Slide bearing

Abstract

To provide a slide bearing which can support a moment load without generating an edge load.SOLUTION: A slide bearing 1 includes an inner ring 2 and an outer ring 3. An inner peripheral surface of the outer ring 3 has a convex surface 3a having an annular protrusion shape protruding to the radial inner side. An outer peripheral surface of the inner ring 2 has a concave surface 2a having an annular recessed shape which is fitted in the convex surface 3a and may slide in a circumferential direction relative to the outer ring 3. The outer ring 3 and the inner ring 2 are annular bodies which are continuous in a circumferential direction. A top part of the convex surface 3a is formed at a center in an axial direction on the inner peripheral surface of the outer ring 3. The convex surface 3a is a curved surface having a diameter increasing from the top part to both sides in the axial direction. A deepest part of the concave surface 2a is formed at a center part in the axial direction on the outer peripheral surface of the inner ring 2. The concave surface 2a is a curved surface having a diameter increasing from the deepest part to both sides in the axial direction.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a sliding bearing, and relates to a sliding bearing used, for example, in a joint of an industrial robot, a service robot, or the like, or in a reducer mounted on such a joint. [Background technology]

[0002] Patent Document 1 discloses a prior art sliding bearing 50 in which the sliding surface 53 between inner and outer rings 51, 52 is tapered, inclined relative to the rotation axis C2, as shown in Figure 12. By tapering the sliding surface 53, this sliding bearing 50 improves the load capacity with a small number of parts, and by dividing the outer ring 52 into outer ring segments in the axial direction, the outer ring width surface can be adjusted to adjust the clearance (preload). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2015 / 172781 [Patent Document 2] Japanese Patent Application Laid-Open No. 2023-169989 Summary of the Invention [Problem to be solved by the invention]

[0004] If the sliding surface 53 is tapered as in the prior art, an edge load may occur when the sliding bearing 50 supports a moment load, which may result in a shorter lifespan of the sliding bearing 50.

[0005] An object of the present invention is to provide a sliding bearing that can support a moment load without generating an edge load. [Means for solving the problem]

[0006] The sliding bearing of the present invention is a sliding bearing comprising an inner ring and an outer ring, the inner peripheral surface of the outer ring has a convexly curved surface having an annular convex portion shape that convex radially inward, The outer peripheral surface of the inner ring has a concavely curved surface with an annular recessed shape that fits into the convexly curved surface and is capable of sliding circumferentially relative to the outer ring, and the outer ring and the inner ring are annular bodies that are continuous in the circumferential direction.

[0007] With this configuration, the moment load can be supported by the convex and concave curved surfaces that fit together, making it possible to support the moment load without generating edge loads, which are localized stress concentrations.This makes it possible to extend the life of the sliding bearing compared to conventional technology.

[0008] an apex of the convexly curved surface is provided at a central portion of the inner peripheral surface of the outer ring in the axial direction, and the convexly curved surface is a curved surface that increases in diameter from the apex to both sides in the axial direction, The outer peripheral surface of the inner ring may have a deepest portion of the concavely curved surface at a central portion in the axial direction, and the concavely curved surface may be a curved surface that increases in diameter from the deepest portion to both sides in the axial direction.

[0009] In this case, because there are contact surfaces in both directions with the apex of the convexly curved surface of the outer ring or the deepest part of the concavely curved surface of the inner ring, both moment loads can be supported. Furthermore, even though it is a plain bearing alone, a large distance between the points of application can be ensured, roughly similar to that of a back-to-back combination of a duplex bearing. This results in superior rigidity against moment loads, enabling an increased load capacity.

[0010] The concavely curved surface and the convexly curved surface may each have a single radius of curvature, and the radius of curvature r1 of the concavely curved surface may be greater than the radius of curvature r2 of the convexly curved surface. In this case, the risk of edge load occurring when the sliding bearing supports a moment load can be more reliably reduced.

[0011] The inner peripheral surface of the outer ring may have an annular flat portion connected to the convexly curved surface and parallel to the axial direction at one end or both ends in the axial direction, and the outer peripheral surface of the inner ring may have an annular flat portion connected to the concavely curved surface and parallel to the axial direction at one end or both ends in the axial direction. In this case, the convexly curved surface of the outer ring can be machined easily and precisely using the flat portion of the outer ring as a reference surface. Similarly, the concavely curved surface of the inner ring can be machined easily and precisely using the flat portion of the inner ring as a reference surface.

[0012] When the axial dimension b of the concavely curved surface is greater than or equal to the axial dimension B of the convexly curved surface, both axial side edges of the convexly curved surface and the outer ring side surfaces continuing from these axial side edges may be smoothly connected. The phrase "smoothly connecting" refers to mitigating stress concentration that may occur at the connecting portions between both axial side edges of the convexly curved surface and the outer ring side surfaces that continue to these axial side edges. If the axial dimension b of the concavely curved surface is greater than or equal to the axial dimension B of the convexly curved surface, there is a risk of an edge load being generated on the outer peripheral surface of the inner ring by the convexly curved surface of the outer ring. For this reason, the edge load can be alleviated by smoothly connecting both axial edges of the convexly curved surface with the side surfaces of the outer ring that continue to those both axial edges.

[0013] When the axial dimension b of the concavely curved surface is smaller than the axial dimension B of the convexly curved surface, both axial side edges of the concavely curved surface and the inner ring side surfaces continuing from these axial side edges may be smoothly connected. The phrase "smoothly connecting" refers to mitigating stress concentration that may occur at both axial side edges of the concavely curved surface and the connecting portions of the inner ring side surfaces that continue to these axial side edges. If the axial dimension b of the concavely curved surface is less than the axial dimension B of the convexly curved surface, there is a risk of edge loading on the inner peripheral surface of the outer ring due to the concavely curved surface of the inner ring. For this reason, the edge loading can be alleviated by smoothly connecting both axial edges of the concavely curved surface with the inner ring side surfaces that continue to those both axial edges.

[0014] The convexly curved surface of the outer ring may be provided with a surface treatment layer that serves as a sliding resistance reduction component. If the sliding bearing is provided with a surface treatment layer that serves as a sliding resistance reduction component, the generation of edge loads on the surface treatment layer should be avoided. For this reason, it is preferable to smoothly round both ends opposite the surface treatment layer. In other words, if the axial dimension b of the concavely curved surface is greater than or equal to the axial dimension B of the convexly curved surface, providing a surface treatment layer on the convexly curved surface of the outer ring can suppress the generation of edge loads on this surface treatment layer. Furthermore, by reducing friction on the sliding surface with a surface treatment layer, power loss in the drive source can be reduced compared to a structure without a surface treatment layer. This also makes it possible to reduce the size of the drive source.

[0015] The concavely curved surface of the inner ring may be provided with a surface treatment layer that serves as a sliding resistance reduction component. When the axial dimension b of the concavely curved surface is less than the axial dimension B of the convexly curved surface, providing a surface treatment layer on the concavely curved surface of the inner ring can suppress the occurrence of edge loads on this surface treatment layer. Furthermore, by reducing friction on the sliding surface with the surface treatment layer, power loss in the drive source can be reduced compared to a structure without a surface treatment layer. This also makes it possible to reduce the size of the drive source. [Effects of the Invention]

[0016] The sliding bearing of the present invention is a sliding bearing comprising an inner ring and an outer ring, wherein the inner peripheral surface of the outer ring has a convexly curved surface with an annular convex portion that convex radially inward, and the outer peripheral surface of the inner ring has a concavely curved surface with an annular concave portion that fits into the convexly curved surface and is capable of sliding circumferentially relative to the outer ring, and the outer ring and the inner ring are annular bodies that are continuous in the circumferential direction. As a result, moment loads can be supported without generating edge loads. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a partial perspective view showing a longitudinal section of a sliding bearing according to a first embodiment of the present invention. [Figure 2A] FIG. 2 is a partial perspective view showing a vertical cross section of the outer ring of the sliding bearing. [Figure 2B]FIG. 2 is a partial perspective view showing a vertical cross section of the inner ring of the sliding bearing. [Figure 3] FIG. 2 is a longitudinal cross-sectional view of the inner and outer rings of the sliding bearing. [Figure 4] FIG. 2 is a longitudinal cross-sectional view illustrating the relationship in axial dimensions of the concavely curved surface and convexly curved surface of the inner and outer rings. [Figure 5A] FIG. 2 is a longitudinal cross-sectional view showing the contact surface of the sliding bearing. [Figure 5B] FIG. 2 is a longitudinal cross-sectional view showing a load application point of the sliding bearing. [Figure 6] FIG. 10 is a partial perspective view showing a longitudinal cross section of a sliding bearing according to a second embodiment of the present invention. [Figure 7A] FIG. 2 is a partial perspective view showing a vertical cross section of the outer ring of the sliding bearing. [Figure 7B] FIG. 2 is a partial perspective view showing a vertical cross section of the inner ring of the sliding bearing. [Figure 8] FIG. 2 is a longitudinal cross-sectional view showing the relationship in axial dimensions between the concavely curved surface and the convexly curved surface of the inner and outer rings of the sliding bearing. [Figure 9] FIG. 10 is a longitudinal sectional view of a sliding bearing according to a third embodiment of the present invention. [Figure 10] FIG. 10 is a longitudinal sectional view of a sliding bearing according to a fourth embodiment of the present invention. [Figure 11A] FIG. 10 is a longitudinal cross-sectional view of the inner ring of a sliding bearing according to a fifth embodiment of the present invention. [Figure 11B] FIG. 2 is a longitudinal sectional view of the sliding bearing. [Figure 12] FIG. 1 is a perspective view of a sliding bearing according to the prior art. DETAILED DESCRIPTION OF THE INVENTION

[0018] [First embodiment] A sliding bearing according to an embodiment of the present invention will be described with reference to Figures 1 to 5. The sliding bearing according to the embodiment is used, for example, in the joint of an industrial robot, service robot, or the like, or as the main bearing of a reducer mounted on said joint. Examples of such reducers include those using a wave gear reducer, an eccentric differential reducer, or a planetary gear reducer. This sliding bearing is used, for example, as a substitute for rolling bearings such as conventional cross roller bearings or four-point contact ball bearings.

[0019] <Schematic configuration of a sliding bearing> As shown in Figure 1, sliding bearing 1 is equipped with an inner ring 2 and an outer ring 3, and is a sliding bearing that can support moment loads on its own; it is not equipped with rolling elements. Throughout this specification, sliding bearings may be simply referred to as "bearings." Sliding bearing 1 may be unlubricated (strictly speaking, it may use a solid lubricant in a resin layer or surface treatment layer, as described below), or it may use a fluid lubricant such as grease or oil.

[0020] The inner peripheral surface of the outer ring 3 has, at or near the axial center, a convexly curved surface 3a with an annular convex shape that convexes radially inward. The outer peripheral surface of the inner ring 2 has a concavely curved surface 2a with an annular concave shape that fits into the convexly curved surface 3a and is capable of sliding circumferentially relative to the outer ring 3. The outer ring 3 and the inner ring 2 are annular bodies that are continuous in the circumferential direction. The inner ring 2 and the outer ring 3 are each formed integrally.

[0021] The portion near the axial center is a section of the inner circumferential surface of the outer ring 3 that is separated from the axial center by a specified length in the axial direction C1. The axial direction of the central axes of the inner ring 2 and outer ring 3 is referred to as the "axial direction" C1. The specified length is set appropriately according to the conditions of use of the sliding bearing 1. The direction perpendicular to the "axial direction" C1 is referred to as the "radial direction." The circumferential direction around the central axis is referred to as the "circumferential direction" or "circumferential direction."

[0022] <Inner and outer rings> As shown in Fig. 2A, an apex 3aa of the convexly curved surface 3a is provided at or near the axial center of the inner peripheral surface of the outer ring 3. The convexly curved surface 3a is a curved surface whose diameter monotonically increases from the apex 3aa toward both sides in the axial direction. As shown in Fig. 3, the convexly curved surface 3a has a single radius of curvature, that is, a single R shape.

[0023] As shown in Fig. 2B, the concavely curved surface 2a has a deepest portion 2aa at or near the axial center of the outer peripheral surface of the inner ring 2. The concavely curved surface 2a is a curved surface whose diameter monotonically increases from the deepest portion 2aa toward both sides in the axial direction. As shown in Fig. 3, the concavely curved surface 2a has a single radius of curvature, i.e., a single R shape.

[0024] When the convexly curved surface 3a and the concavely curved surface 2a each have a single R shape in this way, it is preferable that the radius of curvature r1 of the concavely curved surface 2a is greater than the radius of curvature r2 of the convexly curved surface 3a. By satisfying this relationship of r1 > r2, the risk of edge load occurring when the sliding bearing 1 supports a moment load can be more reliably reduced.

[0025] As shown in Figure 4, the mode of edge load generation varies depending on the relative size of the axial dimension b of the concavely curved surface 2a of the inner ring 2 (referred to as the "inner ring concave groove width") and the axial dimension B of the convexly curved surface 3a of the outer ring 3 (referred to as the "outer ring convex portion width"). When inner ring concave groove width b ≥ outer ring convex portion width B, there is a risk of edge load being generated on the outer peripheral surface of the inner ring 2 by the convexly curved surface 3a of the outer ring 3. Therefore, when inner ring concave groove width b ≥ outer ring convex portion width B, the axial side edges 3ab, 3ab of the convexly curved surface 3a are smoothly connected to the outer ring side surfaces 3b, 3b that respectively continue from these axial side edges 3ab, 3ab. Specifically, for example, the corners of the axial side edges 3ab, 3ab of the convexly curved surface 3a are machined to be smoothly rounded. This can mitigate the edge load.

[0026] When the inner ring recess groove width b < the outer ring protrusion width B, the relationship described above is reversed, so both axial edges 2ab, 2ab of the concavely curved surface 2a are smoothly connected to the inner ring side surfaces 2b, 2b that continue to these axial edges 2ab, 2ab. Specifically, for example, the corners of both axial edges 2ab, 2ab of the concavely curved surface 2a are machined to be smoothly rounded. This can reduce the edge load. As in Patent Document 2, the inner ring and the output shaft may be integrally formed.

[0027] <Bearing materials, etc.> Steel material, lightweight material, or resin material can be used as the material for the inner and outer rings 2, 3. The inner and outer rings 2, 3 may be made of the same material or different materials. The steel material may be, for example, high carbon chromium bearing steel, chromium molybdenum steel, carbon steel for machine structures, stainless steel, etc. Molybdenum disulfide or the like may be added to any one of these. The lightweight material may be, for example, aluminum, ceramics, etc.

[0028] When the resin material is used, the base of the resin composition is, for example, a thermoplastic PI resin, polyether ketone resin (PEK), polyether ether ketone resin (PEEK), polyphenylene sulfide resin (PPS), polyether ketone ether ketone ketone resin (PEKEKK), polyamide imide resin (PAI), polyamide resin (PA), polyethylene resin (PE), polyacetal resin (POM), polytetrafluoroethylene resin (PTFE), or the like.

[0029] When steel or lightweight materials are used as the material for the inner and outer rings 2, 3, a resin layer RS ​​may be formed on both, or one of, the inner circumferential surface of the outer ring 3 and the outer circumferential surface of the inner ring 2 to reduce frictional resistance at the sliding surfaces. Examples of resin layer RS ​​include thermoplastic PI resin, PEK, PEEK, PPS, PEKEKK, PAI, PA, PE, POM, and PTFE. When either or both of the inner and outer rings 2, 3 are made of a resin material, the resin rings are molded, for example, by injection molding. Specifically, when the outer ring 3 is made of a resin material, the inner ring 2 is supported in an injection molding machine (not shown), and the entire outer ring is injection molded into a cavity on the outer periphery of this inner ring 2.

[0030] <Contact surface and load application point> As shown in Figure 5A, sliding bearing 1 has contact surfaces sf, sf on both axial sides of apex 3aa of convexly curved surface 3a of outer ring 3 or deepest part 2aa of concavely curved surface 2a of inner ring 2, and is therefore capable of supporting moment loads in both directions. As shown in Figure 5B, curved shapes for the sliding surfaces of inner and outer rings 2, 3 enable flexible setting of contact angles θ1, θ2 depending on the position of the load application point. The contact angles θ1, θ2 are the angles of load action lines L1, L2 relative to shaft center C.

[0031] <Action and effect> With the sliding bearing 1 described above, the moment load can be supported by the convex curved surface 3a and the concave curved surface 2a that fit together, making it possible to support the moment load without generating an edge load, which is a localized concentration of stress.This makes it possible to extend the life of the sliding bearing 1 compared to prior art.

[0032] The inner peripheral surface of the outer ring 3 has an apex 3aa of the convexly curved surface 3a at or near the axial center. The convexly curved surface 3a is a curved surface whose diameter monotonically increases from the apex 3aa toward both axial sides. The inner peripheral surface of the inner ring 2 has a deepest portion 2aa of the concavely curved surface 2a at or near the axial center. The concavely curved surface 2a is a curved surface whose diameter monotonically increases from the deepest portion 2aa toward both axial sides. In this case, as shown in FIG. 5A , contact surfaces sf, sf are provided on both sides with respect to the apex 3aa of the convexly curved surface 3a of the outer ring 3 or the deepest portion 2aa of the concavely curved surface 2a of the inner ring 2, allowing both moment loads to be supported. Furthermore, despite being a sliding bearing alone, a large distance between the points of application can be ensured, similar to that of a back-to-back duplex bearing. This provides superior rigidity against moment loads and increases load capacity.

[0033] <Other embodiments> In the following description, parts corresponding to matters previously described in each embodiment are given the same reference numerals, and duplicated description will be omitted. When only a part of the configuration is described, the other parts of the configuration are the same as those in the previously described embodiment unless otherwise specified. The same configuration produces the same effects. It is possible to combine not only the parts specifically described in each embodiment, but also partially combine embodiments as long as there is no particular problem with the combination.

[0034] [Second embodiment: flat portion, Figs. 6 to 8] As shown in FIGS. 6 and 7A , the inner peripheral surface of the outer ring 3 has annular flat portions 3c, 3c connected to the convexly curved surface 3a and parallel to the axial direction C1 at both axial ends. Furthermore, as shown in FIGS. 6 and 7B , the outer peripheral surface of the inner ring 2 has annular flat portions 2c, 2c connected to the concavely curved surface 2a and parallel to the axial direction C1 at both axial ends. In this case, the convexly curved surface 3a of the outer ring 3 can be machined easily and precisely using the flat portions 3c, 3c of the outer ring 3 as a reference surface. Furthermore, the concavely curved surface 2a of the inner ring 2 can be machined easily and precisely using the flat portions 2c, 2c of the inner ring 2 as a reference surface. An annular flat portion 3c may be provided on only one axial end of the inner peripheral surface of the outer ring 3 shown in FIGS. 6 and 8 . An annular flat portion 2c may be provided on only one axial end of the outer peripheral surface of the inner ring 2.

[0035] [Third embodiment: seal portion, FIG. 9] As shown in Figure 9, in addition to a configuration in which annular flat portions 2c, 2c and 3c, 3c are provided at both axial ends of the inner and outer rings 2, 3, seal portions 4, 4 may be provided to seal the bearing space between the inner and outer rings 2, 3. In this embodiment, for example, the base end of the seal portion 4 is fitted and fixed to the axially outer portion of each flat portion 3c of the outer ring 3, and the tip end of the seal portion 4 extends radially inward. The tip end of the seal portion 4 may be either a contact type or a non-contact type.

[0036] When the sliding bearing 1 is filled with a fluid lubricant such as the aforementioned grease or oil, the provision of seals 4,4 can prevent leakage of the fluid lubricant from the sliding bearing 1 and prevent foreign matter from entering the sliding bearing 1. Alternatively, the base end of seal 4 may be fitted and fixed to the axially outer part of each flat portion 2c of the inner ring 2, with the tip end of seal 4 extending radially outward.

[0037] [Fourth embodiment: surface treatment layer, FIG. 10] As shown in Figure 10, a surface treatment layer hf serving as a sliding resistance reduction member for lowering friction on the sliding surfaces may be provided on either or both of the convexly curved surface 3a of the outer ring 3 and the concavely curved surface 2a of the inner ring 2. Examples of the surface treatment layer hf include chrome plating, electroplating (copper plating, chrome plating, etc.), electroless plating (Ni-P-based, Ni-B-based, Sn-based, etc., or composite plating of these with PTFE or SiC), shot peening, diamond-like carbon (DLC), or hairline (HL) treatment. However, surface treatments are not necessarily limited to these, and other surface treatments may also be applied depending on the conditions of use of the sliding bearing 1.

[0038] When providing this sliding bearing with a surface treatment layer hf that serves as a sliding resistance reduction component, it is desirable to avoid generating edge loads on the surface treatment layer hf. For this reason, it is preferable to smoothly round both ends of the side facing the surface treatment layer hf. In other words, when the axial dimension b of the concavely curved surface 2a is greater than or equal to the axial dimension B of the convexly curved surface 3a, providing a surface treatment layer hf on the convexly curved surface 3a of the outer ring 3 can suppress the generation of edge loads on this surface treatment layer hf.

[0039] When the axial dimension b of the concavely curved surface 2a is smaller than the axial dimension B of the convexly curved surface 3a, providing a surface treatment layer hf on the concavely curved surface 2a of the inner ring 2 can suppress the occurrence of edge load on this surface treatment layer hf. Furthermore, by reducing friction on the sliding surface with the surface treatment layer hf, power loss in a drive source such as a motor can be reduced compared to a structure without a surface treatment layer. This also makes it possible to reduce the size of the drive source. In a configuration in which the inner peripheral surface of the outer ring 3 has annular flat portions 3c, 3c (FIG. 6) at one end or both ends in the axial direction, and the outer peripheral surface of the inner ring 2 has annular flat portions 2c, 2c (FIG. 6) at one end or both ends in the axial direction, a surface treatment layer hf may be provided on either or both of the convexly curved surface 3a of the outer ring 3 and the concavely curved surface 2a of the inner ring 2.

[0040] [Fifth embodiment: Inner ring division, Figs. 11A and 11B] 11A and 11B, the inner ring 2 may have two divided inner ring segments 2A, 2A aligned in the axial direction C1. The inner ring segments 2A, 2A in this example have the same structure and are symmetrical with respect to the axial direction C1, as shown in FIG. <About gaps> As shown in Figure 11B, the width dimension W of the inner ring 2 can be adjusted by dividing the inner ring 2 into two inner ring segments 2A, 2A. Furthermore, by inserting a spacer (not shown) between the inner ring segments 2A, 2A, the clearance between the inner circumferential surface of the outer ring 3 and the outer circumferential surface of the inner ring 2 can be easily adjusted. Furthermore, to increase the rigidity of the sliding bearing 1, it may be used with a negative clearance (preload).

[0041] <Contact status> When the inner ring is divided into two inner ring segments 2A, 2A as shown in Figure 11A, by making the central positions P1, P2 of the radius of curvature of one inner ring segment 2A and the other inner ring segment 2A different, the concavely curved surface 2a of the inner ring 2 can serve as a guide surface having a compound curved surface, and the inner ring 2 and outer ring 3 can be in two-point contact as shown in Figure 11B.

[0042] The convexly curved surface 3a of the outer ring 3 and the concavely curved surface 2a of the inner ring 2 are not limited to a single R shape, and may be a compound curved surface in which multiple curved surfaces are connected. It is also possible for either the convex curved surface 3a or the concave curved surface 2a to have a single R shape, and for the other to be a compound curved surface. The resin inner ring 2 or outer ring 3 may be produced, for example, by a 3D printer. It is also possible to use sliding bearings for purposes other than robot joints.

[0043] Although the embodiments of the present invention have been described above, the disclosed embodiments are illustrative in all respects and are not limiting. The scope of the present invention is defined by the claims rather than the above description, and it is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0044] 1... sliding bearing, 2... inner ring, 2a... concave curved surface, 2aa... deepest part, 2c... flat part, 3... outer ring, 3a... convex curved surface, 3aa... top, 3c... flat part, hf... surface treatment layer

Claims

1. A sliding bearing comprising an inner ring and an outer ring, the inner peripheral surface of the outer ring has a convexly curved surface having an annular convex portion shape that convex radially inward, a sliding bearing in which the outer peripheral surface of the inner ring has a concavely curved surface with an annular recessed portion that fits into the convexly curved surface and is capable of sliding circumferentially relative to the outer ring, and the outer ring and the inner ring are circumferentially continuous annular bodies.

2. 2. The sliding bearing according to claim 1, wherein an apex of the convexly curved surface is provided at a central portion of the inner peripheral surface of the outer ring in the axial direction, and the convexly curved surface is a curved surface that increases in diameter from the apex to both sides in the axial direction, A sliding bearing comprising: a deepest portion of the concavely curved surface located at the axial center of the outer peripheral surface of the inner ring, the concavely curved surface being a curved surface that increases in diameter from the deepest portion to both sides in the axial direction.

3. 3. The sliding bearing according to claim 1 or 2, wherein the concavely curved surface and the convexly curved surface each have a single radius of curvature, and the radius of curvature r1 of the concavely curved surface is greater than the radius of curvature r2 of the convexly curved surface.

4. The sliding bearing according to claim 1 or 2, wherein one or both axial end portions of the inner peripheral surface of the outer ring have an annular flat portion that is connected to the convexly curved surface and parallel to the axial direction, and one or both axial end portions of the outer peripheral surface of the inner ring have an annular flat portion that is connected to the concavely curved surface and parallel to the axial direction.

5. 3. The sliding bearing according to claim 1 or 2, wherein, when the axial dimension b of the concavely curved surface is greater than or equal to the axial dimension B of the convexly curved surface, both axial side edges of the convexly curved surface are smoothly connected to the outer ring side surfaces that respectively continue from these both axial side edges.

6. 3. The sliding bearing according to claim 1 or 2, wherein, when the axial dimension b of the concavely curved surface is smaller than the axial dimension B of the convexly curved surface, both axial side edges of the concavely curved surface are smoothly connected to the inner ring side surfaces that respectively continue from these both axial side edges.

7. 6. The sliding bearing according to claim 5, wherein a surface treatment layer serving as a sliding resistance reduction member is provided on the convexly curved surface of the outer ring.

8. 7. The sliding bearing according to claim 6, wherein a surface treatment layer serving as a sliding resistance reduction member is provided on the concavely curved surface of the inner ring.

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