equal.

[0031] 3 is an enlarged cross-section of annular groove portion 17 taken along line A1-A1 in FIG. 1, with the depth H of annular groove portion 17 being 3 μm or greater, and preferably 5 μm or greater. If the depth H of annular groove portion 17 is less than 3 μm, annular groove portion 17 cannot be seen from the outside. The width W1 of annular groove portion 17 is 30 μm or greater, and preferably 50 μm or greater. Annular groove portion 17 includes a first slope portion 17A extending from first curved surface portion 16A toward deepest portion 17C, and a second slope portion 17B extending from second curved surface portion 16B toward deepest portion 17C, with first slope portion 17A having a gentler slope than second slope portion 17B. Specifically, the width W11 of the first inclined surface portion 17A is set larger than the width W12 of the second inclined surface portion 17B, and if the ratio of the depth H of the annular groove portion 17 divided by the width W11 is defined as the gradient a of the first inclined surface portion 17A, and the ratio of the depth H of the annular groove portion 17 divided by the width W12 is defined as the gradient b of the second inclined surface portion 17B, then the relationship of gradient a<gradient b holds. If the width W1 is less than 30 μm, the annular groove portion 17 cannot be seen from the outside.

[0032] Next, a method for manufacturing the sintered bearing 1 will be described. The manufacturing method of the sintered bearing 1 includes a green compact molding process in which raw material powder is compressed in a molding die to form the green compact 100, a sintering process in which the raw material powder of the green compact 100 is sintered to form the sintered body 100S, and a recompression process in which the sintered body 100S is recompressed.

[0033] (Powder compact forming process) In the powder compact molding process, a cylindrical powder compact 100 shown in FIGS. 4 and 5 is formed. The inner surface of the powder compact 100 includes a first inner circumferential portion 111 and a reduced diameter portion 112 having an inner diameter smaller than that of the first inner circumferential portion 111. The reduced diameter portion 112 includes a second inner circumferential portion 112A formed away from the first inner circumferential portion 111 and an annular third inner circumferential portion 112B formed between the first inner circumferential portion 111 and the second inner circumferential portion 112A. In the illustrated example, the third inner circumferential portion 112B is formed so that its diameter gradually decreases from the first inner circumferential portion 111 to the second inner circumferential portion 112A. The shape of the third inner circumferential portion 112B is not limited thereto, and it may be formed, for example, as a surface perpendicular to the central axis C.

[0034] Furthermore, the outer surface side of the powder compact 100 is provided with one annular end face portion 151 extending radially outward from the edge that opens outward of the second inner circumferential portion 112A (reduced diameter portion 112), another annular end face portion 152 extending radially outward from the edge that opens outward of the first inner circumferential portion 111 and positioned opposite the one end face portion 151, a first outer circumferential portion 161 extending from the outer peripheral edge of the one end face portion 151, and an expanded diameter portion 162 formed with an outer diameter larger than that of the first outer circumferential portion 161.

[0035] The expanded diameter portion 162 includes a second outer circumferential portion 162A formed apart from the first outer circumferential portion 161, and an annular third outer circumferential portion 162B formed between the first outer circumferential portion 161 and the second outer circumferential portion 162A. The second outer circumferential portion 162A is formed adjacent to the other end surface portion 152.

[0036] As shown in Figure 5, in a longitudinal section of the powder compact 100 passing through the central axis C, at the point where the expanded diameter portion 162 and the first outer peripheral portion 161 are connected, the angle θ1 formed by the extension line Ex1 of the first outer peripheral portion 161, shown by the dotted line, and the third outer peripheral portion 162B is, for example, 10 degrees or more and 90 degrees or less, and preferably 10 degrees or more and 45 degrees or less, makes it easier to form the point where the expanded diameter portion 162 and the first outer peripheral portion 161 are connected compared to when the angle exceeds 45 degrees.

[0037] In the illustrated example, the third outer peripheral portion 162B is formed as a surface whose diameter gradually increases from the end of the first outer peripheral portion 161 to the end of the second outer peripheral portion 162A, but the shape of the third outer peripheral portion 162B is not limited to this and may be formed as a surface perpendicular to the central axis C, for example, with an angle θ1 of 90 degrees.

[0038] In addition, in the longitudinal cross section of the powder compact 100 passing through the central axis C shown in Figure 5, the step Δr1 along the radially outward direction between the first outer peripheral portion 161 and the second outer peripheral portion 162A and the angle θ1 have the following relationship (a) or (b). (a) When the step difference Δr1 is 30 μm or more, the angle θ1 of the connecting portion is set to 30 degrees or more. (b) When the step difference Δr1 is 55 μm or more, the angle θ1 of the connection portion is 10 degrees or more. In addition, in (A), it is more preferable that the step difference Δr1 is 35 μm or more, and in (B), it is more preferable that the step difference Δr1 is 85 μm or more. Note that these relationships also apply to the sintered body 100S produced through the sintering process.

[0039] In this way, by setting the step difference Δr1 to 35 μm or more and the angle θ1 of the connection portion to 30 degrees or more, or by setting the step difference Δr1 to 85 μm or more and the angle θ1 of the connection portion to 10 degrees or more, it is possible to form a clear annular groove portion 17 all around the central axis C in the re-compressed body (sintered bearing 1) produced by subjecting the sintered body to a re-compression process.

[0040] The metal used to make the green compact 100 is not particularly limited, but copper-based powder or iron-copper-based powder is suitable as the raw powder. The copper-based powder is copper powder whose main component is copper or a copper alloy such as copper-tin, copper-phosphorus, or copper-zinc. It may contain 0.1% to 5% by mass of a low-melting-point metal powder (e.g., tin powder) whose melting point is below the sintering temperature, or 0.5% to 5% by mass of a solid lubricant such as graphite. The iron-copper-based powder is 16% to 80% by mass of the copper powder, with the remainder being iron powder, and may contain 0.1% to 5% by mass of the low-melting-point metal powder and 0.5% to 5% by mass of the solid lubricant.

[0041] (Powder compact forming process) The molding die 300 used in the powder compact molding step is not limited, but for example, the molding die 300 shown in Fig. 6 can be used. This molding die 300 has a multi-stage punch configuration with two upper stages and two lower stages, and specifically, the molding die 300 includes a molding die 310, a cylindrical lower outer punch 320 inserted into a through-hole of the molding die 310, a cylindrical lower inner punch 330 inserted into the lower outer punch 320, a molding core rod 340 inserted into the lower inner punch 330, a cylindrical upper outer punch 350 inserted into the through-hole of the molding die 310 with its lower end abutting against the upper end (punch surface 320A) of the lower outer punch 320, and a cylindrical upper inner punch 360 inserted into the upper outer punch 350.

[0042] The molding core rod 340 includes a molding large outer diameter portion 341 and a molding small outer diameter portion 343 that is formed with a diameter smaller than the diameter of the molding large outer diameter portion 341 and is connected to the molding large outer diameter portion 341 via a molding first stage portion 342, and these form a molding stepped outer surface portion 344 that molds the inner periphery of the powder compact 100. The molding large outer diameter portion 341, the molding first stage portion 342, and the molding small outer diameter portion 343 are arranged in this order from top to bottom.

[0043] The upper outer punch 350 has a large forming hole 351 and a small forming hole 353 on its inside, the small forming hole 353 having an inner diameter smaller than that of the large forming hole 351 and connected to the large forming hole 351 via a second forming step 352, which together form a stepped inner surface 354 for forming the outer periphery of the powder compact 100. The large forming hole 351, the second forming step 352, and the small forming hole 353 are arranged in this order from top to bottom. A punch surface 360A of the upper inner punch 360 is held at a position higher than the stepped inner surface 354.

[0044] 7, in a vertical cross section of stepped inner surface portion 354 passing through mold center axis C, molding second step portion 352 has a shape in which the diameter gradually decreases from the end on the molding large hole 351 side to the end on the molding small hole 353 side, and the inclination angle θ2 of molding second step portion 352 with respect to extension line Ex2 of molding small hole 353 shown by the dashed dotted line is, for example, 10 degrees or more and 90 degrees or less, and preferably 10 degrees or more and 45 degrees or less. Note that when angle θ2 is 90 degrees, molding second step portion 352 is formed as a surface perpendicular to the central axis C of molding die 300.

[0045] 7, the stepped inner surface 354 of the upper outer punch 350 has a step Δr2 along the radially outward direction between the small forming hole 353 and the large forming hole 351, and an angle θ2, which correspond to the step Δr1 and angle θ1 of the powder compact 100, and the relationship of (A) or (B) above also applies to the step Δr2 and the angle θ2. In this case, Δr1 in (A) or (B) above should be read as Δr2, and θ1 should be read as θ2.

[0046] In the powder compact molding process, first, as shown in Figure 6, the punch surface 320A of the lower outer punch 320 is arranged at the same height as the die surface 310A, and the stepped outer surface portion 344 is made to protrude upward from the punch surface 330A of the lower inner punch 330. In this state, the lower outer punch 320, the lower inner punch 330, and the molding core rod 340 form a space S for the raw material powder, and the raw material powder is filled into the space S.

[0047] 8, the upper outer punch 350 and the upper inner punch 360 are lowered to compress the raw material powder. At this time, the molding second step portion 352 of the stepped inner surface portion 354 of the upper outer punch 350 is positioned lower than the molding first step portion 342 of the stepped outer surface portion 344 of the molding core rod 340.

[0048] In this manner, the lower outer punch 320 is pressed down sufficiently until the forming small hole 353 of the upper outer punch 350 is positioned radially outward from the first forming step 342 of the forming core rod 340, and the raw material powder is compressed by the stepped inner surface 354 of the upper outer punch 350 and the stepped outer surface 344 of the forming core rod 340, and is also compressed by the upper outer punch 350 and the lower inner punch 330. By this compression, the powder compact 100 is formed into a cylindrical shape having an inner reduced diameter portion 112 formed with a small inner diameter adjacent to one end face portion 151, and an outer expanded diameter portion 162 formed with a large outer diameter adjacent to the other end face portion 152 opposite the one end face portion 151.

[0049] (Sintering process) In the sintering step, the powder compact 100 is sintered at a temperature of, for example, 800° C. to 950° C. As a result, a sintered body 100S is formed. The shape of the sintered body 100S is almost the same as that of the powder compact 100, and in the following description, the components of the sintered body 100S that correspond to the components of the powder compact 100 will have the word "sintered" added to their names and the letter "S" added to their reference symbols, and descriptions of their shapes, etc. will be omitted. After the sintering process, the first inner peripheral portion 111 is formed into the first sintered inner peripheral portion 111S, the reduced diameter portion 112 (second inner peripheral portion 112A, third inner peripheral portion 112B) is formed into the sintered reduced diameter portion 112S (second sintered inner peripheral portion 112AS, third sintered inner peripheral portion 112BS), one end face portion 151 is formed into one annular sintered end face portion 151S, the other end face portion 152 is formed into the other annular sintered end face portion 152S, the first outer peripheral portion 161 is formed into the first sintered outer peripheral portion 161S extending from the outer peripheral edge of one sintered end face portion 151S, and the expanded diameter portion 162 (second outer peripheral portion 162A, third outer peripheral portion 162B) is formed into the sintered expanded diameter portion 162S (second sintered outer peripheral portion 162AS, third sintered outer peripheral portion 162BS).

[0050] (Recompression process) In the re-compressing step, the cylindrical sintered body 100S is re-compressed, the sintered expanded diameter portion 162S is coined, and the sintered reduced diameter portion 112S and the first sintered outer peripheral portion 161S are sized.

[0051] As shown in Figure 9, the recompression mold 400 includes a recompression die 410, a cylindrical recompression lower punch 420 that is inserted into the through-hole 411 of the recompression die 410, a cylindrical recompression upper punch 430 that is formed in a cylindrical shape and inserted into the through-hole of the recompression die 410, and a recompression core rod 440 that is inserted into the recompression lower punch 420 and the recompression upper punch 430.

[0052] The through portion 411 of the recompression die 410 has a large recompression hole portion 411A into which the sintered body 100S fits, a small recompression hole portion 411B having a diameter smaller than the first sintered outer peripheral portion 161S of the sintered body 100S, and a recompression step portion 411C provided between the large recompression hole portion 411A and the small recompression hole portion 411B, the diameter of which gradually decreases from the end on the large recompression hole portion 411A side to the end on the small recompression hole portion 411B side.

[0053] In the recompression process, as shown in Figure 9, first, the lower punch surface 421 of the recompression lower punch 420 is placed in the recompression small hole portion 411B of the recompression die 410, and the sintered body 100S is then placed in the recompression large hole portion 411A of the recompression die 410 so that the sintered expanded diameter portion 162S is positioned lower than the sintered reduced diameter portion 112S.

[0054] 10 , in the recompression step, an upper recompression punch 430 and a recompression core rod 440 are inserted into the through-hole 411 of the recompression die 410. At this time, the sintered compact 100S is forced from the large recompression hole 411A to the small recompression hole 411B, and the recompression core rod 440 is forced into the sintered compact 100S and the lower recompression punch 420. As a result, the sintered compact 100S is compressed by the inner peripheral surface of the small recompression hole 411B of the recompression die 410 and the outer peripheral surface of the recompression core rod 440, and is also compressed by the upper recompression punch 430 and the lower recompression punch 420.

[0055] As a result, the recompression core rod 440 compresses the second sintered inner portion 112AS (sintered reduced diameter portion 112S) of the sintered body 100S radially outward to form a first bearing portion 11 that supports the shaft 90, and by coining using the recompression die 410, the area inside the sintered expanded diameter portion 162S is tightly attached to the recompression core rod 440 to form a second bearing portion 12 that supports the shaft 90.

[0056] In the re-compression process, the upper punch surface 431 compresses the upper sintered end surface portion 151S of the sintered body 100S to form it into an annular first end surface portion 15A, and the lower punch surface 421 compresses the lower sintered end surface portion 152S of the sintered body 100S to form it into an annular second end surface portion 15B.

[0057] Furthermore, by sizing the first sintered outer peripheral portion 161S and coining the sintered expansion diameter portion 162S, the outer peripheral surface of the sintered compact 100S is formed into a first curved surface portion 16A extending from the outer peripheral edge of the first end surface portion 15A and formed with a constant diameter from the central axis C, a second curved surface portion 16B extending from the outer peripheral edge of the second end surface portion 15B and formed with a diameter equal to that of the first curved surface portion 16A, and an annular groove portion 17 provided between the first curved surface portion 16A and the second curved surface portion 16B and extending in the circumferential direction. Note that the annular groove portion 17 is formed when the first sintered outer peripheral portion 161S is pulled inward by a force when the second sintered outer peripheral portion 162AS and third sintered outer peripheral portion 162BS sides of the sintered expansion diameter portion 162S are reduced in diameter by coining in the re-compression process.

[0058] By re-compressing the sintered body 100S in this manner, a central relief portion 13 recessed radially outward is formed between the first bearing portion 11 and the second bearing portion 12, completing the sintered bearing 1. This porous sintered bearing 1 can be used as an oil-impregnated sintered bearing by impregnating it with lubricating oil, for example.

[0059] The sintered bearing 1 of the first embodiment of the present invention has directionality due to the difference between the dimension L1 of the first bearing portion 11 and the dimension L2 of the second bearing portion 12. The directionality of this sintered bearing 1 can be easily identified by the annular groove portion 17 formed on the second end face portion 15B side.

[0060] On the other hand, in conventional cylindrically formed sintered bearings, the annular groove portion 17 is not provided on one end face side or the other end face side of the outer peripheral surface, so it is not possible to identify the directionality of the sintered bearing.

[0061] According to the method for producing the sintered bearing 1 of the first embodiment, by re-compressing the sintered body 100S, it is possible to form the first bearing portion 11, the second bearing portion 12, the first curved surface portion 16A, the second curved surface portion 16B, and also to form the annular groove portion 17, thereby reducing the production costs of the sintered bearing 1. In particular, by using a sintered body 100S in which the step is 30 μm or more and the angle of the connecting portion is 30 degrees, or by using a sintered body 100S in which the step is 55 μm or more and the angle of the connecting portion is 10 degrees or more, the annular groove portion 17 can be clearly formed.

[0062] On the other hand, if additional processing is performed to form grooves on the outer surface of the recompression body in addition to the process of recompressing the sintered body to form a recompression body having a first bearing surface and a second bearing surface, manufacturing becomes complicated and costs increase.

[0063] According to the powder compacting step in the method for producing the sintered bearing 1 of the first embodiment, it is possible to impart an undercut shape to the enlarged diameter portion, thereby reducing processing costs.

[0064] (Modification of the first embodiment) The directionality of the sintered bearing is not limited to the difference between the dimension L1 of the first bearing portion in the direction in which the central axis C of the first bearing portion extends and the dimension L2 of the second bearing portion in the direction in which the central axis C of the second bearing portion extends, but may also be due to other differences in shape between the first bearing portion and the second bearing portion, or differences in shape between the first connecting portion 13A connecting to the first bearing portion 11 of the central relief portion 13 and the second connecting portion 13B connecting to the second bearing portion 12 of the central relief portion 13.

[0065] For example, in the sintered bearing 2 shown in FIG. 11 , the inner diameter φ5 of the first bearing portion 11 and the inner diameter φ6 of the second bearing portion 12 differ by, for example, several tens of μm, giving the sintered bearing 2 directionality. In this case, the sintered bearing 2 also has an annular groove 17 on the second end face 15B side, allowing the user to identify the directionality of the sintered bearing 2. This allows the user to identify that the first bearing portion 11 is located on the first end face 15A side, and also that the second bearing portion 12 is located on the second end face 15B side. Note that in FIG. 11 , the left side of the vertical center line represents a front view of the sintered bearing 2, and the right side represents a cross section. In FIG. 11 , the same components as those in the first embodiment are designated by the same reference numerals, and their description will be omitted.

[0066] In the sintered bearing 3 shown in FIG. 12 , the dimension L3 of the first bearing portion 11 in the direction of its central axis C is the same as the dimension L4 of the second bearing portion 12 in the direction of its central axis C. However, in the central relief portion 13, the dimension L5 of the first connecting portion 13A connected to the first bearing portion 11 in the direction of its central axis C is different from the dimension L6 of the second connecting portion 13B connected to the second bearing portion 12 in the direction of its central axis C. This gives the sintered bearing 3 directional characteristics. Again, the annular groove 17 on the second end face 15B side of the sintered bearing 3 allows the user to identify the directionality of the sintered bearing 3. This allows the user to identify that the first connecting portion 13A is located on the first end face 15A side and that the second connecting portion 13B is located on the second end face 15B side. Note that in FIG. 12 , the left side of the vertical center line represents a front view of the sintered bearing 3, and the right side represents a cross section. In FIG. 12 , the same components as those in the first embodiment are designated by the same reference numerals, and their description will be omitted.

[0067] Furthermore, the first connecting portion 13A and the second connecting portion 13B are not limited to differences in the dimensions L5, L6 in the direction in which the central axis C extends. Although not shown in the figures, for example, the first connecting portion 13A may be an annular surface that extends radially outward, while the second connecting portion 13B may be formed as an inclined surface whose diameter gradually increases from the end of the bottom portion 13C to the end of the second bearing portion 12. Even when the shapes differ, the directionality of the sintered bearing can be identified by the annular groove portion 17.

[0068] Furthermore, although not shown in the figures, examples of shapes in which the first bearing portion and the second bearing portion differ include when the roughness of the first bearing surface and the roughness of the second bearing surface differ, when the number of pores exposed in the first bearing portion (surface porosity) differs from the number of pores exposed in the second bearing portion (surface porosity), etc. In these cases, too, the user can identify the directionality of the sintered bearing because the sintered bearing has annular groove portion 17 near the second end face portion.

[0069] Second Embodiment The center relief type sintered bearing 4 of the second embodiment shown in FIGS. 13 and 14 is formed in a cylindrical shape.

[0070] The inner surface of the sintered bearing 4 is formed in the same manner as the inner surface of the sintered bearing 1, and is provided with a first bearing portion 11, a second bearing portion 12, and a central relief portion 13 formed between the first bearing portion 11 and the second bearing portion 12.

[0071] The outer surface side of the sintered bearing 4 further comprises: an annular first end surface portion 15A extending radially outward from the outwardly opening edge of the first bearing portion 11; an annular second end surface portion 15C extending radially outward from the outwardly opening edge of the second bearing portion 12 and positioned opposite the first end surface portion 15A; a plurality of longitudinal grooves 18 extending parallel to the central axis C from the outer peripheral edge of the second end surface portion 15C and spaced apart around the central axis C; a first curved surface portion 46A extending from the outer peripheral edge of the first end surface portion 15A to the plurality of longitudinal grooves 18; and a plurality of second curved surface portions 46B extending between the adjacent longitudinal grooves 18 from one longitudinal groove 18 to the other longitudinal groove 18, continuing to the first curved surface portion 46A and extending to the outer peripheral edge of the second end surface portion 15C. In FIG. 13 , the boundary between the first curved surface portion 46A and the second curved surface portion 46B is indicated by a dashed line.

[0072] The dimension L1 of the first bearing portion 11 in the direction in which the central axis C of the sintered bearing 4 extends is different from the dimension L2 of the second bearing portion 12, and the sintered bearing 4 has directionality. Note that the same components as in the first embodiment are denoted by the same reference numerals and their description will be omitted.

[0073] In the sintered bearing 4, vertical grooves 18 extending parallel to the central axis C are provided instead of the annular grooves 17 of the sintered bearing 1 of the first embodiment, and as shown in the cross-sectional view of Figure 15, the angle θ3 of each vertical groove 18 about the central axis C is set to be smaller than the angle θ4 of the second curved surface portion 46B adjacent to the vertical groove 18 about the central axis C. Preferably, multiple vertical grooves 18 are formed at equal angular intervals around the central axis C, for example, and the vertical grooves 18 and the second curved surface portions 46B are formed alternately around the central axis C.

[0074] The sintered bearing 4 can be formed by re-compressing, in a re-compression mold 400, a sintered body 200S shown in Fig. 16, in which a sintered groove portion 210S, which has a larger groove width than the longitudinal groove portion 18, is formed in the sintered expanded diameter portion 162S. By sizing the first sintered outer peripheral portion 161S and coining the sintered expanded diameter portion 162S, the outer peripheral surface of the sintered body 200S is formed into the longitudinal groove portion 18, the first curved surface portion 46A, and the second curved surface portion 46B.

[0075] The cross-sectional shape of the sintered body 200S taken along line A5-A5 in Fig. 16 is substantially the same as that of the sintered body 100S, as shown in Fig. 5, but the angle θ1 and the step height Δr1 are not limited to the values ​​described above for forming the annular groove portion 17. Note that the sintered groove portion 210S is omitted from Fig. 5. Also, although not shown, the upper outer punch 350 of the die for molding the powder compact for the sintered body 200S has a convex portion protruding from its inner circumferential surface for forming a recess for the sintered groove portion.

[0076] The vertical grooves 18 formed in the sintered bearing 4 in this manner can be used to identify the directionality of the sintered bearing 4.

[0077] Furthermore, even though the sintered bearing 4 has multiple vertical groove portions 18 arranged around the central axis C, the angle θ3 formed by the vertical groove portion 18 around the central axis C is sufficiently smaller than the angle θ4 of the adjacent second curved surface portion 46B, and therefore when the sintered bearing 4 is press-fitted into a motor housing or the like, it can be made to adhere sufficiently tightly to the housing.

[0078] (Third embodiment) A center relief type sintered bearing 5 according to a third embodiment shown in FIG. 17 is formed in a cylindrical shape.

[0079] The inner surface of the sintered bearing 5 is formed in the same manner as the inner surface of the sintered bearing 1, and is provided with a first bearing portion 11, a second bearing portion 12, and a central relief portion 13 formed between the first bearing portion 11 and the second bearing portion 12.

[0080] Furthermore, the outer surface side of the sintered bearing 5 is provided with an annular first end surface portion 15A extending radially outward from the edge that opens outward of the first bearing portion 11, an annular second end surface portion 15D extending radially outward from the edge that opens outward of the second bearing portion 12 and positioned opposite the first end surface portion 15A, and a curved surface portion 55 extending at a constant outer diameter from the outer peripheral edge of the first end surface portion 15A to the outer peripheral edge of the second end surface portion 15D.

[0081] The dimension L1 of the first bearing portion 11 in the direction in which the central axis C of the sintered bearing 5 extends is different from the dimension L2 of the second bearing portion 12, and the sintered bearing 5 has directionality. Note that the same components as in the first embodiment are given the same reference numerals and their description will be omitted. Note that in Figure 17, the left side of the vertical center line is a front view of the sintered bearing 5, and the right side is a cross-section.

[0082] Furthermore, in the sintered bearing 5, instead of the annular groove 17 of the sintered bearing 1 of the first embodiment, the shape of the first end surface 15A and the shape of the second end surface 15D are different. For example, as shown in FIG. 18, the second end surface 15D has four linear recesses 19 recessed from the flat surface, extending radially from the center axis C. In FIG. 18, the center axis C is represented by a black circle (●). The first end surface 15A does not have any recesses 19, and the entire surface is formed flat from the inner peripheral edge to the outer peripheral edge. Note that the shape, number, and location of at least one of the recesses and protrusions are not limited to those shown in the illustration. The recesses or protrusions may represent letters, symbols, etc. Furthermore, at least one of the recesses and protrusions may have different shapes on both the first end surface 15A and the second end surface 15D.

[0083] In order to form the recess 19 in the recompression step, for example, the recompression lower punch 420 of the recompression die 400 is provided with a protruding convex portion (not shown) on the lower punch surface 421. By recompressing the sintered body with the recompression die 400, it is possible to form the second end surface portion 15D having the recess 19. Alternatively, when the green compact is molded with the molding die 300, a depression for the recess 19 may be formed in one end surface portion of the green compact.

[0084] The recess 19 formed in the second end surface portion 15D of the sintered bearing 5 in this manner can be used to identify the directionality of the sintered bearing 5.

[0085] The present invention can be practiced without being limited to the above-described and illustrated examples.

[0086] The method for manufacturing a sintered bearing is not limited to the above explanation and illustrated examples.

[0087] (Powder compact forming process) As another example of the configuration of a molding die used in the powder compact molding process, for example, the molding die 500 shown in Figure 19 comprises a molding die 510, a cylindrical molding lower punch 520 inserted into the through-hole of the molding die 510, a lower core rod 530 inserted into the lower molding punch 520, a cylindrically formed upper molding punch 540 inserted into the through-hole of the molding die 510, and an upper core rod 550 inserted into the upper molding punch 540.

[0088] The upper core rod 550 includes a large outer diameter portion for molding 551, a small outer diameter portion for molding 552 formed with a smaller outer diameter than the large outer diameter portion for molding 551, and a first step portion for molding 553 formed between the large outer diameter portion for molding 551 and the small outer diameter portion for molding 552, which together form a stepped outer surface portion 554 for molding the inner peripheral surface of the powder compact 100. The molding die 510 includes a large hole for molding 511, a small hole for molding 512 formed with a smaller inner diameter than the large hole for molding 511, and a second step portion for molding 513 formed between the large hole for molding 511 and the small hole for molding 512, which together form a stepped inner surface portion 514 for molding the outer peripheral surface of the powder compact 100. The second forming step 513 in the illustrated example has a shape in which the diameter gradually decreases from the end on the large forming hole 511 side to the end on the small forming hole 512 side. In addition, the stepped outer surface 514 protrudes downward from the punch surface 540A of the upper forming punch 540. In the forming die 500, the large forming outer diameter portion 551, the first forming step 553, and the small forming outer diameter portion 552 are arranged downward in this order, and the large forming hole 511, the second forming step 513, and the small forming hole 512 are also arranged in this order. 21, in the stepped inner surface portion 514, the inclination angle θ5 of the molding second step portion 513 with respect to the extension line Ex3 of the molding small hole portion 512, shown by the dashed line, is, for example, 10 degrees or more and 90 degrees or less, similar to the angle θ2 described above. When the angle θ5 is 90 degrees, the molding second step portion 513 is formed as a surface perpendicular to the central axis C of the molding die 500. In the molding die 510, in a vertical cross section passing through the central axis C shown in FIG. 21, the step Δr3 along the radially outward direction between the molding small hole portion 512 and the molding large hole portion 511 and the angle θ5 desirably satisfies the above-mentioned (A) or (B), similar to the relationship between the step Δr1 and the angle θ1 described above.

[0089] In the powder compact molding process, as shown in FIG. 19, the upper end surface 530A of the lower core rod 530 is arranged at the same height as the die surface 510A, and the punch surface 520A of the lower forming punch 520 is held at a position lower than the second forming stage 513 of the forming die 510, so that the forming die 510, the lower forming punch 520, and the lower core rod 530 form a space for the raw material powder, and the raw material powder is filled into the space S.

[0090] 20 , the upper forming punch 540 and the upper core rod 550 are lowered to sufficiently press the raw material powder and the lower core rod 530 downward until the forming small hole 512 is positioned radially outward from the first forming stage 553, thereby compressing the raw material powder. In this manner, the upper core rod 550 enters the through-hole of the forming die 510 until the first forming stage 553 of the stepped outer surface 554 of the upper core rod 550 is positioned lower than the second forming stage 513 of the stepped inner surface 514 of the forming die 510. This allows the raw material powder to be compressed by the stepped inner surface 514 of the forming die 510 and the stepped outer surface 554 of the upper core rod 550, as well as by the upper forming punch 540 and the lower forming punch 520, thereby forming the green compact 100. The green compact forming process makes it possible to impart an undercut shape to the reduced diameter portion, thereby reducing processing costs.

[0091] (Recompression process) In the re-compression process shown in Figures 9 and 10, the sintered body 100S having the sintered expanded diameter portion 162S is re-compressed, but it is also possible to form a center relief type sintered bearing by including a process of re-compressing the sintered body 250S shown in Figure 22, which does not have the sintered expanded diameter portion 162S, instead of the sintered body 100S.

[0092] The sintered body 250S is a straight cylindrical body, and its inner surface consists of a sintered inner peripheral portion 251S formed with a constant inner diameter, and its outer surface consists of one annular sintered end surface portion 252S extending radially outward from one edge that opens to the outside of the sintered inner peripheral portion 251S, another annular sintered end surface portion 253S located opposite the one sintered end surface portion 252S and extending radially outward from the other edge that opens to the outside of the sintered inner peripheral portion 251S, and a sintered outer peripheral portion 254S formed with a constant outer diameter from the outer peripheral edge of one sintered end surface portion 252S to the outer peripheral edge of the other sintered end surface portion 253S.

[0093] The method for manufacturing a sintered bearing using the straight sintered body 250S includes a first re-compression step of re-compressing the sintered body 250S to form a re-compressed body, and a second re-compression step of re-compressing the re-compressed body.

[0094] In the first re-compressing step, the sintered body 250S is re-compressed using a first re-compressing mold 600 shown in Fig. 22. The first re-compressing mold 600 includes a first die 610, a cylindrical first lower punch 620 inserted into the through-hole of the first die 610, a cylindrical first upper punch 630 inserted into the through-hole of the first die 610, and a first core rod 640 that penetrates the first upper punch 630 and has its tip protruding beyond the first upper punch 630. During the first compression step, the first lower punch 620 is inserted into the first small hole portion 612, and the first upper punch 630 is inserted into the first large hole portion 611 of the first die 610.

[0095] 22, the first core rod 640 includes a large outer diameter portion 641 for recompression, a small outer diameter portion 642 for recompression having a smaller outer diameter than the large outer diameter portion 641 for recompression, and a first step portion 643 for recompression formed between the large outer diameter portion 641 for recompression and the small outer diameter portion 642 for recompression, which together form a stepped outer surface portion 644 for recompression on the inner periphery of the sintered compact 250S. In the illustrated example, the first step portion 643 for recompression has a shape in which the diameter gradually decreases from the end on the large outer diameter portion 641 side to the end on the small outer diameter portion 642 side for recompression. The stepped outer surface portion 644 also protrudes downward from the punch surface 630A of the first upper punch 630. 22, the first die 610 includes a first large hole portion 611 having a large inner diameter, a first small hole portion 612 having a smaller inner diameter than the first large hole portion 611, and a recompression second step portion 613 formed between the first large hole portion 611 and the first small hole portion 612. These are arranged around a stepped outer surface portion 644 and form a stepped inner surface portion 614 for molding the outer periphery of the sintered compact 250S. The recompression second step portion 613 in the illustrated example has a shape in which the diameter gradually decreases from the end on the first large hole portion 611 side to the end on the first small hole portion 612 side. In the first recompression mold 600, the recompression large outer diameter portion 641, the recompression first step portion 643, and the recompression small outer diameter portion 642 are arranged in this order from bottom to top, as well as the first large hole portion 611, the recompression second step portion 613, and the first small hole portion 612.

[0096] 21, which is an enlarged view of the area circled by a dashed line in Fig. 22, in the stepped inner surface portion 614, the inclination angle θ6 of the recompression second stage portion 613 with respect to the extension line Ex4 of the first small hole portion 612 shown by the dashed line is, for example, 10 degrees or more and 90 degrees or less, similar to the angle θ2. Note that when the angle θ6 is 90 degrees, the recompression second stage portion 613 is formed as a surface perpendicular to the central axis C of the first recompression mold 600.

[0097] In the first die 610, in a longitudinal section passing through the central axis C shown in Figure 21, the step Δr4 along the radially outward direction between the first small hole portion 612 and the first large hole portion 611 and the angle θ6 are preferably (a) or (b) as described above, similar to the relationship between the step Δr1 and the angle θ1.

[0098] 22, with the punch surface 620A of the first lower punch 620 positioned lower than the second recompression stage 613 of the first die 610, one sintered end face 252S of the sintered compact 250S is inserted into the first large hole 611 of the first die 610. Then, the first upper punch 630 and the first core rod 640 are lowered to move the first core rod 640 downward until the first recompression stage 643 of the stepped outer surface 644 of the first core rod 640 is lower than the second recompression stage 613 of the stepped inner surface 614 of the first die 610. As a result, as shown in FIG. 23, one sintered end face 252S of the sintered compact 250S is pressed into the first small hole 612 of the first die 610, and the other sintered end face 253S of the sintered compact 250S is pressed into the first large hole 611 until it is accommodated therein. At this time, the first core rod 640 is inserted into the sintered body 250S and the first lower punch 620. The sintered body 250S is compressed by the stepped inner surface portion 614 of the first die 610 and the stepped outer surface portion 644 of the first core rod 640, and is also compressed by the first upper punch 630 and the first lower punch 620.

[0099] In this first compression step, the sintered body 250S is formed into a recompressed body 100B having substantially the same shape as the sintered body 100S shown in FIG. The inner surface of the repressurization body 100B consists of a first sintered inner peripheral portion 111S and a sintered reduced diameter portion 112S (second sintered inner peripheral portion 112AS, third sintered inner peripheral portion 112BS) formed with an inner diameter smaller than that of the first sintered inner peripheral portion 111S, and the outer surface side consists of one annular sintered end surface portion 151S extending radially outward from the outwardly opening edge of the second sintered inner peripheral portion 112AS, the other annular sintered end surface portion 152S extending radially outward from the outward opening edge of the first sintered inner peripheral portion 111S and positioned opposite the one sintered end surface portion 151S, a first sintered outer peripheral portion 161S extending from the outer peripheral edge of the one sintered end surface portion 151S, and a sintered expanded diameter portion 162S (second sintered outer peripheral portion 162AS, third sintered outer peripheral portion 162BS) formed with an outer diameter larger than that of the first outer peripheral portion 161S.

[0100] In the second recompression step, the recompression body 100B is recompressed using a second recompression mold. As the second recompression mold, the recompression mold 400 can be used, as in the recompression step of the first embodiment. In the second recompression step, the recompression body 100B is placed in a second large hole (recompression large hole 411A) of a through-hole formed in a second die (recompression die 410) so that the sintered expanded diameter portion 162S is lower than the sintered reduced diameter portion 112S. Then, the recompression body 100B is pressed into a second small hole (recompression small hole 411B) of the through-hole with a cylindrical second upper punch (recompression upper punch 430), and a second core rod (recompression core rod 440) inserted in the second upper punch is pressed into the recompression body 100B and a cylindrical second lower punch (recompression lower punch 420). As a result, the sintered reduced diameter portion 112S of the repressure body 100B is compressed radially outward by the second core rod and is formed into the first bearing portion 11 that supports the shaft 90.

[0101] The area inside the sintered expanded diameter portion 162S is coined by the second die to form the second bearing portion 12 that adheres closely to the second core rod and supports the shaft 90, and the second upper punch and second lower punch form the annular first end surface portion 15A and the annular second end surface portion 15B located opposite the first end surface portion 15A. Furthermore, by sizing the first sintered outer peripheral portion 161S and coining the sintered expanded diameter portion 162S, the outer peripheral surface of the re-compression body 100B is formed with a first curved surface portion 16A extending from the outer peripheral edge of the first end surface portion 15A and having a constant diameter from the central axis C, a second curved surface portion 16B extending from the outer peripheral edge of the second end surface portion 15B and having a diameter equal to that of the first curved surface portion 16A, and an annular groove portion 17 extending circumferentially between the first curved surface portion 16A and the second curved surface portion 16B. Thus, a center-relief type sintered bearing is completed through the first and second re-compression steps. Adding the sintered expanded diameter portion and sintered reduced diameter portion to the sintered body formed as a cylindrical body in the first re-compression step reduces processing costs.

[0102] (Combination of annular groove and protrusion, etc.) In the sintered bearing of the first embodiment, the first end face portion and the second end face portion may be formed to have different shapes in addition to the annular groove portion 17. In the sintered bearing of the second embodiment, the first end face portion and the second end face portion may be formed to have different shapes in addition to the longitudinal groove portion 18. Furthermore, the sintered bearing of the second embodiment may further be provided with an annular groove portion extending circumferentially on the outer circumferential surface. [Example]

[0103] A number of sintered bodies were prepared by varying the angle θ1 and step height Δr1 of the sintered expansion portion shown in Figure 5, and each was further recompressed to form a recompressed body with a center relief shape. The width W1 and depth H of the groove-like portion formed on the outer surface of each sample were measured, and the visibility of the groove-like portion was evaluated.

[0104] (Measurement of grooved areas) Of the groove-like portions, three locations A, B, and C were targeted, spaced apart by an angle of 120 degrees around the central axis, and the width (dimension along the direction of extension of central axis C) and depth of each were measured. The measurement direction was the axial direction from first curved surface portion 16A to second curved surface portion 16B using a surface roughness measuring instrument. The measured values ​​(width W1 and depth H) of measurement locations A, B, and C of each sample are shown in Table 1.

[0105] (Visibility of grooved areas) The visibility of each of the groove-like areas, measurement points A, B, and C, was checked. This was done by pinching the sample under fluorescent light and changing the angle of the sample to visually inspect measurement points A, B, and C from multiple angles. The evaluation was "good" if the grooves were visible at each angle of the sample, "fair" if the grooves were sometimes visible and sometimes not depending on the angle of the sample, and "poor" if the grooves were not visible at any angle of the sample. This evaluation was carried out by three people, and the visibility evaluation was decided by majority vote. The evaluation of the visibility of the groove-like areas of each sample is shown in Table 1.

[0106] [Table 1]

[0107] When the step difference Δr1 was 30 μm or more and the angle θ1 was 30 degrees or more, or when the step difference Δr1 was 55 μm or more and the angle θ1 was 10 degrees or more, the grooves were visible. In particular, when the angle θ1 was 30 degrees or more, by increasing the step difference Δr1 by 35 μm, the grooves were visible at all of measurement points A, B, and C even when the angle at which the sample was viewed was changed. Furthermore, when the angle θ1 was 10 degrees or more, by increasing the step difference Δr1 by 85 μm or more, the grooves were visible at all of measurement points A, B, and C even when the angle at which the sample was viewed was changed.

[0108] On the other hand, when the step difference Δr1 was less than 30 μm, or when the step difference Δr1 was 30 μm or more and less than 55 μm but the angle was 10 degrees, the groove could not be visually recognized. [Explanation of symbols]

[0109] 1,2,3,4,5 Sintered bearings 11 First bearing part 12 Second bearing part 13 Middle escape area 13A First Connection 13B Second connection part 15A First end section 15B, 15C, 15D Second end section 16A First curved section 16B Second curved surface part 17 Annular groove 18 Vertical groove 19 Recess 100 powder compacts 100B recompression body 100S, 200S, 250S sintered body 112 Reduced diameter part 112S Sintered reduced diameter section 151,152 End section 151S,152S Sintered end face 161 First outer periphery 161S First sintered outer periphery 162 Expanded diameter part 162S Sintered expansion section 300,500 Molding mold 310,510 Molding dies 320 Lower outer punch 330 Lower inner punch 340 molding core rod 341,551 Large outer diameter section for molding 342,553 First stage section for molding 343,552 Small outer diameter section for molding 344,554 Stepped outer surface 350 Upper outer punch 351,511 Large hole for molding 352,513 Second stage section for molding 353,512 Small hole for molding 354,514 Stepped inner surface 360 Upper Inner Punch 400 Recompression mold 410 Recompression Die 411 Penetration 411A Large hole for recompression 411B Small hole for recompression 420 Lower punch for recompression 430 Upper punch for recompression 440 Recompression Core Rod 520 Lower punch for forming 530 Lower Core Rod 540 Upper punch for forming 550 Upper Core Rod 600 First Recompression Mold 610 First Die 611 First large hole 612 First small hole 613 Second stage for recompression 614 Stepped inner surface 620 First Lower Punch 630 First Upper Punch 640 First Core Rod 641 Large outer diameter part for recompression 642 Small outer diameter part for recompression 643 First stage for recompression 644 Stepped external part

Claims

1. A cylindrically formed sintered bearing, The inner side is an annular first bearing portion that supports a shaft that rotates; an annular second bearing portion that supports the shaft; a central relief portion formed between the first bearing portion and the second bearing portion, the central relief portion being recessed radially outward from the first bearing portion and extending in a circumferential direction, the first bearing portion and the second bearing portion have different shapes, or a first connection portion of the central relief portion connected to the first bearing portion and a second connection portion of the central relief portion connected to the second bearing portion have different shapes, The outer side is a first annular end surface portion extending radially outward from an edge of the first bearing portion that opens outward; a second annular end surface portion extending radially outward from an edge of the second bearing portion that opens outward and positioned opposite the first end surface portion; a first curved surface portion extending from an outer peripheral edge of the first end surface portion; a second curved surface portion extending from an outer peripheral edge of the second end surface portion; an annular groove portion formed between the first curved surface portion and the second curved surface portion and extending in a circumferential direction, The sintered bearing is further characterized in that the annular groove is provided on the first end surface side or the second end surface side.

2. 2. The sintered bearing according to claim 1, wherein the annular groove has a depth of 3 μm or more and a width of 30 μm or more.

3. the annular groove portion includes a first inclined surface portion extending from the first curved surface portion toward the deepest portion and a second inclined surface portion extending from the second curved surface portion toward the deepest portion, 3. The sintered bearing according to claim 1, wherein the first inclined surface portion has a gradient gentler than that of the second inclined surface portion.

4. A cylindrically formed sintered bearing, The inner side is an annular first bearing portion that supports a shaft that rotates; an annular second bearing portion that supports the shaft; a central relief portion formed between the first bearing portion and the second bearing portion, the central relief portion being recessed radially outward from the first bearing portion and extending in a circumferential direction, the first bearing portion and the second bearing portion have different shapes, or a first connection portion of the central relief portion connected to the first bearing portion and a second connection portion of the central relief portion connected to the second bearing portion have different shapes, The outer side is a first annular end surface portion extending radially outward from an edge of the first bearing portion that opens outward; a second annular end surface portion extending radially outward from an edge of the second bearing portion that opens outward and positioned opposite the first end surface portion; a plurality of longitudinal grooves extending parallel to the central axis from an outer peripheral edge of one of the first end surface portion and the second end surface portion and spaced apart from one another around the central axis; a first curved surface portion extending from the outer peripheral edge of the other of the first end surface portion and the second end surface portion to the plurality of longitudinal groove portions; a plurality of second curved surface portions extending from one of the longitudinal groove portions to the other of the longitudinal groove portions between the adjacent ones of the longitudinal groove portions, and continuing to the first curved surface portion and extending to the outer peripheral edge of the other of the longitudinal groove portions; A sintered bearing, wherein an angle of the longitudinal groove portion about the central axis is smaller than an angle of the second curved surface portion about the central axis.

5. A cylindrically formed sintered bearing, The inner side is an annular first bearing portion that supports a shaft that rotates; an annular second bearing portion that supports the shaft; a central relief portion formed between the first bearing portion and the second bearing portion, the central relief portion being recessed radially outward from the first bearing portion and extending in a circumferential direction, the first bearing portion and the second bearing portion have different shapes, or a first connection portion of the central relief portion connected to the first bearing portion and a second connection portion of the central relief portion connected to the second bearing portion have different shapes, The outer side is a first annular end surface portion extending radially outward from an edge of the first bearing portion that opens outward; a second annular end surface portion extending radially outward from an edge of the second bearing portion that opens outward and positioned opposite the first end surface portion; a curved surface portion extending from an outer peripheral edge of the first end surface portion to an outer peripheral edge of the second end surface portion, A sintered bearing, characterized in that the first end surface portion and the second end surface portion have different shapes.

6. one of the first end surface portion and the second end surface portion has at least one of a recessed portion recessed from a flat surface and a protruding portion, 6. The sintered bearing according to claim 5, wherein the other of the first end surface portion and the second end surface portion is formed as a flat surface from the inner peripheral edge to the outer peripheral edge.

7. A method for manufacturing a sintered bearing, comprising a recompression step of recompressing a cylindrically formed sintered body in a recompression die, The sintered body has an inner sintered reduced diameter portion formed adjacent to one annular sintered end surface portion and having a small inner diameter, and an outer sintered expanded diameter portion formed adjacent to the other annular sintered end surface portion located opposite the one sintered end surface portion, Furthermore, in a cross section passing through the central axis, the sintered body has a step of 30 μm or more in a radially outward direction of the sintered cone-shaped portion, and at a location where the first sintered outer peripheral portion extending from the outer peripheral edge of the one sintered end face portion and the sintered cone-shaped portion are connected, the angle formed by the sintered cone-shaped portion and an extension line of the first sintered outer peripheral portion is 30 degrees or more and 90 degrees or less, Alternatively, in a cross section passing through the central axis, the sintered body has a step of 55 μm or more in a radially outward direction of the sintered convex portion, and an angle formed between the sintered convex portion and an extension line of the first sintered outer peripheral portion at a point where the sintered convex portion is connected to the first sintered outer peripheral portion is 10 degrees or more and 90 degrees or less, The recompression step A method for manufacturing a sintered bearing, characterized in that the sintered body is placed in a large recompression hole portion of a through-hole formed in a recompression die so that the sintered expanded diameter portion is positioned lower than the sintered reduced diameter portion, and then the sintered body is forced into a small recompression hole portion of the through-hole portion with a cylindrical upper recompression punch, and a recompression core rod inserted in the upper recompression punch is forced into the sintered body and the cylindrical lower recompression punch.

8. Before the recompression step, a powder compact forming step of compressing a raw material powder in a forming die to form a cylindrical powder compact having an inner reduced diameter portion formed adjacent to one annular end face portion and an outer expanded diameter portion formed adjacent to the other annular end face portion opposite the one end face portion; a sintering step of sintering the powder compact to form the sintered body, the molding die comprises a stepped outer surface portion for molding an inner peripheral side of the powder compact, and a stepped inner surface portion disposed around the stepped outer surface portion for molding an outer peripheral side of the powder compact, the stepped outer surface portion comprises a molding large outer diameter portion, a molding small outer diameter portion formed to have an outer diameter smaller than that of the molding large outer diameter portion, and a molding first step portion formed between the molding large outer diameter portion and the molding small outer diameter portion, the stepped inner surface portion comprises a large molding hole portion, a small molding hole portion formed with an inner diameter smaller than that of the large molding hole portion, and a second molding step portion formed between the large molding hole portion and the small molding hole portion, Furthermore, in a cross section passing through the central axis, the stepped inner surface portion has a step between the large molding hole portion and the small molding hole portion in a radially outward direction of 30 μm or more, and an angle formed by the second molding step portion and an extension line of the small molding hole portion at a location where the second molding step portion is connected to the small molding hole portion is 30 degrees or more and 90 degrees or less, Alternatively, in a cross section passing through the central axis, the stepped inner surface portion has a step between the large molding hole portion and the small molding hole portion in a radially outward direction of 55 μm or more, and at a location where the second molding step portion is connected to the small molding hole portion, the angle formed by the second molding step portion and an extension line of the small molding hole portion is 10 degrees or more and 90 degrees or less, the large outer diameter forming portion, the first forming step portion, and the small outer diameter forming portion are arranged in this order upward or downward, and the large forming hole portion, the second forming step portion, and the small forming hole portion are arranged in this order, 8. The method for producing a sintered bearing according to claim 7, wherein the powder compacting step comprises moving at least one of the stepped outer surface portion and the stepped inner surface portion downward until the molding small hole portion is positioned radially outward from the molding first step portion, thereby compressing the raw material powder with the stepped outer surface portion and the stepped inner surface portion.

9. the large outer diameter forming portion, the first step forming portion, and the small outer diameter forming portion are arranged in this order from top to bottom, and the large hole forming portion, the second step forming portion, and the small hole forming portion are arranged in this order, The powder compact forming step includes: a step of putting the raw material powder into a space defined by a cylindrical lower outer punch, a cylindrical lower inner punch inserted into the lower outer punch with its punch surface positioned below the upper end of the lower outer punch, and a molding core rod inserted into the lower inner punch with its stepped outer surface portion protruding above the punch surface of the lower inner punch; 9. A method for manufacturing a sintered bearing according to claim 8, further comprising a step of lowering a cylindrical upper outer punch, the lower end of which abuts against the upper end of the lower outer punch, and an upper inner punch, the punch surface of which is inserted into the upper outer punch and held at a position higher than the stepped inner surface of the upper outer punch, to compress the raw material powder between the stepped outer surface and the stepped inner surface, and also between the punch surface of the lower inner punch and the punch surface of the upper inner punch.

10. the large outer diameter forming portion, the first step forming portion, and the small outer diameter forming portion are arranged downward in this order, and the large hole forming portion, the second step forming portion, and the small hole forming portion are arranged downward in this order, The powder compact forming step includes: a step of putting the raw material powder into a space defined by a forming die in which the large forming hole portion with a larger inner diameter is positioned higher than the small forming hole portion to form the stepped inner surface portion, a lower forming punch inserted into the small forming hole portion and having a punch surface held lower than the second forming step portion of the forming die, and a lower core rod inserted into the lower forming punch; 9. A method for manufacturing a sintered bearing according to claim 8, further comprising the step of lowering a cylindrical upper forming punch whose punch surface at a lower end is inserted into the large forming hole portion, and an upper core rod inserted into the upper forming punch and whose stepped outer surface portion protrudes below the punch surface of the upper forming punch, to compress the raw material powder between the stepped outer surface portion and the stepped inner surface portion, and also between the punch surface of the lower forming punch and the punch surface of the upper forming punch.

11. a first compression step of recompressing the cylindrical sintered body in a first recompression die to form a recompressed body; A second recompression step of recompressing the recompressed body in a second recompression mold, The repressurization body has an inner sintered reduced diameter portion formed adjacent to one annular sintered end surface portion and having a small inner diameter, and an outer sintered expanded diameter portion formed adjacent to the other annular sintered end surface portion located opposite to the one sintered end surface portion, the first re-compression die comprises a stepped outer surface portion for re-compressing the inner periphery of the sintered body and a stepped inner surface portion disposed around the stepped outer surface portion for molding the outer periphery of the sintered body, In the first re-compression step, a cylindrical first lower punch is placed in the first small hole portion of a first die having a stepped inner surface formed therein, and one annular end surface of the sintered body is inserted into the first large hole portion, with the first large hole portion being positioned higher than the first small hole portion having a smaller inner diameter. Then, a cylindrical first upper punch is inserted into the first upper punch, with the punch surface of the lower end being inserted into the first large hole portion. The first upper punch is inserted into the first upper punch, and the re-compression large outer diameter portion is positioned higher than the re-compression small outer diameter portion having a smaller outer diameter. a first core rod with the stepped outer surface portion protruding below the first upper punch is lowered to push the one end surface side of the sintered body into the first small hole portion and the other annular end surface side of the sintered body into the first large hole portion, and the first core rod is inserted into the sintered body and the first lower punch to move the first recompression step portion of the stepped outer surface portion to a position lower than the second recompression step portion of the stepped inner surface portion, thereby compressing the sintered body between the stepped inner surface portion and the stepped outer surface portion; In the second re-compression step, the re-compressed body is placed in a second large hole of a through-hole formed in a second die so that the sintered expanded diameter portion is positioned lower than the sintered contracted diameter portion, and then the re-compressed body is forced into a second small hole of the through-hole with a cylindrical second upper punch, and a second core rod inserted in the second upper punch is forced into the re-compressed body and a cylindrical second lower punch; In the first recompression step, the stepped inner surface portion has a step between the first large hole portion and the first small hole portion in a radially outward direction of 30 μm or more in a cross section passing through the central axis, and an angle formed between the second recompression step portion and an extension line of the first small hole portion at a location where the second recompression step portion is connected to the first small hole portion is 30 degrees or more and 90 degrees or less; Alternatively, in a cross section passing through the central axis, the stepped inner surface portion has a radially outward step between the first large hole portion and the first small hole portion of 55 μm or more, and at the point where the second recompression step portion connects to the first small hole portion, the angle formed by the second recompression step portion and an extension line of the first small hole portion is 10 degrees or more and 90 degrees or less.

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