Plain bearing

Abstract

To provide a sliding bearing that allows for easy adjustment of the axial internal clearance. [Solution] The sliding bearing 1 comprises an inner ring 2 and an outer ring 3. These inner and outer rings 2 and 3 are annular bodies continuous in the circumferential direction. The inner circumferential surface of the outer ring 3 has a convex curved surface 3a with an annular convex shape that is convex radially inward. The inner ring 2 is assembled from two inner ring sections 2A, 2A that are divided and aligned in the axial direction. The outer circumferential surface of the inner ring 2 has a concave curved surface 2a with an annular recess shape that fits into the convex curved surface 3a of the outer ring 3 and is slidable relative to the outer ring 3 in the circumferential direction. A recess Gv is provided in the convex curved surface 3a of the outer ring 3. The recess Gv is a groove in the circumferential direction. Crowning is provided at both axial ends and the bottom of the groove in the concave curved surface 2a of the inner ring 2. Crowning is provided at both axial ends and the tip of the convex curved surface 3a of the outer ring 3.

Description

Technical Field

[0001] The present invention relates to a sliding bearing, for example, a sliding bearing used for a joint portion of an industrial robot, a service robot, or a speed reducer mounted on the joint portion.

Background Art

[0002] As shown in FIG. 10, as a conventional sliding bearing 50, a sliding bearing is disclosed in which a sliding surface 53 between inner and outer rings 51 and 52 has a tapered shape inclined with respect to a rotation axis C2 (Patent Document 1). By forming the sliding surface 53 into the tapered shape, this sliding bearing 50 improves the load capacity with a small number of parts, and further, by forming the outer ring 52 into an outer ring split body that is split in the axial direction, the outer ring width surface can be adjusted to adjust the clearance (preload).

[0003] When the sliding surface 53 has a tapered shape as in the prior art, an edge load may occur when supporting a moment load with this sliding bearing 50, leading to a short life of the sliding bearing 50. Therefore, the applicant of the present application has proposed a sliding bearing that suppresses edge loads by forming the sliding surfaces of the inner and outer rings 2 and 3 into a curved surface shape as shown in FIGS. 11 to 13. This sliding bearing can adjust the mating surface and adjust the axial internal clearance (preload) by splitting the inner ring 2 in the axial direction.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] As shown in FIG. 10, when the sliding surface 53 has a tapered shape, an edge load may occur when supporting a moment load with the sliding bearing 50, leading to a short life of the sliding bearing 50. In the sliding bearing shown in Figure 11, the axial internal clearance can be adjusted by adjusting the mating surfaces of the inner ring segments 2A, 2A. However, this requires adjustment or matching of the mating surfaces of the inner ring segments 2A, 2A with respect to the outer ring 3, which is time-consuming. Therefore, adjusting the axial internal clearance is time-consuming.

[0006] The object of the present invention is to provide a sliding bearing that allows for easy adjustment of the axial internal clearance. [Means for solving the problem]

[0007] The sliding bearing of the present invention is a sliding bearing comprising an inner ring and an outer ring, The inner circumferential surface of the outer ring has a convex curved surface with an annular convex shape that is convex radially inward. The inner ring is assembled from two inner ring sections arranged in the axial direction, and the outer circumferential surface of the inner ring has a concave surface in the shape of an annular recess that fits into the convex curved surface of the outer ring and is slidable relative to the outer ring in the circumferential direction. A recess is provided on the convex curved surface of the outer ring.

[0008] In sliding bearings, the following relationship exists between axial clearance, moment stiffness, and surface pressure: Increasing the preload (≒ decreasing the axial clearance) tends to correlate with increasing moment stiffness and surface pressure. Furthermore, since sliding bearings are used in surface contact, if the axial clearance is "negative clearance," a change of 1 μm significantly alters the surface pressure, making adjustment of the axial clearance difficult.

[0009] Therefore, with this configuration, when setting the allowable surface pressure, a recess is provided on the convex curved surface of the outer ring to deliberately reduce the moment stiffness. As a result, the range of axial internal clearance that satisfies the allowable surface pressure is wider for the sliding bearing with a recess on the convex curved surface compared to the range of axial internal clearance that satisfies the allowable surface pressure for a sliding bearing without a recess on the convex curved surface. Consequently, the adjustment of the axial internal clearance by the mating surface of the inner ring segment can be made easier than with the conventional sliding bearing structure described above, where the axial internal clearance is adjusted.

[0010] The aforementioned recess may be a circumferential groove. In this case, a circumferential groove can be easily machined into the convex curved surface of the outer ring by turning or grinding.

[0011] Crowning may be provided at both axial ends and the groove bottom of the concave surface of the inner ring. In this case, when a moment load is applied, the edge load during skew that occurs at both axial ends and the groove bottom of the concave surface of the inner ring can be mitigated.

[0012] Crowning may be provided at both axial ends and the tip of the convex curved surface of the outer ring. In this case, when a moment load is applied, the edge load during skew that occurs at both axial ends and the tip of the convex curved surface of the outer ring can be mitigated.

[0013] The tip of the convex curved surface of the outer ring may be made into a straight surface, and crowning may be provided at the intersection of this straight surface and the convex curved surface. However, providing crowning on the convex curved surface of the outer ring is difficult to process. Therefore, it is preferable to make the tip of the convex curved surface of the outer ring into a straight surface and provide crowning at the intersection of the straight surface and the convex curved surface. In this case, edge load during skew can be mitigated and the processing time can be reduced. [Effects of the Invention]

[0014] The sliding bearing of the present invention is a sliding bearing provided with an inner ring and an outer ring, wherein the inner peripheral surface of the outer ring has a convex curved surface in the shape of an annular convex portion that protrudes radially inward, the inner ring is assembled from two inner ring split bodies arranged in the axial direction, the outer peripheral surface of the inner ring has a concave curved surface in the shape of an annular concave portion that fits into the convex curved surface of the outer ring and is slidable relative to the outer ring in the circumferential direction, and a recess is provided on the convex curved surface of the outer ring. Therefore, the adjustment of the axial internal clearance can be easily performed.

Brief Description of the Drawings

[0015] [Figure 1] It is a longitudinal sectional view of the sliding bearing according to the first embodiment of the present invention. [Figure 2] It is a diagram showing a comparison of the relationship between the axial internal clearance and the moment rigidity in the sliding bearing with or without a groove. [Figure 3] It is a diagram showing a comparison of the relationship between the axial internal clearance and the surface pressure in the sliding bearing with or without a groove. [Figure 4] It is a diagram for explaining the location where edge loading occurs in the sliding bearing. [Figure 5A] It is a partial enlarged view showing an enlarged view of the main part of the inner ring in the sliding bearing. [Figure 5B] It is a partial enlarged view showing an enlarged view of the main part of the outer ring in the sliding bearing. [Figure 6A] It is a partial enlarged view of a part of the concave curved surface of the inner ring in the sliding bearing according to the first embodiment, where the part is made straight. [Figure 6B] It is a partial enlarged view of a part of the concave curved surface of the inner ring in the same inner ring, where the part is made in an R shape. [Figure 7A] It is a partial enlarged view of a part of the convex curved surface of the outer ring in the sliding bearing, where the part is made straight. [Figure 7B] It is a partial enlarged view of a part of the convex curved surface of the outer ring in the same outer ring, where the part is made in an R shape. [Figure 8A] It is a longitudinal sectional view of the main part of the sliding bearing according to the second embodiment of the present invention. [Figure 8B]It is a longitudinal sectional view of a main part of a sliding bearing according to a third embodiment of the present invention. [Figure 9] It is a longitudinal sectional view of a main part of a sliding bearing according to a fourth embodiment of the present invention. [Figure 10] It is a perspective view of a conventional sliding bearing. [Figure 11] It is a perspective view of a sliding bearing of a reference proposal example. [Figure 12] It is a perspective view of an outer ring in the sliding bearing. [Figure 13] It is a perspective view of an inner ring in the sliding bearing.

Embodiments for Carrying Out the Invention

[0016] [First Embodiment] A sliding bearing according to an embodiment of the present invention will be described with reference to FIGS. 1 to 7B. The sliding bearing according to the embodiment is used, for example, for a main bearing of a joint part of an industrial robot, a service robot, etc. or a speed reducer mounted on the joint part. Examples of the speed reducer include a speed reducer of a harmonic gear speed reducer method, an eccentric differential method, a planetary gear method, etc. This sliding bearing is applied, for example, as a substitute for a rolling bearing such as a conventional cross roller bearing or a four-point contact ball bearing.

[0017] <Schematic Configuration of Sliding Bearing> As shown in FIG. 1, the sliding bearing 1 includes an inner ring 2 and an outer ring 3, and is a sliding bearing capable of supporting a moment load by the bearing alone, and no rolling elements are provided. The moment load is a load acting in a direction to tilt an axis (not shown) fitted and fixed to the inner ring 2. In this specification, the sliding bearing may be simply referred to as a "bearing". The sliding bearing 1 uses, for example, non-lubrication (strictly speaking, a solid lubricant of a resin layer or a surface treatment layer described later) or a fluid lubricant such as grease or oil.

[0018] <Inner and Outer Rings> The inner and outer rings 2 and 3 are continuous annular bodies in the circumferential direction. The inner surface of the outer ring 3 has a convex curved surface 3a with an annular convex shape that is convex radially inward at or near the axial center. The inner ring 2 is assembled from two inner ring sections 2A, 2A aligned in the axial direction by multiple bolts Bt in the circumferential direction. In this example, the inner ring sections 2A, 2A have the same structure, symmetrical in the axial direction C1, except for the bolt holes Bh.

[0019] 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. By adjusting the width of the inner ring segments 2A, the gap between the inner circumferential surface of the outer ring 3 and the outer circumferential surface of the inner ring 2 can be adjusted. The gap between the inner circumferential surface of the outer ring 3 and the outer circumferential surface of the inner ring 2 can also be adjusted by inserting a spacer (not shown) between the inner ring segments 2A, 2A. The outer circumferential surface of the inner ring 2 has a concave curved surface 2a with an annular recess shape that fits into the convex curved surface 3a and is slidable relative to the outer ring 3 in the circumferential direction.

[0020] The portion near the axial center is the part of the inner circumferential surface of the outer ring 3 that is separated from the axial center by a length determined in the axial direction C1. The axial direction of the central axes of the inner ring 2 and the outer ring 3 is defined as "axial direction" C1. The defined length is set appropriately according to the operating conditions of the sliding bearing 1. The direction perpendicular to the "axial direction" C1 is defined as the "radial direction". The direction around the central axis is defined as the "circumferential direction".

[0021] <Dents, etc.> The inner circumferential surface of the outer ring 3 is provided with the apex of a convex curved surface 3a at or near the axial center. The convex curved surface 3a is a curved surface whose diameter increases as it extends from the apex toward both sides in the axial direction. A recess Gv is provided at the apex of the convex curved surface 3a. This recess Gv is a circumferential groove with a substantially rectangular cross-section that is recessed radially outward to a predetermined depth from the apex. The recess Gv is formed, for example, by machining such as turning or grinding.

[0022] The width (axial dimension) W1 of the recess Gv extends a predetermined short distance in both axial directions from the top. The recess Gv is used as a so-called oil reservoir for grease, oil, etc. The radial depth Dp and width W1 of the recess Gv are set based on the relationship between the axial internal clearance, moment stiffness, and surface pressure, which will be described later. In this specification, the axial internal clearance may be referred to as "negative clearance" or simply "clearance".

[0023] The inner circumferential surface of the outer ring 3 has annular flat sections 3c, 3c at both axial ends, connected to the convex curved surface 3a via a stepped portion and parallel to the axial direction C1. The radial heights of these flat sections 3c, 3c are set to be the same, for example. Furthermore, the inner ring 2 has annular flat portions 2c, 2c at both axial ends of its outer circumferential surface, which are connected to the concave curved surface 2a and parallel to the axial direction C1. The radial heights of these flat portions 2c, 2c are set to be the same, for example. However, the radial height of the flat portions 2c is lower than the radial height of the outer ring 3.

[0024] <Regarding bearing materials, etc.> The inner and outer rings 2 and 3 can be made of steel, lightweight materials, or resin materials. The inner and outer rings 2 and 3 may be made of the same material or different materials. As the steel material, for example, high-carbon chromium bearing steel, chromium-molybdenum steel, carbon steel for machine structures, stainless steel, etc. may be used. Molybdenum disulfide or the like may be added to any one of these. As the lightweight material, for example, aluminum, ceramics, etc. may be used.

[0025] When the above-mentioned resin material is applied, the base of the resin composition may be, for example, 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), etc.

[0026] When steel or lightweight materials are used as the material for the inner and outer rings 2 and 3, a resin layer may be formed on both sides, or on either one, of the inner circumferential surface of the outer ring 3 and the outer circumferential surface of the inner ring 2, in order to reduce the frictional resistance of the sliding surfaces. Examples of this resin layer include thermoplastic PI resin, PEK, PEEK, PPS, PEKEKK, PAI, PA, PE, POM, PTFE, etc.

[0027] <Relationship between axial internal clearance, moment stiffness, and surface pressure> The sliding bearing 1 adjusts the axial internal clearance between the inner surface of the outer ring 3 and the outer surface of the inner ring 2 based on the allowable surface pressure. The relationship between the axial internal clearance, moment stiffness, and surface pressure is as shown in Figures 2 and 3. Increasing the preload (≒ decreasing the axial internal clearance) tends to correlate with increasing moment stiffness and surface pressure. Moment stiffness refers to the stiffness against the aforementioned moment load.

[0028] Since the sliding bearing 1 (Figure 1) is used in surface contact, in the case of negative clearance, a change of 1 μm significantly alters the surface pressure, making clearance (preload) adjustment difficult. Therefore, for example, when the allowable surface pressure of the sliding bearing is 6 MPa, a groove is provided on the convex curved surface of the outer ring to deliberately reduce the moment stiffness of the sliding bearing. As a result, compared to the grooveless sliding bearing without a groove on the convex curved surface, the sliding bearing with a groove on the convex curved surface can have a wider range of axial internal clearance B (-3 μm) that satisfies the allowable surface pressure (also called "limit surface pressure").

[0029] Therefore, the axial internal clearance can be easily adjusted by the mating surfaces of the inner ring segments 2A, 2A shown in Figure 1, compared to the conventional sliding bearing structure described above, which adjusts the axial internal clearance. The reduction in moment stiffness is required by the positioning accuracy of the robot. For example, there are applications where positioning accuracy is not required, such as collaborative robots or service robots. In such applications where positioning accuracy is not required, it is permissible to reduce the moment stiffness of the sliding bearing.

[0030] <About Crowning> As shown in Figure 4, when the sliding surfaces of the inner and outer rings 2 and 3 are curved, the sliding surface at a position 90° from the center of the moment load slides in a skewed position. Therefore, edge loads, which are localized stress concentrations, can occur near the junction 3A and near the tip 3B of the convex curved surface of the outer ring, where the axial side edges 2Aa and the bottom of the groove 2Ab of the concave curved surface of the inner ring shown in Figure 5A meet.

[0031] Therefore, crowning Cw, as shown in Figures 6A to 7B, is provided on the round frame portions of the inner and outer rings 2 and 3, as shown in Figure 4. The crowning Cw is a surface shape designed to equalize the contact pressure between the convex curved surface 3a of the outer ring 3 and the concave curved surface 2a of the inner ring 2. Specifically, as shown in Figure 6A, straight-shaped crowning Cw is provided at both axial ends and at the bottom of the groove on the concave curved surface 2a of the inner ring 2. The straight-shaped crowning Cw at both axial ends and at the bottom of the groove connect smoothly to the concave curved surface 2a of the inner ring 2, respectively.

[0032] As shown in Figure 6B, crownings Cw, Cw of a curved shape may be provided at both axial ends and the bottom of the groove on the concave surface 2a of the inner ring 2, respectively. These crownings Cw, Cw are, for example, a single radius of curvature, a so-called single R shape. In this case, the center of curvature of each crowning Cw is located inside the inner ring in Figure 6B. The crownings Cw may also be composite surfaces formed by the connection of multiple curved surfaces. A combination of a single R-shaped crowning Cw and a composite surface crowning Cw may be used at both axial ends and the bottom of the groove on the concave surface 2a of the inner ring 2. The crowning shapes may differ at both axial ends and the bottom of the groove on the concave surface 2a of the inner ring 2.

[0033] The crowning Cw shown in Figures 6A and 6B can be performed using any mechanically controllable method, such as polishing, tumbler, barrel, shot, or superfinishing, or by hand lapping. The crowning Cw of the outer ring 3 shown in Figures 7A and 7B, described later, can be performed in the same manner.

[0034] As shown in Figure 7A, straight crownings Cw are provided at both axial ends and the tip of the convex curved surface 3a of the outer ring 3. The straight crownings Cw at both axial ends and the straight crownings Cw at the tip are smoothly connected to the convex curved surface 3a of the outer ring 3. Furthermore, the straight crownings Cw at the tip are set to be wider by a predetermined length in the axial direction C1 than the width W1 of the recess Gv, for example.

[0035] When crowning is provided on the convex curved surface 3a of the outer ring 3, machining is difficult. Therefore, as shown in Figure 7B, it is preferable to make the tip of the convex curved surface 3a of the outer ring 3 a straight surface Sa, and to provide a curved crowning Cw at the intersection of this straight surface Sa and the convex curved surface 3a. Furthermore, curved crowning Cw is provided at both axial ends of the convex curved surface 3a of the outer ring 3. In this case, edge loading during skew can be mitigated and machining time can be reduced.

[0036] These crownings Cw may, for example, have a single radius of curvature, a so-called single R shape, or they may be composite surfaces formed by the connection of multiple curved surfaces. In this case, the center of curvature of each crowning Cw is located inside the outer ring in Figure 7B. At both axial ends and the intersection points of the convex curved surface 3a of the outer ring 3, a combination of a single R-shaped crowning Cw and a composite surface crowning Cw may be made. At both axial ends and the intersection points of the convex curved surface 3a of the outer ring 3, the crowning shapes may differ.

[0037] As explained above, the crowning Cw of the inner and outer rings may be a straight surface, a curved surface, a combination of a straight surface and a curved surface, or a combination of different curved surfaces. Since the stress acting on the inner and outer rings changes depending on the load conditions, the crowning Cw may be applied only to the inner ring 2 or outer ring 3 in Figure 1. Furthermore, the crowning shape may differ between the inner and outer rings 2 and 3, the sliding surface end faces, and the groove bottom.

[0038] <Effects and Effects> As described above, the sliding bearing 1 has a recess Gv on the convex curved surface 3a of the outer ring 3, which deliberately reduces the moment stiffness. As a result, the sliding bearing 1 with a recess Gv on the convex curved surface 3a has a wider range of axial internal clearance that satisfies the allowable surface pressure compared to a sliding bearing without a recess on the convex curved surface. Therefore, the adjustment of the axial internal clearance by the mating surfaces of the inner ring divisions 2A, 2A can be made easier than with the conventional sliding bearing structure described above, which adjusts the axial internal clearance. The sliding bearing 1 can obtain the desired moment stiffness by adjusting the axial internal clearance.

[0039] The recess Gv is a circumferential groove. In this case, a circumferential groove can be easily machined into the convex curved surface 3a of the outer ring 3 by turning or grinding. The recess Gv is used as a so-called oil reservoir for grease, oil, etc. Therefore, it is possible to supply lubricant such as grease to the sliding surface from the recess Gv and extend the lubrication life.

[0040] When crowning is provided at both axial ends and the groove bottom of the concave curved surface 2a of the inner ring 2, the edge load during skew that occurs at both axial ends and the groove bottom of the concave curved surface 2a of the inner ring 2 when a moment load is applied can be mitigated. When crowning is provided at both axial ends and the tip of the convex curved surface 3a of the outer ring 3, the edge load during skew that occurs at both axial ends and the tip of the convex curved surface 3a of the outer ring 3 when a moment load is applied can be mitigated.

[0041] <Regarding other embodiments> In the following description, parts corresponding to matters previously described in each embodiment will be denoted by the same reference numerals, and redundant explanations will be omitted. When only a part of the configuration is described, the other parts of the configuration will be the same as those in the previously described embodiment unless otherwise specified. Identical configurations will produce the same effects. Not only are combinations of the parts specifically described in each embodiment possible, but partial combinations of embodiments are also possible, provided that there are no particular problems with the combination.

[0042] [Second embodiment: Change of recess position, Figure 8A] In cases where the moment load acting on the sliding bearing 1 is directional, it may be preferable not to provide the recess Gv in the axial center of the inner circumferential surface of the outer ring 3. In this case, the recess Gv is not limited to the axial center of the inner circumferential surface of the outer ring 3. For example, as shown in Figure 8A, the recess Gv may be provided at a position offset in one axial direction (to the right in Figure 8A) from the apex of the convex curved surface 3a. The amount of offset from the apex can be appropriately determined, for example, by testing or simulation.

[0043] [Third embodiment: Seal portion, Figure 8B] As shown in Figure 8B, in addition to the configuration having annular flat portions 2c, 2c, 3c, 3c at both axial ends of the inner and outer rings 2 and 3, the bearing may also be provided with seal portions 4, 4 that seal the bearing space between the inner and outer rings 2 and 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 of the seal portion 4 extends radially inward. The tip of the seal portion 4 may be either a contact type or a non-contact type.

[0044] When a fluid lubricant such as grease is sealed inside the sliding bearing 1, providing seal portions 4, 4 prevents leakage of the fluid lubricant from the sliding bearing 1 and prevents foreign matter from entering the sliding bearing 1. The base end of the seal portion 4 may be fitted and fixed to the axially outer portion of each flat portion 2c of the inner ring 2, and the tip of the seal portion 4 may extend radially outward.

[0045] [Fourth embodiment: No flat section, Figure 9] As shown in Figure 9, the inner and outer rings 2 and 3 may be configured without annular flat portions at both axial ends. In this case, a larger sliding surface area can be secured than in the embodiments described above, and even though it is a single sliding bearing, a large distance between the points of action can be secured, similar to a back-to-back combination of combination bearings. Therefore, it is possible to have superior rigidity against moment loads and increase the load capacity.

[0046] <Other> A surface treatment layer that acts as a sliding resistance reducing member to reduce friction on the sliding surfaces may be provided on either or both of the convex curved surface 3a of the outer ring 3 and the concave curved surface 2a of the inner ring 2. Examples of surface treatment layers include chromium plating, electroplating (copper plating, chromium plating, etc.), electroless plating (Ni-P system, Ni-B system, Sn system, etc., or composite plating including PTFE or SiC), shot peening, diamond-like carbon (DLC), or hairline (HL) treatment. However, the surface treatment is not necessarily limited to these, and other surface treatments may be applied depending on the operating conditions of the sliding bearing 1.

[0047] The recesses are not limited to grooves in the circumferential direction. For example, multiple recesses may be provided on the convex surface of the outer ring at regular or appropriate intervals in the circumferential direction. It is also possible to provide only one recess on the convex surface of the outer ring. Depending on the load conditions, it is also possible to have a configuration without crowning on the inner and outer rings. The inner or outer ring made of resin may be manufactured, for example, using a 3D printer. Sliding bearings can also be used for applications other than robot joints.

[0048] While embodiments for carrying out the present invention have been described above based on the embodiments, the embodiments disclosed herein are illustrative in all respects and are not restrictive. The scope of the present invention is indicated by the claims rather than the foregoing description, and all modifications within the meaning and scope equivalent to the claims are intended to be included. [Explanation of Symbols]

[0049] 1…Sliding bearing, 2…Inner ring, 2A…Inner ring split, 2a…Concave curved surface, 3…Outer ring, 3a…Convex curved surface, Cw…Crowning, Gv…Concave

Claims

1. A sliding bearing comprising an inner ring and an outer ring, The inner circumferential surface of the outer ring has a convex curved surface with an annular convex shape that is convex radially inward. The inner ring is assembled from two inner ring sections arranged in the axial direction, and the outer circumferential surface of the inner ring has a concave surface in the shape of an annular recess that fits into the convex curved surface of the outer ring and is slidable relative to the outer ring in the circumferential direction. A sliding bearing in which a recess is provided on the convex curved surface of the outer ring.

2. A sliding bearing according to claim 1, wherein the recess is a groove in the circumferential direction.

3. A sliding bearing according to claim 1 or claim 2, wherein crowning is provided at both axial ends and the bottom of the groove on the concave curved surface of the inner ring.

4. A sliding bearing according to claim 1 or claim 2, wherein crowning is provided at both axial ends and the tip of the convex curved surface of the outer ring.

5. A sliding bearing according to claim 1 or claim 2, wherein the tip of the convex curved surface of the outer ring is a straight surface, and crowning is provided at the intersection of the straight surface and the convex curved surface.

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